Internet Engineering Task Force Jiri Kuthan Internet Draft GMD Fokus draft-kuthan-midcom-framework-00.txt Jonathan Rosenberg November, 2000 dynamicsoft Expires: May 2001 Middlebox Communication: Framework and Requirements Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. 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. Abstract The purpose of this document is to develop framework and requirements for a protocol that will allow for communicating control data associated with IP/transport-layer data flows or aggregates of them between intermediate packet processing devices and external controllers. The protocol will be extensible in order to allow for communicating arbitrary control data associated with packet flows and defining packet flow processing. It will include provisions for verifying the integrity of each message as well as ensuring authentication of all parties involved in the transactions. A major application of this protocol will be the control of packet processing policies in decomposed firewalls/NATs/NAT-PTs by externalized Application Level Gateways. This particular use will relieve firewalls/NATs from application-layer processing to improve their maintainability and performance. Examples of other possible applications include but are not limited to buffer management and load balancing. J. Kuthan, J. Rosenberg [Page 1] Internet Draft Middlebox Framework and Requirements November 2000 Contents 1 Introduction.....................................................2 2 Terminology......................................................3 3 Case Study: Traversal of Applications Using Session Control Protocols across Firewalls/NATs ..................................5 4 Protocol Requirements for FCP....................................7 4.1 FCP Operational Model..........................................7 4.2 Functional Requirements: Management of Packet Flow Processing States .........................................................7 4.3 Rule Manipulation Operations...................................7 4.4 Soft-state Rule Design.........................................7 4.5 Rule Language..................................................7 4.5.1 Packet Matching Expressions..................................8 4.5.2 Rule Processing Precedence...................................8 4.5.3 Control State Content........................................9 4.6 Feedback......................................................10 4.7 Security......................................................11 4.8 Reliability...................................................11 4.9 Real-time Operation...........................................11 4.10 Extensibility................................................11 4.11 On Support Specific to NAT/NAPT/NAT-PT.......................12 5 Related Issues..................................................13 5.1 Access Control................................................13 5.2 Rule Ownership................................................13 5.3 Default Flows.................................................13 5.4 Location of FCP Controllers...................................14 6 Open Issues.....................................................14 6.1 Location of Intermediate Devices..............................14 7 Performance and Scalability Considerations......................15 8 Security Considerations.........................................15 9 Document Status.................................................15 10 Acknowledgments................................................16 1 Introduction Though the Internet has been designed to provide network layer connectivity end to end [2] it actually consists of mixed network realms. These include IPv4 networks, IPv6 networks, networks hidden behind NATs and firewalls. This problem was referred to as "transparency loss" and discussed in [3]. Applications being run across mixed realms may experience lack of interoperability or suboptimal performance. To assists applications in traversing network boundaries Application Level Gateways (ALGs) embedded in intermediate devices have been used. However, tight coupling of application and network/transport layer processing results in reduced maintainability of the intermediate devices. Built-in application awareness typically requires updates of operating systems with new applications or versions of it. Operators of such systems wanting to support new J. Kuthan, J. Rosenberg [Page 2] Internet Draft Middlebox Framework and Requirements November 2000 applications cannot use software supplied by third parties and are at the mercy of vendors of their equipment. Furthermore, support of numerous application protocols increases complexity of such integrated systems and may affect performance. To deal with this sort of problems we suggest decomposition of application awareness from network/transport layer processing. We assume that applications control network/transport layer processing in intermediate devices through a generic application-independent control protocol. In the common case, the application-awareness resides in proxies ("externalized ALGs") that hide boundary traversals from end-devices. +--------------------+ +------+-----------+ | +----------+ FCP | | Per-Flow | | +----------+|..........| | State | | | ALGs ||..........| FCP | Table | | policy +--------+ +----------+ | unit |-----------| | protocol | policy | | | ACL ~~~~~~~~~~~~~~+ server | __________+------+-----------+ | +--------+ | Intermediate | | Device | +--------------------+ Device | | Interfaces | | ... IN OUT ... Legend: .... FCP ~~~~ policy protocol Figure 1: Middlebox Communication Architecture The rest of this document describes framework and requirements for the missing piece, the control protocol. We refer to this protocol as Flow Processing Control Protocol (FCP). Discussion on how FCP maps or does not map to an existing protocol is out of scope of this document. Section 2 defines terms used throughout this memo. In Section 3, we demonstrate the use of the FCP in a case study. We formulate protocol requirements in Section 4. Issues that are related to protocol operation but do not affect protocol specification are summarized in Section 5. Unresolved issues are identified in Section 6. Section 7 reviews performance issues. Security is considered in Section 8 and status of this memo in Section 9. 