SIP Working Group H. Kaplan Internet Draft Acme Packet Intended status: Standards Track Expires: May 30, 2009 November 30, 2008 A Session Identifier for the Session Initiation Protocol (SIP) draft-kaplan-sip-session-id-01 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/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 May 30, 2009. Copyright and License Notice Copyright (c) 2008 IETF Trust and the persons identified as the document authors. All rights reserved. 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. Abstract There are several reasons for having a globally unique session identifier for the same SIP session, which can be maintained across B2BUA's and other SIP middle-boxes. This draft proposes a new SIP header to carry such a value: Session-ID. Kaplan Expires May 30, 2009 [Page 1] Internet-Draft SIP Session Identifier November 2008 Table of Contents 1. Introduction..................................................2 1.1. Requirements..............................................3 1.2. Example use-cases for Session-ID..........................4 2. Terminology...................................................5 3. Applicability.................................................5 4. Overview of Operation.........................................5 5. Session-ID Behavior...........................................6 5.1. Generating a Session-ID value.............................6 5.2. UAC Behavior..............................................7 5.3. UAS Behavior..............................................7 5.4. Proxy Behavior............................................7 5.5. B2BUA Behavior............................................8 5.5.1 B2BUA Generation of New Session-ID......................8 5.5.2 B2BUA Insertion of Saved Session-ID.....................9 6. Dialog Matching using Session-ID..............................9 6.1. Changes for Dialog-Matching Mechanisms...................10 7. Session-ID Migration and Failure Scenarios...................10 8. New Header...................................................11 8.1. "Session-ID" header......................................12 8.2. Augmented BNF Definitions................................12 9. Example Exchange.............................................12 10. Security Considerations......................................12 10.1. Security considerations for B2BUA vendors and operators..13 10.2. Security considerations for extensions to the Session-ID.14 11. IANA Considerations..........................................15 12. Acknowledgments..............................................15 13. References...................................................15 13.1. Normative References.....................................15 13.2. Informative References...................................15 Author's Address...................................................16 Acknowledgment.....................................................16 1. Introduction The SIP [RFC3261] Call-ID header value is a globally unique identifier, mandatory in all requests/responses, which identifies SIP messages belonging to the same dialog or registration. It provides a portion of the SIP message dialog-matching criteria, and is used in such things as [Replaces] headers and [dialog-events] package for matching to dialogs, and in [SIP-Identity] and [Connected-identity] as one of the inputs for signing. Kaplan Expires - May 2007 [Page 2] SIP Session Identifier November 2008 Unfortunately, the Call-ID is often changed by B2BUA's and other such middle-boxes in the end-to-end message path. A B2BUA logically represents a UAS and UAC, and as such may use a new Call-ID value for the dialog it creates on its UAC half; but there are several use-cases for having a common, consistent end-to-end identifier, as described later in this draft. There are several reasons the Call-ID value is changed by B2BUA's. There are security and privacy reasons, since Call-ID values typically contain UA IP Addresses; some B2BUA's need to change them to keep track of spiraling dialogs; and some need to change them to keep track of separate forks. In fact, some people have argued a B2BUA has no choice but to create a new one, in order to strictly comply with RFC 3261 as a UAC. In general, B2BUA's modify the Call- ID value in both directions, "fixing" it to be what each side of the B2BUA would expect. This works fine if the B2BUA is in the message path, and knows all SIP message or body contents which use or reference the value. However for subsequent out-of-dialog requests, or new SIP uses, a B2BUA often does not or cannot "fix" the value correctly, for example if it is not traversed. Therefore, in order to provide an identifier which will not be modified/replaced by B2BUA's, this draft proposes a new SIP Header "Session-ID", and mandatory rules for the value of such a header. The rules are designed to be such that the value in the Session-ID header is not considered unsafe, private, or have any property which would cause B2BUA's to change it. The goal of this draft is to enable use-cases which need a unique identifier for a given session which can successfully cross B2BUA's, and be used for matching purposes. 