





Internet Engineering Task Force (IETF)                    P. Saint-Andre
Request for Comments: 7247                                          &yet
Category: Standards Track                                       A. Houri
ISSN: 2070-1721                                                      IBM
                                                           J. Hildebrand
                                                     Cisco Systems, Inc.
                                                                May 2014


   Interworking between the Session Initiation Protocol (SIP) and the
           Extensible Messaging and Presence Protocol (XMPP):
              Architecture, Addresses, and Error Handling

Abstract

   As a foundation for the definition of bidirectional protocol mappings
   between the Session Initiation Protocol (SIP) and the Extensible
   Messaging and Presence Protocol (XMPP), this document specifies the
   architectural assumptions underlying such mappings as well as the
   mapping of addresses and error conditions.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7247.

















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Copyright Notice

   Copyright (c) 2014 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.  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. Interdomain Federation ..........................................6
   6. Address Mapping .................................................7
      6.1. Overview ...................................................7
      6.2. Local Part Mapping .........................................9
      6.3. Instance-Specific Mapping .................................11
      6.4. SIP to XMPP ...............................................11
      6.5. XMPP to SIP ...............................................12
   7. Error Mapping ..................................................14
      7.1. XMPP to SIP ...............................................15
      7.2. SIP to XMPP ...............................................17
   8. Security Considerations ........................................20
   9. References .....................................................21
      9.1. Normative References ......................................21
      9.2. Informative References ....................................22
   Appendix A. Acknowledgements ......................................24















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1.  Introduction

   The IETF has worked on two signaling technologies that can be used
   for multimedia session negotiation, instant messaging, presence,
   capabilities discovery, notifications, and other functions served by
   real-time communication applications:

   o  The Session Initiation Protocol [RFC3261], along with various SIP
      extensions developed within the SIP for Instant Messaging and
      Presence Leveraging Extensions (SIMPLE) Working Group.

   o  The Extensible Messaging and Presence Protocol [RFC6120], along
      with various XMPP extensions developed by the IETF as well as by
      the XMPP Standards Foundation (XSF).

   Because these technologies are widely deployed, it is important to
   clearly define mappings between them for the sake of interworking.
   This document provides the basis for a series of SIP-XMPP
   interworking specifications by defining architectural assumptions,
   address translation methods, and error condition mappings.  Other
   documents in this series define mappings for presence, messaging, and
   media sessions.

   The guidelines in this series are based on implementation and
   deployment experience, and they describe techniques that have worked
   well in the field with existing systems.  In practice, interworking
   has been achieved through direct protocol mappings, not through
   mapping to an abstract model as described in, for example, [RFC3859]
   and [RFC3860].  Therefore, this series describes the direct mapping
   approach in enough detail for developers of new implementations to
   achieve practical interworking between SIP systems and XMPP systems.

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).  With regard to this document, we assume that readers are
   familiar with the core specifications for both SIP and XMPP; with
   regard to the other documents in this series, we assume that readers
   are familiar with this document as well as with the relevant SIP and
   XMPP extensions.








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3.  Terminology

   A number of terms used here are explained in [RFC3261] and [RFC6120].

   Several examples use the "XML Notation" from the Internationalized
   Resource Identifier (IRI) specification [RFC3987] to represent
   Unicode characters outside the ASCII range (e.g., the string "ue"
   stands for the Unicode character [UNICODE] LATIN SMALL LETTER U WITH
   DIAERESIS, U+00FC).

   In architectural diagrams, SIP traffic is shown using arrows such as
   "***>", whereas XMPP traffic is shown using arrows such as "...>".

   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

   Protocol translation between SIP and XMPP could occur in a number of
   different entities, depending on the architecture of real-time
   communication deployments.  For example, protocol translation could
   occur within a multiprotocol server (which uses protocol-specific
   connection managers to initiate traffic to and accept traffic from
   clients or other servers natively using SIP/SIMPLE, XMPP, etc.),
   within a multiprotocol client (which enables a user to establish
   connections natively with various servers using SIP/SIMPLE, XMPP,
   etc.), or within a gateway that acts as a dedicated protocol
   translator (which takes one protocol as input and provides another
   protocol as output).

   This document assumes that the protocol translation will occur within
   a gateway, specifically:

   o  When information needs to flow from an XMPP-based system to a SIP-
      based system, protocol translation will occur within an "XMPP-to-
      SIP gateway" that translates XMPP syntax and semantics on behalf
      of an "XMPP server" (together, these two logical functions
      comprise an "XMPP service").

   o  When information needs to flow from a SIP-based system to an XMPP-
      based system, protocol translation will occur within a "SIP-to-
      XMPP gateway" that translates SIP syntax and semantics on behalf
      of a "SIP server" (together, these two logical functions comprise
      a "SIP service").





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   Naturally, these logical functions could occur in one and the same
   actual entity; we differentiate between them mainly for explanatory
   purposes (although, in practice, such gateways are indeed fairly
   common).

      Note: This assumption is not meant to discourage protocol
      translation within multiprotocol clients or servers; instead, this
      assumption is followed mainly to clarify the discussion and
      examples so that the protocol translation principles can be more
      easily understood and can be applied by client and server
      implementors with appropriate modifications to the examples and
      terminology.