2 Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. J. Kuthan, J. Rosenberg [Page 3] Internet Draft Middlebox Framework and Requirements November 2000 o Endpoint address - general term describing source or destination of a packet. This is, depending on context, IP address and/or TCP/UDP port number. o Packet matching expression - expression that specifies values of header fields of packets to be selected. The inspected header fields MAY be but are not limited to source and destination IP addresses, TCP/UDP port numbers, etc. o Packet flow - a sequence of packets matched by a packet matching expression. o Rule - a packet matching expression and control state used to determine processing of packets matched by the expression. o Session - a set of packet flows belonging to an application. o Session control protocol - a protocol used to negotiate endpoint addresses of flows belonging to a session. Examples of such protocols are SIP, H.323, RTSP. o Proxy - an intermediary server that relays application messages from one entity to another one. A proxy acts as server for message senders and as client on behalf of the senders for message receivers. It may be used for enforcement of application-level policies such as content filtering or message translation. With applications using session control protocols, proxies typically relay session control protocols and do not relay data flows belonging to a session. o Packet filter - a network device that examines headers of forwarded packets and allows only packets conforming to a security policy to pass through. The security policy defines which endpoint addresses are considered trustworthy and which are not. For example, it may permit data traffic of an application identified by a port numbe only from/to a trusted proxy. o Network Address Translator (NAT) - a packet processing device that is able to map source and/or destination endpoint addresses of forwarded packet flows to a pool of other addresses. This technique is used to conserve IP address space and/or hide IP address of hosts behind the NATs from outside of the NATs. The NAT concept is described in [10]. o Bind - a pair consisting of an original and translated endpoint address. o Intermediate device - a packet-processing device located along end-to-end path. It may be a packet filter, NAT, intrusion detection system, load balancer, etc. o Firewall - centrally maintained devices set-up to increase network security by putting restrictions on information flows. The restrictions are applied with packet filters at the packet level and/or proxies at the application level. Optionally, NATs may be used. Note that the term "firewall" is sometimes used to refer to packet filters. o Application Level Gateways (ALGs) - application-aware modules that control processing state in firewalls/NATs and manipulate application messages to accomplish firewall/NAT traversal. Typically, ALGs are embedded in firewalls/NATs. They may also be J. Kuthan, J. Rosenberg [Page 4] Internet Draft Middlebox Framework and Requirements November 2000 externalized to remote proxies if a control interface between them and firewalls/NATs is provided [4]. o Flow Processing Control Protocol (FCP) - protocol for communicating flow processing policy between external controllers and intermediate devices. The intermediate devices accept only authorized FCP requests. The authorization can come from an internal access control list or an external policy server. o Access Control List (ACL) - policy defining who may access/manipulate controlled devices with FCP. The ACLs may be outsourced to an external policy server. 3 Case Study: Traversal of Applications Using Session Control Protocols across Firewalls/NATs Firewalls are trusted, administrator-maintained devices used to increase network security by enforcing restrictions on information flows. The restriction policies are centrally defined and maintained by network administrators. The firewalls consist of proxies and packet filters. Proxies are application-aware entities acting on behalf of untrusted hosts at application layer. They examine application protocol flows and allow only messages conformant to security policies to pass through. Optionally, they modify the messages to make them policy-conformant. Packet filters are used to impose security restrictions at lower layers. They usually inspect IP and TCP/UDP packet headers against tables of filtering rules. Only conforming IP packets are allowed to pass through filters. The packet filtering policy may be either 'default-permit' or 'default- deny'. 'Default-permit' policy allows all but explicitly stated IP flows whereas 'default-deny' policy allows only explicitly stated IP flows to pass through. Typically, the latter policy is set up to allow traffic from and to trusted proxies to pass through. It provides higher security by being more restrictive. Thus, it is frequently deployed in corporate networks. Unfortunately, this policy makes firewall traversal difficult for applications using session bundles. This means that such applications (e.g., SIP [5], H.323 [6], RTSP[7], and FTP [8])negotiate IP addresses and port numbers with a session control protocol dynamically and then use the negotiated addresses to establish packet streams for transport of data. This technique is useful, for example, if multiple applications want to receive RTP [9] flows and cannot share the same TCP/UDP port number or an application uses port numbers to demultiplex multiple incoming