Internet Engineering Task Force (IETF)                         H. Kaplan
Request for Comments: 7332                                        Oracle
Category: Standards Track                                     V. Pascual
ISSN: 2070-1721                                                   Quobis
                                                             August 2014

    Loop 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 in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7332.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  B2BUA Loop-Detection Behavior . . . . . . . . . . . . . . . .   3
   5.  B2BUA Max-Forwards Behavior . . . . . . . . . . . . . . . . .   4
   6.  B2BUA Max-Breadth Behavior  . . . . . . . . . . . . . . . . .   4
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  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 in BCP 14, RFC 2119
   [RFC2119].

   B2BUA terminology and taxonomy used in this document is based on
   [RFC7092].

3.  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].

5.  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 in Section 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 in Section 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 in Section 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 odds

   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.  References

9.1.  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", RFC
              7092, December 2013.

Authors' Addresses

   Hadriel Kaplan
   Oracle

   EMail: hadrielk@yahoo.com

   Victor Pascual
   Quobis

   EMail: victor.pascual@quobis.com