AVT A. Begen Internet-Draft D. Wing Intended status: Standards Track Cisco Expires: June 17, 2011 T. VanCaenegem Alcatel-Lucent December 14, 2010 Port Mapping Between Unicast and Multicast RTP Sessions draft-ietf-avt-ports-for-ucast-mcast-rtp-09 Abstract This document presents a port mapping solution that allows RTP receivers to choose their own ports for an auxiliary unicast session in RTP applications using both unicast and multicast services. The solution provides protection against denial-of-service or packet amplification attacks that could be used to cause one or more RTP packets to be sent to a victim client. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on June 17, 2011. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must Begen, et al. Expires June 17, 2011 [Page 1] Internet-Draft Port Mapping December 2010 include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5 3. Token-Based Port Mapping . . . . . . . . . . . . . . . . . . . 6 3.1. Motivating Scenario . . . . . . . . . . . . . . . . . . . 6 3.2. Normative Behavior and Requirements . . . . . . . . . . . 9 4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Port Mapping Request . . . . . . . . . . . . . . . . . . . 12 4.2. Port Mapping Response . . . . . . . . . . . . . . . . . . 13 4.3. Token Verification Request . . . . . . . . . . . . . . . . 15 4.3.1. Where to Include Token . . . . . . . . . . . . . . . . 16 4.4. Token Verification Failure . . . . . . . . . . . . . . . . 16 5. Procedures for Token Construction . . . . . . . . . . . . . . 18 6. Validating Tokens . . . . . . . . . . . . . . . . . . . . . . 20 7. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 21 7.1. The portmapping-req Attribute . . . . . . . . . . . . . . 21 7.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 22 7.3. Example and Discussion . . . . . . . . . . . . . . . . . . 22 8. Address Pooling NATs . . . . . . . . . . . . . . . . . . . . . 25 9. Security Considerations . . . . . . . . . . . . . . . . . . . 26 9.1. Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . 26 9.2. The portmapping-req Attribute . . . . . . . . . . . . . . 26 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 10.1. Registration of SDP Attributes . . . . . . . . . . . . . . 28 10.2. Registration of RTCP Control Packet Types . . . . . . . . 28 10.3. SMT Values for TOKEN Packet Type Registry . . . . . . . . 28 10.4. RAMS Response Code Space Registry . . . . . . . . . . . . 29 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31 12.1. Normative References . . . . . . . . . . . . . . . . . . . 31 12.2. Informative References . . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33 Begen, et al. Expires June 17, 2011 [Page 2] Internet-Draft Port Mapping December 2010 1. Introduction In (any-source or source-specific) multicast RTP applications, destination ports, i.e., the ports on which the multicast receivers receive the RTP and RTCP packets, are defined declaratively. In other words, the receivers cannot choose their receive ports and the sender(s) use the pre-defined ports. In unicast RTP applications, the receiving end needs to choose its ports for RTP and RTCP since these ports are local resources and only the receiving end can determine which ports are available to use. In addition, Network Address Port Translators (NAPT - hereafter simply called NAT) devices are commonly deployed in networks, thus, static port assignments cannot be used. The receiving end may convey its request to the sending end through different ways, one of which is the Offer/Answer Model [RFC3264] for the Session Description Protocol (SDP) [RFC4566]. However, the Offer/Answer Model requires offer/ answer exchange(s) between the endpoints, and the resulting delay may not be desirable in delay-sensitive real-time applications. Furthermore, the Offer/Answer Model may be burdensome for the endpoints that are concurrently running a large number of unicast sessions with other endpoints. In this specification, we consider an RTP application that uses one or more unicast and multicast RTP sessions together. While the declaration and selection of the ports are well defined and work well for multicast and unicast RTP applications, respectively, the usage of the ports introduces complications when a receiving end mixes unicast and multicast RTP sessions within the same RTP application. An example scenario is where the RTP packets are distributed through source-specific multicast (SSM) and a receiver sends unicast RTCP NACK feedback to a local repair server (also functioning as a unicast RTCP feedback target) [RFC5760] asking for a retransmission of the packets it is missing, and the local repair server sends the retransmission packets over a unicast RTP session [RFC4588]. Another scenario is where a receiver wants to rapidly acquire a new primary multicast RTP session and receives one or more RTP burst packets over a unicast session before joining the SSM session [I-D.ietf-avt-rapid-acquisition-for-rtp]. Similar scenarios exist in applications where some part of the content is distributed through multicast while the receivers get additional and/or auxiliary content through one or more unicast connections, as sketched in Figure 1. In this document, we discuss this problem and introduce a solution that we refer to as Port Mapping. This solution allows receivers to choose their desired UDP ports for RTP and RTCP in every unicast Begen, et al. Expires June 17, 2011 [Page 3] Internet-Draft Port Mapping December 2010 session when they are running RTP applications using both unicast and multicast services, and offer/answer exchange is not available. This solution is not applicable in cases where TCP is used as the transport protocol in the unicast sessions. For such scenarios, refer to [RFC4145]. ----------- | Unicast |................ | Source |............. : | (Server) | : : ----------- : : v v ----------- ---------- ----------- | Multicast |------->| Router |---------->|Client RTP | | Source | | |..........>|Application| ----------- ---------- ----------- | : | : ----------- | :..............