Network Working Group R. Sparks Internet-Draft Estacado Systems Updates: 3261 (if approved) June 25, 2007 Intended status: Standards Track Expires: December 27, 2007 Correct transaction handling for 200 responses to Session Initiation Protocol INVITE requests draft-sparks-sip-invfix-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of 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 December 27, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract This document normatively updates RFC 3261, the Session Initiation Protocol (SIP), to address an error in the specified handling of success (200 class) responses to INVITE requests. Elements following RFC 3261 exactly will misidentify retransmissions of the request as a new, unassociated, request. The correction involves modifying the INVITE transaction state machines and changing the way responses that Sparks Expires December 27, 2007 [Page 1]
Internet-Draft invfix June 2007 cannot be matched to an existing transaction are handled. Table of Contents 1. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Reason for Change . . . . . . . . . . . . . . . . . . . . . . 3 4. Summary of Change . . . . . . . . . . . . . . . . . . . . . . 4 5. Consequences if Not Approved . . . . . . . . . . . . . . . . . 4 6. The Change . . . . . . . . . . . . . . . . . . . . . . . . . . 4 6.1. Page 85 . . . . . . . . . . . . . . . . . . . . . . . . . 4 6.2. Page 107 . . . . . . . . . . . . . . . . . . . . . . . . . 5 6.3. Page 114 . . . . . . . . . . . . . . . . . . . . . . . . . 5 6.4. Pages 126 through 128 . . . . . . . . . . . . . . . . . . 5 6.5. Pages 134 to 135 . . . . . . . . . . . . . . . . . . . . . 8 6.6. Page 136 . . . . . . . . . . . . . . . . . . . . . . . . . 8 6.7. Page 137 . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.8. Page 144 . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.9. Page 146 . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.10. Page 265 . . . . . . . . . . . . . . . . . . . . . . . . . 11 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 8. Security Considerations . . . . . . . . . . . . . . . . . . . 11 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 10.1. Normative References . . . . . . . . . . . . . . . . . . . 12 10.2. Informative References . . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 Intellectual Property and Copyright Statements . . . . . . . . . . 13 Sparks Expires December 27, 2007 [Page 2]
Internet-Draft invfix June 2007 1. Conventions and Definitions 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 [RFC2119]. 2. Introduction This document describes an essential correction to the Session Initiation Protocol (SIP), defined in [RFC3261], using the process defined in [I-D.drage-sip-essential-correction]. The change addresses an error in the handling of 200 class responses to INVITE requests that leads to retransmissions of the INVITE being treated as new requests. 3. Reason for Change One use of the INVITE method in SIP is to establish new sessions. These "initial" INVITEs may fork at intermediaries, and more than one receiving endpoint may choose to accept the request. SIP is designed such that the requester receives all of these success responses. Two sets of requirements in [RFC3261] work together to allow multiple 200s to be processed correctly by the requester. First, all elements are required to immediately destroy any INVITE client transaction state upon forwarding a matching 200 OK response. This requirement applies to both proxies and user agents (proxies forward the response upstream, the transaction layer at user agents forward the response to its "UA core"). Second, all proxies are required to statelessly forward any 200 OK responses that do not match an existing transaction, also called stray responses, upstream. The transaction layer at user agents is required to forward these responses to its UA core. Logic in the UA core deals with acknowledging each of these responses. This technique for specifying the behavior was chosen over adjusting INVITE client transaction state machines as a simpler way to specify the correct behavior. Over time, implementation experience demonstrated the existing text is in error. Once any element with a server transaction (say, a proxy in the path of the INVITE) deletes that transaction state, any retransmission of the INVITE will be treated as a new request, potentially forwarded to different locations than the original. Many implementations in the field have made proprietary adjustments to their transaction logic to avoid this error. Sparks Expires December 27, 2007 [Page 3]
Internet-Draft invfix June 2007 The requirement to statelessly forward stray responses has also been identified as a security risk. Through it, elements compliant to [RFC3261] are compelled to do work (forward packets) that is not protected by the admission policies applied to requests. This can be leveraged to, for instance, use a SIP proxy as an anonymizing forwarder of packets in a distributed DOS attack. General internet endpoints can also collude to tunnel non-SIP content through such proxies by wrapping them in an SIP response envelope. 4. Summary of Change This correction document updates [RFC3261], changing the transitions in the INVITE client state machine such that the INVITE client transaction remains in place to receive multiple 200 OK responses. It adds a state to the INVITE server state machine to absorb retransmissions of the INVITE after a 200 OK response has been sent. It also removes the requirement to forward stray responses to INVITE requests (not just 200 OK responses). 