   This document assumes that a gateway will translate directly from one
   protocol to the other.  For the sake of the examples, we further
   assume that protocol translation will occur within a gateway in the
   source domain, so that information generated by the user of an XMPP
   system will be translated by a gateway within the trust domain of
   that XMPP system, and information generated by the user of a SIP
   system will be translated by a gateway within the trust domain of
   that SIP system.  However, nothing in this document ought to be taken
   as recommending against protocol translation at the destination
   domain.

   An architectural diagram for a possible gateway deployment is shown
   below, where the entities have the following significance and the "#"
   character is used to show the boundary of a trust domain:

   o  romeo@example.net -- a SIP user.

   o  example.net -- a SIP server with an associated gateway ("S2X GW")
      to XMPP.

   o  juliet@example.com -- an XMPP user.

   o  example.com -- an XMPP server with an associated gateway ("X2S
      GW") to SIP.














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        #########################################################
        #                                                       #
        #                 +-----+                               #
        #                 | S2X |                               #
        #   +-------------+ GW  |<...........>+-------------+   #
        #   | SIP Server  +-----+             | XMPP Server |   #
        #   | example.net |             +-----+ example.com |   #
        #   +-------------+<***********>| X2S +-------------+   #
        #         *                     | GW  |  :              #
        #         *                     +-----+  :              #
        #         *                              :              #
        #    romeo@example.net             juliet@example.com   #
        #                                                       #
        #########################################################

            Legend:
              XMPP = ... or :
               SIP = *

            Figure 1: Possible Gateway Deployment Architecture

   Note that bidirectional communication between the SIP server and the
   XMPP server can be established over either SIP or XMPP.  If the XMPP
   user initiates the interaction, then the XMPP server would invoke its
   XMPP-to-SIP gateway; thus, the communication would occur over SIP.
   If the SIP user initiates the interaction, then the SIP server would
   invoke its SIP-to-XMPP gateway; thus, the communication would occur
   over XMPP.

5.  Interdomain Federation

   The architectural assumptions underlying this document imply that
   communication between a SIP system and an XMPP system will take place
   using interdomain federation: the SIP server invokes its associated
   SIP-to-XMPP gateway, which communicates with the XMPP server using
   native XMPP server-to-server methods; similarly, the XMPP server
   invokes its associated XMPP-to-SIP gateway, which communicates with
   the SIP server using native SIP server-to-server methods.

   When an XMPP server receives an XMPP stanza whose 'to' address
   specifies or includes a domain other than the domain of the XMPP
   server, it needs to determine whether the destination domain
   communicates via XMPP or SIP.  To do so, it performs one or more DNS
   SRV lookups [RFC2782] for "_xmpp-server" records as specified in
   [RFC6120].  If the response returns a hostname, the XMPP server can
   attempt XMPP communication.  If not, it can determine the appropriate
   location for SIP communication at the target domain using the
   procedures specified in [RFC3263].



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   Similarly, when a SIP server receives a SIP message whose Request-URI
   or To header specifies or includes a domain other than the domain of
   the SIP server, it needs to determine whether the destination domain
   communicates via SIP or XMPP.  To do so, it uses the procedures
   specified in [RFC3263].  If that response returns a hostname, the SIP
   server can attempt SIP communication.  If not, it can perform one or
   more DNS SRV lookups [RFC2782] for "_xmpp-server" records as
   specified in [RFC6120].

   In both cases, the server in question might have previously
   determined that the foreign domain communicates via SIP or XMPP, in
   which case it would not need to perform the relevant DNS lookups.
   The caching of such information is a matter of implementation and
   local service policy and is therefore out of scope for this document.

   Existing SIP and XMPP server implementations do not typically include
   the ability to communicate using the other technology (XMPP for SIP
   implementations, SIP for XMPP implementations).  One common
   architectural pattern is to associate a gateway with the core server
   implementation (e.g., in XMPP such a gateway might be called a
   "connection manager").  How exactly such a gateway interacts with the
   core server to complete tasks such as address lookups and
   communication with systems that use the other technology is a matter
   of implementation (e.g., the gateway might be an add-on module that
   is trusted by the core server to act as a fallback delivery mechanism
   if the remote domain does not support the server's native
   communication technology).

   Because [RFC6120] specifies a binding of XMPP to TCP, a gateway from
   SIP to XMPP will need to support TCP as the underlying transport
   protocol.  By contrast, as specified in [RFC3261], either TCP or UDP
   can be used as the underlying transport for SIP messages, and a given
   SIP deployment might support only UDP; therefore, a gateway from XMPP
   to SIP might need to communicate with a SIP server using either TCP
   or UDP.

6.  Address Mapping

6.1.  Overview

   The basic SIP address format is a 'sip' or 'sips' URI as specified in
   [RFC3261].  When a SIP entity supports extensions for instant
   messaging it might be identified by an 'im' URI as specified in the
   Common Profile for Instant Messaging [RFC3860] (see [RFC3428]), and
   when a SIP entity supports extensions for presence it might be






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   identified by a 'pres' URI as specified in the Common Profile for
   Presence [RFC3859] (see [RFC3856]).  SIP entities typically also
   support the 'tel' URI scheme [RFC3966] and might support other URI
   schemes as well.