RTP flows. It is also necessary if IP address information is dynamic. As a result, dynamically created sessions fail to traverse firewalls deploying static 'default-deny' filtering policies. If network address translation (NAT)[10] is deployed the traversal fails as well because session signaling conveys unroutable IP addresses and port numbers. This problem has been addressed in firewalls/NATs by usage of embedded Application Level Gateways (ALGs) [15]. They adapt J. Kuthan, J. Rosenberg [Page 5] Internet Draft Middlebox Framework and Requirements November 2000 packet processing policy to application needs dynamically. However, embedded ALGs suffer from limited maintainability, increase complexity of firewalls/NATs and fail to operate if message authentication or encryption is used. Thus we suggest using external application proxies that control firewalls/NATs and relieve them from application processing. Arbitrary application proxies may be added to manageable firewalls easily. Hop-by-hop security is enabled. Existing software such as SIP proxies or H.323 gatekeepers may be used without duplicating the applications' protocol stacks in the firewalls/NATs. The reference scheme is depicted in Figure 2:. There are FCP-enabled, trusted, administrator-maintained proxies acting as external ALGs and controlling a packet filter located within the same administrative domain. The packet filter implements the 'default-deny' packet filtering policy. It permits session control traffic from and to the trusted proxies and accepts FCP requests from them. The proxies use their application-awareness to control the packet filter dynamically with FCP. They also enforce application-level policy such as dropping messages infected with known viruses, or lacking user authentication. The policy decisions may be delegated to an external policy server. +--------+ | App. | | Policy | +---------+ SIP | Server |~~~~~~~~~| SIP +_____________ | +--------+ ________| Proxy | \ | / +---------+.. +----+---------------+ | : FCP +------+-----------+ |__ | RSTP +----------+ :...........| | Per-Flow | |__ SIP | ____| RSTP |..............| | State | |__ | / | Proxy |______________| FCP | Table | | | | +----------+ | unit | -------- | | | | FTP +---------+.............| | ACL | | | | _____|FTP Proxy|_____________+------+-----------+ | | | / +---------+ | Packet | | | | -----| Filter |-- +-----------+ /-----| |-- +-----------+| data streams // +----+---------------+ +-----------+||----------->----// | |end-devices||------------<----- | +-----------+ (RTP, ftp-data, etc.) | Inside | Outside Legend: ---- raw data streams ____ application control protocols .... FCP ~~~~ policy protocol Figure 2: Use of FCP for Firewall/NAT Decomposition J. Kuthan, J. Rosenberg [Page 6] Internet Draft Middlebox Framework and Requirements November 2000 4 Protocol Requirements for FCP 4.1 FCP Operational Model In the remainder of this document we assume a model in which packet processing in an intermediate device located at a network edge is determined by an ordered set of rules. The rules consist of packet matching expressions and control state. The packet matching expressions select packets and associated control state determines how selected packets are treated. A packet may match multiple packet matching expressions. If this happens the first one will be taken. The rules are manipulated with FCP dynamically. Multiple FCP controllers MAY control a single intermediate device. It is expected that only a few trusted hosts from a single administrative domain will act as FCP controllers. 4.2 Functional Requirements: Management of Packet Flow Processing States The primary goal of FCP is to allow for remote dynamic management of packet flow processing rules. As a minimum requirement, the FCP MUST enable controllers to permit/forbid processing of specific packet flows and request/release NAT/NAPT/NAT-PT[11] translations. 4.3 Rule Manipulation Operations FCP MUST allow for setting, releasing and querying packet flow processing rules. Operations like modification of existing rules and keeping them alive are most likely to be implemented with the 'set' operation. The 'set' operation MAY either specify values of updated state elements explicitly or omit them to allow controlled entities to assign appropriate values. These MAY be default values (e.g. 0 for packet counter), ephemeral values, or current values if the state elements already exists (useful for keep-alive messages). 4.4 Soft-state Rule Design. To avoid accumulation of stale rules in case of controller failures rules MUST have timers that expire if they are no more refreshed by controllers. FCP MUST enable controllers to refresh rules periodically. FCP MUST also allow controllers to set the timer's length -- it is frequently a controller that knows best what timer length is appropriate. If a controller does not specify timer value explicitly a default value will be assigned. A trivial value for infinite timer MUST be defined. 4.5 Rule Language J. Kuthan, J. Rosenberg [Page 7] Internet Draft Middlebox Framework and Requirements November 2000 This section specifies requirements for the language of packet rules. Note that FCP-controlled hosts have to understand all expressions written in this language but FCP controllers may use only a subset of them. 4.5.1 Packet Matching Expressions A) As a minimum requirement, the language of packet matching expressions MUST allow for specification of the following protocols and their respective header fields: - IPv4: source and destination IP address or group of them determined by a netmask, protocol number, TOS field - IPv6: source and destination IP address or group of them determined by a netmask, next header fields (Note that multiple fields may need to be traversed until a match is found.), traffic class field - UDP: source and destination port numbers or group of them - TCP: source and destination port numbers