NSIS Working Group M. Stiemerling Internet-Draft NEC Intended status: Standards Track H. Tschofenig Expires: September 6, 2007 Siemens C. Aoun E. Davies Folly Consulting March 5, 2007 NAT/Firewall NSIS Signaling Layer Protocol (NSLP) draft-ietf-nsis-nslp-natfw-14.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 6, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Stiemerling, et al. Expires September 6, 2007 [Page 1] Internet-Draft NAT/FW NSIS NSLP March 2007 Abstract This memo defines the NSIS Signaling Layer Protocol (NSLP) for Network Address Translators (NATs) and firewalls. This NSLP allows hosts to signal on the data path for NATs and firewalls to be configured according to the needs of the application data flows. It enables hosts behind NATs to obtain a public reachable address and hosts behind firewalls to receive data traffic. The overall architecture is given by the framework and requirements defined by the Next Steps in Signaling (NSIS) working group. The network scenarios, the protocol itself, and examples for path-coupled signaling are given in this memo. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Terminology and Abbreviations . . . . . . . . . . . . . . 7 1.2. Middleboxes . . . . . . . . . . . . . . . . . . . . . . . 10 1.3. General Scenario for NATFW Traversal . . . . . . . . . . 11 2. Network Deployment Scenarios using the NATFW NSLP . . . . . . 13 2.1. Firewall Traversal . . . . . . . . . . . . . . . . . . . 13 2.2. NAT with two private Networks . . . . . . . . . . . . . . 14 2.3. NAT with Private Network on Sender Side . . . . . . . . . 15 2.4. NAT with Private Network on Receiver Side Scenario . . . 15 2.5. Both End Hosts behind twice-NATs . . . . . . . . . . . . 16 2.6. Both End Hosts Behind Same NAT . . . . . . . . . . . . . 17 2.7. Multihomed Network with NAT . . . . . . . . . . . . . . . 18 2.8. Multihomed Network with Firewall . . . . . . . . . . . . 19 3. Protocol Description . . . . . . . . . . . . . . . . . . . . 20 3.1. Policy Rules . . . . . . . . . . . . . . . . . . . . . . 20 3.2. Basic Protocol Overview . . . . . . . . . . . . . . . . . 21 3.2.1. Message Types . . . . . . . . . . . . . . . . . . . . 25 3.2.2. Classification of RESPONSE Messages . . . . . . . . . 25 3.2.3. NATFW NSLP Signaling Sessions . . . . . . . . . . . . 26 3.3. Basic Message Processing . . . . . . . . . . . . . . . . 27 3.4. Calculation of Signaling Session Lifetime . . . . . . . . 27 3.5. Message Sequencing . . . . . . . . . . . . . . . . . . . 30 3.6. Authentication, Authorization, and Policy Decisions . . . 31 3.7. Protocol Operations . . . . . . . . . . . . . . . . . . . 32 3.7.1. Creating Signaling Sessions . . . . . . . . . . . . . 32 3.7.2. Reserving External Addresses . . . . . . . . . . . . 35 3.7.3. NATFW NSLP Signaling Session Refresh . . . . . . . . 42 3.7.4. Deleting Signaling Sessions . . . . . . . . . . . . . 43 3.7.5. Reporting Asynchronous Events . . . . . . . . . . . . 44 3.7.6. Proxy Mode of Operation . . . . . . . . . . . . . . . 46 Stiemerling, et al. Expires September 6, 2007 [Page 2] Internet-Draft NAT/FW NSIS NSLP March 2007 3.8. De-Multiplexing at NATs . . . . . . . . . . . . . . . . . 49 3.9. Reacting to Route Changes . . . . . . . . . . . . . . . . 51 3.10. Updating Policy Rules . . . . . . . . . . . . . . . . . . 51 4. NATFW NSLP Message Components . . . . . . . . . . . . . . . . 53 4.1. NSLP Header . . . . . . . . . . . . . . . . . . . . . . . 53 4.2. NSLP Objects . . . . . . . . . . . . . . . . . . . . . . 54 4.2.1. Signaling Session Lifetime Object . . . . . . . . . . 55 4.2.2. External Address Object . . . . . . . . . . . . . . . 55 4.2.3. Extended Flow Information Object . . . . . . . . . . 56 4.2.4. Information Code Object . . . . . . . . . . . . . . . 57 4.2.5. Nonce Object . . . . . . . . . . . . . . . . . . . . 60 4.2.6. Message Sequence Number Object . . . . . . . . . . . 60 4.2.7. Data Terminal Information Object . . . . . . . . . . 61 4.2.8. ICMP Types Object . . . . . . . . . . . . . . . . . . 62 4.3. Message Formats . . . . . . . . . . . . . . . . . . . . . 63 4.3.1. CREATE . . . . . . . . . . . . . . . . . . . . . . . 64 4.3.2. EXTERNAL (EXT) . . . . . . . . . . . . . . . . . . . 64 4.3.3. RESPONSE . . . . . . . . . . . . . . . . . . . . . . 65 4.3.4. NOTIFY . . . . . . . . . . . . . . . . . . . . . . . 65 5. Security Considerations . . . . . . . . . . . . . . . . . . . 67 5.1. Authorization Framework . . . . . . . . . . . . . . . . . 67 5.1.1. Peer-to-Peer Relationship . . . . . . . . . . . . . . 67 5.1.2. Intra-Domain Relationship . . . . . . . . . . . . . . 68 5.1.3. End-to-Middle Relationship . . . . . . . . . . . . . 69 5.2. Security Threats and Requirements . . . . . . . . . . . . 70 5.2.1. Data Sender (DS) behind a firewall . . . . . . . . . 70 5.2.2. Data Sender (DS) behind a NAT . . . . . . . . . . . . 71 5.2.3. Data Receiver (DR) behind a firewall . . . . . . . . 71 5.2.4. Data Receiver (DR) behind a NAT . . . . . . . . . . . 73 5.2.5. NSLP Message Injection . . . . . . . . . . . . . . . 74 5.3. Denial-of-Service Attacks . . . . . . . . . . . . . . . . 75 5.3.1. Flooding with CREATE messages from outside . . . . . 75 5.3.2. Flooding with EXT messages from inside . . . . . . . 76 5.4. Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . 76 5.5. Message Modification by non-NSIS on-path node . . . . . . 77 5.6. Message Modification by malicious NSIS node . . . . . . . 77 5.7. Signaling Session Ownership . . . . . . . . . . . . . . . 78 5.7.1. Misuse of Mobility in a NAT Handling Scenario . . . . 78 5.8. Misuse of unreleased signaling sessions . . . . . . . . . 79 5.9. Data Traffic Injection . . . . . . . . . . . . . . . . . 81 5.10. Eavesdropping and Traffic Analysis . . . . . . . . . . . 81 5.11. Security Framework for the NAT/Firewall NSLP . . . . . . 81 5.11.1. Security Protection between neighboring NATFW NSLP Nodes . . . . . . . . . . . . . . . . . . . . . . . . 82 5.11.2. Security Protection between non-neighboring NATFW NSLP Nodes . . . . . . . . . . . . . . . . . . . . . 82 Stiemerling, et al. Expires September 6, 2007 [Page 3] Internet-Draft NAT/FW NSIS NSLP March 2007 6. IAB Considerations on UNSAF . . . . . . . . . . . . . . . . . 85 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 86 8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 88 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 89 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 90 10.1. Normative References . . . . . . . . . . . . . . . . . . 90 10.2. Informative References . . . . . . . . . . . . . . . . . 90 Appendix A. Selecting Signaling Destination Addresses for EXT . 92 Appendix B. Applicability Statement on Data Receivers behind Firewalls . . . . . . . . . . . . . . . . . . . . . 93 Appendix C. Firewall and NAT Resources . . . . . . . . . . . . . 95 C.1. Wildcarding of Policy Rules . . . . . . . . . . . . . . . 95 C.2. Mapping to Firewall Rules . . . . . . . . . . . . . . . . 95 C.3. Mapping to NAT Bindings . . . . . . . . . . . . . . . . . 96 C.4. NSLP Handling of Twice-NAT . . . . . . . . . . . . . . . 96 Appendix D. Assigned Numbers for Testing . . . . . . . . . . . . 98 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 99 Intellectual Property and Copyright Statements . . . . . . . . . 100 Stiemerling, et al. Expires September 6, 2007 [Page 4] Internet-Draft NAT/FW NSIS NSLP March 2007 1. Introduction Firewalls and Network Address Translators (NAT) have both been used throughout the Internet for many years, and they will remain present for the foreseeable future. Firewalls are used to protect networks against certain types of attacks from internal networks and the Internet, whereas NATs provide a virtual extension of the IP address space. Both types of devices may be obstacles to some applications, since they only allow traffic created by a limited set of applications to traverse them, typically those that use protocols with relatively predetermined and static properties (e.g., most HTTP traffic, and other client/server applications). Other applications, such as IP telephony and most other peer-to-peer applications, which have more dynamic properties, create traffic that is unable to traverse NATs and firewalls unassisted. In practice, the traffic of many applications cannot traverse autonomous firewalls or NATs, even when they have additional functionality which attempts to restore the transparency of the network. Several solutions to enable applications to traverse such entities have been proposed and are currently in use. Typically, application level gateways (ALG) have been integrated with the firewall or NAT to configure the firewall or NAT dynamically. Another approach is middlebox communication (MIDCOM). In this approach, ALGs external to the firewall or NAT configure the corresponding entity via the MIDCOM protocol [7]. Several other work-around solutions are available, such as STUN [15]. However, all of these approaches introduce other problems that are generally hard to solve, such as dependencies on the type of NAT implementation (full-cone, symmetric, etc), or dependencies on certain network topologies. NAT and firewall (NATFW) signaling shares a property with Quality of Service (QoS) signaling. The signaling of both must reach any device on the data path that is involved in, respectively, NATFW or QoS treatment of data packets. This means, that for both, NATFW and QoS, it is convenient if signaling travels path-coupled, meaning that the signaling messages follow exactly the same path that the data packets take. RSVP [11] is an example of a current QoS signaling protocol that is path-coupled. [22] proposes the use of RSVP as firewall signaling protocol but does not include NATs. This memo defines a path-coupled signaling protocol for NAT and firewall configuration within the framework of NSIS, called the NATFW NSIS Signaling Layer Protocol (NSLP). The general requirements for NSIS are defined in [5] and the general framework of NSIS is outlined in [4]. It introduces the split between an NSIS transport layer and an NSIS signaling layer. The transport of NSLP messages is handled by an NSIS Network Transport Layer Protocol (NTLP, with General Stiemerling, et al. Expires September 6, 2007 [Page 5] Internet-Draft NAT/FW NSIS NSLP March 2007 Internet Signaling Transport (GIST) [2] being the implementation of the abstract NTLP). The signaling logic for QoS and NATFW signaling is implemented in the different NSLPs. The QoS NSLP is defined in [6]. The NATFW NSLP is designed to request the dynamic configuration of NATs and/or firewalls along the data path. Dynamic configuration includes enabling data flows to traverse these devices without being obstructed, as well as blocking of particular data flows at inbound firewalls. Enabling data flows requires the loading of firewall rules with an action that allows the data flow packets to be forwarded and creating NAT bindings. Blocking of data flows requires the loading of firewalls rules with an action that will deny forwarding of the data flow packets. A simplified example for enabling data flows: A source host sends a NATFW NSLP signaling message towards its data destination. This message follows the data path. Every NATFW NSLP-enabled NAT/firewall along the data path intercepts these messages, processes them, and configures itself accordingly. Thereafter, the actual data flow can traverse all these configured firewalls/NATs. It is necessary to distinguish between two different basic scenarios when operating the NATFW NSLP, independent of the type of the middleboxes to be configured. 1. Both, data sender and data receiver, are NSIS NATFW NSLP aware. This includes the cases where the data sender is logically decomposed from the NSIS initiator or the data receiver logically decomposed from the NSIS receiver, but both sides support NSIS. This scenario assumes deployment of NSIS all over the Internet, or at least at all NATs and firewalls. This scenario is used as base assumption, if not otherwise noted. 2. Only one end host or region of the network is NSIS NATFW NSLP aware, either data receiver or data sender. This scenario is referred to as proxy mode. The NATFW NSLP has two basic signaling messages which are sufficient to cope with the various possible scenarios likely to be encountered before and after widespread deployment of NSIS: CREATE message: The basic message for configuring a path outbound from a data sender to a data receiver. EXTERNAL (EXT) message: Used to locate inbound NATs/firewalls and prime them to expect outbound signaling and at NATs to pre- allocate a public address. This is used for data receivers behind these devices to enable their reachability. Stiemerling, et al. Expires September 6, 2007 [Page 6] Internet-Draft NAT/FW NSIS NSLP March 2007 CREATE and EXT messages are sent by the NSIS initiator (NI) towards the NSIS responder (NR). Both type of messages are acknowledged by a subsequent RESPONSE message. This RESPONSE message is generated by the NR if the requested configuration can be established, otherwise the NR or any of the NSIS forwarders (NFs) can also generate such a message if an error occurs. NFs and the NR can also generate asynchronous messages to notify the NI, the so called NOTIFY messages. If the data receiver resides in a private addressing realm or firewall, and needs to preconfigure the edge-NAT/edge-firewall to provide a (publicly) reachable address for use by the data sender, a combination of EXTERNAL and CREATE messages is used. During the introduction of NSIS, it is likely that one or other of the data sender and receiver will not be NSIS aware. In these cases, the NATFW NSLP can utilize NSIS aware middleboxes on the path between the data sender and data receiver to provide proxy NATFW NSLP services (i.e., the proxy mode operation). Typically, these boxes will be at the boundaries of the realms in which the end hosts are located. The CREATE and EXT messages create NATFW NSLP and NTLP state in NSIS entities. NTLP state allows signaling messages to travel in the forward (outbound) and the reverse (inbound) direction along the path between a NAT/firewall NSLP sender and a corresponding receiver. This state is managed using a soft-state mechanism, i.e., it expires unless it is refreshed from time to time. The NAT bindings and firewall rules being installed during the state setup are bound to the particular signaling session. However, the exact local implementation of the NAT bindings and firewall rules are NAT/ firewall specific. This memo is structured as follows. Section 2 describes the network environment for NATFW NSLP signaling. Section 3 defines the NATFW signaling protocol and Section 4 defines the message components and the overall messages used in the protocol. The remaining parts of the main body of the document, covers security considerations Section 5, IAB considerations on UNilateral Self-Address Fixing (UNSAF) [12] in Section 6 and IANA considerations in Section 7. Please note that readers familiar with firewalls and NATs and their possible location within networks can safely skip Section 2. 1.1. Terminology and Abbreviations The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [1]. Stiemerling, et al. Expires September 6, 2007 [Page 7] Internet-Draft NAT/FW NSIS NSLP March 2007 This document uses a number of terms defined in [5] and [4]. The following additional terms are used: o Policy rule: A policy rule is "a basic building block of a policy- based system. It is the binding of a set of actions to a set of conditions - where the conditions are evaluated to determine whether the actions are performed" [16]. In the context of NSIS NATFW NSLP, the conditions are the specification of a set of packets to which the rule is applied. The set of actions always contains just a single element per rule, and is limited to either action "deny" or action "allow". o Reserved policy rule: A policy rule stored at NATs or firewalls for activation by a later, different signaling exchange. This type of policy rule is kept in the NATFW NSLP and is not loaded into the firewall or NAT engine, i.e., it does not affect the data flow handling. o Installed policy rule: A policy rule in operation at NATs or firewalls. This type of rule is kept in the NATFW NSLP and is loaded into the firewall or NAT engine, i.e., it is affecting the data flow. o Remembered policy rule: A policy rule stored at NATs and firewalls for immediate use, as soon as the signaling exchange is successfully completed. o Firewall: A packet filtering device that matches packets against a set of policy rules and applies the actions. In the context of NSIS NATFW NSLP we refer to this device as a firewall. o Network Address Translator: Network Address Translation is a method by which IP addresses are mapped from one IP address realm to another, in an attempt to provide transparent routing between hosts (see [9]). Network Address Translators are devices that perform this work by modifying packets passing through them. o Middlebox: "A middlebox is defined as any intermediate device performing functions other than the normal, standard functions of an IP router on the datagram path between a source host and a destination host" [10]. In the context of this document, the term middlebox refers to firewalls and NATs only. Other types of middlebox are outside of the scope of this document. o Data Receiver (DR): The node in the network that is receiving the data packets of a flow. Stiemerling, et al. Expires September 6, 2007 [Page 8] Internet-Draft NAT/FW NSIS NSLP March 2007 o Data Sender (DS): The node in the network that is sending the data packets of a flow. o NATFW NSLP peer or peer: An NSIS NATFW NSLP node with which an NSIS adjacency has been created as defined in [2]. o NATFW NSLP signaling session or signaling session: A signaling session defines an association between the NI, NFs, and the NR related to a data flow. All the NATFW NSLP peers on the path, including the NI and the NR, use the same identifier to refer to the state stored for the association. The same NI and NR may have more than one signaling session active at any time. The state for NATFW NSLP consists of NSLP state and associated policy rules at a middlebox. o Edge-NAT: An edge-NAT is a NAT device with a globally routable IP address which is reachable from the public Internet. o Edge-firewall: An edge-firewall is a firewall device that is located on the border line of an administrative domain. o Public Network: "A Global or Public Network is an address realm with unique network addresses assigned by Internet Assigned Numbers Authority (IANA) or an equivalent address registry. This network is also referred as external network during NAT discussions" [9]. o Private/Local Network: "A private network is an address realm independent of external network addresses. Private network may also be referred alternately as Local Network. Transparent routing between hosts in private realm and external realm is facilitated by a NAT router" [9]. o Public/Global IP address: An IP address located in the public network according to Section 2.7 of [9]. o Private/Local IP address: An IP address located in the private network according to Section 2.8 of [9]. o Signaling Destination Address (SDA): An IP address generally taken from the public/global IP address range, although, the SDA may in certain circumstances be part of the private/local IP address range. This address is used in EXT signaling message exchanges, if the data receiver's IP address is unknown. Stiemerling, et al. Expires September 6, 2007 [Page 9] Internet-Draft NAT/FW NSIS NSLP March 2007 1.2. Middleboxes The term middlebox covers a range of devices which intercept the flow of packets between end hosts and perform actions other than standard forwarding expected in an IP router. As such, middleboxes fall into a number of categories with a wide range of functionality, not all of which is pertinent to the NATFW NSLP. Middlebox categories in the scope of this memo are firewalls that filter data packets against a set of filter rules, and NATs that translate packet addresses from one address realm to another address realm. Other categories of middleboxes, such as QoS traffic shapers, are out of scope of this memo. The term NAT used in this document is a placeholder for a range of different NAT flavors. We consider the following types of NATs: o Traditional NAT (basic NAT and NAPT) o Bi-directional NAT o Twice-NAT o Multihomed NAT For definitions and a detailed discussion about the characteristics of each NAT type please see [9]. All types of middleboxes under consideration here, use policy rules to make a decision on data packet treatment. Policy rules consist of a flow identifier which selects the packets to which the policy applies and an associated action; data packets matching the flow identifier are subjected to the policy rule action. A typical flow identifier is the 5-tuple selector which matches the following fields of a packet to configured values: o Source and destination IP addresses o Transport protocol number o Transport source and destination port numbers Actions for firewalls are usually one or more of: o Allow: forward data packet o Deny: block data packet and discard it Stiemerling, et al. Expires September 6, 2007 [Page 10] Internet-Draft NAT/FW NSIS NSLP March 2007 o Other actions such as logging, diverting, duplicating, etc Actions for NATs include (amongst many others): o Change source IP address and transport port number to a globally routeable IP address and associated port number. o Change destination IP address and transport port number to a private IP address and associated port number. It should be noted that a middlebox may contain two logical representations of the policy rule. The policy rule has a representation within the NATFW NSLP, comprising the message routing information (MRI) of the NTLP and NSLP information (such as the rule action). The other representation is the implementation of the NATFW NSLP policy rule within the NAT and firewall engine of the particular device. Refer to Appendix C for further details. 