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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-03) exists of draft-ietf-straw-b2bua-taxonomy-02 ** Downref: Normative reference to an Informational draft: draft-ietf-straw-b2bua-taxonomy (ref. 'I-D.ietf-straw-b2bua-taxonomy') Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 STRAW Working Group H. Kaplan 3 Internet-Draft Oracle 4 Intended status: Standards Track V. Pascual 5 Expires: February 04, 2014 Consultant 6 August 03, 2013 8 Loop Detection Mechanisms for Session Initiation Protocol (SIP) Back-to- 9 Back User Agents (B2BUAs) 10 draft-ietf-straw-b2bua-loop-detection-01.txt 12 Abstract 14 SIP Back-to-Back User Agents (B2BUAs) can cause unending SIP request 15 routing loops because, as User Agent Clients, they can generate SIP 16 requests with new Max-Forwards values. This document discusses the 17 difficulties associated with loop detection for B2BUAs, and 18 requirements for them to prevent infinite loops. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on February 04, 2014. 37 Copyright Notice 39 Copyright (c) 2013 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 2 56 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 57 4. B2BUA Loop-Detection Behavior . . . . . . . . . . . . . . . . 3 58 5. B2BUA Max-Forwards Behavior . . . . . . . . . . . . . . . . . 4 59 6. B2BUA Max-Breadth Behavior . . . . . . . . . . . . . . . . . 4 60 7. Security Considerations . . . . . . . . . . . . . . . . . . . 4 61 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 62 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5 63 10. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 5 64 11. Normative References . . . . . . . . . . . . . . . . . . . . 5 65 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 67 1. Introduction 69 SIP provides a means of preventing infinite request forwarding loops 70 in [RFC3261], and a means of mitigating parallel forking 71 amplification floods in [RFC5393]. Neither document normatively 72 defines specific behavior for B2BUAs, however. 74 Unbounded SIP request loops have actually occurred in SIP 75 deployments, numerous times. The cause of loops is usually mis- 76 configuration, but the reason they have been unbounded/unending is 77 they crossed B2BUAs that reset the Max-Forwards value in the SIP 78 requests they generated on their UAC side. Although such behavior is 79 technically legal per [RFC3261] because a B2BUA is a UAC, the 80 resulting unbounded loops have caused service outages and make 81 troubleshooting difficult. 83 Furthermore, [RFC5393] also provides a mechanism to mitigate the 84 impact of parallel forking amplification issues, through the use of a 85 "Max-Breadth" header field. If a B2BUA does not pass on this header 86 field, parallel forking amplification is not mitigated with the 87 [RFC5393] mechanism. 89 This document defines normative requirements for Max-Forwards and 90 Max-Breadth header field behaviors of B2BUAs, in order to mitigate 91 the effect of loops and parallel forking amplification. 93 2. Conventions 95 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 96 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 97 document are to be interpreted as described in BCP 14, RFC 2119 98 [RFC2119]. 100 B2BUA terminology and taxonomy used in this document is based on 101 [I-D.ietf-straw-b2bua-taxonomy] 103 3. Background 105 Within the context of B2BUAs, the scope of the SIP protocol ends at 106 the UAS side of the B2BUA, and a new one begins on the UAC side. A 107 B2BUA is thus capable of choosing what it wishes to do on its UAC 108 side independently of its UAS side, and still remain compliant to 109 [RFC3261] and its extensions. For example, any B2BUA type defined in 110 [I-D.ietf-straw-b2bua-taxonomy] other than Proxy-B2BUA may create the 111 SIP request on its UAC side without copying any of the Via header 112 field values received on its UAS side. Indeed there are valid 113 reasons for it to do so; however this prevents the Via-based loop- 114 detection mechanism defined in [RFC3261] and updated by [RFC5393] 115 from detecting SIP request loops any earlier than by reaching a Max- 116 Forwards limit. 118 Some attempts have been made by B2BUA vendors to detect request loops 119 in other ways: by keeping track of the number of outstanding dialog- 120 forming requests for a given caller/called URI pair; or by detecting 121 when they receive and send their own media addressing information too 122 many times in certain cases when they are a Media- plane B2BUA; or by 123 encoding a request instance identifier in some field they believe 124 will pass through other nodes, and detecting when they see the same 125 value too many times. 