Network Working Group B. Campbell, Ed.
Internet-Draft Estacado Systems
Expires: August 29, 2006 R. Mahy, Ed.
SIP Edge, LLC
C. Jennings, Ed.
Cisco Systems, Inc.
February 25, 2006
The Message Session Relay Protocol
draft-ietf-simple-message-sessions-14
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes the Message Session Relay Protocol, a
protocol for transmitting a series of related instant messages in the
context of a session. Message sessions are treated like any other
media stream when set up via a rendezvous or session creation
protocol such as the Session Initiation Protocol.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Applicability of MSRP . . . . . . . . . . . . . . . . . . . . 5
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
5. Key Concepts . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. MSRP Framing and Message Chunking . . . . . . . . . . . . 9
5.2. MSRP Addressing . . . . . . . . . . . . . . . . . . . . . 10
5.3. MSRP Transaction and Report Model . . . . . . . . . . . . 10
5.4. MSRP Connection Model . . . . . . . . . . . . . . . . . . 12
6. MSRP URLs . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. MSRP URL Comparison . . . . . . . . . . . . . . . . . . . 15
6.2. Resolving MSRP Host Device . . . . . . . . . . . . . . . 15
7. Method-Specific Behavior . . . . . . . . . . . . . . . . . . . 16
7.1. Constructing Requests . . . . . . . . . . . . . . . . . . 16
7.1.1. Sending SEND Requests . . . . . . . . . . . . . . . . 18
7.1.2. Sending REPORT Requests . . . . . . . . . . . . . . . 20
7.1.3. Generating Success Reports . . . . . . . . . . . . . . 21
7.1.4. Generating Failure Reports . . . . . . . . . . . . . . 21
7.2. Constructing Responses . . . . . . . . . . . . . . . . . 22
7.3. Receiving Requests . . . . . . . . . . . . . . . . . . . 23
7.3.1. Receiving SEND Requests . . . . . . . . . . . . . . . 23
7.3.2. Receiving REPORT Requests . . . . . . . . . . . . . . 25
8. Using MSRP with SIP and SDP . . . . . . . . . . . . . . . . . 26
8.1. SDP Connection and Media Lines . . . . . . . . . . . . . 27
8.2. URL Negotiations . . . . . . . . . . . . . . . . . . . . 27
8.3. Path Attributes with Multiple URLs . . . . . . . . . . . 28
8.4. Updated SDP Offers . . . . . . . . . . . . . . . . . . . 29
8.5. Connection Negotiation . . . . . . . . . . . . . . . . . 30
8.6. Content Type Negotiation . . . . . . . . . . . . . . . . 30
8.7. Example SDP Exchange . . . . . . . . . . . . . . . . . . 32
8.8. MSRP User Experience with SIP . . . . . . . . . . . . . . 32
9. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 33
10. Response Code Descriptions . . . . . . . . . . . . . . . . . . 35
10.1. 200 . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10.2. 400 . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10.3. 403 . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10.4. 408 . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10.5. 413 . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10.6. 415 . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10.7. 423 . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
10.8. 481 . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
10.9. 501 . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
10.10. 506 . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
11.1. Basic IM Session . . . . . . . . . . . . . . . . . . . . 37
11.2. Message with XHTML Content . . . . . . . . . . . . . . . 40
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11.3. Chunked Message . . . . . . . . . . . . . . . . . . . . . 40
11.4. System Message . . . . . . . . . . . . . . . . . . . . . 40
11.5. Positive Report . . . . . . . . . . . . . . . . . . . . . 41
11.6. Forked IM . . . . . . . . . . . . . . . . . . . . . . . . 41
12. Extensibility . . . . . . . . . . . . . . . . . . . . . . . . 45
13. CPIM Compatibility . . . . . . . . . . . . . . . . . . . . . . 45
14. Security Considerations . . . . . . . . . . . . . . . . . . . 46
14.1. Transport Level Protection . . . . . . . . . . . . . . . 46
14.2. S/MIME . . . . . . . . . . . . . . . . . . . . . . . . . 48
14.3. Using TLS in Peer to Peer Mode . . . . . . . . . . . . . 48
14.4. Other Security Concerns . . . . . . . . . . . . . . . . . 50
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 51
15.1. MSRP Method Names . . . . . . . . . . . . . . . . . . . . 52
15.2. MSRP Header Fields . . . . . . . . . . . . . . . . . . . 52
15.3. MSRP Status Codes . . . . . . . . . . . . . . . . . . . . 52
15.4. MSRP Port . . . . . . . . . . . . . . . . . . . . . . . . 53
15.5. MSRP URL Schemes . . . . . . . . . . . . . . . . . . . . 53
15.6. SDP Transport Protocol . . . . . . . . . . . . . . . . . 53
15.7. SDP Attribute Names . . . . . . . . . . . . . . . . . . . 53
15.7.1. Accept Types . . . . . . . . . . . . . . . . . . . . . 53
15.7.2. Wrapped Types . . . . . . . . . . . . . . . . . . . . 54
15.7.3. Max Size . . . . . . . . . . . . . . . . . . . . . . . 54
15.7.4. Path . . . . . . . . . . . . . . . . . . . . . . . . . 54
16. Contributors and Acknowledgments . . . . . . . . . . . . . . . 54
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 55
17.1. Normative References . . . . . . . . . . . . . . . . . . 55
17.2. Informational References . . . . . . . . . . . . . . . . 56
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 58
Intellectual Property and Copyright Statements . . . . . . . . . . 59
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1. Introduction
A series of related instant messages between two or more parties can
be viewed as part of a "message session", that is, a conversational
exchange of messages with a definite beginning and end. This is in
contrast to individual messages each sent independently. Messaging
schemes that track only individual messages can be described as
"page-mode" messaging, whereas messaging that is part of a "session"
with a definite start and end is called "session-mode" messaging.
