SIP K. Johns Internet-Draft CableLabs Intended status: Standards Track January 31, 2007 Expires: August 4, 2007 Routing of mid dialog requests using sip-outbound draft-johns-sip-outbound-middialog-draft-02 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 August 4, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Johns Expires August 4, 2007 [Page 1] Internet-Draft Mid dialog request routing January 2007 Abstract This document describes a solution for routing of mid-dialog requests in the presence of NATs. The solution leverages and extends the concepts described in the Internet Draft titled Managing Client Initiated Connections in the Session Initiation Protocol. This solution is necessary to support routing of mid-dialog requests while preserving Edge Proxy failover within an outbound-proxy-set. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 4 3. Use Case . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Solution Requirements . . . . . . . . . . . . . . . . . . . . 8 5. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 9 5.1. User Agent Proceedures . . . . . . . . . . . . . . . . . . 9 5.2. Edge Proxy Proceedures . . . . . . . . . . . . . . . . . . 9 5.2.1. Processing REGISTER Requests . . . . . . . . . . . . . 9 5.2.2. Record-Routing Dialog Forming Requests . . . . . . . . 9 5.2.3. Forwarding Dialog Forming and Mid-Dialog Requests . . 10 5.3. Registrar/Authoritative Proxy . . . . . . . . . . . . . . 10 5.3.1. Processing REGISTER Requests . . . . . . . . . . . . . 10 5.3.2. Record-Routing Dialog Forming Requests . . . . . . . . 11 5.3.3. Forwarding Mid-Dialog Requests . . . . . . . . . . . . 11 5.4. Limitations . . . . . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 7. Security Considerations . . . . . . . . . . . . . . . . . . . 13 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14 9. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 9.1. changes from 01 Version . . . . . . . . . . . . . . . . . 15 9.2. changes from 00 Version . . . . . . . . . . . . . . . . . 15 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 10.1. Normative References . . . . . . . . . . . . . . . . . . . 16 10.2. Informative References . . . . . . . . . . . . . . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17 Intellectual Property and Copyright Statements . . . . . . . . . . 18 Johns Expires August 4, 2007 [Page 2] Internet-Draft Mid dialog request routing January 2007 1. Introduction The Internet Draft titled Managing Client Initiated Connections in the Session Initiation Protocol [OUTBOUND] describes extensions to the Session Initiation Protocol (SIP) to support NAT traversal. In particular it defines behaviors for User Agents, registrars and proxy servers that allow dialog initiating requests to be delivered on existing flows established by the User Agent at Registration time. However, procedures for the routing of mid-dialog request over an existing flow is explicitly placed out of scope by [OUTBOUND]. This draft highlights some of the issues that may arise due to the lack of guidance on how to route mid-dialog requests in [OUTBOUND] and attempts to present a solution based on existing procedures in [OUTBOUND]. Johns Expires August 4, 2007 [Page 3] Internet-Draft Mid dialog request routing January 2007 2. Terms and Definitions Note: The following definitions are borrowed from [OUTBOUND] Authoritative Proxy: A proxy that handles non-REGISTER requests for a specific Address-of-Record (AOR), performs the logical Location Server lookup described in RFC 3261, and forwards those requests to specific contact URIs. Edge Proxy: An Edge Proxy is any proxy that is located topologically between the registering SIP User Agent (SIP UA) and the registrar. Flow: A Flow is a network protocol layer (layer 4) association between two hosts that is represented by the network address and port number of both ends and by the protocol. For TCP, a flow is equivalent to a TCP connection. For UDP a flow is a bidirectional stream of datagrams between a single pair of IP addresses and ports of both peers. With TCP, a flow often has a one to one correspondence with a single file descriptor in the operating system. Instance-id: This specification uses the word instance-id to refer to the value of the "sip.instance" media feature tag in the Contact header field. This is a Uniform Resource Name (URN) that uniquely identifies this specific UA instance. Outbound-proxy-set: A set of SIP URIs (Uniform Resource Identifiers) that represents each of the outbound proxies (often Edge Proxies) with which the UA will attempt to maintain a direct flow. The first URI in the set is often