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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 5405 (Obsoleted by RFC 8085) -- Obsolete informational reference (is this intentional?): RFC 4566 (Obsoleted by RFC 8866) -- Obsolete informational reference (is this intentional?): RFC 5389 (Obsoleted by RFC 8489) Summary: 2 errors (**), 0 flaws (~~), 2 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group X. Marjou 3 Internet-Draft A. Sollaud 4 Intended status: Standards Track France Telecom 5 Expires: June 11, 2010 December 8, 2009 7 Application Mechanism for maintaining alive the Network Address 8 Translator (NAT) mappings associated to RTP flows. 9 draft-ietf-avt-app-rtp-keepalive-07 11 Abstract 13 This document lists the different mechanisms that enable applications 14 using Real-time Transport Protocol (RTP) to maintain their RTP 15 Network Address Translator (NAT) mappings alive. It also makes a 16 recommendation for a preferred mechanism. This document is not 17 applicable to Interactive Connectivity Establishment (ICE) agents. 19 Status of this Memo 21 This Internet-Draft is submitted to IETF in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF), its areas, and its working groups. Note that 26 other groups may also distribute working documents as Internet- 27 Drafts. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 The list of current Internet-Drafts can be accessed at 35 http://www.ietf.org/ietf/1id-abstracts.txt. 37 The list of Internet-Draft Shadow Directories can be accessed at 38 http://www.ietf.org/shadow.html. 40 This Internet-Draft will expire on June 11, 2010. 42 Copyright Notice 44 Copyright (c) 2009 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 61 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4 62 4. List of Alternatives for Performing RTP Keepalive . . . . . . 5 63 4.1. Transport Packet of 0-byte . . . . . . . . . . . . . . . . 5 64 4.2. RTP Packet with Comfort Noise Payload . . . . . . . . . . 5 65 4.3. RTCP Packets Multiplexed with RTP Packets . . . . . . . . 6 66 4.4. STUN Indication Packet . . . . . . . . . . . . . . . . . . 6 67 4.5. RTP Packet with Incorrect Version Number . . . . . . . . . 6 68 4.6. RTP Packet with Unknown Payload Type . . . . . . . . . . . 6 69 5. Recommended Solution for Keepalive Mechanism . . . . . . . . . 7 70 6. Media Format Exceptions . . . . . . . . . . . . . . . . . . . 7 71 6.1. Real-time Text Payload Format Keepalive Mechanism . . . . 8 72 7. Timing and Transport Considerations . . . . . . . . . . . . . 8 73 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 74 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 75 9.1. Registration of the SDP 'rtp-keepalive' Attribute . . . . 9 76 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 77 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 78 11.1. Normative references . . . . . . . . . . . . . . . . . . . 9 79 11.2. Informative references . . . . . . . . . . . . . . . . . . 10 80 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 82 1. Introduction 84 Documents [RFC4787] and [RFC5382] describe NAT behaviors and point 85 out that two key aspects of NAT are mappings (a.k.a. bindings) and 86 keeping them refreshed. This introduces a derived requirement for 87 applications engaged in a multimedia session involving NAT traversal: 88 they need to generate a minimum of flow activity in order to create 89 NAT mappings and maintain them. 91 When applied to applications using RTP [RFC3550], the RTP media 92 stream packets themselves normally fulfill this requirement. However 93 there exist some cases where RTP does not generate the minimum 94 required flow activity. 96 The examples are: 98 o In some RTP usages, such as SIP, agents can negotiate a 99 unidirectional media stream by using the SDP "recvonly" attribute 100 on one agent and "sendonly" on the peer, as defined in [RFC3264]. 101 RFC 3264 directs implementations not to transmit media on the 102 receiving agent. In case the agent receiving the media is located 103 in the private side of a NAT, it will never receive RTP packets 104 from the public peer if the NAT mapping has not been created. 106 o Similarly, a bidirectional media stream can be "put on hold". 