Page-mode messaging is enabled in SIP via the SIP [4] MESSAGE method
[21]. Session-mode messaging has a number of benefits over page-mode
messaging, however, such as explicit rendezvous, tighter integration
with other media types, direct client-to-client operation, and
brokered privacy and security.
This document defines a session-oriented instant message transport
protocol called the Message Session Relay Protocol (MSRP), whose
sessions can be negotiated with an offer or answer [3] using the
Session Description Protocol(SDP [2]). The exchange is carried by
some signaling protocol, such as the Session Initiation Protocol (SIP
[4]). This allows a communication user agent to offer a messaging
session as one of the possible media types in a session. For
instance, Alice may want to communicate with Bob. Alice doesn't know
at the moment whether Bob has his phone or his IM client handy, but
she's willing to use either. She sends an invitation to a session to
the address of record she has for Bob, sip:bob@example.com. Her
invitation offers both voice and an IM session. The SIP services at
example.com forward the invitation to Bob at his currently registered
clients. Bob accepts the invitation at his IM client and they begin
a threaded chat conversation.
When a user uses an IM URL, RFC 3861 [31] defines how DNS can be used
to map this to a particular protocol to establish the session such as
SIP. SIP can use an offer answer model to transport the MSRP URLs
for the media in SDP. This document defines how the offer/answer
exchange works to establish MSRP connections and how messages are
sent across the MSRP protocol, but it does not deal with the issues
of mapping an IM URL to a session establishment protocol.
This session model allows message sessions to be integrated into
advanced communications applications with little to no additional
protocol development. For example, during the above chat session,
Bob decides Alice really needs to be talking to Carol. Bob can
transfer [20] Alice to Carol, introducing them into their own
messaging session. Messaging sessions can then be easily integrated
into call-center and dispatch environments using third-party call
control [19] and conferencing [18] applications.
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This document specifies MSRP behavior only for peer-to-peer sessions,
that is, sessions crossing only a single hop. MSRP relay devices
[22] (referred to herein as "relays") are specified in a separate
document.
2. Conventions
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 [5].
This document consistently refers to a "message" as a complete unit
of MIME or text content. In some cases, a message is split and
delivered in more than one MSRP request. Each of these portions of
the complete message is called a "chunk".
3. Applicability of MSRP
MSRP is not designed for use as a standalone protocol. MSRP MUST be
used only in the context of a rendezvous mechanism meeting the
following requirements:
The rendezvous mechanism MUST provide both MSRP URLs associated
with an MSRP session to each of the participating endpoints. The
rendezvous mechanism MUST implement mechanisms to provide these
URLs securely - they MUST NOT be made available to an untrusted
third party or be easily discoverable.
The rendezvous mechanism MUST provide mechanisms for the
negotiation of any supported MSRP extensions that are not
backwards compatible.
The rendezvous mechanism MUST be able to natively transport im:
URIs or automatically translate im: URIs [26] into the addressing
identifiers of the rendezvous protocol.
To use a rendezvous mechanism with MSRP, an RFC must be prepared
describing how it exchanges MSRP URLs and meets these requirements
listed here. This document provides such a description for the use
of MSRP in the context of SIP and SDP.
SIP meets these requirements for a rendezvous mechanism. The MSRP
URLs are exchanged using SDP in an offer/answer exchange via SIP.
The exchanged SDP can also be used to negotiate MSRP extensions.
This SDP can be secured using any of the mechanisms available in SIP,
including using the sips mechanism to ensure transport security
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across intermediaries and S/MIME for end-to-end protection of the SDP
entity. SIP can carry arbitrary URIs (including im: URIs) in the
Request-URI, and procedures are available to map im: URIs to sip: or
sips: URIs. It is expected that initial deployments of MSRP will use
SIP as its rendezvous mechanism.
