Network Working Group P. Saint-Andre Internet-Draft Filament Obsoletes: 7248 (if approved) March 20, 2016 Intended status: Standards Track Expires: September 21, 2016 Interworking between the Session Initiation Protocol (SIP) and the Extensible Messaging and Presence Protocol (XMPP): Presence draft-ietf-stox-7248bis-07 Abstract This document defines a bidirectional protocol mapping for the exchange of presence information between the Session Initiation Protocol (SIP) and the Extensible Messaging and Presence Protocol (XMPP). This document obsoletes RFC 7248. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on September 21, 2016. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. Saint-Andre Expires September 21, 2016 [Page 1] Internet-Draft SIP-XMPP Interworking: Presence March 2016 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 Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Intended Audience . . . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Architectural Assumptions . . . . . . . . . . . . . . . . . . 4 5. Subscriptions to Presence Information . . . . . . . . . . . . 5 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5 5.2. XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . 6 5.2.1. Establishing a Presence Subscription . . . . . . . . 6 5.2.2. Refreshing a Presence Subscription . . . . . . . . . 10 5.2.3. Cancelling a Presence Subscription . . . . . . . . . 10 5.3. SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . 13 5.3.1. Establishing a Presence Subscription . . . . . . . . 13 5.3.2. Refreshing a Presence Subscription . . . . . . . . . 16 5.3.3. Cancelling a Presence Subscription . . . . . . . . . 18 6. Notifications of Presence Information . . . . . . . . . . . . 19 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 19 6.2. XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . 20 6.3. SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . 24 7. Polling for Presence Information . . . . . . . . . . . . . . 26 7.1. XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . 26 7.2. SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . 27 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 9. Privacy and Security Considerations . . . . . . . . . . . . . 28 9.1. Amplification Attacks . . . . . . . . . . . . . . . . . . 28 9.2. Presence Leaks . . . . . . . . . . . . . . . . . . . . . 29 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 10.1. Normative References . . . . . . . . . . . . . . . . . . 29 10.2. Informative References . . . . . . . . . . . . . . . . . 30 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 30 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 31 Saint-Andre Expires September 21, 2016 [Page 2] Internet-Draft SIP-XMPP Interworking: Presence March 2016 1. Introduction In order to help ensure interworking between presence systems that conform to the instant message / presence requirements [RFC2779], it is important to clearly define protocol mappings between such systems. Within the IETF, work has proceeded on two presence technologies: o Various extensions to the Session Initiation Protocol ([RFC3261]) for presence, in particular [RFC3856] o The Extensible Messaging and Presence Protocol (XMPP), which consists of a formalization of the core XML streaming protocols developed originally by the Jabber open-source community; the relevant specifications are [RFC6120] for the XML streaming layer and [RFC6121] for basic presence and instant-messaging extensions One approach to helping ensure interworking between these protocols is to map each protocol to the abstract semantics described in [RFC3860]; however, apparently that approach has never been implemented. The approach taken in this document is to directly map semantics from one protocol to another (i.e., from SIP/SIMPLE (SIP for Instant Messaging and Presence Leveraging Extensions) to XMPP and vice versa), because that is how existing systems solve the interworking problem. The architectural assumptions underlying such direct mappings are provided in [RFC7247], including mapping of addresses and error conditions. The mappings specified in this document cover basic presence functionality. Mapping of more advanced functionality (e.g., so-called "rich presence") is out of scope for this document. This document obsoletes RFC 7248. 2. Intended Audience The documents in this series are intended for use by software developers who have an existing system based on one of these technologies (e.g., SIP) and would like to enable communication from that existing system to systems based on the other technology (e.g., XMPP). We assume that readers are familiar with the core specifications for both SIP [RFC3261] and XMPP [RFC6120], with the base document for this series [RFC7247], and with the following presence-related specifications: o "A Presence Event Package for the Session Initiation Protocol" [RFC3856] Saint-Andre Expires September 21, 2016 [Page 3] Internet-Draft SIP-XMPP Interworking: Presence March 2016 o "Presence Information Data Format (PIDF)" [RFC3863] o "Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence" [RFC6121] o "SIP-Specific Event Notification" [RFC6665] 3. Terminology A number of terms used here (user, contact, subscription, notification, etc.) are explained in [RFC3261], [RFC3856], [RFC3857], [RFC6120], and [RFC6121]. This document uses some, but not all, of the presence-related terms defined in the Model for Presence and Instant Messaging [RFC2778]. In particular, the term "presence session" is used as described in [RFC6121] to mean a delimited time period in which an endpoint is online and available for communications. In flow diagrams, SIP traffic is shown using arrows such as "***>", whereas XMPP traffic is shown using arrows such as "...>". As in [RFC7247], the terms "SIP to XMPP Gateway" and "XMPP to SIP Gateway" are abbreviated as "S2X GW" and "X2S GW", respectively. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 4. Architectural Assumptions The fundamental architectural assumptions underlying SIP-XMPP interworking are described in [RFC7247]. In SIP, there are two ways that presence services are deployed on the server side: o Under the first model, described most fully in [RFC3857], a dedicated SIP Presence Server handles subscriptions to the presence event package. Instead of forwarding a SUBSCRIBE message to the SIP user, the Presence Server would act on the basis of a policy set by the SIP User Agent using the 'presence.winfo' event package, to which the SIP user would be subscribed. The SIP User Agent would then authorize the subscriber through some interaction with the Presence Server (for instance using XCAP [RFC4825]). Therefore, presence updates from the SIP User Agent would not be sent as NOTIFY messages to the XMPP user but as PUBLISH messages to the Presence Server, which would then generate NOTIFY messages to all active subscribers. Saint-Andre Expires September 21, 2016 [Page 4] Internet-Draft SIP-XMPP Interworking: Presence March 2016 o Under the second model, a SIP Presence Server acts in proxy mode and merely passes through the SUBSCRIBE and NOTIFY messages to the SIP User Agent. Because the behavior of the XMPP-to-SIP gateway is not changed by the SIP architectural model that is used, the diagrams and protocol flows in this document cover both options by labeling the end entity a "SIP User Agent or Presence Server". 5. Subscriptions to Presence Information 5.1. Overview Both XMPP and presence-aware SIP systems enable entities (often, but not necessarily, human users) to subscribe to the presence of other entities. XMPP presence subscriptions are specified in [RFC6121]. Presence subscriptions using a SIP event package for presence are specified in [RFC3856]. As described in [RFC6121], XMPP presence subscriptions are managed using XMPP stanzas of type "subscribe", "subscribed", "unsubscribe", and "unsubscribed". The main subscription states are: o "none" (neither the user nor the contact is subscribed to the other's presence information) o "from" (the user has a subscription from the contact) o "to" (the user has a subscription to the contact's presence information) o "both" (both user and contact are subscribed to each other's presence information) As described in [RFC3856], SIP presence subscriptions are managed through the use of SIP SUBSCRIBE events sent from a SIP User Agent or Presence Server to an intended recipient who is most generally referenced by a Presence URI of the form but who might be referenced by a SIP or SIPS ((Session Initiation Protocol Secure) URI of the form or . In practice, 'pres' URIs are rarely used, which is why the examples in this document use 'sip' URIs. The subscription models underlying XMPP and SIP differ mainly in the fact that XMPP presence subscriptions are long-lived (indeed permanent if not explicitly cancelled, so that a subscription need never be refreshed during any given presence session [RFC6121]), whereas SIP presence subscriptions are short-lived (the default time- Saint-Andre Expires September 21, 2016 [Page 5] Internet-Draft SIP-XMPP Interworking: Presence March 2016 to-live of a SIP presence subscription is 3600 seconds, as specified in Section 6.4 of [RFC3856], so that a subscription needs to be explicitly refreshed if it will have the appearance of being permanent or even of lasting as long as the duration of a presence session). This disparity has implications for the handling of subscription cancellations in either direction and, from the SIP side, subscription refreshes. 5.2. XMPP to SIP 5.2.1. Establishing a Presence Subscription The following diagram illustrates the protocol flow for establishing a presence subscription from an XMPP user to a SIP contact, as further explained in the text and examples after the diagram. XMPP XMPP SIP SIP UA or Client Server Proxy Presence Server | + X2S GW | | | | | | | (F1) XMPP | | | | subscribe | | | |...........>| | | | | (F2) SIP | | | | SUBSCRIBE | | | |***********>| | | | | (F3) SIP | | | | SUBSCRIBE | | | |***********>| | | | (F4) SIP | | | | 200 OK | | | |<***********| | | (F5) SIP | | | | 200 OK | | | |<***********| | | | | (F6) SIP | | | | NOTIFY | | | | (pending) | | | |<***********| | | (F7) SIP | | | | NOTIFY | | | |<***********| | | | (F8) SIP | | | | 200 OK | | | |***********>| | | | | (F9) SIP | | | | 200 OK | | | |***********>| Saint-Andre Expires September 21, 2016 [Page 6] Internet-Draft SIP-XMPP Interworking: Presence March 2016 | | | (F10) SIP | | | | NOTIFY | | | | (active) | | | |<***********| | | (F11) SIP | | | | NOTIFY | | | |<***********| | | | (F12) SIP | | | | 200 OK | | | |***********>| | | | | (F13) SIP | | | | 200 OK | | | |***********>| | (F14) XMPP | | | | subscribed | | | |<...........| | | | (F15) XMPP | | | | presence | | | |<...........| | | | | | | An XMPP user (e.g., juliet@example.com) initiates a subscription by sending a subscription request to a SIP contact (e.g., romeo@example.net), and the contact either accepts or declines the request. If the SIP contact accepts the request, the XMPP user will have a subscription to the SIP contact's presence information until (1) the XMPP user unsubscribes or (2) the SIP contact cancels the subscription. The subscription request is encapsulated in a stanza of type "subscribe": Example 1: XMPP User Subscribes to SIP Contact (F1) | Upon receiving such a stanza, the XMPP server to which Juliet has connected needs to determine the identity of the domainpart in the 'to' address, which it does by following the procedures explained in Section 5 of [RFC7247]. If the domain is a SIP domain, the XMPP server will hand off the stanza to an associated XMPP-to-SIP gateway or connection manager that natively communicates with presence-aware SIP proxies. Saint-Andre Expires September 21, 2016 [Page 7] Internet-Draft SIP-XMPP Interworking: Presence March 2016 The XMPP-to-SIP gateway is then responsible for translating the XMPP subscription request into a SIP SUBSCRIBE request addressed from the XMPP user to the SIP contact: Example 2: SIP Transformation of XMPP Subscription Request (F2) | SUBSCRIBE sip:romeo@example.net SIP/2.0 | Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bKna998sk | From: ;tag=j89d | Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C | Event: presence | Max-Forwards: 70 | CSeq: 1 SUBSCRIBE | Contact: ;gr=yn0cl4bnw0yr3vym | Accept: application/pidf+xml | Expires: 3600 | Content-Length: 0 Once the SIP proxy has delivered the SIP SUBSCRIBE to the SIP User Agent or Presence Server (F3, no example shown), the SIP User Agent then would send