SIP E. Burger
Internet-Draft This Space For Sale
Obsoletes: RFC 2976 H. Kaplan
(if approved) Acme Packet
Intended status: Standards Track C. Holmberg
Expires: July 6, 2009 Ericsson
January 2, 2009
Session Initiation Protocol (SIP) INFO Method and Package Framework
draft-ietf-sip-info-events-02
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Abstract
This document defines the new SIP INFO method and a mechanism for
defining, negotiating and exchanging Info Packages that use the INFO
method. Applications that need to exchange session-related
information within a SIP INVITE-created dialog, also known as
application level information, use these INFO requests. This draft
addresses issues and open items from RFC 2976 and replaces it.
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].
The terminology in this document conforms to the Internet Security
Glossary [RFC4949].
Be mindful of the terms User Agent Server (UAS) and User Agent Client
(UAC). This document strictly follows RFC 3261 [RFC3261]. The UAC
issues a SIP request and the UAS responds. This terminology may be
confusing when one combines the INFO case with the INVITE case. For
an INVITE, the initiator of the session is the UAC and the target of
the session is the UAS. However, it is possible for the target UA of
the session, the UAS of the INVITE transaction, to send an INFO to
the initiating UA of the session, the UAC of the INVITE transaction.
From the perspective of the INFO, the target UA of the session
(INVITE UAS) is, in fact, the UAC (sender) of the INFO request.
Likewise, from the perspective of the INFO, the initiating UA of the
session (INVITE UAC) is the UAS (recipient) of the INFO request.
Since this document strictly follows RFC 3261, we refer to the UA
that issues the INVITE as the "initiating UA" and the UA that
responds to the INVITE as the "target UA" to remove any confusion.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Info Package Negotiation . . . . . . . . . . . . . . . . . . . 7
3.1. UA Behavior . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Package Versions . . . . . . . . . . . . . . . . . . . . . 9
4. The INFO Method Request . . . . . . . . . . . . . . . . . . . 10
4.1. INFO Requests . . . . . . . . . . . . . . . . . . . . . . 10
4.2. INFO Request Body . . . . . . . . . . . . . . . . . . . . 11
4.3. Responses to the INFO Request Method . . . . . . . . . . . 12
4.4. Routing Behavior . . . . . . . . . . . . . . . . . . . . . 13
4.5. Behavior of Registrars . . . . . . . . . . . . . . . . . . 13
4.6. OPTIONS Processing . . . . . . . . . . . . . . . . . . . . 13
4.7. Order of Delivery . . . . . . . . . . . . . . . . . . . . 14
5. Formal INFO Method Definition . . . . . . . . . . . . . . . . 14
5.1. INFO Method . . . . . . . . . . . . . . . . . . . . . . . 14
5.2. INFO Headers . . . . . . . . . . . . . . . . . . . . . . . 16
5.2.1. Info-Package header . . . . . . . . . . . . . . . . . 16
5.2.2. Recv-Info header . . . . . . . . . . . . . . . . . . . 16
6. Legacy Uses of INFO . . . . . . . . . . . . . . . . . . . . . 16
7. Info Package Requirements . . . . . . . . . . . . . . . . . . 17
7.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 18
7.2. Info Package Name . . . . . . . . . . . . . . . . . . . . 18
7.3. Info Package Parameters . . . . . . . . . . . . . . . . . 18
7.4. Info Package Tags . . . . . . . . . . . . . . . . . . . . 18
7.5. INFO Bodies . . . . . . . . . . . . . . . . . . . . . . . 18
7.6. UAC generation of INFO requests . . . . . . . . . . . . . 18
7.7. UAS processing of INFO requests . . . . . . . . . . . . . 19
7.8. Rate of INFO Requests . . . . . . . . . . . . . . . . . . 19
7.9. IANA Registrations . . . . . . . . . . . . . . . . . . . . 19
7.10. Security Considerations . . . . . . . . . . . . . . . . . 19
7.11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 20
8. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
9.1. Update to Registration of SIP INFO Method . . . . . . . . 21
9.2. Registration of the Info-Package Header Field . . . . . . 21
9.3. Registration of the Recv-Info Header Field . . . . . . . . 21
9.4. Creation of the Info Packages Registry . . . . . . . . . . 22
9.5. Registration of the Info-Package Content-Disposition . . . 22
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Single Info Package . . . . . . . . . . . . . . . . . . . 23
10.2. Multipart INFO Example . . . . . . . . . . . . . . . . . . 24
11. Modifications to SIP Change Process . . . . . . . . . . . . . 24
12. Security Considerations . . . . . . . . . . . . . . . . . . . 25
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
13.1. Normative References . . . . . . . . . . . . . . . . . . . 26
13.2. Informative References . . . . . . . . . . . . . . . . . . 26
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Appendix A. Info Package Considerations . . . . . . . . . . . . . 28
A.1. Appropriateness of Usage . . . . . . . . . . . . . . . . . 28
A.2. Dialog Fate-Sharing . . . . . . . . . . . . . . . . . . . 29
A.3. Messaging Rates and Volume . . . . . . . . . . . . . . . . 29
A.4. Is there a better alternative? . . . . . . . . . . . . . . 29
A.5. Alternatives for Common INFO Use . . . . . . . . . . . . . 31
A.5.1. State Updates . . . . . . . . . . . . . . . . . . . . 31
A.5.2. User Stimulus: Touch Tones and Others . . . . . . . . 32
A.5.3. Direct Signaling Channel . . . . . . . . . . . . . . . 32
A.5.4. Proxy-Aware Signaling . . . . . . . . . . . . . . . . 32
A.5.5. Dialog Probe . . . . . . . . . . . . . . . . . . . . . 33
A.5.6. Malicious Indicator . . . . . . . . . . . . . . . . . 33
Appendix B. Legacy INFO Usages . . . . . . . . . . . . . . . . . 34
B.1. ISUP . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
B.2. QSIG . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
B.3. MSCML . . . . . . . . . . . . . . . . . . . . . . . . . . 34
B.4. MSML . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
B.5. Video Fast Update . . . . . . . . . . . . . . . . . . . . 34
B.6. DTMF . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 34
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36
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1. Introduction
The SIP protocol [RFC3261] defines session control messages used to
setup and tear down a SIP controlled session. In addition, a SIP
User Agent (UA) can use the re-INVITE and UPDATE methods during a
session to change the characteristics of the session. Most often,
this is to change the properties of media flows related to the
session or to update the SIP session timer [RFC4028]. The purpose of
the INFO message [RFC2976] is to carry application level information
along the SIP signaling path. Note the INFO method does not change
the SIP dialog state. It may, however, change application state for
applications using the SIP dialog.
While INFO has been widely adopted for specific application use
cases, such as ISUP and DTMF exchange, RFC 2976 [RFC2976] neither
defined a negotiation mechanism nor a means by which to explicitly
indicate the type of application information contained in the INFO
message. This led to problems associated with static configuration.
In addition, the industry realized there was a potential for
interoperability problems due to undefined content syntax and
semantics. This draft addresses these deficiencies and provides a
framework for explicit negotiation of capabilities and content
context using "Info Packages".
