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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 AVT A. Begen 3 Internet-Draft D. Wing 4 Intended status: Standards Track Cisco 5 Expires: June 10, 2011 T. VanCaenegem 6 Alcatel-Lucent 7 December 7, 2010 9 Port Mapping Between Unicast and Multicast RTP Sessions 10 draft-ietf-avt-ports-for-ucast-mcast-rtp-06 12 Abstract 14 This document presents a port mapping solution that allows RTP 15 receivers to choose their own ports for an auxiliary unicast session 16 in RTP applications using both unicast and multicast services. The 17 solution provides protection against denial-of-service attacks that 18 could be used to cause one or more RTP packets to be sent to a victim 19 client. 21 Status of this Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on June 10, 2011. 38 Copyright Notice 40 Copyright (c) 2010 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5 57 3. Token-Based Port Mapping . . . . . . . . . . . . . . . . . . . 6 58 3.1. Token Request and Retrieval . . . . . . . . . . . . . . . 6 59 3.2. Unicast Session Establishment . . . . . . . . . . . . . . 6 60 3.2.1. Motivating Scenario . . . . . . . . . . . . . . . . . 6 61 3.2.2. Normative Behavior and Requirements . . . . . . . . . 8 62 4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 11 63 4.1. Port Mapping Request . . . . . . . . . . . . . . . . . . . 12 64 4.2. Port Mapping Response . . . . . . . . . . . . . . . . . . 12 65 4.3. Token Verification Request . . . . . . . . . . . . . . . . 14 66 4.4. Token Verification Failure . . . . . . . . . . . . . . . . 15 67 5. Procedures for Token Construction . . . . . . . . . . . . . . 17 68 6. Validating Tokens . . . . . . . . . . . . . . . . . . . . . . 19 69 7. SDP Signaling . . . . . . . . . . . . . . . . . . . . . . . . 20 70 7.1. The portmapping-req Attribute . . . . . . . . . . . . . . 20 71 7.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 20 72 7.3. Example and Discussion . . . . . . . . . . . . . . . . . . 21 73 8. Address Pooling NATs . . . . . . . . . . . . . . . . . . . . . 23 74 9. Security Considerations . . . . . . . . . . . . . . . . . . . 24 75 9.1. Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . 24 76 9.2. The portmapping-req Attribute . . . . . . . . . . . . . . 24 77 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 78 10.1. Registration of SDP Attributes . . . . . . . . . . . . . . 26 79 10.2. Registration of FMT Values . . . . . . . . . . . . . . . . 26 80 10.3. SFMT Values for Port Mapping Messages Registry . . . . . . 26 81 10.4. RAMS Response Code Space Registry . . . . . . . . . . . . 27 82 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28 83 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29 84 12.1. Normative References . . . . . . . . . . . . . . . . . . . 29 85 12.2. Informative References . . . . . . . . . . . . . . . . . . 30 86 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32 88 1. Introduction 90 In (any-source or source-specific) multicast RTP applications, 91 destination ports, i.e., the ports on which the multicast receivers 92 receive the RTP and RTCP packets, are defined declaratively. In 93 other words, the receivers cannot choose their receive ports and the 94 sender(s) use the pre-defined ports. 96 In unicast RTP applications, the receiving end needs to choose its 97 ports for RTP and RTCP since these ports are local resources and only 98 the receiving end can determine which ports are available to use. In 99 addition, Network Address Port Translators (NAPT - hereafter simply 100 called NAT) devices are commonly deployed in networks, thus, static 101 port assignments cannot be used. The receiving may convey its 102 request to the sending end through different ways, one of which is 103 the Offer/Answer Model [RFC3264] for the Session Description Protocol 104 (SDP) [RFC4566]. However, the Offer/Answer Model requires offer/ 105 answer exchange(s) between the endpoints, and the resulting delay may 106 not be desirable in delay-sensitive real-time applications. 107 Furthermore, the Offer/Answer Model may be burdensome for the 108 endpoints that are concurrently running a large number of unicast 109 sessions with other endpoints. 111 In this specification, we consider an RTP application that uses one 112 or more unicast and multicast RTP sessions together. While the 113 declaration and selection of the ports are well defined and work well 114 for multicast and unicast RTP applications, respectively, the usage 115 of the ports introduces complications when a receiving end mixes 116 unicast and multicast RTP sessions within the same RTP application. 118 An example scenario is where the RTP packets are distributed through 119 source-specific multicast (SSM) and a receiver sends unicast RTCP 120 NACK feedback to a local repair server (also functioning as a unicast 121 RTCP feedback target) [RFC5760] asking for a retransmission of the 122 packets it is missing, and the local repair server sends the 123 retransmission packets over a unicast RTP session [RFC4588]. 125 Another scenario is where a receiver wants to rapidly acquire a new 126 primary multicast RTP session and receives one or more RTP burst 127 packets over a unicast session before joining the SSM session 128 [I-D.ietf-avt-rapid-acquisition-for-rtp]. Similar scenarios exist in 129 applications where some part of the content is distributed through 130 multicast while the receivers get additional and/or auxiliary content 131 through one or more unicast connections, as sketched in Figure 1. 133 In this document, we discuss this problem and introduce a solution 134 that we refer to as Port Mapping. This solution allows receivers to 135 choose their desired UDP ports for RTP and RTCP in every unicast 136 session when they are running RTP applications using both unicast and 137 multicast services, and offer/answer exchange is not available. This 138 solution is not applicable in cases where TCP is used as the 139 transport protocol in the unicast sessions. For such scenarios, 140 refer to [RFC4145]. 