Session Initiation Protocol (SIP) INFO Method and Package Framework
SIP gets a method for passing application data inside a call, plus a labelling scheme so both ends know what the data means.
Holmberg, C., Burger, E., and H. Kaplan, "Session Initiation Protocol (SIP) INFO Method and Package Framework", RFC 6086, DOI 10.17487/RFC6086, January 2011, <https://www.rfc-editor.org/info/rfc6086>.
@misc{rfc6086,
series = {Request for Comments},
number = 6086,
howpublished = {RFC 6086},
publisher = {RFC Editor},
doi = {10.17487/RFC6086},
url = {https://www.rfc-editor.org/info/rfc6086},
author = {C. Holmberg and E. Burger and H. Kaplan},
title = {{Session Initiation Protocol (SIP) INFO Method and Package Framework}},
pagetotal = 36,
year = 2011,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) C. Holmberg
Request for Comments: 6086 Ericsson
Obsoletes: 2976 E. Burger
Category: Standards Track Georgetown University
ISSN: 2070-1721 H. Kaplan
Acme Packet
January 2011
Session Initiation Protocol (SIP) INFO Method and Package Framework
Abstract
This document defines a method, INFO, for the Session Initiation
Protocol (SIP), and an Info Package mechanism. This document
obsoletes RFC 2976. For backward compatibility, this document also
specifies a "legacy" mode of usage of the INFO method that is
compatible with the usage previously defined in RFC 2976, referred to
as "legacy INFO Usage" in this document.
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/rfc6086.
Copyright Notice
Copyright (c) 2011 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
1.1. Conventions Used in This Document ..........................4
2. Motivation ......................................................4
3. Applicability and Backward Compatibility ........................5
4. The INFO Method .................................................6
4.1. General ....................................................6
4.2. INFO Request ...............................................6
4.2.1. INFO Request Sender .................................6
4.2.2. INFO Request Receiver ...............................7
4.2.3. SIP Proxies .........................................8
4.3. INFO Message Body ..........................................8
4.3.1. INFO Request Message Body ...........................8
4.3.2. INFO Response Message Body ..........................9
4.4. Order of Delivery ..........................................9
5. Info Packages ...................................................9
5.1. General ....................................................9
5.2. User Agent Behavior .......................................10
5.2.1. General ............................................10
5.2.2. UA Procedures ......................................10
5.2.3. Recv-Info Header Field Rules .......................11
5.2.4. Info Package Fallback Rules ........................12
5.3. REGISTER Processing .......................................12
6. Formal INFO Method Definition ..................................13
6.1. INFO Method ...............................................13
7. INFO Header Fields .............................................15
7.1. General ...................................................15
7.2. Info-Package Header Field .................................15
7.3. Recv-Info Header Field ....................................16
8. Info Package Considerations ....................................16
8.1. General ...................................................16
8.2. Appropriateness of Info Package Usage .....................16
8.3. INFO Request Rate and Volume ..............................16
8.4. Alternative Mechanisms ....................................17
8.4.1. Alternative SIP Signaling Plane Mechanisms .........17
8.4.2. Media Plane Mechanisms .............................18
8.4.3. Non-SIP-Related Mechanisms .........................19
9. Syntax .........................................................19
9.1. General ...................................................19
9.2. ABNF ......................................................19
10. Info Package Requirements .....................................20
10.1. General ..................................................20
10.2. Overall Description ......................................20
10.3. Applicability ............................................20
10.4. Info Package Name ........................................21
10.5. Info Package Parameters ..................................21
10.6. SIP Option-Tags ..........................................22
10.7. INFO Message Body Parts ..................................22
10.8. Info Package Usage Restrictions ..........................22
10.9. Rate of INFO Requests ....................................23
10.10. Info Package Security Considerations ....................23
10.11. Implementation Details ..................................23
10.12. Examples ................................................24
11. IANA Considerations ...........................................24
11.1. Update to Registration of SIP INFO Method ................24
11.2. Registration of the Info-Package Header Field ............24
11.3. Registration of the Recv-Info Header Field ...............24
11.4. Creation of the Info Packages Registry ...................25
11.5. Registration of the Info-Package Content-Disposition .....25
11.6. SIP Response Code 469 Registration .......................26
12. Examples ......................................................26
12.1. Indication of Willingness to Receive INFO Requests
for Info Packages ........................................26
12.1.1. Initial INVITE Request ............................26
12.1.2. Target Refresh ....................................27
12.2. INFO Request Associated with Info Package ................28
12.2.1. Single Payload ....................................28
12.2.2. Multipart INFO ....................................28
13. Security Considerations .......................................30
14. References ....................................................31
14.1. Normative References .....................................31
14.2. Informative References ...................................32
Appendix A. Acknowledgements .....................................35
1. Introduction
This document defines a method, INFO, for the Session Initiation
Protocol (SIP) [RFC3261].
The purpose of the INFO message is to carry application level
information between endpoints, using the SIP dialog signaling path.
Note that the INFO method is not used to update characteristics of a
SIP dialog or session, but to allow the applications that use the SIP
session to exchange information (which might update the state of
those applications).
Use of the INFO method does not constitute a separate dialog usage.
INFO messages are always part of, and share the fate of, an invite
dialog usage [RFC5057]. INFO messages cannot be sent as part of
other dialog usages, or outside an existing dialog.
This document also defines an Info Package mechanism. An Info
Package specification defines the content and semantics of the
information carried in an INFO message associated with the Info
Package. The Info Package mechanism also provides a way for user
agents (UAs) to indicate for which Info Packages they are willing to
receive INFO requests, and which Info Package a specific INFO request
is associated with.
A UA uses the Recv-Info header field, on a per-dialog basis, to
indicate for which Info Packages it is willing to receive INFO
requests. A UA can indicate an initial set of Info Packages during
dialog establishment and can indicate a new set during the lifetime
of the invite dialog usage.
NOTE: A UA can use an empty Recv-Info header field (a header field
without a value) to indicate that it is not willing to receive
INFO requests for any Info Package, while still informing other
UAs that it supports the Info Package mechanism.
When a UA sends an INFO request, it uses the Info-Package header
field to indicate which Info Package is associated with the request.
One particular INFO request can only be associated with a single Info
Package.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Motivation
A number of applications, standardized and proprietary, make use of
the INFO method as it was previously defined in RFC 2976 [RFC2976],
here referred to as "legacy INFO usage". These include but are not
limited to the following:
o RFC 3372 [RFC3372] specifies the encapsulation of ISDN User Part
(ISUP) in SIP message bodies. ITU-T and the Third Generation
Partnership Project (3GPP) have specified similar procedures.
o [ECMA-355] specifies the encapsulation of "QSIG" in SIP message
bodies.
o RFC 5022 [RFC5022] specifies how INFO is used as a transport
mechanism by the Media Server Control Markup Language (MSCML)
protocol. MSCML uses an option-tag in the Require header field to
ensure that the receiver understands the INFO content.
o RFC 5707 [RFC5707] specifies how INFO is used as a transport
mechanism by the Media Server Markup Language (MSML) protocol.
o Companies have been using INFO messages in order to request fast
video update. Currently, a standardized mechanism, based on the
Real-time Transport Control Protocol (RTCP), has been specified in
RFC 5168 [RFC5168].
o Companies have been using INFO messages in order to transport
Dual-Tone Multi-Frequency (DTMF) tones. All mechanisms are
proprietary and have not been standardized.
Some legacy INFO usages are also recognized as being shortcuts to
more appropriate and flexible mechanisms.
Furthermore, RFC 2976 did not define mechanisms that would enable a
SIP UA to indicate (1) the types of applications and contexts in
which the UA supports the INFO method or (2) the types of
applications and contexts with which a specific INFO message is
associated.
Because legacy INFO usages do not have associated Info Packages, it
is not possible to use the Recv-Info and Info-Package header fields
with legacy INFO usages. That is, a UA cannot use the Recv-Info
header field to indicate for which legacy INFO usages it is willing
to receive INFO requests, and a UA cannot use the Info-Package header
field to indicate with which legacy INFO usage an INFO request is
associated.
Due to the problems described above, legacy INFO usages often require
static configuration to indicate the types of applications and
contexts for which the UAs support the INFO method, and the way they
handle application information transported in INFO messages. This
has caused interoperability problems in the industry.
To overcome these problems, the SIP Working Group has spent
significant discussion time over many years coming to agreement on
whether it was more appropriate to fix INFO (by defining a
registration mechanism for the ways in which it was used) or to
deprecate it altogether (with the usage described in RFC 3398
[RFC3398] being grandfathered as the sole legitimate usage).
Although it required substantial consensus building and concessions
by those more inclined to completely deprecate INFO, the eventual
direction of the working group was to publish a framework for
registration of Info Packages as defined in this specification.