1.3. General Scenario for NATFW Traversal The purpose of NSIS NATFW signaling is to enable communication between endpoints across networks, even in the presence of NAT and firewall middleboxes that have not been specially engineered to facilitate communication with the application protocols used. This removes the need to create and maintain application layer gateways for specific protocols that have been commonly used to provide transparency in previous generations of NAT and firewall middleboxes. It is assumed that these middleboxes will be statically configured in such a way that NSIS NATFW signaling messages themselves are allowed to reach the locally installed NATFW NSLP daemon. NSIS NATFW NSLP signaling is used to dynamically install additional policy rules in all NATFW middleboxes along the data path that will allow transmission of the application data flow(s). Firewalls are configured to forward data packets matching the policy rule provided by the NSLP signaling. NATs are configured to translate data packets matching the policy rule provided by the NSLP signaling. An additional capability, that is an exception to the primary goal of NSIS NATFW signaling, is that the NATFW nodes can request blocking of particular data flows instead of enabling these flows at inbound firewalls. The basic high-level picture of NSIS usage is that end hosts are located behind middleboxes, meaning that there is a middlebox on the data path from the end host in a private network and the external network (NATFW in Figure 1). Applications located at these end hosts try to establish communication with corresponding applications on other such end hosts. They trigger the NSIS entity at the local host to control provisioning for middlebox traversal along the prospective Stiemerling, et al. Expires September 6, 2007 [Page 11] Internet-Draft NAT/FW NSIS NSLP March 2007 data path (e.g., via an API call). The NSIS entity in turn uses NSIS NATFW NSLP signaling to establish policy rules along the data path, allowing the data to travel from the sender to the receiver unobstructed. Application Application Server (0, 1, or more) Application +----+ +----+ +----+ | +------------------------+ +------------------------+ | +-+--+ +----+ +-+--+ | | | NSIS Entities NSIS Entities | +-+--+ +----+ +-----+ +-+--+ | +--------+ +----------------------------+ +-----+ | +-+--+ +-+--+ +--+--+ +-+--+ | | ------ | | | | //// \\\\\ | | +-+--+ +-+--+ |/ | +-+--+ +-+--+ | | | | | Internet | | | | | | +--------+ +-----+ +----+ +-----+ | +----+ +----+ |\ | +----+ +----+ \\\\ ///// sender NATFW (1+) ------ NATFW (1+) receiver Note that 1+ refers to one or more NATFW nodes. Figure 1: Generic View of NSIS with NATs and/or Firewalls For end-to-end NATFW signaling, it is necessary that each firewall and each NAT along the path between the data sender and the data receiver implements the NSIS NATFW NSLP. There might be several NATs and FWs in various possible combinations on a path between two hosts. Section 2 presents a number of likely scenarios with different combinations of NATs and firewalls. Stiemerling, et al. Expires September 6, 2007 [Page 12] Internet-Draft NAT/FW NSIS NSLP March 2007 2. Network Deployment Scenarios using the NATFW NSLP This section introduces several scenarios for middlebox placement within IP networks. Middleboxes are typically found at various different locations, including at enterprise network borders, within enterprise networks, as mobile phone network gateways, etc. Usually, middleboxes are placed more towards the edge of networks than in network cores. Firewalls and NATs may be found at these locations either alone, or they may be combined; other categories of middleboxes may also be found at such locations, possibly combined with the NATs and/or firewalls. Using combined middleboxes typically reduces the number of network elements needed. NSIS initiators (NI) send NSIS NATFW NSLP signaling messages via the regular data path to the NSIS responder (NR). On the data path, NATFW NSLP signaling messages reach different NSIS nodes that implement the NATFW NSLP. Each NATFW NSLP node processes the signaling messages according to Section 3 and, if necessary, installs policy rules for subsequent data packets. Each of the following sub-sections introduces a different scenario for a different set of middleboxes and their ordering within the topology. It is assumed that each middlebox implements the NSIS NATFW NSLP signaling protocol. 2.1. Firewall Traversal This section describes a scenario with firewalls only; NATs are not involved. Each end host is behind a firewall. The firewalls are connected via the public Internet. Figure 2 shows the topology. The part labeled "public" is the Internet connecting both firewalls. +----+ //----\\ +----+ NI -----| FW |---| |------| FW |--- NR +----+ \\----// +----+ private public private FW: Firewall NI: NSIS Initiator NR: NSIS Responder Figure 2: Firewall Traversal Scenario Each firewall on the data path must provide traversal service for NATFW NSLP in order to permit the NSIS message to reach the other end host. All firewalls process NSIS signaling and establish appropriate Stiemerling, et al. Expires September 6, 2007 [Page 13] Internet-Draft NAT/FW NSIS NSLP March 2007 policy rules, so that the required data packet flow can traverse them. There are several very different ways to place firewalls in a network topology. To distinguish firewalls located at network borders, such as administrative domains, from others located internally, the term edge-firewall is used. A similar distinction can be made for NATs, with an edge-NAT fulfilling the equivalent role. 2.2. NAT with two private Networks Figure 3 shows a scenario with NATs at both ends of the network. Therefore, each application instance, the NSIS initiator and the NSIS responder, are behind NATs. The outermost NAT, known as the edge-NAT (MB2 and MB3), at each side is connected to the public Internet. The NATs are generically labeled as MBX (for middlebox No. X), since those devices certainly implement NAT functionality, but can implement firewall functionality as well. Only two middleboxes MB are shown in Figure 3 at each side, but in general, any number of MBs on each side must be considered. +----+ +----+ //----\\ +----+ +----+ NI --| MB1|-----| MB2|---| |---| MB3|-----| MB4|--- NR +----+ +----+ \\----// +----+ +----+ private public private MB: Middlebox NI: NSIS Initiator NR: NSIS Responder Figure 3: NAT with two Private Networks Scenario Signaling traffic from NI to NR has to traverse all the middleboxes on the path (MB1 to MB4, in this order), and all the middleboxes must be configured properly to allow NSIS signaling to traverse them. The NATFW signaling must configure all middleboxes and consider any address translation that will result from this configuration in further signaling. The sender (NI) has to know the IP address of the receiver (NR) in advance, otherwise it will not be possible to send any NSIS signaling messages towards the responder. Note that this IP address is not the private IP address of the responder but the NAT's public IP address (here MB3's IP address). Instead a NAT binding (including a public IP address) has to be previously installed on the NAT MB3. This NAT binding subsequently allows packets reaching the Stiemerling, et al. Expires September 6, 2007 [Page 14] Internet-Draft NAT/FW NSIS NSLP March 2007 NAT to be forwarded to the receiver within the private address realm. The receiver might have a number of ways to learn its public IP address and port number (including the NATFW NSLP) and might need to signal this information to the sender using the application level signaling protocol. 2.3. NAT with Private Network on Sender Side This scenario shows an application instance at the sending node that is behind one or more NATs (shown as generic MB, see discussion in Section 2.2). The receiver is located in the public Internet. +----+ +----+ //----\\ NI --| MB |-----| MB |---| |--- NR +----+ +----+ \\----// private public MB: Middlebox NI: NSIS Initiator NR: NSIS Responder Figure 4: NAT with Private Network on Sender Side Scenario The traffic from NI to NR has to traverse middleboxes only on the sender's side. The receiver has a public IP address. The NI sends its signaling message directly to the address of the NSIS responder. Middleboxes along the path intercept the signaling messages and configure the policy rules accordingly. The data sender does not necessarily know whether the receiver is behind a NAT or not, hence, it is the receiving side that has to detect whether itself is behind a NAT or not. 2.4. NAT with Private Network on Receiver Side Scenario The application instance receiving data is behind one or more NATs shown as MB (see discussion in Section 2.2). Stiemerling, et al. Expires September 6, 2007 [Page 15] Internet-Draft NAT/FW NSIS NSLP March 2007 //----\\ +----+ +----+ NI ---| |---| MB |-----| MB |--- NR \\----// +----+ +----+ public private MB: Middlebox NI: NSIS Initiator NR: NSIS Responder Figure 5: NAT with Private Network on Receiver Scenario Initially, the NSIS responder must determine its publicly reachable IP address at the external middlebox and notify the NSIS initiator about this address. One possibility is that an application level protocol is used, meaning that the public IP address is signaled via this protocol to the NI. Afterwards the NI can start its signaling towards the NR and therefore establish the path via the middleboxes in the receiver side private network. This scenario describes the use case for the EXTERNAL message of the NATFW NSLP. 2.5. Both End Hosts behind twice-NATs This is a special case, where the main problem arises from the need to detect that both end hosts are logically within the same address space, but are also in two partitions of the address realm on either side of a twice-NAT (see [9] for a discussion of twice-NAT functionality). Sender and receiver are both within a single private address realm but the two partitions potentially have overlapping IP address ranges. Figure 6 shows the arrangement of NATs. Stiemerling, et al. Expires September 6, 2007 [Page 16] Internet-Draft NAT/FW NSIS NSLP March 2007 public +----+ +----+ //----\\ NI --| MB |--+--| MB |---| | +----+ | +----+ \\----// | | +----+ +--| MB |------------ NR +----+ private MB: Middlebox NI: NSIS Initiator NR: NSIS Responder Figure 6: NAT to Public, Sender and Receiver on either side of a twice-NAT Scenario The middleboxes shown in Figure 6 are twice-NATs, i.e., they map IP addresses and port numbers on both sides, meaning the mapping of source and destination address at the private and public interfaces. This scenario requires the assistance of application level entities, such as a DNS server. The application level entities must handle requests that are based on symbolic names, and configure the middleboxes so that data packets are correctly forwarded from NI to NR. The configuration of those middleboxes may require other middlebox communication protocols, such as MIDCOM [7]. NSIS signaling is not required in the twice-NAT only case, since middleboxes of the twice-NAT type are normally configured by other means. Nevertheless, NSIS signaling might be useful when there are also firewalls on the path. In this case NSIS will not configure any policy rule at twice-NATs, but will configure policy rules at the firewalls on the path. The NSIS signaling protocol must be at least robust enough to survive this scenario. This requires that twice- NATs must implement the NATFW NSLP also and participate in NATFW signaling sessions but they do not change the configuration of the NAT, i.e., they only read the address mapping information out of the NAT and translate the Message Routing Information (MRI, [2]) within the NSLP and NTLP accordingly. For more information see Appendix C.4 2.6. Both End Hosts Behind Same NAT When NSIS initiator and NSIS responder are behind the same NAT (thus being in the same address realm, see Figure 7), they are most likely not aware of this fact. As in Section 2.4 the NSIS responder must determine its public IP address in advance and transfer it to the Stiemerling, et al. Expires September 6, 2007 [Page 17] Internet-Draft NAT/FW NSIS NSLP March 2007 NSIS initiator. Afterwards, the NSIS initiator can start sending the signaling messages to the responder's public IP address. During this process, a public IP address will be allocated for the NSIS initiator at the same middlebox as for the responder. Now, the NSIS signaling and the subsequent data packets will traverse the NAT twice: from initiator to public IP address of responder (first time) and from public IP address of responder to responder (second time). NI public \ +----+ //----\\ +-| MB |----| | / +----+ \\----// NR private MB: Middlebox NI: NSIS Initiator NR: NSIS Responder Figure 7: NAT to Public, Both Hosts Behind Same NAT 2.7. Multihomed Network with NAT The previous sub-sections sketched network topologies where several NATs and/or firewalls are ordered sequentially on the path. This section describes a multihomed scenario with two NATs placed on alternative paths to the public network. +----+ //---\\ NI -------| MB |---| | \ +----+ \\-+-// \ | \ +----- NR \ | \ +----+ //-+-\\ --| MB |---| | +----+ \\---// private public MB: Middlebox NI: NSIS Initiator NR: NSIS Responder Stiemerling, et al. Expires September 6, 2007 [Page 18] Internet-Draft NAT/FW NSIS NSLP March 2007 Figure 8: Multihomed Network with Two NATs Depending on the destination, either one or the other middlebox is used for the data flow. Which middlebox is used, depends on local policy or routing decisions. NATFW NSLP must be able to handle this situation properly, see Section 3.7.2 for an extended discussion of this topic with respect to NATs. 2.8. Multihomed Network with Firewall This section describes a multihomed scenario with two firewalls placed on alternative paths to the public network (Figure 9). The routing in the private and public network decides which firewall is being taken for data flows. Depending on the data flow's direction, either outbound or inbound, a different firewall could be traversed. This is a challenge for the EXT message of the NATFW NSLP where the NSIS responder is located behind these firewalls within the private network. The EXT message is used to block a particular data flow on an inbound firewall. NSIS must route the EXT message inbound from NR to NI probably without knowing which path the data traffic will take from NI to NR (see also Appendix B). +----+ NR -------| MB |\ \ +----+ \ //---\\ \ -| |-- NI \ \\---// \ +----+ | --| MB |-------+ +----+ private private public MB: Middlebox NI: NSIS Initiator NR: NSIS Responder Figure 9: Multihomed Network with two Firewalls Stiemerling, et al. Expires September 6, 2007 [Page 19] Internet-Draft NAT/FW NSIS NSLP March 2007 3. Protocol Description This section defines messages, objects, and protocol semantics for the NATFW NSLP. 3.1. Policy Rules Policy rules, bound to a NATFW NSLP signaling session, are the building blocks of middlebox devices considered in the NATFW NSLP. For firewalls the policy rule usually consists of a 5-tuple, source/ destination addresses, transport protocol, and source/destination port numbers, plus an action, such as allow or deny. For NATs the policy rule consists of the action 'translate this address' and further mapping information, that might be, in the simplest case, internal IP address and external IP address. The NATFW NSLP carries, in conjunction with the NTLP's Message Routing Information (MRI), the policy rules to be installed at NATFW peers. This policy rule is an abstraction with respect to the real policy rule to be installed at the respective firewall or NAT. It conveys the initiator's request and must be mapped to the possible configuration on the particular used NAT and/or firewall in use. For pure firewalls one or more filter rules must be created and for pure NATs one or more NAT bindings must be created. In mixed firewall and NAT boxes, the policy rule must be mapped to filter rules and bindings observing the ordering of the firewall and NAT engine. Depending on the ordering, NAT before firewall or vice versa, the firewall rules must carry public or private IP addresses. However, the exact mapping depends on the implementation of the firewall or NAT which is different for each vendor. The policy rule at the NATFW NSLP level comprises the message routing information (MRI) part, carried in the NTLP, and the information available in the NATFW NSLP. The information provided by the NSLP is stored in the 'extend flow information' (NATFW_EFI) and 'data terminal information' (NATFW_DTINFO) objects, and the message type. Additional information, such as the external IP address and port number, stored in the NAT or firewall, will be used as well. The MRI carries the filter part of the NAT/firewall-level policy rule that is to be installed. The NATFW NSLP specifies two actions for the policy rules: deny and allow. A policy rule with action set to deny will result in all packets matching this rule to be dropped. A policy rule with action set to allow will result in all packets matching this rule to be forwarded. Stiemerling, et al. Expires September 6, 2007 [Page 20] Internet-Draft NAT/FW NSIS NSLP March 2007 3.2. Basic Protocol Overview The NSIS NATFW NSLP is carried over the General Internet Signaling Transport (GIST, the implementation of the NTLP) defined in [2]. NATFW NSLP messages are initiated by the NSIS initiator (NI), handled by NSIS forwarders (NF) and received by the NSIS responder (NR). It is required that at least NI and NR implement this NSLP, intermediate NFs only implement this NSLP when they provide relevant middlebox functions. NSIS forwarders that do not have any NATFW NSLP functions just forward these packets as they have no interest in them. A Data Sender (DS), intending to send data to a Data Receiver (DR) has to start NATFW NSLP signaling. This causes the NI associated with the data sender (DS) to launch NSLP signaling towards the address of data receiver (DR) (see Figure 10). Although it is expected that the DS and the NATFW NSLP NI will usually reside on the same host, this specification does not rule out scenarios where the DS and NI reside on different hosts, the so-called proxy mode (see Section 3.7.6.) +-------+ +-------+ +-------+ +-------+ | DS/NI |<~~~| MB1/ |<~~~| MB2/ |<~~~| DR/NR | | |--->| NF1 |--->| NF2 |--->| | +-------+ +-------+ +-------+ +-------+ ========================================> Data Traffic Direction (outbound) ---> : NATFW NSLP request signaling ~~~> : NATFW NSLP response signaling DS/NI : Data sender and NSIS initiator DR/NR : Data receiver and NSIS responder MB1 : Middlebox 1 and NSIS forwarder 1 MB2 : Middlebox 2 and NSIS forwarder 2 Figure 10: General NSIS signaling The following list shows the normal sequence of NSLP events without detailing the interaction with the NTLP and the interactions on the the NTLP level. o NSIS initiators generate NATFW NSLP CREATE/EXT messages and send these towards the NSIS responder. This CREATE/EXT message is the initial message which creates a new NATFW NSLP signaling session. The NI and the NR will most likely already share an application session before they start the NATFW NSLP signaling session. Note Stiemerling, et al. Expires September 6, 2007 [Page 21] Internet-Draft NAT/FW NSIS NSLP March 2007 the difference between both sessions. o NSLP CREATE/EXT messages are processed each time a NF with NATFW NSLP support is traversed. Each NF that is intercepting a CREATE/ EXT message and is accepting it for further treatment is joining the particular NATFW NSLP signaling session. These nodes process the message, check local policies for authorization and authentication, possibly create policy rules, and forward the signaling message to the next NSIS node. The request message is forwarded until it reaches the NSIS responder. o NSIS responders will check received messages and process them if applicable. NSIS responders generate RESPONSE messages and send them hop-by-hop back to the NI via the same chain of NFs (traversal of the same NF chain is guaranteed through the established reverse message routing state in the NTLP). The NR is also joining the NATFW NSLP signaling session if the CREATE/EXT message is accepted. o The RESPONSE message is processed at each NF that has been included in the prior NATFW NSLP signaling session setup. o If the NI has received a successful RESPONSE message and if the signaling NATFW NSLP session started with a CREATE message, the data sender can start sending its data flow to the data receiver. If the Ni has received a successful RESPONSE message and if the signaling NATFW NSLP session started with a EXT message, the data receiver is ready to receive further CREATE messages. Because NATFW NSLP signaling follows the data path from DS to DR, this immediately enables communication between both hosts for scenarios with only firewalls on the data path or NATs on the sender side. For scenarios with NATs on the receiver side certain problems arise, as described in Section 2.4. When the NR and the NI are located in different address realms and the NR is located behind a NAT, the NI cannot signal to the NR address directly. The DR/NR are not reachable from the NIs using the private address of the NR and thus NATFW signaling messages cannot be sent to the NR/DR's address. Therefore, the NR must first obtain a NAT binding that provides an address that is reachable for the NI. Once the NR has acquired a public IP address, it forwards this information to the DS via a separate protocol. This application layer signaling, which is out of scope of the NATFW NSLP, may involve third parties that assist in exchanging these messages. The same holds partially true for NRs located behind firewalls that block all traffic by default. In this case, NR must tell its inbound Stiemerling, et al. Expires September 6, 2007 [Page 22] Internet-Draft NAT/FW NSIS NSLP March 2007 firewalls of inbound NATFW NSLP signaling and corresponding data traffic. Once the NR has informed the inbound firewalls, it can start its application level signaling to initiate communication with the NI. This application layer signaling, which is out of scope of the NATFW NSLP, may involve third parties that assist in exchanging these messages. This mechanism can be used by machines hosting services behind firewalls as well. In this case, the NR informs the inbound firewalls as described, but does not need to communicate this to the NIs. NATFW NSLP signaling supports this scenario by using the EXT message 1. The DR acquires a public address by signaling on the reverse path (DR towards DS) and thus making itself available to other hosts. This process of acquiring public addresses is called reservation. During this process the DR reserves publicly reachable addresses and ports suitable for further usage in application level signaling and the publicly reachable address for further NATFW NSLP signaling. However, the data traffic will not be allowed to use this address/port initially (see next point). In the process of reservation the DR becomes the NI for the messages necessary to obtain the publicly reachable IP address, i.e., the NI for this specific NATFW NSLP signaling session. 