a response indicating acceptance of the subscription request: Example 3: SIP User Accepts Subscription Request (F4) | SIP/2.0 200 OK | Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk | From: ;tag=j89d | To: ;tag=ffd2 | Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C | CSeq: 1 SUBSCRIBE | Contact: ;gr=dr4hcr0st3lup4c | Expires: 3600 | Content-Length: 0 In accordance with Section 6.7 of [RFC3856], the XMPP-to-SIP gateway needs to consider the subscription state to be "neutral" until it receives a NOTIFY message with a Subscription-State header [RFC6665] whose value is "active". Therefore, the SIP User Agent or Presence Server SHOULD immediately send such a NOTIFY message (see Section 6 below). In case the XMPP-to-SIP gateway initially receives one or more NOTIFY messages with Subscription-State header whose value is "pending" (F6), it MUST respond to them on the SIP side but not generate any presence stanzas towards the XMPP User. Saint-Andre Expires September 21, 2016 [Page 8] Internet-Draft SIP-XMPP Interworking: Presence March 2016 Example 4: SIP User Agent or Presence Server Sends Presence Notification (F10) | NOTIFY sip:juliet@example.com SIP/2.0 | Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk | From: ;tag=j89d | To: ;tag=ffd2 | Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C | Event: presence | Subscription-State: active;expires=499 | Max-Forwards: 70 | CSeq: 2 NOTIFY | Content-Type: application/pidf+xml | Content-Length: 193 | | | | | | open | away | | | Upon receiving the first NOTIFY with a subscription state of active, the XMPP-to-SIP gateway returns a 200 OK to the SIP User Agent or Presence Server (F12, no example shown). The XMPP-to-SIP gateway also generates a stanza of type "subscribed": Example 5: XMPP User Receives Acknowledgement from SIP Contact (F14) | As described in Section 6, the XMPP-to-SIP gateway also generates a presence notification addressed to the XMPP user: Example 6: XMPP User Receives Presence Notification from SIP Contact (F15) Saint-Andre Expires September 21, 2016 [Page 9] Internet-Draft SIP-XMPP Interworking: Presence March 2016 | 5.2.2. Refreshing a Presence Subscription It is the responsibility of the XMPP-to-SIP gateway to set the value of the Expires header and to periodically renew the subscription on the SIMPLE side of the gateway so that the subscription appears to be permanent to the XMPP user. For example, the XMPP-to-SIP gateway SHOULD send a new SUBSCRIBE request to the SIP contact whenever the XMPP user initiates a presence session with the XMPP server by sending initial presence to its XMPP server. The XMPP-to-SIP gateway also SHOULD send a new SUBSCRIBE request to the SIP contact whenever the SIP presence subscription is scheduled to expire during the XMPP user's active presence session. The rules regarding SIP SUBSCRIBE requests for the purpose of establishing and refreshing a presence subscription are provided in [RFC6665]. Those rules also apply to XMPP-to-SIP gateways. Furthermore, an XMPP-to-SIP gateway MUST consider the XMPP subscription to be permanently cancelled (and so inform the XMPP user) if it receives a SIP response of 403, 489, or 603. By contrast, it is appropriate to consider a SIP response of 423 or 481 to be a transient error and to honor the long-lived XMPP presence subscription. [RFC6665] explains more detailed considerations about the handling of SIP responses in relation to subscription requests and refreshes. Finally, see the security considerations section (Section 9) of this document for important information and requirements regarding the security implications of subscription refreshes. 5.2.3. Cancelling a Presence Subscription The following diagram illustrates the protocol flow for cancelling an XMPP user's presence subscription to a SIP contact, as further explained in the text and examples after the diagram. XMPP XMPP SIP SIP UA or Client Server Proxy Presence Server | + X2S GW | | | | | | | (F16) XMPP | | | |unsubscribe | | | |...........>| | | | | (F17) SIP | | | | SUBSCRIBE | | Saint-Andre Expires September 21, 2016 [Page 10] Internet-Draft SIP-XMPP Interworking: Presence March 2016 | | Expires: 0 | | | |***********>| | | | | (F18) SIP | | | | SUBSCRIBE | | | | Expires: 0 | | | |***********>| | | | (F19) SIP | | | | 200 OK | | | |<***********| | | (F20) SIP | | | | 200 OK | | | |<***********| | | (F21) XMPP | | | |unsubscribed| | | |<...........| | | | | (F22) SIP | | | | NOTIFY | | | | terminated | | | |***********>| | | | | (F23) SIP | | | | NOTIFY | | | | terminated | | | |***********>| | | | (F24) SIP | | | | 200 OK | | | |<***********| | | (F25) SIP | | | | 200 OK | | | |<***********| | | | | | At any time after subscribing, the XMPP user can unsubscribe from the contact's presence. This is done by sending a stanza of type "unsubscribe": Example 7: XMPP User Unsubscribes from SIP Contact (F16) | The XMPP-to-SIP gateway is responsible for translating the unsubscribe command into a SIP SUBSCRIBE request with the Expires header set to a value of zero: Saint-Andre Expires September 21, 2016 [Page 11] Internet-Draft SIP-XMPP Interworking: Presence March 2016 Example 8: SIP Transformation of XMPP Unsubscribe (F17) | SUBSCRIBE sip:romeo@example.net SIP/2.0 | Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk | From: ;tag=j89d | To: ;tag=ffd2 | Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C | Event: presence | Max-Forwards: 70 | CSeq: 42 SUBSCRIBE | Contact: ;gr=yn0cl4bnw0yr3vym | Accept: application/pidf+xml | Expires: 0 | Content-Length: 0 Upon receiving the SIP 200 OK acknowleding the unsubscribe, the XMPP- to-SIP gateway SHOULD send a stanza of type "unsubscribed" addressed to the XMPP user: Example 9: XMPP User Receives Unsubscribed Notification (F21) | In accordance with Section 4.4.1 of [RFC6665], the XMPP-to-SIP gateway is then responsible for sending a NOTIFY with a "Subscription-State" of "terminated" in order to formally end the subscription and the associated SIP dialog. Example 10: XMPP-to-SIP Gateway Sends Presence Notification to Terminate Subscription (F25) | NOTIFY sip:juliet@example.com SIP/2.0 | Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk | From: ;tag=j89d | To: ;tag=ffd2 | Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C | Event: presence | Subscription-State: terminated | Max-Forwards: 70 | CSeq: 43 NOTIFY | Content-Type: application/pidf+xml | Content-Length: 0 Saint-Andre Expires September 21, 2016 [Page 12] Internet-Draft SIP-XMPP Interworking: Presence March 2016 5.3. SIP to XMPP 5.3.1. Establishing a Presence Subscription The following diagram illustrates the protocol flow for establishing a presence subscription from a SIP user to an XMPP contact, as further explained in the text and examples after the diagram. SIP SIP XMPP XMPP UA Proxy Server Client | + S2X GW | | | | | | | (F26) SIP | | | | SUBSCRIBE | | | |**********>| | | | (F27) SIP | | | | 200 OK | | | |<**********| | | | | (F28) XMPP | | | | subscribe | | | |...........