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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 4634 (Obsoleted by RFC 6234) ** Obsolete normative reference: RFC 5342 (Obsoleted by RFC 7042) -- Obsolete informational reference (is this intentional?): RFC 4941 (Obsoleted by RFC 8981) Summary: 2 errors (**), 0 flaws (~~), 1 warning (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 AVT A. Begen 3 Internet-Draft Cisco 4 Updates: 3550 (if approved) C. Perkins 5 Intended status: Standards Track University of Glasgow 6 Expires: July 30, 2011 D. Wing 7 Cisco 8 January 26, 2011 10 Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names 11 (CNAMEs) 12 draft-ietf-avt-rtp-cnames-05 14 Abstract 16 The RTP Control Protocol (RTCP) Canonical Name (CNAME) is a 17 persistent transport-level identifier for an RTP endpoint. While the 18 Synchronization Source (SSRC) identifier of an RTP endpoint may 19 change if a collision is detected, or when the RTP application is 20 restarted, its RTCP CNAME is meant to stay unchanged, so that RTP 21 endpoints can be uniquely identified and associated with their RTP 22 media streams. For proper functionality, RTCP CNAMEs should be 23 unique within the participants of an RTP session. However, the 24 existing guidelines for choosing the RTCP CNAME provided in the RTP 25 standard are insufficient to achieve this uniqueness. This memo 26 updates those guidelines to allow endpoints to choose unique RTCP 27 CNAMEs. 29 Status of this Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at http://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on July 30, 2011. 46 Copyright Notice 48 Copyright (c) 2011 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 64 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . . 3 65 3. Deficiencies with Earlier Guidelines for Choosing an RTCP 66 CNAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 67 4. Choosing an RTCP CNAME . . . . . . . . . . . . . . . . . . . . 4 68 4.1. Persistent RTCP CNAMEs vs. Per-Session RTCP CNAMEs . . . . 4 69 4.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5 70 5. Procedure to Generate a Unique Identifier . . . . . . . . . . . 6 71 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 72 6.1. Considerations on Uniqueness of RTCP CNAMEs . . . . . . . . 7 73 6.2. Session Correlation Based on RTCP CNAMEs . . . . . . . . . 7 74 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 75 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8 76 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 77 9.1. Normative References . . . . . . . . . . . . . . . . . . . 8 78 9.2. Informative References . . . . . . . . . . . . . . . . . . 9 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 81 1. Introduction 83 In Section 6.5.1 of the RTP specification, [RFC3550], there are a 84 number of recommendations for choosing a unique RTCP CNAME for an RTP 85 endpoint. However, in practice, some of these methods are not 86 guaranteed to produce a unique RTCP CNAME. This memo updates 87 guidelines for choosing RTCP CNAMEs, superceding those presented in 88 Section 6.5.1 of [RFC3550]. 90 2. Requirements Notation 92 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 93 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 94 "OPTIONAL" in this document are to be interpreted as described in 95 [RFC2119]. 97 3. Deficiencies with Earlier Guidelines for Choosing an RTCP CNAME 99 The recommendation in [RFC3550] is to generate an RTCP CNAME of the 100 form "user@host" for multiuser systems, or "host" if the username is 101 not available. The "host" part is specified to be the fully 102 qualified domain name (FQDN) of the host from which the real-time 103 data originates. While this guidance was appropriate at the time 104 [RFC3550] was written, FQDNs are no longer necessarily unique, and 105 can sometimes be common across several endpoints in large service 106 provider networks. This document replaces the use of FQDN as an RTCP 107 CNAME by alternative mechanisms. 