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Versions: 00 01 02 03 04 05 RFC 3262

Internet Engineering Task Force                                   SIP WG
Internet Draft                                               J.Rosenberg
                                                             Columbia U.
February 21, 2002
Expires: August 2002

              Reliability of Provisional Responses in SIP


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see


   This document specifies an extension to the Session Initiation
   Protocol (SIP) providing reliable provisional response messages. This
   extension uses the option tag 100rel.

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                           Table of Contents

   1          Introduction ........................................    3
   2          Terminology .........................................    3
   3          UAS Behavior ........................................    4
   4          UAC Behavior ........................................    6
   5          The Offer/Answer Model and PRACK ....................    8
   6          Definition of the PRACK Method ......................    8
   7          Header Field Definitions ............................    9
   7.1        RSeq ................................................    9
   7.2        RAck ................................................    9
   8          IANA Registration of the 100rel Option Tag ..........    9
   9          Security Considerations .............................   12
   10         IANA Considerations .................................   12
   11         Collected BNF .......................................   12
   12         Acknowledgements ....................................   12
   13         Author's Addresses ..................................   12
   14         Normative References ................................   13
   15         Non-Normative References ............................   13

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1 Introduction

   (NOTE TO RFC-EDITOR: Please replace instances of BBBB in this
   document with the RFC number for draft-ietf-sip-rfc2543bis.)

   The Session Initiation Protocol (SIP) (RFC BBBB [1]) is a request-
   response protocol for initiating and managing communications
   sessions. SIP defines two types of responses, provisional and final.
   Final responses convey the result of the request processing, and are
   sent reliably. Provisional responses provide information on progress
   of the request processing, but are not sent reliably in RFC BBBB.

   It was later observed that reliability was important in several
   cases, including interoperability scenarios with the PSTN. Therefore,
   an optional capability was needed to support reliable transmission of
   provisional responses. That capability is provided in this

   The reliability mechanism works by mirroring the current reliability
   mechanisms for 2xx final responses to INVITE. Those requests are
   transmitted periodically by the TU until a separate transaction, ACK,
   is received that indicates reception of the 2xx by the UAC. The
   reliability for the 2xx responses to INVITE and ACK messages are
   end-to-end. In order to achieve reliability for provisional
   responses, we do nearly the same thing. Reliable provisional
   responses are retransmitted by the TU with an exponential backoff.
   Those retransmissions cease when a PRACK message is received. The
   PRACK request plays the same role as ACK, but for provisional
   responses. There is an important difference, however. PRACK is a
   normal SIP message, like BYE. As such, its own reliability is ensured
   hop-by-hop through each stateful proxy. Also like BYE, but unlike
   ACK, PRACK has its own response. If this were not the case, the PRACK
   message could not traverse proxy servers compliant to RFC 2543 [4].

   Each provisional response is given a sequence number, carried in the
   RSeq header field in the response. The PRACK messages contain an RAck
   header field, which indicates the sequence number of the provisional
   response that is being acknowledged. The acknowledgements are not
   cumulative, and the specifications recommend a single outstanding
   provisional response at a time, for purposes of congestion control.

2 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
   indicate requirement levels for compliant SIP implementations.

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3 UAS Behavior

   A UAS MAY send any non-100 provisional response to INVITE reliably,
   so long as the initial INVITE request (the request whose provisional
   response is being sent reliably) contained a Supported header field
   with the option tag 100rel. While this specification does not allow
   reliable provisional responses for any method but INVITE, extensions
   that define new methods that can establish dialogs may make use of
   the mechanism.

   The UAS MUST send any non-100 provisional response reliably if the
   initial request contained a Require header field with the option tag
   100rel. If the UAS is unwilling to do so, it MUST reject the initial
   request with a 420 (Bad Extension) and include an Unsupported header
   field containing the option tag 100rel.

