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Internet Engineering Task Force (IETF)                         H. KaplanRequest for Comments: 7332                                        OracleCategory: Standards Track                                     V. PascualISSN: 2070-1721                                                   Quobis                                                             August 2014Loop Detection Mechanisms for Session Initiation Protocol (SIP)Back-to-Back User Agents (B2BUAs)Abstract   SIP Back-to-Back User Agents (B2BUAs) can cause unending SIP request   routing loops because, as User Agent Clients, they can generate SIP   requests with new Max-Forwards values.  This document discusses the   difficulties associated with loop detection for B2BUAs and the   requirements for them to prevent infinite loops.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7332.Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Kaplan & Pascual             Standards Track                    [Page 1]

RFC 7332                Loop Detection for B2BUAs            August 2014Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .23.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .34.  B2BUA Loop-Detection Behavior . . . . . . . . . . . . . . . .35.  B2BUA Max-Forwards Behavior . . . . . . . . . . . . . . . . .46.  B2BUA Max-Breadth Behavior  . . . . . . . . . . . . . . . . .47.  Security Considerations . . . . . . . . . . . . . . . . . . .48.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .59.  References  . . . . . . . . . . . . . . . . . . . . . . . . .51.  Introduction   SIP provides a means of preventing infinite request forwarding loops   in [RFC3261], and a means of mitigating parallel forking   amplification floods in [RFC5393].  Neither document normatively   defines specific behavior for B2BUAs, however.   Unbounded SIP request loops have actually occurred in SIP deployments   numerous times.  The cause of loops is usually misconfiguration, but   the reason they have been unbounded/unending is they crossed B2BUAs   that reset the Max-Forwards value in the SIP requests they generated   on their User Agent Client (UAC) side.  Although such behavior is   technically legal per [RFC3261] because a B2BUA is a UAC, the   resulting unbounded loops have caused service outages and make   troubleshooting difficult.   Furthermore, [RFC5393] also provides a mechanism to mitigate the   impact of parallel forking amplification issues, through the use of a   "Max-Breadth" header field.  If a B2BUA does not pass this header   field on, parallel forking amplification is not mitigated with the   [RFC5393] mechanism.   This document defines normative requirements for Max-Forwards and   Max-Breadth header field behaviors of B2BUAs, in order to mitigate   the effect of loops and parallel forking amplification.2.  Conventions   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inBCP 14,RFC 2119   [RFC2119].   B2BUA terminology and taxonomy used in this document is based on   [RFC7092].Kaplan & Pascual             Standards Track                    [Page 2]

RFC 7332                Loop Detection for B2BUAs            August 20143.  Background   Within the context of B2BUAs, the scope of the SIP protocol ends at   the User Agent Server (UAS) side of the B2BUA, and a new one begins   on the UAC side.  A B2BUA is thus capable of choosing what it wishes   to do on its UAC side independently of its UAS side, and still   remains compliant with [RFC3261] and its extensions.  For example,   any B2BUA type defined in [RFC7092] other than Proxy-B2BUA may create   the SIP request on its UAC side without copying any of the Via header   field values received on its UAS side.  Indeed there are valid   reasons for it to do so; however, this prevents the Via-based loop-   detection mechanism defined in [RFC3261] and updated by [RFC5393]   from detecting SIP request loops any earlier than by reaching a Max-   Forwards limit.   Some attempts have been made by B2BUA vendors to detect request loops   in other ways: by keeping track of the number of outstanding dialog-   forming requests for a given caller/called URI pair; or by detecting   when they receive and send their own media addressing information too   many times in certain cases when they are a signaling/media-plane   B2BUA; or by encoding a request instance identifier in some field   they believe will pass through other nodes, and detecting when they   see the same value too many times.   All of these methods are brittle and prone to error, however.  They   are brittle because it is very hard to accurately define when a value   has been seen "too many times".  Requests can and do fork before and   after B2BUAs process them, and requests legitimately spiral in some   cases, leading to incorrect determination of loops.  The mechanisms   are prone to error because there can be other B2BUAs in the loop's   path that interfere with the particular mechanism being used.   Ultimately, the last defense against loops becoming unbounded is to   limit how many SIP hops any request can traverse, which is the   purpose of the SIP Max-Forwards field value.  If B2BUAs were to at   least copy and decrement the Max-Forwards header field value from   their UAS to the UAC side, loops would not continue indefinitely.4.  B2BUA Loop-Detection Behavior   It is RECOMMENDED that B2BUAs implement the loop-detection mechanism   for the Via header field, as defined for a proxy in [RFC5393].Kaplan & Pascual             Standards Track                    [Page 3]