The INFO method as defined by RFC 2976 did not provide any context
for the information the request carried. While it may sometimes be
clear what the content is based on the Content-Type, this is only
true where there is only one contextual usage of the content-type.
For example, if the Content-Type is "image/jpeg", the MIME-attached
content is a JPEG image. However, there is no indication what the
purpose of the image is. The image could be a caller-id picture, a
contact icon, a photo for sharing, and so on. The sender does not
know which JPEG to give the receiver if the receiver supports a JPEG
content type, and the receiver does not know which JPEG the client is
sending if the receiver supports receiving more than one JPEG content
type. Thus we need a well defined and documented statement of what
the information sent is for. This situation is identical to the
context issue in Internet Mail [RFC3458]. RFC 3458 goes into this
and other issues in detail.
Event Packages [RFC3265] perform the role of disambiguating the
context of a message for subscription-based events. This document
provides a similar framework for INVITE-based application level
information exchange. The mechanism defined in this draft has no
relationship to the SUBSCRIBE and NOTIFY methods. The mechanism
defined here neither creates a separate subscription dialog nor a
subscription usage within an existing dialog. Instead, it uses the
INVITE method and its responses to indicate and negotiate supported
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Info Packages, and the INFO method to convey the Info Packages. This
mechanism is not appropriate for IANA-registered Subscribe Event
[RFC3265] package types. Info Package definitions and registrations
indicate support for this mechanism when one registers them with
IANA.
Each UA enumerates which Info Packages it can receive. If the far
end indicates it can receive a package offered by the near end, the
near end can send INFO methods containing the payload for that
package. Likewise, if the near end indicates it can receive a
package, the far end can send INFO methods containing the payload for
that package. The Recv-Info header indicates which packages a UA is
willing to receive. The Info-Package header indicates which package
a particular INFO method request belongs to. There is a reserved
Info Package, "nil", which indicates the UA conforms to this
document, but does not wish to receive Info Packages. This enables
other UAs that conform with this document to detect legacy UAs, as
the legacy UA will not include a Recv-Info header in their SIP dialog
establishment or modification requests. Section 3 describes the
negotiation in detail.
This document does not describe any specific Info Package type
extensions. One must extend this protocol by other documents, herein
referred to as "Info Packages". Section 7 describes guidelines for
creating these extensions.
The INFO method does not 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.
Applications need to be aware that application level information
transported by the INFO method constitutes mid-dialog signaling.
These messages traverse the post-session-setup SIP signaling path.
This is the path taken by SIP re-INVITEs, BYEs, and other SIP
requests within an individual dialog. SIP proxy servers will
receive, and potentially act on, mid-dialog signaling information.
Application designers need to understand this can be a feature, as
when the User Agents are exchanging information that elements in the
SIP signaling path need to be aware of. Conversely, this can be a
problem, as messages these network elements have no interest in can
put a significant burden on those element's ability to process other
traffic. Moreover, such network elements may not be able to read
end-to-end encrypted INFO bodies.
2. Applicability
This document replaces the [RFC2976] SIP INFO method document
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[RFC2976] to include explicit negotiation of supported Info Packages
in the INVITE transaction and indication of the Info Package to use
by using a new header field in the INFO request.
3. Info Package Negotiation
To be abundantly clear, as stated in the Conventions section, the
term UAC refers to the UAC (sender) of the INFO method and UAS refers
to the recipient of the INFO method. "Initiating UA" refers to the
sender of an initial INVITE to establish a session and "target UA"
refers to the recipient of that INVITE request.
3.1. UA Behavior
A UA supporting this document MUST advertise a set of Recv-Info
packages in the initial INVITE exchange. This includes the initial
INVITE request, as well as provisional 1xx, final 2xx responses, and
the ACK. The initiating UA (UAC of the INVITE) may choose not to
offer any packages in the initial INVITE and negotiate packages from
the target UA's subsequent responses and the ACK, in order to support
third-party call control [RFC3725].
Info Package negotiation may occur any time the UAs negotiate session
parameters. There are two cases to consider for SIP dialog parameter
negotiation: the initial INVITE transaction and subsequent
renegotiation. By subsequent renegotiation, we mean procedures such
as re-INVITE and UPDATE [RFC3311]. In the first case, dialog
establishment (the initial INVITE transaction), the UAC MUST NOT send
INFO requests for a given Info Package until the UAS lists the given
Info Package in a Recv-Info header. If the UAS sends a subsequent
message in the dialog establishment exchange that removes a listed
Info Package, the UAC MUST NOT send INFO requests for that package.
In the second case, dialog renegotiation, the UAC MUST NOT send INFO
requests for a newly listed Info Package until the dialog
renegotiation exchange successfully completes and the newly listed
Info Package is in the UAS' final renegotiation exchange message.
A UAS lists multiple packages by enumerating the package name(s),
separated by commas, as values for the Recv-Info header in the
session establishment exchange. A UAS can also list multiple
packages by including multiple Recv-Info headers. The UAS can also
combine multiple Recv-Info headers with one or more packages in each
header value. If the UAS has a preference for receiving one package
over another, the UAS MUST list the preferred Info Package lexically
earlier in the message. That is, by listing it earlier in a list
within a given Recv-Info header or listing it in a previous Recv-Info
header in a given message. Listing a package multiple times does no
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harm. As far as a hint to the UAC, the first appearance is what the
UAC uses for determining the UAS' preference. Note this order is
only a hint to the UAC, as there is no meaningful way of enforcing
the use of a preferred package at the UAC.
If a UAS does not wish to receive any Info Packages, the UAS MUST
indicate this by including one and only one Recv-Info header with the
value 'nil'. This enables the UAC to discern the difference between
the UAS understanding Info Packages but not wishing to receive any
from a UAS that does not understand Info Packages. A UAC conforming
to this document can always send or receive legacy INFO usages
without packages.
NOTE: We could allow an empty Recv-Info header to indicate the UAS
does not wish to receive Info Packages. Semantically that is what
this means, as there is no null package. However, this is sloppy
and we may find we need an explicit value here in the event we
require a richer negotiation strategy. Since mandating nil at
this time is no burden, and it will be a burden in the future if
we do not specify it now, we specify it now.
Info Package capability negotiation occurs within the context of a
single session negotiation exchange. Moreover, the last capability
set received within the exchange is the one the receiver applies
against its advertised capability set. For example, if in an INVITE,
the initiating UA offers the following.
INVITE ...
...
Recv-Info: P, R
...
The target UA responds with a 200 OK, and the initiating UA then
confirms in an ACK, as shown.
ACK
...
Recv-Info: R, T
...
The target UA can now send from package T to the initiating UA.
Moreover, in this example, the target UA may not send form package P,
as P no longer is in the initiating UA's Info Package set.
The limitation on requiring the negotiation to occur within the
context of a session negotiation exchange means that if the
initiating UA issues a re-INVITE (after the above ACK) with the
following.
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INVITE ...
...
Recv-Info: P, R, T
...
The target UA MUST NOT send any package P INFO methods until the
target UA sees P in the final ACK from the initiating UA.