107 This is accomplished by using the SDP "sendonly" or "inactive" 108 attributes. Again RFC 3264 directs implementations to cease 109 transmission of media in these cases. However, doing so may cause 110 NAT bindings to timeout, and media won't be able to come off hold. 112 o In case of audio media, if silence suppression is in use, long 113 periods of silence may cause media transmission to cease 114 sufficiently long for NAT bindings to time out. 116 o Some RTP payload formats, such as the payload format for text 117 conversation [RFC4103], may send packets so infrequently that the 118 interval exceeds the NAT binding timeouts. 120 To solve these problems, an agent therefore needs to periodically 121 send keepalive data within the outgoing RTP session of an RTP media 122 stream regardless of whether the media stream is currently inactive, 123 sendonly, recvonly or sendrecv, and regardless of the presence or 124 value of the bandwidth attribute. 126 It is important to note that the above examples also require the 127 agents to use symmetric RTP [RFC4961] in addition to RTP keepalive. 129 This document first states the requirements that must be supported to 130 perform RTP keepalives (Section 3). In a second step, the document 131 reports the different mechanisms to overcome this problem (Section 4) 132 and makes recommendations about their use. Section 5 finally states 133 the recommended solution for RTP keepalive. 135 The scope of the draft is limited to non-ICE agents. Indeed, ICE 136 agents need to follow the RTP keepalive mechanism specified in the 137 ICE specification [DRAFT-ICE]. 139 The scope of the draft is also limited to RTP flows. In particular, 140 this document does not address keepalive activity related to: 142 o Session signaling flows, such as the Session Initiation Protocol 143 (SIP). 145 o RTCP flows. 146 Recall that [RFC3550] recommends a minimum interval of 5 147 seconds and that "on hold" procedures of [RFC3264] do not 148 impact RTCP transmissions. Therefore, when in use, there is 149 always some RTCP flow activity. 151 Note that if a given media uses a codec that already integrates a 152 keepalive mechanism, no additional keepalive mechanism is required at 153 the RTP level. 155 As mentionned in Section 3.5 of [RFC5405] "It is important to note 156 that keep-alive messages are NOT RECOMMENDED for general use -- they 157 are unnecessary for many applications and can consume significant 158 amounts of system and network resources." 160 2. Terminology 162 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 163 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 164 and "OPTIONAL" are to be interpreted as described in RFC 2119 165 [RFC2119]. 167 3. Requirements 169 This section outlines the key requirements that need to be satisfied 170 in order to provide RTP media keepalive. 172 REQ-1 Some data is sent periodically within the outgoing RTP session 173 for the whole duration of the RTP media stream. 175 REQ-2 Any type of transport (e.g. UDP, TCP) MUST be supported. 177 REQ-3 Any media type (e.g. audio, video, text) MUST be supported. 179 REQ-4 Any media format (e.g. G.711, H.263) MUST be supported. 181 REQ-5 Session signaling protocols SHOULD NOT be impacted. 183 REQ-6 Impacts on existing software SHOULD be minimized. 185 REQ-7 Remote peer SHOULD NOT be impacted. 187 REQ-8 The support for RTP keepalive SHOULD be described in the SDP. 189 REQ-9 More than one mechanism MAY exist. 191 4. List of Alternatives for Performing RTP Keepalive 193 This section lists, in no particular order, some alternatives that 194 can be used to perform a keepalive message within RTP media streams. 196 4.1. Transport Packet of 0-byte 198 The application sends an empty transport packet (e.g. UDP packet, 199 DCCP packet). 201 Cons: 202 o This alternative is specific to each transport protocol. 204 4.2. RTP Packet with Comfort Noise Payload 206 The application sends an RTP packet with a comfort-noise payload 207 [RFC3389]. 209 Cons: 210 o This alternative is limited to audio formats only. 211 o Comfort Noise needs to be supported by the remote peer. 212 o Comfort Noise needs to be signalled in SDP offer/answer. 213 o The peer is likely to render comfort noise at the other side, so 214 the content of the payload (the noise level) needs to be carefully 215 chosen. 217 4.3. RTCP Packets Multiplexed with RTP Packets 219 The application sends RTCP packets in the RTP media path itself (i.e. 220 same tuples for both RTP and RTCP packets) [DRAFT-RTP-RTCP]. RTCP 221 packets therefore maintain the NAT mappings open. 223 Cons: 224 o Multiplexing RTP and RTCP must be supported by the remote peer. 225 o Multiplexing RTP and RTCP must be signalled in SDP offer/answer. 