4. Protocol Overview
MSRP is a text-based, connection-oriented protocol for exchanging
arbitrary (binary) MIME[8] content, especially instant messages.
This section is a non-normative overview of how MSRP works and how it
is used with SIP.
MSRP sessions are typically arranged using SIP the same way a session
of audio or video media is set up. One SIP user agent (Alice) sends
the other (Bob) a SIP invitation containing an offered session-
description which includes a session of MSRP. The receiving SIP user
agent can accept the invitation and include an answer session-
description which acknowledges the choice of media. Alice's session
description contains an MSRP URL that describes where she is willing
to receive MSRP requests from Bob, and vice-versa. (Note: Some lines
in the examples are removed for clarity and brevity.)
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Alice sends to Bob:
INVITE sip:bob@atlanta.example.com SIP/2.0
To: <sip:bob@biloxi.example.com>
From: <sip:alice@atlanta.example.com>;tag=786
Call-ID: 3413an89KU
Content-Type: application/sdp
c=IN IP4 atlanta.example.com
m=message 7654 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://atlanta.example.com:7654/jshA7we;tcp
Bob sends to Alice:
SIP/2.0 200 OK
To: <sip:bob@biloxi.example.com>;tag=087js
From: <sip:alice@atlanta.example.com>;tag=786
Call-ID: 3413an89KU
Content-Type: application/sdp
c=IN IP4 biloxi.example.com
m=message 12763 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://biloxi.example.com:12763/kjhd37s2s2;tcp
Alice sends to Bob:
ACK sip:bob@atlanta.example.com SIP/2.0
To: <sip:bob@biloxi.example.com>;tag=087js
From: <sip:alice@atlanta.example.com>;tag=786
Call-ID: 3413an89KU
MSRP defines two request types, or methods. SEND requests are used
to deliver a complete message or a chunk (a portion of a complete
message), while REPORT requests report on the status of a previously
sent message, or a range of bytes inside a message. When Alice
receives Bob's answer, she checks to see if she has an existing
connection to Bob. If not, she opens a new connection to Bob using
the URL he provided in the SDP. Alice then delivers a SEND request
to Bob with her initial message, and Bob replies indicating that
Alice's request was received successfully.
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MSRP a786hjs2 SEND
To-Path: msrp://biloxi.example.com:12763/kjhd37s2s2;tcp
From-Path: msrp://atlanta.example.com:7654/jshA7we;tcp
Message-ID: 87652
Byte-Range: 1-25/25
Content-Type: text/plain
Hey Bob, are you there?
-------a786hjs2$
MSRP a786hjs2 200 OK
To-Path: msrp://atlanta.example.com:7654/jshA7we;tcp
From-Path: msrp://biloxi.example.com:12763/kjhd37s2s2;tcp
Byte-Range: 1-25/25
-------a786hjs2$
Alice's request begins with the MSRP start line, which contains a
transaction identifier that is also used for request framing. Next
she includes the path of URLs to the destination in the To-Path
header field, and her own URL in the From-Path header field. In this
typical case there is just one "hop", so there is only one URL in
each path header field. She also includes a message ID which she can
use to correlate status reports with the original message. Next she
puts the actual content. Finally she closes the request with an end-
line of seven hyphens, the transaction identifier and a "$" to
indicate this request contains the end of a complete message.
If Alice wants to deliver a very large message, she can split the
message into chunks and deliver each chunk in a separate SEND
request. The message ID corresponds to the whole message, so the
receiver can also use it to reassemble the message and tell which
chunks belong with which message. Chunking is described in more
detail in Section 5.1. The Byte-Range header field identifies the
portion of the message carried in this chunk and the total size of
the message.
Alice can also specify what type of reporting she would like in
response to her request. If Alice requests positive acknowledgments,
Bob sends a REPORT request to Alice confirming the delivery of her
complete message. This is especially useful if Alice sent a series
of SEND request containing chunks of a single message. More on
requesting types of reports and errors is described in Section 5.3.
Alice and Bob generally choose their MSRP URLs in such a way that is
difficult to guess the exact URL. Alice and Bob can reject requests
to URLs they are not expecting to service, and can correlate the
specific URL with the probable sender. Alice and Bob can also use
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TLS [1] to provide channel security over this hop. To receive MSRP
requests over a TLS protected connection, Alice or Bob could
advertise URLs with the "msrps" scheme instead of "msrp."
MSRP is designed with the expectation that MSRP can carry URLs for
nodes on the far side of relays. For this reason, a URL with the
"msrps" scheme makes no assertion about the security properties of
other hops, just the next hop. The user agent knows the URL for each
hop, so it can verify that each URL has the desired security
properties.