In the case of a SIP dialog refresh, if the initiating UA and target
UA wishes to keep their Info Package set active, the UAs MUST include
the Recv-Info header with the appropriate values. Otherwise, if the
UA neglects to include the Recv-Info header, the other UA in the dial
will assume the first UA no longer supports INFO as specified by this
document.
INFO itself does not necessitate the use of Require or Proxy-Require
headers. There is no token defined for "Supported" headers. If
necessary, clients may probe for the support of this version of INFO
using the OPTIONS request defined in SIP [RFC3261]. One could
envision a particular Info Package implementation that relied on
either of these headers. See Section 7 for more on this issue.
The presence of the Recv-Info header in a message is sufficient to
indicate support for this version of INFO. The "methods" parameter
for Contact [RFC3841] is not sufficient to determine if the endpoints
support Info Packages, as the INFO method supported might be the
obsolete RFC 2976 [RFC2976] version.
For Info Packages, this draft does not provide a means of requiring
support for a specific Info Package. If the far-end UA does not
indicate support for an Info Package that the local server requires,
the server MAY terminate the session with a CANCEL or BYE request.
3.2. Package Versions
The protocol mechanism described herein does not provide for a
package versioning mechanism. This is for two reasons. The first is
that if an Info Package has a capability for forward and backward
compatibility in the Info Package payload, then that compatibility
comes from the application level semantics of the information. This
means it is the responsibility of the application to hanlde such
compatibility and not the INFO framework. For example, one could use
XML versioning techniques in the payload to indicate verisons of the
Info Package.
The second reason we do not have a package versioning system is if
the payload is not sufficient to carry payload versions, then it is
highly unlikely payloads would be backwards compatible. That is,
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what one really is defining is a new Info Package. This is more
especially so when one considers User Agents can negotiate package
support but cannot negotiate package version support.
4. The INFO Method Request
4.1. INFO Requests
The INFO method provides additional, application level information
that can further enhance a SIP application. It is important to note
there are some types of application information for which using INFO
messages are inappropriate. See Appendix A for a discussion of this.
The UAC MUST include the Info-Package header field when it sends an
INFO request carrying an Info Package. The Info-Package header field
value in an INFO request MUST contain a single Info Package token.
That Info Package token MUST match one of the Info Packages the UAS
indicated support for during the negotiation described in Section 3.
The UAC MAY send an INFO in a legacy usage context. See Appendix B
for examples of legacy usages. In general a legacy usage is where
there is no Info-Package header. In this case, if the UAS has never
offered a Recv-Info header or never offered a Recv-Info header with a
package of a similar function to the legacy INFO usage, the UAC MAY
send an INFO without an Info-Package header field and a body
appropriate to the said legacy usage.
A UAC MUST NOT use the INFO method outside an INVITE dialog usage.
The INFO method has no lifetime or usage of its own, as it is
inexorably linked to that of the INVITE. When the INVITE-created
dialog terminates, that signals the termination of the negotiated
Info Packages. A UAS that receives an INFO message after the INVITE
dialog usage terminates MUST respond with a 481 Call Does Not Exist.
The dialog identifiers defined in RFC 3261 [RFC3261] must match those
of the provisional or final responses to the INVITE. As a result,
INFO requests cannot fork. The UAC may send INFO requests once the
UAS has sent the Recv-Info header field value, indicating what the
UAS supports.
The converse is not true during initial session establishment. The
initiating UA of the first INVITE MUST be prepared to receive
multiple INFO requests, as the first INVITE may fork. Since dialog
negotiation has not completed, and we allow early INFO requests,
multiple target UAs may respond. This initial dialog establishment
phase is the only time the UAS need be prepared to receive multiple
INFO requests, as we require post-session-establishment negotiation
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to fully complete before a UAC can send an INFO request.
The INFO request MUST NOT carry a Recv-Info header. The UAC can only
negotiate Info Packages using the procedures of Section 3.
The signaling path for the INFO method is the signaling path
established as a result of the dialog setup. This can be 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.
4.2. INFO Request Body
The purpose of the INFO request is to carry application level
information between SIP user agents. The INFO message body SHOULD
carry this information, unless the message headers carry the
information of interest. Note this is not an invitation to invent
SIP headers for the purposes of application level information
exchange. Rather, one could envision circumstances where existing
SIP headers already convey the information the application has
interest in.
If the Info Package defines a payload, and the UAC determines it is
appropriate to send that payload to the UAS, the UAC MUST include the
payload, with the MIME type specified by the Info Package.
If the Info Package allows the UAC to send a request without a
payload, the UAC MAY send the INFO request without a body.
Some SIP extensions, which are orthogonal to INFO proper, may insert
body parts unrelated to the INFO payload. User Agents MUST conform
to RFC 3261 as updated by body-handling [I-D.ietf-sip-body-handling]
to support multipart MIME handling. If there are bodies unrelated to
the Info Package, and the Info Package also has a payload, the UAC
MUST bundle these elements into a multipart MIME body. In this case,
the UAS needs a means to unambiguously identify the body part
belonging to the Info Package. To do this, the UAC MUST identify the
Info Package payload MIME body part with a Content-Disposition of
'Info-Package'.
If the payload of an Info Package is already a multipart MIME body,
the UAC MUST identify the payload with a Content-Disposition of
'Info-Package' in the headers for the appropriate MIME body part.
If there is no payload in the INFO request unrelated to the Info
Package and the payload of the Info Package is not a multipart MIME,
the UAC MUST identfiy the message, at the SIP header level, with a
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Content-Disposition of 'Info-Package'.
If there is no payload for the Info Package, they UAC MAY omit the
Content-Disposition header.
NOTE: We could be lazy and even save 33 octets by allowing the UAC
to construct a non-multipart MIME payload without a Content-
Disposition header. However, mandating the presence makes parsing
considerably easier, and it is easier to have it required now than
run into a problem later.
NOTE: One could offer that the Info-Package header is redundant,
as we could have the Info Package name be a parameter for Content-
Disposition. However, there could be corner cases with legacy
INFO usage that makes this a poor choice.
4.3. Responses to the INFO Request Method
If a UAS receives an INFO request it MUST send a final response. A
UAS MUST send a 200 OK response for an INFO request with no message
body and no Info-Package header if the UAS received the INFO request
on an existing dialog. This protocol action supports legacy use of
INFO as a keep-alive mechanism.
If the UAS receives an INFO request with an Info-Package the UAS
advertised with a Recv-Info in the last dialog state update and the
body of the INFO request is an appropriate MIME type for the Info
Package, the UAS MUST send a 200 OK response.
If the INFO request contains a body the server does not understand
then, in the absence of Info Package associated processing rules for
the body, including the absence of an Info-Package header, the server
MUST respond with a 415 Unsupported Media Type message.
If the INFO request indicates an Info Package type the server does
not understand, then the server MUST respond with a 489 Bad Event.
The server then MUST terminate the INVITE dialog, as this represents
a protocol failure.
NOTE: Some may think "Bad Event" implies there is a link between
INFO and NOTIFY. However, what this does is refine 489 to mean,
"Received some package in some context that I do not understand,"
where today the possible contexts are INFO and NOTIFY. The text
is irrelevant and the meaning is clear from the context.