   A UAS MUST NOT attempt to send a 100 (Trying) response reliably. Only
   provisional responses numbered 101 to 199 may be sent reliably. If
   the request did not include either a Supported or Require header
   field indicating this feature, the UAS MUST NOT send the provisional
   response reliably.

        100 (Trying) responses are hop-by-hop only. For this
        reason, the reliability mechanisms described here, which
        are end-to-end, cannot be used.

   An element that can act as a proxy can also send reliable provisional
   responses. In this case, it acts as a UAS for purposes of that
   transaction. However, it MUST NOT attempt to do so for any request
   that contains a tag in the To field. That is, a proxy cannot generate
   reliable provisional responses to requests sent within the context of
   a dialog. Of course, unlike a UAS, when the proxy element receives a
   PRACK that does not match any outstanding reliable provisional
   response, the PRACK MUST be proxied.

   There are several reasons why a UAS might want to send a reliable
   provisional response. One reason is if the INVITE transaction will
   take some time to generate a final response. As discussed in Section
   13.3 of RFC BBBB, the UAS will need to send periodic provisional
   responses to request an "extension" of the transaction at proxies.
   The requirement is that a proxy receive them every three minutes, but
   the UAS needs to send them more frequently (once a minute is
   recommended) because of the possibility of packet loss. As a more
   efficient alternative, the UAS can send the response reliably, in
   which case the UAS SHOULD send provisional responses once every three
   minutes. Use of reliable provisional responses for extending
   transactions is RECOMMENDED.

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   The rest of this discussion assumes that the initial request
   contained a Supported or Require header field listing 100rel, and
   that there is a provisional response to be sent reliably.

   The provisional response to be sent reliably is constructed by the
   UAS core according to the procedures of Section 8.2.6 of RFC BBBB. In
   addition, it MUST contain a Require header field containing the
   option tag 100rel, and MUST include an RSeq header field. The value
   of the header field for the first reliable provisional response in a
   transaction MUST be between 1 and 2**31 - 1. It is RECOMMENDED that
   it be chosen uniformly in this range. The RSeq numbering space is
   within a single transaction. This means that provisional responses
   for different requests MAY use the same values for the RSeq number.

   The reliable provisional response MAY contain a body. The usage of
   session descriptions is described in Section 5.

   The reliable provisional response is passed to the transaction layer
   periodically with an interval that starts at T1 seconds and doubles
   for each retransmission (T1 is defined in Section 17 of RFC BBBB).
   Once passed to the server transaction, it is added to an internal
   list of unacknowledged reliable provisional responses. The
   transaction layer will forward each retransmission passed from the
   UAS core.

        This differs from retransmissions of 2xx responses, whose
        intervals cap at T2 seconds. This is because
        retransmissions of ACK are triggered on receipt of a 2xx,
        but retransmissions of PRACK take place independently of
        reception of 1xx.

   Retransmissions of the reliable provisional response cease when a
   matching PRACK is received by the UA core. PRACK is like any other
   request within a dialog, and the UAS core processes it according to
   the procedures of Sections 8.2 and 12.2.2 of RFC BBBB. A matching
   PRACK is defined as one within the same dialog as the response, and
   whose method, CSeq-num, and response-num in the RAck header field
   match, respectively, the method and sequence number from the CSeq and
   sequence number from the RSeq of the reliable provisional response.

   If a PRACK request is received by the UA core that does not match any
   unacknowledged reliable provisional response, the UAS MUST respond to
   the PRACK with a 481 response. If the PRACK does match an
   unacknowledged reliable provisional response, it MUST be responded to
   with a 2xx response. The UAS can be certain at this point that the
   provisional response has been received in order. It SHOULD cease
   retransmissions of the reliable provisional response, and MUST remove

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   it from the list of unacknowledged provisional responses.

   If a reliable provisional response is retransmitted for 64*T1 seconds
   without reception of a corresponding PRACK, the UAS SHOULD reject the
   original request with a 5xx response.