MSRP URLs are discussed in more detail in Section 6.
An adjacent pair of busy MSRP nodes (for example two relays) can
easily have several sessions, and exchange traffic for several
simultaneous users. The nodes can use existing connections to carry
new traffic with the same destination host, port, transport protocol,
and scheme. MSRP nodes can keep track of how many sessions are using
a particular connection and close these connections when no sessions
have used them for some period of time. Connection management is
discussed in more detail in Section 5.4.
5. Key Concepts
5.1. MSRP Framing and Message Chunking
Messages sent using MSRP can be very large and can be delivered in
several SEND requests, where each SEND request contains one chunk of
the overall message. Long chunks may be interrupted in mid-
transmission to ensure fairness across shared transport connections.
To support this, MSRP uses a boundary based framing mechanism. The
start line of an MSRP request contains a unique identifier that is
also used to indicate the end of the request. Included at the end of
the end-line, there is a flag that indicates whether this is the last
chunk of data for this message or whether the message will be
continued in a subsequent chunk. There is also a Byte-Range header
field in the request that indicates the overall position of this
chunk inside the complete message.
For example, the following snippet of two SEND requests demonstrates
a message that contains the text "abcdEFGH" being sent as two chunks.
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MSRP dkei38sd SEND
Message-ID: 456
Byte-Range: 1-4/8
Content-Type: text/plain
abcd
-------dkei38sd+
MSRP dkei38ia SEND
Message-ID: 456
Byte-Range: 5-8/8
Content-Type: text/plain
EFGH
-------dkei38ia$
This chunking mechanism allows a sender to interrupt a chunk part of
the way through sending it. The ability to interrupt messages allows
multiple sessions to share a TCP connection, and for large messages
to be sent efficiently while not blocking other messages that share
the same connection, or even the same MSRP session. Any chunk that
is larger than 2048 octets MUST be interruptible. While MSRP would
be simpler to implement if each MSRP session used its own TCP
connection, that approach would circumvent the congestion avoidance
features of TCP.
The chunking mechanism only applies to the SEND method, as it is the
only method used to transfer message content.
5.2. MSRP Addressing
MSRP entities are addressed using URLs. The MSRP URL schemes are
defined in Section 6. The syntax of the To-Path and From-Path header
fields each allow for a list of URLs. This was done to allow the
protocol to work with relays, which are defined in a separate
document, to provide a complete path to the end recipient. When two
MSRP nodes communicate directly they need only one URL in the To-Path
list and one URL in the From-Path list.
5.3. MSRP Transaction and Report Model
A sender sends MSRP requests to a receiver. The receiver MUST
quickly accept or reject the request. If the receiver initially
accepted the request, it still may then do things that take
significant time to succeed or fail. For example, if the receiver is
an MSRP to XMPP [29] gateway, it may forward the message over XMPP.
The XMPP side may later indicate that the request did not work. At
this point, the MSRP receiver may need to indicate that the request
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did not succeed. There are two important concepts here: first, the
hop by hop delivery of the request may succeed or fail; second, the
end result of the request may be successfully processed or not. The
first type of status is referred to as "transaction status" and may
be returned in response to a request. The second type of status is
referred to as "delivery status" and may be returned in a REPORT
transaction.
The original sender of a request can indicate if they wish to receive
reports for requests that fail, and can independently indicate if
they wish to receive reports for requests that succeed. A receiver
only sends a success REPORT if it knows that the request was
successfully delivered, and the sender requested a success report. A
receiver only sends a failure REPORT if the request failed to be
delivered and the sender requested failure reports.
This document describes the behavior of MSRP endpoints. MSRP
relays will introduce additional conditions that indicate a
failure REPORT should be sent, such as the failure to receive a
positive response from the next hop.
Two header fields control the sender's desire to receive reports.
The header field "Success-Report" can have a value of "yes" or "no"
and the "Failure-Report" header field can have a value of "yes",
"no", or "partial".
The combinations of reporting are needed to meet the various
scenarios of currently deployed IM systems. Success-Report might be
"no" in many public systems to reduce load but might be "yes" in
certain enterprise systems, such as systems used for securities
trading. A Failure-Report value of "no" is useful for sending system
messages such as "the system is going down in 5 minutes" without
causing a response explosion to the sender. A Failure-Report of
"yes" is used by many systems that wish to notify the user if the
message failed. A Failure-Report of "partial" is a way to report
errors other than timeouts. The timeout error reporting requires the
sending hop to run a timer and the receiving hop to send an
acknowledgment to stop the timer. Some systems don't want the
overhead of doing this. "Partial" allows them to choose not to do
so, but still allows error responses to be sent in many cases.