3. Applicability and Backward Compatibility
This document defines a method, INFO, for the Session Initiation
Protocol (SIP) [RFC3261], and an Info Package mechanism. This
document obsoletes RFC 2976 [RFC2976]. For backward compatibility,
this document also specifies a "legacy" mode of usage of the INFO
method that is compatible with the usage previously defined in
RFC 2976, here referred to as "legacy INFO Usage".
For backward compatibility purposes, this document does not deprecate
legacy INFO usages, and does not mandate users to define Info
Packages for such usages. However:
1. A UA MUST NOT insert an Info-Package header field in a legacy
INFO request (as described in Section 4.2.1, an INFO request
associated with an Info Package always contains an Info-Package
header field).
2. Any new usage MUST use the Info Package mechanism defined in this
specification, since it does not share the issues associated with
legacy INFO usage, and since Info Packages can be registered with
IANA.
3. UAs are allowed to enable both legacy INFO usages and Info
Package usages as part of the same invite dialog usage, but UAs
SHALL NOT mix legacy INFO usages and Info Package usages in order
to transport the same application level information. If
possible, UAs SHALL prefer the usage of an Info Package.
4. The INFO Method
4.1. General
The INFO method provides a mechanism for transporting application
level information that can further enhance a SIP application.
Section 8 gives more details on the types of applications for which
the use of INFO is appropriate.
This section describes how a UA handles INFO requests and responses,
as well as the message bodies included in INFO messages.
4.2. INFO Request
4.2.1. INFO Request Sender
An INFO request can be associated with an Info Package (see
Section 5), or associated with a legacy INFO usage (see Section 2).
The construction of the INFO request is the same as any other
non-target refresh request within an existing invite dialog usage as
described in Section 12.2 of RFC 3261.
When a UA sends an INFO request associated with an Info Package, it
MUST include an Info-Package header field that indicates which Info
Package is associated with the request. A specific INFO request can
be used only for a single Info Package.
When a UA sends an INFO request associated with a legacy INFO usage,
there is no Info Package associated with the request, and the UA MUST
NOT include an Info-Package header field in the request.
The INFO request MUST NOT contain a Recv-Info header field. A UA can
only indicate a set of Info Packages for which it is willing to
receive INFO requests by using the SIP methods (and their responses)
listed in Section 5.
A UA MUST NOT send an INFO request outside an invite dialog usage and
MUST NOT send an INFO request for an Info Package inside an invite
dialog usage if the remote UA has not indicated willingness to
receive that Info Package within that dialog.
If a UA receives a 469 (Bad Info Package) response to an INFO
request, based on RFC 5057 [RFC5057], the response represents a
Transaction Only failure, and the UA MUST NOT terminate the invite
dialog usage.
Due to the possibility of forking, the UA that sends the initial
INVITE request MUST be prepared to receive INFO requests from
multiple remote UAs during the early dialog phase. In addition, the
UA MUST be prepared to receive different Recv-Info header field
values from different remote UAs.
NOTE: If the User Agent Server (UAS) (receiver of the initial
INVITE request) sends an INFO request just after it has sent the
response that creates the dialog, the UAS needs to be prepared for
the possibility that the INFO request will reach the User Agent
Client (UAC) before the dialog-creating response, and might
therefore be rejected by the UAC. In addition, an INFO request
might be rejected due to a race condition, if a UA sends the INFO
request at the same time that the remote UA sends a new set of
Info Packages for which it is willing to receive INFO requests.
4.2.2. INFO Request Receiver
If a UA receives an INFO request associated with an Info Package that
the UA has not indicated willingness to receive, the UA MUST send a
469 (Bad Info Package) response (see Section 11.6), which contains a
Recv-Info header field with Info Packages for which the UA is willing
to receive INFO requests. The UA MUST NOT use the response to update
the set of Info Packages, but simply to indicate the current set. In
the terminology of multiple dialog usages [RFC5057], this represents
a Transaction Only failure, and does not terminate the invite dialog
usage.
If a UA receives an INFO request associated with an Info Package, and
the message body part with Content-Disposition "Info-Package" (see
Section 4.3.1) has a Multipurpose Internet Mail Extensions (MIME)
type that the UA supports but not in the context of that Info
Package, it is RECOMMENDED that the UA send a 415 (Unsupported Media
Type) response.
The UA MAY send other error responses, such as Request Failure (4xx),
Server Failure (5xx), and Global Failure (6xx), in accordance with
the error-handling procedures defined in RFC 3261.
Otherwise, if the INFO request is syntactically correct and well
structured, the UA MUST send a 200 (OK) response.
NOTE: If the application needs to reject the information that it
received in an INFO request, that needs to be done on the
application level. That is, the application needs to trigger a
new INFO request, which contains information that the previously
received application data was not accepted. Individual Info
Package specifications need to describe the details for such
procedures.
4.2.3. SIP Proxies
Proxies need no additional behavior beyond that described in RFC 3261
to support INFO.
4.3. INFO Message Body
4.3.1. INFO Request Message Body
The purpose of the INFO request is to carry application level
information between SIP UAs. The application information data is
carried in the payload of the message body of the INFO request.
NOTE: An INFO request associated with an Info Package can also
include information associated with the Info Package using
Info-Package header field parameters.
If an INFO request associated with an Info Package contains a message
body part, the body part is identified by a Content-Disposition
header field "Info-Package" value. The body part can contain a
single MIME type, or it can be a multipart [RFC5621] that contains
other body parts associated with the Info Package.
UAs MUST support multipart body parts in accordance with RFC 5621.
NOTE: An INFO request can also contain other body parts that are
meaningful within the context of an invite dialog usage but are
not specifically associated with the INFO method and the
application concerned.
When a UA supports a specific Info Package, the UA MUST also support
message body MIME types in accordance with that Info Package.
However, in accordance with RFC 3261, the UA still indicates the
supported MIME types using the Accept header.
4.3.2. INFO Response Message Body
A UA MUST NOT include a message body associated with an Info Package
in an INFO response. Message bodies associated with Info Packages
MUST only be sent in INFO requests.
A UA MAY include a message body that is not associated with an Info
Package in an INFO response.
4.4. Order of Delivery
The Info Package mechanism does not define a delivery order
mechanism. Info Packages can rely on the CSeq header field [RFC3261]
to detect if an INFO request is received out of order.
If specific applications need additional mechanisms for order of
delivery, those mechanisms, and related procedures, are specified as
part of the associated Info Package (e.g., the use of sequence
numbers within the application data).
5. Info Packages
5.1. General
An Info Package specification defines the content and semantics of
the information carried in an INFO message associated with an Info
Package. The Info Package mechanism provides a way for UAs to
indicate for which Info Packages they are willing to receive INFO
requests, and with which Info Package a specific INFO request is
associated.
5.2. User Agent Behavior
5.2.1. General
This section describes how a UA handles Info Packages, how a UA uses
the Recv-Info header field, and how the UA acts in re-INVITE rollback
situations.
5.2.2. UA Procedures
A UA that supports the Info Package mechanism MUST indicate, using
the Recv-Info header field, the set of Info Packages for which it is
willing to receive INFO requests for a specific session. A UA can
list multiple Info Packages in a single Recv-Info header field, and
the UA can use multiple Recv-Info header fields. A UA can use an
empty Recv-Info header field, i.e., a header field without any header
field values.
A UA provides its set of Info Packages for which it is willing to
receive INFO requests during the dialog establishment. A UA can
update the set of Info Packages during the invite dialog usage.
If a UA is not willing to receive INFO requests for any Info
Packages, during dialog establishment or later during the invite
dialog usage, the UA MUST indicate this by including an empty
Recv-Info header field. This informs other UAs that the UA still
supports the Info Package mechanism.
Example: If a UA has previously indicated Info Packages "foo" and
"bar" in a Recv-Info header field, and the UA during the lifetime of
the invite dialog usage wants to indicate that it does not want to
receive INFO requests for any Info Packages anymore, the UA sends a
message with an empty Recv-Info header field.
Once a UA has sent a message with a Recv-Info header field containing
a set of Info Packages, the set is valid until the UA sends a new
Recv-Info header field containing a new, or empty, set of Info
Packages.
Once a UA has indicated that it is willing to receive INFO requests
for a specific Info Package, and a dialog has been established, the
UA MUST be prepared to receive INFO requests associated with that
Info Package until the UA indicates that it is no longer willing to
receive INFO requests associated with that Info Package.
For a specific dialog usage, a UA MUST NOT send an INFO request
associated with an Info Package until it has received an indication
that the remote UA is willing to receive INFO requests for that Info
Package, or after the UA has received an indication that the remote
UA is no longer willing to receive INFO requests associated with that
Info Package.
NOTE: When a UA sends a message that contains a Recv-Info header
field with a new set of Info Packages for which the UA is willing
to receive INFO requests, the remote UA might, before it receives
the message, send an INFO request based on the old set of Info
Packages. In this case, the receiver of the INFO requests
rejects, and sends a 469 (Bad Info Package) response to, the INFO
request.
