RFC 2976 PROPOSED STANDARD VOIP · SIGNALING · SESSION CONTROL
The SIP INFO Method
A way for SIP calls to exchange mid-call control information.
RFC EDITOR
Donovan, S., "The SIP INFO Method", RFC 2976, DOI 10.17487/RFC2976, October 2000, <https://www.rfc-editor.org/info/rfc2976>.
BIBTEX
@misc{rfc2976,
series = {Request for Comments},
number = 2976,
howpublished = {RFC 2976},
publisher = {RFC Editor},
doi = {10.17487/RFC2976},
url = {https://www.rfc-editor.org/info/rfc2976},
author = {S. Donovan},
title = {{The SIP INFO Method}},
pagetotal = 9,
year = 2000,
month = oct,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Network Working Group S. Donovan
Request for Comments: 2976 dynamicsoft
Category: Standards Track October 2000
The SIP INFO Method
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document proposes an extension to the Session Initiation
Protocol (SIP). This extension adds the INFO method to the SIP
protocol. The intent of the INFO method is to allow for the carrying
of session related control information that is generated during a
session. One example of such session control information is ISUP and
ISDN signaling messages used to control telephony call services.
This and other example uses of the INFO method may be standardized in
the future.
Table of Contents
1 Introduction................................................2
1.1 Example Uses................................................2
2 INFO Method.................................................3
2.1 Header Field Support for INFO Method........................3
2.2 Responses to the INFO Request Method........................4
2.3 Message Body Inclusion......................................5
2.4 Behavior of SIP User Agents.................................6
2.5 Behavior of SIP Proxy and Redirect Servers..................6
2.5.1 Proxy Server................................................6
2.5.2 Forking Proxy Server........................................6
2.5.3 Redirection Server..........................................6
3. INFO Message Bodies.........................................6
4. Guidelines for extensions making use of INFO................7
5. Security Considerations.....................................7
6. References..................................................8
7. Acknowledgments.............................................8
8. Author's Address............................................8
Full Copyright Statement....................................9
1. Introduction
The SIP protocol described in [1] defines session control messages
used during the setup and tear down stages of a SIP controlled
session.
In addition, the SIP re-INVITE can be used during a session to change
the characteristics of the session. This is generally to change the
properties of media flows related to the session or to update the SIP
session timer.
However, there is no general-purpose mechanism to carry session
control information along the SIP signaling path during the session.
The purpose of the INFO message is to carry application level
information along the SIP signaling path.
The INFO method is not used to change the state of SIP calls, or the
parameters of the sessions SIP initiates. It merely sends optional
application layer information, generally related to the session.
It is necessary that the mid-session signaling information traverse
the post session setup SIP signaling path. This is the path taken by
SIP re-INVITEs, BYEs and other SIP requests that are tied to an
individual session. This allows SIP proxy servers to receive, and
potentially act on, the mid-session signaling information.
This document proposes an extension to SIP by defining the new INFO
method. The INFO method would be used for the carrying of mid-call
signaling information along the session signaling path.
1.1 Example Uses
The following are a few of the potential uses of the INFO message:
- Carrying mid-call PSTN signaling messages between PSTN
gateways.
- Carrying DTMF digits generated during a SIP session.
- Carrying wireless signal strength information in support of
wireless mobility applications.
- Carrying account balance information.
- Carrying images or other non streaming information between the
participants of a session.
These are just potential uses; this document does not specify such
uses nor does it necessarily recommend them.
It can also be envisioned that there will be other telephony and
non-telephony uses of the INFO method.
2. INFO Method
The INFO method is used for communicating mid-session signaling
information along the signaling path for the call.
The INFO method is not used to change the state of SIP calls, nor
does it change the state of sessions initiated by SIP. Rather, it
provides additional optional information which can further enhance
the application using SIP.
The signaling path for the INFO method is the signaling path
established as a result of the call setup. This can be either direct
signaling between the calling and called user agents or a signaling
path involving SIP proxy servers that were involved in the call setup
and added themselves to the Record-Route header on the initial INVITE
message.
The mid-session information can be communicated in either an INFO
message header or as part of a message body. The definition of the
message body and/or message headers used to carry the mid-session
information is outside the scope of this document.
There are no specific semantics associated with INFO. The semantics
are derived from the body or new headers defined for usage in INFO.
2.1 Header Field Support for INFO Method
Tables 1 and 2 add a column to tables 4 and 5 in the [1]. Refer
to Section 6 of [1] for a description of the content of the
tables. Note that the rules defined in the enc. and e-e columns
in tables 4 and 5 in [1] also apply to use of the headers in the
INFO request and responses to the INFO request.