142 ----------- 143 | Unicast |................ 144 | Source |............. : 145 | (Server) | : : 146 ----------- : : 147 v v 148 ----------- ---------- ----------- 149 | Multicast |------->| Router |---------->|Client RTP | 150 | Source | | |..........>|Application| 151 ----------- ---------- ----------- 152 | : 153 | : ----------- 154 | :..............>|Client RTP | 155 +---------------->|Application| 156 ----------- 158 -------> Multicast RTP Flow 159 .......> Unicast RTP Flow 161 Figure 1: RTP applications simultaneously using both unicast and 162 multicast services 164 In the remainder of this document, we refer to the RTP endpoints that 165 serve other RTP endpoints over a unicast session as the Servers. The 166 receiving RTP endpoints are referred to as Clients. 168 2. Requirements Notation 170 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 171 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 172 document are to be interpreted as described in [RFC2119]. 174 3. Token-Based Port Mapping 176 Token-based Port Mapping consists of two steps: (i) Token request 177 and retrieval, and (ii) unicast session establishment. These are 178 described below. 180 3.1. Token Request and Retrieval 182 This first step is required to be completed only once. Once a Token 183 is retrieved from a particular server, it can be used for all the 184 unicast sessions the client will be running with this particular 185 server. By default, Tokens are server specific. However, the client 186 can use the same Token to communicate with different servers if these 187 servers are provided with the same secret key and algorithm used to 188 generate the Token and are at least loosely clock-synchronized. The 189 Token becomes invalid if client's public IP address changes or when 190 the server expires the Token. In these cases, the client has to 191 request a new Token. 193 The Token is essentially an opaque encapsulation that is based on 194 client's IP address (as seen by the server). When a request is 195 received, the server creates a Token for this particular client, and 196 sends it back to the client. Later, when the client wants to 197 establish a unicast session, the Token will be validated by the 198 server, making sure that the IP address information matches. This is 199 effective against DoS attacks, e.g., an attacker cannot simply spoof 200 another client's IP address and start a unicast transmission towards 201 random clients. 203 3.2. Unicast Session Establishment 205 The second step is the unicast session establishment. We illustrate 206 this step with an example. First, we describe the motivation 207 scenario and then define the normative behavior and requirements. 209 3.2.1. Motivating Scenario 211 Consider an SSM distribution network where a distribution source 212 multicasts RTP packets to a large number of clients, and one or more 213 retransmission servers function as feedback targets to collect 214 unicast RTCP feedback from these clients [RFC5760]. The 215 retransmission servers also join the multicast session to receive the 216 multicast packets and cache them for a certain time period. When a 217 client detects missing packets in the multicast session, it requests 218 a retransmission from one of the retransmission servers by using an 219 RTCP NACK message [RFC4585]. The retransmission server pulls the 220 requested packet(s) out of the cache and retransmits them to the 221 requesting client [RFC4588]. 223 The RTP and RTCP flows pertaining to the scenario described above are 224 sketched in Figure 2. Between the client and server, there can be 225 one or more NAT devices [RFC4787]. 227 -------------- --- ---------- 228 | |-------------------------------| |-->|P1 | 229 | |-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-| |.->|P2 | 230 | | | | | | 231 | Distribution | ---------------- | | | | 232 | Source | | | | | | | 233 | |---->|P1 | | | | | 234 | |.-.->|P2 | | | | | 235 | | | | | | | | 236 -------------- | P3|<.=.=.=.| |=.=|*c0 | 237 | P3|<~~~~~~~| |~~~|*c1 | 238 MULTICAST RTP | | | | | | 239 SESSION with | | | | | | 240 UNICAST FEEDBACK | | | N | | | 241 | Retransmission | | A | | Client | 242 - - - - - - - - - - -| - - - - - - - -| - - - -| - |- -| - - - - -|- 243 | Server | | T | | | 244 | | | | | | 245 PORT MAPPING | PT|<~~~~~~~| |~~>|*cT | 246 | | | | | | 247 - - - - - - - - - - -| - - - - - - - -| - - - -| - |- -| - - - - -|- 248 | | | | | | 249 AUXILIARY UNICAST | | | | | | 250 RTP SESSION | | | | | | 251 | P3|........| |..>|*c1 | 252 | P3|=.=.=.=.| |=.>|*c1 | 253 | P4|<.=.=.=.| |=.=|*c2 | 254 | | | | | | 255 ---------------- --- ---------- 257 -------> Multicast RTP Flow 258 .-.-.-.> Multicast RTCP Flow 259 .=.=.=.> Unicast RTCP Reports 260 ~~~~~~~> Unicast RTCP Feedback Messages 261 .......> Unicast RTP Flow 263 Figure 2: Example scenario showing an SSM distribution with support 264 for retransmissions from a server 266 3.2.2. Normative Behavior and Requirements 268 In Figure 2, we have the following multicast and unicast ports: 270 o Ports P1 and P2 denote the destination RTP and RTCP ports in the 271 multicast session, respectively. The clients listen to these 272 ports to receive the multicast RTP and RTCP packets. Ports P1 and 273 P2 are defined declaratively. 275 o Port P3 denotes the RTCP port on the feedback target running on 276 the retransmission server to collect any RTCP packet sent by the 277 clients including feedback messages, and RTCP receiver and 278 extended reports. This is also the port that the retransmission 279 server uses to send the RTP packets and RTCP sender reports in the 280 unicast session. Port P3 is defined declaratively. 282 o Port P4 denotes the RTCP port on the retransmission server used to 283 collect the RTCP receiver and extended reports for the unicast 284 session. Port P4 is defined declaratively and MUST be different 285 from port P3. 