If a UA indicates multiple Info Packages that provide similar
functionality, it is not possible to indicate a priority order of the
Info Packages, or to indicate that the UA wishes to only receive INFO
requests for one of the Info Packages. It is up to the application
logic associated with the Info Packages, and particular Info Package
specifications, to describe application behavior in such cases.
For backward compatibility purposes, even if a UA indicates support
of the Info Package mechanism, it is still allowed to enable legacy
INFO usages. In addition, if a UA indicates support of the INFO
method using the Allow header field [RFC3261], it does not implicitly
indicate support of the Info Package mechanism. A UA MUST use the
Recv-Info header field in order to indicate that it supports the Info
Package mechanism. Likewise, even if a UA uses the Recv-Info header
field to indicate that it supports the Info Package mechanism, in
addition the UA still indicates support of the INFO method using the
Allow header.
This document does not define a SIP option-tag [RFC3261] for the Info
Package mechanism. However, an Info Package specification can define
an option-tag associated with the specific Info Package, as described
in Section 10.6.
5.2.3. Recv-Info Header Field Rules
The text below defines rules on when a UA is required to include a
Recv-Info header field in SIP messages. Section 7.1 lists the SIP
methods for which a UA can insert a Recv-Info header field in
requests and responses.
o The sender of an initial INVITE request MUST include a Recv-Info
header field in the initial INVITE request, even if the sender is
not willing to receive INFO requests associated with any Info
Package.
o The receiver of a request that contains a Recv-Info header field
MUST include a Recv-Info header field in a reliable 18x/2xx
response to the request, even if the request contains an empty
Recv-Info header field, and even if the header field value of the
receiver has not changed since the previous time it sent a
Recv-Info header field.
o A UA MUST NOT include a Recv-Info header field in a response if
the associated request did not contain a Recv-Info header field.
NOTE: In contrast to the rules for generating Session Description
Protocol (SDP) answers [RFC3264], the receiver of a request is not
restricted to generating its own set of Info Packages as a subset
of the Info Package set received in the Info-Package header field
of the request.
As with SDP answers, the receiver can include the same Recv-Info
header field value in multiple responses (18x/2xx) for the same
INVITE/re-INVITE transaction, but the receiver MUST use the same
Recv-Info header field value (if included) in all responses for the
same transaction.
5.2.4. Info Package Fallback Rules
If the receiver of a request that contains a Recv-Info header field
rejects the request, both the sender and receiver of the request MUST
roll back to the set of Info Packages that was used before the
request was sent. This also applies to the case where the receiver
of an INVITE/re-INVITE request has included a Recv-Info header field
in a provisional response, but later rejects the request.
NOTE: The dialog state rollback rules for Info Packages might
differ from the rules for other types of dialog state information
(SDP, target, etc.).
5.3. REGISTER Processing
This document allows a UA to insert a Recv-Info header field in a
REGISTER request. However, a UA SHALL NOT include a header value for
a specific Info Package unless the particular Info Package
specification describes how the header field value shall be
interpreted and used by the registrar, e.g., in order to determine
request targets.
Rather than using the Recv-Info header field in order to determine
request targets, it is recommended to use more appropriate
mechanisms, e.g., based on RFC 3840 [RFC3840]. However, this
document does not define a feature tag for the Info Package
mechanism, or a mechanism to define feature tags for specific Info
Packages.
6. Formal INFO Method Definition
6.1. INFO Method
This document describes one new SIP method: INFO. This document
replaces the definition and registrations found in RFC 2976
[RFC2976].
This table expands on Tables 2 and 3 in RFC 3261 [RFC3261].
Header field where INFO
--------------------------------------------
Accept R o
Accept 415 o
Accept-Encoding R o
Accept-Encoding 2xx o
Accept-Encoding 415 c
Accept-Language R o
Accept-Language 2xx o
Accept-Language 415 o
Accept-Resource-Priority 2xx,417 o
Alert-Info -
Allow R o
Allow 405 m
Allow r o
Authentication-Info 2xx o
Authorization R o
Call-ID c m
Call-Info o
Contact -
Content-Disposition o
Content-Encoding o
Content-Language o
Content-Length o
Content-Type *
CSeq c m
Date o
Error-Info 3xx-6xx o
Expires -
From c m
Geolocation R o
Geolocation-Error r o
Max-Breadth R -
Max-Forwards R o
MIME-Version o
Min-Expires -
Organization -
Priority R -
Privacy o
Proxy-Authenticate 401 o
Proxy-Authenticate 407 m
Proxy-Authorization R o
Proxy-Require R o
Reason R o
Record-Route R o
Record-Route 2xx,18x o
Referred-By R o
Request-Disposition R o
Require o
Resource-Priority o
Retry-After R -
Retry-After 404,413,480,486 o
Retry-After 500,503 o
Retry-After 600,603 o
Route R o
Security-Client R o
Security-Server 421,494 o
Security-Verify R o
Server r o
Subject R o
Supported R o
Supported 2xx o
Timestamp o
To c m (w/ Tag)
Unsupported 420 o
User-Agent o
Via m
Warning r o
WWW-Authenticate 401 m
WWW-Authenticate 407 o
Table 1: Summary of Header Fields
7. INFO Header Fields
7.1. General
This table expands on Tables 2 and 3 in RFC 3261 [RFC3261].
Header field where proxy ACK BYE CAN INV OPT REG PRA INF MSG UPD
------------------------------------------------------------------
Info-Package R - - - - - - - m* - -
Recv-Info R - - - m - o o - - o
Recv-Info 2xx - - - o** - - o***- - o***
Recv-Info 1xx - - - o** - - - - - -
Recv-Info 469 - - - - - - - m* - -
Recv-Info r - - - o - - o - - o
Header field where SUB NOT RFR
--------------------------------
Info-Package R - - -
Recv-Info R - - -
Recv-Info 2xx - - -
Recv-Info 1xx - - -
Recv-Info 469 - - -
Recv-Info r - - -
Table 2: INFO-Related Header Fields
The support and usage of the Info-Package and Recv-Info header fields
are not applicable to UAs that only support legacy INFO usages.
* Not applicable to INFO requests and responses associated with
legacy INFO usages.
** Mandatory in at least one reliable 18x/2xx response, if sent, to
the INVITE request, if the associated INVITE request contained a
Recv-Info header field.
*** Mandatory if the associated request contained a Recv-Info header
field.
As defined in Section 20 of RFC 3261, a "mandatory" header field MUST
be present in a request, and MUST be understood by the UAS receiving
the request.
7.2. Info-Package Header Field
This document adds "Info-Package" to the definition of the element
"message-header" in the SIP message grammar [RFC3261]. Section 4
describes the Info-Package header field usage.
For the purposes of matching Info Package types indicated in
Recv-Info with those in the Info-Package header field value, one
compares the Info-package-name portion of the Info-package-type
portion of the Info-Package header field octet by octet with that of
the Recv-Info header field value. That is, the Info Package name is
case sensitive. Info-package-param is not part of the comparison-
checking algorithm.
This document does not define values for Info-Package types.
Individual Info Package specifications define these values.
7.3. Recv-Info Header Field
This document adds Recv-Info to the definition of the element
"message-header" in the SIP message grammar [RFC3261]. Section 5
describes the Recv-Info header field usage.
8. Info Package Considerations
8.1. General
This section covers considerations to take into account when deciding
whether the usage of an Info Package is appropriate for transporting
application information for a specific use-case.
8.2. Appropriateness of Info Package Usage
When designing an Info Package, for application level information
exchange, it is important to consider: is signaling, using INFO
requests, within a SIP dialog, an appropriate mechanism for the use-
case? Is it because it is the most reasonable and appropriate
choice, or merely because "it's easy"? Choosing an inappropriate
mechanism for a specific use-case can cause negative effects in SIP
networks where the mechanism is used.
8.3. INFO Request Rate and Volume
INFO messages differ from many other sorts of SIP messages in that
they carry application information, and the size and rate of INFO
messages are directly determined by the application. This can cause
application information traffic to interfere with other traffic on
that infrastructure, or to self-interfere when data rates become too
high.
There is no default throttling mechanism for INFO requests. Apart
from the SIP session establishment, the number of SIP messages
exchanged during the lifetime of a normal SIP session is rather
small.
Some applications, like those sending Dual-Tone Multi-Frequency
(DTMF) tones, can generate a burst of up to 20 messages per second.
Other applications, like constant GPS location updates, could
generate a high rate of INFO requests during the lifetime of the
invite dialog usage.
A designer of an Info Package, and the application that uses it, need
to consider the impact that the size and the rate of the INFO
messages have on the network and on other traffic, since it normally
cannot be ensured that INFO messages will be carried over a
congestion-controlled transport protocol end-to-end. Even if an INFO
message is sent over such a transport protocol, a downstream SIP
entity might forward the message over a transport protocol that does
not provide congestion control.