If a server receives an INFO request with a body it understands, but
it has no Info-Package header, the UAS MAY use the body as it sees
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fit. The UAS SHOULD respond to the INFO request with a 200 OK. This
enables legacy use of INFO. The UAS MAY reject the request with a
489 if the UAS needs to enforce strict compliance with the current
INFO framework described here.
The UAS MUST send a 481 Call Leg/Transaction Does Not Exist message
if the INFO request does not match any existing INVITE-initiated
dialog.
The UAS MAY send other responses, such as Request Failure (4xx),
Server Failure (5xx) and Global Failure (6xx) as appropriate for the
request.
4.4. Routing Behavior
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 RFC 3261 [RFC3261] as defined for the BYE request.
The INFO message MUST NOT change the state of the SIP dialog. Of
course, outside the INFO machinery specific failure responses as
documented in the SIP dialog usages document [RFC5057], may cause the
INVITE dialog to terminate.
4.5. Behavior of Registrars
Registrars receiving a REGISTER request that includes Recv-Info
headers MAY store such information and use it for routing purposes.
How the registrar uses this information is beyond the scope of this
document.
4.6. OPTIONS Processing
A UAC, the sender of the OPTIONS request, SHOULD include Recv-Info
headers, populated appropriately for the packages the UAC supports.
The UAS SHOULD include its set of Recv-Info packages. These
strictures are of "should" strength because local policy might
restrict the advertisement of full capabilities, the UA may know the
best choice of equivalent packages to list from local configuration,
and so on.
The UAS and UAC MUST NOT consider the OPTIONS request to be part of a
capabilities negotiation. The OPTIONS request is purely a probe.
For the UAC or UAS to renegotiate package support, they must use the
procedures described in Section 3.
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4.7. Order of Delivery
The INFO method does not define mechanisms for ensuring in-order
delivery for overlapping INFO requests. That is, the UAC can send
another INFO request before receiving a transaction response from the
UAS to a prior INFO request. While the UAC will increment the CSeq
header upon the transmission of new INFO messages, the UAS cannot use
the CSeq 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.
It is up to the individual Info Package definition to specify what
happens when there are overlapping INFO requests. However, since it
is legal SIP to have overlapping requests, the application must be
able to handle the reception of overlapping requests, even if the
Info Package does not allow for it. Since overlapping requests can
occur even if the application (Info Package) does not allow it, the
Info Package needs to define the appropriate response. This is more
especially so given the UAC could send from multiple Info Packages.
Some of those packages may allow overlapping INFO requests, while
others do not. In this situation, it would be hard to tell if the
non-overlapping packages were being violated or not.
5. Formal INFO Method Definition
5.1. INFO Method
This document describes one new SIP method: INFO. This document
replaces the definition and registrations found in [RFC2976].
This table expands on Tables 2 and 3 in [RFC3261].
Table 1: Summary of Header Fields
Header Where INFO
------ ----- ----
Accept R o
Accept-Encoding R o
Accept-Encoding 2xx o
Accept-Encoding 415 c
Accept-Language R o
Accept-Language 2xx o
Accept-Language 415 c
Allow R o
Allow 200 -
Allow 405 o
Allow-Events R o
Allow-Events r o
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Authentication-Info 2xx o
Authorization R o
Call-ID gc m
Call-Info R o
Call-Info r o
Contact R -
Contact 1xx -
Contact 2xx -
Contact 3xx -
Contact 485 -
Content-Disposition e o
Content-Encoding e o
Content-Language e o
Content-Length e o
Content-Type e *
CSeq gc m
Date g o
Error-Info 3xx-6xx o
Expires g -
From gc m
Geolocation R o
Max-Breadth R -
Max-Forwards R o
MIME-Version R o
MIME-Version r o
Organization g o
Privacy R o
Proxy-Authenticate 407 o
Proxy-Authorization R o
Proxy-Require R o
Reason R o
Record-Route R o
Record-Route 2xx o
Require R o
Require r o
Retry-After R -
Retry-After 404,480,486 o
Retry-After 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 g o
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To gc(1) m
Unsupported 420 o
User-Agent g o
Via gc(2) m
Warning r o
WWW-Authenticate 401 o
5.2. INFO Headers
This table expands on tables 2 and 3 in [RFC3261].
Header field where ACK BYE CAN INV OPT REG PRA INF MSG UPD SUB NOT
--------------------------------------------------------------------
Info-Package R - - - - - - - m - - - -
Recv-Info R o - - o o o o - - o - -
Recv-Info 2xx o - - o o - o - - o - -
Recv-Info 1xx o - - o o - o - - o - -
Recv-Info r o - - - o - o - - o - -
5.2.1. Info-Package header
This document adds Info-Package to the definition of the element
"message-header" in the SIP message grammar.
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 octet-by-octet with that of the Recv-Info header
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 Packages define these values. Such documents MUST
register such values with IANA. These values are Specification
Required [RFC5226].
5.2.2. Recv-Info header
This document adds Recv-Info to the definition of the element
"general-header" in the SIP [RFC3261] message grammar. Section 3
describes the Recv-Info header usage.
6. Legacy Uses of INFO
Several RFC-defined and other standards-defined uses of RFC 2976 INFO
[RFC2976] exist and are in use, as well as numerous proprietary uses.
Appendix B describes some of these usages. By definition,
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identifying such uses has relied on either static local configuration
or implicit context determination based on the body Content-Type or
Content-Disposition value or some proprietary mechanism. This draft
cannot forbid nor avoid such uses, since local configuration can
always override standardized mechanisms.
To maintain backward compatibility with the extant standardized uses
of INFO, a server MAY interpret an INFO request with no "Info-
Package" header as being of such legacy use.
It should be noted that such legacy use will not "break" the
mechanism in this draft. For example, if a UA supports SIP-T
[RFC3372], it does so based on static local configuration or based on
acceptance of the application/isup body. If it adds support for this
draft's Info Package negotiation mechanism, the local configuration
still applies, and the UA will send/receive INFO messages based on
SIP-T regardless of the Info Package negotiation. It will also be
able to send/receive INFO messages based on the Info Packages it
negotiated. If, at some future time, an Info Package is defined for
SIP-T, the UA can indicate such in the negotiation, and again local
configuration would supersede if need be. The UA would not lose the
ability to use SIP-T with legacy devices. Rather, it would gain the
ability to use it with devices which support this draft and with
which it did not have such local configuration set, and could avoid
failures related to unsupported bodies.
It is the hope of this draft's authors that vendors that implement
proprietary INFO uses submit their mechanisms as Info Package
extension documents, and follow the Info Package negotiation
mechanism defined in this draft.
7. Info Package Requirements
Info Packages SHOULD NOT reiterate any of the behavior described in
this document, unless required for clarity or emphasis. However,
such packages MUST describe the behavior that extends or modifies the
behavior described in this document.
Info Packages MUST NOT weaken any behavior designated with "SHOULD"
or "MUST" in this document. However, Info Packages MAY strengthen
"SHOULD", "MAY", or "RECOMMENDED" requirements to "MUST" strength if
the application requires it.
In addition to the normal sections expected in standards-track RFCs
and SIP extension documents, authors of Info Packages need to address
each of the issues detailed in the following subsections, whenever
applicable.