287 o Ports *c0, *c1 and *c2 are chosen by the client. *c0 denotes the 288 port on the client used to send the RTCP reports for the multicast 289 session. *c1 denotes the port on the client used to send the 290 unicast RTCP feedback messages in the multicast session and to 291 receive the RTP packets and RTCP sender reports in the unicast 292 session. *c2 denotes the port on the client used to send the RTCP 293 receiver and extended reports in the unicast session. Ports c0, 294 c1 and c2 MAY be the same port or different ports. There are two 295 advantages of using the same port for both c0 and c1: 297 1. Some NATs only keep bindings active when a packet goes from 298 the inside to the outside of the NAT (See REQ-6 of Section 4.3 299 of [RFC4787]). When the gap between retransmission requests 300 (or other traffic sent from the client to the server) is long, 301 this can exceed that timeout. If c0=c1, the occasional 302 (periodic) RTCP receiver reports sent from port c0 (for the 303 multicast session's RTCP port P3) will ensure the NAT does not 304 time out the public port associated with the incoming unicast 305 traffic to port c1. 307 2. Having c0=c1 conserves NAT port bindings. 309 Thus, it is strongly RECOMMENDED that the client uses the same 310 port for c0 and c1. 312 A client cannot keep using the same receive port for different 313 unicast sessions since there could be packet leakage when 314 switching from one unicast session to another unless each received 315 unicast stream has its own distinct Synchronization Source (SSRC) 316 identifier to allow the client to filter out the undesired 317 packets. Unless this is guaranteed (which is not often easy), a 318 client SHOULD use separate receive ports for subsequent unicast 319 sessions. After a sufficient time, a previously used receive port 320 could be used again. 322 o Ports PT and cT denote the ports through which the Token request 323 and retrieval occur at the server and client sides, respectively. 324 Port PT is declared on a per unicast session basis, although its 325 value MAY be the same for two or more unicast sessions sourced by 326 the server. A Token once requested and retrieved by a client from 327 port PT remains valid until its expiration time. Port PT MAY be 328 equal to port P3. Port cT MAY also be equal to ports c0 and c1. 330 In addition to the ports, we use the following notation: 332 o DS: IP address of the distribution source 334 o G: Destination multicast address 336 o S: IP address of the retransmission server 338 o C: IP address of the client 340 o C': Public IP address of the client (as seen by the server) 342 We assume that the information declaratively defined is available as 343 part of the session description information and is provided to the 344 clients. The Session Description Protocol (SDP) [RFC4566] and other 345 session description methods can be used for this purpose. 347 The following steps summarize the Token-based solution: 349 1. The client ascertains server address (S) and port numbers (P3 and 350 P4) from the session description. 352 2. The client selects its local port numbers (*c0, *c1 and *c2). 354 3. If the client does not have a Token (or the existing Token has 355 expired): 357 A. The client first sends a message to the server via a new RTCP 358 message, called Port Mapping Request to port PT. This 359 message is sent from port cT on the client side. The server 360 learns client's public IP address (C') from the received 361 message. The client can send this message anytime it wants 362 (e.g., during initialization), and does not normally ever 363 need to re-send this message (See Section 6). 365 B. The server generates an opaque encapsulation (i.e., the 366 Token) based on certain information including client's IP 367 address. 369 C. The server sends the Token back to the client using a new 370 RTCP message, called Port Mapping Response. This message 371 MUST be sent from port PT to port cT. 373 4. The client needs to provide the Token to the server using a new 374 RTCP message, called Token Verification Request, whenever the 375 client sends an RTCP feedback message for triggering or 376 controlling a unicast session (See Section 4.3). Note that the 377 unicast session is only established after the server has received 378 a feedback message (along with a valid Token) from the client for 379 which it needs to react by sending unicast data. Until a unicast 380 session is established, neither the server nor the client needs 381 to send RTCP reports for the unicast session. 383 5. Normal flows ensue as shown in Figure 2. Note that in the 384 unicast session, traffic from the server to the client (i.e., 385 both the RTP and RTCP packets sent from port P3 to port c1) MUST 386 be multiplexed on the (same) port c1. If the client uses the 387 same port for both c0 and c1, the RTCP reports sent for the 388 multicast session keep the P3->c1(=c0) binding alive. If the 389 client uses different ports for c0 and c1, the client needs to 390 periodically send an explicit keep-alive message 391 [I-D.ietf-avt-app-rtp-keepalive] to keep the P3->c1 binding alive 392 during the lifetime of the unicast session if the unicast 393 session's lifetime is likely to exceed the NAT's timeout value. 395 4. Message Formats 397 This section defines the formats of the RTCP transport-layer feedback 398 messages that are exchanged between a server and a client for the 399 purpose of Token-based port mapping. Four RTCP messages are defined: 401 1. Port Mapping Request 403 2. Port Mapping Response 405 3. Token Verification Request 407 4. Token Verification Failure 409 These are all payload-independent RTCP feedback messages with a 410 common format defined in Section 6.1 of [RFC4585], also sketched in 411 Figure 3. 