Furthermore, SIP messages tend to be relatively small, on the order
of 500 Bytes to 32K Bytes. SIP is a poor mechanism for direct
exchange of bulk data beyond these limits, especially if the headers
plus body exceed the User Datagram Protocol (UDP) MTU [RFC0768].
Appropriate mechanisms for such traffic include the Hypertext
Transfer Protocol (HTTP) [RFC2616], the Message Session Relay
Protocol (MSRP) [RFC4975], or other media plane data transport
mechanisms.
RFC 5405 [RFC5405] provides additional guidelines for applications
using UDP that may be useful background reading.
8.4. Alternative Mechanisms
8.4.1. Alternative SIP Signaling Plane Mechanisms
8.4.1.1. General
This subsection describes some alternative mechanisms for
transporting application information on the SIP signaling plane,
using SIP messages.
8.4.1.2. SUBSCRIBE/NOTIFY
An alternative for application level interaction is to use
subscription-based events [RFC3265] that use the SIP SUBSCRIBE and
NOTIFY methods. Using that mechanism, a UA requests state
information, such as keypad presses from a device to an application
server, or key-map images from an application server to a device.
Event Packages [RFC3265] perform the role of disambiguating the
context of a message for subscription-based events. The Info Package
mechanism provides similar functionality for application information
exchange using invite dialog usages [RFC5057].
While an INFO request is always part of, and shares the fate of, an
existing invite dialog usage, a SUBSCRIBE request creates a separate
dialog usage [RFC5057], and is normally sent outside an existing
dialog usage.
The subscription-based mechanism can be used by SIP entities to
receive state information about SIP dialogs and sessions, without
requiring the entities to be part of the route set of those dialogs
and sessions.
As SUBSCRIBE/NOTIFY messages traverse through stateful SIP proxies
and back-to-back user agents (B2BUAs), the resource impact caused by
the subscription dialogs needs to be considered. The number of
subscription dialogs per user also needs to be considered.
As for any other SIP-signaling-plane-based mechanism for transporting
application information, the SUBSCRIBE/NOTIFY messages can put a
significant burden on intermediate SIP entities that are part of the
dialog route set, but do not have any interest in the application
information transported between the end users.
8.4.1.3. MESSAGE
The MESSAGE method [RFC3428] defines one-time instant message
exchange, typically for sending MIME contents for rendering to the
user.
8.4.2. Media Plane Mechanisms
8.4.2.1. General
In SIP, media plane channels associated with SIP dialogs are
established using SIP signaling, but the data exchanged on the media
plane channel does not traverse SIP signaling intermediates, so if
there will be a lot of information exchanged, and there is no need
for the SIP signaling intermediaries to examine the information, it
is recommended to use a media plane mechanism, rather than a SIP-
signaling-based mechanism.
A low-latency requirement for the exchange of information is one
strong indicator for using a media channel. Exchanging information
through the SIP routing network can introduce hundreds of
milliseconds of latency.
8.4.2.2. MRCP
One mechanism for media plane exchange of application data is the
Media Resource Control Protocol (MRCP) [SPEECHSC-MRCPv2], where a
media plane connection-oriented channel, such as a Transmission
Control Protocol (TCP) [RFC0793] or Stream Control Transmission
Protocol (SCTP) [RFC4960] stream is established.
8.4.2.3. MSRP
MSRP [RFC4975] defines session-based instant messaging as well as
bulk file transfer and other such large-volume uses.
8.4.3. Non-SIP-Related Mechanisms
Another alternative is to use a SIP-independent mechanism, such as
HTTP [RFC2616]. In this model, the UA knows about a rendezvous point
to which it can direct HTTP requests for the transfer of information.
Examples include encoding of a prompt to retrieve in the SIP Request
URI [RFC4240] or the encoding of a SUBMIT target in a VoiceXML
[W3C.REC-voicexml21-20070619] script.
9. Syntax
9.1. General
This section describes the syntax extensions to the ABNF syntax
defined in RFC 3261 required for the INFO method, and adds
definitions for the Info-Package and Recv-Info header fields. The
previous sections describe the semantics. The ABNF defined in this
specification is conformant to RFC 5234 [RFC5234].
9.2. ABNF
INFOm = %x49.4E.46.4F ; INFO in caps
Method =/ INFOm
message-header =/ (Info-Package / Recv-Info) CRLF
Info-Package = "Info-Package" HCOLON Info-package-type
Recv-Info = "Recv-Info" HCOLON [Info-package-list]
Info-package-list = Info-package-type *( COMMA Info-package-type )
Info-package-type = Info-package-name *( SEMI Info-package-param )
Info-package-name = token
Info-package-param = generic-param
10. Info Package Requirements
10.1. General
This section provides guidance on how to define an Info Package, and
what information needs to exist in an Info Package specification.
If, for an Info Package, there is a need to extend or modify the
behavior described in this document, that behavior MUST be described
in the Info Package specification. It is bad practice for Info
Package specifications to repeat procedures defined in this document,
unless needed for purposes of clarification or emphasis.
Info Package specifications MUST NOT weaken any behavior designated
with "SHOULD" or "MUST" in this specification. However, Info Package
specifications MAY strengthen "SHOULD", "MAY", or "RECOMMENDED"
requirements to "MUST" if applications associated with the Info
Package require it.
Info Package specifications MUST address the issues defined in the
following subsections, or document why an issue is not applicable to
the specific Info Package.
Section 8.4 describes alternative mechanisms, which should be
considered as part of the process for solving a specific use-case,
when there is a need for transporting application information.
10.2. Overall Description
The Info Package specification MUST contain an overall description of
the Info Package: what type of information is carried in INFO
requests associated with the Info Package, and for what types of
applications and functionalities UAs can use the Info Package.
If the Info Package is defined for a specific application, the Info
Package specification MUST state which application UAs can use the
Info Package with.
10.3. Applicability
The Info Package specification MUST describe why the Info Package
mechanism, rather than some other mechanism, has been chosen for the
specific use-case to transfer application information between SIP
endpoints. Common reasons can be a requirement for SIP proxies or
back-to-back user agents (B2BUAs) to see the transported application
information (which would not be the case if the information was
transported on a media path), or that it is not seen as feasible to
establish separate dialogs (subscription) in order to transport the
information.
Section 8 provides more information and describes alternative
mechanisms that one should consider for solving a specific use-case.
10.4. Info Package Name
The Info Package specification MUST define an Info Package name,
which UAs use as a header field value (e.g., "infoX") to identify the
Info Package in the Recv-Info and Info-Package header fields. The
header field value MUST conform to the ABNF defined in Section 9.2.
The Info Package mechanism does not support package versioning.
Specific Info Package message body payloads can contain version
information, which is handled by the applications associated with the
Info Package. However, such a feature is outside the scope of the
generic Info Package mechanism.
NOTE: Even if an Info Package name contains version numbering
(e.g., foo_v2), the Info Package mechanism does not distinguish a
version number from the rest of the Info Package name.
10.5. Info Package Parameters
The Info Package specification MAY define Info Package parameters,
which can be used in the Recv-Info or Info-Package header fields,
together with the header field value that indicates the Info Package
name (see Section 10.4).
The Info Package specification MUST define the syntax and semantics
of the defined parameters. In addition, the specification MUST
define whether a specific parameter is applicable to only the
Recv-Info header field, only the Info-Package header field, or to
both.
By default, an Info Package parameter is only applicable to the Info
Package for which the parameter has been explicitly defined.
Info Package parameters defined for specific Info Packages can share
the name with parameters defined for other Info Packages, but the
parameter semantics are specific to the Info Package for which they
are defined. However, when choosing the name of a parameter, it is
RECOMMENDED to not use the same name as an existing parameter for
another Info Package, if the semantics of the parameters are
different.
10.6. SIP Option-Tags
The Info Package specification MAY define SIP option-tags, which can
be used as described in RFC 3261.
The registration requirements for option-tags are defined in RFC 5727
[RFC5727].
10.7. INFO Message Body Parts
The Info Package specification MUST define which message body part
MIME types are associated with the Info Package. The specification
MUST either define those body parts, including the syntax, semantics,
and MIME type of each body part, or refer to other documents that
define the body parts.
If multiple message body part MIME types are associated with an Info
Package, the Info Package specification MUST define whether UAs need
to use multipart body parts, in order to include multiple body parts
in a single INFO request.
10.8. Info Package Usage Restrictions
If there are restrictions on how UAs can use an Info Package, the
Info Package specification MUST document such restrictions.
There can be restrictions related to whether UAs are allowed to send
overlapping (outstanding) INFO requests associated with the Info
Package, or whether the UA has to wait for the response for a
previous INFO request associated with the same Info Package.
