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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SIP WG R. Mahy 3 Internet-Draft Cisco Systems, Inc. 4 Expires: November 30, 2003 Jun 2003 6 Connection Reuse in the Session Initiation Protocol (SIP) 7 draft-mahy-sip-connect-reuse-00.txt 9 Status of this Memo 11 This document is an Internet-Draft and is in full conformance with 12 all provisions of Section 10 of RFC2026. 14 Internet-Drafts are working documents of the Internet Engineering 15 Task Force (IETF), its areas, and its working groups. Note that other 16 groups may also distribute working documents as Internet-Drafts. 18 Internet-Drafts are draft documents valid for a maximum of six months 19 and may be updated, replaced, or obsoleted by other documents at any 20 time. It is inappropriate to use Internet-Drafts as reference 21 material or to cite them other than as "work in progress." 23 The list of current Internet-Drafts can be accessed at http:// 24 www.ietf.org/ietf/1id-abstracts.txt. 26 The list of Internet-Draft Shadow Directories can be accessed at 27 http://www.ietf.org/shadow.html. 29 This Internet-Draft will expire on November 30, 2003. 31 Copyright Notice 33 Copyright (C) The Internet Society (2003). All Rights Reserved. 35 Abstract 37 When SIP entities use a connection oriented protocol to send a 38 request, they typically originate their connections from an ephemeral 39 port. The SIP protocol includes mechanisms which insure that 40 responses to a request, and new requests sent in the original 41 direction reuse an existing connection. However, new requests sent 42 in the opposite direction are unlikely to reuse the existing 43 connection. This frequently causes a pair of SIP entities to use one 44 connection for requests sent in each direction, and can result in 45 potential scaling and performance problems. This document proposes 46 requirements and a mechanism which address this deficiency. 48 Table of Contents 50 1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 51 2. Introduction and Problem Statement . . . . . . . . . . . . . . 3 52 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5 53 4. Overview of Proposed Mechanism . . . . . . . . . . . . . . . . 6 54 4.1 Authorizing an alias request . . . . . . . . . . . . . . . . . 8 55 4.2 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 9 56 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 57 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 58 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 59 Normative References . . . . . . . . . . . . . . . . . . . . . 9 60 Informational References . . . . . . . . . . . . . . . . . . . 10 61 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10 62 Intellectual Property and Copyright Statements . . . . . . . . 11 64 1. Conventions 66 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 67 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 68 document are to be interpreted as described in RFC-2119 [2]. 70 2. Introduction and Problem Statement 72 SIP [1] entities can communicate using either unreliable/ 73 connectionless (ex: UDP) or reliable/connection-oriented (ex: TCP, 74 SCTP [11]) transport protocols. When SIP entities use a 75 connection-oriented protocol (such as TCP or SCTP) to send a request, 76 they typically originate their connections from an ephemeral port. 78 In the following example, Entity A listens for SIP requests over TLS 79 [4] on TCP port 5061 (the default port for SIP over TLS over TCP), 80 but uses an ephemeral port (port 8293) for a new connection to Entity 81 B. These entities could be SIP User Agents or SIP Proxy Servers. 83 +-----------+ 8293 (UAC) 5061 (UAS) +-----------+ 84 | |--------------------------->| | 85 | Entity | | Entity | 86 | A | | B | 87 | | 5061 (UAS) | | 88 +-----------+ +-----------+ 90 The SIP protocol includes mechanisms which insure that responses to a 91 request reuse the existing connection which is typically still 92 available, and also includes provisions for reusing existing 93 connections for other requests sent by the originator of the 94 connection. However, new requests sent in the opposite direction 95 (routed from the target of the original connection toward the 96 originator of the original connection) are unlikely to reuse the 97 existing connection. This frequently causes a pair of SIP entities 98 to use one connection for requests sent in each direction, as shown 99 below. 101 +-----------+ 8293 5061 +-----------+ 102 | |.......................