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7.1. Applicability
This section, which MUST be present, describes why any of the other
established user-to-user data transfer protocols are not appropriate
for the given Info Package. Common reasons can be a requirement for
SIP Proxies or back-to-back User Agents (B2BUAs) to see the
application level information. Consideration in this section MUST
describe what happens if one or both endpoints encrypt the payload.
7.2. Info Package Name
This section, which MUST be present, defines the token name that
designates the Info Package. The name MUST conform to the token-
nodot ABNF production described in Section 8. It MUST include the
information that appears in the IANA registration of the token. For
information on registering such types, see Section 9.
7.3. Info Package Parameters
If the "Info-Package" header allows parameters to modify the behavior
of the Info Package, this section MUST clearly define the syntax and
semantics of such parameters.
7.4. Info Package Tags
If useful for the Info Package to have SIP option tags, this is the
place to define the tag. Note that if the Info Package defines a SIP
option tag, the Info Package must conform to the SIP Change Process
[RFC3427].
7.5. INFO Bodies
Each Info Package MUST define what type or types of bodies are
expected in INFO requests. Such packages MUST specify or cite
detailed specifications for the syntax and semantics associated with
such a body.
The UAS MUST enumerate every MIME type associated with the Info
Packages advertised in the UAS' Recv-Info header the UAS is willing
to receive. If a UAC sends a body that includes something not
enumerated by the UAS, this is a protocol error and the UAS MUST
respond appropriately.
7.6. UAC generation of INFO requests
Each Info Package MUST describe the process by which a UA generates
and sends an INFO request. This includes detailed information about
what events cause the UA to send an INFO request.
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If the Info Package does not allow overlapping (outstanding) INFO
requests the Info Package MUST disclose this in the section
describing UA generation of INFO requests. Note the generic protocol
machinery of the INFO method has no way of enforcing such a
requirement. Section 7.7 describes this situation.
7.7. UAS processing of INFO requests
The Info Package MAY describe the process followed by the UA upon
receipt of an INFO request. Since INFO does not change SIP state,
and may not even change application state, there may be no useful
guidance required in the Info Package specification for UA
processing.
If the info Package does not permit overlapping INFO requests, it is
important to note the issuance of overlapping INFO requests is an
application-layer protocol failure and not an INFO method failure.
Therefore, in the event a UAC issues overlapping INFO requests for an
Info Package, the UAS MUST NOT return an error response. This
section of the Info Package specification MUST describe the
application level response to overlapping INFO requests. Examples
include a new INFO request back to the offending UAC indicating an
application error, ignoring the overlapping request and processing it
to the UAS' best effort, or terminating the entire SIP dialog.
7.8. Rate of INFO Requests
Each Info Package MUST define a requirement of MUST strength which
defines an absolute maximum on the rate at which an Info Package of a
given type can generate INFO messages by a UA in a dialog.
If possible, a package MUST define a throttle mechanism that allows
UAs to further limit the rate of INFO messages.
7.9. IANA Registrations
The Info Package MUST have an IANA Considerations section that
includes definitions for the Info Package Name and, if needed,
supported MIME types.
7.10. Security Considerations
The INFO mechanism transports application level information. One
implication of this is INFO messages may require a higher level of
protection than the underlying SIP-based session signaling. If the
application transports sensitive information, such as credit card
numbers, health history, personal identifiers, and so on, the Info
Package MUST document security procedures that exceed the default
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procedures presented in this document. In most circumstances it is
not sufficient for a package to attempt to mandate TLS for the
signaling channel to secure the data carried by the INFO. This is
because there are few protocol mechanisms to enforce this
requirement. It may be possible for an Info Package to inform the
SIP transport layer stack to be "secure." However, the only way to
ensure secure transport at the application level is to have the
security be part of the Info Package itself. The most common method
of achieving this is to use end-to-end security techniques such as
S/MIME [RFC3851].
7.11. Examples
We RECOMMEND Info Packages include several demonstrative message flow
diagrams paired with several typical, syntactically correct, and
complete messages.
Documents describing Info Packages MUST clearly indicate the examples
are informative and not normative, with instructions that
implementers refer to the main text of the document for exact
protocol details.
8. Syntax
This section describes the syntax extensions required for user-to-
user data exchange in SIP. The previous sections describe the
semantics. Note the formal syntax definitions described in this
document use the ABNF format used in SIP [RFC3261] and contain
references to elements defined therein.
The Augmented BNF definitions for the various new and modified syntax
elements follow. The notation is as used in SIP [RFC3261]. See SIP
for any elements not defined in this section.
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INFOm = %x49.4E.46.4F ; INFO in caps
extension-method = INFOm / token
Info-Package = "Info-Package" HCOLON Info-package-type
Recv-Info = "Recv-Info" HCOLON "nil"
/ Info-package-type
*( COMMA Info-package-type )
Info-package-type = Info-package-name *( "." Info-package-param)
Info-package-name = token-nodot
Info-package-param = token-nodot
token-nodot = 1*( alphanum / "-" / "!" / "%" / "*"
/ "_" / "+" / "`" / "'" / "~" )
NOTE on the Recv-Info production: if the value is "nil", there can be
one and only one Recv-Info header in the SIP message.
9. IANA Considerations
9.1. Update to Registration of SIP INFO Method
Please update the existing registration in the SIP Methods and
Response Codes registry under the SIP Parameters registry that
states:
Method: INFO
Reference: [RFC2976]
to:
Method: INFO
Reference: [RFCXXXX]
9.2. Registration of the Info-Package Header Field
Please add the following new SIP header field in the Header Fields
subregistry under the SIP Parameters registry.
Header Name: Info-Package
Compact Form: (none)
Reference: [RFCXXXX]
9.3. Registration of the Recv-Info Header Field
Please add the following new SIP header field in the Header Fields
subregistry under the SIP Parameters registry.
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Header Name: Recv-Info
Compact Form: (none)
Reference: [RFCXXXX]
(postamble)
9.4. Creation of the Info Packages Registry
Please create a subregistry in the SIP Parameters registry for Info
Packages. This subregistry has a modified First Come First Served
[RFC5226] policy.
The following data elements populate the Info Package 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 Info Package Payload MIME Types: A list of zero or more registered
MIME types from the MIME Type Registry.
o Standards Status: Informational or Standards Track. See below for
a discussion and rules on this field.This registry is First Come
First Served, which means one does not need a published, standards
track RFC to register an Info Package. However,
o Reference: The Reference is the published specification describing
the Info Package.
The Standards Status field is an indicator of the level of community
review for the Info Package. Values are "Standards Track" and
"Informational". To attain a "Standards Track" designation, the
Reference must meet the IANA Guidelines [RFC5226] for Specification
Required.
This document reserves the Info Package Name "nil" to represent "no
Info Package present" and as such IANA shall not honor a request to
register the "nil" Info Package.
The initial population of this table shall be:
Name MIME Type Standards Status Reference
nil Standards Track [RFCXXXX]
9.5. Registration of the Info-Package Content-Disposition
Please add the following registration to the Content-Disposition
registry. The description suitable for the IANA registry is as
follows.
The payload of the message carrying this Content-Disposition header
field value is the payload of an Info Package.