   The XMPP address format is specified in [RFC6122] (although note that
   XMPP URIs [RFC5122] are not used natively on the XMPP network); in
   addition, [RFC6121] encourages instant messaging and presence
   applications of XMPP to also support 'im' and 'pres' URIs as
   specified in [RFC3860] and [RFC3859], respectively, although such
   support might simply involve leaving resolution of such addresses up
   to an XMPP server.

   In this document we primarily describe mappings for addresses of the
   form <user@domain>; however, we also provide guidelines for mapping
   the addresses of specific user agent instances, which take the form
   of Globally Routable User Agent URIs (GRUUs) in SIP and
   "resourceparts" in XMPP.  Mapping of protocol-specific identifiers
   (such as telephone numbers) is out of scope for this specification.
   In addition, we have ruled the mapping of domain names as out of
   scope for now, since that is a matter for the Domain Name System;
   specifically, the issue for interworking between SIP and XMPP relates
   to the translation of fully internationalized domain names (IDNs)
   into non-internationalized domain names (IDNs are not allowed in the
   SIP address format but are allowed in the XMPP address via
   Internationalized Domain Names in Applications; see [RFC6122] and
   [XMPP-ADDRESS-FORMAT]).  Therefore, in the following sections we
   focus primarily on the local part of an address (these are called
   variously "usernames", "instant inboxes", "presentities", and
   "localparts" in the protocols at issue), secondarily on the instance-
   specific part of an address, and not at all on the domain-name part
   of an address.

   The 'sip'/'sips', 'im'/'pres', and XMPP address schemes allow
   different sets of characters (although all three allow alphanumeric
   characters and disallow both spaces and control characters).  In some
   cases, characters allowed in one scheme are disallowed in others;
   these characters need to be mapped appropriately in order to ensure
   interworking across systems.












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6.2.  Local Part Mapping

   The local part of a 'sip'/'sips' URI inherits from the "userinfo"
   rule in [RFC3986] with several changes; here we discuss the SIP
   "user" rule only (using ABNF as defined in [RFC5234]):

      user             =  1*( unreserved / escaped / user-unreserved )
      user-unreserved  =  "&" / "=" / "+" / "$" / "," / ";" / "?" / "/"
      unreserved       =  alphanum / mark
      mark             =  "-" / "_" / "." / "!" / "~" / "*" / "'"
                          / "(" / ")"

   Here we make the simplifying assumption that the local part of an
   'im'/'pres' URI inherits from the "dot-atom-text" rule in [RFC5322]
   rather than the more complicated "local-part" rule:

      dot-atom-text =  1*atext *("." 1*atext)
      atext         =  ALPHA / DIGIT /    ; Any character except
                       "!" / "#" / "$" /  ; controls, SP, and
                       "%" / "&" / "'" /  ; specials.  Used for
                       "*" / "+" / "-" /  ; atoms.
                       "/" / "=" / "?" /
                       "^" / "_" / "`" /
                       "{" / "|" / "}" /
                       "~"

   The local part of an XMPP address allows any ASCII character except
   space, controls, and the " & ' / : < > @ characters.























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   To summarize the foregoing information, the following table lists the
   allowed and disallowed characters in the local part of identifiers
   for each protocol (aside from the alphanumeric, space, and control
   characters), in order by hexadecimal character number (where each "A"
   row shows the allowed characters and each "D" row shows the
   disallowed characters).

                 +---+----------------------------------+
                 | SIP/SIPS CHARACTERS                  |
                 +---+----------------------------------+
                 | A | !  $ &'()*+,-./ ; = ?     _    ~ |
                 | D |  "# %          : < > @[\]^ `{|}  |
                 +---+----------------------------------+
                 | IM/PRES CHARACTERS                   |
                 +---+----------------------------------+
                 | A | ! #$%&'  *+ - /   = ?    ^_`{|}~ |
                 | D |  "     ()  , . :;< > @[\]        |
                 +---+----------------------------------+
                 | XMPP CHARACTERS                      |
                 +---+----------------------------------+
                 | A | ! #$%  ()*+,-.  ; = ? [\]^_`{|}~ |
                 | D |  "   &'       /: < > @           |
                 +---+----------------------------------+

                Table 1: Allowed and Disallowed Characters

   When transforming the local part of an address from one address
   format to another, an application SHOULD proceed as follows:

   1.  Unescape any escaped characters in the source address (e.g., from
       SIP to XMPP unescape "%23" to "#" per [RFC3986], and from XMPP to
       SIP unescape "\27" to "'" per [XEP-0106]).

   2.  Leave unmodified any characters that are allowed in the
       destination scheme.

   3.  Escape any characters that are allowed in the source scheme but
       reserved in the destination scheme, as escaping is defined for
       the destination scheme.  In particular:

       *  Where the destination scheme is a URI (i.e., an 'im', 'pres',
          'sip', or 'sips' URI), each reserved character MUST be
          percent-encoded to "%hexhex" as specified in Section 2.5 of
          [RFC3986] (e.g., when transforming from XMPP to SIP, encode
          "#" as "%23").