refereed to as the primary outbound proxy and the second as the secondary outbound proxy. There is no difference between any of the URIs in this set, nor does the primary/secondary terminology imply that one is preferred over the other. Note: The following definition is added by this document Stream-id: (This needs a better name) An identifier created by the registrar which identifies a set of flows between the Registrar and all Edge Proxies the UA has registered through. Johns Expires August 4, 2007 [Page 4] Internet-Draft Mid dialog request routing January 2007 3. Use Case Section 5.3 of [OUTBOUND] states: "Note that techniques to ensure that mid-dialog requests are routed over an existing flow are out of scope and therefore not part of this specification. However, an approach such as having the Edge Proxy Record-Route with a flow token is one way to ensure that mid-dialog requests are routed over the correct flow." Consider the following network architecture as presented in [OUTBOUND]in figure 1 below. +---------+ |Registrar| |Proxy | +---------+ / \ / \ / \ +-----+ +-----+ |Edge1| |Edge2| +-----+ +-----+ \ / \ / ----------------------------NAT/FW \ / \ / +------+ |User | |Agent | +------+ Figure 1: Example network architecture In this scenario the User Agent is configured with an outbound-proxy- set that consists of "sip:edge1.example.com;lr;keepalive=stun" and "sip:edge2.example.com;lr;keepalive=stun" and has registered through each. A session is established through the primary edge proxy which follows the suggestion in outbound to include a flow token in the Record- Route entry. Should the primary edge proxy fail mid-call, the User Agents will not be affected by this failure until the session is cleared. Please see figure 2 below for an illustration of this Johns Expires August 4, 2007 [Page 5] Internet-Draft Mid dialog request routing January 2007 scenario. Callee Edge2 Edge1 Caller | | | | | | | | | | | | | | |(1) INVITE | | | |<----------------| | | |Edge1 record routes | | | | | | |INVITE with Flow Token | | | | |(2) INVITE | | | |<----------------------------------| | |(3) 180 Ringing | | | |---------------------------------->| | | | |(4) 180 Ringing | | | |---------------->| |(5) 200 OK | | | |---------------------------------->| | | | |(6) 200 OK | | | |---------------->| | | |(7) ACK | | | |<----------------| |(8) ACK | | | |<----------------------------------| | | | X - Crash | | | | |(9) BYE | | |----------------------------------> No response | | | | | | | |(10) BYE | | |---------------->| | | |How does the Callee determine | | | | | |it shoud now send the BYE | | | | | |to Edge2? | | | | | | | | | | Figure 2: Routing of Mid-Dialog Requests Johns Expires August 4, 2007 [Page 6] Internet-Draft Mid dialog request routing January 2007 Message 1 is a normal INVITE with the exception that Edge1 adds a Record-Route header with a flow token. Record-Route: In message 9, the BYE is sent to Edge1 per the route set. Given that Edge1 has failed it will not respond to the BYE reqeust from the caller. As previously stated, [OUTBOUND] does not discuss how the caller determines it should send the BYE request to Edge2. As such this document discusses two issues related to following the suggestion in [OUTBOUND] for routing of mid-dialog requests: 1. How to route to Edge2 when Edge1 has failed and added a Record- Route entry when the dialog was established. 2. How does Edge2 determine which flow to forward a mid-dialog request on if the dialog was established via Edge1. Johns Expires August 4, 2007 [Page 7] Internet-Draft Mid dialog request routing January 2007 4. Solution Requirements Before presenting a solution it is useful to present the requirements a solution must satisfy. As such, any solution that attempts to solve this use case should adhere to the following requirements: 1. The flow token is unique to a flow, the flow can be recovered from the token, and the token can not be modified by attackers (this requirement is taken from [OUTBOUND]); 2. work in the presence of multiple Edge Proxies supporting redundant flows to the registrar; 3. support the use case identified in this document for the routing of mid-dialog requests; 4. work for the case where the SIP UA registers multiple AORs from the same contact or different contact. 