There can also be restrictions related to whether UAs need to support
and use other SIP extensions and capabilities when they use the Info
Package, and if there are restrictions related to how UAs can use the
Info Package together with other Info Packages.
As the SIP stack might not be aware of Info Package specific
restrictions, it cannot be assumed that overlapping requests would be
rejected. As defined in Section 4.2.2, UAs will normally send a 200
(OK) response to an INFO request. The application logic associated
with the Info Package needs to handle situations where UAs do not
follow restrictions associated with the Info Package.
10.9. Rate of INFO Requests
If there is a maximum or minimum rate at which UAs can send INFO
requests associated with the Info Package within a dialog, the Info
Package specification MUST document the rate values.
If the rates can vary, the Info Package specification MAY define Info
Package parameters that UAs can use to indicate or negotiate the
rates. Alternatively, the rate information can be part of the
application data information associated with the Info Package.
10.10. Info Package Security Considerations
If the application information carried in INFO requests associated
with the Info Package requires a certain level of security, the Info
Package specification MUST describe the mechanisms that UAs need to
use in order to provide the required security.
If the Info Package specification does not require any additional
security, other than what the underlying SIP protocol provides, this
MUST be stated in the Info Package specification.
NOTE: In some cases, it may not be sufficient to mandate Transport
Layer Security (TLS) [RFC5246] in order to secure the Info Package
payload, since intermediaries will have access to the payload, and
because beyond the first hop, there is no way to assure subsequent
hops will not forward the payload in clear text. The best way to
ensure secure transport at the application level is to have the
security at the application level. One way of achieving this is
to use end-to-end security techniques such as Secure/Multipurpose
Internet Mail Extensions (S/MIME) [RFC5751].
10.11. Implementation Details
It is strongly RECOMMENDED that the Info Package specification define
the procedure regarding how implementors shall implement and use the
Info Package, or refer to other locations where implementors can find
that information.
NOTE: Sometimes an Info Package designer might choose to not
reveal the details of an Info Package. However, in order to allow
multiple implementations to support the Info Package, Info Package
designers are strongly encouraged to provide the implementation
details.
10.12. Examples
It is RECOMMENDED that the Info Package specification provide
demonstrative message flow diagrams, paired with complete messages
and message descriptions.
Note that example flows are by definition informative, and do not
replace normative text.
11. IANA Considerations
11.1. Update to Registration of SIP INFO Method
IANA updated the existing registration in the "Methods and Response
Codes" registry under "Session Initiation Protocol (SIP) Parameters"
from:
Method: INFO
Reference: [RFC2976]
to:
Method: INFO
Reference: [RFC6086]
11.2. Registration of the Info-Package Header Field
IANA added the following new SIP header field in the "Header Fields"
registry under "Session Initiation Protocol (SIP) Parameters".
Header Name: Info-Package
Compact Form: (none)
Reference: [RFC6086]
11.3. Registration of the Recv-Info Header Field
IANA added the following new SIP header field in the "Header Fields"
registry under "Session Initiation Protocol (SIP) Parameters".
Header Name: Recv-Info
Compact Form: (none)
Reference: [RFC6086]
11.4. Creation of the Info Packages Registry
IANA created the following registry under "Session Initiation
Protocol (SIP) Parameters":
Info Packages
Note to the reviewer:
The policy for review of Info Packages is "Specification
Required", as defined in [RFC5226]. This policy was selected
because Info Packages re-use an existing mechanism for transport
of arbitrary session-associated data within SIP; therefore, new
Info Packages do not require the more extensive review required by
specifications that make fundamental protocol changes. However,
the reviewer is expected to verify that each Info Package
registration is in fact consistent with this definition. Changes
to the SIP protocol and state machine are outside of the allowable
scope for an Info Package and are governed by other procedures
including RFC 5727 and its successors, if any.
The following data elements populate the Info Packages Registry.
o Info Package Name: The Info Package Name is a case-sensitive
token. In addition, IANA shall not register multiple Info Package
names that have identical case-insensitive values.
o Reference: A reference to a specification that describes the Info
Package.
The initial population of this table shall be:
Name Reference
11.5. Registration of the Info-Package Content-Disposition
IANA added the following new header field value to the "Mail Content
Disposition Values" registry under "Mail Content Disposition Values
and Parameters".
Name: info-package
Description: The body contains information associated with an
Info Package
Reference: RFC6086
11.6. SIP Response Code 469 Registration
IANA registered the following new response code in the "Session
Initiation Protocol (SIP) Parameters" -- "Response Codes" registry.
Response Code: 469
Default Reason Phrase: Bad Info Package
Reference: RFC6086
12. Examples
12.1. Indication of Willingness to Receive INFO Requests for Info
Packages
12.1.1. Initial INVITE Request
The UAC sends an initial INVITE request, where the UAC indicates that
it is willing to receive INFO requests for Info Packages P and R.
INVITE sip:bob@example.com SIP/2.0
Via: SIP/2.0/TCP pc33.example.com;branch=z9hG4bK776
Max-Forwards: 70
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.example.com
CSeq: 314159 INVITE
Recv-Info: P, R
Contact: <sip:alice@pc33.example.com>
Content-Type: application/sdp
Content-Length: ...
...
The UAS sends a 200 (OK) response back to the UAC, where the UAS
indicates that it is willing to receive INFO requests for Info
Packages R and T.
SIP/2.0 200 OK
Via: SIP/2.0/TCP pc33.example.com;branch=z9hG4bK776;
received=192.0.2.1
To: Bob <sip:bob@example.com>;tag=a6c85cf
From: Alice <sip:alice@example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.example.com
CSeq: 314159 INVITE
Contact: <sip:bob@pc33.example.com>
Recv-Info: R, T
Content-Type: application/sdp
Content-Length: ...
...
The UAC sends an ACK request.
ACK sip:bob@pc33.example.com SIP/2.0
Via: SIP/2.0/TCP pc33.example.com;branch=z9hG4bK754
Max-Forwards: 70
To: Bob <sip:bob@example.com>;tag=a6c85cf
From: Alice <sip:alice@example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.example.com
CSeq: 314159 ACK
Content-Length: 0
12.1.2. Target Refresh
The UAC sends an UPDATE request within the invite dialog usage, where
the UAC indicates (using an empty Recv-Info header field) that it is
not willing to receive INFO requests for any Info Packages.
UPDATE sip:bob@pc33.example.com SIP/2.0
Via: SIP/2.0/TCP pc33.example.com;branch=z9hG4bK776
Max-Forwards: 70
To: Bob <sip:bob@example.com>;tag=a6c85cf
From: Alice <sip:alice@example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.example.com
CSeq: 314163 UPDATE
Recv-Info:
Contact: <sip:alice@pc33.example.com>
Content-Type: application/sdp
Content-Length: ...
...
The UAS sends a 200 (OK) response back to the UAC, where the UAS
indicates that it is willing to receive INFO requests for Info
Packages R and T.
SIP/2.0 200 OK
Via: SIP/2.0/TCP pc33.example.com;branch=z9hG4bK893;
received=192.0.2.1
To: Bob <sip:bob@example.com>;tag=a6c85cf
From: Alice <sip:alice@example.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.example.com
CSeq: 314163 INVITE
Contact: <sip:alice@pc33.example.com>
Recv-Info: R, T
Content-Type: application/sdp
Content-Length: ...
...
12.2. INFO Request Associated with Info Package
12.2.1. Single Payload
The UA sends an INFO request associated with Info Package "foo".
INFO sip:alice@pc33.example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.2:5060;branch=z9hG4bKnabcdef
To: Bob <sip:bob@example.com>;tag=a6c85cf
From: Alice <sip:alice@example.com>;tag=1928301774
Call-Id: a84b4c76e66710@pc33.example.com
CSeq: 314333 INFO
Info-Package: foo
Content-type: application/foo
Content-Disposition: Info-Package
Content-length: 24
I am a foo message type
12.2.2. Multipart INFO
12.2.2.1. Non-Info Package Body Part
SIP extensions can sometimes add body part payloads into an INFO
request, independent of the Info Package. In this case, the Info
Package payload gets put into a multipart MIME body, with a
Content-Disposition header field that indicates which body part is
associated with the Info Package.
INFO sip:alice@pc33.example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.2:5060;branch=z9hG4bKnabcdef
To: Alice <sip:alice@example.net>;tag=1234567
From: Bob <sip:bob@example.com>;tag=abcdefg
Call-Id: a84b4c76e66710@pc33.example.com
CSeq: 314400 INFO
Info-Package: foo
Content-Type: multipart/mixed;boundary="theboundary"
Content-Length: ...
--theboundary
Content-Type: application/mumble
...