2. Now on the side of DS, the NI creates a new NATFW NSLP signaling session and signals directly to the public IP address of DR. This public IP address is used as NR's address, as the NI would do if there is no NAT in between, and creates policy rules at middleboxes. Note, that the reservation will only allow forwarding of signaling messages, but not data flow packets. Policy rules allowing forwarding of data flow packets set up by the prior EXT message signaling will be activated when the signaling from NI towards NR is confirmed with a positive RESPONSE message. The EXTERNAL (EXT) message is described inSection 3.7.2. Stiemerling, et al. Expires September 6, 2007 [Page 23] Internet-Draft NAT/FW NSIS NSLP March 2007 +-------+ +-------+ +-------+ +-------+ | DS/NI |<~~~| MB1/ |<~~~| NR | | DR | | |--->| NF1 |--->| | | | +-------+ +-------+ +-------+ +-------+ ========================================> Data Traffic Direction (outbound) ---> : NATFW NSLP request signaling ~~~> : NATFW NSLP response signaling DS/NI : Data sender and NSIS initiator DR/NR : Data receiver and NSIS responder MB1 : Middlebox 1 and NSIS forwarder 1 MB2 : Middlebox 2 and NSIS forwarder 2 Figure 11: A NSIS proxy mode signaling The above usage assumes that both ends of a communication support NSIS, but fails when NSIS is only deployed at one end of the path. In this case only one of the receiving or sending side is NSIS aware and not both at the same time. NATFW NSLP supports this scenario (i.e., the DR does not support NSIS) by using a proxy mode, as described in Section 3.7.6; the proxy mode operation also supports scenarios with a data sender that does not support NSIS, i.e. the data receiver must act to enable data flows towards itself. The basic functionality of the NATFW NSLP provides for opening firewall pin holes and creating NAT bindings to enable data flows to traverse these devices. Firewalls are normally expected to work on a 'deny-all' policy, meaning that traffic not explicitly matching any firewall filter rule will be blocked. Similarly, the normal behavior of NATs is to block all traffic that does not match any already configured/installed binding or NATFW NSLP session. However, some scenarios require support of firewalls having 'allow-all' policies, allowing data traffic to traverse the firewall unless it is blocked explicitly. Data receivers can utilize NATFW NSLP's EXT message with action set to 'deny' to install policy rules at inbound firewalls to block unwanted traffic. The protocol works on a soft-state basis, meaning that whatever state is installed or reserved on a middlebox will expire, and thus be de- installed or forgotten after a certain period of time. To prevent premature removal of state that is needed for ongoing communication, the NATFW NI involved will have to specifically request a NATFW NSLP signaling session extension. An explicit NATFW NSLP state deletion capability is also provided by the protocol. Stiemerling, et al. Expires September 6, 2007 [Page 24] Internet-Draft NAT/FW NSIS NSLP March 2007 If the actions requested by a NATFW NSLP message cannot be carried out, NFs and the NR must return a failure, such that appropriate actions can be taken. They can do this either during a the request message handling (synchronously) by sending an error RESPONSE message, or at any time (asynchronously) by sending a notification message. The next sections define the NATFW NSLP message types and formats, protocol operations, and policy rule operations. 3.2.1. Message Types The protocol uses four messages types: o CREATE: a request message used for creating, changing, refreshing, and deleting NATFW NSLP signaling sessions, i.e., open the data path from DS to DR. o EXTERNAL (EXT): a request message used for reserving, changing, refreshing, and deleting EXT NATFW NSLP signaling sessions. EXT messages are forwarded to the edge-NAT or edge-firewall and allow inbound CREATE messages to be forwarded to the NR. Additionally, EXT messages reserve an external address and, if applicable, port number at an edge-NAT. o NOTIFY: an asynchronous message used by NATFW peers to alert inbound NATFW peers about specific events (especially failures). o RESPONSE: used as a response to CREATE and EXT request messages. 3.2.2. Classification of RESPONSE Messages RESPONSE messages will be generated synchronously to CREATE and EXT messages by NSIS Forwarders and Responders to report success or failure of operations or some information relating to the NATFW NSLP signaling session or a node. RESPONSE messages MUST NOT be generated for any other message, such as NOTIFY and RESPONSE. All RESPONSE messages MUST carry a NATFW_INFO object which contains a severity class code and a response code (see Section 4.2.4). This section defines terms for groups of RESPONSE messages depending on the severity class. o Successful RESPONSE: Messages carrying NATFW_INFO with severity class 'Success' (0x2). o Informational RESPONSE: Messages carrying NATFW_INFO with severity class 'Informational' (0x1) (only used with NOTIFY messages). Stiemerling, et al. Expires September 6, 2007 [Page 25] Internet-Draft NAT/FW NSIS NSLP March 2007 o Error RESPONSE: Messages carrying NATFW_INFO with severity class other than 'Success' or 'Informational'. 3.2.3. NATFW NSLP Signaling Sessions A NATFW NSLP signaling session defines an association between the NI, NFs, and the NR related to a data flow. This association is created when the initial CREATE or EXT message is successfully received at the NFs or the NR. There is signaling NATFW NSLP session state stored at the NTLP layer and at the NATFW NSLP level. The NATFW NSLP signaling session state for the NATFW NSLP comprises NSLP state and the associated policy rules at a middlebox. The NATFW NSLP signaling session is identified by the session ID (plus other information at the NTLP level). The session ID is generated by the NI before the initial CREATE or EXT message is sent. The value of the session ID MUST generated in a random way, i.e., the output MUST NOT be easily guessable by third parties. The session ID is not stored in any NATFW NSLP message but passed on to the NTLP. A NATFW NSLP signaling session can conceptually be in different states, implementations may use other or even more states. The signaling session can have these states at a node: o Pending: The NATFW NSLP signaling session has been created and the node is waiting for a RESPONSE message to the CREATE or EXT message. A NATFW NSLP signaling session in state 'Pending' MUST be marked as 'Dead' if no corresponding RESPONSE message has been received within the time of the locally granted NATFW NSLP signaling session lifetime of the forwarded CREATE or EXT message (as described in Section 3.4). o Established: The NATFW NSLP signaling session is established, i.e, the signaling has been successfully performed and the lifetime of NATFW NSLP signaling session is counted from now on. A NATFW NSLP signaling session in state 'Established' MUST be marked as 'Dead' if no refresh message has been received within the time of the locally granted NATFW NSLP signaling session lifetime of the RESPONSE message (as described in Section 3.4). o Dead: Either the NATFW NSLP signaling session is timed out or the node has received an error RESPONSE message for the NATFW NSLP signaling session and the NATFW NSLP signaling session can be deleted. o Transit: The node has received an asynchronous message, i.e., a NOTIFY, and can delete the NATFW NSLP signaling session if needed. When a node has received a NOTIFY message (for instance, Stiemerling, et al. Expires September 6, 2007 [Page 26] Internet-Draft NAT/FW NSIS NSLP March 2007 indicating a route change) it marks it as 'Transit' and deletes this NATFW NSLP signaling session if it is unused for some time specific to the local node. This idle time does not need to be fixed, since it can depend on the node local maintenance cycle, i.e., the NATFW NSLP signaling session could be deleted if the node runs it garbage collection cycle. 3.3. Basic Message Processing All NATFW messages are subject to some basic message processing when received at a node, independent of message type. Initially, the syntax of the NSLP message is checked and a RESPONSE message with an appropriate error of class 'Protocol error' (0x1) code is generated if any problem is detected. If a message is delivered to the NATFW NSLP, this implies that the NTLP layer has been able to correlate it with the SID and MRI entries in its database. There is therefore enough information to identify the source of the message and routing information to route the message back to the NI through an established chain of NTLP messaging associations. The message is not further forwarded if any error in the syntax is detected. The specific response codes stemming from the processing of objects are described in the respective object definition section (see Section 4). After passing this check, the NATFW NSLP node performs authentication/authorization related checks described in Section 3.6. Further processing is executed only if these tests have been successfully passed, otherwise the processing stops and an error RESPONSE is returned. Further message processing stops whenever an error RESPONSE message is generated, and the EXT or CREATE message is discarded. 3.4. Calculation of Signaling Session Lifetime NATFW NSLP signaling sessions, and the corresponding policy rules which may have been installed, are maintained via a soft-state mechanism. Each signaling session is assigned a signaling session lifetime and the signaling session is kept alive as long as the lifetime is valid. After the expiration of the signaling session lifetime, signaling sessions and policy rules MUST be removed automatically and resources bound to them MUST be freed as well. Signaling session lifetime is handled at every NATFW NSLP node. The NSLP forwarders and NSLP responder MUST NOT trigger signaling session lifetime extension refresh messages (see Section 3.7.3): this is the task of the NSIS initiator. The NSIS initiator MUST choose a NATFW NSLP signaling session lifetime value (expressed in seconds) before sending any message, including the initial message which creates the NATFW NSLP signaling Stiemerling, et al. Expires September 6, 2007 [Page 27] Internet-Draft NAT/FW NSIS NSLP March 2007 session, to other NSLP nodes. The NATFW NSLP signaling session lifetime value is calculated based on: o the number of lost refresh messages that NFs should cope with; o the end-to-end delay between the NI and NR; o network vulnerability due to NATFW NSLP signaling session hijacking ([8]), NATFW NSLP signaling session hijacking is made easier when the NI does not explicitly remove the NATFW NSLP signaling session); o the user application's data exchange duration, in terms of time and networking needs. This duration is modeled as M x R, with R the message refresh period (in seconds) and M as a multiplier for R; o the load on the signalling plane. Short lifetimes imply more frequent signaling messages. o the acceptable time for a NATFW NSLP signaling session to be present after it is no longer actually needed. For example, if the existence of the NATFW NSLP signaling session implies a monetary cost and teardown cannot be guaranteed, shorter lifetimes would be preferable. o the lease time of the NI's IP address. The chosen NATFW NSLP signaling session lifetime must be larger than the lease time, otherwise the IP address can be re-assigned to a different node. This node may receive unwanted traffic, although it never has requested a NAT/firewall configuration, which might be an issue in mobile environments. The RSVP specification [11] provides an appropriate algorithm for calculating the NATFW NSLP signaling session lifetime as well as means to avoid refresh message synchronization between NATFW NSLP signaling sessions. [11] recommends: 1. The refresh message timer to be randomly set to a value in the range [0.5R, 1.5R]. 2. To avoid premature loss of state, lt (with lt being the NATFW NSLP signaling session lifetime) must satisfy lt >= (K + 0.5)*1.5*R, where K is a small integer. Then in the worst case, K-1 successive messages may be lost without state being deleted. Currently K = 3 is suggested as the default. However, it may be necessary to set a larger K value for hops with high loss rate. Other algorithms could be used to define the relation between the Stiemerling, et al. Expires September 6, 2007 [Page 28] Internet-Draft NAT/FW NSIS NSLP March 2007 NATFW NSLP signaling session lifetime and the refresh message period; the algorithm provided is only given as an example. This requested NATFW NSLP signaling session lifetime value lt is stored in the NATFW_LT object of the NSLP message. NSLP forwarders can execute the following behavior with respect to the lifetime handling: Requested signaling session lifetime acceptable: No changes to the NATFW NSLP signaling session lifetime values are needed. The CREATE or EXT message is forwarded. Signaling session lifetime can be lowered: The NSLP responder MAY also lower the requested NATFW NSLP signaling session lifetime to an acceptable value (based on its local policies). If an NF changes the NATFW NSLP signaling session lifetime value, it MUST store the new value in the NATFW_LT object. The CREATE or EXT message is forwarded. Requested signaling session lifetime is too big: The NSLP responder MAY reject the requested NATFW NSLP signaling session lifetime value as being too big and MUST generate an error RESPONSE message of class 'Signaling session failures' (0x6) with response code 'Requested lifetime is too big' (0x02) upon rejection. Lowering the lifetime is preferred instead of generating an error message. Requested signaling session lifetime is too small: The NSLP responder MAY reject the requested NATFW NSLP signaling session lifetime value as being to small and MUST generate an error RESPONSE message of class 'Signaling session failures' (0x6) with response code 'Requested lifetime is too small' (0x10) upon rejection. NFs MUST NOT increase the NATFW NSLP signaling session lifetime value. Messages can be rejected on the basis of the NATFW NSLP signaling session lifetime being too long when a NATFW NSLP signaling session is first created and also on refreshes. The NSLP responder generates a successful RESPONSE for the received Stiemerling, et al. Expires September 6, 2007 [Page 29] Internet-Draft NAT/FW NSIS NSLP March 2007 CREATE or EXT message, sets the NATFW NSLP signaling session lifetime value in the NATFW_LT object to the above granted lifetime and sends the message back towards NSLP initiator. Each NSLP forwarder processes the RESPONSE message, reads and stores the granted NATFW NSLP signaling session lifetime value. The forwarders MUST accept the granted NATFW NSLP signaling session lifetime, as long as this value is less than or equal to the acceptable value. The acceptable value refers to the value accepted by the NSLP forwarder when processing the CREATE or EXT message. For received values greater than the acceptable value, NSLP forwarders MUST generate a RESPONSE message of class 'Signaling session failures' (0x6) with response code 'Requested lifetime is too big' (0x02). For received values lower than the values acceptable by the node local policy, NSLP forwarders MUST generate a RESPONSE message of class 'Signaling session failures' (0x6) with response code 'Requested lifetime is too small' (0x10). Figure 12 shows the procedure with an example, where an initiator requests 60 seconds lifetime in the CREATE message and the lifetime is shortened along the path by the forwarder to 20 seconds and by the responder to 15 seconds. When the NSLP forwarder receives the RESPONSE message with a NATFW NSLP signaling session lifetime value of 15 seconds it checks whether this value is lower or equal to the acceptable value. +-------+ CREATE(lt=60s) +-------------+ CREATE(lt=20s) +--------+ | |---------------->| NSLP |---------------->| | | NI | | forwarder | | NR | | |<----------------| check 15<20 |<----------------| | +-------+ RESPONSE(lt=15s)+-------------+ RESPONSE(lt=15s)+--------+ lt = lifetime Figure 12: Signaling Session Lifetime Setting Example 3.5. Message Sequencing NATFW NSLP messages need to carry an identifier so that all nodes along the path can distinguish messages sent at different points in time. Messages can be lost along the path or duplicated. So all NATFW NSLP nodes should be able to identify either old messages that have been received before (duplicated), or the case that messages have been lost before (loss). For message replay protection it is necessary to keep information about messages that have already been received and requires every NATFW NSLP message to carry a message sequence number (MSN), see also Section 4.2.6. The MSN MUST be set by the NI and MUST NOT be set or modified by any Stiemerling, et al. Expires September 6, 2007 [Page 30] Internet-Draft NAT/FW NSIS NSLP March 2007 other node. The initial value for the MSN MUST be generated randomly and MUST be unique only within the NATFW NSLP signaling session for which it is used. The NI MUST increment the MSN by one for every message sent. Once the MSN has reached the maximum value, the next value it takes is zero. All NATFW NSLP nodes MUST use the algorithm defined in [3] to detect MSN wrap-arounds. NSIS forwarders and the responder store the MSN from the initial CREATE or EXT packet which creates the NATFW NSLP signaling session as the start value for the NATFW NSLP signaling session. NFs and NRs MUST include the received MSN value in the corresponding RESPONSE message that they generate. When receiving a CREATE or EXT message, a NATFW NSLP node uses the MSN given in the message to determine whether the state being requested is different to the state already installed. The message MUST be discarded if the received MSN value is equal to or lower than the stored MSN value. Such a received MSN value can indicate a duplicated and delayed message or replayed message. If the received MSN value is greater than the already stored MSN value, the NATFW NSLP MUST update its stored state accordingly, if permitted by all security checks (see Section 3.6), and stores the updated MSN value accordingly. 3.6. Authentication, Authorization, and Policy Decisions NATFW NSLP nodes receiving signaling messages MUST first check whether this message is authenticated and authorized to perform the requested action. NATFW NSLP nodes requiring more information than provided MUST generate an error RESPONSE of class 'Permanent failure' (0x5) with response code 'Authentication failed' (0x01) or with response code 'Authorization failed' (0x02). The NATFW NSLP is expected to run in various environments, such as IP-based telephone systems, enterprise networks, home networks, etc. The requirements on authentication and authorization are quite different between these use cases. While a home gateway, or an Internet cafe, using NSIS may well be happy with a "NATFW signaling coming from inside the network" policy for authorization of signaling, enterprise networks are likely to require more strongly authenticated/authorized signaling. This enterprise scenario may require the use of an infrastructure and administratively assigned identities to operate the NATFW NSLP. Once the NI is authenticated and authorized, another step is performed. The requested policy rule for the NATFW NSLP signaling session is checked against a set of policy rules, i.e., whether the requesting NI is allowed to request the policy rule to be loaded in Stiemerling, et al. Expires September 6, 2007 [Page 31] Internet-Draft NAT/FW NSIS NSLP March 2007 the device. If this fails the NF or NR must send an error RESPONSE of class 'Permanent failure' (0x5) and with response code 'Authorization failed' (0x02). 3.7. Protocol Operations This section defines the protocol operations including, how to create NATFW NSLP signaling sessions, maintain them, and how to reserve addresses. 3.7.1. Creating Signaling Sessions Allowing two hosts to exchange data even in the presence of middleboxes is realized in the NATFW NSLP by use of the CREATE message. The NI (either the data sender or a proxy) generates a CREATE message as defined in Section 4.3.1 and hands it to the NTLP. The NTLP forwards the whole message on the basis of the message routing information (MRI) towards the NR. Each NSIS forwarder along the path that implements NATFW NSLP, processes the NSLP message. Forwarding is thus managed NSLP hop-by-hop but may pass transparently through NSIS forwarders which do not contain NATFW NSLP functionality and non-NSIS aware routers between NSLP hop way points. When the message reaches the NR, the NR can accept the request or reject it. The NR generates a response to CREATE and this response is transported hop-by-hop towards the NI. NATFW NSLP forwarders may reject requests at any time. Figure 13 sketches the message flow between NI (DS in this example), a NF (e.g., NAT), and NR (DR in this example). NI Private Network NF Public Internet NR | | | | CREATE | | |----------------------------->| | | | | | | | | | CREATE | | |--------------------------->| | | | | | RESPONSE | | RESPONSE |<---------------------------| |<-----------------------------| | | | | | | | Figure 13: CREATE message flow with success RESPONSE There are several processing rules for a NATFW peer when generating Stiemerling, et al. Expires September 6, 2007 [Page 32] Internet-Draft NAT/FW NSIS NSLP March 2007 and receiving CREATE messages, since this message type is used for creating new NATFW NSLP signaling session, updating existing, extending the lifetime and deleting NATFW NSLP signaling session. The three latter functions operate in the same way for all kinds of CREATE message, and are therefore described in separate sections: o Extending the lifetime of NATFW NSLP signaling sessions is described in Section 3.7.3. o Deleting NATFW NSLP signaling sessions is described in Section 3.7.4. o Updating policy rules is described in Section 3.10. For an initial CREATE message creating a new NATFW NSLP signaling session, the processing of CREATE messages is different for every NATFW node type: o NSLP initiator: An NI only generates CREATE messages and hands them over to the NTLP. The NI should never receive CREATE messages and MUST discard it. o NATFW NSLP forwarder: NFs that are unable to forward the CREATE message to the next hop MUST generate an error RESPONSE of class 'Permanent failure' (0x6) with response code 'Did not reach the NR' (0x07). This case may occur if the NTLP layer cannot find an NATFW NSLP peer, either another NF or the NR, and returns an error via the GIST API. The NSLP message processing at the NFs depends on the middlebox type: * NAT: When the initial CREATE message is received at the public side of the NAT, it looks for a reservation made in advance, by using a EXT message (see Section 3.7.2). The matching process considers the received MRI information and the stored MRI information, as described in Section 3.8. If no matching reservation can be found, i.e. no reservation has been made in advance, the NSLP MUST return an error RESPONSE of class 'Signaling session failure' (0x6) with response code 'No reservation found matching the MRI of the CREATE request' (0x03) MUST be generated. If there is a matching reservation, the NSLP stores the data sender's address (and if applicable port number) as part of the source address of the policy rule ('the remembered policy rule') to be loaded and forwards the message with the destination address set to the internal (private in most cases) address of NR. When the initial CREATE message is received at the private side, the NAT binding is allocated, but not activated (see also Appendix C.3). The MRI information is updated to reflect the address, and if Stiemerling, et al. Expires September 6, 2007 [Page 33] Internet-Draft NAT/FW NSIS NSLP March 2007 applicable port, translation. The NSLP message is forwarded towards the NR with source address set to the NAT's external address from the newly remembered binding. * Firewall: When the initial CREATE message is received, the NSLP just remembers the requested policy rule, but does not install any policy rule. Afterwards, the message is forwarded towards the NR. * Combined NAT and firewall: Processing at combined firewall and NAT middleboxes is the same as in the NAT case. No policy rules are installed. Implementations MUST take into account the order of packet processing in the firewall and NAT functions within the device. This will be referred to as 'order of functions' and is generally different depending on whether the packet arrives at the external or internal side of the middlebox. o NSLP receiver: NRs receiving initial CREATE messages MUST reply with a success RESPONSE of class 'Success' (0x2) with response code set to 'All successfully processed' (0x01), if they accept the CREATE message. Otherwise they MUST generate a RESPONSE message with a suitable response code. RESPONSE messages are sent back NSLP hop-by-hop towards the NI, irrespective of the response codes, either success or error. Remembered policy rules at middleboxes MUST be only installed upon receiving a corresponding successful RESPONSE message with the same SID and MSN as the CREATE message that caused them to be remembered. This is a countermeasure to several problems, for example, wastage of resources due to loading policy rules at intermediate NFs when the CREATE message does not reach the final NR for some reason. Processing of a RESPONSE message is different for every NSIS node type: o NSLP initiator: After receiving a successful RESPONSE, the data path is configured and the DS can start sending its data to the DR. After receiving an error RESPONSE message, the NI MAY try to generate the CREATE message again or give up and report the failure to the application, depending on the error condition. o NSLP forwarder: NFs install the remembered policy rules, if a successful RESPONSE message with matching SID and MSN is received. If an ERROR RESPONSE message with matching SID and MSN is received, the NATFW NSLP session is marked as dead, no policy rule is installed and the remembered rule is discarded. Stiemerling, et al. Expires September 6, 2007 [Page 34] Internet-Draft NAT/FW NSIS NSLP March 2007 o NSIS responder: The NR should never receive RESPONSE messages and MUST silently drop any such messages received. 