109 IPv4 addresses are also suggested for use in RTCP CNAMEs in 110 [RFC3550], where the "host" part of the RTCP CNAME is the numeric 111 representation of the IPv4 address of the interface from which the 112 RTP data originates. As noted in [RFC3550], the use of private 113 network address space [RFC1918] can result in hosts having network 114 addresses that are not globally unique. Additionally, this shared 115 use of the same IPv4 address can also occur with public IPv4 116 addresses if multiple hosts are assigned the same public IPv4 address 117 and connected to a Network Address Translation (NAT) device 118 [RFC3022]. When multiple hosts share the same IPv4 address, whether 119 private or public, using the IPv4 address as the RTCP CNAME leads to 120 RTCP CNAMEs that are not necessarily unique. 122 It is also noted in [RFC3550] that if hosts with private addresses 123 and no direct IP connectivity to the public Internet have their RTP 124 packets forwarded to the public Internet through an RTP-level 125 translator, they may end up having non-unique RTCP CNAMEs. The 126 suggestion in [RFC3550] is that such applications provide a 127 configuration option to allow the user to choose a unique RTCP CNAME, 128 and puts the burden on the translator to translate RTCP CNAMEs from 129 private addresses to public addresses if necessary to keep private 130 addresses from being exposed. Experience has shown that this does 131 not work well in practice. 133 4. Choosing an RTCP CNAME 135 It is difficult, and in some cases impossible, for a host to 136 determine if there is a NAT between itself and its RTP peer. 137 Furthermore, even some public IPv4 addresses can be shared by 138 multiple hosts in the Internet. Using the numeric representation of 139 the IPv4 address as the "host" part of the RTCP CNAME is NOT 140 RECOMMENDED. 142 4.1. Persistent RTCP CNAMEs vs. Per-Session RTCP CNAMEs 144 The RTCP CNAME can either be persistent across different RTP sessions 145 for an RTP endpoint, or it can be unique per session, meaning that an 146 RTP endpoint chooses a different RTCP CNAME for each RTP session. 148 An RTP endpoint that is emitting multiple related RTP streams that 149 require synchronization at the other endpoint(s) MUST use the same 150 RTCP CNAME for all streams that are to be synchronized. This 151 requires a short-term persistent RTCP CNAME that is common across 152 several RTP flows, and potentially across several related RTP 153 sessions. A common example of such use occurs when lip-syncing audio 154 and video streams in a multimedia session, where a single participant 155 has to use the same RTCP CNAME for its audio RTP session and for its 156 video RTP session. Another example might be to synchronize the 157 layers of a layered audio codec, where the same RTCP CNAME has to be 158 used for each layer. 160 A longer-term persistent RTCP CNAME is sometimes useful to facilitate 161 third-party monitoring, consistent with [RFC3550]. One such use 162 might be to allow network management tools to correlate the ongoing 163 quality of service for a participant across multiple RTP sessions for 164 fault diagnosis, and to understand long-term network performance 165 statistics. An implementation that wishes to discourage this type of 166 third-party monitoring can generate a unique RTCP CNAME for each RTP 167 session, or group of related RTP sessions, that it joins. Such a 168 per-session RTCP CNAME cannot be used for traffic analysis, and so 169 provides some limited form of privacy (note that there are non-RTP 170 means that can be used by a third-party to correlate RTP sessions, so 171 the use of per-session RTCP CNAMEs will not prevent a determined 172 traffic analyst). 174 This memo defines several different ways by which an implementation 175 can choose an RTCP CNAME. It is possible, and legitimate, for 176 independent implementations to make different choices of RTCP CNAME 177 when running on the same host. This can hinder third-party 178 monitoring, unless some external means is provided to configure a 179 persistent choice of RTCP CNAME for those implementations. 181 Note that there is no backwards compatibility issue (with [RFC3550]- 182 compatible implementations) introduced in this memo, since the RTCP 183 CNAMEs are opaque strings to remote peers. 185 4.2. Requirements 187 RTP endpoints will choose to generate RTCP CNAMEs that are persistent 188 or per-session. An RTP endpoint that wishes to generate a persistent 189 RTCP CNAME MUST use one of the following two methods: 191 o To produce a long-term persistent RTCP CNAME, an RTP endpoint MUST 192 generate and store a Universally Unique IDentifier (UUID) 193 [RFC4122] for use as the "host" part of its RTCP CNAME. The UUID 194 MUST be version 1, 2 or 4 described in [RFC4122], with the 195 "urn:uuid:" stripped, resulting in a 36-octet printable string 196 representation. 