>| | | | | (F29) XMPP| | | | subscribe | | | |..........>| | | | (F30) XMPP| | | | subscribed| | | |<..........| | | (F31) XMPP | | | | subscribed | | | |<...........| | | (F32) SIP | | | | NOTIFY | | | | (active) | | | |<**********| | | | (F33) SIP | | | | 200 OK | | | |**********>| | | | | | | A SIP User Agent initiates a subscription to a contact's presence information by sending a SIP SUBSCRIBE request to the contact. The following is an example of such a request: Example 11: SIP User Subscribes to XMPP Contact (F26) Saint-Andre Expires September 21, 2016 [Page 13] Internet-Draft SIP-XMPP Interworking: Presence March 2016 | SUBSCRIBE sip:juliet@example.com SIP/2.0 | Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk | From: ;tag=xfg9 | To: | Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11 | Event: presence | Max-Forwards: 70 | CSeq: 1 SUBSCRIBE | Contact: ;gr=dr4hcr0st3lup4c | Accept: application/pidf+xml | Content-Length: 0 Notice that the Expires header was not included in the SUBSCRIBE request; this means that the default value of 3600 (i.e., 3600 seconds = 1 hour) applies. Upon receiving the SUBSCRIBE, the SIP proxy needs to determine the identity of the domain portion of the Request-URI, which it does by following the procedures explained in Section 5 of [RFC7247]. If the domain is an XMPP domain, the SIP proxy will hand off the SUBSCRIBE to an associated SIP-to-XMPP gateway or connection manager that natively communicates with XMPP servers. The SIP-to-XMPP gateway is then responsible for translating the SUBSCRIBE into an XMPP subscription request addressed from the SIP user to the XMPP contact: Example 12: XMPP Transformation of SIP SUBSCRIBE (F28) | In accordance with [RFC6121], the XMPP user's server delivers the presence subscription request to the XMPP user (or, if a subscription already exists in the XMPP user's roster, the XMPP server SHOULD auto-reply with a stanza of type 'subscribed'). The "happy path" is for the XMPP user to approve the subscription request by generating a stanza of type "subscribed" (F30). The XMPP server then stamps that presence stanza with the 'from' address of the XMPP contact and sends it to the SIP user (F31). Upon receiving the stanza, the SIP-to-XMPP gateway generates an empty SIP NOTIFY message with a Subscription-State header [RFC6665] of "active", which serves to inform the SIP user that the subscription request has been approved (F32). Saint-Andre Expires September 21, 2016 [Page 14] Internet-Draft SIP-XMPP Interworking: Presence March 2016 Example 13: XMPP User Approves Subscription Request (F31) | Example 14: Subscription Request Approved (F32) | NOTIFY sip:romeo@example.net SIP/2.0 | Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk | From: ;tag=xfg9 | To: ;tag=ur93 | Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11 | Event: presence | Subscription-State: active | Max-Forwards: 70 | CSeq: 2 NOTIFY | Content-Length: 0 As an alternative to the "happy path", the XMPP user could decline the subscription request by generating a stanza of type "unsubscribed". The XMPP server would stamp that presence stanza with the 'from' address of the XMPP contact and would send it to the SIP user. The SIP-to-XMPP gateway then transforms that stanza into an empty SIP NOTIFY message with a Subscription-State header [RFC6665] of "terminated" and a reason of "rejected": Example 15: XMPP User Rejects Subscription Request | Saint-Andre Expires September 21, 2016 [Page 15] Internet-Draft SIP-XMPP Interworking: Presence March 2016 Example 16: Subscription Request Rejected | NOTIFY sip:romeo@example.net SIP/2.0 | Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk | From: ;tag=xfg9 | To: ;tag=ur93 | Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11 | Event: presence | Subscription-State: terminated;reason=rejected | Max-Forwards: 70 | CSeq: 2 NOTIFY | Content-Length: 0 5.3.2. Refreshing a Presence Subscription For as long as a SIP user is online and interested in receiving presence notifications from the XMPP contact, the user's SIP User Agent is responsible for periodically refreshing the subscription by sending an updated SUBSCRIBE request with an appropriate value for the Expires header. In response, the presence-aware SIP-to-XMPP gateway sends a SIP NOTIFY to the user agent (per [RFC6665]); if the SIP-to-XMPP gateway has meaningful information about the availability state of the XMPP user (e.g., obtained from the core presence session in the XMPP server or learned by sending a presence probe as described under Section 7) then the NOTIFY communicates that information (e.g., by including a PIDF body [RFC3863] with the relevant data), whereas if the SIP-to-XMPP gateway does not have meaningful information about the availability state of the XMPP user then the NOTIFY MUST be empty as allowed by [RFC6665]. Note: In SIP there is no necessary connection between a presence session and an outbound subscription to a contact's presence; for example, a SIP user agent could end its subscription to the contact's presence yet still keep sending presence notifications to the contact, or it could send a presence notification indicating that it is offline (i.e., by sending a PIDF document with a basic status of "closed") while still maintaining a subscription to the contact's presence. However, to simplify the description of handling notifications and subscriptions this document assumes that if a SIP user goes offline it will also terminate its outbound subscriptions. Saint-Andre Expires September 21, 2016 [Page 16] Internet-Draft SIP-XMPP Interworking: Presence March 