or group of them, "SYN packets" permission - ICMP: type and code - IGMP: type B) FCP controllers MUST be able to specify in which direction packets may traverse controlled devices. This requires notion of device interfaces. The notion of interface is abstract and independent on interface technology and assigned IP address. Support for generic predefined interface names "in", "out", "loopback" (synonym for senders and receivers located at the controlled device), and "DMZ" (demilitarized zone) is REQUIRED. Packet traversal direction may be expressed in various ways, for example by inbound and outbound interfaces or by interface and direction in which packets pass it. 4.5.2 Rule Processing Precedence The ability to indicate desired rule processing precedence is REQUIRED to enable controlled devices to resolve conflicts between multiple applicable matching rules in a predictable manner. If no precedence is specified for a rule, default precedence will be assigned by FCP-controlled device. Multiple rules MAY share a single precedence. Note: Though precedence sharing leaves processing order of rules with the same precedence undetermined it greatly simplifies certain common cases. For example, allocating a single precedence for all dynamically generated firewall pinholes does not affect firewall's behavior because all the pinhole rules result in the same action, which is packet forwarding. Then none of multiple FCP controllers needs to determine at which position a new rule will be inserted in a rule base. J. Kuthan, J. Rosenberg [Page 8] Internet Draft Middlebox Framework and Requirements November 2000 4.5.3 Control State Content The control state associated with a packet matching expression in a rule keeps information related to a packet flow. It MAY include flow processing actions, timer information, number of matched packets, traffic limitations, etc. Members of the control states are subject to future extensions. The following control state members are REQUIRED: A) "Action" defines whether matched packets are forwarded. It takes the values 'pass packets', 'drop packets with or without ICMP notification'. B) "Rule timer" defines time remaining until state expiration. See also discussion of soft-state rule design. C) "Logging" of asynchronous events related to a rule. The protocol MUST allow FCP controllers to request logging of asynchronous events such as packet match and timer expiration. The protocol MUST enable controllers to specify log level and frequency. The log frequency is used to avoid voluminous logging if an event occurs frequently. Choice of the notification/logging mechanism is a configuration option that does not need to be specified with FCP. D) Unique "flow state identifiers" are REQUIRED unless matching expressions are uniquely identifiable. Otherwise, state modification/releasing could not work consistently. The identifiers may be generated either by controllers or by controlled devices. They may be random or ephemeral. If controllers generate them, they MUST be random to avoid collisions with identifiers generated by other controllers. If controlled devices generate identifiers, they MAY be ephemeral. Ephemeral identifiers are typically shorter but lose their uniqueness under a failure. E) "Packet modifier" allows to describe one or more rules for re- writing header fields of matched accepted packets. The modifier rules will consist of identification of the packet fields to be changed, operators (at least the assignment operator is REQUIRED) and operands. In particular, the modifier MUST be able to change the following protocol header fields: - IPv4: IP addresses, TOS field - IPv6: IP addresses, traffic class field - UDP: port numbers - TCP: port numbers Note that if modifiers are used packet checksums MUST be recalculated. J. Kuthan, J. Rosenberg [Page 9] Internet Draft Middlebox Framework and Requirements November 2000 The following control state members are RECOMMENDED: F) "Packet counter" keeps number of packets matched by a rule. G) "Maximum packets per second" specifies the maximum allowed packet rate of a flow. Packets exceeding this rate will be dropped. H) "Maximum bitrate" specifies the maximum allowed bitrate of a flow. Packets exceeding this rate will be dropped. I) "Inactivity timer" specifies period of time after which a rule is released if no packet matches. J) "Reflexive Rules": In order to allow replies to TCP/UDP data flows originated from the internal side of firewall while still keeping the filtering policy as restrictive as possible, so- called reflexive rules are used. Reflexive rules are rules that allow packet flows reverse to explicitly permitted active flows. They are defined implicitly by permitting their generation within specification of the original explicit rules. They specify the same protocol, IP addresses, port numbers as flows matching the original rule do except the addresses and port numbers are swapped. If permitted, packet filters generate a reflexive rule whenever a new flow matches an explicit rule. No controller's intervention is needed. The reflexive rules are valid only temporarily. They MUST be released when an inactivity timer expires, the flow is terminated explicitly (e.g., by a FIN segment with TCP) or the original rule is deleted. If creation of reflexive rules is supported, FCP MUST allow to specify values of their control state members. Note: If endpoint address modifiers are enabled in the original rules they MUST be reflected in the reflexive rules; namely packet-matching expressions of the reflexive rules MUST match flows reverse to modified flows and modifiers MUST be enabled to translate endpoint addresses of reverse flow to addresses before modification. 