   If the PRACK contained a session description, it is processed as
   described in Section 5 of this document. If the PRACK instead
   contained any other type of body, the body is treated in the same way
   that body in an ACK would be treated.

   After the first reliable provisional response for a request has been
   acknowledged, the UAS MAY send additional reliable provisional
   responses. The UAS MUST NOT send a second reliable provisional
   response until the first is acknowledged. After the first, it is
   RECOMMENDED that the UAS not send an additional reliable provisional
   response until the previous is acknowledged. The first reliable
   provisional response receives special treatment because it conveys
   the initial sequence number. If additional reliable provisional
   responses were sent before the first was acknowledged, the UAS could
   not be certain these were received in order.

   The value of the RSeq in each subsequent reliable provisional
   response for the same request MUST be greater by exactly one.  RSeq
   numbers MUST NOT wrap around. Because the initial one is chosen to be
   less than 2**31 - 1, but the maximum is 2**32 - 1, there can be up to
   2**31 reliable provisional responses per request, which is more than

   The UAS MAY send a final response to the initial request before
   having recevied PRACKs for all unacknowleged reliable provisional
   responses, unless any of the unacknowleged reliable provisional
   responses contained a session description. In that case, it MUST NOT
   send a final response until those provisional responses are
   acknowledged. If the UAS does send a final response when reliable
   responses are still unacknowledged, it SHOULD NOT continue to
   retransmit the unacknowledged reliable provisional responses, but it
   MUST be prepared to process PRACK requests for those outstanding
   responses. A UAS MUST NOT send new reliable provisional responses (as
   opposed to retransmissions of unacknowledged ones) after sending a
   final response to a request.

4 UAC Behavior

   When the UAC creates a new request, it can insist on reliable
   delivery of provisional responses for that request. To do that, it
   inserts a Require header field into the request with the option tag
   100rel. This header field MUST NOT be present in any requests

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   excepting INVITE, although extensions to SIP may allow its usage with
   other request methods.

   If the UAC does not wish to insist on usage of reliable provisional
   responses, but merely indicate that it supports them if the UAS needs
   to send one, a Supported header MUST be included in the request with
   the option tag 100rel. The UAC SHOULD include this in all INVITE

   If a provisional response is received for an initial request, and
   that response contains a Require header field containing the option
   tag 100rel, the response is to be sent reliably. If the response is a
   100 (Trying) (as opposed to 101 to 199), this option tag MUST be
   ignored, and the procedures below MUST NOT be used.

   The provisional response MUST establish a dialog if one is not yet

   Assuming the response is to be transmitted reliably, the UAC MUST
   create a new request with method PRACK. This request is sent within
   the dialog associated with the provisional response (indeed, the
   provisional response may have created the dialog). PRACK requests MAY
   contain bodies, which are interpreted according to their type and

   Note that the PRACK is like any other non-INVITE request within a
   dialog. In particular, a UAC SHOULD NOT retransmit the PRACK request
   when it receives a retransmission of the provisional response being
   acknowledged, although doing so does not create a protocol error.

   Once a reliable provisional response is received, retransmissions of
   that response MUST be discarded. A response is a retransmission when
   its dialog ID, CSeq, and RSeq match the original response. The UAC
   MUST maintain a sequence number that indicates the most recently
   received in-order reliable provisional response for the initial
   request. This sequence number MUST be maintained until a final
   response is received for the initial request. Its value MUST be
   initialized to the RSeq header field in the first reliable
   provisional response received for the initial request.

   Handling of subsequent reliable provisional responses for the same
   initial request follows the same rules as above, with the following
   difference: reliable provisional responses are guaranteed to be in
   order. As a result, if the UAC receives another reliable provisional
   response to the same request, and its RSeq value is not one higher
   than the value of the sequence number, that response MUST NOT be
   acknowledged with a PRACK, and MUST NOT be processed further by the
   UAC. An implementation MAY discard the response, or MAY cache the

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   response in the hopes of receiving the missing responses.