1.1. Requirements The following requirements drive the need for Session-ID: REQ1: It must be possible to identify a set of dialogs which have a direct correlation with each other such that they represent the same SIP session, with as high a probability as possible. REQ2: It must be possible for a SIP device to use the identifier in out-of-dialog requests, to match existing dialogs at B2BUA's and/or UAS's, if the Call-ID and tags the device believes are correct do not in fact match, with as high a probability as possible. REQ3: It must be possible to pass the identifier through B2BUA's, with as high a probability as possible. This requirement drives the following requirements: Kaplan Expires - May 2007 [Page 3] SIP Session Identifier November 2008 REQ3a: The identifier must not reveal any information related to any SIP device or domain identity, including IP Address, port, hostname, domain name, username, Address-of-Record, MAC address, IP address family, transport type, etc. REQ3b: The identifier must not reveal to the receiver of it that the Call-ID, tags, or any other SIP header or body portion have been changed by middle-boxes, with as high a probability as possible. One should note that REQ2 and REQ3b are at odds with each other to some degree, as described in the Security section. Rationale is given for why B2BUA's should not be concerned with such a contradiction in the "Security Considerations for B2BUA vendors and operators" section. 1.2. Example use-cases for Session-ID The need for a unique identifier is driven by the following use- cases: 1. Certain SIP applications need to reference dialogs in out-of- dialog requests at a layer above the SIP message dialog matching rules, and wish it to work across B2BUA domains. For example, the SIP Media Control Channel Framework [media-ctrl] needs to reference a dialog identifier used between a UAC and UAS by a third party. The mechanism originally used the Call-ID and remote/local-tags for such matching, but changed due to concerns over B2BUA's changing them, and now uses a new "cfw-id" SDP attribute instead which does not rely on the Call-ID value. 2. Multiple RFC 3265 Event packages use the Call-ID value in their package bodies to reference established sessions, even though they don't actually need to match a Call-ID per se - and should work across B2BUA domains. These packages could be updated to include a Session-ID mechanism as a secondary, optional matching criteria. 3. Several proposed and documented identity verification mechanisms need a hard-to-guess dialog identifier for verification. For example, [RFC4474] and [RFC4916] use the Call-ID header value in its signature to prevent replay/copy-paste attacks, even though they do not need a Call-ID value per se; they just need a unique dialog identifier. Likewise, [draft-derive] wishes to perform a reverse dialog-verification to verify a caller identity based on some unique identifier for the dialog; and [draft-pass] creates a header-parameter to perform something similar. 4. Some SIP service providers implement call admission control (CAC) for bandwidth, and only allow SIP INVITE requests if the network has sufficient bandwidth for the given SDP. If a call request is forked by B2BUA's, or crosses them, however, the CAC model treats Kaplan Expires - May 2007 [Page 4] SIP Session Identifier November 2008 each fork as a separate call because there is no identifier to tie them together. This leads to rejected forks due to CAC, when they should have been allowed to proceed. A common identifier would provide the necessary information to correlate the forked requests. Currently proprietary SIP headers are used for this purpose. 5. Troubleshooting SIP sessions is more complicated if multiple legs of the session are on different sides of B2BUA's, due to the lack of a common identifier to tie the legs together. Currently proprietary mechanisms are used to achieve this. 6. When SIP requests cross B2BUA's, the only form of loop detection that will stop a loop is the Max-Forwards hop-count limit being reached (value reaching zero). Via header values are removed by B2BUA's, so both spirals and simple loops cannot be detected by Via branch-parameter matching. A Session-ID value could be used to detect loops by imposing a limit on the number of times the same Session-ID can cross the same B2BUA. This would be a local decision, and an optimization, but it would be useful. 2. 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". 3. Applicability This draft proposes a new SIP header for all requests and responses. 