5. Consequences if Not Approved Implementations strictly conformant to [RFC3261] will process retransmitted initial INVITE requests as new requests. Proxies may forward them to different locations than the original. Proxies may also be used as anonymizing forwarders of bulk traffic. 6. The Change 6.1. Page 85 Section 13.3.1.4 paragraph 4 is replaced entirely by Once the response has been constructed, it is passed to the INVITE server transaction. In order to ensure reliable end-to-end transport of the response, it is necessary to periodically pass the response directly to the transport until the ACK arrives. The 2xx response is passed to the transport with an interval that starts at T1 seconds and doubles for each retransmission until it reaches T2 seconds (T1 and T2 are defined in Section 17). Response retransmissions cease when an ACK request for the response is received. This is independent of whatever transport protocols are used to send the response. Sparks Expires December 27, 2007 [Page 4]
Internet-Draft invfix June 2007 6.2. Page 107 Section 16.7 paragraphs 1 and 2 are replaced entirely by When a response is received by an element, it first tries to locate a client transaction (Section 17.1.3) matching the response. If none is found, the element MUST NOT forward the response. If a response is found, the response is handed to the client transaction. 6.3. Page 114 Section 16.7, part 9, first paragraph. Replace this sentence If the server transaction is no longer available to handle the transmission, the element MUST forward the response statelessly by sending it to the server transport. with If the server transaction is no longer available to handle the transmission, the response is simply discarded. 6.4. Pages 126 through 128 Section 17.1.1.2. Replace paragraph 7 (starting "When in either") through the end of the section with When in either the "Calling" or "Proceeding" states, reception of a response with status code from 300-699 MUST cause the client transaction to transition to "Completed". The client transaction MUST pass the received response up to the TU, and the client transaction MUST generate an ACK request, even if the transport is reliable (guidelines for constructing the ACK from the response are given in Section 17.1.1.3) and then pass the ACK to the transport layer for transmission. The ACK MUST be sent to the same address, port, and transport to which the original request was sent. When in either the "Calling" or "Preceeding" states, reception of a 2xx response MUST cause the client transaction to transition to "Completed", and the response MUST be passed up to the TU. The client transaction MUST NOT generate an ACK to the 2xx response - its handling is delegated to the TU. A UAC core will send an ACK to the 2xx response using a new transaction. A proxy core will always forward the 2xx response upstream. Sparks Expires December 27, 2007 [Page 5]
Internet-Draft invfix June 2007 The client transaction SHOULD start timer D when it enters the "Completed" state for any reason, with a value of at least 32 seconds for unreliable transports, and a value of zero seconds for reliable transports. Timer D reflects the amount of time that the server transaction can remain in the "Completed" state when unreliable transports are used. This is equal to Timer H in the INVITE server transaction, whose default is 64*T1, and is also equal to the time a UAS core will wait for an ACK once it sends a 2xx response. However, the client transaction does not know the value of T1 in use by the server transaction or any downstream UAS cores, so an absolute minimum of 32s is used instead of basing Timer D on T1. Any retransmissions of a response with status code 300-699 that are received while in the "Completed" state MUST cause the ACK to be re-passed to the transport layer for retransmission, but the newly received response MUST NOT be passed up to the TU. Any retransmissions of a 2xx response that are received while in the "Completed" state MUST be passed up to the TU. The client transaction MUST NOT generate an ACK to the 2xx response. The client transaction takes no further action. A retransmission of the response is defined as any response which would match the same client transaction based on the rules of Section 17.1.3. If timer D fires while the client transaction is in the "Completed" state, the client transaction MUST move to the terminated state. The client transaction MUST be destroyed the instant it enters the "Terminated" state. Replace Figure 5 with Sparks Expires December 27, 2007 [Page 6]
Internet-Draft invfix June 2007 |INVITE from TU Timer A fires |INVITE sent Reset A, V Timer B fires INVITE sent +-----------+ or Transport Err. +---------| |---------------+inform TU | | Calling | | +-------->| |-----------+ | +-----------+ 2xx | | | | 2xx to TU | | | |1xx | | 300-699 +---------------+ |1xx to TU | | ACK sent | | | | resp. to TU | 1xx V | | | 1xx to TU -----------+ | | | +---------| | | | | | |Proceeding |---------->| | | +-------->| | 2xx | | | +-----------+ 2xx to TU | | | 300-699 | | | | ACK sent, | | | | resp. to TU| +-------------+ | | | | | NOTE: | 300-699 V V Transport Err. | | ACK sent +-----------+Inform TU | transitions | +---------| |-------------->| labeled with | | | Completed | | the event | +-------->| |---+ | over the action | +-----------+ | 2xx | to take | ^ | ^ | 2xx to TU | | | | | | | +--------------+ | +-----+ | | | | Timer D fires | | - | V | +-----------+ | | | | | Terminated|<--------------+ | | +-----------+ INVITE client transaction Sparks Expires December 27, 2007 [Page 7]