   The UAC MAY acknowledge reliable provisional responses received after
   the final response or MAY discard them.

5 The Offer/Answer Model and PRACK

   RFC BBBB describes guidelines for the sets of messages in which
   offers and answers [3] can appear. Based on those guidelines, this
   extension provides additional opportunities for offer/answer

   If the INVITE contained an offer, the UAS MAY generate an answer in a
   reliable provisional response (assuming these are supported by the
   UAC). That results in the establishment of the session before
   completion of the call. Similarly, if a reliable provisional response
   is the first reliable message sent back to the UAC, and the INVITE
   did not contain an offer, one MUST appear in that reliable
   provisional response.

   If the UAC receives a reliable provisional response with an offer
   (this would occur if the UAC sent an INVITE without an offer, in
   which case the first reliable provisional response will contain the
   offer), it MUST generate an answer in the PRACK. If the UAC receives
   a reliable provisional response with an answer, it MAY generate an
   additional offer in the PRACK. If the UAS receives a PRACK with an
   offer, it MUST place the answer in the 2xx to the PRACK.

   Once an answer has been sent or received, the UA SHOULD establish the
   session based on the parameters of the offer and answer, even if the
   original INVITE itself has not been responded to.

   If the UAS had placed a session description in any reliable
   provisional response that is unacknowledged when the INVITE is
   accepted, the UAS MUST delay sending the 2xx until the provisional
   response is acknowledged. Otherwise, the reliability of the 1xx
   cannot be guaranteed, and reliability is needed for proper operation
   of the offer/answer exchange.

   All user agents that support this extension MUST support all
   offer/answer exchanges that are possible based on the rules in
   Section 13.2 of RFC BBBB, based on the existence of INVITE and PRACK
   as requests, and 2xx and reliable 1xx as non-failure reliable

6 Definition of the PRACK Method

   This specification defines a new SIP method, PRACK. The semantics of

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   this method are described above. Tables 1 and 2 extend Tables 2 and 3
   from RFC BBBB for this new method.

7 Header Field Definitions

   This specification defines two new header fields, RAck and RSeq.
   Table 3 extends Tables 2 and 3 from RFC BBBB for these headers.

7.1 RSeq

   The RSeq header is used in provisional responses in order to transmit
   them reliably. It contains a single numeric value from 1 to 2**32 -
   1. For details on its usage, see Section 3.


     RSeq: 988789

7.2 RAck

   The RAck header is sent in a PRACK request to support reliability of
   provisional responses. It contains two numbers and a method tag. The
   first number is the value from the RSeq header in the provisional
   response that is being acknowledged. The next number, and the method,
   are copied from the CSeq in the response that is being acknowledged.
   The method name in the RAck header is case sensitive.


     RAck: 776656 1 INVITE

8 IANA Registration of the 100rel Option Tag

   This specification registers a single option tag, 100rel. The
   required information for this registration, as specified in RFC BBBB,

        Name: 100rel

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                    Header field          where   PRACK
                    Accept                  R       o
                    Accept                 2xx      -
                    Accept                 415      c
                    Accept-Encoding         R       o
                    Accept-Encoding        2xx      -
                    Accept-Encoding        415      c
                    Accept-Language         R       o
                    Accept-Language        2xx      -
                    Accept-Language        415      c
                    Alert-Info              R       -
                    Alert-Info             180      -
                    Allow                   R       o
                    Allow                  2xx      o
                    Allow                   r       o
                    Allow                  405      m
                    Authentication-Info    2xx      o
                    Authorization           R       o
                    Call-ID                 c       m
                    Call-Info                       -
                    Contact                 R       -
                    Contact                1xx      -
                    Contact                2xx      -
                    Contact                3xx      o
                    Contact                485      o
                    Content-Disposition             o
                    Content-Encoding                o
                    Content-Language                o
                    Content-Length                 t
                    Content-Type                    *
                    CSeq                    c       m
                    Date                            o
                    Error-Info           300-699    o
                    Expires                         -
                    From                    c       m
                    In-Reply-To             R       -
                    Max-Forwards            R      m
                    Min-Expires            423      -
                    MIME-Version                    o
                    Organization                    -