The "partial" value allows a compromise between no reporting of
failures, and reporting all failures. For example, with
"partial", an sending device does not have to keep transaction
state around waiting for a positive acknowledgment. But it still
allows devices to report other types of errors. For example, the
receiving device could still report a policy violation such as an
unacceptable content-type, or an ICMP error trying to connect to a
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downstream device.
5.4. MSRP Connection Model
When an MSRP endpoint wishes to send a request to a peer identified
by an MSRP URL, it first needs a transport connection, with the
appropriate security properties, to the host specified in the URL.
If the sender already has such a connection, that is, one associated
with the same host, port, and URL scheme, then it SHOULD reuse that
connection.
When a new MSRP session is created, the offerer MUST act as the
"active" endpoint, meaning that it is responsible for opening the
transport connection to the answerer, if a new connection is
required. However, this requirement MAY be weakened if standardized
mechanisms for negotiating the connection direction become available,
and is implemented by both parties to the connection.
Likewise, the active endpoint MUST immediately issue a SEND request.
This initial SEND request MAY have a body if the sender has content
to send, or it MAY have no body at all.
The first SEND request serves to bind a connection to an MSRP
session from the perspective of the passive endpoint. If the
connection is not authenticated with TLS, and the active endpoint
did not send an immediate request, the passive endpoint would have
no way to determine who had connected, and would not be able to
safely send any requests towards the active party until after the
active party sends its first request.
When an element needs to form a new connection, it looks at the URL
to decide on the type of connection (TLS, TCP, etc.) then connects to
the host indicated by the URL, following the URL resolution rules in
Section 6.2. Connections using the msrps: scheme MUST use TLS. The
SubjectAltName in the received certificate MUST match the hostname
part of the URL and the certificate MUST be valid, including having a
date that is valid and being signed by an acceptable certificate
authority. At this point the device that initiated the connection
can assume that this connection is with the correct host.
If the connection used mutual TLS authentication, and the TLS client
presented a valid certificate, then the element accepting the
connection can immediately know the identity of the connecting host.
When mutual TLS authentication is not used, the listening device MUST
wait until it receives a request on the connection, at which time it
infers the identity of the connecting device from the associated
session description.
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When the first request arrives, its To-Path header field should
contain a URL that the listening element provided in the SDP for a
session. The element that accepted the connection looks up the URL
in the received request, and determines which session it matches. If
a match exists, the node MUST assume that the host that formed the
connection is the host to which this URL was given. If no match
exists, the node MUST reject the request with a 481 response. The
node MUST also check to make sure the session is not already in use
on another connection. If the session is already in use, it MUST
reject the request with a 506 response.
If it were legal to have multiple connections associated with the
same session, a security problem would exist. If the initial SEND
request is not protected, an eavesdropper might learn the URL, and
use it to insert messages into the session via a different
connection.
If a connection fails for any reason, then an MSRP endpoint MUST
consider any sessions associated with the connection as also having
failed. When either endpoint notices such a failure, it MAY attempt
to re-create any such sessions. If it chooses to do so, it MUST use
a new SDP exchange, for example, in a SIP re-INVITE . If a
replacement session is successfully created, endpoints MAY attempt to
resend any content for which delivery on the original session could
not be confirmed. If it does this, the Message-ID values for the
resent messages MUST match those used in the initial attempts. If
the receiving endpoint receives more than one message with the same
Message-ID, it SHOULD assume that the messages are duplicates. The
specific action that an endpoint takes when it receives a duplicate
message is a matter of local policy, except that it SHOULD NOT
present the duplicate messages to the user without warning of the
duplication. Note that acknowledgments as needed based on the
Failure-Report and Success-Report settings are still necessary even
for requests containing duplicate content.
When endpoints create a new session in this fashion, the chunks for a
given logical message MAY be split across the sessions. However,
endpoints SHOULD NOT split chunks between sessions under non-failure
circumstances.
If an endpoint attempts to re-create a failed session in this manner,
it MUST NOT assume that the MSRP URLs in the SDP will be the same as
the old ones.
A connection SHOULD NOT be closed while there are sessions associated
with it.
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6. MSRP URLs
URLs using the MSRP and MSRPS schema are used to identify a session
of instant messages at a particular MSRP device. MSRP URLs are
ephemeral; an MSRP device will generally use a different MSRP URL for
each distinct session. An MSRP URL generally has no meaning outside
of the associated session.
An MSRP URL follows a subset of the URL syntax in Appendix A of
RFC3986 [10], with a scheme of "msrp" or "msrps". The syntax is
described in Section 9.
The constructions for "userinfo", and "unreserved" are detailed in
RFC3986 [10]. In order to allow IPV6 addressing, the construction
for hostport is that used for SIP in RFC3261. URLs designating MSRP
over TCP MUST include the "tcp" transport parameter.