2.2 Responses to the INFO Request Method
If a server receives an INFO request it MUST send a final
response.
A 200 OK response MUST be sent by a UAS for an INFO request with
no message body if the INFO request was successfully received for
an existing call. Beyond that, no additional operations are
required.
Header Where INFO
------ ----- ----
Accept R o
Accept-Encoding R o
Accept-Language R o
Allow 200 -
Allow 405 o
Authorization R o
Call-ID gc m
Contact R o
Contact 1xx -
Contact 2xx -
Contact 3xx -
Contact 485 -
Content-Encoding e o
Content-Length e o
Content-Type e *
CSeq gc m
Date g o
Encryption g o
Expires g o
From gc m
Hide R o
Max-Forwards R o
Organization g o
Table 1 Summary of header fields, A-0
Handling of INFO messages that contain message bodies is outside
the scope of this document. The documents defining the message
bodies will also need to define the SIP protocol rules associated
with those message bodies.
A 481 Call Leg/Transaction Does Not Exist message MUST be sent by
a UAS if the INFO request does not match any existing call leg.
If a server receives an INFO request with a body it understands,
but it has no knowledge of INFO associated processing rules for
the body, the body MAY be rendered and displayed to the user. The
INFO is responded to with a 200 OK.
If the INFO request contains a body that the server does not
understand then, in the absence of INFO associated processing
rules for the body, the server MUST respond with a 415 Unsupported
Media Type message.
Header Where INFO
------ ----- ----
Priority R o
Proxy-Authenticate 407 o
Proxy-Authorization R o
Proxy-Require R o
Require R o
Retry-After R -
Retry-After 404,480,486 o
Retry-After 503 o
Retry-After 600,603 o
Response-Key R o
Record-Route R o
Record-Route 2xx o
Route R o
Server r o
Subject R o
Timestamp g o
To gc(1) m
Unsupported 420 o
User-Agent g o
Via gc(2) m
Warning r o
WWW-Authenticate 401 o
Table 2 Summary of header fields, P-Z
Bodies which imply a change in the SIP call state or the sessions
initiated by SIP MUST NOT be sent in an INFO message.
Other request failure (4xx), Server Failure (5xx) and Global
Failure (6xx) responses MAY be sent for the INFO Request.
2.3 Message Body Inclusion
The INFO request MAY contain a message body.
2.4 Behavior of SIP User Agents
Unless stated otherwise, the protocol rules for the INFO request
governing the usage of tags, Route and Record-Route,
retransmission and reliability, CSeq incrementing and message
formatting follow those in [1] as defined for the BYE request.
An INFO request MAY be cancelled. A UAS receiving a CANCEL for an
INFO request SHOULD respond to the INFO with a "487 Request
Cancelled" response if a final response has not been sent to the
INFO and then behave as if the request were never received.
However, the INFO message MUST NOT change the state of the SIP
call, or the sessions initiated by SIP.
2.5 Behavior of SIP Proxy and Redirect Servers
2.5.1 Proxy Server
Unless stated otherwise, the protocol rules for the INFO
request at a proxy are identical to those for a BYE request as
specified in [1].
2.5.2 Forking Proxy Server
Unless stated otherwise, the protocol rules for the INFO
request at a proxy are identical to those for a BYE request as
specified in [1].
2.5.3 Redirection Server
Unless stated otherwise, the protocol rules for the INFO
request at a proxy are identical to those for a BYE request as
specified in [1].
3. INFO Message Bodies
The purpose of the INFO message is to carry mid-session information
between SIP user agents. This information will generally be carried
in message bodies, although it can be carried in headers in the INFO
message.
The definition of the message bodies or any new headers created for
the INFO method is outside the scope of this document. It is
expected that separate documents will be created to address
definition of these entities.
In addition, the INFO method does not define additional mechanisms
for ensuring in-order delivery. While the CSeq header will be
incremented upon the transmission of new INFO messages, this should
not be used to determine the sequence of INFO information. This is
due to the fact that there could be gaps in the INFO message CSeq
count caused by a user agent sending re-INVITES or other SIP
messages.
4. Guidelines for extensions making use of INFO
The following are considerations that should be taken into account
when defining SIP extensions that make use of the INFO method.
- Consideration should be taken on the size of message bodies to be
carried by INFO messages. The message bodies should be kept small
due to the potential for the message to be carried over UDP and the
potential for fragmentation of larger messages.