198 o To produce a short-term persistent RTCP CNAME, an RTP endpoint 199 MUST use either (a) the numeric representation of the layer-2 200 (MAC) address of the interface that is used to initiate the RTP 201 session as the "host" part of its RTCP CNAME or (b) generate an 202 identifier by following the procedure described in Section 5. In 203 either case, the procedure is performed once per initialization of 204 the software. After obtaining a identifier by doing (a) or (b), 205 the least significant 48 bits are converted to the standard colon- 206 separated hexadecimal format [RFC5342], e.g., "00:23:32:af:9b:aa", 207 resulting in a 17-octet printable string representation. 209 In the two cases above, the "user@" part of the RTCP CNAME MAY be 210 omitted on single-user systems, and MAY be replaced by an opaque 211 token on multi-user systems, to preserve privacy. 213 An RTP endpoint that wishes to generate a per-session RTCP CNAME MUST 214 use the following method: 216 o For every new RTP session, a new CNAME is generated following the 217 procedure described in Section 5. After performing that 218 procedure, the least significant 96 bits are used to generate an 219 identifier (to compromise between packet size and security) which 220 is converted ASCII using Base64 encoding [RFC4648]. This results 221 in a 16-octet string representation. The RTCP CNAME cannot change 222 over the life of an RTP session [RFC3550], hence, only the initial 223 SSRC value chosen by the endpoint is used. The "user@" part of 224 the RTCP CNAME is omitted when generating per-session RTCP CNAMEs. 226 It is believed that obtaining uniqueness (with a high probability) is 227 an important property that requires careful evaluation of the method. 228 This document provides a number of methods, at least one of which 229 would be suitable for all deployment scenarios. This document 230 therefore does not provide for the implementor to define and select 231 an alternative method. 233 A future specification might define an alternative method for 234 generating RTCP CNAMEs as long as the proposed method has appropriate 235 uniqueness, and there is consistency between the methods used for 236 multiple RTP sessions that are to be correlated. However, such a 237 specification needs to be reviewed and approved before deployment. 239 The mechanisms described in this document are to be used to generate 240 RTCP CNAMEs, and they are not to be used for generating general- 241 purpose unique identifiers. 243 5. Procedure to Generate a Unique Identifier 245 The algorithm described below is intended to be used for locally- 246 generated unique identifier. 248 1. Obtain the current time of day in 64-bit NTP format [RFC5905]. 250 2. Obtain a modified EUI-64 identifier from the system running this 251 algorithm [RFC4291]. If this does not exist, one can be created 252 from a 48-bit MAC address as specified in [RFC4291]. If one 253 cannot be obtained or created, a suitably unique identifier, 254 local to the node, should be used (e.g., system serial number). 256 3. Concatenate the time of day with the system-specific identifier 257 in order to create a key. 259 4. If generating a per-session CNAME, also concatenate RTP 260 endpoint's initial SSRC, the source and destination IP addresses, 261 and ports to the key. 263 5. Compute an SHA-256 digest on the key as specified in [RFC4634], 264 which outputs 256 bits. 266 6. Security Considerations 268 The security considerations of [RFC3550] apply to this memo. 270 6.1. Considerations on Uniqueness of RTCP CNAMEs 272 The recommendations on RTCP CNAME generation in this document ensure 273 that a set of cooperating participants in an RTP session will have 274 unique RTCP CNAMEs with very high probability. However, neither 275 [RFC3550] nor this document provides any way to ensure that 276 participants will choose RTCP CNAMEs appropriately, and thus 277 implementations MUST NOT rely on the uniqueness of CNAMEs for any 278 essential security services. This is consistent with [RFC3550], 279 which does not require that RTCP CNAMEs are unique within a session, 280 but instead says that condition SHOULD hold. As described in the 281 Security Considerations section of [RFC3550], because each 282 participant in a session is free to choose its own RTCP CNAME, they 283 can do so in such a way as to impersonate another participant. That 284 is, participants are trusted to not impersonate each other. No 285 recommendation for generating RTCP CNAMEs can prevent this 286 impersonation, because an attacker can neglect the stipulation. 