Since this document only specifies MSRP over TCP, all MSRP URLs
herein use the "tcp" transport parameter. Documents that provide
bindings on other transports should define respective parameters
for those transports.
An MSRP URL hostport field identifies a participant in a particular
MSRP session. If the hostport contains a numeric IP address, it MUST
also contain a port. The session-id part identifies a particular
session of the participant. The absence of the session-id part
indicates a reference to an MSRP host device, but does not
specifically refer to a particular session.
A scheme of "msrps" indicates the underlying connection MUST be
protected with TLS.
MSRP has an IANA registered recommended port defined in Section 15.4.
This value is not a default, as the URL negotiation process described
herein will always include explicit port numbers. However, the URLs
SHOULD be configured so that the recommended port is used whenever
appropriate. This makes life easier for network administrators who
need to manage firewall policy for MSRP.
The hostport will typically not contain a userinfo component, but MAY
do so to indicate a user account for which the session is valid.
Note that this is not the same thing as identifying the session
itself. If a userinfo component exists, it MUST be constructed only
from "unreserved" characters, to avoid a need for escape processing.
Escaping MUST NOT be used in an MSRP URL. Furthermore, a userinfo
part MUST NOT contain password information.
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The limitation of userinfo to unreserved characters is an
additional restriction to the userinfo definition in RFC3986.
That version allows reserved characters. The additional
restriction is to avoid the need for escaping.
The following is an example of a typical MSRP URL:
msrp://host.example.com:8493/asfd34;tcp
6.1. MSRP URL Comparison
MSRP URL comparisons MUST be performed according to the following
rules:
1. The scheme must match. Scheme comparison is case insensitive.
2. If the hostpart contains an explicit IP address, and/or port,
these are compared for address and port equivalence. Otherwise,
hostpart is compared as a case insensitive character string.
3. If the port exists explicitly in either URL, then it must match
exactly. A URL with an explicit port is never equivalent to
another with no port specified.
4. The session-id part is compared as case sensitive. A URL without
a session-id part is never equivalent to one that includes one.
5. URLs with different "transport" parameters never match. Two URLs
that are identical except for transport are not equivalent. The
transport parameter is case-insensitive.
6. Userinfo parts are not considered for URL comparison.
Path normalization is not relevant for MSRP URLs. Escape
normalization is not required due to character restrictions in the
formal syntax.
6.2. Resolving MSRP Host Device
An MSRP host device is identified by the hostport of an MSRP URL.
If the hostport contains a numeric IP address and port, they MUST be
used as listed.
If the hostport contains a host name and a port, the connecting
device MUST determine a host address by doing an A or AAAA DNS query,
and use the port as listed.
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If a connection attempt fails, the device SHOULD attempt to connect
to the addresses returned in any additional A or AAAA records, in the
order the records were presented.
This process assumes that the connection port is always known
prior to resolution. This is always true for the MSRP URL uses
described in this document, that is, URLs exchanged in the SDP
offer and answer. The introduction of relays may create
situations where this is not the case. For example, the MSRP URL
that a user enters into a client to configure it to use a relay
may be intended to be easily remembered and communicated by
humans, and therefore is likely to omit the port. Therefore, the
relay specification [22] may describe additional steps to resolve
the port number.
MSRP devices MAY use other methods for discovering other such
devices, when appropriate. For example, MSRP endpoints may use other
mechanisms to discover relays, which are beyond the scope of this
document.
7. Method-Specific Behavior
7.1. Constructing Requests
To form a new request, the sender creates a unique transaction
identifier and uses this and the method name to create an MSRP
request start line. Next, the sender places the target URL in a To-
Path header field, and the sender's URL in a From-Path header field.
If multiple URLs are present in the To-Path, the leftmost is the
first URL visited; the rightmost URL is the last URL visited. The
processing then becomes method specific. Additional method-specific
header fields are added as described in the following sections.
After any method-specific header fields are added, processing
continues to handle a body, if present. If the request has a body,
it must contain a Content-Type header field. It may contain other
MIME-specific header fields. The Content-Type header field MUST be
the last field in the message header section. The body MUST be
separated from the header fields with an extra CRLF.
Non-SEND requests are not intended to carry message content, and are
therefore not interruptible. Non-SEND request bodies MUST NOT be
larger than 10240 octets.
Although this document does not discuss any particular usage of
bodies in non-SEND requests, they may be useful in the future for
carrying security or identity information, information about a
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message in progress, etc. The 10K size limit was chosen to be
large enough for most of such applications, but small enough to
avoid the fairness issues caused by sending arbitrarily large
content in non-interruptible method bodies.
A request with no body MUST NOT include a Content-Type header field.
Note that, if no body is present, no extra CRLF will be present
between the header section and the end-line.