4. Overview of Operation The general concept is that the UAC generating an out-of-dialog request generates a new, pseudo-random, unique value which remains constant for the duration of the transaction, any dialog created from the request, or a registration. The value is based on the rules for creating a fixed-length pseudo-random value, and is inserted in a new Session-ID header defined in this draft. The UAC and UAS then reflect this value in all messages for the duration of the dialog. To aid in migration of deployments, a B2BUA or Proxy may also generate and/or insert the value on behalf of a UAC or UAS, if one or the other does not support this document's mechanism. Kaplan Expires - May 2007 [Page 5] SIP Session Identifier November 2008 This Session-ID is not used for message dialog-matching rules in RFC 3261, nor does it change the Call-ID usage, nor does it replace the Call-ID value. Instead this new header value provides an identifier for other uses, some of which are similar to Call-ID. In particular, a UA or B2BUA can provide a matching function whereby out-of-dialog requests are "matched" to established dialogs using the Session-ID value and remote-tag, as defined later in this document. This is to enable SIP uses of Call-ID matching for out- of-dialog requests to function in scenarios they cannot today, for example in specific scenarios with the dialog-events package. The specific mechanisms to be enabled for such use are to be documented separately, in updates to their relevant RFCs as appropriate. [Open issue: or should we document and extend them in this document?] 5. Session-ID Behavior 5.1. Generating a Session-ID value This draft proposes the Session-ID header value be generated based on a defined hash mechanism for creating a 128-bit pseudo-random value, and encode it as its lower-case hex representation. The reason for specifying the mechanism, and its length, is to make it impossible to determine the manufacturer of the device which generated it by looking at its format or value. For example, the theoretically random "session id" value in SDP origin line has been found to be fairly vendor-specific in nature, and one can narrow the vendor that generated the SDP simply by the origin session id value. (In fact, this drove some SBC's to modify that SDP field for "anonymization" purposes) In order to perform matching of out-of-dialog requests to established dialogs, for example for [dialog-events], a generator and B2BUA's need to remember the Session-ID for a given dialog(s). For example by having a Session-ID-to-dialogs table. This is described in more detail in following sections of this document. The Session-ID value is generated by taking the Call-ID header value, and SHA-1 hashing it based on [RFC2104] HMAC using a locally generated pseudo-random 128-bit system secret key, to create a 128- bit resultant HMAC value. The secret key makes the resultant HMAC value not re-creatable by other parties, which is necessary to prevent detection of Call-ID's being changed, as required by Req-3b. Otherwise, middle-boxes may have motivation to remove the Session-ID in order to hide the fact that they changed the Call-ID. Per [RFC2104], the algorithm is thus HMAC-SHA-1-128(Call-ID_value, secret_key), and the 128-bit result is encoded using lowercase Kaplan Expires - May 2007 [Page 6] SIP Session Identifier November 2008 alphanumeric hex representation, as defined in the ABNF section of this document. 5.2. UAC Behavior The rules for when a UAC generates a new Session-ID value are similar as those for Call-ID value: a UAC supporting this draft MUST generate a *new* unique Session-ID value whenever it generates an out-of-dialog request, or for a new Registration. The UAC MUST re- use the same Session-ID for in-dialog messages, and for any out-of- dialog request it retransmits or re-generates in response to a 3xx, or it re-formulates due to failure responses. This follows the rules in [RFC3261] for Call-ID generation. Session-ID values in Registration "refreshes" - REGISTER requests which are used to update the expiry time but not to register a new contact - MUST use the same Session-ID value as previous REGISTER requests. New Registrations, which add or change the Contact URI for the AoR, but not simply to delete them, MUST use a new Session- ID value. This follows the behavior of Call-ID per RFC 3261 and thus the hash mechanism should by definition produce the correct Session-ID; it is re-iterated here because some devices incorrectly change their Call-ID value for every re-Registration, and MUST NOT do the same to the Session-ID. The UAC MUST include the Session-ID header value in every SIP message it transmits. This serves both a troubleshooting purpose, and may be used in specific identity verification mechanisms which are beyond the scope of this draft. 