3.7.2. Reserving External Addresses NSIS signaling is intended to travel end-to-end, even in the presence of NATs and firewalls on-path. This works well in cases where the data sender is itself behind a NAT or a firewall as described in Section 3.7.1. For scenarios where the data receiver is located behind a NAT or a firewall and it needs to receive data flows from outside its own network (usually referred to as inbound flows, see Figure 5) the problem is more troublesome. NSIS signaling, as well as subsequent data flows, are directed to a particular destination IP address that must be known in advance and reachable. Data receivers must tell the local NSIS infrastructure (i.e., the inbound firewalls/NATs) about incoming NATFW NSLP signaling and data flows before they can receive these flows. It is necessary to differentiate between data receivers behind NATs and behind firewalls for understanding the further NATFW procedures. Data receivers that are only behind firewalls already have a public IP address and they need only to be reachable for NATFW signaling. Unlike data receivers behind just firewalls, data receivers behind NATs do not have public IP addresses; consequently they are not reachable for NATFW signaling by entities outside their addressing realm. The preceding discussion addresses the situation where a DR node that wants to be reachable is unreachable because the NAT lacks a suitable rule with the 'allow' action which would forward inbound data. However, in certain scenarios, a node situated behind inbound firewalls that do not block inbound data traffic (firewalls with "default to allow") unless requested might wish to prevent traffic being sent to it from specified addresses. In this case, NSIS NATFW signaling can be used to achieve this by installing a policy rule with its action set to 'deny' using the same mechanisms as for 'allow' rules. The required result is obtained by sending a EXTERNAL (EXT) message in the inbound direction of the intended data flow. When using this functionality the NSIS initiator for the 'Reserve External Address' signaling is typically the node that will become the DR for the eventual data flow. To distinguish this initiator from the usual case where the NI is associated with the DS, the NI is denoted by NI+ and the NSIS responder is similarly denoted by NR+. Stiemerling, et al. Expires September 6, 2007 [Page 35] Internet-Draft NAT/FW NSIS NSLP March 2007 Public Internet Private Address Space Edge NI(DS) NAT/FW NAT NR(DR) NR+ NI+ | | | | | | | | | | | | | | EXT[(DTInfo)] | EXT[(DTInfo)] | | |<----------------------|<----------------------| | | | | | |RESPONSE[Success/Error]|RESPONSE[Success/Error]| | |---------------------->|---------------------->| | | | | | | | | ============================================================> Data Traffic Direction Figure 14: Reservation message flow for DR behind NAT or firewall Figure 14 shows the EXT message flow for enabling inbound NATFW NSLP signaling messages. In this case the roles of the different NSIS entities are: o The data receiver (DR) for the anticipated data traffic is the NSIS initiator (NI+) for the EXTERNAL (EXT) message, but becomes the NSIS responder (NR) for following CREATE messages. o The actual data sender (DS) will be the NSIS initiator (NI) for later CREATE messages and may be the NSIS target of the signaling (NR+). o It may be necessary to use a signaling destination address (SDA) as the actual target of the EXT message (NR+) if the DR is located behind a NAT and the address of the DS is unknown. The SDA is an arbitrary address in the outermost address realm on the other side of the NAT from the DR. Typically this will be a suitable public IP address when the 'outside' realm is the public Internet. This choice of address causes the EXT message to be routed through the NATs towards the outermost realm and would force interception of the message by the outermost NAT in the network at the boundary between the private address and the public address realm (the edge-NAT). It may also be intercepted by other NATs and firewalls on the path to the edge-NAT. Stiemerling, et al. Expires September 6, 2007 [Page 36] Internet-Draft NAT/FW NSIS NSLP March 2007 Basically, there are two different signaling scenarios. Either 1. the DR behind the NAT/firewall knows the IP address of the DS in advance, 2. or the address of DS is not known in advance. Case 1 requires the NATFW NSLP to request the path-coupled message routing method (PC-MRM) from the NTLP. The EXT message MUST be sent with PC-MRM (see Section 5.8.1 in [2]) with the direction set to 'upstream' (inbound). The handling of case 2 depends on the situation of DR: If DR is solely located behind a firewall, the EXT message MUST be sent with the PC-MRM, direction 'upstream' (inbound), and data flow source IP address set to wildcard. If DR is located behind a NAT, the EXT message MUST be sent with the loose-end message routing method (LE-MRM, see Section 5.8.2 in [2]), the destination- address set to the signaling destination address (SDA, see also Appendix A). For scenarios with DR being behind a firewall, special conditions apply (applicability statement, Appendix B). The data receiver is challenged to determine whether it is solely located behind firewalls or NATs, for choosing the right message routing method. This decision can depend on a local configuration parameter, possibly given through DHCP, or it could be discovered through other non-NSLP related testing of the network configuration. For case 2 with NAT, the NI+ (which could be on the data receiver DR or on any other host within the private network) sends the EXT message targeted to the signaling destination address. The message routing for the EXT message is in the reverse direction to the normal message routing used for path-coupled signaling where the signaling is sent outbound (as opposed to inbound in this case). When establishing NAT bindings (and an NATFW NSLP signaling session) the signaling direction does not matter since the data path is modified through route pinning due to the external IP address at the NAT. Subsequent NSIS messages (and also data traffic) will travel through the same NAT boxes. However, this is only valid for the NAT boxes, but not for any intermediate firewall. That is the reason for having a separate CREATE message enabling the reservations made with EXT at the NATs and either enabling prior reservations or creating new pinholes at the firewalls which are encountered on the outbound path depending on whether the inbound and outbound routes coincide. The EXT signaling message creates an NSIS NATFW signaling session at any intermediate NSIS NATFW peer(s) encountered, independent of the message routing method used. Furthermore, it has to be ensured that the edge-NAT or edge-firewall device is discovered as part of this process. The end host cannot be assumed to know this device - instead the NAT or firewall box itself is assumed to know that it is Stiemerling, et al. Expires September 6, 2007 [Page 37] Internet-Draft NAT/FW NSIS NSLP March 2007 located at the outer perimeter of the network. Forwarding of the EXT message beyond this entity is not necessary, and MUST be prohibited as it may provide information on the capabilities of internal hosts. It should be noted, that it is the outermost NAT or firewall that is the edge-device that must be found during this discovery process. For instance, when there are a NAT and afterwards a firewall on the outbound path at the network border, the firewall is the edge- firewall. All messages must be forwarded to the topology-wise outermost edge-device, to ensure that this devices knows about the NATFW NSLP signaling sessions for incoming CREATE messages. However, the NAT is still the edge-NAT because it has a public globally routable IP address on its public side: this is not affected by any firewall between the edge-NAT and the public network. Possible edge arrangements are: Public Net ----------------- Private net -------------- | Public Net|--|Edge-FW|--|FW|...|FW|--|DR| | Public Net|--|Edge-FW|--|Edge-NAT|...|NAT or FW|--|DR| | Public Net|--|Edge-NAT|--|NAT or FW|...|NAT or FW|--|DR| The edge-NAT or edge-firewall device closest to the public realm responds to the EXT message with a successful RESPONSE message. An edge-NAT includes an NATFW_EXT_IP object (see Section 4.2.2), carrying the public reachable IP address, and if applicable port number. There are several processing rules for a NATFW peer when generating and receiving EXT messages, since this message type is used for creating new reserve NATFW NSLP signaling sessions, updating existing, extending the lifetime and deleting NATFW NSLP signaling session. The three latter functions operate in the same way for all kinds of CREATE and EXT messages, and are therefore described in separate sections: o Extending the lifetime of NATFW NSLP signaling sessions is described in Section 3.7.3. o Deleting NATFW NSLP signaling sessions is described in Section 3.7.4. o Updating policy rules is described in Section 3.10. The NI+ MUST include a NATFW_DTINFO object in the EXT message when Stiemerling, et al. Expires September 6, 2007 [Page 38] Internet-Draft NAT/FW NSIS NSLP March 2007 using the LE-MRM. The LE-MRM does not include enough information for some types of NATs (basically, those NATs which also translate port numbers) to perform the address translation. This information is provided in the NATFW_DTINFO (see Section 4.2.7). This information MUST include at least the 'dst port number' and 'protocol' fields, in the NATFW_DTINFO object as these may be required by en-route NATs, depending on the type of the NAT. All other fields MAY be set by the NI+ to restrict the set of possible NIs. An edge-NAT will use the information provided in the NATFW_DTINFO object to allow only NATFW CREATE message with the MRI matching ('src IPv4/v6 address', 'src port number', 'protocol') to be forwarded. A NAT requiring information carried in the NATFW_DTINFO can generate a number of error RESPONSE messages of class 'Signaling session failures' (0x6): o 'Requested policy rule denied due to policy conflict' (0x04) o 'NATFW_DTINFO object is required' (0x07) o 'Requested value in sub_ports field in NATFW_EFI not permitted' (0x08) o 'Requested IP protocol not supported' (0x09) o 'Plain IP policy rules not permitted -- need transport layer information' (0x0A) o 'source IP address range is too large' (0x0C) o 'destination IP address range is too large' (0x0D) o 'source L4-port range is too large' (0x0E) o 'destination L4-port range is too large' (0x0F) Processing of EXT messages is specific to the NSIS node type: o NSLP initiator: NI+ only generate EXT messages. When the data sender's address information is known in advance the NI+ can include a NATFW_DTINFO object in the EXT message, if not anyway required to do so (see above). When the data sender's IP address is not known, the NI+ MUST NOT include a NATFW_DTINFO object. The NI should never receive EXT messages and MUST silently discard it. o NSLP forwarder: The NSLP message processing at NFs depends on the middlebox type: * NAT: NATs check whether the message is received at the external (public in most cases) address or at the internal (private) Stiemerling, et al. Expires September 6, 2007 [Page 39] Internet-Draft NAT/FW NSIS NSLP March 2007 address. If received at the external an NF MUST generate an error RESPONSE of class 'Protocol error' (0x3) with response code 'Received EXT request message on external side' (0x0B). If received at the internal (private address) and the NATFW_EFI object contains the action 'deny', an error RESPONSE of class 'Protocol error' (0x3) with response code 'Requested rule action not applicable' (0x06) MUST be generated. If received at the internal address, an IP address, and if applicable one or more ports, are reserved. If it is an edge-NAT and there is no edge-firewall beyond, the NSLP message is not forwarded any further and a successful RESPONSE message is generated containing an NATFW_EXT_IP object holding the translated address, and if applicable port, information from the binding reserved as a result of the EXT message. The RESPONSE message is sent back towards the NI+. If it is not an edge-NAT, the NSLP message is forwarded further using the translated IP address as signaling source address in the LE-MRM and translated port in the NATFW_DTINFO object in the field 'DR port number', i.e., the NATFW_DTINFO object is updated to reflect the translated port number. The edge-NAT or any other NAT MUST reject EXT messages not carrying a NATFW_DTINFO object or if the address information within this object is invalid or is not compliant with local policies (e.g., the information provided relates to a range of addresses ('wildcarded') but the edge-NAT requires exact information about DS' IP address and port) with the above mentioned response codes. * Firewall: Non edge-firewalls remember the requested policy rule, keep NATFW NSLP signaling session state, and forward the message. Edge-firewalls stop forwarding the EXT message. The policy rule is immediately loaded if the action in the NATFW_EFI object is set to 'deny' and the node is an edge- firewall. The policy rule is remembered, but not activated, if the action in the NATFW_EFI object is set to 'allow'. In both cases, a successful RESPONSE message is generated. If the action is 'allow', and the NATFW_DTINFO object is included, and the MRM is set to LE-MRM in the request, additionally an NATFW_EXT_IP object is included in the RESPONSE message, holding the translated address, and if applicable port, information. This information is obtained from the NATFW_DTINFO object's 'DR port number' and the source-address of the LE-MRM. * Combined NAT and firewall: Processing at combined firewall and NAT middleboxes is the same as in the NAT case. Stiemerling, et al. Expires September 6, 2007 [Page 40] Internet-Draft NAT/FW NSIS NSLP March 2007 o NSLP receiver: This type of message should never be received by any NR+ and it MUST generate an error RESPONSE message of class 'Permanent failure' (0x5) with response code 'No edge-device here' (0x06). Processing of a RESPONSE message is different for every NSIS node type: o NSLP initiator: Upon receiving a successful RESPONSE message, the NI+ can rely on the requested configuration for future inbound NATFW NSLP signaling sessions. If the response contains an NATFW_EXT_IP object, the NI can use IP address and port pairs carried for further application signaling. After receiving a error RESPONSE message, the NI+ MAY try to generate the EXT message again or give up and report the failure to the application, depending on the error condition. o NSLP forwarder: NFs simply forward this message as long as they keep state for the requested reservation, if the RESPONSE message contains NATFW_INFO object with class set to 'Success' (0x2). If the RESPONSE message contains NATFW_INFO object with class set not to 'Success' (0x2), the NATFW NSLP signaling session is marked as dead. o NSIS responder: This type of message should never be received by any NR+. The NF should never receive response messages and MUST silently discard it. Reservations with action 'allow' made with EXT MUST be enabled by a subsequent CREATE message. A reservation made with EXT (independent of selected action) is kept alive as long as the NI+ refreshes the particular NATFW NSLP signaling session and it can be reused for multiple, different CREATE messages. An NI+ may decide to teardown a reservation immediately after receiving a CREATE message. This implies that a new NATFW NSLP signaling session must be created for each new CREATE message. The CREATE message does not re-use the NATFW NSLP signaling session created by REA. Without using CREATE Section 3.7.1 or EXT in proxy mode Section 3.7.6 no data traffic will be forwarded to DR beyond the edge-NAT or edge- firewall. The only function of EXT is to ensure that subsequent CREATE messages traveling towards the NR will be forwarded across the public-private boundary towards the DR. Correlation of incoming CREATE messages to EXT reservation states is described in Section 3.8. Stiemerling, et al. Expires September 6, 2007 [Page 41] Internet-Draft NAT/FW NSIS NSLP March 2007 3.7.3. NATFW NSLP Signaling Session Refresh NATFW NSLP signaling sessions are maintained on a soft-state basis. After a specified timeout, sessions and corresponding policy rules are removed automatically by the middlebox, if they are not refreshed. Soft-state is created by CREATE and EXT and the maintenance of this state must be done by these messages. State created by CREATE must be maintained by CREATE, state created by EXT must be maintained by EXT. Refresh messages, are messages carrying the same session ID as the initial message and a NATFW_LT lifetime object with a lifetime greater than zero. Messages with the same SID but carrying a different MRI are treated as updates of the policy rules and are processed as defined in Section 3.10. Every refresh CREATE or EXT message MUST be acknowledged by an appropriate response message generated by the NR. Upon reception by each NSIS forwarder, the state for the given session ID is extended by the NATFW NSLP signaling session refresh period, a period of time calculated based on a proposed refresh message period. The lifetime extension of a NATFW NSLP signaling session is calculated as current local time plus proposed lifetime value (NATFW NSLP signaling session refresh period). Section 3.4 defines the process of calculating lifetimes in detail. NI Public Internet NAT Private address NR | | space | | CREATE[lifetime > 0] | | |----------------------------->| | | | | | | | | | CREATE[lifetime > 0] | | |--------------------------->| | | | | | RESPONSE[Success/Error] | | RESPONSE[Success/Error] |<---------------------------| |<-----------------------------| | | | | | | | Figure 16: Successful Refresh Message Flow, CREATE as example Processing of NATFW NSLP signaling session refresh CREATE and EXT messages is different for every NSIS node type: o NSLP initiator: The NI/NI+ can generate NATFW NSLP signaling session refresh CREATE/EXT messages before the NATFW NSLP Stiemerling, et al. Expires September 6, 2007 [Page 42] Internet-Draft NAT/FW NSIS NSLP March 2007 signaling session times out. The rate at which the refresh CREATE/EXT messages are sent and their relation to the NATFW NSLP signaling session state lifetime is discussed further in Section 3.4. o NSLP forwarder: Processing of this message is independent of the middlebox type and is as described in Section 3.4. o NSLP responder: NRs accepting a NATFW NSLP signaling session refresh CREATE/EXT message generate a successful RESPONSE message, including the granted lifetime value of Section 3.4 in a NATFW_LT object. 3.7.4. Deleting Signaling Sessions NATFW NSLP signaling sessions can be deleted at any time. NSLP initiators can trigger this deletion by using a CREATE or EXT messages with a lifetime value set to 0, as shown in Figure 17. Whether a CREATE or EXT message type is used, depends on how the NATFW NSLP signaling session was created. NI Public Internet NAT Private address NR | | space | | CREATE[lifetime=0] | | |----------------------------->| | | | | | | CREATE[lifetime=0] | | |--------------------------->| | | | Figure 17: Delete message flow, CREATE as example NSLP nodes receiving this message delete the NATFW NSLP signaling session immediately. Policy rules associated with this particular NATFW NSLP signaling session MUST be also deleted immediately. This message is forwarded until it reaches the final NR. The CREATE/EXT message with a lifetime value of 0, does not generate any response, neither positive nor negative, since there is no NSIS state left at the nodes along the path. NSIS initiators can use CREATE/EXT message with lifetime set to zero in an aggregated way, such that a single CREATE or EXT message is terminating multiple NATFW NSLP signaling sessions. NIs can follow this procedure if the like to aggregate NATFW NSLP signaling session deletion requests: The NI uses the CREATE or EXT message with the session ID set to zero and the MRI's source-address set to its used Stiemerling, et al. Expires September 6, 2007 [Page 43] Internet-Draft NAT/FW NSIS NSLP March 2007 IP address. All other fields of the respective NATFW NSLP signaling sessions to be terminated are set as well, otherwise these fields are completely wildcarded. The NSLP message is transferred to the NTLP requesting 'explicit routing' as described in Sections 5.2.1 and 7.1.4. in [2]. The outbound NF receiving such an aggregated CREATE or EXT message MUST reject it with an error RESPONSE of class 'Permanent failure' (0x5) with response code 'Authentication failed' (0x01) if the authentication fails and with an error RESPONSE of class 'Permanent failure' (0x5) with response code 'Authorization failed' (0x02) if the authorization fails. Per NATFW NSLP signaling session proof of ownership, as it is defined in this memo, is not possible anymore when using this aggregated way. However, the outbound NF can use the relationship between the information of the received CREATE or EXT message and the GIST messaging association where the request has been received. The outbound NF MUST only accept this aggregated CREATE or EXT message through already established GIST messaging associations with the NI. The outbound NF MUST NOT propagate this aggregated CREATE or EXT message but it MAY generate and forward per NATFW NSLP signaling session CREATE or EXT messages. 