>|Client RTP | +---------------->|Application| ----------- -------> Multicast RTP Flow .......> Unicast RTP Flow Figure 1: RTP applications simultaneously using both unicast and multicast services In the remainder of this document, we refer to the RTP endpoints that serve other RTP endpoints over a unicast session as the Servers. The receiving RTP endpoints are referred to as Clients. Begen, et al. Expires June 17, 2011 [Page 4] Internet-Draft Port Mapping December 2010 2. Requirements Notation 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 [RFC2119]. Begen, et al. Expires June 17, 2011 [Page 5] Internet-Draft Port Mapping December 2010 3. Token-Based Port Mapping Token-based Port Mapping consists of two steps: (i) Token request and retrieval, and (ii) unicast session establishment. The first step is required to be completed only once. Once a Token is retrieved from a particular server, it can be used for all the unicast sessions the client will be running with this particular server. By default, Tokens are server specific. However, the client can use the same Token to communicate with different servers if these servers are provided with the same secret key and algorithm used to generate the Token and are at least loosely clock-synchronized. The Token is essentially an opaque encapsulation that is based on client's IP address (as seen by the server). When a Token request is received, the server creates a Token for this particular client, and sends it back to the client. The Token becomes invalid if client's IP address (as seen by the server) changes (note that the client cannot necessarily detect this in a timely manner) or when the server expires the Token. In these cases, the client has to request a new Token. As the second step, when the client wants to establish a unicast session, the client includes the Token with its RTCP feedback message. The server validates the Token, making sure that the IP address information matches. This is effective against DoS attacks, e.g., an attacker cannot simply spoof another client's IP address and start a unicast transmission towards random clients. If the validation passes, the unicast session gets established. Otherwise, the server notifies the client that the validation has failed, and in this case, the unicast session will not be established. Upon successfull validation and once the unicast session is established, all the RTP and RTCP rules specified in [RFC3550] and other relevant specifications also apply in this session until it is terminated. During its lifetime, certain actions and messages by the client might need to be authorized by the server by requiring a valid Token. These are explained later in this document. Below, we first present a motivating scenario for port mapping and then describe the normative behavior and requirements. 3.1. Motivating Scenario Consider an SSM distribution network where a distribution source multicasts RTP packets to a large number of clients, and one or more retransmission servers function as feedback targets to collect unicast RTCP feedback from these clients [RFC5760]. The Begen, et al. Expires June 17, 2011 [Page 6] Internet-Draft Port Mapping December 2010 retransmission servers also join the multicast session to receive the multicast packets and cache them for a certain time period. When a client detects missing packets in the multicast session, it requests a retransmission from one of the retransmission servers by using an RTCP NACK message [RFC4585]. The retransmission server pulls the requested packet(s) out of the cache and retransmits them to the requesting client [RFC4588]. The RTP and RTCP flows pertaining to the scenario described above are sketched in Figure 2. Between the client and server, there can be one or more NAT devices [RFC4787]. The multicast and unicast sessions are clearly identified with their individual RTP and RTCP flows and port numbers. Begen, et al. Expires June 17, 2011 [Page 7] Internet-Draft Port Mapping December 2010 -------------- --- ---------- | |-------------------------------| |-->|P1 | | |-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-| |.->|P2 | | | | | | | | Distribution | ---------------- | | | | | Source | | | | | | | | |---->|P1 | | | | | | |.-.->|P2 | | | | | | | | | | | | | -------------- | P3|<.=.=.=.| |=.=|*c0 | | P3|<~~~~~~~| |~~~|*c1 | MULTICAST RTP | | | | | | SESSION with | | | N | | | UNICAST FEEDBACK | | | A | | | | Retransmission | | T | | Client | - - - - - - - - - - -| - - - - - - - -| - - - -| - |- -| - - - - -|- | Server | | | | | | | | | | | PORT MAPPING | PT|<~~~~~~~| |~~>|*cT | | | | | | | - - - - - - - - - - -| - - - - - - - -| - - - -| - |- -| - - - - -|- | | | | | | AUXILIARY UNICAST | | | | | | RTP SESSION | | | | | | | P3|........| |..>|*c1 | | P3|=.=.=.=.| |=.>|*c1 | | P4|<.=.=.=.| |=.=|*c2 | | | | | | | ---------------- --- ---------- -------> Multicast RTP Flow .-.-.-.> Multicast RTCP Flow .=.=.=.> Unicast RTCP Reports ~~~~~~~> Unicast RTCP (Feedback) Messages .......> Unicast RTP Flow Figure 2: Example scenario showing an SSM distribution with support for retransmissions from a server In Figure 2, we have the following multicast and unicast ports: o Ports P1 and P2 denote the destination RTP and RTCP ports in the multicast session, respectively. The clients listen to these ports to receive the multicast RTP and RTCP packets. Ports P1 and P2 are defined declaratively. Begen, et al. Expires June 17, 2011 [Page 8] Internet-Draft Port Mapping December 2010 o Port P3 denotes the RTCP port on the feedback target running on the retransmission server to collect any RTCP packet sent by the clients including feedback messages, and RTCP receiver and extended reports. This is also the port that the retransmission server uses to send the RTP packets and RTCP sender reports in the unicast session. Port P3 is defined declaratively. o Port P4 denotes the RTCP port on the retransmission server used to collect the RTCP receiver and extended reports for the unicast session. Port P4 is defined declaratively. o Ports *c0, *c1 and *c2 are chosen by the client. *c0 denotes the port on the client used to send the RTCP reports