RFC 7332                Loop Detection for B2BUAs            August 20145.  B2BUA Max-Forwards Behavior   This section applies for dialog-forming and out-of-dialog SIP   requests.  B2BUAs MAY perform the same actions for in-dialog   requests, but doing so may cause issues with devices that set Max-   Forwards values based upon the number of received Via or Record-Route   headers.   All B2BUA types MUST copy the received Max-Forwards header field from   the received SIP request on their UAS side, to any request(s) they   generate on their UAC side, and decrement the value, as if they were   a proxy following the requirements described in [RFC3261].   Being a UAS, B2BUAs MUST also check the received Max-Forwards header   field and reject or respond to the request if the value is zero, as   defined in [RFC3261].   If the received request did not contain a Max-Forwards header field,   one MUST be created in any request generated in the UAC side, as   described for proxies inSection 16.6, Step 3 of [RFC3261].  As in   that specification, the value of the new Max-Forwards header SHOULD   be 70.6.  B2BUA Max-Breadth Behavior   All B2BUA types MUST copy the received Max-Breadth header field from   the received SIP request on their UAS side, to any request(s) they   generate on their UAC side, as if they were a proxy following the   requirements described in [RFC5393].   B2BUAs of all types MUST follow the requirements imposed on Proxies   as described inSection 5.3.3 of [RFC5393], including generating the   header field if none is received, limiting its maximum value, etc.   B2BUAs that generate parallel requests on their UAC side for a single   incoming request on the UAS side MUST also follow the rules for Max-   Breadth handling in [RFC5393] as if they were a parallel forking   proxy.7.  Security Considerations   The security implications for parallel forking amplification are   documented inSection 7 of [RFC5393].  This document does not   introduce any additional issues beyond those discussed in [RFC5393].   Some B2BUAs reset the Max-Forwards and Max-Breadth header field   values in order to obfuscate the number of hops a request has already   traversed, as a privacy or security concern.  Such goals are at oddsKaplan & Pascual             Standards Track                    [Page 4]

RFC 7332                Loop Detection for B2BUAs            August 2014   with the mechanisms in this document, and administrators can decide   which they consider more important: obfuscation vs. loop detection.   In order to comply with this RFC, manufacturers MUST comply with the   normative rules defined herein by default, but MAY provide user-   configurable overrides as they see fit.8.  Acknowledgments   Thanks to Brett Tate (Broadsoft), Andrew Hutton (Unify), and Anton   Roman (Quobis) for their review of the document.9.  References9.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,              A., Peterson, J., Sparks, R., Handley, M., and E.              Schooler, "SIP: Session Initiation Protocol",RFC 3261,              June 2002.   [RFC5393]  Sparks, R., Lawrence, S., Hawrylyshen, A., and B. Campen,              "Addressing an Amplification Vulnerability in Session              Initiation Protocol (SIP) Forking Proxies",RFC 5393,              December 2008.9.2.  Informative References   [RFC7092]  Kaplan, H. and V. Pascual, "A Taxonomy of Session              Initiation Protocol (SIP) Back-to-Back User Agents",RFC7092, December 2013.Authors' Addresses   Hadriel Kaplan   Oracle   EMail: hadrielk@yahoo.com   Victor Pascual   Quobis   EMail: victor.pascual@quobis.comKaplan & Pascual             Standards Track                    [Page 5]