413 0 1 2 3 414 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 1 415 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 416 |V=2|P| FMT | PT | length | 417 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 418 | SSRC of packet sender | 419 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 420 | SSRC of media source | 421 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 422 : Feedback Control Information (FCI) : 423 : : 425 Figure 3: The common packet format for the RTCP feedback messages 427 Each feedback message has a fixed-length field for version, padding, 428 feedback message type (FMT), packet type (PT), length, SSRC of packet 429 sender, SSRC of media source as well as a variable-length field for 430 feedback control information (FCI). 432 In the new messages defined in this section, the PT field is set to 433 RTPFB (205) and the FMT field is set to Port Mapping (7). Individual 434 Port Mapping messages are identified by a sub-field called Sub 435 Feedback Message Type (SFMT). Any Reserved field SHALL be set to 436 zero and ignored. 438 Following the rules specified in [RFC3550], all integer fields in the 439 messages defined below are carried in network-byte order, that is, 440 most significant byte (octet) first, also known as big-endian. 441 Unless otherwise stated, numeric constants are in decimal (base 10). 443 Note that RTCP is not a timely or reliable protocol. The RTCP 444 packets might get lost or re-ordered in the network. When a client 445 sends a Port Mapping Request or Token Verification Request message 446 but it does not receive a response back from the server (either a 447 Port Mapping Response or Token Verification Failure message), it MAY 448 resend its request when it is eligible to do so based on the timer 449 rules defined in [RFC4585]. 451 4.1. Port Mapping Request 453 The Port Mapping Request message is identified by SFMT=1. This 454 message is a unicast feedback message transmitted by the client to a 455 dedicated server port to request a Token. In the Port Mapping 456 Request message, the client MUST set both the packet sender SSRC and 457 media source SSRC fields to its own SSRC since the Port Mapping 458 Request message is not necessarily linked to any specific media 459 source. The FCI field has the structure depicted in Figure 4. 461 0 1 2 3 462 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 1 463 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 464 | SFMT=1 | Random Nonce : 465 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 466 : Random Nonce | 467 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 469 Figure 4: The FCI field of Port Mapping Request message 471 o Random Nonce (56 bits): Mandatory field that contains a random 472 nonce value generated by the client following the procedures of 473 [RFC4086]. This nonce is taken into account by the server when 474 generating a Token for the client to enable better security for 475 clients that share the same IP address. If the Port Mapping 476 Request message is transmitted multiple times for redundancy 477 reasons, the random nonce value MUST remain the same in these 478 duplicated messages. However, the client MUST generate a new 479 random nonce for every new Port Mapping Request message. 481 4.2. Port Mapping Response 483 The Port Mapping Response message is identified by SFMT=2. This 484 message is sent by the server and delivers the Token to the client as 485 a response to the Port Mapping Request message. In the Port Mapping 486 Response message, the packet sender SSRC and media sender SSRC fields 487 are both set to the client's SSRC since the Port Mapping Response 488 message is not necessarily linked to any specific media source. The 489 FCI field has the structure depicted in Figure 5. 491 0 1 2 3 492 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 1 493 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 494 | SFMT=2 | Associated Nonce : 495 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 496 : Associated Nonce | 497 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 498 : Token Element : 499 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 500 | Absolute | 501 | Expiration Time | 502 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 503 | Relative Expiration Time | 504 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 506 Figure 5: FCI field syntax for the Port Mapping Response message 508 o Associated Nonce (56 bits): Mandatory field that contains the 509 nonce received in the Port Mapping Request message and used in 510 Token construction. 512 o Token Element (Variable size): Mandatory element that is used to 513 carry the Token generated by the server. This element is a 514 Length-Value element. The Length field, which is 8 bits, 515 indicates the length (in octets) of the Value field that follows 516 the Length field. The Value field carries the Token (or more 517 accurately, the output of the encoding process on the server). 519 o Absolute Expiration Time (64 bits): Mandatory field that contains 520 the absolute expiration time of the Token. The absolute 521 expiration time is expressed as a Network Time Protocol (NTP) 522 timestamp value in seconds since year 1900 [RFC5905]. The client 523 does not need to use this element directly, thus, does not need to 524 synchronize its clock with the server. However, the client needs 525 to send this element back to the server along with the associated 526 nonce in the Token Verification Request message, thus, needs to 527 keep it associated with the Token. 529 o Relative Expiration Time (32 bits): Mandatory field that contains 530 the relative expiration time of the Token. The relative 531 expiration time is expressed in seconds from the time the Token 532 was generated. A relative expiration time of zero indicates that 533 the accompanying Token is not valid. 