Internet-Draft invfix June 2007 6.5. Pages 134 to 135 Section 17.2.1 paragraph 4 is replaced with If, while in the "Proceeding" state, the TU passes a 2xx response to the server transaction, the server transaction MUST pass this response to the transport layer for transmission. It is not retransmitted by the server transaction; retransmissions of 2xx responses are handled by the TU. The server transaction MUST then transition to the "Accepted" state. 6.6. Page 136 Replace Figure 7 with Sparks Expires December 27, 2007 [Page 8]
Internet-Draft invfix June 2007 |INVITE |pass INV to TU INVITE V send 100 if TU won't in 200ms send response+-----------+ +--------| |--------+101-199 from TU | | Proceeding| |send response +------->| |<-------+ | | Transport Err. | | Inform TU | |--------------->+ +-----------+ | 300-699 from TU | |2xx from TU | send response | |send response | +--------------+ +-------------+ | | | | INVITE V Timer G fires | | send response+-----------+ send response | | +--------| |--------+ | | | | Completed | | | | +------->| |<-------+ INVITE | | +-----------+ - | | | | ACK +-----+ | | | | - | | V | | +--+ | +------------+ | | | | | | | | V +->| Accepted | | Timer H fires | +-----------+ | | | or | | | +------------+ | Transport Error | | Confirmed | | ^ | | Inform TU | | | +--+ | | | | +-----------+ | +-----+ | | | | 2xx from TU | | | Timer I fires | send response | | | - | | | | | Timer L fires | | | | - | | | V | | | +------------+ | | +---->| | | +---------->| Terminated |<--------------+ | | +------------+ INVITE server transaction Sparks Expires December 27, 2007 [Page 9]
Internet-Draft invfix June 2007 6.7. Page 137 Section 17.2.1 - Replace the last paragraph (starting "Once the transaction") with The purpose of the "Accepted" state is to absorb retransmissions of an accepted INVITE request. Any such retransmissions are absorbed entirely within the server transaction. They are not passed up to the TU since any downstream UAS cores that accepted the request have taken responsibility for reliability and will already retransmit their 2xx responses if neccessary. While in the "Accepted" state, if the TU passes a 2xx response, the server transaction MUST pass the response to the transport layer for transmission. When the INVITE server transaction enters the "Accepted" state, Timer L MUST be set to fire in 64*T1 for all transports. This value matches both Timer B in the next upstream client state machine (the amount of time the previous hop will wait for a response when no provisionals have been sent) and the amount of time this (or any downstream) UAS core might be retransmitting the 2xx while waiting for an ACK. If Timer L fires while the INVITE server transaction is in the "Accepted" state, the transaction MUST transition to the "Terminated" state. Once the transaction is in the "Terminated" state, it MUST be destroyed immediately. 6.8. Page 144 Section 18.1.2 - Replace the second paragraph with The client transport uses the matching procedures of Section 17.1.3 to attempt to match the response to an existing transaction. If there is a match, the response MUST be passed to that transaction. Otherwise, the response MUST be silently discarded. 6.9. Page 146 Section 18.2.1 - Replace the last paragraph with Next, the server transport attempts to match the request to a server transaction. It does so using the matching rules described in Section 17.2.3. If a matching server transaction is found, the request is passed to that transaction for processing. If no match is found, the request is passed to the core, which may decide to Sparks Expires December 27, 2007 [Page 10]
Internet-Draft invfix June 2007 construct a new server transaction for that request. 6.10. Page 265 Add to Table 4: Timer L 64*T1 Section 17.2.1 Wait time for accepted INVITE request retransmits 7. IANA Considerations None. 8. Security Considerations This document makes two changes to the Session Initiation Protocol to address the error discussed in Section 3. It changes the behavior of both the client and server INVITE transaction state machines, and it changes the way "stray" responses (those that don't match any existing transaction) are handled. The changes to the state machines cause elements to hold onto each INVITE transaction state longer (32 seconds) than what was specified in RFC 3261. This will have a direct impact on the amount of work an attacker leveraging state exhaustion will have to exert against the system. However, this additional state is necessary to achieve correct operation. RFC 3261 required SIP proxies to forward any stray 200 class responses to an INVITE request upstream statelessly. As a result, conformant proxies can be forced to forward packets (that look sufficiently like SIP responses) to destinations of the sender's choosing. Section 3 discusses some of the malicious behavior this enables. This document reverses the stateless forwarding requirement, making it a violation of the specification to forward stray responses. 9. Acknowledgments Pekka Pessi reported the improper handling of INVITE retransmissions. 10. References Sparks Expires December 27, 2007 [Page 11]
Internet-Draft invfix June 2007 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. 10.2. Informative References [I-D.drage-sip-essential-correction] Drage, K., "A Process for Handling Essential Corrections to the Session Initiation Protocol (SIP)", draft-drage-sip-essential-correction-00 (work in progress), October 2006. Author's Address Robert Sparks Estacado Systems 17210 Campbell Road Suite 250 Dallas, Texas 75254-4203 USA Email: RjS@nostrum.com Sparks Expires December 27, 2007 [Page 12]
Internet-Draft invfix June 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Sparks Expires December 27, 2007 [Page 13]