Network Working Group H. Kaplan Internet Draft Acme Packet Intended status: Standards Track Victor Pascual Expires: April 21, 2012 Acme Packet October 24, 2011 Loop Detection Mechanisms for Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs) draft-kaplan-dispatch-b2bua-loop-detection-00 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 to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 24, 2011. Copyright and License Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. Kaplan Expires April 24, 2012 [Page 1] Internet-Draft Loop Detection for B2BUAs October 2011 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 the Trust Legal Provisions and are provided without warranty as described in the BSD License. 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 Kaplan Expires - April 2011 [Page 2] Internet-Draft Loop Detection for B2BUAs October 2011 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. Kaplan Expires - April 2011 [Page 3] Internet-Draft Loop Detection for B2BUAs October 2011 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 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 with a default value of 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. Kaplan Expires - April 2011 [Page 4] Internet-Draft Loop Detection for B2BUAs October 2011 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). 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- dispatch-b2bua-taxonomy-00, October 15, 2011. Kaplan Expires - April 2011 [Page 5] Internet-Draft Loop Detection for B2BUAs October 2011 Authors' Addresses Hadriel Kaplan Acme Packet Email: hkaplan@acmepacket.com Victor Pascual Acme Packet Email: vpascual@acmepacket.com Kaplan Expires - April 2011 [Page 6]