535 The server conveys the relative expiration time in the clear to 536 the client to allow the client to request a new Token well before 537 the expiration time. 539 4.3. Token Verification Request 541 The Token Verification Request message is identified by SFMT=3. This 542 message contains the Token and accompanies any RTCP message that 543 would trigger a new or control an existing unicast session. 544 Currently, the following RTCP messages are REQUIRED to be accompanied 545 by a Token Verification Request message: 547 o Messages that trigger a new unicast session: 549 * NACK messages [RFC4585] 551 * RAMS-R messages [I-D.ietf-avt-rapid-acquisition-for-rtp] 553 o Messages that control an existing unicast session associated with 554 a multicast session: 556 * BYE messages [RFC3550] 558 * RAMS-T messages [I-D.ietf-avt-rapid-acquisition-for-rtp] 560 * CCM messages [RFC5104] 562 Other RTCP messages defined in the future, which could be abused to 563 cause packet amplification attacks, SHOULD also be authenticated 564 using the mechanism described in this document. The Token 565 Verification Request message might also be bundled with packets 566 carrying RTCP receiver or extended reports. While such packets do 567 not have a strong security impact, a specific application might 568 desire to have a more controlled reporting scheme from the clients. 570 In the Token Verification Request message, the client MUST set both 571 the packet sender SSRC and media source SSRC fields to its own SSRC 572 since the media source SSRC may not be known. The client MUST NOT 573 send a Token Verification Request message with a Token that has 574 expired. The FCI field has the structure depicted in Figure 6. 576 0 1 2 3 577 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 1 578 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 579 | SFMT=3 | Associated Nonce : 580 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 581 : Associated Nonce | 582 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 583 : Token Element : 584 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 585 | Associated Absolute | 586 | Expiration Time | 587 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 589 Figure 6: FCI field syntax for the Token Verification message 591 o Associated Nonce (56 bits): Mandatory field that contains the 592 nonce associated with the Token above. 594 o Token Element (Variable size): Mandatory Token element that was 595 previously received in the Port Mapping Response message. 597 o Associated Absolute Expiration Time (64 bits): Mandatory field 598 that contains the absolute expiration time associated with the 599 Token above. 601 4.4. Token Verification Failure 603 The Token Verification Failure message is identified by SFMT=4. This 604 message is sent by the server and notifies the client that the Token 605 was invalid or that the client did not include a Token Verification 606 Request message in the RTCP packet although it was supposed to. In 607 the Token Verification Failure message, the packet sender SSRC and 608 media sender SSRC fields are both set to the client's SSRC. The FCI 609 field has the structure depicted in Figure 6. 611 0 1 2 3 612 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 1 613 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 614 | SFMT=4 | Associated Nonce : 615 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 616 : Associated Nonce | 617 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 619 Figure 7: FCI field syntax for the Token Failure message 621 o Associated Nonce (56 bits): Mandatory field that contains the 622 nonce received in the Token Verification Request message. If 623 there was no Token Verification Request message included by the 624 client, this field is set to 0. 626 5. Procedures for Token Construction 628 The Token encoding is known to the server but opaque to the client. 629 Implementations MUST encode the following information into the Token 630 as a minimum, in order to provide adequate security: 632 o Client's IP address as seen by the server (32/128 bits for IPv4/ 633 IPv6 addresses) 635 o The nonce generated and inserted in the Port Mapping Request 636 message by the client (56 bits) 638 o The absolute expiration time chosen by the server indicated as an 639 NTP timestamp value in seconds since year 1900 [RFC5905] (64 bits, 640 to protect against replay attacks) 642 An example way for constructing Tokens is to perform HMAC-SHA1 643 [RFC2104] on the concatenated values of the information listed above. 644 The HMAC key should be at least 160 bits long, and generated using a 645 cryptographically secure random source [RFC4086]. However, 646 implementations MAY adopt different approaches and are encouraged to 647 encode whatever additional information is deemed necessary or useful. 648 For example, key rollover is simplified by encoding a key-id into the 649 Token. As another example, a cluster of anycast servers could find 650 advantage by encoding a server identifier into the Token. As another 651 example, if HMAC-SHA1 has been compromised, a replacement HMAC 652 algorithm could be used instead (e.g., HMAC-SHA256). 654 To protect from offline attacks, the server SHOULD occasionally 655 choose a new HMAC key. To ease implementation, a key-id can be 656 assigned to each HMAC key. This can be encoded as simply as one bit 657 (where the new key is X (e.g., 1) and the old key is the inverted 658 value of X (e.g., 0)), or if several keys are supported at once could 659 be encoded into several bits. As the encoding of the Token is 660 entirely private to the server and opaque to the clients, any 661 encoding can be used. By encoding the key-id into the Token element, 662 the server can reject an old key without bothering to do HMAC 663 validation (saving CPU cycles). The key-id can be encoded into the 664 Value field of the Token element by simply concatenating the 665 (plaintext) key-id with the hashed information (i.e., the Token 666 itself). 