Requests with no bodies are useful when a client wishes to send
"traffic", but does not wish to send content to be rendered to the
peer user. For example, the offerer must send a SEND request
immediately upon establishing a connection. If it has nothing to
say at the moment, it can send a request with no body. Bodiless
requests may also be used in certain applications to keep NAT
bindings alive, etc.
Bodiless requests are distinct from requests with empty bodies. A
request with an empty body will have a Content-Type header field
value, and will generally be rendered to the recipient according
to the rules for that type.
The end-line that terminates the request MUST be composed of seven
"-" (minus sign) characters, the transaction ID as used in the start
line, and a flag character. If a body is present, the end-line must
be preceded by a CRLF that is not part of the body. If the chunk
represents the data that forms the end of the complete message, the
flag value MUST be a "$". If sender is aborting an incomplete
message, and intends to send no further chunks in that message, it
MUST be a "#". Otherwise it MUST be a "+".
If the request contains a body, the sender MUST ensure that the end-
line (seven hyphens, the transaction identifier, and a continuation
flag) is not present in the body. If the end-line is present in the
body, the sender MUST choose a new transaction identifier that is not
present in the body, and add a CRLF if needed, and the end-line,
including the "$", "#", or "+" character.
Some implementations may choose to scan for the closing sequence as
they send the body, and if it is encountered, simply interrupt the
chunk at that point and start a new transaction with a different
transaction identifier to carry the rest of the body. Other
implementation may choose to scan the data an ensure that the body
does not contain the transaction identifier before they start sending
the transaction.
Finally, requests which have no body MUST NOT contain a Content-Type
header field or any other MIME specific header field. Requests
without bodies MUST contain a end-line after the final header field.
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Once a request is ready for delivery, the sender follows the
connection management (Section 5.4) rules to forward the request over
an existing open connection or create a new connection.
7.1.1. Sending SEND Requests
When an endpoint has a message to deliver, it first generates a new
Message-ID. This ID MUST be globally unique. If necessary, it
breaks the message into chunks. It then generates a SEND request for
each chunk, following the procedures for constructing requests
(Section 7.1).
The Message-ID header field provides a globally unique message
identifier that refers to a particular version of a particular
message. The term "Message" in this context refers to a unit of
content that the sender wishes to convey to the recipient. While
such a message may be broken into chunks, the Message-ID refers to
the entire message, not a chunk of the message.
The uniqueness of the message identifier is guaranteed by the host
that generates it. This message identifier is intended to be
machine readable and not necessarily meaningful to humans. A
message identifier pertains to exactly one version of a particular
message; subsequent revisions to the message each receive new
message identifiers.
Each chunk of a message MUST contain a Message-ID header field
containing the Message-ID. If the sender wishes non-default status
reporting, it MUST insert a Failure-Report and/or Success-Report
header field with an appropriate value. All chunks of the same
message MUST use the same Failure-Report and Success-Report values in
their SEND requests.
If success reports are requested, i.e. the value of the Success-
Report header field is "yes", the sending device MAY wish to run a
timer of some value that makes sense for its application and take
action if a success Report is not received in this time. There is no
universal value for this timer. For many IM applications, it may be
2 minutes while for some trading systems it may be under a second.
Regardless of whether such a timer is used, if the success report has
not been received by the time the session is ended, the device SHOULD
inform the user.
If the value of "Failure-Report" is set to "yes", then the sender of
the request runs a timer. If a 200 response to the transaction is
not received within 30 seconds from the time the last byte of the
transaction is sent, or submitted to the operating system for
sending, the element MUST inform the user that the request probably
failed. If the value is set to "partial", then the element sending
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the transaction does not have to run a timer, but MUST inform the
user if it receives a non-recoverable error response to the
transaction.
If no Success-Report header field is present in a SEND request, it
MUST be treated the same as a Success-Report header field with value
of "no". If no Failure-Report header field is present, it MUST be
treated the same as a Failure-Report header field with value of
"yes". If an MSRP endpoint receives a REPORT for a Message-ID it
does not recognize, it SHOULD silently ignore the REPORT.
Success-Report and Failure-Report header fields MUST NOT be present
in REPORT requests. MSRP nodes MUST NOT send REPORT requests in
response to REPORT requests. MSRP Nodes MUST NOT send MSRP responses
to REPORT requests.
The Byte-Range header field value contains a starting value (range-
start) followed by a "-", an ending value (range-end) followed by a
"/", and finally the total length. The first octet in the message
has a position of one, rather than a zero.
The first chunk of the message SHOULD, and all subsequent chunks MUST
include a Byte-Range header field. The range-start field MUST
indicate the position of the first byte in the body in the overall
message (for the first chunk this field will have a value of one).