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       *  Where the destination scheme is a native XMPP address, each
          reserved character MUST be encoded to "\hexhex" as specified
          in [XEP-0106] (e.g., when transforming from SIP to XMPP,
          encode "'" as "\27").

6.3.  Instance-Specific Mapping

   The meaning of a resourcepart in XMPP (i.e., the portion of a Jabber
   ID (JID) after the slash character, such as "foo" in
   "user@example.com/foo") matches that of a Globally Routable User
   Agent URI (GRUU) in SIP [RFC5627].  In both cases, these constructs
   identify a particular device associated with the bare JID
   ("localpart@domainpart") of an XMPP entity or with the Address of
   Record (AOR) of a SIP entity.  Therefore, it is reasonable to map the
   value of a "gr" URI parameter to an XMPP resourcepart and vice versa.

   The mapping described here does not apply to temporary GRUUs -- only
   to GRUUs associated with an Address of Record.

   The "gr" URI parameter in SIP can contain only characters from the
   ASCII range (although characters outside the ASCII range can be
   percent-encoded in accordance with [RFC3986]), whereas an XMPP
   resourcepart can contain nearly any Unicode character [UNICODE].
   Therefore, Unicode characters outside the ASCII range need to be
   mapped to characters in the ASCII range, as described below.

6.4.  SIP to XMPP

   The following is a high-level algorithm for mapping a 'sip', 'sips',
   'im', or 'pres' URI to an XMPP address:

   1.  Remove URI scheme.

   2.  Split at the first '@' character into local part and hostname
       (mapping the latter is out of scope).

   3.  Translate any percent-encoded strings ("%hexhex") to percent-
       decoded octets.

   4.  Treat result as a UTF-8 string.

   5.  Translate "&" to "\26", "'" to "\27", and "/" to "\2f",
       respectively in order to properly handle the characters
       disallowed in XMPP addresses but allowed in 'sip'/'sips' URIs and
       'im'/'pres' URIs as shown in Table 1 above (this is consistent
       with [XEP-0106]).





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   6.  Apply Nodeprep profile of stringprep [RFC3454] or its replacement
       (see [RFC6122] and [XMPP-ADDRESS-FORMAT]) for canonicalization
       (OPTIONAL).

   7.  Recombine local part with mapped hostname to form a bare JID
       ("localpart@domainpart").

   8.  If the (SIP) address contained a "gr" URI parameter, append a
       slash character "/" and the "gr" value to the bare JID to form a
       full JID ("localpart@domainpart/resourcepart").

   Several examples follow, illustrating steps 3, 5, and 8 described
   above.

      +----------------------------+--------------------------+
      | SIP URI                    |  XMPP Address            |
      +----------------------------+--------------------------+
      | sip:f%C3%BC@sip.example    |  f&#xFC;@sip.example     |
      +----------------------------+--------------------------+
      | sip:o'malley@sip.example   |  o\27malley@sip.example  |
      +----------------------------+--------------------------+
      | sip:foo@sip.example;gr=bar |  foo@sip.example/bar     |
      +----------------------------+--------------------------+

   In the first example, the string "%C3%BC" is a percent-encoded
   representation of the UTF-8-encoded Unicode character LATIN SMALL
   LETTER U WITH DIAERESIS (U+00FC), whereas the string "&#xFC;" is the
   same character shown for documentation purposes using the XML
   Notation defined in [RFC3987] (in XMPP, it would be sent directly as
   a UTF-8-encoded Unicode character and not percent-encoded as in a SIP
   URI to comply with the URI syntax defined in [RFC3986]).

6.5.  XMPP to SIP

   The following is a high-level algorithm for mapping an XMPP address
   to a 'sip', 'sips', 'im', or 'pres' URI:

   1.  Split XMPP address into localpart (mapping described in remaining
       steps), domainpart (hostname; mapping is out of scope), and
       resourcepart (specifier for particular device or connection, for
       which an OPTIONAL mapping is described below).

   2.  Apply Nodeprep profile of stringprep [RFC3454] or its replacement
       (see [RFC6122] and [XMPP-ADDRESS-FORMAT]) for canonicalization of
       the XMPP localpart (OPTIONAL).

   3.  Translate "\26" to "&", "\27" to "'", and "\2f" to "/",
       respectively (this is consistent with [XEP-0106]).



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   4.  Determine if the foreign domain supports 'im' and 'pres' URIs
       (discovered via [RFC2782] lookup as specified in [RFC6121]), else
       assume that the foreign domain supports 'sip'/'sips' URIs.

   5.  If converting into 'im' or 'pres' URI, for each byte, if the byte
       is in the set (),.;[\] or is a UTF-8 character outside the ASCII
       range, then percent-encode that byte to "%hexhex" format.  If
       converting into 'sip' or 'sips' URI, for each byte, if the byte
       is in the set #%[\]^`{|} or is a UTF-8 character outside the
       ASCII range, then percent-encode that byte to "%hexhex" format.

   6.  Combine resulting local part with mapped hostname to form
       local@domain address.

   7.  Prepend with the string 'im:' (for XMPP <message/> stanzas) or
       'pres:' (for XMPP <presence/> stanzas) if foreign domain supports
       these, else prepend with the string 'sip:' or 'sips:' according
       to local service policy.