<mumble stuff>
--theboundary
Content-Type: application/foo-x
Content-Disposition: Info-Package
Content-length: 59
I am a foo-x message type, and I belong to Info Package foo
--theboundary--
12.2.2.2. Info Package with Multiple Body Parts inside Multipart Body
Part
Multiple body part payloads can be associated with a single Info
Package. In this case, the body parts are put into a multipart MIME
body, with a Content-Disposition header field that indicates which
body part is associated with the Info Package.
INFO sip:alice@pc33.example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.2:5060;branch=z9hG4bKnabcdef
To: Alice <sip:alice@example.net>;tag=1234567
From: Bob <sip:bob@example.com>;tag=abcdefg
Call-Id: a84b4c76e66710@pc33.example.com
CSeq: 314423 INFO
Info-Package: foo
Content-Type: multipart/mixed;boundary="theboundary"
Content-Disposition: Info-Package
Content-Length: ...
--theboundary
Content-Type: application/foo-x
Content-length: 59
I am a foo-x message type, and I belong to Info Package foo
<mumble stuff>
--theboundary
Content-Type: application/foo-y
Content-length: 59
I am a foo-y message type, and I belong to Info Package foo
--theboundary--
12.2.2.3. Info Package with Single Body Part inside Multipart Body Part
The body part payload associated with the Info Package can have a
Content-Disposition header field value other than "Info-Package". In
this case, the body part is put into a multipart MIME body, with a
Content-Disposition header field that indicates which body part is
associated with the Info Package.
INFO sip:alice@pc33.example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.2:5060;branch=z9hG4bKnabcdef
To: Alice <sip:alice@example.net>;tag=1234567
From: Bob <sip:bob@example.com>;tag=abcdefg
Call-Id: a84b4c76e66710@pc33.example.com
CSeq: 314423 INFO
Info-Package: foo
Content-Type: multipart/mixed;boundary="theboundary"
Content-Disposition: Info-Package
Content-Length: ...
--theboundary
Content-Type: application/foo-x
Content-Disposition: icon
Content-length: 59
I am a foo-x message type, and I belong to Info Package foo
--theboundary--
13. Security Considerations
By eliminating multiple usages of INFO messages without adequate
community review, and by eliminating the possibility of rogue SIP UAs
confusing another UA by purposely sending unrelated INFO requests, we
expect this document's clarification of the use of INFO to improve
the security of the Internet. While rogue UAs can still send
unrelated INFO requests, this mechanism enables the UAS and other
security devices to associate INFO requests with Info Packages that
have been negotiated for a session.
If the content of the Info Package payload is private, UAs will need
to use end-to-end encryption, such as S/MIME, to prevent access to
the content. This is particularly important, as transport of INFO is
likely not to be end-to-end, but through SIP proxies and back-to-back
user agents (B2BUAs), which the user may not trust.
The INFO request transports application level information. One
implication of this is that INFO messages may require a higher level
of protection than the underlying SIP dialog signaling. In
particular, if one does not protect the SIP signaling from
eavesdropping or authentication and repudiation attacks, for example
by using TLS transport, then the INFO request and its contents will
be vulnerable as well. Even with SIP/TLS, any SIP hop along the path
from UAC to UAS can view, modify, or intercept INFO requests, as they
can with any SIP request. This means some applications may require
end-to-end encryption of the INFO payload, beyond, for example, hop-
by-hop protection of the SIP signaling itself. Since the application
dictates the level of security required, individual Info Packages
have to enumerate these requirements. In any event, the Info Package
mechanism described by this document provides the tools for such
secure, end-to-end transport of application data.
One interesting property of Info Package usage is that one can re-use
the same digest-challenge mechanism used for INVITE-based
authentication for the INFO request. For example, one could use a
quality-of-protection (qop) value of authentication with integrity
(auth-int), to challenge the request and its body, and prevent
intermediate devices from modifying the body. However, this assumes
the device that knows the credentials in order to perform the INVITE
challenge is still in the path for the INFO request, or that the far-
end UAS knows such credentials.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[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.
[RFC5621] Camarillo, G., "Message Body Handling in the Session
Initiation Protocol (SIP)", RFC 5621, September 2009.
[RFC5727] Peterson, J., Jennings, C., and R. Sparks, "Change Process
for the Session Initiation Protocol (SIP) and the Real-
time Applications and Infrastructure Area", BCP 67,
RFC 5727, March 2010.
14.2. Informative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2976] Donovan, S., "The SIP INFO Method", RFC 2976,
October 2000.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3398] Camarillo, G., Roach, A., Peterson, J., and L. Ong,
"Integrated Services Digital Network (ISDN) User Part
(ISUP) to Session Initiation Protocol (SIP) Mapping",
RFC 3398, December 2002.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
[RFC3372] Vemuri, A. and J. Peterson, "Session Initiation Protocol
for Telephones (SIP-T): Context and Architectures",
BCP 63, RFC 3372, September 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
and D. Gurle, "Session Initiation Protocol (SIP) Extension
for Instant Messaging", RFC 3428, December 2002.
[RFC4240] Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
Media Services with SIP", RFC 4240, December 2005.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message
Session Relay Protocol (MSRP)", RFC 4975, September 2007.
[RFC5022] Van Dyke, J., Burger, E., and A. Spitzer, "Media Server
Control Markup Language (MSCML) and Protocol", RFC 5022,
September 2007.
[RFC5057] Sparks, R., "Multiple Dialog Usages in the Session
Initiation Protocol", RFC 5057, November 2007.
[RFC5168] Levin, O., Even, R., and P. Hagendorf, "XML Schema for
Media Control", RFC 5168, March 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.
[RFC5707] Saleem, A., Xin, Y., and G. Sharratt, "Media Server Markup
Language (MSML)", RFC 5707, February 2010.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[W3C.REC-voicexml21-20070619]
Porter, B., Oshry, M., Rehor, K., Auburn, R., Bodell, M.,
Carter, J., Burke, D., Baggia, P., Candell, E., Burnett,
D., McGlashan, S., and A. Lee, "Voice Extensible Markup
Language (VoiceXML) 2.1", World Wide Web Consortium
Recommendation REC-voicexml21-20070619, June 2007,
<http://www.w3.org/TR/2007/REC-voicexml21-20070619>.
[SPEECHSC-MRCPv2]
Burnett, D. and S. Shanmugham, "Media Resource Control
Protocol Version 2 (MRCPv2)", Work in Progress,
November 2010.
[ECMA-355]
"Standard ECMA-355 Corporate Telecommunication Networks -
Tunnelling of QSIG over SIP", ECMA http://
www.ecma-international.org/publications/standards/
Ecma-355.htm, June 2008.
Appendix A. Acknowledgements
The work on this document was influenced by "The Session Initiation
Protocol (SIP) INFO Considered Harmful" (26 December 2002) written by
Jonathan Rosenberg, and by "Packaging and Negotiation of INFO Methods
for the Session Initiation Protocol" (15 January 2003) written by
Dean Willis.
The following individuals have been involved in the work, and have
provided input and feedback on this document:
Adam Roach, Anders Kristensen, Andrew Allen, Arun Arunachalam, Ben
Campbell, Bob Penfield, Bram Verburg, Brian Stucker, Chris
Boulton, Christian Stredicke, Cullen Jennings, Dale Worley, Dean
Willis, Eric Rescorla, Frank Miller, Gonzalo Camarillo, Gordon
Beith, Henry Sinnreich, Inaki Baz Castillo, James Jackson, James
Rafferty, Jeroen van Bemmel, Joel Halpern, John Elwell, Jonathan
Rosenberg, Juha Heinanen, Keith Drage, Kevin Attard Compagno,
Manpreet Singh, Martin Dolly, Mary Barnes, Michael Procter, Paul
Kyzivat, Peili Xu, Peter Blatherwick, Raj Jain, Rayees Khan,
Robert Sparks, Roland Jesske, Roni Even, Salvatore Loreto, Sam
Ganesan, Sanjay Sinha, Spencer Dawkins, Steve Langstaff, Sumit
Garg, and Xavier Marjoum.
John Elwell and Francois Audet helped with QSIG references. In
addition, Francois Audet provided text for the revised abstract.
Keith Drage provided comments and helped immensely with Table 1.
Arun Arunachalam, Brett Tate, John Elwell, Keith Drage, and Robert
Sparks provided valuable feedback during the working group last call
process, in order to prepare this document for publication.
Adam Roach, Dean Willis, John Elwell, and Paul Kyzivat provided
valuable input in order to sort out the message body part usage for
Info Packages.
Authors' Addresses
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas, 02420
Finland
EMail: christer.holmberg@ericsson.com
Eric W. Burger
Georgetown University
EMail: eburger@standardstrack.com
URI: http://www.standardstrack.com
Hadriel Kaplan
Acme Packet
100 Crosby Drive
Bedford, MA 01730
USA
EMail: hkaplan@acmepacket.com
ISO Plain Language
On this page
§1Introduction
This document defines a method, INFO, for the Session Initiation Protocol, RFC 3261. The purpose of the INFO message is to carry application level information between endpoints, using the SIP dialog signaling path. The INFO method is not used to update characteristics of a SIP dialog or session; it allows the applications that use the session to exchange information, which might update the state of those applications.