2016 When the SIP user ends its presence session by going offline (e.g., by sending a basic status of "closed") and terminating its outbound subscriptions, it is the responsibility of the presence-aware SIP-to- XMPP gateway to properly handle the difference between short-lived SIP presence subscriptions and long-lived XMPP presence subscriptions. The SIP-to-XMPP gateway has two options when the SIP user's subscription expires: o Cancel the subscription (i.e., treat it as temporary) and send an XMPP stanza of type "unsubscribe" to the XMPP contact; this honors the SIP semantic but will seem strange to the XMPP contact (because it will appear that the SIP user has cancelled a long-lived subscription). o Treat the SIP subscription as if it were long-lived and: 1. send a SIP NOTIFY request to the SIP User Agent containing a PIDF document specifying that the XMPP contact now has a basic status of "closed", including a Subscription-State header [RFC6665] of "terminated" with a reason of "timeout" 2. send an XMPP stanza of type "unavailable" to the XMPP contact; this will seem more natural to the XMPP contact Which of these options a presence-aware SIP-to-XMPP gateway chooses is up to the implementation. If the implementation chooses the first option, the protocol generated would be as follows: Example 17: XMPP Handling of Temporary Subscription Expiry | If the implementation chooses the second option, the protocol generated would be as follows: Example 18: SIP Handling of Long-Lived Subscription Expiry Saint-Andre Expires September 21, 2016 [Page 17] Internet-Draft SIP-XMPP Interworking: Presence March 2016 | NOTIFY sip:romeo@example.net SIP/2.0 | Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk | From: ;tag=xfg9 | To: ;tag=ur93 | Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11 | Event: presence | Subscription-State: terminated;reason=timeout | Max-Forwards: 70 | CSeq: 6 NOTIFY | Content-Type: application/pidf+xml | Content-Length: 194 | | | | | | closed | | | Example 19: XMPP Handling of Long-Lived Subscription Expiry | 5.3.3. Cancelling a Presence Subscription At any time, the SIP User Agent can cancel the subscription by sending a SUBSCRIBE message whose Expires header is set to a value of zero ("0"): Example 20: SIP User Cancels Subscription Saint-Andre Expires September 21, 2016 [Page 18] Internet-Draft SIP-XMPP Interworking: Presence March 2016 | SUBSCRIBE sip:juliet@example.com SIP/2.0 | Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk | From: ;tag=xfg9 | To: ;tag=ur93 | Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11 | Event: presence | Max-Forwards: 70 | CSeq: 66 SUBSCRIBE | Contact: ;gr=dr4hcr0st3lup4c | Expires: 0 | Content-Length: 0 As above, upon receiving such a request, a presence-aware SIP-to-XMPP gateway is responsible for doing one of the following: o Cancel the subscription (i.e., treat it as temporary) and send an XMPP stanza of type "unsubscribe" to the XMPP contact. o Treat the SIP subscription as it were long-lived and 1. send a SIP NOTIFY request to the SIP User Agent containing a PIDF document specifying that the XMPP contact now has a basic status of "closed", including a Subscription-State header [RFC6665] of "terminated" with a reason of "timeout" 2. send an XMPP stanza of type "unavailable" to the XMPP contact 6. Notifications of Presence Information 6.1. Overview Both XMPP and presence-aware SIP systems enable entities (often, but not necessarily, human users) to send presence notifications to other entities. At its most basic, the term "presence" refers to information about an entity's "on/off" availability for communication on a network. Often, this basic concept is supplemented by information that further specifies the entity's context or status while available for communication; these availability states commonly include "away" and "do not disturb". Some systems and protocols extend the concepts of presence and availability even further and refer to any relatively ephemeral information about an entity as a kind of presence; categories of such "extended presence" include geographical location (e.g., GPS coordinates), user mood (e.g., grumpy), user activity (e.g., walking), and ambient environment (e.g., noisy). This document focuses on the "least common denominator" of network availability only, although future documents Saint-Andre Expires September 21, 2016 [Page 19] Internet-Draft SIP-XMPP Interworking: Presence March 2016 might address broader notions of presence, including availability states and extended presence. [RFC6121] defines how XMPP stanzas can indicate availability (via absence of a 'type' attribute) or lack of availability (via a 'type' attribute with a value of "unavailable"). SIP presence using a SIP event package for presence is specified in [RFC3856]. As described in [RFC6121], XMPP presence information about an entity is communicated by means of an XML stanza sent over an XML stream. This document assumes that such a stanza is sent from an XMPP client to an XMPP server over an XML stream negotiated between the client and the server, and that the client is controlled by a human user. In general, XMPP presence is sent by the user to the user's server and then broadcast to all entities who are subscribed to the user's presence information. As described in [RFC3856], presence information about an entity is communicated by means of a SIP NOTIFY event sent from a SIP User Agent to an intended recipient who is most generally referenced by a Presence URI of the form but who might be referenced by a SIP or SIPS URI of the form or . This document addresses basic presence or network availability only, not the various extensions to SIP and XMPP for "rich presence" such as [RFC4480], [XEP-0107], and [XEP-0108]. 