   8.  If the XMPP address included a resourcepart and the destination
       URI scheme is 'sip' or 'sips', optionally append the slash
       character '/' and then append the resourcepart (making sure to
       percent-encode any UTF-8 characters outside the ASCII range) as
       the "gr" URI parameter.

   Several examples follow, illustrating steps 3, 5, and 8 described
   above.

      +---------------------------+---------------------------------+
      | XMPP Address              |  SIP URI                        |
      +---------------------------+---------------------------------+
      | m\26m@xmpp.example        |  sip:m&m@xmpp.example           |
      | tsch&#xFC;ss@xmpp.example |  sip:tsch%C3%BCss@xmpp.example  |
      | baz@xmpp.example/qux      |  sip:baz@xmpp.example;gr=qux    |
      +---------------------------+---------------------------------+

   As above, in the first example the string "&#xFC;" is the Unicode
   character LATIN SMALL LETTER U WITH DIAERESIS (U+00FC) shown for
   documentation purposes using the XML Notation defined in [RFC3987]
   (in XMPP, it would be sent directly as a UTF-8-encoded Unicode
   character and not percent-encoded, whereas the string "%C3%BC" is a
   percent-encoded representation of the same character.









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7.  Error Mapping

   Various differences between XMPP error conditions and SIP response
   codes make it hard to provide a comprehensive and consistent mapping
   between the protocols:

   o  Whereas the set of XMPP error conditions is fixed in the core XMPP
      specification (and supplemented where needed by application-
      specific extensions), the set of SIP response codes is more open
      to change, as evidenced by the IANA registry of SIP response
      codes.

   o  XMPP has defined fewer error conditions related to stanza handling
      (22 are defined in [RFC6120]) than SIP has defined response codes
      related to message handling (at the date of this writing, 71 SIP
      response codes are registered with IANA as defined in [RFC3261]
      and numerous SIP extensions).

   o  In many cases, the SIP response codes are more specific than the
      XMPP error conditions (e.g., from an XMPP perspective the SIP
      codes "413 Request Entity Too Large" and "414 Request-URI Too
      Long" are simply two different types of response to the same bad
      request, and the SIP codes "415 Unsupported Media Type" and "416
      Unsupported URI Scheme" are two different responses to a request
      that is not acceptable).

   o  SIP differentiates between responses about a particular endpoint
      or resource (the 4xx series) and responses about a user, i.e., all
      of a user's endpoints or resources (the 6xx series).  There is no
      such distinction in XMPP, since the same error condition can be
      returned in relation to the "bare JID" (localpart@domainpart) of a
      user or the "full JID" (localpart@domainpart/resourcepart) of a
      particular endpoint or resource, depending on the 'to' address of
      the original request.

   As a result of these and other factors, the mapping of error
   conditions and response codes is more of an art than a science.  This
   document provides suggested mappings, but implementations are free to
   deviate from these mappings if needed.  Also, because no XMPP error
   conditions are equivalent to the provisional (1xx) and successful
   (2xx) response codes in SIP, this document suggests mappings only for
   the SIP redirection (3xx), request failure (4xx), server failure
   (5xx), and global failure (6xx) response code families.

   Supplementary information about SIP response codes can be expressed
   in the "Reason-Phrase" in the Status-Line header, and detailed
   information about XMPP error conditions can be expressed in the
   <text/> child of the <error/> element.  Although the semantics of



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   these constructs are specified in a slightly different way, it is
   reasonable for a gateway to map these constructs to each other if
   they are found in a SIP response or XMPP error stanza.

7.1.  XMPP to SIP

   The mapping of specific XMPP error conditions to SIP response codes
   SHOULD be as described in the following table.

         +------------------------------+---------------------+
         |  XMPP Error Condition        |  SIP Response Code  |
         +------------------------------+---------------------+
         |  <bad-request/>              | 400                 |
         +------------------------------+---------------------+
         |  <conflict/>                 | 400                 |
         +------------------------------+---------------------+
         |  <feature-not-implemented/>  | 405 or 501 (1)      |
         +------------------------------+---------------------+
         |  <forbidden/>                | 403 or 603 (2)      |
         +------------------------------+---------------------+
         |  <gone/>                     | 301 or 410 (3)      |
         +------------------------------+---------------------+
         |  <internal-server-error/>    | 500                 |
         +------------------------------+---------------------+
         |  <item-not-found/>           | 404 or 604 (2)      |
         +------------------------------+---------------------+
         |  <jid-malformed/>            | 400                 |
         +------------------------------+---------------------+
         |  <not-acceptable/>           | 406 or 606 (2)      |
         +------------------------------+---------------------+
         |  <not-allowed/>              | 403                 |
         +------------------------------+---------------------+
         |  <not-authorized/>           | 401                 |
         +------------------------------+---------------------+
         |  <policy-violation/>         | 403                 |
         +------------------------------+---------------------+
         |  <recipient-unavailable/>    | 480 or 600 (2)      |
         +------------------------------+---------------------+
         |  <redirect/>                 | 302                 |
         +------------------------------+---------------------+
         |  <registration-required/>    | 407                 |
         +------------------------------+---------------------+
         |  <remote-server-not-found/>  | 404 or 408 (4)      |
         +------------------------------+---------------------+
         |  <remote-server-timeout/>    | 408                 |
         +------------------------------+---------------------+