3.7.5. Reporting Asynchronous Events NATFW NSLP forwarders and NATFW NSLP responders must have the ability to report asynchronous events to other NATFW NSLP nodes, especially to allow reporting back to the NATFW NSLP initiator. Such asynchronous events may be premature NATFW NSLP signaling session termination, changes in local policies, route change or any other reason that indicates change of the NATFW NSLP signaling session state. NFs and NRs may generate NOTIFY messages upon asynchronous events, with a NATFW_INFO object indicating the reason for event. These reasons can be carried in the NATFW_INFO object (class MUST be set to 'Informational' (0x1)) within the NOTIFY message. This list shows the response codes and the associated actions to take at NFs and the NI: o 'Route change: possible route change on the outbound path' (0x01): Follow instructions in Section 3.9. This MUST be sent inbound. o 'Re-authentication required' (0x02): The NI should re-send the authentication. This MUST be sent inbound. o 'NATFW node is going down soon' (0x03): The NI and other NFs should be prepared for a service interruption at any time. This message MAY be sent inbound and outbound. Stiemerling, et al. Expires September 6, 2007 [Page 44] Internet-Draft NAT/FW NSIS NSLP March 2007 o 'NATFW signaling session lifetime expired' (0x04): The NATFW signaling session has been expired and the signaling session is invalid now. NFs MUST mark the signaling session as 'Dead'. This message MAY be sent inbound and outbound. NOTIFY messages are always sent hop-by-hop inbound towards NI until they reach NI or outbound towards the NR as indicated in the list above. The initial processing when receiving a NOTIFY message is the same for all NATFW nodes: NATFW nodes MUST only accept NOTIFY messages through already established NTLP messaging associations. The further processing is different for each NATFW NSLP node type and depends on the events notified: o NSLP initiator: NIs analyze the notified event and behave appropriately based on the event type. NIs MUST NOT generate NOTIFY messages. o NSLP forwarder: NFs analyze the notified event and behave based on the above description per response code. NFs SHOULD generate NOTIFY messages upon asynchronous events and forward them inbound towards the NI or outbound towards the NR, depending on the received direction, i.e., inbound messages MUST be forwarded further inbound and outbound messages MUST be forwarded further inbound. NFs MUST silently discard NOTIFY messages that have been received outbound but are only allowed to be sent inbound, e.g. 'Re-authentication required' (0x02). o NSLP responder: NRs SHOULD generate NOTIFY messages upon asynchronous events including a response code based on the reported event. The NR MUST silently discard NOTIFY messages that have been received outbound but are only allowed to be sent inbound, e.g. 'Re-authentication required' (0x02), NATFW NSLP forwarders, keeping multiple NATFW NSLP signaling sessions at the same time, can experience problems when shutting down service suddenly. This sudden shutdown can be result of node local failure, for instance, due to a hardware failure. This NF generates NOTIFY messages for each of the NATFW NSLP signaling sessions and tries to send them inbound. Due to the number of NOTIFY messages to be sent, the shutdown of the node may be unnecessarily prolonged, since not all messages can be sent at the same time. This case can be described as a NOTIFY storm, if a multitude of NATFW NSLP signaling sessions is involved. To avoid the need of generating per NATFW NSLP signaling session NOTIFY messages in such a scenario described or similar cases, NFs Stiemerling, et al. Expires September 6, 2007 [Page 45] Internet-Draft NAT/FW NSIS NSLP March 2007 SHOULD follow this procedure: The NF uses the NOTIFY message with the session ID in the NTLP set to zero, with the MRI completely wildcarded, using the 'explicit routing' as described in Sections 5.2.1 and 7.1.4. in [2]. The inbound NF receiving this type of NOTIFY immediately regards all NATFW NSLP signaling sessions from that peer matching the MRI as void. This message will typically result in multiple NOTIFY messages at the inbound NF, i.e., the NF can generate per terminated NATFW NSLP signaling session a NOTIFY message. However, a NF MAY aggregate again the NOTIFY messages as described here. 3.7.6. Proxy Mode of Operation Some migration scenarios need specialized support to cope with cases where NSIS is only deployed in same areas of the Internet. End-to- end signaling is going to fail without NSIS support at or near both data sender and data receiver terminals. A proxy mode of operation is needed. This proxy mode of operation must terminate the NATFW NSLP signaling as topologically close to the terminal for which it is proxying and proxy all messages. This NATFW NSLP node doing the proxying of the signaling messages becomes either the NI or the NR for the particular NATFW NSLP signaling session, depending on whether it is the DS or DR that does not support NSIS. Typically, the edge- NAT or the edge-firewall would be used to proxy NATFW NSLP messages. This proxy mode operation does not require any new CREATE or EXT message type, but relies on extended CREATE and EXT message types. They are called respectively CREATE-PROXY and EXT-PROXY and are distinguished by setting the P flag in the NSLP header to P=1. This flag instructs edge-NATs and edge-firewalls receiving them to operate in proxy mode for the NATFW NSLP signaling session in question. The semantics of the CREATE and EXT message types are not changed and the behavior of the various node types is as defined in Section 3.7.1 and Section 3.7.2, except for the proxying node. The following paragraphs describe the proxy mode operation for data receivers behind middleboxes and data senders behind middleboxes. 3.7.6.1. Proxying for a Data Sender The NATFW NSLP gives the NR the ability to install state on the inbound path towards the data sender for outbound data packets, even when only the receiving side is running NSIS (as shown in Figure 18). The goal of the method described is to trigger the edge-NAT/ edge-firewall to generate a CREATE message on behalf of the data receiver. In this case, an NR can signal towards the network border as it is performed in the standard EXT message handling scenario as in Section 3.7.2. The message is forwarded until the edge-NAT/ edge-firewall is reached. A public IP address and port number is Stiemerling, et al. Expires September 6, 2007 [Page 46] Internet-Draft NAT/FW NSIS NSLP March 2007 reserved at an edge-NAT/edge-firewall. As shown in Figure 18, unlike the standard EXT message handling case, the edge-NAT/edge-firewall is triggered to send a CREATE message on a new reverse path which traverse several firewalls or NATs. The new reverse path for CREATE is necessary to handle routing asymmetries between the edge-NAT/ edge-firewall and DR. It must be stressed that the semantics of the CREATE and EXT messages is not changed, i.e., each is processed as described earlier. DS Public Internet NAT/FW Private address NR No NI NF space NI+ NR+ | | EXT-PROXY[(DTInfo)] | | |<------------------------- | | | RESPONSE[Error/Success] | | | ---------------------- > | | | CREATE | | | ------------------------> | | | RESPONSE[Error/Success] | | | <---------------------- | | | | Figure 18: EXT Triggering Sending of CREATE Message A NATFW_NONCE object, carried in the EXT and CREATE message, is used to build the relationship between received CREATEs at the message initiator. An NI+ uses the presence of the NATFW_NONCE object to correlate it to the particular EXT-PROXY. The absence of a NONCE object indicates a CREATE initiated by the DS and not by the edge- NAT. Therefore, these processing rules of EXT-PROXY messages are added to the regular EXT processing: o NSLP initiator (NI+): The NI+ MUST choose a random value and place it in the NATFW_NONCE object. o NSLP forwarder being either edge-NAT or edge-firewall: When the NF accepts a EXT_PROXY message, it generates a successful RESPONSE message as if it were the NR and additionally, it generates a CREATE message as defined in Section 3.7.1 and includes a NATFW_NONCE object having the same value as of the received NATFW_NONCE object. The NF MUST not generate a CREATE-PROXY message. The NF MUST refresh the CREATE message signaling session only if a EXT-PROXY refresh message has been received first. This also includes tearing down signaling sessions, i.e., the NF must teardown the CREATE signaling session only if a EXT-PROXY message with lifetime set to 0 has been received first. Stiemerling, et al. Expires September 6, 2007 [Page 47] Internet-Draft NAT/FW NSIS NSLP March 2007 The scenario described in this section challenges the data receiver because it must make a correct assumption about the data sender's ability to use NSIS NATFW NSLP signaling. It is possible for the DR to make the wrong assumption in two different ways: a) the DS is NSIS unaware but the DR assumes the DS to be NSIS aware and b) the DS is NSIS aware but the DR assumes the DS to be NSIS unaware. Case a) will result in middleboxes blocking the data traffic, since DS will never send the expected CREATE message. Case b) will result in the DR successfully requesting proxy mode support by the edge-NAT or edge-firewall. The edge-NAT/edge-firewall will send CREATE messages and DS will send CREATE messages as well. Both CREATE messages are handled as separated NATFW NSLP signaling sessions and therefore the common rules per NATFW NSLP signaling session apply; the NATFW_NONCE object is used to differentiate CREATE messages generated by the edge-NAT/edge-firewall from NI initiated CREATE messages. It is the NR's responsibility to decide whether to teardown the EXT-PROXY signaling sessions in the case where the data sender's side is NSIS aware, but was incorrectly assumed not to be so by the DR. It is RECOMMENDED that a DR behind NATs uses the proxy mode of operation by default, unless the DR knows that the DS is NSIS aware. The DR MAY cache information about data senders which it has found to be NSIS aware in past NATFW NSLP signaling sessions. There is a possible race condition between the RESPONSE message to the EXT-PROXY and the CREATE message generated by the edge-NAT. The CREATE message can arrive earlier than the RESPONSE message. An NI+ MUST accept CREATE messages generated by the edge-NAT even if the RESPONSE message to the EXT-PROXY was not received. 3.7.6.2. Proxying for a Data Receiver As with data receivers behind middleboxes, data senders behind middleboxes can require proxy mode support. The issue here is that there is no NSIS support at the data receiver's side and, by default, there will be no response to CREATE messages. This scenario requires the last NSIS NATFW NSLP aware node to terminate the forwarding and to proxy the response to the CREATE message, meaning that this node is generating RESPONSE messages. This last node may be an edge-NAT/ edge-firewall, or any other NATFW NSLP peer, that detects that there is no NR available (probably as a result of GIST timeouts but there may be other triggers). Stiemerling, et al. Expires September 6, 2007 [Page 48] Internet-Draft NAT/FW NSIS NSLP March 2007 DS Private Address NAT/FW Public Internet NR NI Space NF no NR | | | | CREATE-PROXY | | |------------------------------>| | | | | | RESPONSE[SUCCESS/ERROR] | | |<------------------------------| | | | | Figure 19: Proxy Mode CREATE Message Flow The processing of CREATE-PROXY messages and RESPONSE messages is similar to Section 3.7.1, except that forwarding is stopped at the edge-NAT/edge-firewall. The edge-NAT/edge-firewall responds back to NI according the situation (error/success) and will be the NR for future NATFW NSLP communication. The NI can choose the proxy mode of operation although the DR is NSIS aware. The CREATE-PROXY mode would not configure all NATs and firewalls along the data path, since it is terminated at the edge- device. Any device beyond this point will never receive any NATFW NSLP signaling for this flow. 3.8. De-Multiplexing at NATs Section 3.7.2 describes how NSIS nodes behind NATs can obtain a public reachable IP address and port number at a NAT and and how the resulting mapping rule can be activated by using CREATE messages (see Section 3.7.1). The information about the public IP address/port number can be transmitted via an application level signaling protocol and/or third party to the communication partner that would like to send data toward the host behind the NAT. However, NSIS signaling flows are sent towards the address of the NAT at which this particular IP address and port number is allocated and not directly to the allocated IP address and port number. The NATFW NSLP forwarder at this NAT needs to know how the incoming NSLP CREATE messages are related to reserved addresses, meaning how to de- multiplex incoming NSIS CREATE messages. The de-multiplexing method uses information stored at the local NATFW NSLP node and the of the policy rule. The policy rule uses the LE- MRM MRI source-address (see [2]) as the flow destination IP address and the network-layer-version as IP version. The external IP address at the NAT is stored as the external flow destination IP address. All other parameters of the policy rule other than the flow destination IP address are wildcarded if no NATFW_DTINFO object is Stiemerling, et al. Expires September 6, 2007 [Page 49] Internet-Draft NAT/FW NSIS NSLP March 2007 included in the EXT message. The LE-MRM MRI destination-address MUST NOT be used in the policy rule, since it is solely a signaling destination address. If the NATFW_DTINFO object is included in the EXT message, the policy rule is filled with further information. The 'dst port number' field of the NATFW_DTINFO is stored as the flow destination port number. The 'protocol' field is stored as the flow protocol. The 'src port number' field is stored as the flow source port number. The 'data sender's IPv4 address' is stored as the flow source IP address. Note that some of these field can contain wildcards. When receiving a CREATE message at the NATFW NSLP it uses the flow information stored in the MRI to do the matching process. This table shows the parameters to be compared against each others. Note that not all parameters can be present in a MRI at the same time. +-------------------------------+--------------------------------+ | Flow parameter (Policy Rule) | MRI parameter (CREATE message) | +-------------------------------+--------------------------------+ | IP version | network-layer-version | | | | | Protocol | IP-protocol | | | | | source IP address (w) | source-address (w) | | | | | external IP address | destination-address | | | | | destination IP address (n/u) | N/A | | | | | source port number (w) | L4-source-port (w) | | | | | external port number (w) | L4-destination-port (w) | | | | | destination port number (n/u) | N/A | | | | | IPsec SPI | ipsec-SPI | +-------------------------------+--------------------------------+ Table entries marked with (w) can be wildcarded and entries marked with (n/u) are not used for the matching. Table 1 Stiemerling, et al. Expires September 6, 2007 [Page 50] Internet-Draft NAT/FW NSIS NSLP March 2007 3.9. Reacting to Route Changes The NATFW NSLP needs to react to route changes in the data path. This assumes the capability to detect route changes, to perform NAT and firewall configuration on the new path and possibly to tear down NATFW NSLP signaling session state on the old path. The detection of route changes is described in Section 7 of [2] and the NATFW NSLP relies on notifications about route changes by the NTLP. This notification will be conveyed by the API between NTLP and NSLP, which is out of scope of this memo. A NATFW NSLP node other than the NI or NI+ detecting a route change, by means described in the NTLP specification or others, generates a NOTIFY message indicating this change and sends this inbound towards NI. Intermediate NFs on the way to the NI can use this information to decide later if their NATFW NSLP signaling session can be deleted locally, if they do not receive an update within a certain time period, as described in Section 3.2.3. It is important to consider the transport limitations of NOTIFY messages as mandated in Section 3.7.5. The NI receiving this NOTIFY message MAY generate a new CREATE or EXT message and sends it towards the NATFW NSLP signaling session's NI as for the initial message using the same session ID. All the remaining processing and message forwarding, such as NSLP next hop discovery, is subject to regular NSLP processing as described in the particular sections. Normal routing will guide the new CREATE or EXT message to the correct NFs along the changed route. NFs that were on the original path receiving these new CREATE or EXT messages (see also Section 3.10), can use the session ID to update the existing NATFW NSLP signaling session, whereas NFs that were not on the original path will create new state for this NATFW NSLP signaling session. The next section describes how policy rules are updated. 3.10. Updating Policy Rules NSIS initiators can request an update of the installed/reserved policy rules at any time within a NATFW NSLP signaling session. Updates to policy rules can be required due to node mobility (NI is moving from one IP address to another), route changes (this can result in a different NAT mapping at a different NAT device), or the wish of the NI to simply change the rule. NIs can update policy rules in existing NATFW NSLP signaling sessions by sending an appropriate CREATE or EXT message (similar to Section 3.4) with modified message routing information (MRI) as compared with that installed previously, but using the existing session ID to identify the intended target of the update. With respect to authorization and authentication, this update CREATE or EXT message is treated in Stiemerling, et al. Expires September 6, 2007 [Page 51] Internet-Draft NAT/FW NSIS NSLP March 2007 exactly the same way as any initial message. Therefore, any node along in the NATFW NSLP signaling session can reject the update with an error RESPONSE message, as defined in the previous sections. The message processing and forwarding is executed as defined in the particular sections. A NF or the NR receiving an update, simply replaces the installed policy rules installed in the firewall/NAT. The local procedures on how to update the MRI in the firewall/NAT is out of scope of this memo Stiemerling, et al. Expires September 6, 2007 [Page 52] Internet-Draft NAT/FW NSIS NSLP March 2007 4. NATFW NSLP Message Components A NATFW NSLP message consists of a NSLP header and one or more objects following the header. The NSLP header is carried in all NATFW NSLP message and objects are Type-Length-Value (TLV) encoded using big endian (network ordered) binary data representations. Header and objects are aligned to 32 bit boundaries and object lengths that are not multiples of 32 bits must be padded to the next higher 32 bit multiple. The whole NSLP message is carried as payload of a NTLP message. Note that the notation 0x is used to indicate hexadecimal numbers. 4.1. NSLP Header All GIST NSLP-Data objects for the NATFW NSLP MUST contain this common header as the first 32 bits of the object (this is not the same as the GIST Common Header). It contains two fields, the NSLP message type and a reserved field. The total length is 32 bits. The layout of the NSLP header is defined by Figure 20. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message type |P| reserved | reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 20: Common NSLP header The reserved field MUST be set to zero in the NATFW NSLP header before sending and MUST be ignored during processing of the header. The defined messages types are: o IANA-TBD(1) : CREATE o IANA-TBD(2) : EXTERNAL(EXT) o IANA-TBD(3) : RESPONSE o IANA-TBD(4) : NOTIFY If a message with another type is received, an error RESPONSE of class 'Protocol error' (0x3) with response code 'Illegal message type' (0x01) MUST be generated. Stiemerling, et al. Expires September 6, 2007 [Page 53] Internet-Draft NAT/FW NSIS NSLP March 2007 The P flag indicates the usage of proxy mode. If proxy mode is used it MUST be set to 1. Proxy mode usage is only allowed in combination with the message types CREATE and EXT, P=1 MUST NOT be set with message types other than CREATE and EXT. The P flag MUST be ignored when processing messages with type RESPONSE. An error RESPONSE message of class 'Protocol error' (0x3) and type 'Bad flags value' (0x03) MUST be generated, if the P flag is set in NOTIFY messages. 4.2. NSLP Objects NATFW NSLP objects use a common header format defined by Figure 21. The object header contains two fields, the NSLP object type and the object length. Its total length is 32 bits. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |A|B|r|r| Object Type |r|r|r|r| Object Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 21: Common NSLP object header The object length field contains the total length of the object without the object header. The unit is a word, consisting of 4 octets. The particular values of type and length for each NSLP object are listed in the subsequent sections that define the NSLP objects. An error RESPONSE of class 'Protocol error' (0x3) with response code 'Wrong object length' (0x07) MUST be generated if the length given for the object in the object header did not match the length of the object data present. The two leading bits of the NSLP object header are used to signal the desired treatment for objects whose treatment has not been defined in this memo (see [2], Section A.2.1), i.e., the Object Type has not been defined. NATFW NSLP uses a subset of the categories defined in GIST: o AB=00 ("Mandatory"): If the object is not understood, the entire message containing it MUST be rejected with an error RESPONSE of class 'Protocol error' (0x3) with response code 'Unknown object present' (0x06). o AB=01 ("Optional"): If the object is not understood, it should be deleted and then the rest of the message processed as usual. o AB=10 ("Forward"): If the object is not understood, it should be retained unchanged in any message forwarded as a result of message processing, but not stored locally. Stiemerling, et al. Expires September 6, 2007 [Page 54] Internet-Draft NAT/FW NSIS NSLP March 2007 The combination AB=11 MUST NOT be used and an error RESPONSE of class 'Protocol error' (0x3) with response code 'Invalid Flag-Field combination' (0x09) MUST be generated. The following sections do not repeat the common NSLP object header, they just list the type and the length. 