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10. Examples
10.1. Single Info Package
In the following example, Alice initiates a call to Bob. Alice can
support sending or receiving "foo" Info Packages, and sending "bar"
Info Packages.
Alice generates the following: (note: much has been left out for
simplicity)
INVITE sip:bob@example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.1:5060;branch=z9hG4bKnashds10
From: Alice <sip:alice@example.net>;tag=1234567
To: Bob <sip:bob@example.com>
Call-Id: 123456mcmxcix
CSeq: 1 INVITE
Contact: <sip:alice@192.0.2.1>
Recv-Info: foo
Bob does not support anything, so he says so.
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP 192.0.2.1:5060;branch=z9hG4bKnashds10
From: Alice <sip:alice@example.net>;tag=1234567
To: Bob <sip:bob@example.com>;tag=abcdefg
Call-Id: 123456mcmxcix
CSeq: 1 INVITE
Recv-Info: nil
Bob answers, but still does not support anything.
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.0.2.1:5060;branch=z9hG4bKnashds10
From: Alice <sip:alice@example.net>;tag=1234567
To: Bob <sip:bob@example.com>;tag=abcdefg
Call-Id: 123456mcmxcix
CSeq: 1 INVITE
Contact: <sip:bob@192.0.2.2>
Recv-Info: nil
Alice could have sent an Info Package as soon as she received the
180, but in this example she would not have been able to do so since
Bob didn't say he could receive any Info Packages in his 180
response. Bob, on the other hand, may send an INFO:
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INFO sip:alice@192.0.2.1 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: 123456mcmxcix
CSeq: 2 INFO
Contact: <sip:bob@192.0.2.2>
Info-Package: foo
10.2. Multipart INFO Example
This is for where there is a single INFO payload in a multipart/mime.
INFO ....
To: ....
From: ....
Info-Package: foo
Mumble: <cid:abcd9999qq>
Content-Type: multipart/mixed;boundary="theboundary"
...
--theboundary
Content-Type: application/mumble
Content-Id: abcd9999qq
...
<mumble stuff>
--theboundary
Content-Type: application/foo
Content-Disposition: Info-Package
<foo body>
--theboundary--
11. Modifications to SIP Change Process
[EDITOR'S NOTE: This section may become a separate document in the
future.]
This document updates RFC 3427 [RFC3427] to add a process for
registering new Info Packages. The process for registering new Info
Packages follows the process outlined in Section 4.3 of RFC 3427 for
the registration of SIP Event Packages. Namely, the registration of
a new SIP Info Package requires the SIPPING chairs to assign an
individual to perform expert review of the proposal if the work is
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not a SIPPING work item in itself.
12. Security Considerations
By eliminating multiple uses of INFO messages without adequate
community review and by eliminating the possibility for rogue SIP
User Agents from confusing another User Agent by purposely sending
unrelated INFO messages, we expect this document's clarification of
the use of INFO to improve the security of the Internet. Whilst
rogue UACs can still send unrelated INFO messages, this framework
provides mechanisms for which the UAS and other security devices can
filter for approved Info Packages.
If the content of the Info Package payload is private, User Agents
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 (B2BUA's), which the user may not trust.
The INFO mechanism transports application level information. One
implication of this is INFO messages may require a higher level of
protection than the underlying SIP-based session 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 Framework described by this document provides the tools for
such secure, end-to-end transport of application data.
One interesting property of Info Package use is one can reuse 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
which knows the credentials in order to perform the INVITE challenge
is still in the path for the INFO, or that the far-end UAS knows such
credentials.
13. References
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13.1. Normative References
[I-D.ietf-sip-body-handling]
Camarillo, G., "Message Body Handling in the Session
Initiation Protocol (SIP)",
draft-ietf-sip-body-handling-05 (work in progress),
October 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
13.2. Informative References
[I-D.ietf-speechsc-mrcpv2]
Shanmugham, S. and D. Burnett, "Media Resource Control
Protocol Version 2 (MRCPv2)",
draft-ietf-speechsc-mrcpv2-17 (work in progress),
November 2008.
[I-D.saleem-msml]
Saleem, A., "Media Server Markup Language (MSML)",
draft-saleem-msml-07 (work in progress), August 2008.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[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.
[RFC2976] Donovan, S., "The SIP INFO Method", RFC 2976,
October 2000.
[RFC3080] Rose, M., "The Blocks Extensible Exchange Protocol Core",
RFC 3080, March 2001.
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[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
[RFC3372] Vemuri, A. and J. Peterson, "Session Initiation Protocol
for Telephones (SIP-T): Context and Architectures",
BCP 63, RFC 3372, September 2002.
[RFC3427] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J.,
and B. Rosen, "Change Process for the Session Initiation
Protocol (SIP)", BCP 67, RFC 3427, December 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.
[RFC3458] Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
Context for Internet Mail", RFC 3458, January 2003.
[RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
Camarillo, "Best Current Practices for Third Party Call
Control (3pcc) in the Session Initiation Protocol (SIP)",
BCP 85, RFC 3725, April 2004.
[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[RFC4028] Donovan, S. and J. Rosenberg, "Session Timers in the
Session Initiation Protocol (SIP)", RFC 4028, April 2005.
[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.
[RFC4240] Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
Media Services with SIP", RFC 4240, December 2005.
[RFC4497] Elwell, J., Derks, F., Mourot, P., and O. Rousseau,
"Interworking between the Session Initiation Protocol
(SIP) and QSIG", BCP 117, RFC 4497, May 2006.
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[RFC4730] Burger, E. and M. Dolly, "A Session Initiation Protocol
(SIP) Event Package for Key Press Stimulus (KPML)",
RFC 4730, November 2006.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[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.
[W3C.REC-voicexml21-20070619]
Porter, B., McGlashan, S., Lee, A., Burnett, D., Carter,
J., Oshry, M., Bodell, M., Baggia, P., Rehor, K., Burke,
D., Candell, E., and R. Auburn, "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>.
Appendix A. Info Package Considerations
This section covers several issues that one should take into
consideration when proposing new Info Packages.
A.1. Appropriateness of Usage
When designing an Info Package using the method described in this
document for application level information exchange, it is important
to consider: is INFO and, more importantly, is signaling within a SIP
dialog, an appropriate mechanism for the problem set? Is it because
it is the most reasonable and appropriate choice, or merely because
"it's easy"?
These are difficult issues to consider, especially when presented
with real-world deadlines and implementation cost issues. However,
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choosing to use INFO for inappropriate uses *will* lead to issues in
the real world, not the least of which are certain types of
middleboxes which will remove the device from the network if it is
found to cause damage to other SIP nodes.
Therefore, the following sections provide consideration guidelines
and alternatives to INFO use.
A.2. Dialog Fate-Sharing
INFO, by design, is a method within an INVITE dialog usage. RFC 5057
[RFC5057] enumerates the problems with using dialogs for multiple
usages, and we strongly urge the reader to review RFC 5057. The most
relevant issue is a failure of transmission or processing of an INFO
request may render the INVITE dialog terminated, depending on the
type of failure. Prior to RFC 5057 it was not clear if the INFO
usage was a separate usage or not. RFC 5057 clarifies the INFO
method is always part of the INVITE usage.