5.3. UAS Behavior A UAS compliant with this document MUST copy a received Session-ID value in a request, into responses and subsequent upstream requests sent within the dialog. If an out-of-dialog request is received without a Session-ID header field, the UAS SHOULD generate a new one for subsequent use in the transaction and dialog, as defined for a UAC, and use the value in all responses and upstream in-dialog requests. 5.4. Proxy Behavior A Proxy MUST NOT remove or modify the Session-ID header values it receives, if one is in the message. By definition, an RFC 3261 compliant Proxy would not modify or remove such a header. Kaplan Expires - May 2007 [Page 7] SIP Session Identifier November 2008 A Proxy compliant with this draft MAY generate a new Session-ID or insert a previously saved one, if and only if none existed in a message, following the rules for doing so as a B2BUA defined later. If the Proxy forks a request, it MUST copy the same Session-ID value into all the forked request copies. If the Proxy recurses requests due to 3xx redirection, or regenerates requests due to failures, it MUST use the same Session-ID value as the original request, just as the UAC does. If the Proxy locally generates any response or request based on a received request, including 100 Trying, it MUST insert any received Session-ID value from the original request into the message it locally creates. This serves both a troubleshooting purpose, and may be used in specific identity verification mechanisms which are beyond the scope of this draft. 5.5. B2BUA Behavior A B2BUA compliant with this document MUST copy the Session-ID it receives in requests as a UAS into the related requests it generates as a UAC; and any Session-ID value it receives in responses as a UAC into the correlated responses it generates as a UAS. If the B2BUA forks or creates multiple requests as a UAC, from a request it received as a UAS, the B2BUA MUST copy the same Session- ID header value it received into all the forks/requests. If the B2BUA recurses requests due to 3xx redirection, or regenerates requests due to failures, it MUST use the same Session-ID value, just as the UAC does. If the B2BUA locally generates any response or request based on a received request, including 100 Trying, it MUST insert any received Session-ID value from the original request into the message it locally creates. A B2BUA MUST remember the received Session-ID value for the duration of the transaction and dialog, for the purpose of re-insertion, and for the purpose of matching out-of- dialog requests to established dialogs on the B2BUA as defined later. In all cases, if the SIP message received by a B2BUA contained a Session-ID header field, a B2BUA compliant with this document MUST NOT remove, modify or replace the header value. 5.5.1 B2BUA Generation of New Session-ID If an out-of-dialog request is received by a B2BUA compliant with this document, and the request does *not* contain a Session-ID header field, the B2BUA MUST generate a new Session-ID. It MUST then Kaplan Expires - May 2007 [Page 8] SIP Session Identifier November 2008 insert it in any requests or responses it generates, as if it had actually received the new Session-ID from the UAC, following the rules previously defined for a B2BUA. This allows for a B2BUA to provide a migration to Session-ID deployment, on behalf of upstream nodes that do not yet support it. As defined previously, if any received message already had a Session-ID, a B2BUA compliant with this document would not replace it. 5.5.2 B2BUA Insertion of Saved Session-ID If a Session-ID was received in an out-of-dialog request, or the B2BUA locally generated one because none existed, the B2BUA SHOULD insert the same Session-ID value into all responses and upstream in- dialog requests if and only if a Session-ID is not already in them. This allows for a B2BUA to provide a migration to Session-ID deployment, on behalf of downstream nodes that do not yet support it. 6. Dialog Matching using Session-ID A UAC, UAS or B2BUA complying with this document MAY perform a dialog matching function, whereby out-of-dialog requests are matched to established dialogs using the Session-ID value. For example, if a UA receives a Replaces header in an out-of-dialog INVITE, and the Call-ID in the Replaces header value does not match any dialog the UA knows about, it could attempt to match an embedded Session-ID value in the Replaces header to its Session-ID table. For the matching function to work, the UA/B2BUA needs to have stored the Session-ID values for all dialogs. How they do so is implementation specific, but the UA/B2BUA needs to be able to internally find any and all dialogs for a given Session-ID value. One problem with performing matching in this way is that the B2BUA will not know which specific dialog a Session-ID value refers to. A B2BUA has at least a UAC and UAS dialog associated with the same Session-ID value; if the