4.2.1. Signaling Session Lifetime Object The signaling session lifetime object carries the requested or granted lifetime of a NATFW NSLP signaling session measured in seconds. Type: NATFW_LT (IANA-TBD) Length: 1 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NATFW NSLP signaling session lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 22: Signaling Session Lifetime object 4.2.2. External Address Object The external address object can be included in RESPONSE messages (Section 4.3.3) only. It carries the publicly reachable IP address, and if applicable port number, at an edge-NAT. Type: NATFW_EXT_IP (IANA-TBD) Length: 2 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | port number | reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IPv4 address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 23: External Address Object for IPv4 addresses Please note that the field 'port number' MUST be set to 0 if only an Stiemerling, et al. Expires September 6, 2007 [Page 55] Internet-Draft NAT/FW NSIS NSLP March 2007 IP address has been reserved, for instance, by a traditional NAT. A port number of 0 MUST be ignored in processing this object. 4.2.3. Extended Flow Information Object In general, flow information is kept in the message routing information (MRI) of the NTLP. Nevertheless, some additional information may be required for NSLP operations. The 'extended flow information' object carries this additional information about the action of the policy rule for firewalls/NATs and contiguous port . Type: NATFW_EFI (IANA-TBD) Length: 1 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | rule action | sub_ports | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 24: Extended Flow Information This object has two fields, 'rule action' and 'sub_ports'. The 'rule action' field has these meanings: o 0x0001: Allow: A policy rule with this action allows data traffic to traverse the middlebox and the NATFW NSLP MUST allow NSLP signaling to be forwarded. o 0x0002: Deny: A policy rule with this action blocks data traffic from traversing the middlebox and the NATFW NSLP MUST NOT allow NSLP signaling to be forwarded. If the 'rule action' field contains neither 0x0001 nor 0x0002, an error RESPONSE of class 'Signaling session error' (0x6) with response code 'Unknown policy rule action' (0x05) MUST be generated. The 'sub_ports' field contains the number of contiguous transport layer ports to which this rule applies. The default value of this field is 0, i.e., only the port specified in the NTLP's MRM or NATFW_DTINFO object is used for the policy rule. A value of 1 indicates that additionally to the port specified in the NTLP's MRM (port1), a second port (port2) is used. This value of port 2 is calculated as: port2 = port1 + 1. Other values than 0 or 1 MUST NOT be used in this field and an error RESPONSE of class 'Signaling session error' (0x6) with response code 'Requested value in sub_ports Stiemerling, et al. Expires September 6, 2007 [Page 56] Internet-Draft NAT/FW NSIS NSLP March 2007 field in NATFW_EFI not permitted' (0x08) MUST be generated. Further version of this memo may allow other values for the 'sub_ports' field. This two contiguous port numbered ports, can be used by legacy voice over IP equipment. This legacy equipment assumes that two adjacent port numbers for its RTP/RTCP flows respectively. 4.2.4. Information Code Object This object carries the response code, which may be indications for either a successful or failed CREATE or EXT message depending on the value of the 'response code' field. Type: NATFW_INFO (IANA-TBD) Length: 1 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Resv. | Class | Response Code |r|r|r|r| Object Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 25: Information Code Object The field 'resv.' is reserved for future extensions and MUST be set to zero when generating such an object and MUST be ignored when receiving. The 'Object Type' field contains the type of the object causing the error. The value of 'Object Type' is set to 0, if no object is concerned and the leading fours bits marked with 'r' are always set to zero and ignored. The 4 bit class field contains the severity class. The following classes are defined: o 0x1: Informational (NOTIFY only) o 0x2: Success o 0x3: Protocol error o 0x4: Transient failure o 0x5: Permanent failure o 0x6: Signaling session failures Within each severity class a number of responses values are defined Stiemerling, et al. Expires September 6, 2007 [Page 57] Internet-Draft NAT/FW NSIS NSLP March 2007 o Informational: * 0x01: Route change: possible route change on the outbound path. * 0x02: Re-authentication required. * 0x03: NATFW node is going down soon. o Success: * 0x01: All successfully processed. o Protocol error: * 0x01: Illegal message type: the type given in the Message Type field of the NSLP header is unknown. * 0x02: Wrong message length: the length given for the message in the NSLP header does not match the length of the message data. * 0x03: Bad flags value: an undefined flag or combination of flags was set in the NSLP header. * 0x04: Mandatory object missing: an object required in a message of this type was missing. * 0x05: Illegal object present: an object was present which must not be used in a message of this type. * 0x06: Unknown object present: an object of an unknown type was present in the message. * 0x07: Wrong object length: the length given for the object in the object header did not match the length of the object data present. * 0x08: Unknown object field value: a field in an object had an unknown value. * 0x09: Invalid Flag-Field combination: An object contains an invalid combination of flags and/or fields. * 0x0A: Duplicate object present. * 0x0B: Received EXT request message on external side. Stiemerling, et al. Expires September 6, 2007 [Page 58] Internet-Draft NAT/FW NSIS NSLP March 2007 o Transient failure: * 0x01: Requested resources temporarily not available. o Permanent failure: * 0x01: Authentication failed. * 0x02: Authorization failed. * 0x03: Unable to agree transport security with peer. * 0x04: Internal or system error. * 0x05: No NAT here. * 0x06: No edge-device here. * 0x07: Did not reach the NR. o Signaling session failures: * 0x01: Session terminated asynchronously. * 0x02: Requested lifetime is too big. * 0x03: No reservation found matching the MRI of the CREATE request. * 0x04: Requested policy rule denied due to policy conflict. * 0x05: Unknown policy rule action. * 0x06: Requested rule action not applicable. * 0x07: NATFW_DTINFO object is required. * 0x08: Requested value in sub_ports field in NATFW_EFI not permitted. * 0x09: Requested IP protocol not supported. * 0x0A: Plain IP policy rules not permitted -- need transport layer information. * 0x0B: ICMP type value not permitted. Stiemerling, et al. Expires September 6, 2007 [Page 59] Internet-Draft NAT/FW NSIS NSLP March 2007 * 0x0C: source IP address range is too large. * 0x0D: destination IP address range is too large. * 0x0E: source L4-port range is too large. * 0x0F: destination L4-port range is too large. * 0x10: Requested lifetime is too small. 4.2.5. Nonce Object This object carries the nonce value as described in Section 3.7.6. Type: NATFW_NONCE (IANA-TBD) Length: 1 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 26: Nonce Object 4.2.6. Message Sequence Number Object This object carries the MSN value as described in Section 3.5. Type: NATFW_MSN (IANA-TBD) Length: 1 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | message sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 27: Message Sequence Number Object Stiemerling, et al. Expires September 6, 2007 [Page 60] Internet-Draft NAT/FW NSIS NSLP March 2007 4.2.7. Data Terminal Information Object The 'data terminal information' object carries additional information possibly needed during EXT operations. EXT messages are transported by the NTLP using the Loose-End message routing method (LE-MRM). The LE-MRM contains only DR's IP address and a signaling destination address (destination address). This destination address is used for message routing only and is not necessarily reflecting the address of the data sender. This object contains information about (if applicable) DR's port number (the destination port number), DS' port number (the source port number), the used transport protocol, the prefix length of the IP address, and DS' IP address. Type: NATFW_DTINFO (IANA-TBD) Length: variable. Maximum 3. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |I|P|S| reserved | sender prefix | protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : DR port number | DS port number : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : IPsec SPI : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | data sender's IPv4 address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 28: Data Terminal IPv4 Address Object The flags are: o I: I=1 means that 'protocol' should be interpreted. o P: P=1 means that 'dst port number' and 'src port number' are present and should be interpreted. o S: S=1 means that SPI is present and should be interpreted. The SPI field is only present if S is set. The port numbers are only present if P is set. The flags P and S MUST NOT be set at the same time. An error RESPONSE of class 'Protocol error' (0x3) with response code 'Invalid Flag-Field combination' (0x09) MUST be generated if they are both set. If either P or S is set, I MUST be set as well and the protocol field MUST carry the particular protocol. An error RESPONSE of class 'Protocol error' (0x3) with Stiemerling, et al. Expires September 6, 2007 [Page 61] Internet-Draft NAT/FW NSIS NSLP March 2007 response code 'Invalid Flag-Field combination' (0x09) MUST be generated if S or P is set but I is not set. The fields MUST be interpreted according these rules: o (data) sender prefix: This parameter indicates the prefix length of the 'data sender's IP address' in bits. For instance, a full IPv4 address requires 'sender prefix' to be set to 32. A value of 0 indicates an IP address wildcard. o protocol: The IPv4 protocol field. This field MUST be interpreted if I=1, otherwise it MUST be set to 0 and MUST be ignored. o DR port number: The port number at the data receiver (DR), i.e., the destination port. A value of 0 indicates a port wildcard, i.e., the destination port number is not known. Any other value indicates the destination port number. o DS port number: The port number at the data sender (DS), i.e., the source port. A value of 0 indicates a port wildcard, i.e., the source port number is not known. Any other value indicates the source port number. o data sender's IPv4 address: The source IP address of the data sender. This field MUST be set to zero if no IP address is provided, i.e., a complete wildcard is desired (see dest prefix field above). 4.2.8. ICMP Types Object The 'ICMP types' object contains additional information needed to configure a NAT of firewall with rules to control ICMP traffic. The object contains a number of values of the ICMP Type field for which a filter action should be set up: Type: NATFW_ICMP_TYPES (IANA-TBD) Length: Variable = ((Number of Types carried + 1) + 3) DIV 4 Where DIV is an integer division. Stiemerling, et al. Expires September 6, 2007 [Page 62] Internet-Draft NAT/FW NSIS NSLP March 2007 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Count | Type | Type | ........ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ................ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ........ | Type | (Padding) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 29: ICMP Types Object The fields MUST be interpreted according these rules: count: 8 bit integer specifying the number of 'Type' entries in the object. type: 8 bit field specifying an ICMP Type value to which this rule applies. padding: Sufficient 0 bits to pad out the last word so that the total size of object is an even multiple of words. Ignored on reception. 4.3. Message Formats This section defines the content of each NATFW NSLP message type. The message types are defined in Section 4.1. Basically, each message is constructed of NSLP header and one or more NSLP objects. The order of objects is not defined, meaning that objects may occur in any sequence. Objects are marked either with mandatory (M) or optional (O). Where (M) implies that this particular object MUST be included within the message and where (O) implies that this particular object is OPTIONAL within the message. Objects defined in this memo carry always the flag combination AB=00 in the NSLP object header. An error RESPONSE message of class 'Protocol error' (0x3) with response code 'Mandatory object missing' (0x02) MUST be generated if a mandatory declared object is missing. An error RESPONSE message of class 'Protocol error' (0x3) with response code 'Illegal object present' (0x05) MUST be generated if an object was present which must not be used in a message of this type. An error RESPONSE message of class 'Protocol error' (0x3) with response code 'Duplicate object present' (0x0A) MUST be generated if an object appears more than once in a message. Each section elaborates the required settings and parameters to be set by the NSLP for the NTLP, for instance, how the message routing Stiemerling, et al. Expires September 6, 2007 [Page 63] Internet-Draft NAT/FW NSIS NSLP March 2007 information is set. 4.3.1. CREATE The CREATE message is used to create NATFW NSLP signaling sessions and to create policy rules. Furthermore, CREATE messages are used to refresh NATFW NSLP signaling sessions and to delete them. The CREATE message carries these objects: o Signaling Session Lifetime object (M) o Extended flow information object (M) o Message sequence number object (M) o Nonce object (M) if P flag set to 1 in the NSLP header, otherwise (O) o ICMP Types Object (O) The message routing information in the NTLP MUST be set to DS as source address and DR as destination address. All other parameters MUST be set according the required policy rule. CREATE messages MUST be transported by using the path-coupled MRM with direction set to 'downstream' (outbound). 4.3.2. EXTERNAL (EXT) The EXTERNAL (EXT) message is used to a) reserve an external IP address/port at NATs, b) to notify firewalls about NSIS capable DRs, or c) to block incoming data traffic at inbound firewalls. The EXT message carries these objects: o Signaling Session Lifetime object (M) o Message sequence number object (M) o Extended flow information object (M) o Data terminal information object (M) o Nonce object [M if P flag set to 1 in the NSLP header, otherwise (O) o ICMP Types Object (O) Stiemerling, et al. Expires September 6, 2007 [Page 64] Internet-Draft NAT/FW NSIS NSLP March 2007 The selected message routing method of the EXT message depends on a number of considerations. Section 3.7.2 describes it exhaustively how to select the correct method. EXT messages can be transported via the path-coupled message routing method (PC-MRM) or via the loose-end message routing method (LE-MRM). In the case of PC-MRM, the source-address is set to DS' address and the destination address is set to DR's address, the direction is set to inbound. In the case of LE-MRM, the destination-address is set to DR's address or to the signaling destination address. The source-address is set to DS's address. 4.3.3. RESPONSE RESPONSE messages are responses to CREATE and EXT messages. RESPONSE messages MUST NOT be generated for any other message, such as NOTIFY and RESPONSE. The RESPONSE message for the class 'Success' (0x2) carries these objects: o Signaling Session Lifetime object (M) o Message sequence number object (M) o Information code object (M) o External address object (O) The RESPONSE message for other classes than 'Success' (0x2) carries these objects: o Message sequence number object (M) o Information code object (M) This message is routed towards the NI hop-by-hop, using existing NTLP messaging associations. The MRM used for this message MUST be the same as MRM used by the corresponding CREATE or EXT message. 4.3.4. NOTIFY The NOTIFY messages is used to report asynchronous events happening along the signaled path to other NATFW NSLP nodes. The NOTIFY message carries this object: o Information code object (M). Stiemerling, et al. Expires September 6, 2007 [Page 65] Internet-Draft NAT/FW NSIS NSLP March 2007 The NOTIFY message is routed towards the NI hop-by-hop using the existing inbound node messaging association entry within the node's Message Routing State table. The MRM used for this message MUST be the same as MRM used by the corresponding CREATE or EXT message. Stiemerling, et al. Expires September 6, 2007 [Page 66] Internet-Draft NAT/FW NSIS NSLP March 2007 5. Security Considerations Security is of major concern particularly in case of firewall traversal. This section provides security considerations for the NAT/firewall traversal and is organized as follows. In Section 5.1 we describe the participating entities relate to each other from a security point of view. This subsection also motivates a particular authorization model. Security threats that focus on NSIS in general are described in [8] and they are applicable to this document as well. In Section 5.2 we extend this threat investigation by considering NATFW NSLP specific threats in more detail. Based on the investigated security threats we derive security requirements. Finally, we illustrate how the security requirements that were created based on the security threats can be fulfilled by specific security mechanisms. These aspects will be elaborated in Section 5.11. 5.1. Authorization Framework The NATFW NSLP is a protocol which may involve a number of NSIS nodes and is, as such, not a two-party protocol. Figure 1 and Figure 2 of [8] already depict the possible set of communication patterns. In this section we will re-evaluate these communication patters with respect to the NATFW NSLP protocol interaction. The security solutions for providing authorization have a direct impact on the treatment of different NSLPs. As it can be seen from the QoS NSLP [6] and the corresponding Diameter QoS work [20] accounting and charging seems to play an important role for QoS reservations, whereas monetary aspects might only indirectly effect authorization decisions for NAT and firewall signaling. Hence, there are differences in the semantic of authorization handling between QoS and NATFW signaling. A NATFW aware node will most likely want to authorize the entity (e.g., user or machine) requesting the establishment of pinholes or NAT bindings. The outcome of the authorization decision is either allowed or disallowed whereas a QoS authorization decision might indicate that a different set of QoS parameters is authorization (see [20] as an example). 5.1.1. Peer-to-Peer Relationship Starting with the simplest scenario, it is assumed that neighboring nodes are able to authenticate each other and to establish keying material to protect the signaling message communication. The nodes Stiemerling, et al. Expires September 6, 2007 [Page 67] Internet-Draft NAT/FW NSIS NSLP March 2007 will have to authorize each other, additionally to the authentication. We use the term 'Security Context' as a placeholder for referring to the entire security procedure, the necessary infrastructure that needs to be in place in order for this to work (e.g., key management) and the established security related state. The required long-term key (symmetric or asymmetric keys) used for authentication are either made available using an out-of-band mechanism between the two NSIS NATFW nodes or they are dynamically established using mechanisms not further specified in this document. Note that the deployment environment will most likely have an impact on the choice of credentials being used. The choice of these credentials used is also outside the scope of this document. +------------------------+ +-------------------------+ |Network A | | Network B| | +---------+ +---------+ | | +-///-+ Middle- +---///////----+ Middle- +-///-+ | | | | box 1 | Security | box 2 | | | | | +---------+ Context +---------+ | | | | Security | | Security | | | | Context | | Context | | | | | | | | | +--+---+ | | +--+---+ | | | Host | | | | Host | | | | A | | | | B | | | +------+ | | +------+ | +------------------------+ +-------------------------+ Figure 30: Peer-to-Peer Relationship Figure 30 shows a possible relationship between participating NSIS aware nodes. Host A might be, for example, a host in an enterprise network that has keying material established (e.g., a shared secret) with the company's firewall (Middlebox 1). The network administrator of Network A (company network) has created access control lists for Host A (or whatever identifiers a particular company wants to use). Exactly the same procedure might also be used between Host B and Middlebox 2 in Network B. For the communication between Middlebox 1 and Middlebox 2 a security context is also assumed in order to allow authentication, authorization and signaling message protection to be successful. 5.1.2. Intra-Domain Relationship In larger corporations, for example, a middlebox is used to protect individual departments. In many cases, the entire enterprise is controlled by a single (or a small number of) security department, Stiemerling, et al. Expires September 6, 2007 [Page 68] Internet-Draft NAT/FW NSIS NSLP March 2007 which gives instructions to the department administrators. In such a scenario, the previously discussed peer-to-peer relationship might be prevalent. Sometimes it might be necessary to preserve authentication and authorization information within the network. As a possible solution, a centralized approach could be used, whereby an interaction between the individual middleboxes and a central entity (for example a policy decision point - PDP) takes place. As an alternative, individual middleboxes exchange the authorization decision with another middlebox within the same trust domain. Individual middleboxes within an administrative domain may exploit their relationship instead of requesting authentication and authorization of the signaling initiator again and again. Figure 31 illustrates a network structure which uses a centralized entity. +-----------------------------------------------------------+ | Network A | | +---------+ +---------+ | +----///--------+ Middle- +------///------++ Middle- +--- | | Security | box 2 | Security | box 2 | | | Context +----+----+ Context +----+----+ | +----+----+ | | | | | Middle- +--------+ +---------+ | | | | box 1 | | | | | | +----+----+ | | | | | | Security | +----+-----+ | | | | Context | | Policy | | | | +--+---+ +-----------+ Decision +----------+ | | | Host | | Point | | | | A | +----------+ | | +------+ | +-----------------------------------------------------------+ Figure 31: Intra-domain Relationship The interaction between individual middleboxes and a policy decision point (or AAA server) is outside the scope of this document. 