>| | 103 | Entity | | Entity | 104 | A | 5061 9741 | B | 105 | |<-----------------------| | 106 +-----------+ +-----------+ 108 This extra pair of connections can result in potential scaling and 109 performance problems. For example, each new connection using TLS 110 requires a TCP 3-way handshake, a handful of round-trips to establish 111 TLS, and (typically) expensive asymetric authentication and key 112 generation algorithms, and certificate verification. This 113 effectively doubles the load on each entity. Setting up a second 114 connection can also cause excessive delay (especially in networks 115 with long round-trip times) for subsequent requests, even requests in 116 the context of an existing dialog (for example a reINVITE or BYE 117 after an initial INVITE, or a NOTIFY after a SUBSCRIBE [8] or a REFER 118 [9]). 120 Consider the call flow shown below where Proxy A and Proxy B use the 121 Record-Route mechanism to stay involved in a dialog. Proxy B will 122 establish a new TLS connection just to send a BYE request. 124 INVITE -> create connection 1 125 <- 200 response over connection 1 126 ACK -> reuse connection 1 128 <- BYE create connection 2 129 -> 200 response over connection 2 131 ReINVITEs are expected to be handled automatically and rapidly in 132 order to avoid media and session state from being out of step. If a 133 reINVITE requires a new TLS connection, the reINVITE could be delayed 134 by several extra round-trip times. Depending on the round-trip time, 135 this combined delay could be perceptible or even annoying to a human 136 user. This is especially problematic for some common SIP call flows 137 (for example, the recommended example flow in figure number 4 in 3pcc 138 [7]) use many reINVITEs. 140 Consider also a call flow where a handheld organizer sends a REFER 141 request which establishes a dialog to a SIP phone. Typically this 142 would require a second connection back to the handheld to be 143 established. 145 REFER -> connection 1 146 <- 202 connection 1 147 <- NOTIFY connection 2 148 200 -> connection 2 149 INVITE -> 150 <- 200 151 <- NOTIFY connection 2 152 200 -> connection 2 154 Likewise when clusters or farms of cooperating SIP servers (for 155 example proxy servers) are configured together, SIP entities have no 156 way to prefer a server with an existing connection. For example, 157 Proxy server B has no mechanism to choose an existing connection with 158 Proxy cluster A. 160 +-----------+ 161 | | 162 | Proxy | 163 | A1 | +-----------+ 164 | | | | 165 +-----------+ | Proxy | 166 +-----------+ 8293 5061 | B | 167 | |----------------------->| | 168 | Proxy | +-----------+ 169 | A2 | 170 | | 171 +-----------+ 173 As a result, Proxy B might open a new connection to another proxy 174 server for requests sent in the opposite direction. 176 +-----------+ 177 | | 178 | Proxy | 179 | A1 | 5061 9741 +-----------+ 180 | |<.......................| | 181 +-----------+ | Proxy | 182 +-----------+ 8293 5061 | B | 183 | |----------------------->| | 184 | Proxy | +-----------+ 185 | A2 | 186 | | 187 +-----------+ 189 The rules for handling the Transport layer described in Section 18 of 190 SIP [1] do not associate incoming connections with the listening port 191 which corresponds to the same SIP entity. If the Tranport layer had 192 some way to associate these connections, then request and responses 193 originated from either node could reuse existing connections as shown 194 below. 196 +-----------+ +-----------+ 197 | | | | 198 | Node A | 8293 5061 | Node B | 199 | |<======================>| | 200 | | | | 201 +-----------+ +-----------+ 203 3. Requirements 205 1. A connection sharing mechanism SHOULD allow SIP entities to reuse 206 existing connections for requests and repsonses originated from 207 either peer in the connection. 209 2. A connection sharing mechanism SHOULD allow SIP entities to reuse 210 existing connections with closely coupled nodes which act as a 211 single SIP entity (for example a cluster of nodes acting as a 212 proxy server). 214 3. A connection sharing mechanism MUST NOT require UACs (clients) to 215 send all traffic from well-know SIP ports. 217 4. A connection sharing mechanism MUST NOT require configuring 218 ephemeral port numbers in DNS. 220 5. A connection sharing mechanism MUST prevent unauthorized 221 hijacking of other connections. 223 6. Connection sharing SHOULD persist across SIP transactions and 224 dialogs. 226 4. Overview of Proposed Mechanism 228 The proposed mechanism uses a new Via header field parameter. The 229 "alias" parameter is included in a Via header field value to indicate 230 that the originator of the request wants to create a transport layer 231 alias, so that the sent-by address becomes mapped to the current 232 connection. 