6.2. XMPP to SIP When Juliet interacts with her XMPP client to modify her presence information (or when her client automatically updates her presence information, e.g., via an "auto-away" feature), her client generates an XMPP stanza. The syntax of the stanza, including required and optional elements and attributes, is defined in [RFC6121]. The following is an example of such a stanza: Example 21: XMPP User Sends Presence Notification | Upon receiving such a stanza, the XMPP server to which Juliet has connected broadcasts it to all subscribers who are authorized to receive presence notifications from Juliet (this is similar to the SIP NOTIFY method). For each subscriber, broadcasting the presence notification involves adding the 'to' address of the subscriber and Saint-Andre Expires September 21, 2016 [Page 20] Internet-Draft SIP-XMPP Interworking: Presence March 2016 then either delivering the notification to a local recipient (if the hostname in the subscriber's address matches one of the hostnames serviced by the XMPP server) or attempting to route it to the foreign domain that services the hostname in the subscriber's address. If the notification is bound for an address at a foreign domain, the XMPP server needs to determine the identity of the domainpart in the 'to' address, which it does by following the procedures discussed in [RFC7247]. If the domain is a SIP domain, the XMPP server will hand off the stanza to an associated XMPP-to-SIP gateway or connection manager that natively communicates with presence-aware SIP proxy (no example shown). The XMPP-to-SIP gateway is then responsible for translating the XMPP stanza into a SIP NOTIFY request and included PIDF document from the XMPP user to the SIP contact. Example 22: SIP Transformation of XMPP Presence Notification | NOTIFY sip:juliet@example.com SIP/2.0 | Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bKna998sk | From: ;tag=gh19 | To: | Contact: ;gr=yn0cl4bnw0yr3vym | Call-ID: 2B44E147-3B53-45E4-9D48-C051F3216D14 | Event: presence | Subscription-State: active;expires=599 | Max-Forwards: 70 | CSeq: 2 NOTIFY | Content-Type: application/pidf+xml | Content-Length: 192 | | | | | | open | away | | | The mapping of XMPP syntax elements to SIP syntax elements MUST be as shown in the following table. (Mappings for elements not mentioned are undefined.) Saint-Andre Expires September 21, 2016 [Page 21] Internet-Draft SIP-XMPP Interworking: Presence March 2016 +-----------------------------+---------------------------+ | XMPP Element or Attribute | SIP Header or PIDF Data | +-----------------------------+---------------------------+ | stanza | "Event: presence" (1) | +-----------------------------+---------------------------+ | XMPP resource identifier | tuple 'id' attribute (2) | +-----------------------------+---------------------------+ | from | From | +-----------------------------+---------------------------+ | id | no mapping (3) | +-----------------------------+---------------------------+ | to | To | +-----------------------------+---------------------------+ | type | basic status (4) (5) | +-----------------------------+---------------------------+ | xml:lang | Content-Language | +-----------------------------+---------------------------+ | | priority for tuple (6) | +-----------------------------+---------------------------+ | | no mapping (7) | +-----------------------------+---------------------------+ | | | +-----------------------------+---------------------------+ Table 1: Presence Syntax Mapping from XMPP to SIP Note the following regarding these mappings: 1. Only an XMPP stanza that lacks a 'type' attribute or whose 'type' attribute has a value of "unavailable" MUST be mapped by an XMPP-to-SIP gateway to a SIP NOTIFY request, because those are the only stanzas that represent notifications. 2. The PIDF schema defines the tuple 'id' attribute as having a datatype of "xs:ID"; because this datatype is more restrictive than the "xs:string" datatype for XMPP resourceparts (in particular, a number is not allowed as the first character of an ID), it is RECOMMENDED to prepend the resourcepart with "ID-" or some other alphabetic string when mapping from XMPP to SIP. 3. In practice, XMPP stanzas often do not include the 'id' attribute. 4. Because the lack of a 'type' attribute indicates that an XMPP entity is available for communications, the XMPP-to-SIP gateway MUST map that information to a PIDF basic status of "open". Saint-Andre Expires September 21, 2016 [Page 22] Internet-Draft SIP-XMPP Interworking: Presence March 2016 Because a 'type' attribute with a value of "unavailable" indicates that an XMPP entity is not available communications, the XMPP-to-SIP gateway MUST map that information to a PIDF status of "closed". 5. When the XMPP-to-SIP gateway receives XMPP presence of type "unavailable" from the XMPP contact, it sends a SIP NOTIFY request from the XMPP contact to the SIP User Agent containing a PIDF document specifying that the XMPP contact now has a basic status of "closed". 6. The value of the XMPP element is an integer between -128 and +127, whereas the value of the PIDF element's 'priority' attribute is a decimal number from zero to one inclusive, with a maximum of three decimal places. If the value of the XMPP element is negative, an XMPP-to-SIP gateway MUST NOT map the value. If an XMPP-to-SIP gateway maps positive values, it SHOULD treat XMPP priority 0 as PIDF priority 0 and XMPP priority 127 as PIDF priority 1, mapping intermediate values appropriately so that they are unique (e.g., XMPP priority 1 to PIDF priority 0.007, XMPP priority 2 to PIDF priority 0.015, and so on up through mapping XMPP priority 126 to PIDF priority 0.992; note that this is an example only and that the exact mapping is up to the implementation). 7. Some implementations support custom extensions to encapsulate detailed information about availability; however, there is no need to standardize a PIDF extension for this purpose, because PIDF is already extensible and thus the element (qualified by the 'jabber:client' namespace) can be included directly in the PIDF XML. The examples in this document illustrate this usage, which is RECOMMENDED. The most useful values are likely "away" and "dnd", although note that the latter value merely means "busy" and does not imply that a server or client ought to block incoming traffic while the user is in that state. Naturally, an XMPP-to-SIP gateway can choose to translate a custom extension into an established value of the element [RFC6121] or translate a element into a custom extension that the XMPP-to-SIP gateway knows is supported by the user agent of the intended recipient. Unfortunately, this behavior does not guarantee that information will not be lost; to help prevent information loss, an XMPP-to-SIP gateway ought to include both