4.6 Feedback FCP controllers need to receive feedback on their control messages in order to learn about results of requested operations. Both positive and negative responses are REQUIRED. Positive responses indicate successful operation and MAY possibly describe content of the controlled states or part of them. Per-flow control state or part of it is always returned if it was asked for by 'query' operation. J. Kuthan, J. Rosenberg [Page 10] Internet Draft Middlebox Framework and Requirements November 2000 Negative responses indicate failures and describe reasons for them. Minimum negative responses REQUIRED are "Authentication failed", "Not permitted", "Syntax Error". 4.7 Security In order to prevent unauthorized entities from manipulating the state of controlled device the FCP channel MUST be secure. It MUST include provisions for verifying the integrity of each message as well as ensuring authentication of all parties involved in the transaction. It is RECOMMENDED that the FCP channel is private so that a malicious listener cannot find out packet processing policy easily. The security protocols may take place either at lower layers (IPSec, TSL) or at the FCP layer. Though IP-address based authentication may be satisfactory in particular cases cryptographic authentication is REQUIRED generally. Note that we discuss the security between FCP controllers and controlled device. Security mechanisms used by applications to communicate with FCP controllers are a separate issue out of scope of this memo. 4.8 Reliability As with almost any other control protocol reliability is REQUIRED regardless if it is implemented by FCP itself or an underlying transport protocol. 4.9 Real-time Operation The protocol transactions must be fast in terms of RTT to avoid introducing delays to applications. Unless network loss results in retransmission, total transaction time SHOULD be one RTT. Note: If TCP is used as underlying protocol to provide reliability, pre-established TCP connections may be used to reduce transaction time. 4.10 Extensibility Protocol extensibility is REQUIRED in order to enable reuse of FCP for control of a variety of packet-processing mechanisms. In particular, adding new control state fields (e.g., buffer management information), new reply codes and elements of packet matching expression language MUST be supported. J. Kuthan, J. Rosenberg [Page 11] Internet Draft Middlebox Framework and Requirements November 2000 The protocol MUST convey protocol version number in order to make transition to potential future versions easier. 4.11 On Support Specific to NAT/NAPT/NAT-PT One of the FCP purposes is to communicate NAT/NAPT/NAT-PT binds between controllers and controlled devices. Knowledge of the binds is necessarily needed by session control proxies to operate properly. The primary question is who creates the binds. One alternative is controllers request a new bind and NATs create and return it. The other choice is the controllers create a bind and instruct NATs to use it. Locating the bind logic in NATs follows the decomposition concept "IP/transport intelligence in controlled devices, application intelligence out of them". It relieves controllers such as SIP proxies from maintenance of the address pools and making bind assignments. It avoids collisions that would be due if multiple controllers would access a single device. (We do not consider splitting a pool of public addresses among multiple controllers a solution. It would beat the purpose of NAT which is address sharing.) A minor drawback of this logic placement is it requires two-stage transactions if NATs are co-located with firewalls. In the first stage, a controller must find out, if NAT applies to a given address and request a bind to include in its signaling. In the second stage, when application signaling is over, it permits a packet flow using the reserved translation. With bind logic residing in controllers, both operations may be done jointly in the second phase and the first phase can be skipped. A specific scenario where locating the bind logic to controllers is advantageous is if a controller wants to make sure the same address translation is applied in multiple controlled devices. Clearly, this would not be possible if the devices assigned the binds independently. We leave the answer to location of bind intelligence to configuration. It is REQUIRED that FCP supports both alternatives. The following protocol operations are REQUIRED: o FCP controllers MUST be able to request NAT translations. If NAT is used, controlled devices allocate an address translation and return it. o FCP controllers MUST be able to set NAT translations. (Note that this can be accomplished with modifiers.) Controlled devices MUST be able to indicate if the translation cannot be set because it is already reserved. J. Kuthan, J. Rosenberg [Page 12] Internet Draft Middlebox Framework and Requirements November 2000 o With NAPT, allocating port blocks is REQUIRED, i.e., FCP controllers MUST be able to request a translation of a contiguous block of port numbers and controlled devices MUST allocate a contiguous block of translated port numbers to such request. The least significant bit of both private and translated port numbers MUST be same. It is needed, for example, by RTP [9], which recommends allocation of even port numbers for itself, subsequent port number for RTCP and contiguous block of port numbers for layered encoding applications. o The controllers MUST be able to release the allocated bindings. o The allocated address bindings are subject to timer expiration in a similar way as soft-state packet-processing rules are. 5 Related Issues This Section explicitly names related features that are out of scope of protocol requirements and are matter of implementation, administrative policy or anything else. 5.1 Access Control There may be different access control lists (ACLs) defining who may access and modify what rules in an intermediate device. For example, an ACL may specify that an FCP controller may only control rules describing traffic to and from a specific subnet. Additionally, it may define in which way the controller is required to authenticate and which precedence it may use for its rules. The access control policies may be stored and applied locally or they may be outsourced to an external policy server using a policy protocol. In either case they are out of scope of FCP. The only required FCP feature is authentication support. 