Use of the INFO method does not constitute a separate dialog usage. INFO messages are always part of, and share the fate of, an invite dialog usage, as described in RFC 5057. They cannot be sent as part of other dialog usages, or outside an existing dialog.
The document also defines an Info Package mechanism. An Info Package specification defines the content and semantics of the information carried in an INFO message associated with that package, and the mechanism gives user agents a way to say which Info Packages they are willing to receive INFO requests for, and which package a specific INFO request belongs to.
A user agent uses the Recv-Info header field, on a per-dialog basis, to indicate for which Info Packages it is willing to receive INFO requests. It can indicate an initial set during dialog establishment and a new set during the lifetime of the invite dialog usage. An empty Recv-Info header field, that is, a header field without a value, means the agent is not willing to receive INFO requests for any Info Package while still informing other agents that it supports the mechanism. When a user agent sends an INFO request, it uses the Info-Package header field to say which package the request is associated with, and one particular INFO request can only be associated with a single Info Package.
The method carries application data along the signaling path of a call that already exists. Two header fields do the labelling: one says what the receiver is willing to accept, the other says what a particular message contains.
§2Motivation
A number of applications, standardized and proprietary, made use of the INFO method as it was defined in RFC 2976, called legacy INFO usage in this document. RFC 3372 specifies the encapsulation of ISDN User Part in SIP message bodies, and the ITU-T and the Third Generation Partnership Project have specified similar procedures. ECMA-355 specifies the encapsulation of QSIG in SIP message bodies. RFC 5022 specifies how INFO is used as a transport mechanism by the Media Server Control Markup Language, which uses an option-tag in the Require header field to ensure the receiver understands the INFO content, and RFC 5707 does the same for the Media Server Markup Language. Companies have also used INFO messages to request fast video update, for which a standardized mechanism based on RTCP now exists in RFC 5168, and to transport Dual-Tone Multi-Frequency tones, for which all mechanisms are proprietary and have not been standardized.
RFC 2976 did not define mechanisms that would let a user agent indicate the types of applications and contexts in which it supports the INFO method, or the types of applications and contexts with which a specific INFO message is associated. Because legacy INFO usages have no associated Info Packages, it is not possible to use the Recv-Info and Info-Package header fields with them: a user agent cannot use Recv-Info to say which legacy usages it will accept, and cannot use Info-Package to say which legacy usage a request belongs to.
Legacy usages therefore often require static configuration of the types of applications and contexts for which the agents support INFO and the way they handle application information carried in it. This has caused interoperability problems in the industry. To overcome these problems, the SIP Working Group spent years reaching agreement on whether it was more appropriate to fix INFO by defining a registration mechanism for the ways it was used, or to deprecate it altogether. The eventual direction was to publish a framework for the registration of Info Packages.
Several unrelated uses of the same method had grown up, and nothing recorded what any of them meant. Two devices could both follow the rules and still disagree about what a message contained. The fix is a registry of named uses, so that the meaning is written down somewhere both ends can point to.
§4The INFO Method
An INFO request can be associated with an Info Package or with a legacy INFO usage. Its construction is the same as any other non-target refresh request within an existing invite dialog usage, as described in Section 12.2 of RFC 3261. When a user agent sends an INFO request associated with an Info Package, it MUST include an Info-Package header field saying which package the request is associated with, and a specific request can be used for only a single package. When the request is associated with a legacy usage, there is no package and the agent MUST NOT include the Info-Package header field. The INFO request MUST NOT contain a Recv-Info header field.
A user agent MUST NOT send an INFO request outside an invite dialog usage, and MUST NOT send a request for an Info Package inside an invite dialog usage if the remote agent has not indicated willingness to receive that package within that dialog. If a user agent receives a 469 response to an INFO request, the response represents a Transaction Only failure, and the agent MUST NOT terminate the invite dialog usage. Because of the possibility of forking, the agent that sends the initial INVITE request MUST be prepared to receive INFO requests from multiple remote agents during the early dialog phase, and MUST be prepared to receive different Recv-Info header field values from different remote agents.
On the receiving side, if a user agent receives an INFO request associated with an Info Package that it has not said it will accept, it MUST send a 469 response, which contains a Recv-Info header field with the packages it is willing to receive. The agent MUST NOT use that response to update the set of packages; it only indicates the current set. If the request is associated with a package and the body part with Content-Disposition Info-Package has a MIME type that the agent supports but not in the context of that package, it is RECOMMENDED that the agent send a 415 response. The agent MAY send other error responses, such as 4xx, 5xx and 6xx, in accordance with the error-handling procedures of RFC 3261. Otherwise, if the request is syntactically correct and well structured, the agent MUST send a 200 response. If the application needs to reject the information it received, that has to be done at the application level, by triggering a new INFO request that says the previous data was not accepted.
The application information is carried in the payload of the message body. A body part associated with an Info Package is identified by a Content-Disposition header field with the value Info-Package. It can hold a single MIME type or be a multipart body that contains other body parts associated with the package, and user agents MUST support multipart body parts. A user agent MUST NOT include a message body associated with an Info Package in an INFO response, though it MAY include a body that is not associated with a package.
The Info Package mechanism does not define a delivery order mechanism. Packages can rely on the CSeq header field to detect an INFO request received out of order, and if an application needs more, that is specified as part of the associated package.
The sender names the package in the request, and the receiver either accepts it with a 200 response or rejects it with 469 if it never agreed to that package. A rejection of this kind does not end the call. The content itself travels in the message body, marked with a content disposition of Info-Package.
| Response | When it is sent |
|---|---|
| 200 (OK) THIS RFC | The request is syntactically correct and well structured |
| 469 (Bad Info Package) | The request is for a package the receiver never said it would accept |
| 415 (Unsupported Media Type) | The body type is supported but not in the context of that package |
| 4xx, 5xx, 6xx | Other errors, following the error handling of RFC 3261 |
§5Info Packages and User Agent Behaviour
A user agent that supports the Info Package mechanism MUST indicate, using the Recv-Info header field, the set of Info Packages for which it is willing to receive INFO requests for a specific session. It can list several packages in one header field, can use several header fields, and can use an empty header field. The set is provided during dialog establishment and can be updated during the invite dialog usage. The set stays valid until the agent sends a new Recv-Info header field, and once the agent has said it will accept a package and a dialog exists, it MUST be prepared to receive requests for that package until it says otherwise.
For a specific dialog usage, an agent MUST NOT send an INFO request for an Info Package until it has received an indication that the remote agent is willing to receive requests for that package, or after it has received an indication that the remote agent is no longer willing to receive them. When a remote agent sends a new set of packages, it might send an INFO request based on the old set in the meantime, in which case the receiver rejects it with a 469 response. If an agent indicates several packages that provide similar functionality, there is no way to indicate a priority order among them or that only one should be used; that is left to the application logic.
For backward compatibility, an agent that indicates support of the INFO method in the Allow header field does not implicitly indicate support of the Info Package mechanism: the Recv-Info header field is required for that, and the Allow header is still used to indicate support of the method itself.
The rules for including Recv-Info are specific. The sender of an initial INVITE request MUST include a Recv-Info header field, even if it is not willing to receive requests for any package. The receiver of a request that contains a Recv-Info header field MUST include one in a reliable 18x or 2xx response, even if the request contained an empty field and even if the receiver's set has not changed. An agent MUST NOT include a Recv-Info header field in a response if the associated request did not contain one. In contrast to the rules for session description answers, the receiver is not restricted to generating its own set as a subset of the set it received. The receiver may include the same Recv-Info value in several responses for the same transaction, but it MUST use the same value in all responses for that transaction.
There are also fallback rules. If the receiver of a request that contains a Recv-Info header field rejects the request, both sender and receiver MUST roll back to the set of packages used before the request was sent, and the same applies when the receiver of an INVITE or re-INVITE has included Recv-Info in a provisional response but later rejects the request.
A user agent may insert a Recv-Info header field in a REGISTER request, but it SHALL NOT include a header value for a specific package unless that package's specification says how the registrar should interpret and use the value, for example to determine request targets. More appropriate mechanisms, such as those in RFC 3840, are recommended instead.
Each side states, in a header field, which kinds of package it will accept for this call, and can change that list later. A request for a package the other side never agreed to is refused. If a call setup fails, both sides go back to the list they had before.
- INVITE with Recv-Info: P, R
- 200 OK with Recv-Info: R, T
- ACK
- INFO with Info-Package: foo
- 200 OK
§9Syntax
The syntax extensions to the ABNF of RFC 3261 needed for the INFO method, and the definitions of the Info-Package and Recv-Info header fields, are given in Augmented Backus-Naur Form conformant to RFC 5234. The method is added to the Method production, and the two header fields are added to the message-header production. The Info-Package header field carries an Info-package-type, which is an Info-package-name followed by any number of Info-package-param values, where the name is a token and the parameters are generic parameters. The Recv-Info header field carries an optional list of Info-package-type values separated by commas.