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         +------------------------------+---------------------+
         |  <resource-constraint/>      | 500                 |
         +------------------------------+---------------------+
         |  <service-unavailable/>      | see note (5) below  |
         +------------------------------+---------------------+
         |  <subscription-required/>    | 400                 |
         +------------------------------+---------------------+
         |  <undefined-condition/>      | 400                 |
         +------------------------------+---------------------+
         |  <unexpected-request/>       | 491 or 400          |
         +------------------------------+---------------------+

     Table 2: Mapping of XMPP Error Conditions to SIP Response Codes

   (1)  If the error relates to a "full JID" (localpart@domainpart/
        resourcepart), the SIP 405 response code is RECOMMENDED.  If the
        error relates to a "bare JID" (localpart@domainpart), the SIP
        501 response code is RECOMMENDED.

   (2)  If the error relates to a "full JID" (localpart@domainpart/
        resourcepart), the SIP response code from the 4xx series is
        RECOMMENDED.  If the error relates to a "bare JID"
        (localpart@domainpart), the SIP response code from the 6xx
        series is RECOMMENDED.

   (3)  If the <gone/> element includes XML character data specifying
        the new address, the error MUST be mapped to SIP 301; if not, it
        MUST be mapped to SIP 410.

   (4)  The XMPP <remote-server-not-found/> error can mean that the
        remote server either (a) does not exist or (b) cannot be
        resolved.  SIP has two different response codes here: 404 to
        cover (a) and 408 to cover (b).

   (5)  The XMPP <service-unavailable/> error condition is widely used
        to inform the requesting entity that the intended recipient does
        not support the relevant feature, to signal that a server cannot
        perform the requested service either generally or in relation to
        a particular user, and to avoid disclosing whether a given
        account exists at all.  This is quite different from the
        semantics of the SIP 503 Service Unavailable response code,
        which is used to signal that communication with a server is
        impossible (e.g., even if the XMPP <service-unavailable/> error
        condition is returned in relation to a specific user, the SIP
        503 response code will be interpreted as applying to all future
        requests to this server, not just requests for the specific
        user).  Therefore, mapping the XMPP <service-unavailable/> error
        condition to the SIP 503 Service Unavailable response code is



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        NOT RECOMMENDED.  Although no precise mapping is available, the
        SIP 403 Forbidden and 405 Method Not Allowed response codes are
        closest in meaning to the XMPP <service-unavailable/> error
        condition.

7.2.  SIP to XMPP

   The mapping of SIP response codes to XMPP error conditions SHOULD be
   as described in the following table.  If a gateway encounters a SIP
   response code that is not listed below, it SHOULD map a 3xx-series
   code to <redirect/>, a 4xx-series code to <bad-request/>, a 5xx-
   series code to <internal-server-error>, and a 6xx-series code to
   <recipient-unavailable/>.

        +---------------------+---------------------------------+
        |  SIP Response Code  |  XMPP Error Condition           |
        +---------------------+---------------------------------+
        |  3xx                |  <redirect/>                    |
        +---------------------+---------------------------------+
        |  300                |  <redirect/>                    |
        +---------------------+---------------------------------+
        |  301                |  <gone/> (1)                    |
        +---------------------+---------------------------------+
        |  302                |  <redirect/>                    |
        +---------------------+---------------------------------+
        |  305                |  <redirect/>                    |
        +---------------------+---------------------------------+
        |  380                |  <not-acceptable/>              |
        +---------------------+---------------------------------+
        |  4xx                |  <bad-request/>                 |
        +---------------------+---------------------------------+
        |  400                |  <bad-request/>                 |
        +---------------------+---------------------------------+
        |  401                |  <not-authorized/>              |
        +---------------------+---------------------------------+
        |  402                |  <bad-request/> (2)             |
        +---------------------+---------------------------------+
        |  403                |  <forbidden/> (3)               |
        +---------------------+---------------------------------+
        |  404                |  <item-not-found/> (4)          |
        +---------------------+---------------------------------+
        |  405                |  <feature-not-implemented/>     |
        +---------------------+---------------------------------+
        |  406                |  <not-acceptable/>              |
        +---------------------+---------------------------------+
        |  407                |  <registration-required/>       |
        +---------------------+---------------------------------+