234 Assuming the Via header field value shown below from the most recent 235 request arrived over a connection from 60.54.32.1 port 8241: 237 Via: SIP/2.0/TLS 60.54.32.1:5061;branch=z9hG4bKa7c8dze ;alias 239 The transport layer creates an alias, such that any requests going to 240 the "advertised address" (60.54.32.1 port 5061) are instead sent over 241 the existing connection (to the "target" of the alias) which is 242 coming from port 8241. This sharing continues as long as the target 243 connection stays up. The SIP community recommends that servers keep 244 connections up unless they need to reclaim resources, and that 245 clients keep connections up as long as they are needed. Connection 246 reuse works best when the client and the server maintain their 247 connections for long periods of time. SIP entities therefore SHOULD 248 NOT drop connections on completion of a transaction or termination of 249 a dialog. 251 Likewise when clusters or farms of cooperating SIP servers (for 252 example proxy servers) are configured together, the proposed 253 mechanism allows a SIP entity to select a server with an existing 254 connection. With the proposed mechanism, Proxy B sends requests for 255 Proxy cluster A to node A2 with whom it shares an existing 256 connection. 258 +-----------+ 259 | | 260 | Proxy | 261 | A1 | +-----------+ 262 | | | | 263 +-----------+ | Proxy | 264 +-----------+ 8293 5061 | B | 265 | |<======================>| | 266 | Proxy | +-----------+ 267 | A2 | 268 | | 269 +-----------+ 271 For example, on receipt of a message with the topmost Via header 272 shown below, the transport layer creates an alias such that requests 273 going to the advertised address (host-a.atlanta.com) are sent over 274 the target connection (from 60.54.32.1:8241). 276 Via: SIP/2.0/TLS host-a.atlanta.com;branch=z9hG4bK7c8dze ;alias 278 As a result, this has an important interaction with the DNS 279 resolution mechanisms for SIP described in RFC3263 [6]. When new 280 requests arrive for host-a.atlanta.com, proxy B still needs to 281 perform a DNS NAPTR lookup to select the transport. Once the 282 transport is selected, an SRV lookup would ordinarily occur to find 283 the appropriate port number. In this case, the transport layer uses a 284 connection reuse alias instead of performing the SRV query. 286 Below is a partial DNS zone file for atlanta.com. 288 ; NAPTR queries for the current domain (atlanta.com) 289 ; 290 ; order pref flags service regexp replacement 291 @ IN NAPTR 50 50 "s" "SIPS+D2T" "" _sips._tcp. 293 ; SRV records for the proxy use 5060/5061 294 ; 295 ;; Priority Weight Port Target 296 _sips._tcp.host-a IN SRV 0 1 5061 host-a1 297 _sips._tcp.host-a IN SRV 0 1 5061 host-a2 299 host-a1 IN A 60.54.32.1 300 host-a2 IN A 60.54.32.2 301 The existence of an alias parameter is treated as a request which 302 asks the transport layer to create an alias (named by the sent-by 303 parameter, which could be a hostname) that points to the alias 304 target (the current connection) 306 This mechanism is fully backwards compatible with existing 307 implementations. If the proposed Via parameter is not understood by 308 the recipient, it will be ignored and the two implementations will 309 revert to current behavior (two connections). 311 4.1 Authorizing an alias request 313 Authorizing connection aliases is essential to prevent connection 314 hijacking. For example a program run by a malicious user of a 315 multiuser system could attempt to hijack SIP requests destined for 316 the well-known SIP port from a large relay proxy. 318 To correctly authorize an alias, both the active connection and the 319 alias need to authenticate using the same credentials. This could be 320 accomplished using one of two mechanisms. The first (and preferred) 321 mechanism is using TLS mutual authentication, such that the 322 subjectAltName of the originator certificate corresponds to both the 323 current connection and the target address of the alias. The Via 324 sent-by address needs to be within the scope protected by the 325 certificate presented by the originator during TLS mutual 326 authentication and the received IP address needs be a valid IP 327 address for the sent-by host or hosts. In other words, the sent-by 328 address and port combination MUST be resolvable from the 329 subjectAltName of the originator certificate, and the received IP 330 address MUST be resolvable from the sent-by address. This is in 331 addition to other requirements for TLS authentication and 332 authorization discussed in SIP [1] and Locating SIP Servers [6]. 