Some uses of INFO can tolerate this fate sharing of the INFO message
over the entire dialog. For example, in the SIP-T usage, it may be
acceptable for a call to fail, or to tear down the call, if one
cannot deliver the associated SS7 information. The same is usually
true for DTMF. However, it may not be acceptable for a call to fail
if, for example, a DTMF buffer overflows. Then again, for some
services, that may be the exact desired behavior.
A.3. Messaging Rates and Volume
There is no throttling mechanism for INFO. Consider that most call
signaling occurs on the order of 7-10 messages per 3 minutes,
although with a burst of 5-7 messages in one second during call
setup. DTMF tones occur in bursts at a rate of up to 20 messages per
second. This is a considerably higher rate than for call signaling.
Sending constant GPS location updates, on the other hand, would incur
an undue burden on SIP Proxies along the path.
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 UDP MTU [RFC0768]. Appropriate mechanisms for
such traffic include MSRP [RFC4975], COMEDIA [RFC4145], or HTTP
[RFC2616].
A.4. Is there a better alternative?
The first alternative for application level interaction is SIP
Events, also known as SUBSCRIBE/NOTIFY [RFC3265]. In this model, a
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user agent requests state information, such as key pad presses from a
device to an application server or key map images from an application
server to a device. The SUBSCRIBE creates a new dialog that does not
share the fate of the related INVITE-initiated dialog. Moreover,
using the SUBSCRIBE model enables multiple applications to receive
state updates. These applications can be outside the media path and
potentially outside the INVITE-initiated dialog's proxy path. In
fact, SIUBSCRIBE/NOTIFY is your only option if you need to exchange
data outside a communications session.
SUBSCRIBE/NOTIFY messages pass through the SIP signaling
infrastructure, such as SIP Proxies and B2BUAs. Application
designers need to understand this can be a feature, as when the User
Agents are exchanging information that elements in the SIP signaling
path need to be aware of. Conversely, this can be a problem, as
messages these network elements have no interest in can put a
significant burden on those element's ability to process other
traffic. Moreover, such network elements may not be able to read
end-to-end encrypted SUBSCRIBE or NOTIFY bodies.
Implementers do need to be aware the price of having a protocol that
works in all cases, can scale, can easily load balance, and will not
mysteriously fail a session in the event of state synchronization
failure does come at a cost. Session establishment is a minimum of
two messages in addition to the INVITE dialog establishment. If the
SUBSCRIBE application is co-resident with the INVITE application, the
application will have to manage two SIP dialogs instead of one.
Tracking the application level state dominates memory and processing
for some applications, and as such the doubling of SIP dialogs is not
an issue. However, for other applications, this may be an issue.
The MESSAGE method [RFC3428] defines one-time instant message
exchange, typically for sending MIME contents for rendering to the
user.
Another model for application level information exchange is to
establish a communication channel in the media plane. One model for
this is MRCPv2 [I-D.ietf-speechsc-mrcpv2]. Here, the INVITE-
initiated dialog establishes a separate reliable, connection-oriented
channel, such as a TCP [RFC0793] or SCTP [RFC4960] stream. One uses
SIP to locate the remote endpoint, but uses a direct connection for
the UUI. One then can create whatever protocol one wishes, whether
from scratch (as in MRCPv2) or using a substrate such as BEEP
[RFC3080].
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
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milliseconds of latency. Also, if there will be a lot of information
exchanged, and there is no need for the SIP routing network to
examine the information, one should use a separate media channel.
Another model is to use a totally externally signaled channel, such
as HTTP [RFC2616]. In this model, the user agent knows about a
rendezvous point to direct HTTP requests to for the transfer of
information. Examples include encoding of a prompt to retrieve in
the SIP Request URI in RFC 4240 [RFC4240] or the encoding of a SUBMIT
target in a VoiceXML [W3C.REC-voicexml21-20070619] script.
MSRP [RFC4975] defines session-based instant messaging as well as
bulk file transfer and other such large-volume uses. It is part of
an INVITE-based session, similar to other media. Unlike INFO, MSRP
follows a direct media path, rather than through the network elements
composing the SIP signaling path.
A common reason people in the past used INFO for application level
information exchange is the negotiation is very lightweight compared
to SUBSCRIBE/NOTIFY. This is more especially so if it is not certain
if there will be application level information exchange. The
SUBSCRIBE/NOTIFY machinery requires the user agents to exchange rich
capabilities and maintain state for additional SIP dialogs. However,
this is a weak argument if there is a high likelihood of application
level information exchange. In this case, we recommend the use of a
more robust application level information exchange protocol.
A.5. Alternatives for Common INFO Use
What alternatives to INFO are there for UA-to-UA application session
signaling? As noted above, there are three broad classes of session
signaling available. The choice depends on the circumstances.
Following is a list of situations that have used INFO in the past.
o State updates
o User stimulus
o Direct signaling channel
o Proxy-aware signaling
o Dialog probe
A.5.1. State Updates
This is the broad class of one User Agent updating another with
changes in state. The design goal of the SUBSCRIBE/NOTIFY [RFC3265]
event framework is to meet just this need.
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A.5.2. User Stimulus: Touch Tones and Others
This is the class of the user entering stimulus at one User Agent,
and the User Agent transporting that stimulus to the other. A key
thing to realize is key presses on the telephone keypad is user
stimulus. Thus, the appropriate mechanism to use here is KPML
[RFC4730].
A.5.3. Direct Signaling Channel
State updates and user stimulus tend to have relatively few messages
per session. Sometimes, User Agents need to exchange a relatively
high number of messages. In addition, User Agents may have a need
for a relatively low-latency exchange of messages. In this latter
case, the User Agent may not be able to tolerate the latency
introduced by intermediate proxies. Likewise, the intermediate
proxies may have no interest in processing all of that data.
In this case, establishing a separate, direct control channel, as in
MSRP [RFC4975] or MRCPv2 [I-D.ietf-speechsc-mrcpv2] is appropriate.
In addition, not every situation requires a SIP solution. Some
signaling is really just one-shot to third-party endpoints. That
situation may better be handled using an appropriate protocol, such
as HTTP [RFC2616].
A.5.4. Proxy-Aware Signaling
Sometimes, one does want proxies to be in the signaling path for UA-
to-UA application signaling. In this case, the use of a SIP request
is appropriate. To date, there are no mechanisms for completely
disambiguating INFO requests. For example, one could create a
registry of INFO packages. The definition of the package would
define the contexts for the various MIME Content-Types, as well as
the context of the request itself. However, a package can have
multiple content types. Moreover, having the context, or package
identifier, at the SIP level precludes bundling multiple contexts
responding in the same INFO request. For example, a User Agent might
want to bundle two different responses in a multipart/mixed MIME body
type.
Because there is no difference in either the protocol machinery or
registration process due to these factors, we will not create an INFO
framework. If one needs a SIP User Agent-to-SIP User Agent
application session signaling transport protocol that touches all
Record-Route proxies in a path, one MUST create a new SIP method as
described in Section 27.4 of RFC 3261 [RFC3261].