5. ensure that if a edge proxy inserted a URI into a Record-Route header field, that it should continue to see that URI in the Route header field of mid-dialog request, even if the mid-dialog request is sent to a backup edge proxy; 6. not require that edge proxies have to replicate any kind of dynamic state between them; 7. not require the SIP UA to register through all edge proxies served by a given Registrar/Authoritative Proxy. Johns Expires August 4, 2007 [Page 8] Internet-Draft Mid dialog request routing January 2007 5. Proposed Solution The following sections propose a solution which satisfies the majority of the above requirements. In summary the solution relies on each Edge Proxy adding a Record-Route entry to each dialog establishing request. The entry contains a flow token as suggested by outbound. However, the flow token used is the sip.instance value provided in the original REGISTER request. This ensures that any Edge Proxy the UA may have registered through, would understand the flow token and be able to forward the Request properly. The solution also requires the Register to remember which edge proxies a UA registers through. This is facilitated by have the registrar associate each REGISTER for the same sip.instance with a stream-id. The stream-id in turn identifies which edge proxies were used for the REGISTER request. The Registrar returns this stream-id in the Service-Route header for each successful REGISTER request. The UA includes the received service-route information in its dialog forming request. This forces all dialog forming requests through the Registrar which allows the Registrar to add a Record-Route entry with the stream-id. Should the edge proxy which the dialog was established through fail, the Registrar can easily route to the next edge proxy identified by the stream-id in the resulting Route set. 5.1. User Agent Proceedures This document imposes no new requirements on the user agent other then those already specified by [OUTBOUND]. 5.2. Edge Proxy Proceedures 5.2.1. Processing REGISTER Requests This document imposes no new requirement on the Edge Proxy for processing Initial REGISTER or Re-REGISTER requests other then those already defined by [OUTBOUND]. 5.2.2. Record-Routing Dialog Forming Requests If the request is a dialog forming request, an edge proxy MUST record-route. The edge proxy MUST insert a flow token in the user portion of the URI. The edge proxy MUST use the SIP UA provided instance-id in the contact header of the REGISTER request as the flow token. However, this does not protect the flow token from modification by attackers. To protect against modification by attackers the flow token should be generated as follows: The Edge Proxy (both Primary and Secondary are configured with the same random 20 byte key called Johns Expires August 4, 2007 [Page 9] Internet-Draft Mid dialog request routing January 2007 K. The HMAC of the SIP UA provided instance-id is computed using the key K and the HMAC-SHA1-80 algorithm, as defined in [RFC2104]. The concatenation of the HMAC and instance-id are base64 encoded, as defined in [RFC3548], and used as the flow identifier. The requirement that the flow be recoverable from the token cannot be satisfied if Edge Proxy failover is desired as the flow itself is specific to the Edge Proxy and cannot be generalized. 5.2.3. Forwarding Dialog Forming and Mid-Dialog Requests There are no changes to how the Edge Proxy forwards requests. The Edge Proxy can verify that the flow token has not been tampered by verifying the instance-id in the user part of the route header by calculating the HMAC and comparing to the HMAC in the flow token, if they match the instance-id can be considered valid and the request forwarded on the proper flow. To cover the case where an Edge Proxy may have crashed and since recovered but lost its dialog state information (and associated flows), it is important that should the Edge Proxy receive a mid- dialog request which contains a flow token which it does not understand, it return a 430 response as defined by outbound. 5.3. Registrar/Authoritative Proxy 5.3.1. Processing REGISTER Requests Upon receipt of a REGISTER Request, the Registrar first checks to see if it contains a sip.instance indicator in the Contact header. If no such indicator is present, then the Registrar continues to process the REGISTER per RFC 3261. If the REGISTER contains the sip.instance indicator, the Registrar retrieves the sip.instance value and searches for an existing registration with the same value. If no match is found, this is an initial Registration attempt by that specific client instance. If the registration attempt is successful, the Registrar creates a stream-id and associates the tuple the REGISTER was received on with it. It MUST include a Service-Route header in response to the REGISTER Request. The Service-Route header MUST contain the stream-id in the user portion of the URI inserted in the Service- Route header. If the Registrar finds an existing Registration for the same sip.instance value, this is either a Re-Registration or a new Registration through a different Edge Proxy. In this case, if the REGISTER Request is successful, the Registrar adds the tuple the Johns Expires August 4, 2007 [Page 10] Internet-Draft Mid dialog request routing January 2007 REGISTER was received on to the list of tuples associated with the stream-id. It MUST include a Service-Route header in response to the REGISTER Request. The Service-Route header MUST contain the stream-id in the user portion of the URI inserted in the Service- Route header. 