668 For example, the Value field in the Token element can be computed as: 670 key-id || hash-alg (client-ip | nonce | abs-expiration) 672 During Token construction, the expiration time has to be chosen 673 carefully based on the intended service duration. Tokens that are 674 valid for an unnecessarily long period of time (e.g., several hours) 675 might impose security risks. Depending on the application and use 676 cases, a reasonable value needs to be chosen by the server. Note 677 that using shorter lifetimes requires the clients to acquire Tokens 678 more frequently. However, since a client can acquire a new Token 679 well before it will need to use it, the client will not necessarily 680 be penalized for the acquisition delay. 682 Finally, be aware that NTP timestamps will wrap around in year 2036 683 and implementations might need to handle this eventually. Refer to 684 Section 6 of [RFC5905] for further details. 686 6. Validating Tokens 688 Upon receipt of an RTCP feedback message along with the Token 689 Verification Request message that contains a Token, nonce and 690 absolute expiration time, the server MUST validate the Token. 692 The server first applies the its own procedure for constructing the 693 Tokens by using client's IP address from the received Token 694 Verification Request message, and the nonce and absolute expiration 695 time values reported in the received Token Verification Request 696 message. The server then compares the resulting output with the 697 Token sent by the client in the Token Verification Request message. 698 If they match and the absolute expiration time has not passed yet, 699 the server declares that the Token is valid. 701 Note that if the client's IP address changes, the Token will not 702 validate. Similarly, if the client inserts an incorrect nonce or 703 absolute expiration time value in the Token Verification Request 704 message, validation will fail. It is also possible that the server 705 wants to expire the Token prematurely. In these cases, the server 706 MUST reply back to the client with a Token Verification Failure 707 message (that goes from port P3 on the server to port c1 on the 708 client). 710 In addition to the Token Verification Failure message, it is 711 RECOMMENDED that applications define an application-specific error 712 response to be sent by the server when the server detects that the 713 Token is invalid. For applications using 714 [I-D.ietf-avt-rapid-acquisition-for-rtp], this document defines a new 715 4xx-level response code in the RAMS Response Code Space Registry. A 716 client that received a Token Verification Failure message can request 717 a new Token from the server. 719 7. SDP Signaling 721 7.1. The portmapping-req Attribute 723 This new SDP attribute is used declaratively to indicate the port and 724 optionally the address for obtaining a Token. Its presence indicates 725 that a Token MUST be included in the feedback messages sent to the 726 server triggering or controlling a unicast session (See Section 4.3 727 for details). 729 The formal description of the 'portmapping-req' attribute is defined 730 by the following ABNF [RFC5234] syntax: 732 portmapping-req-attribute = "a=portmapping-req:" port [nettype space 733 addrtype space connection-address] CRLF 735 Here, 'port', 'nettype', 'addrtype' and 'connection-address' are 736 defined as specified in Section 9 of [RFC4566]. The 'portmapping- 737 req' attribute is used as a session-level or media-level attribute. 738 If used at a media level, the attribute MUST be used for a unicast 739 media stream. In the optional address value, only unicast addresses 740 are allowed; multicast addresses SHOULD NOT be used without 741 evaluating the additional security risks such as non-legit servers 742 generating fake Tokens. If the address is not specified, the 743 (source) address in the "c" line corresponding to the unicast media 744 stream is implied. 746 7.2. Requirements 748 The use of SDP for the port mapping solution normatively requires the 749 support for: 751 o The SDP grouping framework and flow identification (FID) semantics 752 [RFC5888] 754 o The RTP/AVPF profile [RFC4585] 756 o The RTCP extensions for SSM sessions with unicast feedback 757 [RFC5760] 759 o The 'multicast-rtcp' attribute [I-D.ietf-avt-rtcp-port-for-ssm] 761 o Multiplexing RTP and RTCP on a single port on both endpoints in 762 the unicast session [RFC5761] 764 7.3. Example and Discussion 766 The declarative SDP describing the scenario given in Figure 2 is 767 written as: 769 v=0 770 o=ali 1122334455 1122334466 IN IP4 nack.example.com 771 s=Local Retransmissions 772 t=0 0 773 a=group:FID 1 2 774 a=rtcp-unicast:rsi 775 m=video 41000 RTP/AVPF 98 776 i=Multicast Stream 777 c=IN IP4 233.252.0.2/255 778 a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1 ; Note 1 779 a=rtpmap:98 MP2T/90000 780 a=multicast-rtcp:41500 ; Note 1 781 a=rtcp:42000 IN IP4 192.0.2.1 ; Note 2 782 a=rtcp-fb:98 nack ; Note 2 783 a=mid:1 784 m=video 42000 RTP/AVPF 99 ; Note 3 785 i=Unicast Retransmission Stream 786 c=IN IP4 192.0.2.1 787 a=sendonly 788 a=rtpmap:99 rtx/90000 789 a=rtcp-mux ; Note 4 790 a=rtcp:42500 ; Note 5 791 a=fmtp:99 apt=98; rtx-time=5000 792 a=portmapping-req:30000 ; Note 6 793 a=mid:2 795 Figure 8: SDP describing an SSM distribution with support for 796 retransmissions from a local server 798 In this description, we highlight the following notes: 800 Note 1: The source stream is multicast from a distribution source 801 with a source IP address of 198.51.100.1 (DS) to the multicast 802 destination address of 233.252.0.2 (G) and port 41000 (P1). The 803 associated RTCP packets are multicast in the same group to port 41500 804 (P2). 806 Note 2: A retransmission server including feedback target 807 functionality with an IP address of 192.0.2.1 (S) and port of 42000 808 (P3) is specified with the 'rtcp' attribute. The feedback 809 functionality is enabled for the RTP stream with payload type 98 810 through the 'rtcp-fb' attribute [RFC4585]. 