for the multicast session. *c1 denotes the port on the client used to send the unicast RTCP feedback messages in the multicast session and to receive the RTP packets and RTCP sender reports in the unicast session. *c2 denotes the port on the client used to send the RTCP receiver and extended reports in the unicast session. Ports c0, c1 and c2 could be the same port or different ports. There are two advantages of using the same port for both c0 and c1: 1. Some NATs only keep bindings active when a packet goes from the inside to the outside of the NAT (See REQ-6 of Section 4.3 of [RFC4787]). When the gap between the packets sent from the client to the server is long, this can exceed that timeout. If c0=c1, the occasional (periodic) RTCP receiver reports sent from port c0 (for the multicast session's RTCP port P3) will ensure the NAT does not time out the public port associated with the incoming unicast traffic to port c1. 2. Having c0=c1 conserves NAT port bindings. o Ports PT and *cT denote the ports through which the Token request and retrieval occur at the server and client sides, respectively. Port PT is declared on a per unicast session basis, although the same port could be used for two or more unicast sessions sourced by the server. A Token once requested and retrieved by a client from port PT remains valid until its expiration time. We assume that the information declaratively defined is available as part of the session description information and is provided to the clients. The Session Description Protocol (SDP) [RFC4566] and other session description methods can be used for this purpose. 3.2. Normative Behavior and Requirements In this section, we describe the normative behavior and requirements. To simplify the presentation, we refer to the port numbers described Begen, et al. Expires June 17, 2011 [Page 9] Internet-Draft Port Mapping December 2010 in the example presented in Figure 2. However, the behavior and requirements described here are not specific to that particular example, and can be applied to any scenario where analogous ports can be identified. First of all, a client compliant with this specification MUST be able to include a Token with any type of RTCP message (as described below) when it is needed. That is, clients MUST NOT implement this specification with only a specific use case in mind. Second, the solution provided in this specification is not applicable in cases where there is RTP traffic flowing from the client to the server in the unicast session. In other words, the direction of RTP traffic MUST be only from the server to the client in the unicast session. If the client wants to send RTP traffic back to the server, the regular session establishment methods such as [RFC3264] need to be used. The following steps summarize the Token-based solution: 1. The client ascertains server address and port numbers (P3, P4 and PT) from the session description. Port P4 MUST be different from port P3. Port PT MAY be equal to port P3. 2. The client selects its local port numbers (*c0, *c1, *c2 and *cT). It is strongly RECOMMENDED that the client uses the same port for c0 and c1. Port cT MAY be equal to ports c0 and c1. A client cannot keep using the same receive port for different unicast sessions since there could be packet leakage when switching from one unicast session to another unless each received unicast stream has its own distinct Synchronization Source (SSRC) identifier to allow the client to filter out the undesired packets. Unless this is guaranteed (which is not often easy), a client SHOULD use separate receive ports for subsequent unicast sessions. After a sufficient time, a previously used receive port could be used again. 3. If the client does not have a Token (or the existing Token has expired): A. The client first sends a message to the server via a new RTCP message, called Port Mapping Request to port PT. This message is sent from port *cT on the client side. The server learns client's IP address from the received message. The client can send this message anytime it wants (e.g., during initialization), and does not normally ever need to re-send this message (See Section 6). Begen, et al. Expires June 17, 2011 [Page 10] Internet-Draft Port Mapping December 2010 B. The server generates an opaque encapsulation (i.e., the Token) based on certain information including client's IP address. C. The server sends the Token back to the client using a new RTCP message, called Port Mapping Response. This message MUST be sent from port PT to port cT. 4. The client needs to provide the Token to the server using a new RTCP message, called Token Verification Request, whenever the client sends an RTCP feedback message for triggering or controlling a unicast session (See Section 4.3.1). If the Token is invalid or missing, the server sends a new RTCP message, called Token Verification Failure, to the client. Note that the unicast session is only established after the server has received a feedback message (along with a valid Token) from the client for which it needs to react by sending unicast data. Until a unicast session is established, neither the server nor the client needs to send RTCP reports for the unicast session. 