When matching the Info Package types listed in Recv-Info with the one in the Info-Package header field, the name portion is compared octet by octet, so the Info Package name is case sensitive, and the parameters are not part of the comparison. This document does not define values for Info Package types; individual package specifications define them.
The method and the two header fields are written out in the formal syntax used for SIP, so that a parser can be built from the document alone. The package name is compared exactly, including its case.
INFOm = %x49.4E.46.4F ; INFO in caps
Method =/ INFOm
message-header =/ (Info-Package / Recv-Info) CRLF
Info-Package = "Info-Package" HCOLON Info-package-type
Recv-Info = "Recv-Info" HCOLON [Info-package-list]
Info-package-list = Info-package-type *( COMMA Info-package-type )
Info-package-type = Info-package-name *( SEMI Info-package-param )
Info-package-name = token
Info-package-param = generic-param§8Choosing Whether to Use INFO
This section covers the considerations to take into account when deciding whether the use of an Info Package is appropriate for a particular use case.
When designing an Info Package it is important to ask whether signaling with INFO requests within a SIP dialog is an appropriate mechanism, and whether it is the most reasonable choice or merely because it is easy. Choosing an inappropriate mechanism can have negative effects on the SIP networks where it is used.
INFO messages differ from many other SIP messages in carrying application information whose size and rate are determined by the application. This can cause application traffic to interfere with other traffic on the same infrastructure, or to interfere with itself when the data rate becomes too high. There is no default throttling mechanism for INFO requests, and apart from session establishment the number of SIP messages exchanged during a normal session is small. Some applications, such as those sending Dual-Tone Multi-Frequency tones, can generate a burst of up to 20 messages per second, and others, such as constant GPS location updates, generate a high rate of requests. A designer needs to consider the effect of the size and rate of the messages on the network and on other traffic, since it normally cannot be ensured that INFO messages travel over a congestion-controlled transport end to end.
SIP messages tend to be relatively small, on the order of 500 bytes to 32K bytes, and SIP is a poor mechanism for direct exchange of bulk data beyond these limits, especially if the headers plus body exceed the UDP MTU. Appropriate mechanisms for such traffic include HTTP, MSRP, or other media plane data transport mechanisms.
The RFC then describes alternatives. For application level interaction on the signaling plane, the subscription-based events mechanism uses the SUBSCRIBE and NOTIFY methods, in which a user agent requests state information such as keypad presses from a device or key-map images from an application server. Event Packages play the role of disambiguating the context of a message for subscription-based events, in the same way the Info Package mechanism does for application information exchange. While an INFO request is always part of an existing invite dialog usage, a SUBSCRIBE request creates a separate dialog usage and is normally sent outside an existing dialog. The subscription mechanism can also be used to receive state information about dialogs and sessions without requiring the entities to be part of the route set of those dialogs. As SUBSCRIBE and NOTIFY messages traverse stateful SIP proxies and back-to-back user agents, the resource impact of the subscription dialogs, and the number of subscription dialogs per user, need to be considered. The MESSAGE method defines one-time instant message exchange, typically for sending MIME contents for rendering to the user.
For the media plane, the RFC notes that the data exchanged on a media channel does not traverse the SIP signaling intermediaries, so when a lot of information is exchanged and the intermediaries have no need to examine it, a media plane mechanism is recommended. A low-latency requirement is one strong indicator, since exchanging information through the SIP routing network can introduce hundreds of milliseconds of latency. The Media Resource Control Protocol is one such mechanism, where a connection-oriented channel such as TCP or SCTP is established, and MSRP defines session-based instant messaging as well as bulk file transfer. A SIP-independent mechanism such as HTTP is another option, where the user agent knows a rendezvous point to which it can direct HTTP requests.
The method is for application data, so an application that sends a lot of it can overload the signaling network. The RFC lists what else to use instead: subscriptions for state information, a media channel for anything large or latency-sensitive, and plain HTTP for anything outside SIP.
§10What an Info Package Specification Must Cover
An Info Package specification defines the content and semantics of the information carried in an INFO message associated with the package. If a package needs to extend or modify the behaviour described in this document, that behaviour MUST be described in the package specification, and it is bad practice for a package specification to repeat the procedures defined here unless it is needed for clarity or emphasis. Package specifications MUST NOT weaken any behaviour designated with SHOULD or MUST, though they MAY strengthen SHOULD, MAY or RECOMMENDED requirements to MUST if the applications require it. Each specification MUST address the issues in the subsections that follow, or document why an issue does not apply to it.
The specification MUST contain an overall description: what type of information is carried in INFO requests associated with the package, and for what types of applications and functionalities user agents can use it. If the package is defined for a specific application, the specification MUST state which applications can use it.
It MUST describe why the Info Package mechanism, rather than some other mechanism, was chosen for the use case. Common reasons are a requirement for SIP proxies or back-to-back user agents to see the transported application information, which would not be the case if the information travelled on a media path, or that it is not seen as feasible to establish separate dialogs in order to transport the information.
It MUST define an Info Package name, used as a header field value such as infoX, conforming to the ABNF in Section 9.2. The mechanism does not support package versioning; specific message body payloads can contain version information handled by the applications, and even a name containing version numbering does not make the mechanism distinguish a version number from the rest of the name.
The specification MAY define Info Package parameters used in the Recv-Info or Info-Package header fields, and MUST define their syntax and semantics and say whether a parameter applies to Recv-Info, to Info-Package or to both. By default a parameter applies only to the package for which it was defined, and while parameters of different packages may share a name, their semantics are specific to the package, so it is RECOMMENDED not to reuse a name whose semantics differ.
The specification MAY define SIP option-tags, whose registration requirements are in RFC 5727. It MUST define which message body part MIME types are associated with the package, either by defining them or by referring to other documents, and if several MIME types are associated it MUST say whether user agents need to use multipart body parts to include several in one request. It MUST document any restrictions on use, such as whether user agents may send overlapping outstanding requests for the package or have to wait for the response to a previous one, and whether they must support other SIP extensions when using the package. Because the SIP stack may not know about package-specific restrictions, the application logic has to handle the case where user agents do not follow them.
If there is a maximum or minimum rate at which requests may be sent within a dialog, the specification MUST document the rate values, and MAY define parameters for negotiating them. If the application information requires a certain level of security, the specification MUST describe the mechanisms user agents need to use, or state that no additional security beyond the underlying SIP protocol is required. It is strongly RECOMMENDED that the specification define implementation details, or refer to another location, so that several implementations can support the package, and it is RECOMMENDED that it provide demonstrative message flow diagrams paired with complete messages.
The document says exactly what a package specification has to contain: what the data is, why this method was chosen, what it is called, what shape the data has, how often it may be sent, and how it is protected. The rules here cannot be weakened by a package, only tightened.
- Overall descriptionWhat kind of information the package carries and what applications can use it.
- ApplicabilityWhy the Info Package mechanism was chosen rather than another one.
- NameThe token used in the Recv-Info and Info-Package header fields. The name is case sensitive.
- ParametersAny parameters, their syntax, their meaning, and which header field they apply to.
- Body partsWhich MIME types belong to the package, and whether multipart is needed.
- Usage restrictionsWhether requests may overlap, and what other extensions are required.
- RateAny maximum or minimum rate at which requests may be sent.
- SecurityThe mechanisms needed, or a statement that plain SIP security is enough.
§13Security Considerations
By eliminating multiple usages of INFO messages without adequate community review, and by eliminating the possibility of rogue user agents confusing another agent by purposely sending unrelated INFO requests, the RFC expects this clarification of the use of INFO to improve the security of the Internet. While rogue agents can still send unrelated INFO requests, the mechanism enables the receiving agent and other security devices to associate INFO requests with the Info Packages that were negotiated for the session.
If the content of an Info Package payload is private, user agents will need to use end-to-end encryption such as S/MIME to prevent access to it. This matters because the transport of INFO is likely not to be end to end, but to pass through SIP proxies and back-to-back user agents that the user may not trust.
The INFO request transports application level information, and one implication is that INFO messages may require a higher level of protection than the underlying SIP dialog signaling. If the SIP signaling is not protected from eavesdropping or from authentication and repudiation attacks, for example by using TLS transport, then the INFO request and its contents are vulnerable as well. Even with SIP over TLS, any SIP hop along the path may be a security concern. The Info Package mechanism itself does not define a SIP option-tag, but an individual package specification can define one.
Because the messages travel through proxies that may not be trusted, private content has to be protected from end to end, and the signalling underneath has to be protected too. Naming the packages also helps, because a receiver can check that a request belongs to something the session agreed on.