226 o Some RTCP monitoring tools expect that RTCP are not multiplexed. 228 4.4. STUN Indication Packet 230 The application sends a STUN [RFC5389] Binding Indication packet as 231 specified in ICE [DRAFT-ICE]. 233 Thanks to the RTP validity check, STUN packets will be ignored by the 234 RTP stack. 236 Cons: 237 o The sending agent needs to support STUN. 239 4.5. RTP Packet with Incorrect Version Number 241 The application sends an RTP packet with an incorrect version number, 242 which value is zero. 244 Based on RTP specification [RFC3550], the peer should perform a 245 header validity check, and therefore ignore these types of packet. 247 Cons: 248 o Only four version numbers are possible. Using one of them for RTP 249 keepalive would be wasteful. 250 o [RFC4566] and [RFC3264] mandate not to send media with inactive 251 and recvonly attributes, however this is mitigated as no real 252 media is sent with this mechanism. 254 4.6. RTP Packet with Unknown Payload Type 256 The application sends an RTP packet of 0 length with a dynamic 257 payload type that has not been negotiated by the peers (e.g. not 258 negotiated within the SDP offer/answer, and thus not mapped to any 259 media format). 261 The sequence number is incremented by one for each packet, as it is 262 sent within the same RTP session as the actual media. The timestamp 263 contains the same value a media packet would have at this time. The 264 marker bit is not significant for the keepalive packets and is thus 265 set to zero. 267 Normally the peer will ignore this packet, as RTP [RFC3550] states 268 that "a receiver MUST ignore packets with payload types that it does 269 not understand". 271 Cons: 272 o [RFC4566] and [RFC3264] mandate not to send media with inactive 273 and recvonly attributes, however this is mitigated as no real 274 media is sent with this mechanism. 276 5. Recommended Solution for Keepalive Mechanism 278 Some mechanisms do not meet the requirements as they are either 279 specific to the transport (Section 4.1), or specific to a media type 280 (Section 4.2), or waste one RTP version number (Section 4.5). These 281 mechanisms are thus NOT RECOMMENDED. 283 Other mechanisms are dependent on the capabilities of the peer 284 (Section 4.3, Section 4.4). Among these mechanisms, RTCP packets 285 multiplexed with RTP packets (Section 4.3) is desirable because it 286 reduces the number of ports used. 288 The RECOMMENDED solution is thus the "RTCP packets multiplexed with 289 RTP packets" (Section 4.3). However, when this mechanism cannot be 290 negotiated, it is RECOMMENDED to use the fallback "RTP Packet with 291 Unknown Payload Type" mechanism (Section 4.6) as it will always work. 293 When using SDP, an agent supporting the fallback solution MUST 294 indicate its support by adding an a=rtp-keepalive SDP attribute. 295 This attribute is declarative only and can not be negotiated. It 296 indicates that the fallback solution will be used if the recommended 297 solution can not be used. 299 When using the SDP offer-answer [RFC3264], the agent SHOULD offer 300 both the "a=rtcp-mux" and "a=rtp-keepalive" attributes. If "a=rtcp- 301 mux" attribute is present in the answer, the agent uses RTCP packets 302 being multiplexed on the RTP port as a keepalive. Otherwise, the 303 agent uses RTP packets with an invalid payload type as a keepalive. 305 6. Media Format Exceptions 307 When a given media format does not allow the keepalive solution 308 recommended in Section 5, an alternative mechanism SHOULD be defined 309 in the payload format specification for this media format. 311 Real-time text payload format [RFC4103] is an example of such a media 312 format. 314 6.1. Real-time Text Payload Format Keepalive Mechanism 316 Real-time text payload format [RFC4103] does not allow different 317 payloads within a same RTP session, so the fallback mechanism does 318 not work. 320 For real-time text, the RECOMMENDED solution is the "RTCP packets 321 multiplexed with RTP packets". When this mechanism cannot be 322 negotiated, it is RECOMMENDED to use an empty T140block containing no 323 data in the same manner as for the idle procedure defined in 324 [RFC4103]. 