the element and the custom extension if it cannot suitably translate the custom value into a value. However, there is no guarantee that the SIP receiver will render a standard XMPP value or custom extension. Saint-Andre Expires September 21, 2016 [Page 23] Internet-Draft SIP-XMPP Interworking: Presence March 2016 In XMPP, a user can connect with multiple devices at the same time [RFC6120]; for presence notification purposes [RFC6121], each device is associated with a distinct resourcepart [RFC7622] and a contact's user agent will receive a separate presence notification from each of the user's devices. Although the interpretation of multiple presence notifications from a single user is a matter of implementation by the contact's user agent, typically the user agent will show the "most available" status for the contact (e.g., if the user is online with three devices, one of which is away, one of which is in do not disturb mode, and one of which is available with no qualifications, then the status shown will simply be available. In SIP, it is reasonable for a user agent to model multiple presence notifications from an XMPP user in the same way that it would handle multiple tuples from a SIP user. 6.3. SIP to XMPP When Romeo changes his presence, his SIP User Agent generates a SIP NOTIFY request for any active subscriptions. The syntax of the NOTIFY request is defined in [RFC3856]. The following is an example of such a request: Example 23: SIP User Sends Presence Notification | NOTIFY sip:romeo@example.net SIP/2.0 | Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk | From: ;tag=yt66 | To: ;tag=bi54 | Contact: ;gr=dr4hcr0st3lup4c | Call-ID: C33C6C9D-0F4A-42F9-B95C-7CE86B526B5B | Event: presence | Subscription-State: active;expires=499 | Max-Forwards: 70 | CSeq: 8 NOTIFY | Content-Type: application/pidf+xml | Content-Length: 193 | | | | | | closed | | | Saint-Andre Expires September 21, 2016 [Page 24] Internet-Draft SIP-XMPP Interworking: Presence March 2016 Upon receiving the NOTIFY, the SIP proxy needs to determine the identity of the domain portion of the Request-URI, which it does by following the procedures discussed in [RFC7247]. If the domain is an XMPP domain, the SIP proxy will hand off the NOTIFY to an associated SIP-to-XMPP gateway or connection manager that natively communicates with XMPP servers. The SIP-to-XMPP gateway is then responsible for translating the NOTIFY into an XMPP stanza addressed from the SIP user to the XMPP contact: Example 24: XMPP Transformation of SIP Presence Notification | The mapping of SIP syntax elements to XMPP syntax elements MUST be as shown in the following table. (Mappings for elements not mentioned are undefined.) +---------------------------+-----------------------------+ | SIP Header or PIDF Data | XMPP Element or Attribute | +---------------------------+-----------------------------+ | basic status | type (1) | +---------------------------+-----------------------------+ | Content-Language | xml:lang | +---------------------------+-----------------------------+ | From | from | +---------------------------+-----------------------------+ | priority for tuple | (2) | +---------------------------+-----------------------------+ | To | to | +---------------------------+-----------------------------+ | | | +---------------------------+-----------------------------+ | | (3) | +---------------------------+-----------------------------+ Table 2: Presence Syntax Mapping from SIP to XMPP Note the following regarding these mappings: 1. A PIDF basic status of "open" MUST be mapped to no 'type' attribute, and a PIDF basic status of "closed" MUST be mapped to a 'type' attribute whose value is "unavailable". Saint-Andre Expires September 21, 2016 [Page 25] Internet-Draft SIP-XMPP Interworking: Presence March 2016 2. See the notes following Table 1 of this document regarding mapping of presence priority. 3. If a SIP implementation supports the element (qualified by the 'jabber:client' namespace) as a PIDF extension for availability status as described in the notes following Table 1 of this document, the SIP-to-XMPP gateway is responsible for including that element in the XMPP presence notification. 7. Polling for Presence Information Both SIP and XMPP provide methods for explicitly requesting one-time information about the current presence status of another entity. These are "polling" methods as opposed to the standing subscription methods described in the rest of this document. 7.1. XMPP to SIP In XMPP, an explicit request for information about current presence status is completed by sending a stanza of type "probe": Example 25: XMPP Server Sends Presence Probe on Behalf of XMPP User | Note: As described in [RFC6121], presence probes are used by XMPP servers to request presence on behalf of XMPP users; XMPP clients are discouraged from sending presence probes, because retrieving presence is a service that servers provide. A SIP-to-XMPP gateway would transform the presence probe into its SIP equivalent, which is a SUBSCRIBE request with an Expires header value of zero: Example 26: SIP Transformation of XMPP Presence Probe Saint-Andre Expires September 21, 2016 [Page 26] Internet-Draft SIP-XMPP Interworking: Presence March 2016 | SUBSCRIBE sip:romeo@example.net SIP/2.0 | Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bKna998sk | From: ;tag=j89d | Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C | Event: presence | Max-Forwards: 70 | CSeq: 1 SUBSCRIBE | Contact: ;gr=yn0cl4bnw0yr3vym | Accept: application/pidf+xml | Expires: 0 | Content-Length: 0 As described in [RFC3856], this cancels any subscription but causes a NOTIFY to be sent to the subscriber, just as a presence probe does (the transformation rules for presence notifications have been previously described in Section 6.2 of this document). 