334 The second mechanism is to accept an alias if the target address of 335 the alias is equivalent (using SIP comparison rules) to a valid 336 Contact already registered by the same user. This user could be 337 authenticated through any SIP or TLS mechanism (ex: user certificate, 338 or Kerberos [10]), but would typically use Digest authentication [5]. 339 For example, if Alice registers a Contact of 123.45.67.89:5061, she 340 could inform Proxy 1 of the existence of a connection to her from 341 Proxy 2. This would allow her to preemptively originate TLS 342 connections, as her user agent may not have access to a site 343 certificate with which to authenticate incoming TLS connections. 345 +-----------+ 346 | | 347 | Proxy | 348 +-----------+ 8672 5061 | 1 | 349 | |----------------------->| | 350 | Alice | +-----------+ 351 | | +-----------+ 352 | |----------------------->| | 353 +-----------+ 8293 5061 | Proxy | 354 | 2 | 355 | | 356 +-----------+ 358 4.2 Formal Syntax 360 The following syntax specification uses the augmented Backus-Naur 361 Form (BNF) as described in RFC-2234 [3]. This document proposes to 362 extend via-params to include a a new via-alias defined below. 364 via-params = via-ttl / via-maddr / via-received / via-branch / 365 via-alias / via-extension 367 via-alias = "alias" 369 5. Security Considerations 371 This document presents requirements and a mechanism for reusing 372 existing connections easily. Connection reuse presents many 373 opportunities for abuse and hijacking, but these attacks can be 374 prevented if the guidelines in the authorization section are 375 followed. 377 6. IANA Considerations 379 This document introduces no additional considerations for IANA. 381 7. Acknowledgments 383 Thanks to Jon Peterson for helpful answers about certificate behavior 384 with SIP, Jonathan Rosenberg for his initial support of this concept, 385 and Cullen Jennings for providing a sounding board for this idea. 387 Normative References 389 [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 390 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 391 Session Initiation Protocol", RFC 3261, June 2002. 393 [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement 394 Levels", BCP 14, RFC 2119, March 1997. 396 [3] Crocker, D. and P. Overell, "Augmented BNF for Syntax 397 Specifications: ABNF", RFC 2234, November 1997. 399 [4] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and 400 P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January 401 1999. 403 [5] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., 404 Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication: 405 Basic and Digest Access Authentication", RFC 2617, June 1999. 407 [6] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol 408 (SIP): Locating SIP Servers", RFC 3263, June 2002. 410 Informational References 412 [7] Rosenberg, J., Schulzrinne, H., Camarillo, G. and J. Peterson, 413 "Best Current Practices for Third Party Call Control in the 414 Session Initiation Protocol", draft-ietf-sipping-3pcc-03 (work 415 in progress), March 2003. 417 [8] Roach, A., "Session Initiation Protocol (SIP)-Specific Event 418 Notification", RFC 3265, June 2002. 420 [9] Sparks, R., "The Session Initiation Protocol (SIP) Refer 421 Method", RFC 3515, April 2003. 423 [10] Kohl, J. and B. Neuman, "The Kerberos Network Authentication 424 Service (V5)", RFC 1510, September 1993. 426 [11] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, 427 H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson, 428 "Stream Control Transmission Protocol", RFC 2960, October 2000. 430 Author's Address 432 Rohan Mahy 433 Cisco Systems, Inc. 434 101 Cooper Street 435 Santa Cruz, CA 95060 436 USA 438 EMail: rohan@cisco.com 440 Intellectual Property Statement 442 The IETF takes no position regarding the validity or scope of any 443 intellectual property or other rights that might be claimed to 444 pertain to the implementation or use of the technology described in 445 this document or the extent to which any license under such rights 446 might or might not be available; neither does it represent that it 447 has made any effort to identify any such rights. Information on the 448 IETF's procedures with respect to rights in standards-track and 449 standards-related documentation can be found in BCP-11. 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