5. Normal flows ensue as shown in Figure 2. Note that in the unicast session, traffic from the server to the client (i.e., both the RTP and RTCP packets sent from port P3 to port c1) MUST be multiplexed on the (same) port c1. If the client uses the same port for both c0 and c1, the RTCP reports sent for the multicast session keep the P3->c1(=c0) binding alive. If the client uses different ports for c0 and c1, the client needs to periodically send an explicit keep-alive message [I-D.ietf-avt-app-rtp-keepalive] to keep the P3->c1 binding alive during the lifetime of the unicast session if the unicast session's lifetime is likely to exceed the NAT's timeout value. Begen, et al. Expires June 17, 2011 [Page 11] Internet-Draft Port Mapping December 2010 4. Message Formats This section defines the formats of the RTCP messages that are exchanged between a server and a client for the purpose of port mapping. A new RTCP control packet type is introduced and four port mapping messages using this control packet are defined: 1. Port Mapping Request 2. Port Mapping Response 3. Token Verification Request 4. Token Verification Failure Each message has a fixed-length field for version (V), padding (P), sub-message type (SMT), packet type (PT), length and SSRC of packet sender. Messages have other fields as defined below. In all messages defined in this section, the PT field is set to TOKEN (210). Individual messages are identified by the SMT field. The length field indicates the message size in 32-bit words minus one, including the header and any padding. This definition is in line with the definition of the length field used in RTCP sender and receiver reports. In all messages, any Reserved field SHALL be set to zero and ignored. Following the rules specified in [RFC3550], all integer fields in the messages defined below are carried in network-byte order, that is, most significant byte (octet) first, also known as big-endian. Unless otherwise stated, numeric constants are in decimal (base 10). Note that RTCP is not a timely or reliable protocol. The RTCP packets might get lost or re-ordered in the network. When sending a new Port Mapping Request message, the scheduling rules that apply to sending initial RTCP messages [RFC4585] apply. When a client sends a Port Mapping Request or Token Verification Request message but it does not receive a response back from the server (either a Port Mapping Response or Token Verification Failure message), it MAY resend its request by following the timer rules defined for RTCP feedback messages in Section 3.5 of [RFC4585] as a good practice. When sending an RTCP (feedback) message bundled with a Token Verification Request message, the timer rules of [RFC4585] apply as usual. 4.1. Port Mapping Request The Port Mapping Request message is identified by SMT=1. This message is transmitted by the client to a dedicated server port (and Begen, et al. Expires June 17, 2011 [Page 12] Internet-Draft Port Mapping December 2010 possibly an address) to request a Token. In the Port Mapping Request message, the packet sender SSRC is set to the client's SSRC. The packet format has the structure depicted in Figure 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P| SMT=1 | PT=TOKEN=210 | Length=3 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of Packet Sender | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Random | | Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Packet format for the Port Mapping Request message o Random Nonce (64 bits): Field that contains a random nonce value generated by the client following the procedures of [RFC4086]. This nonce is taken into account by the server when generating a Token for the client to enable better security for clients that share the same IP address. If the Port Mapping Request message is transmitted multiple times for redundancy reasons, the random nonce value MUST remain the same in these duplicated messages. However, the client MUST generate a new random nonce for every new Port Mapping Request message. 4.2. Port Mapping Response The Port Mapping Response message is identified by SMT=2. This message is sent by the server and delivers the Token to the client as a response to the Port Mapping Request message. In the Port Mapping Response message, the packet sender SSRC is set to the server's SSRC. The packet format has the structure depicted in Figure 4. Begen, et al. Expires June 17, 2011 [Page 13] Internet-Draft Port Mapping December 2010 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P| SMT=2 | PT=TOKEN=210 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of Packet Sender | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of Requesting Client | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Associated | | Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : Token Element : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Absolute | | Expiration Time | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Relative Expiration Time | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Packet format for the Port Mapping Response message o SSRC of Requesting Client (32 bits): Field that contains the SSRC of the client who sent the request. o Associated Nonce (64 bits): Field that contains the nonce received in the Port Mapping Request message and used in Token construction. o Token Element (Variable size): Element that is used to carry the Token generated by the server. This element is a 32-bit aligned Length-Value element. The Length field, which is 8 bits, indicates the length (in octets) of the Value field that follows the Length field. This length does not include any padding that is required for alignment. The Value field carries the Token (or more accurately, the output of the encoding process on the server). If the Token element does not fall on a 32-bit boundary, the last word MUST be padded to the boundary using further bits set to zero. o Absolute Expiration Time (64 bits): Field that contains the absolute expiration time of the Token. The absolute expiration time is expressed as a Network Time Protocol (NTP) timestamp value in seconds since year 1900 [RFC5905]. The client does not need to use this element directly, thus, does not need to synchronize its clock with the server. However, the client needs to send this element back to the server along with the associated nonce in the Token Verification Request message, thus, needs to keep it Begen, et al. Expires June 17, 2011 [Page 14] Internet-Draft Port Mapping December 2010 associated with the Token. o Relative Expiration Time (32 bits): Field that contains the relative expiration time of the Token. The relative expiration time is expressed in seconds from the time the Token was generated. A relative expiration time of zero indicates that the accompanying Token is not valid. The server conveys the relative expiration time in the clear to the client to allow the client to request a new Token well before the expiration time. 4.3. Token Verification Request The Token Verification Request message is identified by SMT=3. This message contains the Token and accompanies any RTCP message that would trigger a new or control an existing unicast session. For a list of such messages, see Section 4.3.1. In the Token Verification Request message, the packet sender SSRC is set to the client's SSRC. The client MUST NOT send a Token Verification Request message with a Token that has expired. The packet format has the structure depicted in Figure 5. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P| SMT=3 | PT=TOKEN=210 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of Packet Sender | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Associated | | Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : Token Element : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Associated Absolute | | Expiration Time | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: Packet format for the Token Verification Request message o Associated Nonce (64 bits): Field that contains the nonce associated with the Token above. o Token Element (Variable size): Token element that was previously received in the Port Mapping Response message. Begen, et al. Expires June 17, 2011 [Page 15] Internet-Draft Port Mapping December 2010 o Associated Absolute Expiration Time (64 bits): Field that contains the absolute expiration time associated with the Token above. 4.3.1. Where to Include Token The following RTCP messages are REQUIRED to be accompanied by a Token Verification Request message when the Token capability is declared in the session description: o Messages that trigger a new unicast session: * NACK messages [RFC4585] * RAMS-R messages [I-D.ietf-avt-rapid-acquisition-for-rtp] o Messages that control an existing unicast session associated with a multicast session: * BYE messages [RFC3550] * RAMS-T messages [I-D.ietf-avt-rapid-acquisition-for-rtp] * CCM messages [RFC5104] A server might determine that other RTCP messages also need to be authenticated using the mechanism described in this document. Thus, clients need to support including a Token with any necessary RTCP message (See Section 3.2). The Token Verification Request message might also be bundled with packets carrying RTCP receiver or extended reports. While such packets do not have a strong security impact, a specific application might desire to have a more controlled reporting scheme from the clients. 4.4. Token Verification Failure The Token Verification Failure message is identified by SMT=4. This message is sent by the server and notifies the client that the Token was invalid or that the client did not include a Token Verification Request message in the RTCP packet although it was supposed to. In the Token Verification Failure message, the packet sender SSRC is set to the server's SSRC. The packet format has the structure depicted in Figure 6. Begen, et al. Expires June 17, 2011 [Page 16] Internet-Draft Port Mapping December 2010 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P| SMT=4 | PT=TOKEN=210 | Length=5 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of Packet Sender | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of Requesting Client | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PT of Request | Req. FMT| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Associated | | Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Packet format for the Token Verification Failure message o SSRC of Requesting Client (32 bits): Field that contains the SSRC of the client who sent the verification request. o PT of Request (8 bits): Field that indicates the type of the RTCP packet that caused this failure message. o Req. FMT (5 bits): Field that indicates the feedback message type (FMT) value of the RTCP packet that caused this failure. Together with the field above, the client can infer which RTCP message it has previously sent caused this failure message to be sent by the server. For example, if the client did not include a valid Token with an RTCP NACK message, the PT of Request field will indicate 205 (RTPFB) and the Req. FMT field will indicate 1 (Generic NACK). If the RTCP message did not have an associated FMT value (such as an RTCP BYE message), the Req. FMT field SHALL be set to zero. o Associated Nonce (64 bits): Field that contains the nonce received in the Token Verification Request message. If there was no Token Verification Request message included by the client, this field is set to 0. Begen, et al. Expires June 17, 2011 [Page 17] Internet-Draft Port Mapping December 2010 5. Procedures for Token Construction The Token encoding is known to the server but opaque to the client. Implementations MUST encode the following information into the Token as a minimum, in order to provide adequate security: o Client's IP address as seen by the server (32/128 bits for IPv4/ IPv6 addresses) o The nonce generated and inserted in the Port Mapping Request message by the client (64 bits) o The absolute expiration time chosen by the server indicated as an NTP timestamp value in seconds since year 1900 [RFC5905] (64 bits, to protect against replay attacks) An example way for constructing Tokens is to perform HMAC-SHA1 [RFC2104] on the concatenated values of the information listed above. The HMAC key should be at least 160 bits long, and generated using a cryptographically secure random source [RFC4086]. However, implementations MAY adopt different approaches and are encouraged to encode whatever additional information is deemed necessary or useful. For example, key rollover is simplified by encoding a key-id into the Token. As another example, a cluster of anycast servers could find advantage by encoding a server identifier into the Token. As another example, if HMAC-SHA1 has been compromised, a replacement HMAC algorithm could be used instead (e.g., HMAC-SHA256). To protect from offline attacks, the server SHOULD occasionally choose a new HMAC key. To ease implementation, a key-id can be assigned to each HMAC key. This can be encoded as simply as one bit (where the new key is X (e.g., 1) and the old key is the inverted value of X (e.g., 0)), or if several keys