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

SIP provides a means of preventing infinite request forwarding loops in , and a means of mitigating parallel forking amplification floods in . 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 because a B2BUA is a UAC, the resulting unbounded loops have caused service outages and make troubleshooting difficult. Furthermore, 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 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.

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 . B2BUA terminology and taxonomy used in this document is based on

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 and its extensions. For example, any B2BUA type defined in 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 and updated by 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.

It is RECOMMENDED that B2BUAs implement the loop-detection mechanism for the Via header field, as defined for a Proxy in .

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 . 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 . 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 .

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 , 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 as if they were a parallel forking Proxy.

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

This document makes no request of IANA.

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.