STRAW Working Group                                            H. Kaplan
Internet-Draft                                                    Oracle
Intended status: Standards Track                              V. Pascual
Expires: February 04, 2014                                    Consultant
                                                         August 03, 2013

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


   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
   requirements for them to prevent infinite loops.

Status of This Memo

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   This Internet-Draft will expire on February 04, 2014.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   10. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   11. Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   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 mis-
   configuration, 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 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 on this header
   field, 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",

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   document are to be interpreted as described in BCP 14, RFC 2119

   B2BUA terminology and taxonomy used in this document is based on

3.  Background

   Within the context of B2BUAs, the scope of the SIP protocol ends at
   the 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 remain compliant to
   [RFC3261] and its extensions.  For example, any B2BUA type defined in
   [I-D.ietf-straw-b2bua-taxonomy] 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 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 the definition of when a value has been seen "too
   many times" is very hard to accurately determine; 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

   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

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   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

   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 [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 requests generated in the UAC side, which
   SHOULD be 70, as described for Proxies in section 16.6 part 3 of

   For B2BUAs that remove Record-Route headers, they MUST only perform
   the copying and checking rules above for out-of-dialog requests.  The
   reason for this is other User Agents might send in-dialog requests
   using a very low Max-Forwards value, based on the number of Record-
   Route headers they received.

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

   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

7.  Security Considerations

   The security implications for parallel forking amplification are
   documented in section 7 of [RFC5393].  This document does not add any
   additional issues beyond those discussed in [RFC5393].

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   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.  IANA Considerations

   This document makes no request of IANA.

9.  Acknowledgments

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).  Thanks to Brett Tate for his
   review of the document.

10.  Change Log

   [RFC EDITOR NOTE: Please remove this section when publishing]

   draft-ietf-straw-b2bua-loop-detection-00 -- WG version.  Editorial

11.  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.

              Kaplan, H., "A Taxonomy of Session Initiation Protocol
              (SIP) Back-to-Back User Agents", draft-ietf-straw-b2bua-
              taxonomy-02 (work in progress), February 2013.

Authors' Addresses

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   Hadriel Kaplan


   Victor Pascual


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