5.2 Rule Ownership Multiple controllers may control a single device with FCP. It is desirable to avoid modification of per-flow control states by other entities than those that created them (perhaps with exception of a network manager). Thus, the controlled devices MUST implement the notion of rule ownership. The only required protocol functionality is authentication. 5.3 Default Flows If a packet does not match any of matching expressions maintained in a packet filter a default rule has to be applied. Otherwise, packet handling would be undefined. Thus, all packet filters controlled by FCP must always maintain the default rule. The matching expression of the rule matches all packets at lowest priority so that any other matching rules take precedence over it. The content of the default control state MAY be modified with FCP, the matching expression MUST NOT. J. Kuthan, J. Rosenberg [Page 13] Internet Draft Middlebox Framework and Requirements November 2000 5.4 Location of FCP Controllers FCP controllers may be located on any side of controlled network device. Their location with respect to the controlled device does not affect protocol specification but may result in different protocol flows. For example, an application proxy located on the private side of a NAT needs to set up a single permanent translation that enables it to receive inbound messages and forward them to their destinations. If the proxy is located on the public side, it needs to set up multiple translations for inbound messages forwarded to individual destinations located on the private side. 6 Open Issues 6.1 Location of Intermediate Devices Determining which intermediate device a controller should control is out of the scope of this document. Administrators can accomplish this task manually. Alternatively, a discovery protocol could be used. A difficult problem arises if packet flows may take path through multiple intermediate devices at the network edge. FCP controllers cannot easily determine which of them they should control. The problem is illustrated in the example depicted in Figure 3: IN | OUT +-----+ SIP +------------+ +-------+ | SIP |____________| firewall 1 |____________| SIP | |proxy|............| | | proxy | +-----+ : +------------+ +-------+ | : FCP ... | | | MGCP : +------------+ MGCP | | :.......|firewall i | | +--------+ : +------------+ +-------+ |media | : ... | ?<-------|media | |gateway |--->? : +------------+ |gateway| +--------+ :..|firewall N | +-------+ +------------+ | Figure 3: Controlling Multiple Intermediate Devices In this example, multiple firewalls 1 .. N are present in a network. A SIP proxy relays SIP signaling, has knowledge of all the firewalls and is authorized to control them. It knows source and destination endpoints of data flows belonging to a session but does not know which of the firewalls they will traverse. It cannot calculate it J. Kuthan, J. Rosenberg [Page 14] Internet Draft Middlebox Framework and Requirements November 2000 because it does not know routing tables along the entire end-to-end path. Solutions are still sought. A possible solution is to let controllers instruct all controlled devices in parallel, most likely using multicast. This solution scales only for a small number of controlled devices. With NAT, it assumes the translation assignments to be communicated from FCP controllers to controlled devices. 7 Performance and Scalability Considerations The ability to add processing rules to control packet-processing devices dynamically may result in creation of large and rapidly changing rule tables. For example, if FCP is used to open pinholes in a 'default-deny-and-dynamic-open' firewall for Internet telephony sessions the table size grows with number of sessions linearly. The lookup processing overhead grows as well and may lead to increased packet latency. Maintenance of per-flow states makes use of FCP meaningful only in network edges. Mechanisms for fast rule lookup in large, frequently changing filter databases are needed. Results of some recent research in this area were published in [12], [13], and [14]. Use of packet modification may also affect processing performance. A performance improvement may be reached administratively by definition of an application-aware rule precedence policy. A controller may request that rules for packet flows with higher expected packet rate will be assigned a higher precedence than rules for packet flows with lower packet rate. Then, the most commonly accessed rules will be processed first and average packet processing time will decrease. Note that this mechanism is not extremely fair to streams with low bandwidth consumption since their processing time will increase. 8 Security Considerations The security requirements for the control protocol are described in Section 4.7. Note that security of the protocol does not help alone. Additionally, security of the entire control system is subject to security of the FCP controllers and access control in FCP-controlled devices (see Section 5.1.). 