287 Secure RTP (SRTP) [RFC3711] keeps unauthorized entities out of an RTP 288 session, but it does not not aim to prevent impersonation attacks 289 from unauthorized entities. 291 This document uses a hash function to ensure the uniqueness of RTCP 292 CNAMEs. A cryptographic hash function is used because such functions 293 provide the randomness properties that are needed. However, no 294 security assumptions are made on the hash function. The hash 295 function is not assumed to be collision-resistant, preimage-resistant 296 or second-preimage resistant in an adversarial setting; as described 297 above, an attacker attempting an impersonation attack could merely 298 set the RTCP CNAME directly rather than attacking the hash function. 299 Similarly, the hash function is not assumed to be a one-way function, 300 or pseudorandom in a cryptographic sense. 302 No confidentiality is provided on the data used as input to the RTCP 303 CNAME generation algorithm. It might be possible for an attacker who 304 observes an RTCP CNAME to determine the inputs that were used to 305 generate that value. 307 6.2. Session Correlation Based on RTCP CNAMEs 309 In some environments, notably telephony, a fixed RTCP CNAME value 310 allows separate RTP sessions to be correlated and eliminates the 311 obfuscation provided by IPv6 privacy addresses [RFC4941] or IPv4 NAPT 312 [RFC3022]. SRTP [RFC3711] can help prevent such correlation by 313 encrypting Secure RTCP (SRTCP) but it should be noted that SRTP only 314 mandates SRTCP integrity protection (not encryption). Thus, RTP 315 applications used in such environments should consider encrypting 316 their SRTCP or generate a per-session RTCP CNAME as discussed in 317 Section 4.1. 319 7. IANA Considerations 321 No IANA actions are required. 323 8. Acknowledgments 325 Thanks to Marc Petit-Huguenin who suggested to use UUIDs in 326 generating RTCP CNAMEs. Also thanks to David McGrew for providing 327 text for the Security Considerations section. 329 9. References 331 9.1. Normative References 333 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 334 Jacobson, "RTP: A Transport Protocol for Real-Time 335 Applications", STD 64, RFC 3550, July 2003. 337 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 338 Requirement Levels", BCP 14, RFC 2119, March 1997. 340 [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally 341 Unique IDentifier (UUID) URN Namespace", RFC 4122, 342 July 2005. 344 [RFC4634] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms 345 (SHA and HMAC-SHA)", RFC 4634, July 2006. 347 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 348 Encodings", RFC 4648, October 2006. 350 [RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network 351 Time Protocol Version 4: Protocol and Algorithms 352 Specification", RFC 5905, June 2010. 354 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing 355 Architecture", RFC 4291, February 2006. 357 [RFC5342] Eastlake, D., "IANA Considerations and IETF Protocol Usage 358 for IEEE 802 Parameters", BCP 141, RFC 5342, 359 September 2008. 361 9.2. Informative References 363 [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and 364 E. Lear, "Address Allocation for Private Internets", 365 BCP 5, RFC 1918, February 1996. 367 [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network 368 Address Translator (Traditional NAT)", RFC 3022, 369 January 2001. 371 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 372 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 373 RFC 3711, March 2004. 375 [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy 376 Extensions for Stateless Address Autoconfiguration in 377 IPv6", RFC 4941, September 2007. 379 Authors' Addresses 381 Ali Begen 382 Cisco 383 181 Bay Street 384 Toronto, ON M5J 2T3 385 CANADA 387 Email: abegen@cisco.com 389 Colin Perkins 390 University of Glasgow 391 School of Computing Science 392 Glasgow, G12 8QQ 393 UK 395 Email: csp@csperkins.org 396 Dan Wing 397 Cisco Systems, Inc. 398 170 West Tasman Dr. 399 San Jose, CA 95134 400 USA 402 Email: dwing@cisco.com