Call-ID and tags don't match one, then it cannot be sure which one the UAC sending the out-of-dialog request wishes to match with. A UA or B2BUA which performs the matching function MUST only match a Session-ID to an established dialog if the requested remote-tag matching criteria also matches, for uses that have such a tag identifier (which are all known uses except the In-Reply-To header of [RFC3261]). Note the "remote-tag" is the term as defined by [RFC3261]: for a UAC it is the to-tag received from a dialog-creating response, while for Kaplan Expires - May 2007 [Page 9] SIP Session Identifier November 2008 a UAS it is the from-tag received in a dialog-creating request. For a B2BUA, it is those same tags on its respective UAC and UAS sides. Otherwise, if no dialog is found with the same Session-ID and remote-tag value, the UA/B2BUA MUST process the request as if it did not find a match. For example, if the Replaces header field has an embedded Session-ID value that matches a dialog set known to the UAC, but the Replaces "from-tag" parameter value does not match a remote-tag for any of the dialogs in the set, the UAC would reject the request with a 481 Call/Transaction Does Not Exist response. Note this means that middle-boxes in an end-to-end path which change the original tags would sometimes prevent upstream/downstream UA's/B2BUA's from matching dialogs successfully with this new mechanism. Mandating middle-boxes to not change tags is beyond the scope of this document. 6.1. Changes for Dialog-Matching Mechanisms This document does not update the current SIP extensions which need dialog-matching for out-of-dialog requests. It is expected that a separate document will be created to do so. [Open issue: should this doc do it?] The following are the known list of currently defined uses which need this property: o In-Reply-To [RFC3261] o Replaces [Replaces] o Join [Join] o Target-Dialog [Target-Dialog] o Event Package for INVITE-Initiated Dialog [dialog-events] o Event Package for Conference State [RFC4575] o Event Package for Key Press Stimulus [KPML] [Open issue: should we give guidance on future extensions which need matching?] [Note: one possible way to do so would be to make the new out-of- dialog message use the same Session-ID header value in the message itself for its Session-ID value; in other words doing a transfer could keep the same Session-ID for the new call] 7. Session-ID Migration and Failure Scenarios SIP is already widely deployed on the Internet, and it is impractical to expect all UA's to be upgraded to support this document's mechanism in the near future. A solution for gradual migration is necessary, which this document provides by allowing B2BUA's or Proxies to perform the Session-ID generator and inserter role. Even within those device types, it is impractical to expect Kaplan Expires - May 2007 [Page 10] SIP Session Identifier November 2008 all B2BUA's to support this mechanism all at once, or any time in the near future. Therefore, it is expected that some B2BUA's and/or UA's will support generating and inserting Session-ID, while others will not support Session-ID at all. Due to the varying types of B2BUAs, such as SBCs, Application Servers, Feature Servers, and Softswitches of various flavors, and the numerous SIP deployment models in use, there are going to be cases in which Session-ID will fail to be a consistent value for all related dialogs, or fail to successfully match. The goal of this draft is to improve current deployments as much as possible - not to cover all possible scenarios - and in this author's opinion that is the best that can be done given the constraints. For example, if the UAC or UAS do not support this draft's mechanism, there is the possibility for out-of-dialog requests which need the matching behavior to fail if they reach the UAC or UAS without crossing a B2BUA which supports the matching mechanism. And the more hops away from the UAC/UAS such matching B2BUA's are, the more likely such a case will occur. Another example is for forked requests: if a UAC which does not support this mechanism sends a request to a Proxy or B2BUA which also does not support this mechanism, each fork could reach B2BUA's or UAS's which *do* support this mechanism. In such a case, each of those forked-to B2BUA/UAS will generate unique Session-IDs and put them in their responses, leading to two different Session-ID values for the same related dialogs temporarily. Eventually the UAC would only accept one of the dialogs (typically), and only one Session-ID would remain. These are just some examples; the number of possible scenarios involving devices which do not support this mechanism is unbounded. However the Session-ID mechanism, as defined in this document, should not cause any of them to fail if they would not have already failed without Session-ID, and should cause some of them to succeed that would otherwise have failed. 