Loop Detection Mechanisms for Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
RFC 7332

Document Type RFC - Proposed Standard (August 2014; Errata)
Last updated 2015-10-14
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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.

   This document is subject to BCP 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 2014

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].

Kaplan & Pascual             Standards Track                    [Page 2]
RFC 7332                Loop Detection for B2BUAs            August 2014
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