7.2. SIP to XMPP In SIP, an explicit request for information about current presence status is effectively completed by sending a SUBSCRIBE with an Expires header value of zero: Example 27: SIP User Sends Presence Request | SUBSCRIBE sip:juliet@example.com SIP/2.0 | Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk | From: ;tag=yt66 | Call-ID: 717B1B84-F080-4F12-9F44-0EC1ADE767B9 | Event: presence | Max-Forwards: 70 | CSeq: 1 SUBSCRIBE | Contact: ;gr=dr4hcr0st3lup4c | Expires: 0 | Content-Length: 0 When honoring the long-lived semantics of an XMPP presence subscription, a presence-aware SIP-to-XMPP gateway translates such a SIP request into a stanza of type "probe" if it does not already have presence information about the contact: Saint-Andre Expires September 21, 2016 [Page 27] Internet-Draft SIP-XMPP Interworking: Presence March 2016 Example 28: XMPP Transformation of SIP Presence Request | 8. IANA Considerations This document makes no requests of IANA. 9. Privacy and Security Considerations Detailed privacy and security considerations for presence protocols are given in [RFC2779], for SIP-based presence in [RFC3856] (see also [RFC3261]), and for XMPP-based presence in [RFC6121] (see also [RFC6120]). 9.1. Amplification Attacks The mismatch between long-lived XMPP presence subscriptions and short-lived SIP presence subscriptions introduces the possibility of an amplification attack launched from the XMPP network against a SIP presence server (because each long-lived XMPP presence subscription would typically result in multiple subscription refresh requests on the SIP side of an XMPP-to-SIP gateway). Therefore, access to an XMPP-to-SIP gateway SHOULD be restricted in various ways; for example: o Only an XMPP service that carefully controls account provisioning and provides effective methods for the administrators to control the behavior of registered users ought to host an XMPP-to-SIP gateway (e.g., not a service that offers open account registration). o An XMPP-to-SIP gateway ought to be associated only with a single domain or trust realm. For example, an XMPP-to-SIP gateway hosted at simple.example.com ought to allow only users within the example.com domain to access the XMPP-to-SIP gateway, not users within example.org, example.net, or any other domain (unless they are part of the same multi-tenanted environment as example.com). This helps to prevent the gateway equivalent of open relays that are shared across XMPP domains from different trust realms. If a SIP presence server receives communications through an XMPP-to- SIP gateway from users who are not associated with a domain that is so related to the hostname of the XMPP-to-SIP gateway, it SHOULD (based on local service provisioning) refuse to service such users or Saint-Andre Expires September 21, 2016 [Page 28] Internet-Draft SIP-XMPP Interworking: Presence March 2016 refuse to receive traffic from the XMPP-to-SIP gateway. As a further check, whenever an XMPP-to-SIP gateway seeks to refresh an XMPP user's long-lived subscription to a SIP user's presence, it first sends an XMPP stanza of type "probe" from the address of the XMPP-to-SIP gateway to the "bare JID" (user@domain.tld) of the XMPP user, to which the user's XMPP server responds in accordance with [RFC6121]; this puts an equal burden on the XMPP server and the SIP proxy. 9.2. Presence Leaks Presence notifications can contain sensitive information (e.g., about network availability). In addition, it is possible in both SIP and XMPP for an entity to send different presence notifications to different subscribers. Therefore, a gateway MUST honor data about the intended recipient of a presence notification (as represented by the 'to' address for XMPP and by the Request-URI for SIP) and it MUST NOT route or deliver a presence notification to any other entities, because it does not possess information about authorization to receive presence notifications for such entities - that information resides at the user's home service, not at the receiving gateway). 10. References 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. [RFC3856] Rosenberg, J., "A Presence Event Package for the Session Initiation Protocol (SIP)", RFC 3856, August 2004. [RFC3857] Rosenberg, J., "A Watcher Information Event Template- Package for the Session Initiation Protocol (SIP)", RFC 3857, August 2004. [RFC3863] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and J. Peterson, "Presence Information Data Format (PIDF)", RFC 3863, August 2004. [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, March 2011. Saint-Andre Expires September 21, 2016 [Page 29] Internet-Draft SIP-XMPP Interworking: Presence March 2016 [RFC6121] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence", RFC 6121, March 2011. [RFC6665] Roach, A., "SIP-Specific Event Notification", RFC 6665, July 2012. [RFC7247] Saint-Andre, P., Houri, A., and J. Hildebrand, "Interworking between the Session Initiation Protocol (SIP) and the Extensible Messaging and Presence Protocol (XMPP): Architecture, Addresses, and Error Handling", RFC 7247, May 2014. [RFC7622] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Address Format", RFC 7622, DOI 10.17487/ RFC7622, September 2015, . 10.2. Informative References [RFC2778] Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence and Instant Messaging", RFC 2778, February 2000. [RFC2779] Day, M., Aggarwal, S., and J. Vincent, "Instant Messaging / Presence Protocol Requirements", RFC 2779, February 2000. [RFC3860] Peterson, J., "Common Profile for Instant Messaging (CPIM)", RFC 3860, August 2004. [RFC4480] Schulzrinne, H., Gurbani, V., Kyzivat, P., and J. Rosenberg, "RPID: Rich Presence Extensions to the Presence Information Data Format (PIDF)", RFC 4480, July 2006. [RFC4825] Rosenberg, J., "The Extensible Markup Language (XML) Configuration Access Protocol (XCAP)", RFC 4825, May 2007. [XEP-0107] Saint-Andre, P. and R. Meijer, "User Mood", XSF XEP 0107, October 2008, . [XEP-0108] Meijer, R. and P. Saint-Andre, "User Activity", XSF XEP 0108, October 2008, . Appendix A. Acknowledgements Saint-Andre Expires September 21, 2016 [Page 30] Internet-Draft SIP-XMPP Interworking: Presence March 2016 Thanks to the authors, contributors, and other individuals acknowledged in RFC 7248. Thanks to Saul Ibarra Corretge and Markus Isomaki for their reviews during working group consideration. Special thanks to Ben Campbell for identifying the underlying discrepancy that resulted in the need to obsolete RFC 7248. Thanks also to Markus Isomaki and Yana Stamcheva as the working group chairs and Alissa Cooper as the sponsoring Area Director. Author's Address Peter Saint-Andre Filament Email: peter@filament.com URI: https://filament.com/ Saint-Andre Expires September 21, 2016 [Page 31]