326 7. Timing and Transport Considerations 328 An application supporting this specification MUST transmit either 329 keepalive packets or media packets at least once every Tr seconds 330 during the whole duration of the media session. The minimum 331 RECOMMENDED Tr value is 15 seconds, and Tr SHOULD be configurable to 332 larger values. 334 When using the "RTCP packets multiplexed with RTP packets" solution 335 for keepalive, Tr MUST comply with the RTCP timing rules of 336 [RFC3550]. The fallback "RTP Packet with Unknown Payload Type" 337 solution uses RTP, and thus does not have these RTCP constraints. 339 Keepalive packets within a particular RTP session MUST use the tuple 340 (source IP address, source TCP/UDP ports, target IP address, target 341 TCP/UDP Port) of the regular RTP packets. 343 The agent SHOULD only send RTP keepalive when it does not send 344 regular RTP packets. 346 8. Security Considerations 348 The RTP keepalive packets are sent on the same path as regular RTP 349 media packets and may be perceived as an attack by a peer. However, 350 [RFC3550] mandates a peer to "ignore packets with payload types that 351 it does not understand". A peer that does not understand the 352 keepalive message will thus appropriately drop the received packets. 354 9. IANA Considerations 356 9.1. Registration of the SDP 'rtp-keepalive' Attribute 358 This section instructs the IANA to register the following SDP att- 359 field under the Session Description Protocol (SDP) Parameters 360 registry: 362 Contact name: xavier.marjou@orange-ftgroup.com 364 Attribute name: rtp-keepalive 366 Long-form attribute name: RTP keepalive 368 Type of attribute Media level 370 Subject to charset: No 372 Purpose of attribute: The 'rtp-keepalive' attribute declares that the 373 agent supports the fallback RTP keepalive mechanism (Section 4.6). 375 Allowed attribute values: None 377 10. Acknowledgements 379 Jonathan Rosenberg provided the major inputs for this draft via the 380 ICE specification. In addition, thanks to Alfred E. Heggestad, Colin 381 Perkins, Dan Wing, Gunnar Hellstrom, Hadriel Kaplan, Randell Jesup, 382 Remi Denis-Courmont, and Steve Casner for their useful inputs and 383 comments. 385 11. References 387 11.1. Normative references 389 [DRAFT-RTP-RTCP] 390 Perkins, C. and M. Magnus, "Multiplexing RTP Data and 391 Control Packets on a Single Port", 392 draft-ietf-avt-rtp-and-rtcp-mux-07 (work in progress), 393 August 2007. 395 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 396 Requirement Levels", BCP 14, RFC 2119, March 1997. 398 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 399 Jacobson, "RTP: A Transport Protocol for Real-Time 400 Applications", STD 64, RFC 3550, July 2003. 402 [RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)", 403 BCP 131, RFC 4961, July 2007. 405 [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines 406 for Application Designers", BCP 145, RFC 5405, 407 November 2008. 409 11.2. Informative references 411 [DRAFT-ICE] 412 Rosenberg, J., "Interactive Connectivity Establishment 413 (ICE): A Methodology for Network Address Translator (NAT) 414 Traversal for Offer/Answer Protocols", 415 draft-ietf-mmusic-ice-19 (work in progress), October 2007. 417 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 418 with Session Description Protocol (SDP)", RFC 3264, 419 June 2002. 421 [RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for 422 Comfort Noise (CN)", RFC 3389, September 2002. 424 [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text 425 Conversation", RFC 4103, June 2005. 427 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 428 Description Protocol", RFC 4566, July 2006. 430 [RFC4787] Audet, F. and C. Jennings, "Network Address Translation 431 (NAT) Behavioral Requirements for Unicast UDP", BCP 127, 432 RFC 4787, January 2007. 434 [RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. 435 Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, 436 RFC 5382, October 2008. 438 [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, 439 "Session Traversal Utilities for NAT (STUN)", RFC 5389, 440 October 2008. 442 Authors' Addresses 444 Xavier Marjou 445 France Telecom 446 2, avenue Pierre Marzin 447 Lannion 22307 448 France 450 Email: xavier.marjou@orange-ftgroup.com 452 Aurelien Sollaud 453 France Telecom 454 2, avenue Pierre Marzin 455 Lannion 22307 456 France 458 Email: aurelien.sollaud@orange-ftgroup.com