Internet DRAFT - draft-kaplan-straw-b2bua-loop-detection
draft-kaplan-straw-b2bua-loop-detection
Network Working Group H. Kaplan
Internet Draft Acme Packet
Intended status: Standards Track Victor Pascual
Expires: August 30, 2013 Acme Packet
February 25, 2013
Loop Detection Mechanisms for
Session Initiation Protocol (SIP)
Back-to-Back User Agents (B2BUAs)
draft-kaplan-straw-b2bua-loop-detection-01
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
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Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Terminology...................................................2
2. Introduction..................................................2
3. Background....................................................3
4. B2BUA Loop-Detection Behavior.................................4
5. B2BUA Max-Forwards Behavior...................................4
6. B2BUA Max-Breadth Behavior....................................4
7. Security Considerations.......................................5
8. IANA Considerations...........................................5
9. Acknowledgments...............................................5
10. References...................................................5
10.1. Informative References..................................5
Authors' Addresses................................................6
1. Terminology
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 RFC 2119. The
terminology in this document conforms to RFC 2828, "Internet
Security Glossary".
B2BUA terminology and taxonomy used in this document is based on
[draft-b2bua-taxonomy].
2. 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
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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.
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 [draft-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 used.
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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
A Proxy-B2BUA, as defined in [draft-b2bua-taxonomy], MUST implement
the loop-detection mechanism for the Via header field, as defined
for a Proxy in [RFC5393].
[Note: should we require all B2BUAs to perform Via-header loop-
detection as well, even if they themselves don't forward on the Via
headers?]
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
[RFC3261].
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
[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.
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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].
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. References
10.1. Informative References
[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., et al, "Addressing an Amplification
Vulnerability in Session Initiation Protocol (SIP) Forking
Proxies", RFC 5393, December 2008.
[draft-b2bua-taxonomy] Kaplan, H., "A Taxonomy of Session Initiation
Protocol (SIP) Back-to-Back User Agents", draft-kaplan-straw-
b2bua-taxonomy-00, July 30, 2012.
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Authors' Addresses
Hadriel Kaplan
Acme Packet
Email: hkaplan@acmepacket.com
Victor Pascual
Acme Packet
Email: vpascual@acmepacket.com
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