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        +---------------------+---------------------------------+
        |  408                |  <remote-server-timeout/> (5)   |
        +---------------------+---------------------------------+
        |  410                |  <gone/> (1)                    |
        +---------------------+---------------------------------+
        |  413                |  <policy-violation/>            |
        +---------------------+---------------------------------+
        |  414                |  <policy-violation/>            |
        +---------------------+---------------------------------+
        |  415                |  <not-acceptable/>              |
        +---------------------+---------------------------------+
        |  416                |  <not-acceptable/>              |
        +---------------------+---------------------------------+
        |  420                |  <feature-not-implemented/>     |
        +---------------------+---------------------------------+
        |  421                |  <not-acceptable/>              |
        +---------------------+---------------------------------+
        |  423                |  <resource-constraint/>         |
        +---------------------+---------------------------------+
        |  430                |  <recipient-unavailable/> (6)   |
        +---------------------+---------------------------------+
        |  439                |  <feature-not-implemented/> (6) |
        +---------------------+---------------------------------+
        |  440                |  <policy-violation/> (7)        |
        +---------------------+---------------------------------+
        |  480                |  <recipient-unavailable/>       |
        +---------------------+---------------------------------+
        |  481                |  <item-not-found/>              |
        +---------------------+---------------------------------+
        |  482                |  <not-acceptable/>              |
        +---------------------+---------------------------------+
        |  483                |  <not-acceptable/>              |
        +---------------------+---------------------------------+
        |  484                |  <item-not-found/>              |
        +---------------------+---------------------------------+
        |  485                |  <item-not-found/>              |
        +---------------------+---------------------------------+
        |  486                |  <recipient-unavailable/>       |
        +---------------------+---------------------------------+
        |  487                |  <recipient-unavailable/>       |
        +---------------------+---------------------------------+
        |  488                |  <not-acceptable/>              |
        +---------------------+---------------------------------+
        |  489                |  <policy-violation/> (8)        |
        +---------------------+---------------------------------+
        |  491                |  <unexpected-request/>          |
        +---------------------+---------------------------------+




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        +---------------------+---------------------------------+
        |  493                |  <bad-request/>                 |
        +---------------------+---------------------------------+
        |  5xx                |  <internal-server-error/>       |
        +---------------------+---------------------------------+
        |  500                |  <internal-server-error/>       |
        +---------------------+---------------------------------+
        |  501                |  <feature-not-implemented/>     |
        +---------------------+---------------------------------+
        |  502                |  <remote-server-not-found/>     |
        +---------------------+---------------------------------+
        |  503                |  <internal-server-error/> (9)   |
        +---------------------+---------------------------------+
        |  504                |  <remote-server-timeout/>       |
        +---------------------+---------------------------------+
        |  505                |  <not-acceptable/>              |
        +---------------------+---------------------------------+
        |  513                |  <policy-violation/>            |
        +---------------------+---------------------------------+
        |  6xx                |  <recipient-unavailable/>       |
        +---------------------+---------------------------------+
        |  600                |  <recipient-unavailable/>       |
        +---------------------+---------------------------------+
        |  603                |  <recipient-unavailable/>       |
        +---------------------+---------------------------------+
        |  604                |  <item-not-found/>              |
        +---------------------+---------------------------------+
        |  606                |  <not-acceptable/>              |
        +---------------------+---------------------------------+

     Table 3: Mapping of SIP Response Codes to XMPP Error Conditions

   (1)  When mapping SIP 301 to XMPP <gone/>, the <gone/> element MUST
        include XML character data specifying the new address.  When
        mapping SIP 410 to XMPP <gone/>, the <gone/> element MUST NOT
        include XML character data specifying a new address.

   (2)  The XMPP <payment-required/> error condition was removed in
        [RFC6120].  Therefore, a mapping to XMPP <bad-request/> is
        suggested instead.

   (3)  Depending on the scenario, other possible translations for
        SIP 403 are <not-allowed/> and <policy-violation/>.

   (4)  Depending on the scenario, another possible translation for
        SIP 404 is <remote-server-not-found/>.





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   (5)  Depending on the scenario, another possible translation for
        SIP 408 is <remote-server-not-found/>.

   (6)  Codes 430 and 439 are defined in [RFC5626].

   (7)  Code 440 is defined in [RFC5393].

   (8)  Code 489 is defined in [RFC6665].

   (9)  Regarding the semantic mismatch between XMPP
        <service-unavailable/> and SIP code 503, see note (5) in
        Section 7.1 of this document.

8.  Security Considerations

   Detailed security considerations for SIP and XMPP are given in
   [RFC3261] and [RFC6120], respectively.

   To protect information sent between SIP and XMPP systems, deployed
   gateways SHOULD use Transport Layer Security (TLS) [RFC5246] when
   communicating over TCP and Datagram Transport Layer Security (DTLS)
   [RFC6347] when communicating over UDP.

   As specified in Section 26.4.4 of [RFC3261] and updated by [RFC5630],
   a To header or a Request-URI containing a Session Initiation Protocol
   Secure (SIPS) URI is used to indicate that all hops in a
   communication path need to be protected using TLS.  Because XMPP
   lacks a way to signal that all hops need to be protected, if the To
   header or Request-URI of a SIP message is a SIPS URI then the SIP-to-
   XMPP gateway MUST NOT translate the SIP message into an XMPP stanza
   and MUST NOT route it to the destination XMPP server (there might be
   exceptions to such a policy, such as explicit agreement among two
   operators to enforce per-hop security, but currently they are quite
   rare).