5.1.3. End-to-Middle Relationship The peer-to-peer relationship between neighboring NSIS NATFW NSLP nodes might not always be sufficient. Network B might require additional authorization of the signaling message initiator (in addition to the authorization of the neighboring node). If authentication and authorization information is not attached to the initial signaling message then the signaling message arriving at Middlebox 2 would result in an error message being created, which indicates the additional authorization requirement. In many cases Stiemerling, et al. Expires September 6, 2007 [Page 69] Internet-Draft NAT/FW NSIS NSLP March 2007 the signaling message initiator might already be aware of the additionally required authorization before the signaling message exchange is executed. Figure 32 shows this scenario. +--------------------+ +---------------------+ | Network A | |Network B | | | Security | | | +---------+ Context +---------+ | | +-///-+ Middle- +---///////----+ Middle- +-///-+ | | | | box 1 | +-------+ box 2 | | | | | +---------+ | +---------+ | | | |Security | | | Security | | | |Context | | | Context | | | | | | | | | +--+---+ | | | +--+---+ | | | Host +----///----+------+ | | Host | | | | A | | Security | | B | | | +------+ | Context | +------+ | +--------------------+ +---------------------+ Figure 32: End-to-Middle Relationship 5.2. Security Threats and Requirements This section describes NATFW specific security threats and requirements. 5.2.1. Data Sender (DS) behind a firewall +------------------------------+ | | | +-----+ create +-----+ | | DS | --------------> | FW | | +-----+ +-----+ | | +------------------------------+ DS behind a firewall DS sends a CREATE message to request the traversal of a data flow. The following attacks are possible: o DS could open a firewall pinhole with a source address different from its own host. Stiemerling, et al. Expires September 6, 2007 [Page 70] Internet-Draft NAT/FW NSIS NSLP March 2007 o DS could open firewall pinholes for incoming data flows that are not supposed to enter the network. o DS could request installation of any policy rules and allow all traffic go through. SECURITY REQUIREMENT: The middlebox MUST authenticate and authorize the neighboring NAT/FW NSLP node requesting an action. Authentication and authorization of the initiator SHOULD be provided to NATs and firewalls along the path. 5.2.2. Data Sender (DS) behind a NAT The case 'DS behind a NAT' is analogous to the case 'DS behind a firewall'. Figure 34 illustrates such a scenario: +------------------------------+ | | | +------+ CREATE | | | NI_1 | ------\ +-----+ CREATE +-----+ | +------+ \------> | NAT |-------->| MB | | +-----+ +-----+ | +------+ | | | NI_2 | | | +------+ | +------------------------------+ Figure 34: Several NIs behind a NAT In this case the middlebox MB does not know who is the NSIS Initiator since both NI_1 and NI_2 are behind a NAT (which is also NSIS aware). Authentication needs to be provided by other means such as the NSLP or the application layer. SECURITY REQUIREMENT: The middlebox MUST authenticate and ensure that the neighboring NAT/FW NSLP node is authorized to request an action. Authentication and authorization of the initiator (which is the DR in this scenario) to the non-neighboring middleboxes SHOULD be provided. 5.2.3. Data Receiver (DR) behind a firewall In this case a CREATE message comes from an entity DS outside the network towards the DR inside the network. Stiemerling, et al. Expires September 6, 2007 [Page 71] Internet-Draft NAT/FW NSIS NSLP March 2007 +------------------------------+ | | +-----+ CREATE +-----+ CREATE +-----+ | | DS | -------------> | FW | -------------> | DR | | +-----+ <------------- +-----+ <------------- +-----+ | successful RESPONSE | successful RESPONSE | | | +------------------------------+ DR behind a firewall Since policy rules at middleboxes must only be installed after receiving a successful response it is necessary that the middlebox waits until the Data Receiver DR confirms the request of the Data Sender DS with a successful RESPONSE message. This confirmation implies that the data receiver is expecting the data flow. At this point we differentiate two cases: 1. DR knows the (publicly reachable) IP address and port number of the DS (for instance because of some previous application layer signaling) and is expecting the data flow. 2. DR might be expecting the data flow (for instance because of some previous application layer signaling) but does not know the (publicly reachable) IP address of the Data Sender DS. For the second case, Figure 36 illustrates a possible attack: an adversary Mallory M could be sniffing the application layer signaling and thus knows the address and port number where DR is expecting the data flow. Thus it could pretend to be DS and send a CREATE message towards DR with the data flow description (M -> DR). Since DR does not know the IP address of DS, it is not able to recognize that the request is coming from the "wrong guy". It will send a success RESPONSE message back and the middlebox will install policy rules that will allow Mallory M to inject its data into the network. Stiemerling, et al. Expires September 6, 2007 [Page 72] Internet-Draft NAT/FW NSIS NSLP March 2007 Application Layer signaling <------------------------------------> / \ / +-----------------\------------+ / | \ | +-----+ +-----+ +-----+ | | DS | -> | FW | | DR | | +-----+ / +-----+ +-----+ | CREATE / | | +-----+ / +-------------------------------+ | M |---------- +-----+ Figure 36: DR behind a firewall with an adversary Network administrators will probably not rely on a DR to check the IP address of the DS. Thus we have to assume the worst case with an attack such as in Figure 36. Many operators might not allow NSIS signaling message to traverse the firewall in Figure 36 without having the DR to interact with the FW first. SECURITY REQUIREMENT: No requirements are created by this scenario. 5.2.4. Data Receiver (DR) behind a NAT When a data receiver DR behind a NAT sends a EXTERNAL (EXT) message to get a public reachable address, this address can be used as a contact address by an arbitrary data sender, if the DR was unable to restrict the future data sender. The NAT reserves an external address and port number and sends them back to DR. The NAT adds an address mapping entry in its reservation list which links the public and private addresses as follows: (DR_ext <=> DR_int) (accept data from any IP address, i.e., wildcard). The NAT sends a RESPONSE message with the external address' object back to the DR with the address DR_ext. DR informs DS about the public address that it has recently received, for instance, by means of application layer signaling. When a data sender sends a CREATE message towards DR_ext then the message will be forwarded to the DR. The data receiver might want to update the NAT binding stored at the edge-NAT to make it more restrictive. We assume that the adversary Mallory M obtains the contact address (i.e., external address and port) allocated at the NAT possibly by Stiemerling, et al. Expires September 6, 2007 [Page 73] Internet-Draft NAT/FW NSIS NSLP March 2007 eavesdropping on the application layer signaling and sends a CREATE message. As a consequence Mallory would be able to communicate with DR (if M is authorized by the edge-NAT and if the DR accepts CREATE and returns a RESPONSE. Application Layer signaling <------------------------------------------> / \ / +----------------------\-------+ v | EXT v | +-----+ +-----+ <----------- +-----+ | | DS | -> | NAT | -----------> | DR | | +-----+ / +-----+ DR_external +-----+ | CREATE / | | +-----+ / +-------------------------------+ | M |---------- +-----+ DR behind a NAT with an adversary SECURITY REQUIREMENT: The DR MUST be able to specify which data sender is allowed to traverse the NAT in order to be forwarded to DR's address. 5.2.5. NSLP Message Injection Malicious hosts, located either off-path or on-path, could inject arbitrary NATFW NSLP messages into the signaling path. These problems apply when no proper authorization and authentication scheme is available. By injecting a bogus CREATE message with NATFW NSLP signaling session lifetime set to zero, a malicious host could try to teardown NATFW NSLP signaling session state partially or completely on a data path, causing a service interruption. By injecting a bogus responses or NOTIFY message, for instance, NATFW NSLP signaling session timeout, a malicious host could try to teardown NATFW NSLP signaling session state as well. This could affect the data path partially or totally, causing a service interruption. SECURITY REQUIREMENT: Messages, such as NOTIFY, can be misused by malicious hosts, and therefore MUST be authorized by the respective NATFW NLSP entities. Stiemerling, et al. Expires September 6, 2007 [Page 74] Internet-Draft NAT/FW NSIS NSLP March 2007 5.3. Denial-of-Service Attacks In this section we describe several ways how an adversary could launch a Denial of service (DoS) attack on networks running NSIS for middlebox configuration to exhaust their resources. 5.3.1. Flooding with CREATE messages from outside 5.3.1.1. Attacks due to NSLP state A CREATE message requests the NSLP to store state information such as a NAT binding or a policy rule. The policy rules requested in the CREATE message will be installed at the arrival of a confirmation from the Data Receiver with a success RESPONSE message. A successful RESPONSE message includes the session ID. So the NSLP looks up the NATFW NSLP signaling session and installs the requested policy rules. An adversary could launch a DoS attack with an arbitrary number of CREATE messages. For each of these messages the middlebox needs to store state information such as the policy rules to be loaded, i.e., the middlebox could run out of memory. This kind of attack is also mentioned in [8] Section 4.8. SECURITY REQUIREMENT: A NAT/FW NSLP node MUST authorize the establishment of state information. 5.3.1.2. Attacks due to authentication complexity This kind of attack is possible if authentication is based on mechanisms that require computing power, for example, digital signatures. For a more detailed treatment of this kind of attack, the reader is encouraged to see [8]. SECURITY REQUIREMENT: A NAT/FW NSLP node MUST NOT introduce new denial of service attacks based on authentication or key exchange mechanisms. 5.3.1.3. Attacking Endpoints The NATFW NSLP requires firewalls to forward NSLP messages, a malicious node may keep sending NSLP messages to a target. This may consume the access network resources of the victim, drain the battery of the victim's terminal and may force the victim to pay for the received although undesired data. Stiemerling, et al. Expires September 6, 2007 [Page 75] Internet-Draft NAT/FW NSIS NSLP March 2007 This threat may be more particularly be relevant in networks where access link is a limited resource, for instance in cellular networks, and where the terminal capacities are limited. SECURITY REQUIREMENT: A NATFW NSLP node MUST be configurable to block unauthorized signaling messages. 5.3.2. Flooding with EXT messages from inside Although we are more concerned with possible attacks from outside the network, we need also to consider possible attacks from inside the network. An adversary inside the network could send arbitrary EXTERNAL messages. At a certain point the NAT will run out of port numbers and the access for other users to the outside will be disabled. SECURITY REQUIREMENT: The NAT/FW NSLP node MUST authorize state creation for the EXTERNAL message. Furthermore, the NAT/FW NSLP implementation MUST prevent denial of service attacks involving the allocation of an arbitrary number of NAT bindings or the installation of a large number of packet filters. 5.4. Man-in-the-Middle Attacks Figure 38 illustrates a possible man-in-the-middle attack using the EXTERNAL (EXT) message. This message travels from DR towards the public Internet. The message might not be intercepted because there are no NSIS aware middleboxes. Imagine such an NSIS signaling message is then intercepted by an adversary Mallory (M). M returns a faked RESPONSE message whereby the adversary pretends that a NAT binding was created. This NAT binding is returned with the RESPONSE message. Mallory might insert it own IP address in the response, the IP address of a third party or the address of a black hole. In the first case, the DR thinks that the address of Mallory M is its public address and will inform the DS about it. As a consequence, the DS will send the data traffic to Mallory M. The data traffic from the DS to the DR will re-directed to Mallory M. M will be able to read, modify or block the data traffic (if the end- to-end communication itself does not experience protection). Eavesdropping and modification is only possible if the data traffic is itself unprotected. Stiemerling, et al. Expires September 6, 2007 [Page 76] Internet-Draft NAT/FW NSIS NSLP March 2007 +-----+ +-----+ +-----+ | DS | | M | | DR | +-----+ +-----+ +-----+ | | | | | EXT | | | <------------------ | | | | | | RESPONSE | | | ------------------> | | | | | data traffic | | |===============>| data traffic | | |====================>| Figure 38: MITM attack using the EXTERNAL message SECURITY REQUIREMENT: Mutual authentication between neighboring NATFW NSLP MUST be provided. To ensure that only legitimate nodes along the path act as NSIS entities the initiator MUST authorize the responder. In the example in Figure 38 the firewall FW must perform an authorization with the neighboring entities. 5.5. Message Modification by non-NSIS on-path node An unauthorized on-path node along the path towards the destination could easily modify, inject or just drop an NSIS message. It could also hijack or disrupt the communication. SECURITY REQUIREMENT: Message integrity, replay protection and data origin authentication between neighboring NAT/FW NSLPs MUST be provided. 5.6. Message Modification by malicious NSIS node Message modification by an NSIS node that became malicious is more serious. An adversary could easily create arbitrary pinholes or NAT bindings. For example: o NATs need to modify the source/destination of the data flow in the 'create session' message. o Each middlebox along the path may change the requested lifetime in the CREATE message to fit their needs and/or local policy. Stiemerling, et al. Expires September 6, 2007 [Page 77] Internet-Draft NAT/FW NSIS NSLP March 2007 SECURITY REQUIREMENT: Malicious NSIS NATs and firewalls will not be addressed by this specification. 5.7. Signaling Session Ownership Section 4.10 in [8] describes a threat where an adversary is able to modify or delete previously installed state information at NATFW NSLP nodes along the path. An adversary therefore needs to know NATFW NSLP signaling session specific information, such as the NATFW NSLP signaling session identifier and MRI information. SECURITY REQUIREMENT: Off-path adversaries MUST NOT be able to modify or delete sessions without proper authorization. 5.7.1. Misuse of Mobility in a NAT Handling Scenario Another kind of NATFW NSLP signaling session modification is related to mobility scenarios. The NSIS protocol suite offers interworking with mobility protocol and a mobile node might need to update state along NATFW NSLP nodes. Whenever a host behind a NAT initiates a data transfer, it is assigned an external IP and port number. In typical mobility scenarios, the DR might also obtain a new address according to the topology and it should convey its new IP address to the NAT. The NAT is assumed to modify these NAT bindings based on the new IP address conveyed by the end host. Public Private Address Internet space +----------+ +----------+ +----------| NAT |------------------|End host | | | | | +----------+ +----------+ | | +----------+ \--------------------|Malicious | |End host | +----------+ data traffic <======================== Figure 39: Misuse of mobility in NAT binding A NAT binding can be changed with the help of NSIS signaling. When a DR moves to a new location and obtains a new IP address, it sends an Stiemerling, et al. Expires September 6, 2007 [Page 78] Internet-Draft NAT/FW NSIS NSLP March 2007 NSIS signaling message to modify the NAT binding. It would use the Session-ID and the new flow-id to update the state. The NAT updates the binding and the DR continues to receive the data traffic. Consider the scenario in Figure 39 where an the end host(DR) and the adversary are behind a NAT. The adversary pretending that it is the end host could generate a spurious signaling message to update the state at the NAT. This could be done for these purposes: o Redirecting packets to the attacker as in Figure 40. o Third party flooding by redirecting packets to arbitrary hosts o Service disruption by redirecting to non-existing hosts +----------+ +----------+ +----------+ | NAT | |End host | |Malicious | | | | | |End host | +----------+ +----------+ +----------+ | | | | Data Traffic | | |--------->----------| | | | Spurious | | | NAT binding update | |---------<----------+--------<------------| | | | | Data Traffic | | |--------->----------+-------->------------| | | | Figure 40: Connection Hijacking SECURITY REQUIREMENT: A NAT/FW signaling message MUST be authenticated, integrity and replay protected between neighboring NAT/FW NSLP nodes. The NSIS NATFW NSLP aware NAT MUST authorize the end host to insure that the messages are indeed belonging to the previously established NATFW NSLP signaling session. 5.8. Misuse of unreleased signaling sessions Assume that DS (N1) initiates NATFW NSLP signaling session with DR (N2) through a series of middleboxes as in Figure 41. When the DS is sending data to DR, it might happen that the DR disconnects from the network (crashes or moves out of the network in mobility scenarios). In such cases, it is possible that another node N3 (which recently entered the network protected by the same firewall) is assigned the same IP address that was previously allocated to N2. The DS could take advantage of the firewall policies installed already, if the Stiemerling, et al. Expires September 6, 2007 [Page 79] Internet-Draft NAT/FW NSIS NSLP March 2007 refresh interval time is very high. The DS can flood the node (N3), which will consume the access network resources of the victim forcing it to pay for unwanted traffic as shown in Figure 42. Note that here we make the assumption that the data receiver has to pay for receiving data packets. Public Internet +--------------------------+ | | +-------+ CREATE +---+-----+ +-------+ | | |-------------->------| |---->---| | | | N1 |--------------<------| FW |----<---| N2 | | | | successful RESPONSE | | | | | | |==============>======| |====>===| | | +-------+ Data Traffic +---+-----+ +-------+ | | | +--------------------------+ Figure 41: Before mobility Public Internet +--------------------------+ | | +-------+ +---+-----+ +-------+ | | | | | | | | | N1 |==============>======| FW |====>===| N3 | | | | Data Traffic | | | | | +-------+ +---+-----+ +-------+ | | | +--------------------------+ Figure 42: After mobility Also, this threat is valid for the other direction as well. The DS which is communicating with the DR may disconnect from the network and this IP address may be assigned to a new node that had recently entered the network. This new node could pretend to be the DS and send data traffic to the DR in conformance with the firewall policies and cause service disruption. SECURITY REQUIREMENT: In order to allow firewalls to verify that a legitimate end host indeed transmitted data traffic it is necessary to provide data origin authentication. This is, however, outside the scope of this document. Hence, there are no security requirements imposed by this threat, which will be addressed by the NATFW NSLP. Stiemerling, et al. Expires September 6, 2007 [Page 80] Internet-Draft NAT/FW NSIS NSLP March 2007 5.9. Data Traffic Injection In some environments, such as enterprise networks, it is still common to perform authorization for access to a service based on the source IP address of the service requester. There is no doubt that this practice by itself represents a security weakness. Using IP spoofing an adversary is able to reach the target machines if they match, using the existing firewall rules. The adversary is able to inject its own data traffic in conformance with the firewall policies simultaneously along with the genuine DS. SECURITY REQUIREMENT: Since IP spoofing is a general limitation of non-cryptographic packet filters no countermeasures need to be taken by the NAT/FW NSLP. Techniques such as ingress filtering (see [19]) and, if necessary, data origin authentication (such as provided with IPsec based VPNs) can help mitigate this threat. This aspect is, however, outside the scope of this document. 5.10. Eavesdropping and Traffic Analysis By collecting NSLP messages, an adversary is able to learn policy rules for packet filters and knows which ports are open. It can use this information to inject its own data traffic due to the IP spoofing capability already mentioned in Section 5.9. An on-path adversary could also observe the data traffic and he could conclude that it is possible to traverse a firewall. An adversary could learn authorization tokens included in CREATE messages and use them to launch replay-attacks or to create a NATFW NSLP signaling session with its own address as source address. This threat is discussed in the respective document suggesting the usage of authorization token in the NSIS protocol suite. SECURITY REQUIREMENT: The threat of eavesdropping itself does not mandate the usage of confidentiality protection since an adversary can also eavesdrop on data traffic. In the context of a particular security solutions (e.g., authorization tokens) it MAY be necessary to offer confidentiality protection. The latter aspect is outside the scope of this document. 