are supported at once could be encoded into several bits. As the encoding of the Token is entirely private to the server and opaque to the clients, any encoding can be used. By encoding the key-id into the Token element, the server can reject an old key without bothering to do HMAC validation (saving CPU cycles). The key-id can be encoded into the Value field of the Token element by simply concatenating the (plaintext) key-id with the hashed information (i.e., the Token itself). For example, the Value field in the Token element can be computed as: key-id || hash-alg (client-ip | nonce | abs-expiration) During Token construction, the expiration time has to be chosen carefully based on the intended service duration. Tokens that are Begen, et al. Expires June 17, 2011 [Page 18] Internet-Draft Port Mapping December 2010 valid for an unnecessarily long period of time (e.g., several hours) might impose security risks. Depending on the application and use cases, a reasonable value needs to be chosen by the server. Note that using shorter lifetimes requires the clients to acquire Tokens more frequently. However, since a client can acquire a new Token well before it will need to use it, the client will not necessarily be penalized for the acquisition delay. Finally, be aware that NTP timestamps will wrap around in year 2036 and implementations might need to handle this eventually. Refer to Section 6 of [RFC5905] for further details. Begen, et al. Expires June 17, 2011 [Page 19] Internet-Draft Port Mapping December 2010 6. Validating Tokens Upon receipt of an RTCP feedback message along with the Token Verification Request message that contains a Token, nonce and absolute expiration time, the server MUST validate the Token. The server first applies its own procedure for constructing the Tokens by using client's IP address from the received Token Verification Request message, and the nonce and absolute expiration time values reported in the received Token Verification Request message. The server then compares the resulting output with the Token sent by the client in the Token Verification Request message. If they match and the absolute expiration time has not passed yet, the server declares that the Token is valid. Note that if the client's IP address changes, the Token will not validate. Similarly, if the client inserts an incorrect nonce or absolute expiration time value in the Token Verification Request message, validation will fail. It is also possible that the server wants to expire the Token prematurely. In these cases, the server MUST reply back to the client with a Token Verification Failure message (that goes from port P3 on the server to port c1 on the client). In addition to the Token Verification Failure message, it is RECOMMENDED that applications define an application-specific error response to be sent by the server when the server detects that the Token is invalid. For applications using [I-D.ietf-avt-rapid-acquisition-for-rtp], this document defines a new 4xx-level response code in the RAMS Response Code Space Registry. A client that received a Token Verification Failure message can request a new Token from the server. Begen, et al. Expires June 17, 2011 [Page 20] Internet-Draft Port Mapping December 2010 7. SDP Signaling 7.1. The portmapping-req Attribute This new SDP attribute is used declaratively to indicate the port and optionally the address for obtaining a Token. Its presence indicates that (i) a Token MUST be included in certain feedback messages sent to the server triggering or controlling a unicast session (The list of those feedback messages is in Section 4.3.1), and (ii) the client MUST receive the unicast session's RTP and RTCP packets from the server on the port from which it sent the RTCP message triggering or controlling the unicast session. The formal description of the 'portmapping-req' attribute is defined by the following ABNF [RFC5234] syntax: portmapping-req-attr = "a=portmapping-req" [":" port [SP nettype SP addrtype SP connection-address]] CRLF Here, 'port', 'nettype', 'addrtype' and 'connection-address' are defined as specified in Section 9 of [RFC4566]. The 'portmapping- req' attribute SHALL be used as a media-level attribute. Note: This does not imply that Token Verification Request messages need to be sent in the unicast session. Token Verification Request messages accompany RTCP messages that trigger or control this unicast session, and are sent either in the multicast session or the unicast session, depending on the RTCP message (See Section 4.3.1). In the optional address value, only unicast addresses SHOULD be used unless one wants to use a multicast address after evaluating the additional security risks such as non-legit servers generating fake Tokens. If the address is not specified, the (source) address in the "c" line corresponding to the unicast media stream is implied. When using this attribute in SDP offer/answer exchanges [RFC3264], the following needs to be considered. First of all, this attribute is used declaratively, and there are no offer/answer exchanges between the client consuming the SDP description and the actual server providing the service. At media level, this applies to the RTCP feedback messages declared by this media block. An offerer that desires the answerer to use Tokens in RTCP messages sent to the server, i.e., received by the server, the attribute is included. In case an offerer desires to declare support for using Tokens as defined in this specification but do not need Tokens to be included for any RTCP messages to be received by the server, it can Begen, et al. Expires June 17, 2011 [Page 21] Internet-Draft Port Mapping December 2010 include the 'portmapping-req' attribute without any parameters, neither port nor address. An answerer receiving an SDP offer with the "a=portmapping-req" line with a port number SHALL use that port number and the address, either explicitly provided in the attribute or implicitly provided by the "c" line, for any needed Token request. If the "a=portmapping-req" line attribute does not contain a port, the answer SHALL take note of the capability. When sending an answer, if the 'portmapping-req' attribute has been present in the offer including a port number and the answerer supports this specification, then the answerer MUST include the attribute in its answer. The answer may or may not include a port and address. This depends on the application and the desire of the answerer. The answerer includes a port and possibly an address when it requires to receive Tokens in RTCP messages. If it only supports this specification but does not need Tokens to be included, the attribute is included without any port or address. 