   A gateway between SIP and XMPP (in either direction) effectively acts
   as a SIP back-to-back user agent ("B2BUA").  The amplification
   vulnerability described in [RFC5393] can manifest itself with B2BUAs
   (see also [B2BUA-LOOP-DETECT]), and a gateway SHOULD implement the
   loop-detection methods defined in that specification to help mitigate
   the possibility of amplification attacks.  Note that although it
   would be possible to signal the Max-Forwards and Max-Breadth SIP
   headers over XMPP using the Stanza Headers and Internet Metadata
   (SHIM) extension [XEP-0131], that extension is not widely
   implemented; therefore, defenses against excessive looping and
   amplification attacks when messages pass back and forth through SIP
   and XMPP networks are out of scope for this document.  However, it




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   ought to be addressed in the future, and implementations are strongly
   encouraged to incorporate appropriate countermeasures wherever
   possible.

   The ability to use a wide range of Unicode characters [UNICODE] can
   lead to security issues, especially the possibility of user confusion
   over identifiers containing visually similar characters (also called
   "confusable characters" or "confusables").  The PRECIS framework
   specification [PRECIS] describes some of these security issues, and
   additional guidance can be found in [UTS39].

9.  References

9.1.  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.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5393]  Sparks, R., Lawrence, S., Hawrylyshen, A., and B. Campen,
              "Addressing an Amplification Vulnerability in Session
              Initiation Protocol (SIP) Forking Proxies", RFC 5393,
              December 2008.

   [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent URIs (GRUUs) in the Session Initiation Protocol
              (SIP)", RFC 5627, October 2009.



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   [RFC5630]  Audet, F., "The Use of the SIPS URI Scheme in the Session
              Initiation Protocol (SIP)", RFC 5630, October 2009.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6122]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Address Format", RFC 6122, March 2011.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [UNICODE]  The Unicode Consortium, "The Unicode Standard,
              Version 6.3", 2013,
              <http://www.unicode.org/versions/Unicode6.3.0/>.

9.2.  Informative References

   [B2BUA-LOOP-DETECT]
              Kaplan, H. and V. Pascual, "Loop Detection Mechanisms for
              Session Initiation Protocol (SIP) Back-to-Back User Agents
              (B2BUAs)", Work in Progress, February 2014.

   [PRECIS]   Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation and Comparison of Internationalized Strings in
              Application Protocols", Work in Progress, April 2014.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC3428]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
              and D. Gurle, "Session Initiation Protocol (SIP) Extension
              for Instant Messaging", RFC 3428, December 2002.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [RFC3856]  Rosenberg, J., "A Presence Event Package for the Session
              Initiation Protocol (SIP)", RFC 3856, August 2004.

   [RFC3859]  Peterson, J., "Common Profile for Presence (CPP)",
              RFC 3859, August 2004.

   [RFC3860]  Peterson, J., "Common Profile for Instant Messaging
              (CPIM)", RFC 3860, August 2004.




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   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, December 2004.

   [RFC5122]  Saint-Andre, P., "Internationalized Resource Identifiers
              (IRIs) and Uniform Resource Identifiers (URIs) for the
              Extensible Messaging and Presence Protocol (XMPP)",
              RFC 5122, February 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5626]  Jennings, C., Mahy, R., and F. Audet, "Managing Client-
              Initiated Connections in the Session Initiation Protocol
              (SIP)", RFC 5626, October 2009.

   [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.

   [UTS39]    The Unicode Consortium, "Unicode Technical Standard #39:
              Unicode Security Mechanisms", November 2013,
              <http://unicode.org/reports/tr39/>.

   [XEP-0106] Hildebrand, J. and P. Saint-Andre, "JID Escaping",
              XSF XEP 0106, June 2007,
              <http://www.xmpp.org/extensions/xep-0106.html>.

   [XEP-0131] Saint-Andre, P. and J. Hildebrand, "Stanza Headers and
              Internet Metadata", XSF XEP 0131, July 2006,
              <http://xmpp.org/extensions/xep-0131.html>.

   [XMPP-ADDRESS-FORMAT]
              Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Address Format", Work in Progress,
              March 2014.













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Appendix A.  Acknowledgements

   The authors wish to thank the following individuals for their
   feedback: Mary Barnes, Dave Cridland, Dave Crocker, Mike De Vries,
   Fabio Forno, Adrian Georgescu, Philipp Hancke, Saul Ibarra Corretge,
   Markus Isomaki, Olle Johansson, Paul Kyzivat, Salvatore Loreto,
   Daniel-Constantin Mierla, Tory Patnoe, and Robert Sparks.

   Dan Romascanu reviewed the document on behalf of the General Area
   Review Team.

   During IESG review, Stephen Farrell, Ted Lemon, Pete Resnick, and
   Sean Turner caught several issues that needed to be addressed.

   The authors gratefully acknowledge the assistance of Markus Isomaki
   and Yana Stamcheva as the working group chairs and Gonzalo Camarillo
   as the sponsoring Area Director.

   Peter Saint-Andre wishes to acknowledge Cisco Systems, Inc., for
   employing him during his work on earlier versions of this document.

Authors' Addresses

   Peter Saint-Andre
   &yet

   EMail: ietf@stpeter.im


   Avshalom Houri
   IBM
   Rorberg Building, Pekris 3
   Rehovot  76123
   Israel

   EMail: avshalom@il.ibm.com


   Joe Hildebrand
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO  80202
   USA

   EMail: jhildebr@cisco.com






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