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A.5.5. Dialog Probe
Some implementations in the wild use INFO to probe if an INVITE-
initiated dialog is alive. While this works, it is NOT RECOMMENDED.
In particular, RFC 4028 [RFC4028] describes how to ensure an INVITE-
initiated dialog is alive.
A.5.6. Malicious Indicator
Take the case of Malicious Indicator. This is where a subscriber
receives a call, realizes it is a malicious call (threatening, SPIT,
etc.). They then press the SPIT button (or press *xx), which tells
their service provider to mark the UAC as a bad actor. One might be
tempted to think that INFO would be a great option for this service.
It follows the return path of the INVITE, and so the INFO will hit
the caller's inbound proxy, which it can learn the caller is
(statistically) a bad actor. That way the inbound proxy can do stuff
like notify law enforcement, add a vote to "this is a SPIT source,"
or other useful action.
However, consider a few issues. First, since INFO lives exclusively
within an established dialog, there is no way to assert this message
after the call completes. Second, this mechanism relies on an active
service provider topology. If there is no proxy in the chain that
will eat the INFO, the caller will see the "this is a bad guy"
message, which may have consequences in the real world. Third, there
is no a'priori way for the UAS to know whether or not it can issue
the INFO. The caller certainly will not advertise, "please tell me
if I am bad, particularly I know in advance that I *am* a bad actor."
One approach is for the service provider's proxy to SUBSCRIBE for the
SPIT event at the UAS. At this point, life is good, interoperable,
and works across networks. This enables events after the dialog is
torn down, as presumably the SPIT event will refer not to, "this
dialog," which does not exist, but to "that dialog identifier," which
exists (and is theoretically unique) forever.
Another approach that saves considerably on the overhead of
subscriptions would be for the service provider to insert a HTTP URI
in the initial INVITE, noting it is for reporting malicious behavior.
When the subscriber presses the SPIT button, an HTTP POST gets
executed, delivering the call information to the service provider.
The service provider can encode basic call information in the HTTP
URI and can instruct the device to send whatever arbitrary data is
necessary in the POST. This method has the added benefit of being
entirely outside the real-time SIP proxy network.
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Appendix B. Legacy INFO Usages
We do not intend this section to be a comprehensive catalog of INFO
usages. However, it should give the reader a flavor for current INFO
usages.
B.1. ISUP
SIP-T uses Content-Type to identify ISUP protocol elements in an INFO
message. See RFC3372 [RFC3372].
B.2. QSIG
QSIG uses Content-Type to identify QSIG protocol elements in an INFO
message. See RFC4497 [RFC4497].
B.3. MSCML
MSCML uses a Require to ensure the UAS understands that INFO messages
of the MSCML type are in fact MSCML messages. See RFC5022 [RFC5022].
B.4. MSML
MSML endpoints just know the INFO messages carry MSML and from the
Content-Type of the given INFO method request. See the MSML
[I-D.saleem-msml] draft.
B.5. Video Fast Update
Microsoft, Polycom, and Radvision used INFO messages as an interim
solution for requesting fast video update before the ability to
request I-Frames in RTCP was available. See the XML Schema for Media
Control [RFC5168] for more information.
B.6. DTMF
[EDITOR'S NOTE: Are there public references? The AS5300
documentation from Cisco describes Cisco's use of INFO to carry DTMF.
Anyone else want to belly up to the bar and have us collect your
proprietary DTMF INFO payload here?]
Appendix C. Acknowledgements
We are standing on the shoulders of giants. Jonathan Rosenberg did
the original "INFO Considered Harmful" Internet Draft on 26 December
2002, which influenced the work group and this document. Likewise,
Dean Willis influenced the text from his Internet Draft, "Packaging
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and Negotiation of INFO Methods for the Session Initiation Protocol"
of 15 January 2003. Four paragraphs come from Jonathan Rosenberg's
INFO Litmus draft. My, we have been working on this for a long time!
This and other related drafts have elicited well over 450 messages on
the SIP list. People who have argued with its thesis, supported its
thesis, added to the examples, or argued with the examples, include
the following individuals:
Adam Roach, Bram Verburg, Brian Stucker, Chris Boulton, Cullen
Jennings, Dale Worley, Dean Willis, Frank Miller, Gonzalo
Camarillo, Gordon Beith, Henry Sinnreich, James Jackson, James
Rafferty, Jeroen van Bemmel, Joel Halpern, John Elwell, Johnathan
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, Salvatore Loreto, Sam Ganesan,
Sanjay Sinha, Spencer Dawkins, Steve Langstaff, Sumit Garg, and
Xavier Marjou.
John Elwell and Francois Audet helped with QSIG references. In
addition, Francois Audet provided actual text for the revised
abstract. Keith Drage gave lots of excellent comments and helped
immensely with Figure 1.
The work group version of this document benefited from the close
readings and comments from
John Elwell, Paul Kyzivat, Dean Willis, Francois Audet, Dale
Worley, Andrew Allen, Adam Roach, Anders Kristensen, Gordon Beith,
Ben Campbell, Bob Penfield, Keith Drage, Jeroen van Bemmel, Mary
Barnes, and Salvatore Loreto.
Since publication of the first work group version of this document,
we have had over 329 messages. New voices in addition to those
included above include
Arun Arunachalam, Christian Stredicke, Eric Rescorla, Inaki Baz
Castillo, and Roni Evan.
However, any errors and issues we missed are still our own.
Appendix D. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing]
Changes from -01
o One and only one Info Package per INFO
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o Removed Send-Info header, greatly simplifying negotiation
o Multiple body part identification through Content-Disposition:
Info-Package
o Note that forking INVITEs may result in multiple INFO's coming
back to INVITE originator
o Describe how a UAS can enforce strict adherence to this document
o Remove CANCEL INFO faux pas
o Better explained overlapping INFO issues and resolutions
o Token names are now really case sensitive
o Moved Info Package Considerations to an Appendix
o Introduced stronger, yet more open, IANA registration process
o Took a few more paragraphs from INFO Litmus to cover all bases.
o Added RFC 5168 to legacy usages
Changes from -00
o Corrected ABNF.
o Enabled sending of legacy INFO messages. Receiving legacy INFO
messages was already here.
o Negotiation is not Offer/Answer, it is Offer/Offer.
o Created the explicit "nil" Info Package to indicate no info
package.
o Fixed CANCEL impacting future transactions.
o Added Registrar behavior.
o Added OPTIONS processing.
o Clarified overlapping INFO method processing.
o Described multiple INFO bodies in a single INFO method.
o Took out Info-Package as a header for responses to the INFO
method.
o Expanded on risks of using INFO and filled-in more on the
alternatives
o Moved definitions of INFO into the body of the text and cleaned up
IANA Considerations section
o Added legacy usages descriptions
Authors' Addresses
Eric W. Burger
This Space For Sale
USA
Email: eburger@standardstrack.com
URI: http://www.standardstrack.com
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Hadriel Kaplan
Acme Packet
71 Third Ave.
Burlington, MA 01803
USA
Phone:
Fax:
Email: hkaplan@acmepacket.com
URI:
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas, 02420
Finland
Phone:
Fax:
Email: christer.holmberg@ericsson.com
URI:
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