   Table 1: Summary of header fields, A--O

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                Header field              where      PRACK
                Priority                    R          -
                Proxy-Authenticate         407         m
                Proxy-Authenticate         401         o
                Proxy-Authorization         R          o
                Proxy-Require               R          o
                Record-Route                R          o
                Record-Route             2xx,18x       o
                Reply-To                               -
                Require                                c
                Retry-After          404,413,480,486   o
                                         500,503       o
                                         600,603       o
                Route                       R          c
                Server                      r          o
                Subject                     R          -
                Supported                   R          o
                Supported                  2xx         o
                Timestamp                              o
                To                          c          m
                Unsupported                420         m
                User-Agent                             o
                Via                         c          m
                Warning                     r          o
                WWW-Authenticate           401         m

   Table 2: Summary of header fields, P--Z; (1):  copied  with  possible
   addition of tag

          Header field  where  proxy ACK BYE CAN INV OPT REG PRA

          RAck            R           -   -   -   -   -   -   m
          RSeq           1xx          -   -   -   o   -   -   -

   Table 3: RAck and RSeq Header Fields

        Description: This option tag is for reliability of provisional
             responses. When present in a Supported header, it indicates
             that the UA can send or receive reliable provisional
             responses. When present in a Require header in a request,
             it indicates that the UAS MUST send all provisional
             responses reliably. When present in a Require header in a
             reliable provisional response, it indicates that the
             response is to be sent reliably.

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        New Headers: The RSeq and RAck header fields are defined by this

        Change Control: IETF.

        Reference: RFCXXXX [Note to IANA: Fill in with the RFC number of
             this specification.]

        Contact Information: Jonathan Rosenberg, jdrosen@jdrosen.net. 72
             Eagle Rock Avenue, First Floor, East Hanover, NJ, 07936,

9 Security Considerations

   The PRACK request can be injected by attackers to force
   retransmissions of reliable provisional responses to cease. As these
   responses can convey important information, PRACK messages SHOULD be
   authenticated as any other request.

10 IANA Considerations

   There are no IANA considerations associated with this specification.

11 Collected BNF

   The BNF for the RAck and RSeq headers and the PRACK method are
   defined here.

        PRACKm        =  %x50. ; PRACK in caps
        Method        =  INVITEm / ACKm / OPTIONSm / BYEm
                         / CANCELm / REGISTERm / PRACKm
                         / extension-method
        RAck          =  "RAck" HCOLON response-num LWS CSeq-num LWS Method
        response-num  =  1*DIGIT
        CSeq-num      =  1*DIGIT
        RSeq          =  "RSeq" HCOLON response-num

12 Acknowledgements

   The authors would like to thank Jo Hornsby, Jonathan Lennox, Rohan
   Mahy, Adam Roach, and Tim Schroeder for the comments on this

13 Author's Addresses

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   Jonathan Rosenberg
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com

   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY 10027-7003
   email: schulzrinne@cs.columbia.edu

14 Normative References

   [1] J. Rosenberg, H. Schulzrinne, et al.  , "SIP: Session initiation
   protocol," Internet Draft, Internet Engineering Task Force, Oct.
   2001.  Work in progress.

   [2] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," Request for Comments 2119, Internet Engineering Task Force,
   Mar. 1997.

   [3] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
   SDP," Internet Draft, Internet Engineering Task Force, Jan. 2002.
   Work in progress.

15 Non-Normative References

   [4] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   session initiation protocol," Request for Comments 2543, Internet
   Engineering Task Force, Mar. 1999.

   Full Copyright Statement

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   the copyright notice or references to the Internet Society or other
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