SIP Max-Forwards

INVITE sip:bob@biloxi.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com:5060;branch=z9hG4bK776asdhds
Max-Forwards: 30
To: Bob <sip:bob@biloxi.com>
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Contact: <sip:alice@pc33.atlanta.com>
Content-Type: application/sdp
Content-Length: 142
(Alice’s SDP not shown)

Max-Forwards is decremented by each proxy that forwards the request.

When count goes to zero, request is discarded and 483 Hops Too Many response is sent.

Used for stateless loop detection.

Slide Notes

For preventing loops when using a B2BUA see RFC 7332:

Hadriel Kaplan and Victor Pascual, Loop Detection Mechanisms for Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs), Internet Engineering Task Force (IETF), ISSN: 2070-1721, Request for Comments 7332, August 2014 Available: http://www.rfc-editor.org/rfc/rfc7332.txt Links to an external site.

mitigating parallel forking amplification floods is addressed in RFC 5393:

R. Sparks, S. Lawrence, A. Hawrylyshen, and B. Campen, ‘Addressing an Amplification Vulnerability in Session Initiation Protocol (SIP) Forking Proxies’, Internet Request for Comments, vol. RFC 5393 (Proposed Standard), Dec. 2008 [Online]. Available: http://www.rfc-editor.org/rfc/rfc5393.txt Links to an external site.

Transcript

[slide136] We use the max forwards, in the same way we use the TTL, and now we can do stateless loop detection.