812 Note 3: The port specified in the second "m" line (for the unicast 813 stream) does not mean anything in this scenario as the client does 814 not send any RTP traffic back to the server. 816 Note 4: The server multiplexes RTP and RTCP packets on the same port 817 (c1 in Figure 2). 819 Note 5: The server uses port 42500 (P4) for the unicast sessions. 821 Note 6: The "a=portmapping-req" line indicates that a Token needs to 822 be retrieved first before a unicast session associated to the 823 multicast session can be established and that the Port Mapping 824 Request message needs to be sent to port 30000 (PT). Since there is 825 no address indiciated in this line, the client needs to retrieve the 826 Token from the address specified in the "c" line. 828 8. Address Pooling NATs 830 Large-scale NAT devices have a pool of public IPv4 addresses and map 831 internal hosts to one of those public IPv4 addresses. As long as an 832 internal host maintains an active mapping in the NAT, the same IPv4 833 address is assigned to new connections. However, once all of the 834 host's mappings have been deleted (e.g., because of timeout), it is 835 possible that a new connection from that same host will be assigned a 836 different IPv4 address from the pool. When that occurs, the Token 837 will be considered invalid by the server, causing an additional round 838 trip for the client to acquire a fresh Token. 840 Any traffic from the host which traverses the NAT will prevent this 841 problem. As the host is sending RTCP receiver reports at least every 842 5 seconds (Section 6.2 of [RFC3550]) for the multicast session it is 843 receiving, those RTCP messages will be sufficient to prevent this 844 problem. 846 9. Security Considerations 848 9.1. Tokens 850 The Token, which is generated based on a client's IP address and 851 expiration date, provides protection against denial-of-service (DoS) 852 attacks. An attacker using a certain IP address cannot cause one or 853 more RTP packets to be sent to a victim client who has a different IP 854 address. However, if the attacker acquires a valid Token for a 855 victim and can spoof the victim's source address, this approach 856 becomes vulnerable to replay attacks. This is especially easy if the 857 attacker and victim are behind a large-scale NAT and share the same 858 IP address. 860 Multicast is deployed on managed networks - not the Internet. These 861 managed networks will choose to enable network ingress filtering 862 [RFC2827] or not. If ingress filtering is enabled on a network, an 863 attacker attacker cannot spoof a victim's IP address to use a Token 864 to initiate an attack against a victim. However, if ingress 865 filtering is not enabled on a network, an attacker could obtain a 866 Token and spoof the victim's address, causing traffic to flood the 867 victim. On such a network, the server can reduce the time period for 868 such an attack by expiring a Token in a short period of time. In the 869 extreme case, the server can expire the Token in such a short period 870 of time, such that the client will have to acquire a new Token 871 immediately before using it in a Token Verification Request message. 873 HMAC-SHA1 provides a level of security that is widely regarded as 874 being more than sufficient for providing message authentication. It 875 is believed that the economic cost of breaking that algorithm is 876 significantly higher than the cost of more direct approaches to 877 violating system security, e.g., theft, bribery, wiretapping, and 878 other forms of malfeasance. HMAC-SHA1 is secure against all known 879 cryptanalytic attacks that use computational resources that are 880 currently economically feasible. 882 9.2. The portmapping-req Attribute 884 The 'portmapping-req' attribute is not believed to introduce any 885 significant security risk to multimedia applications. A malevolent 886 third party could use this attribute to redirect the Port Mapping 887 Request messages by altering the port number or cause the unicast 888 session establishment to fail by removing it from the SDP 889 description. But, this requires intercepting and rewriting the 890 packets carrying the SDP description; and if an interceptor can do 891 that, many more attacks are possible, including a wholesale change of 892 the addresses and port numbers at which the media will be sent. 894 In order to avoid attacks of this sort, the SDP description needs to 895 be integrity protected and provided with source authentication. This 896 can, for example, be achieved on an end-to-end basis using S/MIME 897 [RFC5652] when SDP is used in a signaling packet using MIME types 898 (application/sdp). Alternatively, HTTPS [RFC2818] or the 899 authentication method in the Session Announcement Protocol (SAP) 900 [RFC2974] could be used as well. 902 10. IANA Considerations 904 The following contact information shall be used for all registrations 905 in this document: 907 Ali Begen 908 abegen@cisco.com 910 Note to the RFC Editor: In the following, please replace "XXXX" with 911 the number of this document prior to publication as an RFC. 913 10.1. Registration of SDP Attributes 915 This document registers a new attribute name in SDP. 917 SDP Attribute ("att-field"): 918 Attribute name: portmapping-req 919 Long form: Port for requesting Token 920 Type of name: att-field 921 Type of attribute: Either session or media level 922 Subject to charset: No 923 Purpose: See this document 924 Reference: [RFCXXXX] 925 Values: See this document 927 10.2. Registration of FMT Values 929 Within the RTPFB range, the following format (FMT) value is 930 registered: 932 Name: Port Mapping 933 Long name: Port Mapping Between Unicast and Multicast RTP Sessions 934 Value: 7 935 Reference: [RFCXXXX] 937 10.3. SFMT Values for Port Mapping Messages Registry 939 This document creates a new sub-registry for the sub-feedback message 940 type (SFMT) values to be used with the FMT value registered for Port 941 Mapping messages. The registry is called the SFMT Values for Port 942 Mapping Messages Registry. This registry is to be managed by the 943 IANA according to the Specification Required policy of [RFC5226]. 