8. New Header The following table updates Table 2 in [RFC3261] and other defined extensions. Hdr-field when ACK BYE CAN INV OPT REG PRA INF REF UPD SUB NOT MSG ----------------------------------------------------------------- Session-ID R m m m m m m m m m m m m m Session-ID r m m m m m m m m m m m m m Kaplan Expires - May 2007 [Page 11] SIP Session Identifier November 2008 8.1. "Session-ID" header This draft proposes "Session-ID" to be added to the definition of the element "message-header" in the SIP message grammar. The Session-ID header is a single-instance header. The compact form of the header is requested to be: h [for "Hide from middleboxes" or "Help us help you" :) ] 8.2. Augmented BNF Definitions Session-ID = "Session-ID" HCOLON sess-id *( SEMI generic-param ) sess-id = 32(DIGIT / %x61-7A) ; 32 chars of [0-9a-z] NOTE: The sess-id value is case-SENSITIVE, just as Call-ID is, however only lower-case characters are defined and allowed. See the Security Considerations section for discussion about using header parameters in Session-ID header fields. 9. Example Exchange In the following example, Alice initiates a call to Bob. Alice generates a Session-ID header in the out-of-dialog INVITE. Alice generates the following: (note: much has been left out for simplicity) INVITE sip:bob@example.com SIP/2.0 Via: SIP/2.0/UDP 192.0.2.1:5060;branch=z9hG4bKnashds10 From: Alice ;tag=1234567 To: Bob Call-Id: 123456mcmxcix@1.2.3.4 Session-ID: f81d4fae7dec11d0a76500a0c91e6bf6 CSeq: 1 INVITE Contact: 10. Security Considerations There are several security considerations surrounding this document's mechanism. The Session-ID's value is created from the Call-ID using a hashing mechanism based on [RFC2104], using SHA-1 and a secret key known only to the system generating the Session-ID. Because the algorithm is defined in this document, it should be fairly secure from Kaplan Expires - May 2007 [Page 12] SIP Session Identifier November 2008 detecting the generator of the Session-ID, in terms of manufacturer or code base. The Session-ID generation algorithm should provide a reasonably random 128-bit Session-ID value, to avoid collisions, and would not let one re-create the original Call-ID. The secret key MUST only be used for the Session-ID mechanism, in case a weakness is found which reveals the key. One such weakness may be that a UAC generates one or more Call-ID's which have a property that makes determining the key more likely. Using a Session-ID value for out-of-dialog request matching criteria is potentially not as secure as using the Call-ID and tags. For one, Call-ID and tag values are potentially harder to guess due to their undefined lengths, depending on their generating systems. But more importantly, dialog matching mechanisms performed by SIP extensions typically use the Call-ID and both local and remote tags for matching. If only the Session-ID value were used for matching, for example, then all recipients of forked requests would have sufficient information to successfully match the dialogs. While some people may consider this a "feature" not a weakness, this document assumes otherwise and requires both the Session-ID and remote-tag to match, preventing any recipient but the matching forked one from being able to use it. But it does not use the local-tag, in order to increase likelihood of success. [Open issue: is that a problem? I don't think so, but it needs scrutiny] 10.1. Security considerations for B2BUA vendors and operators One of the requirements for the Session-ID is to be an identifier which cannot be used by a recipient to identify if the Call-ID has been changed by middle-boxes. That requirement is somewhat contradictory with the requirement to be usable for out-of-dialog matching purposes, because clearly a UA/B2BUA which performs the matching mechanism in this draft would detect the Call-ID and/or local-tag are not correct while the Session-ID and remote-tag are, and thus surmise that a middle-box had changed the Call-ID and/or local-tag. Some future mechanism similar to dialog-event package could also perform a check to see if the Call-ID had been changed by first trying only the Call-ID for a match, and upon failure trying the Session-ID, and if it succeeds they could surmise the Call-ID had been changed by a middle-box. B2BUA vendors and operators should note, however, that the only time a Call-ID would be changed by a B2BUA such that an out-of-dialog request using it does not successfully match is when a UA could send an out-of-dialog request to some node other than that B2BUA; if a B2BUA cannot or does not wish to prevent that from happening, then a Kaplan Expires - May 2007 [Page 13] SIP Session Identifier November 2008 change could be detected even *without* the Session-ID, simply by assuming a failed Call-ID match means a middle-box changed it. Therefore, Session-ID