Internet Engineering Task Force SIP WG
Internet Draft J.Rosenberg
dynamicsoft
H.Schulzrinne
Columbia U.
draft-ietf-sip-100rel-05.txt
February 21, 2002
Expires: August 2002
Reliability of Provisional Responses in SIP
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
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Abstract
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
J.Rosenberg et. al. [Page 2]
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
specification.
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",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
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
sufficient.
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
requests.
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
created.
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
disposition.
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
exchanges.
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
responses.
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.
Example:
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.
Example:
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,
is:
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
option.
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,
USA.
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 = %x50.52.41.43.4B ; 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
document.
13 Author's Addresses
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Internet Draft 100 Reliability February 21, 2002
Jonathan Rosenberg
dynamicsoft
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.
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