7.2. Requirements The use of SDP for the port mapping solution normatively requires the support for: o The SDP grouping framework and flow identification (FID) semantics [RFC5888] o The RTP/AVPF profile [RFC4585] o Multiplexing RTP and RTCP on a single port on both endpoints in the unicast session [RFC5761] 7.3. Example and Discussion The declarative SDP describing the scenario given in Figure 2 is written as: Begen, et al. Expires June 17, 2011 [Page 22] Internet-Draft Port Mapping December 2010 v=0 o=ali 1122334455 1122334466 IN IP4 nack.example.com s=Local Retransmissions t=0 0 a=group:FID 1 2 a=rtcp-unicast:rsi m=video 41000 RTP/AVPF 98 i=Multicast Stream c=IN IP4 233.252.0.2/255 a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1 ; Note 1 a=rtpmap:98 MP2T/90000 a=multicast-rtcp:41500 ; Note 1 a=rtcp:42000 IN IP4 192.0.2.1 ; Note 2 a=rtcp-fb:98 nack ; Note 2 a=mid:1 m=video 42000 RTP/AVPF 99 ; Note 3 i=Unicast Retransmission Stream c=IN IP4 192.0.2.1 a=sendonly a=rtpmap:99 rtx/90000 a=rtcp-mux ; Note 4 a=rtcp:42500 ; Note 5 a=fmtp:99 apt=98; rtx-time=5000 a=portmapping-req:30000 ; Note 6 a=mid:2 Figure 7: SDP describing an SSM distribution with support for retransmissions from a local server In this description, we highlight the following notes: Note 1: The source stream is multicast from a distribution source with a source IP address of 198.51.100.1 to the multicast destination address of 233.252.0.2 and port 41000 (P1). The associated RTCP packets are multicast in the same group to port 41500 (P2). Note 2: A retransmission server including feedback target functionality with an IP address of 192.0.2.1 and port of 42000 (P3) is specified with the 'rtcp' attribute. The feedback functionality is enabled for the RTP stream with payload type 98 through the 'rtcp-fb' attribute [RFC4585]. Note 3: The port specified in the second "m" line (for the unicast stream) does not mean anything in this scenario as the client does not send any RTP traffic back to the server. Note 4: The server multiplexes RTP and RTCP packets on the same port (c1 in Figure 2). Begen, et al. Expires June 17, 2011 [Page 23] Internet-Draft Port Mapping December 2010 Note 5: The server uses port 42500 (P4) for the unicast sessions. Note 6: The "a=portmapping-req" line indicates that a Token needs to be retrieved first before a unicast session associated to the multicast session can be established and that the Port Mapping Request message needs to be sent to port 30000 (PT). Since there is no address indicated in this line, the client needs to send the Token request to the address specified in the "c" line. Begen, et al. Expires June 17, 2011 [Page 24] Internet-Draft Port Mapping December 2010 8. Address Pooling NATs Large-scale NAT devices have a pool of public IPv4 addresses and map internal hosts to one of those public IPv4 addresses. As long as an internal host maintains an active mapping in the NAT, the same IPv4 address is assigned to new connections. However, once all of the host's mappings have been deleted (e.g., because of timeout), it is possible that a new connection from that same host will be assigned a different IPv4 address from the pool. When that occurs, the Token will be considered invalid by the server, causing an additional round trip for the client to acquire a fresh Token. Any traffic from the host which traverses the NAT will prevent this problem. As the host is sending RTCP receiver reports at least every 5 seconds (Section 6.2 of [RFC3550]) for the multicast session it is receiving, those RTCP messages will be sufficient to prevent this problem. Begen, et al. Expires June 17, 2011 [Page 25] Internet-Draft Port Mapping December 2010 9. Security Considerations 9.1. Tokens The Token, which is generated based on a client's IP address and expiration date, provides protection against off-path denial-of- service (DoS) attacks. An attacker using a certain IP address cannot cause one or more RTP packets to be sent to a victim client who has a different IP address. However, if the attacker acquires a valid Token for a victim and can spoof the victim's source address, this approach becomes vulnerable to replay attacks. This is especially easy if the attacker and victim are behind a large-scale NAT and share the same IP address. Multicast is deployed on managed networks - not the Internet. These managed networks will choose to enable network ingress filtering [RFC2827] or not. If ingress filtering is enabled on a network, an attacker attacker cannot spoof a victim's IP address to use a Token to initiate an attack against a victim. However, if ingress filtering is not enabled on a network, an attacker could obtain a Token and spoof the victim's address, causing traffic to flood the victim. On such a network, the server can reduce the time period for such an attack by expiring a Token in a short period of time. In the extreme case, the server can expire the Token in such a short period of time, such that the client will have to acquire a new Token immediately before using it in a Token Verification Request message. HMAC-SHA1 provides a level of security that is widely regarded as being more than sufficient for providing message authentication. It is believed that the economic cost of breaking that algorithm is significantly higher than the cost of more direct approaches to violating system security, e.g., theft, bribery, wiretapping, and other forms of malfeasance. HMAC-SHA1 is secure against all known cryptanalytic attacks that use computational resources that are currently economically feasible. 