5.3.2. Record-Routing Dialog Forming Requests When a dialog is created the UA will place the received Service-Route header in the Route header. When the registrar receives the dialog request, the route header will identify the set of flows which the UA has registered through (the different edge proxies which the UA has registered through). The registrar MUST add a record-route entry with the user part being the stream-id along with extra information identifying which of the flows the dialog was established via. [TODO: Need to work out exactly how this will be done, as part of the user part Reg-id+stream-id, as a separate opaque parameter flow-id=x, or in some other way.] 5.3.3. Forwarding Mid-Dialog Requests When the registrar receives a mid-dialog request, it uses the stream-id which is located in the user portion of its Route header entry to forward the Request. If the edge proxy which established the dialog is no longer available, the Registrar MUST forward the request to the next Edge Proxy identified by the stream-id. 5.4. Limitations As stated above the use of the instance ID does not allow the flow to be recovered from the flow token. Additionally if a SIP UA is registering multiple AORs, this solution would require they all be registered over the same flow as they will all be registered using the same instance ID. If the SIP UA wanted to register multiple AORs against different contacts, it would require a different instance ID for each contact. Johns Expires August 4, 2007 [Page 11] Internet-Draft Mid dialog request routing January 2007 6. IANA Considerations There are no IANA Considerations Johns Expires August 4, 2007 [Page 12] Internet-Draft Mid dialog request routing January 2007 7. Security Considerations Outbound does not contemplate the idea of the flow token being known by the client. The solution proposed in this document relies on the Edge Proxy populating the record-route header with not only its URI but the flow token associated with the client it is providing service to. The end result is that the remote client will now know flow token. It is unclear what benefit this provides the remote client. For unchanged Outbound procedures, the threats listed in [OUTBOUND] are also applicable to this document. Johns Expires August 4, 2007 [Page 13] Internet-Draft Mid dialog request routing January 2007 8. Acknowledgements The author would like to thank the following individuals for their feedback, comments and recommendations (in alphabetical order): Cullen Jennings, David Hancock and Jean-Francois Mule. Johns Expires August 4, 2007 [Page 14] Internet-Draft Mid dialog request routing January 2007 9. Changes Note to RFC Editor: Please remove this entire section. 9.1. changes from 01 Version Added proceedures for the Registrar to determine which edge proxy to forward a mid-dialog request to should the edge proxy which established the dialog fail. Significant rework of the requirements and problem statement was also done. 9.2. changes from 00 Version Updated the figure to better illustrate the use case. Removed the sections after the figure as they were no longer relvant. Expaned text in section 5 intro. Johns Expires August 4, 2007 [Page 15] Internet-Draft Mid dialog request routing January 2007 10. References 10.1. Normative References [OUTBOUND] Jennings, C. and R. Mahy, "Managing Client Initiated Connections in the Session Initiation Protocol(SIP)", March 2006. 10.2. Informative References [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 3548, July 2003. Johns Expires August 4, 2007 [Page 16] Internet-Draft Mid dialog request routing January 2007 Author's Address Kevin Johns CableLabs 858 Coal Creek Circle Louisville, CO 80027 USA Email: k.johns@cablelabs.com Johns Expires August 4, 2007 [Page 17] Internet-Draft Mid dialog request routing January 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. 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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). Johns Expires August 4, 2007 [Page 18]