945 The length of the SFMT field in the Port Mapping messages is a single 946 octet, allowing 256 values. The registry is initialized with the 947 following entries: 949 Value Name Reference 950 ----- -------------------------------------------------- ------------- 951 0 Reserved [RFCXXXX] 952 1 Port Mapping Request [RFCXXXX] 953 2 Port Mapping Response [RFCXXXX] 954 3 Token Verification Request [RFCXXXX] 955 4 Token Verification Failure [RFCXXXX] 956 5-254 Assignable - Specification Required 957 255 Reserved [RFCXXXX] 959 The SFMT values 0 and 255 are reserved for future use. 961 Any registration for an unassigned SFMT value needs to contain the 962 following information: 964 o Contact information of the one doing the registration, including 965 at least name, address, and email. 967 o A detailed description of what the new SFMT represents and how it 968 shall be interpreted. 970 10.4. RAMS Response Code Space Registry 972 This document adds the following entry to the RAMS Response Code 973 Space Registry. 975 Code Description Reference 976 ----- -------------------------------------------------- ------------- 977 405 Invalid Token [RFCXXXX] 979 This response code is used when the Token included by the RTP_Rx in 980 the RAMS-R message is invalid. 982 11. Acknowledgments 984 The approach presented in this document came out after discussions 985 with various individuals in the AVT and MMUSIC WGs, and the breakout 986 session held in the Anaheim meeting. We thank each of these 987 individuals, in particular to Magnus Westerlund and Colin Perkins. 989 12. References 991 12.1. Normative References 993 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 994 Jacobson, "RTP: A Transport Protocol for Real-Time 995 Applications", STD 64, RFC 3550, July 2003. 997 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 998 Requirement Levels", BCP 14, RFC 2119, March 1997. 1000 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 1001 Description Protocol", RFC 4566, July 2006. 1003 [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, 1004 "Extended RTP Profile for Real-time Transport Control 1005 Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, 1006 July 2006. 1008 [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control 1009 Protocol (RTCP) Extensions for Single-Source Multicast 1010 Sessions with Unicast Feedback", RFC 5760, February 2010. 1012 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1013 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1015 [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness 1016 Requirements for Security", BCP 106, RFC 4086, June 2005. 1018 [RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network 1019 Time Protocol Version 4: Protocol and Algorithms 1020 Specification", RFC 5905, June 2010. 1022 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- 1023 Hashing for Message Authentication", RFC 2104, 1024 February 1997. 1026 [I-D.ietf-avt-rtcp-port-for-ssm] 1027 Begen, A., "RTP Control Protocol (RTCP) Port for Source- 1028 Specific Multicast (SSM) Sessions", 1029 draft-ietf-avt-rtcp-port-for-ssm-03 (work in progress), 1030 October 2010. 1032 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 1033 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 1035 [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and 1036 Control Packets on a Single Port", RFC 5761, April 2010. 1038 12.2. Informative References 1040 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 1041 with Session Description Protocol (SDP)", RFC 3264, 1042 June 2002. 1044 [RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in 1045 the Session Description Protocol (SDP)", RFC 4145, 1046 September 2005. 1048 [I-D.ietf-avt-rapid-acquisition-for-rtp] 1049 Steeg, B., Begen, A., Caenegem, T., and Z. Vax, "Unicast- 1050 Based Rapid Acquisition of Multicast RTP Sessions", 1051 draft-ietf-avt-rapid-acquisition-for-rtp-17 (work in 1052 progress), November 2010. 1054 [RFC4787] Audet, F. and C. Jennings, "Network Address Translation 1055 (NAT) Behavioral Requirements for Unicast UDP", BCP 127, 1056 RFC 4787, January 2007. 1058 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. 1059 Hakenberg, "RTP Retransmission Payload Format", RFC 4588, 1060 July 2006. 1062 [I-D.ietf-avt-app-rtp-keepalive] 1063 Marjou, X. and A. Sollaud, "Application Mechanism for 1064 keeping alive the Network Address Translator (NAT) 1065 mappings associated to RTP flows.", 1066 draft-ietf-avt-app-rtp-keepalive-09 (work in progress), 1067 September 2010. 1069 [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering: 1070 Defeating Denial of Service Attacks which employ IP Source 1071 Address Spoofing", BCP 38, RFC 2827, May 2000. 1073 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 1074 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 1075 May 2008. 1077 [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, 1078 "Codec Control Messages in the RTP Audio-Visual Profile 1079 with Feedback (AVPF)", RFC 5104, February 2008. 1081 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 1082 RFC 5652, September 2009. 1084 [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. 1086 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 1087 Announcement Protocol", RFC 2974, October 2000. 1089 Authors' Addresses 1091 Ali Begen 1092 Cisco 1093 181 Bay Street 1094 Toronto, ON M5J 2T3 1095 Canada 1097 Email: abegen@cisco.com 1099 Dan Wing 1100 Cisco Systems, Inc. 1101 170 West Tasman Dr. 1102 San Jose, CA 95134 1103 USA 1105 Email: dwing@cisco.com 1107 Tom VanCaenegem 1108 Alcatel-Lucent 1109 Copernicuslaan 50 1110 Antwerpen, 2018 1111 Belgium 1113 Email: Tom.Van_Caenegem@alcatel-lucent.com