- There is potential that INFO messages could be forked by a SIP
Proxy Server. The implications of this forking of the information
in the INFO message need to be taken into account.
- The use of multi-part message bodies may be helpful when defining
the message bodies to be carried by the INFO message.
- The extensions that use the INFO message MUST NOT rely on the
INFO message to do anything that effects the SIP call state or the
state of related sessions.
- The INFO extension defined in this document does not depend on
the use of the Require or Proxy-Require headers. Extensions using
the INFO message may need the use of these mechanisms. However,
the use of Require and Proxy-Require should be avoided, if
possible, in order to improve interoperability between SIP
entities.
5. Security Considerations
If the contents of the message body are private then end-to-end
encryption of the message body can be used to prevent unauthorized
access to the content.
There are no other security issues specific to the INFO method.
The security requirements specified in the SIP specification apply
to the INFO method.
6. References
[1] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
7. Acknowledgements
The author would like to thank Matthew Cannon for his contributions
to this document. In addition, the author would like to thank the
members of the MMUSIC and SIP working groups, especially Jonathan
Rosenberg, for comments and suggestions on how to improve the
document.
8. Author's Address
Steve Donovan
dynamicsoft
5100 Tennyson Parkway, Suite 200
Plano, Texas 75024
Email: sdonovan@dynamicsoft.com
9. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
PROBLEMSIP handles call setup and teardown, but once a call is established, there is no standard way to send session-related control information like DTMF tones or signaling between gateways.
MECHANISMRFC 2976 adds the INFO method, which lets SIP endpoints send optional information along the established call signaling path without changing the call state.
LATER REFINEDINFO provides a general-purpose mechanism for mid-call signaling, though it was later refined in RFC 6086.
§1What INFO Solves
The SIP protocol handles call setup (INVITE), teardown (BYE) and mid-call changes (re-INVITE). However, once a call is established, there is no general way to send session control information along the SIP signaling path. The INFO method fills this gap.
INFO carries application-level information related to the session without changing the call state or session parameters. Examples include DTMF tones during calls, PSTN signaling between gateways, wireless signal strength information, or account balance updates.
Your phone system can set up calls and hang up calls, but it needs a way to send little messages during the call. INFO is that way.
§2How INFO Works
The INFO method follows the same signaling path that re-INVITE, BYE and other in-session requests use. This path is established during call setup and may involve SIP proxy servers that added themselves to the Record-Route header during the original INVITE.
An INFO request may carry information in message headers or in a message body. A 200 OK response indicates successful delivery. The protocol does not define the format of the body; that is left to extensions using INFO. INFO messages MUST be responded to with a final response (not provisional responses).
INFO messages travel the same route as the rest of the call. You send one, and the other side sends back a message saying it got it.
§3Header Fields and Message Body
INFO requests support the same header fields as other SIP requests like BYE, with a few specific rules. The Call-ID, CSeq, From, To and Via headers are mandatory. Content-Type and Content-Length headers apply when a message body is present.
The RFC defines rules for handling different response codes: a 200 OK for successful delivery, a 481 Call Leg Does Not Exist if the call is not found, and a 415 Unsupported Media Type if the message body cannot be handled. The CSeq header is incremented for each INFO sent, though the RFC notes that gaps may occur due to re-INVITEs or other messages.
INFO messages look like other SIP messages but use special rules for what headers must be present and what to do if the message arrives at the wrong call.
§4Restrictions and Guidelines
INFO messages MUST NOT be cancelled with a CANCEL request; instead, the recipient sends a 487 Request Cancelled response if no final response has been sent. An INFO request MUST NOT change the state of the SIP call or the sessions it initiates.
Extensions using INFO should keep message bodies small to avoid fragmentation over UDP. They should be aware that SIP proxy servers may fork INFO messages, and that extending INFO should avoid using the Require or Proxy-Require headers where possible to improve interoperability.
INFO messages are meant to carry extra information, not to break or change the call. Keep them small and simple, and remember that multiple copies might arrive if the network branches them.
§5The INFO Method in Practice
The INFO method provides a standard way for mid-session information to travel alongside SIP calls. RFC 2976 defines the mechanism but leaves the body format to extensions. Later, RFC 6086 refined the definition for better interoperability. The original method is still valid and widely used as the basis for INFO-carrying extensions.
INFO became the standard way for SIP calls to exchange extra messages during the call. Other documents have since clarified how to use it.