127 All of these methods are brittle and prone to error, however. They 128 are brittle because the definition of when a value has been seen "too 129 many times" is very hard to accurately determine; requests can and do 130 fork before and after B2BUAs process them, and requests legitimately 131 spiral in some cases, leading to incorrect determination of loops. 132 The mechanisms are prone to error because there can be other B2BUAs 133 in the loop's path that interfere with the particular mechanism being 134 used. 136 Ultimately, the last defense against loops becoming unbounded is to 137 limit how many SIP hops any request can traverse, which is the 138 purpose of the SIP Max-Forwards field value. If B2BUAs were to at 139 least copy and decrement the Max-Forwards header field value from 140 their UAS to the UAC side, loops would not continue indefinitely. 142 4. B2BUA Loop-Detection Behavior 143 It is RECOMMENDED that B2BUAs implement the loop-detection mechanism 144 for the Via header field, as defined for a Proxy in [RFC5393]. 146 5. B2BUA Max-Forwards Behavior 148 All B2BUA types MUST copy the received Max-Forwards header field from 149 the received SIP request on their UAS side, to any request(s) they 150 generate on their UAC side, and decrement the value, as if they were 151 a Proxy following [RFC3261]. 153 Being a UAS, B2BUAs MUST also check the received Max-Forwards header 154 field and reject or respond to the request if the value is zero, as 155 defined in [RFC3261]. 157 If the received request did not contain a Max-Forwards header field, 158 one MUST be created in any requests generated in the UAC side, which 159 SHOULD be 70, as described for Proxies in section 16.6 part 3 of 160 [RFC3261]. 162 For B2BUAs that remove Record-Route headers, they MUST only perform 163 the copying and checking rules above for out-of-dialog requests. The 164 reason for this is other User Agents might send in-dialog requests 165 using a very low Max-Forwards value, based on the number of Record- 166 Route headers they received. 168 6. B2BUA Max-Breadth Behavior 170 All B2BUA types MUST copy the received Max-Breadth header field from 171 the received SIP request on their UAS side, to any request(s) they 172 generate on their UAC side, as if they were a Proxy following 173 [RFC5393]. 175 B2BUAs of all types MUST follow the requirements imposed on Proxies 176 as described in section 5.3.3 of [RFC5393], including generating the 177 header field if none is received, limiting its maximum value, etc. 179 B2BUAs that generate parallel requests on their UAC side for a single 180 incoming request on the UAS side MUST also follow the rules for Max- 181 Breadth handling in [RFC5393] as if they were a parallel forking 182 Proxy. 184 7. Security Considerations 186 The security implications for parallel forking amplification are 187 documented in section 7 of [RFC5393]. This document does not add any 188 additional issues beyond those discussed in [RFC5393]. 190 Some B2BUAs reset the Max-Forwards and Max-Breadth header field 191 values in order to obfuscate the number of hops a request has already 192 traversed, as a privacy or security concern. Such goals are at odds 193 with the mechanisms in this document, and administrators can decide 194 which they consider more important: obfuscation vs. loop detection. 195 In order to comply with this RFC, manufacturers MUST comply with the 196 normative rules defined herein by default, but MAY provide user- 197 configurable overrides as they see fit. 199 8. IANA Considerations 201 This document makes no request of IANA. 203 9. Acknowledgments 205 Funding for the RFC Editor function is provided by the IETF 206 Administrative Support Activity (IASA). Thanks to Brett Tate for his 207 review of the document. 209 10. Change Log 211 [RFC EDITOR NOTE: Please remove this section when publishing] 213 draft-ietf-straw-b2bua-loop-detection-00 -- WG version. Editorial 214 updates 216 11. Normative References 218 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 219 Requirement Levels", BCP 14, RFC 2119, March 1997. 221 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 222 A., Peterson, J., Sparks, R., Handley, M., and E. 223 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 224 June 2002. 226 [RFC5393] Sparks, R., Lawrence, S., Hawrylyshen, A., and B. Campen, 227 "Addressing an Amplification Vulnerability in Session 228 Initiation Protocol (SIP) Forking Proxies", RFC 5393, 229 December 2008. 231 [I-D.ietf-straw-b2bua-taxonomy] 232 Kaplan, H., "A Taxonomy of Session Initiation Protocol 233 (SIP) Back-to-Back User Agents", draft-ietf-straw-b2bua- 234 taxonomy-02 (work in progress), February 2013. 236 Authors' Addresses 237 Hadriel Kaplan 238 Oracle 240 Email: hadriel.kaplan@oracle.com 242 Victor Pascual 243 Consultant 245 Email: victor.pascual.avila@gmail.com