does not create a new "leak" of information that a Call-ID was changed by a B2BUA. To the contrary, it enables scenarios with such "leaks" to fix themselves, so that they do not create failed requests. If operators of the B2BUAs actually *want* such cases to fail, then they will choose to do whatever they want to make that happen, and nothing can stop them. If they do so by removing or changing the Session-ID, they would simply not be compliant with this document. It would be preferable for them to leave the Session-ID alone, and prevent B2BUA circumvention in a different way. Other techniques exist for doing so, but are beyond the scope of this draft. Furthermore, it is highly unlikely that SIP users or vendors would perform such a Call-ID check for anything but beneficial reasons, for example for troubleshooting purposes or to make the application work; it would be foolish to do otherwise because so many devices in modern deployments modify the Call-ID value, that it cannot reasonably be considered malicious for middle-boxes modify it. 10.2. Security considerations for extensions to the Session-ID In general, B2BUA behavior cannot be dictated by standards. They do whatever their owners/operators wish them to do, or whatever is necessary to make their applications work. This document attempts to normatively specify B2BUA behavior, by creating a SIP header value for which the properties are such that B2BUA's should have no legitimate reason to interfere. This effectively creates a "promise" that future uses of this Session-ID header field, including its value *and* future defined parameters, maintain this benign property. Any future extensions to the Session-ID mechanism and header field MUST maintain this property, or else B2BUA's will begin to modify it again or remove it, and its value will be lost. Manufacturers of SIP devices should note that there is no guarantee that a B2BUA will not inspect the Session-ID header field, and remove it if it does not comply with this document's value restrictions. Because of this, any uses for Session-ID header parameters MUST be documented in RFCs. Kaplan Expires - May 2007 [Page 14] SIP Session Identifier November 2008 11. IANA Considerations This document asks IANA for a new SIP header field, in long and compact form. 12. Acknowledgments Thanks to Raphael Coeffic, Bob Penfield, Dale Worley, Paul Kyzivat, and Ian Elz for their input. The list of current uses which need matching was taken from [draft-transparent-b2bua]. 13. References 13.1. Normative 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. [RFC2104] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. 13.2. Informative References [RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific Event Notification", RFC 3265, June 2002 [SIP-Identity] Peterson, J., Jennings, C., "Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP)", RFC 4474, August 2006. [Connected-Identity] Elwell, J., "Connected Identity in the Session Initiation Protocol (SIP)", RFC 4916, June 2007. [Replaces] Elwell, J., "The Session Initiation Protocol (SIP) "Replaces" Header", RFC 3891, September 2004. [Join] Mahy, R. and D. Petrie, "The Session Initiation Protocol (SIP) "Join" Header", RFC 3911, October 2004. [Target-Dialog] Rosenberg, J., "Request Authorization through Dialog Identification in the Session Initiation Protocol (SIP)", RFC 4538, June 2006. [dialog-events] Elwell, J., "The Session Initiation Protocol (SIP) "Replaces" Header", RFC 3891, September 2004. Kaplan Expires - May 2007 [Page 15] SIP Session Identifier November 2008 [RFC4575] Rosenberg, J., Schulzrinne, H., and O. Orit, "A Session Initiation Protocol (SIP) Event Package for Conference State", RFC 4575, August 2006. [KPML] Burger, E. and M. Dolly, "A Session Initiation Protocol (SIP) Event Package for Key Press Stimulus (KPML)", RFC 4730, November 2006. [RFC4122] Leach, P., Mealling, M., Salz, R., "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, July 2005. [media-ctrl] Boulton, C., Melanchuk, T., McGlashan, S., "Media Control Channel Framework", draft-ietf-mediactrl-sip-control- framework-06, October 2008. [draft-derive] Kuthan, J., Sisalem, D., Coeffic, R., Pascual V., "Dialog Event foR Identity VErification", draft-kuthan-sip- derive-00, October 2008. [draft-pass] Kaplan, H., "Private Extensions to the Session Initiation Protocol (SIP) for Asserter Identification within Trusted Networks", draft-kaplan-sip-asserter-identity-00, July 2008. [draft-transparent-b2bua] Marjou, X., Elz, I., Musgrave, P., "Best Current Practices for a Session Initiation Protocol (SIP) Transparent Back-To-Back User-Agent (B2BUA)", draft-marjou- sipping-b2bua-01, July 2007. Author's Address Hadriel Kaplan Acme Packet 71 Third Ave. Burlington, MA 01803, USA Email: hkaplan@acmepacket.com Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Kaplan Expires - May 2007 [Page 16]