9 Document Status This document is in the stage of collection of requirements and open issues. Numerous updates result from discussions on the foglamps mailing list. Previous versions were issued as draft-kuthan-fcp- {00|01}.txt. J. Kuthan, J. Rosenberg [Page 15] Internet Draft Middlebox Framework and Requirements November 2000 10 Acknowledgments Numerous people have been contributing to collection of these requirements. Many document clarifications and enhancements resulted from discussions on the foglamps mailing list. We specially acknowledge the following people for their help: Scott Bradner, Stefan Foeckel, Melinda Shore, Dave Oran, and Jon Peterson. The firewall traversal problem was stated in [15], [16]. Appendixes A Examples This section shows how to use FCP by examples. Many of the examples refer to the application described in Section 3 and use SIP as a prominent example of a session control protocol. A.1 FCP Transaction Examples This section illustrates how FCP requests could look like. The requests in the following examples use abstract syntax in this form: PME= [ [=] ...] The syntax of packet matching expression is borrowed from tcpdump. An additional keyword 'if' specifies interface to whose incoming queue the matching expression is applied. A similar syntax is used for definition of packet modifiers. Discussion on how these abstract FCP examples map or do not map to existing protocols is out of scope of this document. In the examples bellow, a protected host behind a firewall has the address 10.1.1.1, an outside host has the address 130.149.17.15 and the firewall's outbound interface has 193.174.152.25. Example 1: Opening a Pinhole in a Packet Filter for an Outgoing Flow In this example, a controller opens a pinhole for a packet flow being sent from the inside to the outside host. SET PME='if in and udp src port 55 and src host 10.1.1.1 and udp dst port 77 and dst host 130.149.17.15' action=pass => REPLY: OK Example 2: Opening a Pinhole in a Packet Filter w/NAPT for an Outgoing Flow J. Kuthan, J. Rosenberg [Page 16] Internet Draft Middlebox Framework and Requirements November 2000 In this example, a controller queries a NAT bind and opens a pinhole for a translated packet flow being sent from the outside to the inside host through a NAT. QUERY_NAT_TRANSLATION udp:10.1.1.1:55 => REPLY: NAT_OK, udp:10.1.1.1:55=udp:193.174.152.25:48374 SET PME='dst host 10.1.1.1 and udp dst port 55 and if out and src host 130.149.17.15 and udp src 77' action=PASS => REPLY: OK Example 3: TOS Control The controller instructs the controlled device to set TOS of matched packets to hexadecimal value 0x10. SET PME='if in and udp src port 55 and src host 10.1.1.1 and udp dst port 77 and dst host 130.149.17.15' modifier='tos=0x10' => REPLY: OK Example 4: Querying Number of Matched Packets QUERY PME='if in and udp src port 55 and src host 10.1.1.1 and udp dst port 77 and dst host 130.149.17.15' packet_count => REPLY: OK, packet_count=333 Example 5: Refreshing Per-Flow State SET PME='if in and udp src port 55 and src host 10.1.1.1 and udp dst port 77 and dst host 130.149.17.15' => REPLY: OK Example 6: Network Ingress Filtering See [17] for more details on this scenario. The first rule denies all packets on the "in" interface. The second rule with higher priority explicitly permits packets from the 10.1.2 network. J. Kuthan, J. Rosenberg [Page 17] Internet Draft Middlebox Framework and Requirements November 2000 SET PME='if in' precedence=default action=drop(no_ICMP) => REPLY: OK SET PME='if in and src net 10.1.2' precedence=high action=pass => REPLY: OK Example 7: Reflexive HTTP Rules The next rule allows controlled packet filters to create temporary rules that permit inbound TCP packets for HTTP transactions initiated from the internal side of a firewall. SET PME='if in and tcp dst port 80' REFLEXIVE='permit=yes, timer=180s, if=out' => REPLY: OK If an HTTP request from 10.1.1.1:37313 to 130.149.17.15:80 matches this rule a reflexive rule of the following form is generated: PME='if=out and tcp src port 80 and src host 130.149.17.15 and tcp dst port 37313 and dst host 10.1.1.1' Example 8: Packet Redirection In this scenario, all HTTP traffic from inside network is redirected to a Web proxy (10.1.4.4) transparently. This scenario is sometimes also referred as 'transparent proxy'. The rule allows for automatic creation of reflexive rules. SET PME='if in and tcp dst port 80' modifier='ip dst host = 10.1.4.4' reflexive_rules='permit=yes, inactivity_timer=240s, if=dmz' => REPLY: OK If an HTTP request from 10.1.1.1:37313 to 130.149.17.15:80 matches this rule a reflexive rule of the following form is generated: PME='if=dmz and tcp src port 80 and src host 10.1.4.4 and tcp dst port 37313 and dst host 10.1.1.1' J. Kuthan, J. Rosenberg [Page 18] Internet Draft Middlebox Framework and Requirements November 2000 modifier='ip src host=130.149.17.15 A.2 Using FCP to Get an Outgoing SIP/SDP Session through a 'Default- Deny' Firewall w/NAPT This example illustrates how FCP can be used to get an outgoing SIP call through a firewall deploying 'default-deny' packet filtering policy. Network configuration as displayed in Figure 1 is assumed: The packet filter allows SIP signaling only from/to a SIP proxy, the proxy rejects calls considered untrustworthy, and instructs the packet filter to open pinholes for RTP streams belonging to trustworthy SIP/SDP sessions for the time of session duration. Additionally, NAPT is deployed. Precise timing of opening and closing pinholes in SIP sessions and issues such as 183 provisional media and re-invites are subject to discussion which is out of scope of this document. Management of RTCP and ICMP pinholes is omitted for the sake of simplification. Note that the pinholes in the packet filter are quite 'wide'. This means they allow packets with arbitrary source address and port number to pass through because SDP does not communicate source endpoint addresses. Notation: In the diagram "INV 10.1.1.1:55" means an INVITE message with the SDP body indicating IP address 10.1.1.1 with port 55 as the receiving address and port for an incoming media-stream. Similarly "200 OK 130.149.17.15:77" indicates an OK response with IP address 130.149.17.15 and port 77 for receiving media. The value 0.0.0.0:0 stands for any IP address and port number. Per-flow control states in this example are identified by packet matching expressions. +---------------------------------------------+--------------------+ | INSIDE | OUTSIDE | +---------------------------------------------+--------------------+ 10.1.1.1 193.174.152.25 130.149.17.15 UAC SIP Proxy AuthServer NAT/FW UAS | | | | | | | | | | /* process SIP invitation, bind to a public address for receiving media, modify the invitation accordingly; do not open firewall pinholes until both parties agree to establish a call; note that binding of source address for outgoing media is not done because SDP does not care about source addresses */ | ----------------->| | | | | INV 10.1.1.1:55 | | | | | | ------> | | | | | auth ? | | | | | <------ | | | J. Kuthan, J. Rosenberg [Page 19] Internet Draft Middlebox Framework and Requirements November 2000 | | OK auth | | | | | | | | | | ----------------------> | | | |FCP query_nat | | | | udp :10.1.1.1:55 | | | | <---------------------- | | | |OK udp:193.174.152.25:66 | | | | -------------------------------------------> | | | INV 193.174.152.25:66 | /* process SIP OK, open NAT-enabled pinholes for outgoing and incoming media as soon as SIP ACK arrives */ | | <------------------------------------------- | | | 200 OK 130.149.17.15:77 | | <-----------------| | | | 200 OK 130.149.17.15:77 | | | ----------------->|--------------------------------------------->| | ACK | ----------------------> | | | |FCP SET | | | |PME='dst udp 130.149.17.15:77 | | | src udp 0.0.0.0:0 | | | | if=in', action=PASS | | | | <---------------------- | | | | FCP OK | | | | ----------------------> | | | |FCP SET | | | |PME='dst udp 10.1.1.1:55 | | | | src udp 0.0.0.0:0 | | | | if=out', action=PASS | | | | <---------------------- | | | | FCP OK | | | | -------------------------------------------> | | | ACK | | | ...............................................................> | | UDP/RTP DST 130.149.17.15:77 | | <...........................................~................... | | UDP/RTP DST 10.1.1.1:55 UDP/RTP DST 193.174.152.25:66| /* close pinholes when either party sends SIP BYE */ | | <------------------------------------------- | | <---------------- | BYE | | | BYE | | | | ----------------->| | | | 200 OK | ----------------------> | | | |FCP RELEASE | | | |PME='dst udp 130.149.17.15:77 | | | src udp 0.0.0.0:0 | | | | if=in' | | | | <---------------------- | | J. Kuthan, J. Rosenberg [Page 20] Internet Draft Middlebox Framework and Requirements November 2000 | | FCP OK | | | | ----------------------> | | | |FCP RELEASE | | | |PME='dst udp 10.1.1.1:55 | | | | src udp 0.0.0.0:0 | | | | if=out' | | | | <---------------------- | | | | FCP OK | | | | ----------------------> | | | |FCP release_bind | | | | udp 10.1.1.1:55 | | | | <---------------------- | | | | OK | | | | -------------------------------------------> | | | 200 OK | | Figure 4: Protocol Flow for "SIP Session over NAT" B Bibliography 1 S. Bradner: " The Internet Standards Process -- Revision 3", RFC 1602, IETF, October 1996. 2 B. Carpenter: "Achitectural Principle of the Internet", RFC 1958, IETF, June 1996. 3 B. Carpenter: "Internet Transparency", RFC 2775, IETF, February 2000. 4 P. Srisuresh: " Framework for interfacing with Network Address Translator", IETF, Internet Draft, July 2000. Work in progress. 5 M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg: "SIP: Session Initiation Protocol", RFC 2543, IETF, March 1999. 6 ITU-T Recommendation H.323. "Packet-based Multimedia Communications Systems," 1998. 7 H. Schulzrinne, A. Rao, R. Lanphier: "Real Time Streaming Protocol", RFC 2326, IETF, April 1998. 8 Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)", RFC 959, IETF. October 1985. 9 Schulzrinne, Casner, Frederick, Jacobson: "RTP: A Transport Protocol for Real-Time Applications", Internet Draft, Internet Engineering Task Force, March 2000, Work in progress. 10 P. Srisuresh and M. Holdrege: "IP network address translator (NAT) terminology and considerations", RFC 2663, IETF, August 1999. 11 G. Tsirtsis, P. Srisuresh: "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, IETF, February 2000. 12 A. Feldmann, S. Muthukrishnann: "Tradeoffs for Packet Classification", In Proc. IEEE INFOCOM 2000, 2000. 13 V. Srinivasan, S. Suri, G. Varghese: "Packet Classification Using Tuple Space Search", In Proc. ACM SIGCOMM '99, 1999. J. Kuthan, J. Rosenberg [Page 21] Internet Draft Middlebox Framework and Requirements November 2000 14 P. Gupta, N. McKeown: "Packet Classification on Multiple Fields", In Proc. ACM SIGCOMM '99, 1999. 15 J. Rosenberg, D. Drew, H. Schulzrinne: "Getting SIP through Firewalls and NATs", Internet Draft, Internet Engineering Task Force, Feb. 2000. Work in progress. 16 M. Shore: "H.323 and Firewalls: Problem Statement and Solution Framework", Internet Engineering Task Force, Feb. 2000. Work in progress. 17 P. Ferguson, D. Senie: "Network Ingress Filtering: Defeating Denial of Service Attacks which Employ IP Source Address Spoofing", RFC 2827, IETF, May 2000. C Glossary of Abbreviations ACL Access Control List ALG Application Level Gateway DMZ Demilitarized Zone FCP Flow Processing Control Protocol FTP File Transfer Protocol IP Internet Protocol HTTP Hypertext Transfer Protocol MGCP Media Gateway Control Protocol NAPT Network Address Port Translation NAT Network Address Translation NAT-PT Network Address Translation - Protocol Translation RTP Transport Protocol for Real-time Applications RTSP Real Time Streaming Protocol RTT Round Trip Time SDP Session Description Protocol SIP Session Initiation Protocol TCP Transmission Control Protocol TOS Type of Service UDP User Datagram Protocol D Authors' Addresses Jiri Kuthan GMD Fokus Kaiserin-Augusta-Allee 31 D-10589 Berlin, Germany E-mail: kuthan@fokus.gmd.de Jonathan Rosenberg dynamicsoft 200 Executive Drive Suite 120 West Orange, NJ 07052 email: jdrosen@dynamicsoft.com J. Kuthan, J. Rosenberg [Page 22] Internet Draft Middlebox Framework and Requirements November 2000 E Full Copyright Statement Copyright (c) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. J. Kuthan, J. Rosenberg [Page 23]