Internet DRAFT - draft-ietf-straw-b2bua-loop-detection
draft-ietf-straw-b2bua-loop-detection
STRAW Working Group H. Kaplan
Internet-Draft Oracle
Intended status: Standards Track V. Pascual
Expires: August 14, 2014 Quobis
February 10, 2014
Loop Detection Mechanisms for Session Initiation Protocol (SIP) Back-to-
Back User Agents (B2BUAs)
draft-ietf-straw-b2bua-loop-detection-04
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
requirements for them to prevent infinite loops.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 14, 2014.
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
Kaplan & Pascual Expires August 14, 2014 [Page 1]
Internet-Draft Loop Detection for B2BUAs February 2014
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. B2BUA Loop-Detection Behavior . . . . . . . . . . . . . . . . 4
5. B2BUA Max-Forwards Behavior . . . . . . . . . . . . . . . . . 4
6. B2BUA Max-Breadth Behavior . . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . 4
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
10.1. Normative References . . . . . . . . . . . . . . . . . . 5
10.2. Informative 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.
Kaplan & Pascual Expires August 14, 2014 [Page 2]
Internet-Draft Loop Detection for B2BUAs February 2014
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 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
[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 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
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.
Kaplan & Pascual Expires August 14, 2014 [Page 3]
Internet-Draft Loop Detection for B2BUAs February 2014
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 [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, which
SHOULD be 70, as described for Proxies in section 16.6 part 3 of
[RFC3261].
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
[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 add any
additional issues beyond those discussed in [RFC5393].
Kaplan & Pascual Expires August 14, 2014 [Page 4]
Internet-Draft Loop Detection for B2BUAs February 2014
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
(Broadsoft) and Andrew Hutton (Unify) and Anton Roman (Quobis) for
their review of the document.
10. References
10.1. 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.
10.2. 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
Kaplan & Pascual Expires August 14, 2014 [Page 5]
Internet-Draft Loop Detection for B2BUAs February 2014
Hadriel Kaplan
Oracle
Email: hadriel.kaplan@oracle.com
Victor Pascual
Quobis
Email: victor.pascual@quobis.com
Kaplan & Pascual Expires August 14, 2014 [Page 6]