5.11. Security Framework for the NAT/Firewall NSLP Based on the identified threats a list of security requirements has been created. Stiemerling, et al. Expires September 6, 2007 [Page 81] Internet-Draft NAT/FW NSIS NSLP March 2007 5.11.1. Security Protection between neighboring NATFW NSLP Nodes Based on the analyzed threats it is necessary to provide, between neighboring NATFW NSLP nodes, the following mechanism: o data origin authentication o replay protection o integrity protection and o optionally confidentiality protection To consider the aspect of authentication and key exchange the security mechanisms provided in [2] between neighboring nodes MUST be enabled when sending NATFW signaling messages. The proposed security mechanisms at GIST provide support for authentication and key exchange in addition to denial of service protection. Depending on the chosen security protocol, support for multiple authentication protocols might be provided. If security between neighboring nodes is desired then the usage of C-MODE for the delivery of data packets and the usage of D-MODE only to discover the next NATFW NSLP aware node along the path is demanded. Almost all security threats at the NATFW NSLP layer can be prevented by using a mutually authenticated Transport Layer secured connection and by relying on authorization by the neighboring NATFW NSLP entities. The NATFW NSLP relies an established security association between neighboring peers to prevent unauthorized nodes to modify or delete installed state. Between non-neighboring nodes the session ID (SID) carried in the NTLP is used to show ownership of a NATFW NSLP signaling session. The session ID MUST be generated in a random way and thereby prevent an off-path adversary to mount targeted attacks. Hence, an adversary would have to learn the randomly generated Session ID to perform an attack. In a mobility environment a former on-path node that is now off-path can perform an attack. The cost- benefit tradeoff to counter this attack does not seem to be high enough to provide a solution. Messages for a particular NATFW NSLP signaling session are handled by the NTLP to the NATFW NSLP for further processing. Messages carrying a different session ID not associated with any NATFW NSLP are subject to the regular processing for new NATFW NSLP signaling sessions. 5.11.2. Security Protection between non-neighboring NATFW NSLP Nodes Based on the security threats and the listed requirements it was noted that some threats also demand authentication and authorization of a NATFW signaling entity (including the initiator) towards a non- Stiemerling, et al. Expires September 6, 2007 [Page 82] Internet-Draft NAT/FW NSIS NSLP March 2007 neighboring node. This mechanism mainly demands entity authentication. Additionally, security protection of certain payloads may be required between non-neighboring signaling entities and the Cryptographic Message Syntax (CMS) [14] migh be a potential solution. Payload protection using CMS is not described in this document. The most important information exchanged at the NATFW NSLP is information related to the establishment for firewall pinholes and NAT bindings. This information can, however, not be protected over multiple NSIS NATFW NSLP hops since this information might change depending on the capability of each individual NATFW NSLP node. Some scenarios might also benefit from the usage of authorization tokens. Their purpose is to associate two different signaling protocols (e.g., SIP and NSIS) and their authorization decision. These tokens are obtained by non-NSIS protocols, such as SIP or as part of network access authentication. When a NAT or firewall along the path receives the token it might be verified locally or passed to the AAA infrastructure. Examples of authorization tokens can be found in RFC 3520 [17] and RFC 3521 [18]. Figure 43 shows an example of this protocol interaction. An authorization token is provided by the SIP proxy, which acts as the assertion generating entity and gets delivered to the end host with proper authentication and authorization. When the NATFW signaling message is transmitted towards the network, the authorization token is attached to the signaling messages to refer to the previous authorization decision. The assertion verifying entity needs to process the token or it might be necessary to interact with the assertion granting entity using HTTP (or other protocols). As a result of a successfully authorization by a NATFW NSLP node, the requested action is executed and later a RESPONSE message is generated. Stiemerling, et al. Expires September 6, 2007 [Page 83] Internet-Draft NAT/FW NSIS NSLP March 2007 +----------------+ Trust Relationship +----------------+ | +------------+ |<.......................>| +------------+ | | | Protocol | | | | Assertion | | | | requesting | | HTTP, SIP Request | | Granting | | | | authz | |------------------------>| | Entity | | | | assertions | |<------------------------| +------------+ | | +------------+ | Artifact/Assertion | Entity Cecil | | ^ | +----------------+ | | | ^ ^| | | | . || HTTP, | | | Trust . || other | API Access | Relationship. || protocols | | | . || | | | . || | | | v |v | v | +----------------+ | +------------+ | | +------------+ | | | Protocol | | NSIS NATFW CREATE + | | Assertion | | | | using authz| | Assertion/Artifact | | Verifying | | | | assertion | | ----------------------- | | Entity | | | +------------+ | | +------------+ | | Entity Alice | <---------------------- | Entity Bob | +----------------+ RESPONSE +----------------+ Figure 43: Authorization Token Usage Threats against the usage of authorization tokens have been mentioned in [8] and also in Section 5.2. Hence, it is required to provide confidentiality protection to avoid allowing an eavesdropper to learn the token and to use it in another NATFW NSLP signaling session (replay attack). The token itself also needs to be protected against tempering. To harmonize the usage of authorization tokens in NSLPs a separate document is available, see [21]. Stiemerling, et al. Expires September 6, 2007 [Page 84] Internet-Draft NAT/FW NSIS NSLP March 2007 6. IAB Considerations on UNSAF UNilateral Self-Address Fixing (UNSAF) is described in [12] as a process at originating endpoints that attempt to determine or fix the address (and port) by which they are known to another endpoint. UNSAF proposals, such as STUN [15] are considered as a general class of workarounds for NAT traversal and as solutions for scenarios with no middlebox communication. This memo specifies a path-coupled middlebox communication protocol, i.e., the NSIS NATFW NSLP. NSIS in general and the NATFW NSLP are not intended as a short-term workaround, but more as a long-term solution for middlebox communication. In NSIS, endpoints are involved in allocating, maintaining, and deleting addresses and ports at the middlebox. However, the full control of addresses and ports at the middlebox is at the NATFW NSLP daemon located to the respective NAT. Therefore, this document addresses the UNSAF considerations in [12] by proposing a long-term alternative solution. Stiemerling, et al. Expires September 6, 2007 [Page 85] Internet-Draft NAT/FW NSIS NSLP March 2007 7. IANA Considerations This section provides guidance to the Internet Assigned Numbers Authority (IANA) regarding registration of values related to the NATFW NSLP, in accordance with BCP 26 RFC 2434 [13]. The NATFW NSLP requires IANA to create a number of new registries. These registries may require further coordination with the registries of the NTLP [2] and the QoS NSLP [6]. NATFW NSLP Message Type Registry The NATFW NSLP Message Type is a 8 bit value. The allocation of values for new message types requires standards action. Updates and deletion of values from the registry is not possible. This specification defines four NATFW NSLP message types, which form the initial contents of this registry. IANA is requested to add these four NATFW NSLP Message Types: CREATE, EXT, RESPONSE, and NOTIFY. NATFW NSLP Header Flag Registry NATFW NSLP messages have a messages-specific 8 bit flags/reserved field in their header. The registration of flags is subject to IANA registration. The allocation of values for flag types requires standards action. Updates and deletion of values from the registry is not possible. This specification defines only one flag, the P flag in Figure 20. NSLP Object Type Registry [Delete this part if already done by another NSLP: A new registry is to be created for NSLP Message Objects. This is a 12-bit field (giving values from 0 to 4095). This registry is shared between a number of NSLPs. Allocation policies are as follows: 0-1023: Standards Action 1024-1999: Specification Required 2000-2047: Private/Experimental Use 2048-4095: Reserved When a new object is defined, the extensbility bits (A/B) must also be defined.] This document defines 8 objects for the NATFW NSLP: NATFW_LT, Stiemerling, et al. Expires September 6, 2007 [Page 86] Internet-Draft NAT/FW NSIS NSLP March 2007 NATFW_EXT_IP, NATFW_EFI, NATFW_INFO, NATFW_NONCE, NATFW_MSN, NATFW_DTINFO, NATFW_ICMP_TYPES. IANA is request to assigned values for them from NSLP Object Type registry and to replace the corresponding IANA-TBD tags with the numeric values. NSLP Response Code Registry In addition it defines a number of Response Codes for the NATFW NSLP. These can be found in Section 4.2.4 and are to be assigned values from NSLP Response Code registry. The allocation of values for Response Codes Codes requires standards action. IANA is request to assigned values for them from NSLP Response Code registry. Furthermore, IANA is requested to add a new value to the NSLP Identifiers (NSLPID) registry defined in [2] for the NATFW NSLP. Stiemerling, et al. Expires September 6, 2007 [Page 87] Internet-Draft NAT/FW NSIS NSLP March 2007 8. Open Issues A more detailed list of open issue can be found at: https://kobe.netlab.nec.de/roundup/nsis-natfw-nslp/index Stiemerling, et al. Expires September 6, 2007 [Page 88] Internet-Draft NAT/FW NSIS NSLP March 2007 9. Acknowledgments We would like to thank the following individuals for their contributions to this document at different stages: o Marcus Brunner and Henning Schulzrinne for work on work on IETF drafts which lead us to start with this document, o Miquel Martin for his help on the initial version of this document, o Srinath Thiruvengadam and Ali Fessi work for their work on the NAT/firewall threats draft, o Henning Peters for his comments and suggestions, o and the NSIS working group. Stiemerling, et al. Expires September 6, 2007 [Page 89] Internet-Draft NAT/FW NSIS NSLP March 2007 10. References 10.1. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Schulzrinne, H. and R. Hancock, "GIST: General Internet Signaling Transport", draft-ietf-nsis-ntlp-11 (work in progress), August 2006. [3] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982, August 1996. 10.2. Informative References [4] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080, June 2005. [5] Brunner, M., "Requirements for Signaling Protocols", RFC 3726, April 2004. [6] Manner, J., "NSLP for Quality-of-Service Signaling", draft-ietf-nsis-qos-nslp-12 (work in progress), October 2006. [7] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A. Rayhan, "Middlebox communication architecture and framework", RFC 3303, August 2002. [8] Tschofenig, H. and D. Kroeselberg, "Security Threats for Next Steps in Signaling (NSIS)", RFC 4081, June 2005. [9] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [10] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and Issues", RFC 3234, February 2002. [11] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification", RFC 2205, September 1997. [12] Daigle, L. and IAB, "IAB Considerations for UNilateral Self- Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. [13] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Stiemerling, et al. Expires September 6, 2007 [Page 90] Internet-Draft NAT/FW NSIS NSLP March 2007 Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [14] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852, July 2004. [15] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN - Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)", RFC 3489, March 2003. [16] Westerinen, A., Schnizlein, J., Strassner, J., Scherling, M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry, J., and S. Waldbusser, "Terminology for Policy-Based Management", RFC 3198, November 2001. [17] Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh, "Session Authorization Policy Element", RFC 3520, April 2003. [18] Hamer, L-N., Gage, B., and H. Shieh, "Framework for Session Set-up with Media Authorization", RFC 3521, April 2003. [19] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, May 2000. [20] Alfano, F., "Diameter Quality of Service Application", draft-alfano-aaa-qosprot-05 (work in progress), October 2005. [21] Manner, J., "Authorization for NSIS Signaling Layer Protocols", draft-manner-nsis-nslp-auth-02 (work in progress), October 2006. [22] Roedig, U., Goertz, M., Karten, M., and R. Steinmetz, "RSVP as firewall Signalling Protocol", Proceedings of the 6th IEEE Symposium on Computers and Communications, Hammamet, Tunisia pp. 57 to 62, IEEE Computer Society Press, July 2001. Stiemerling, et al. Expires September 6, 2007 [Page 91] Internet-Draft NAT/FW NSIS NSLP March 2007 Appendix A. Selecting Signaling Destination Addresses for EXT As with all other message types, EXT messages need a reachable IP address of the data sender on the GIST level. For the path-coupled MRM the source-address of GIST is the reachable IP address (i.e., the real IP address of the data sender, or a wildcard). While this is straight forward, it is not necessarily so for the loose-end MRM. Many applications do not provide the IP address of the communication counterpart, i.e., either the data sender or both a data sender and receiver. For the EXT messages, the case of data sender is of interest only. The rest of this section is giving informational guidance about determining a good destination-address of the LE-MRM in GIST for EXT messages. This signaling destination address (SDA, the destination-address in GIST) can be the data sender, but for applications which do not provide an address upfront, the destination address has to be chosen independently, as it is unknown at the time when the NATFW NSLP signaling has to start. Choosing the 'correct' destination IP address may be difficult and it is possible that there is no 'right answer' for all applications relying on the NATFW NSLP. Whenever possible it is RECOMMENDED to chose the data sender's IP address as SDA. It necessary to differentiate between the received IP addresses on the data sender. Some application level signaling protocols (e.g., SIP) have the ability to transfer multiple contact IP addresses of the data sender. For instance, private IP address, public IP address at NAT, and public IP address at a relay. It is RECOMMENDED to use all non-private IP addresses as SDAs. A different SDA must be chosen, should the IP address of the data sender be unknown. This can have multiple reasons: The application level signaling protocol cannot determine any data sender IP address at this point of time or the data receiver is server behind a NAT, i.e., accepting inbound packets from any host. In this case, the NATFW NSLP can be instructed to use the public IP address of an application server or any other node. Choosing the SDA in this case is out of the scope of the NATFW NSLP and depends on the application's choice. The local network can provide a network-SDA, i.e., a SDA which is only meaningful to the local network. This will ensure that GIST packets with destination-address set to this network-SDA are going to be routed to a edge-NAT or edge-firewall. Stiemerling, et al. Expires September 6, 2007 [Page 92] Internet-Draft NAT/FW NSIS NSLP March 2007 Appendix B. Applicability Statement on Data Receivers behind Firewalls Section 3.7.2 describes how data receivers behind middleboxes can instruct inbound firewalls/NATs to forward NATFW NSLP signaling towards them. Finding an inbound edge-NAT in address environment with NAT'ed addresses is quite easy. It is only required to find some edge-NAT, as the data traffic will be route-pinned to the NAT, which is done with the LE-MRM. Locating the appropriate edge- firewall with the PC-MRM, sent inbound is difficult. For cases with a single, symmetric route from the Internet to the data receiver, it is quite easy; simply follow the default route in the inbound direction. +------+ Data Flow +-------| EFW1 +----------+ <=========== | +------+ ,--+--. +--+--+ / \ NI+-----| FW1 | (Internet )----NR+/NI/DS NR +--+--+ \ / | +------+ `--+--' +-------| EFW2 +----------+ +------+ ~~~~~~~~~~~~~~~~~~~~~> Signaling Flow Figure 44: Data receiver behind multiple, parallel located firewalls When a data receiver, and thus NR, is located in a network site that is multihomed with several independently firewalled connections to the public Internet (as shown in Figure 44), the specific firewall through which the data traffic will be routed has to be ascertained. NATFW NSLP signaling messages sent from the NI+/NR during the EXT message exchange towards the NR+ must be routed by the NTLP to the edge-firewall that will be passed by the data traffic as well. The NTLP would need to be aware about the routing within the Internet to determine the path between DS and DR. Out of this, the NTLP could determine which of the edge-firewalls, either EFW1 or EFW2, must be selected to forward the NATFW NSLP signaling. Signaling to the wrong edge-firewall, as shown in Figure 44, would install the NATFW NSLP policy rules at the wrong device. This causes either a blocked data flow (when the policy rule is 'allow') or an ongoing attack (when the policy rule is 'deny'). Requiring the NTLP to know all about the routing within the Internet is definitely a tough challenge and usually not possible. In such described case, the NTLP must Stiemerling, et al. Expires September 6, 2007 [Page 93] Internet-Draft NAT/FW NSIS NSLP March 2007 basically give up and return an error to the NSLP level, indicating that the next hop discovery is not possible. Stiemerling, et al. Expires September 6, 2007 [Page 94] Internet-Draft NAT/FW NSIS NSLP March 2007 Appendix C. Firewall and NAT Resources This section gives some examples on how NATFW NSLP policy rules could be mapped to real firewall or NAT resources. The firewall rules and NAT bindings are described in a natural way, i.e., in a way one will find it in common implementation. C.1. Wildcarding of Policy Rules The policy rule/MRI to be installed can be wildcarded to some degree. Wildcarding applies to IP address, transport layer port numbers, and the IP payload (or next header in IPv6). Processing of wildcarding splits into the NTLP and the NATFW NSLP layer. The processing at the NTLP layer is independent of the NSLP layer processing and per layer constraints apply. For wildcarding in the NTLP see Section 5.8 of [2]. Wildcarding at the NATFW NSLP level is always a node local policy decision. A signaling message carrying a wildcarded MRI (and thus policy rule) arriving at an NSLP node can be rejected if the local policy does not allow the request. For instance, a MRI with IP addresses set (not wildcarded), transport protocol TCP, and TCP port numbers completely wildcarded. Now the local policy allows only requests for TCP with all ports set and not wildcarded. The request is going to be rejected. C.2. Mapping to Firewall Rules This section describes how a NSLP policy rule signaled with a CREATE message is mapped to a firewall rule. The MRI is set as follows: o network-layer-version=IPv4 o source-address=192.0.2.100, prefix-length=32 o destination-address=192.0.50.5, prefix-length=32 o IP-protocol=UDP o L4-source-port=34543, L4-destination-port=23198 The NATFW_EFI object is set to action=allow and sub_ports=0. The resulting policy rule (firewall rule) to be installed might look like: allow udp from 192.0.2.100 port=34543 to 192.0.50.5 port=23198 Stiemerling, et al. Expires September 6, 2007 [Page 95] Internet-Draft NAT/FW NSIS NSLP March 2007 C.3. Mapping to NAT Bindings This section describes how a NSLP policy rule signaled with a EXT message is mapped to a NAT binding. It is assumed that the EXT message is sent by a NI+ being located behind a NAT and does contain a NATFW_DTINFO object. The MRI is set following using the signaling destination address, since the IP address of the real data sender is not known: o network-layer-version=IPv4 o source-address= 192.168.5.100 o destination-address=SDA o IP-protocol=UDP The NATFW_EFI object is set to action=allow and sub_ports=0. The NATFW_DTINFO object contains these parameters: o P=1 o dest prefix=0 o protocol=UDP o dst port number = 20230, src port number=0 o src IP=0.0.0.0 The edge-NAT allocates the external IP 192.0.2.79 and port 45000. The resulting policy rule (NAT binding) to be installed could look like: translate from any to 192.0.2.79 port=45000 to 192.168.5.100 port=20230 C.4. NSLP Handling of Twice-NAT The dynamic configuration of twice-NATs requires application level support, as stated in Section 2.5. The NATFW NSLP cannot be used for configuring twice-NATs if application level support is needed. Assuming application level support performing the configuration of the twice-NAT and the NATFW NSLP being installed at this devices, the NATFW NSLP must be able to traverse it. The NSLP is probably able to traverse the twice-NAT, as any other data traffic, but the flow information stored in the NTLP's MRI will be invalidated through the translation of source and destination address. The NATFW NSLP implementation on the twice-NAT MUST intercept NATFW NSLP and NTLP Stiemerling, et al. Expires September 6, 2007 [Page 96] Internet-Draft NAT/FW NSIS NSLP March 2007 signaling messages as any other NATFW NSLP node does. For the given signaling flow, the NATFW NSLP node MUST look up the corresponding IP address translation and modify the NTLP/NSLP signaling accordingly. The modification results in an updated MRI with respect to the source and destination IP addresses. Stiemerling, et al. Expires September 6, 2007 [Page 97] Internet-Draft NAT/FW NSIS NSLP March 2007 Appendix D. Assigned Numbers for Testing NOTE: This section MUST be removed before publication. This section defines temporarily used values of the NATFW NSLP for testing the different implementations. Values for the NATFW NSLP message types: o CREATE: 0x01 o EXT: 0x02 o RESPONSE: 0x03 o NOTIFY: 0x04 Values for the NSLP object types o NATFW_LT: 0x00F1 o NATFW_EXT_IP: 0x00F2 o NATFW_EFI: 0x00F3 o NATFW_INFO: 0x00F4 o NATFW_NONCE: 0x00F5 o NATFW_MSN: 0x00F6 o NATFW_DTINFO: 0x00F7 o NATFW_ICMP_TYPES: 0x00F9 1345 Stiemerling, et al. Expires September 6, 2007 [Page 98] Internet-Draft NAT/FW NSIS NSLP March 2007 Authors' Addresses Martin Stiemerling Network Laboratories, NEC Europe Ltd. Kurfuersten-Anlage 36 Heidelberg 69115 Germany Phone: +49 (0) 6221 4342 113 Email: stiemerling@netlab.nec.de URI: http://www.stiemerling.org Hannes Tschofenig Siemens AG Otto-Hahn-Ring 6 Munich 81739 Germany Phone: Email: Hannes.Tschofenig@siemens.com URI: http://www.tschofenig.com Cedric Aoun Paris France Email: cedric@caoun.net Elwyn Davies Folly Consulting Soham UK Phone: +44 7889 488 335 Email: elwynd@dial.pipex.com Stiemerling, et al. Expires September 6, 2007 [Page 99] Internet-Draft NAT/FW NSIS NSLP March 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Stiemerling, et al. Expires September 6, 2007 [Page 100]