9.2. The portmapping-req Attribute The 'portmapping-req' attribute is not believed to introduce any significant security risk to multimedia applications. A malevolent third party could use this attribute to redirect the Port Mapping Request messages by altering the port number or cause the unicast session establishment to fail by removing it from the SDP description. But, this requires intercepting and rewriting the packets carrying the SDP description; and if an interceptor can do that, many more attacks are possible, including a wholesale change of the addresses and port numbers at which the media will be sent. Begen, et al. Expires June 17, 2011 [Page 26] Internet-Draft Port Mapping December 2010 In order to avoid attacks of this sort, the SDP description needs to be integrity protected and provided with source authentication. This can, for example, be achieved on an end-to-end basis using S/MIME [RFC5652] when SDP is used in a signaling packet using MIME types (application/sdp). Alternatively, HTTPS [RFC2818] or the authentication method in the Session Announcement Protocol (SAP) [RFC2974] could be used as well. Begen, et al. Expires June 17, 2011 [Page 27] Internet-Draft Port Mapping December 2010 10. IANA Considerations The following contact information shall be used for all registrations in this document: Ali Begen abegen@cisco.com Note to the RFC Editor: In the following, please replace "XXXX" with the number of this document prior to publication as an RFC. 10.1. Registration of SDP Attributes This document registers a new attribute name in SDP. SDP Attribute ("att-field"): Attribute name: portmapping-req Long form: Port for requesting Token Type of name: att-field Type of attribute: Media level Subject to charset: No Purpose: See this document Reference: [RFCXXXX] Values: See this document 10.2. Registration of RTCP Control Packet Types In accordance with Section 15 of [RFC3550], this specification adds the following value to the RTCP Control Packet types sub-registry of the Real-Time Transport Protocol (RTP) Parameters registry: Value Abbrev. Name Reference -------- --------- --------------------------------------- --------- 210 TOKEN Port Mapping [RFCXXXX] 10.3. SMT Values for TOKEN Packet Type Registry This document creates a new sub-registry for the sub-message type (SMT) values to be used with the TOKEN packet type. The registry is called the SMT Values for TOKEN Packet Type Registry. This registry is to be managed by the IANA according to the IETF Review policy of [RFC5226]. The length of the SMT field is five bits, allowing 32 values. The Begen, et al. Expires June 17, 2011 [Page 28] Internet-Draft Port Mapping December 2010 registry is initialized with the following entries: Value Name Reference ----- -------------------------------------------------- ------------- 0 Reserved [RFCXXXX] 1 Port Mapping Request [RFCXXXX] 2 Port Mapping Response [RFCXXXX] 3 Token Verification Request [RFCXXXX] 4 Token Verification Failure [RFCXXXX] 5-30 Unassigned IETF Review 31 Reserved [RFCXXXX] The SMT values 0 and 31 are reserved for future use. 10.4. RAMS Response Code Space Registry This document adds the following entry to the RAMS Response Code Space Registry. Code Description Reference ----- -------------------------------------------------- ------------- 405 Invalid Token [RFCXXXX] This response code is used when the Token included by the RTP_Rx in the RAMS-R message is invalid. Begen, et al. Expires June 17, 2011 [Page 29] Internet-Draft Port Mapping December 2010 11. Acknowledgments The approach presented in this document came out after discussions with various individuals in the AVT and MMUSIC WGs, and the breakout session held in the Anaheim meeting. We thank each of these individuals, in particular to Magnus Westerlund and Colin Perkins. Begen, et al. Expires June 17, 2011 [Page 30] Internet-Draft Port Mapping December 2010 12. References 12.1. Normative References [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, "Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July 2006. [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control Protocol (RTCP) Extensions for Single-Source Multicast Sessions with Unicast Feedback", RFC 5760, February 2010. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description Protocol (SDP) Grouping Framework", RFC 5888, June 2010. [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", RFC 5761, April 2010. 12.2. Informative References [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. Begen, et al. Expires June 17, 2011 [Page 31] Internet-Draft Port Mapping December 2010 [RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in the Session Description Protocol (SDP)", RFC 4145, September 2005. [I-D.ietf-avt-rapid-acquisition-for-rtp] Steeg, B., Begen, A., Caenegem, T., and Z. Vax, "Unicast- Based Rapid Acquisition of Multicast RTP Sessions", draft-ietf-avt-rapid-acquisition-for-rtp-17 (work in progress), November 2010. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. Hakenberg, "RTP Retransmission Payload Format", RFC 4588, July 2006. [I-D.ietf-avt-app-rtp-keepalive] Marjou, X. and A. Sollaud, "Application Mechanism for keeping alive the Network Address Translator (NAT) mappings associated to RTP flows.", draft-ietf-avt-app-rtp-keepalive-09 (work in progress), September 2010. [RFC2827] 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. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, "Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)", RFC 5104, February 2008. [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, September 2009. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000. Begen, et al. Expires June 17, 2011 [Page 32] Internet-Draft Port Mapping December 2010 Authors' Addresses Ali Begen Cisco 181 Bay Street Toronto, ON M5J 2T3 Canada Email: abegen@cisco.com Dan Wing Cisco Systems, Inc. 170 West Tasman Dr. San Jose, CA 95134 USA Email: dwing@cisco.com Tom VanCaenegem Alcatel-Lucent Copernicuslaan 50 Antwerpen, 2018 Belgium Email: Tom.Van_Caenegem@alcatel-lucent.com Begen, et al. Expires June 17, 2011 [Page 33]