The range-end field SHOULD indicate the position of the last byte in
the body, if known. It MUST take the value of "*" if the position is
unknown, or if the request needs to be interruptible. The total
field SHOULD contain the total size of the message, if known. The
total field MAY contain a "*" if the total size of the message is not
known in advance. The sender MUST send all chunks in Byte-Range
order. (However, the receiver cannot assume the requests will be
delivered in order, as intervening relays may have changed the
order.)
To ensure fairness over a connection, senders MUST NOT send chunks
with a body larger than 2048 octets unless they are prepared to
interrupt them (meaning that any chunk with a body of greater than
2048 octets will have a "*" character in the range-end field). A
sender can use one of the following two strategies to satisfy this
requirement. The sender is STRONGLY RECOMMENDED to send messages
larger than 2048 octets using as few chunks as possible, interrupting
chunks (at least 2048 octets long) only when other traffic is waiting
to use the same connection. Alternatively, the sender MAY simply
send chunks in 2048 octet increments until the final chunk. Note
that the former strategy results in markedly more efficient use of
the connection. All MSRP nodes MUST be able to receive chunks of any
size from zero octets to the maximum number of octets they can
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receive for a complete message. Senders SHOULD NOT break messages
into chunks smaller than 2048 octets, except for the final chunk of a
complete message.
A SEND request is interrupted while a body is in the process of being
written to the connection by simply noting how much of the message
has already been written to the connection, then writing out the end-
line to end the chunk. It can then be resumed in a another chunk
with the same Message-ID and a Byte-Range header field range start
field containing the position of the first byte after the
interruption occurred.
SEND requests larger than 2048 octets MUST be interrupted if the
sender needs to send pending responses or REPORT requests. If
multiple SEND requests from different sessions are concurrently being
sent over the same connection, the device SHOULD implement some
scheme to alternate between them such that each concurrent request
gets a chance to send some fair portion of data at regular intervals
suitable to the application.
The sender MUST NOT assume that a message is received by the peer
with the same chunk allocation with which it was sent. An
intervening relay could possibly break SEND requests into smaller
chunks, or aggregate multiple chunks into larger ones.
The default disposition of bodies is "render". If the sender wants
different disposition, it MAY insert a Content-Disposition[9] header
field. Since MSRP can carry unencoded binary payloads, transfer
encoding is always "binary", and transfer-encoding parameters MUST
NOT be present.
7.1.2. Sending REPORT Requests
REPORT requests are similar to SEND requests, except that report
requests MUST NOT include Success-Report or Failure-Report header
fields, and MUST contain a Status header field. REPORT requests MUST
contain the Message-ID header field from the original SEND request.
If an MSRP element receives a REPORT for a Message-ID it does not
recognize, it SHOULD silently ignore the REPORT.
An MSRP endpoint MUST be able to generate success REPORT requests.
REPORT requests will normally not include a body, as the REPORT
request header fields can carry sufficient information in most cases.
However, REPORT requests MAY include a body containing additional
information about the status of the associated SEND request. Such a
body is informational only, and the sender of the REPORT request
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SHOULD NOT assume that the recipient pays any attention to the body.
REPORT requests are not interruptible.
7.1.3. Generating Success Reports
An endpoint MUST send a success report if it successfully receives a
SEND request which contained a Success-Report value of "yes" and
either contains a complete message, or contains the last chunk needed
to complete the message. This request is sent following the normal
procedures (Section 7.1), with a few additional requirements.
The endpoint inserts a To-Path header field containing the From-Path
value from the original request, and a From-Path header field
containing the URL identifying itself in the session. The endpoint
then inserts a Status header field with a namespace of "000", a
short-status of "200" and an implementation defined comment phrase.
It also inserts a Message-ID header field containing the value from
the original request.
The namespace field denotes the context of the short-status field.
The namespace value of "000" means the short-status should be
interpreted in the same way as the matching MSRP transaction
response code. If a future specification uses the short-status
field for some other purpose, it MUST define a new namespace field
value.
The endpoint MUST NOT send a success report for a SEND request that
either contained no Success-Report header field, or contained such a
field with a value of "no". That is, if no Success-Report header
field is present, it is treated identically to one with a value of
"no."
7.1.4. Generating Failure Reports
If an MSRP endpoint receives a SEND request that it cannot process
for some reason, and the Failure-Report header field either was not
present in the original request, or had a value of "yes", it SHOULD
simply include the appropriate error code in the transaction
response. However, there may be situations where the error cannot be
determined quickly, such as when the endpoint is a gateway that must
wait for a downstream network to indicate an error. In this
situation, it MAY send a 200 OK response to the request, and then
send a failure REPORT request when the error is detected.
If the endpoint receives a SEND request with a Failure-Report header
field value of "no", then it MUST NOT send a failure REPORT request,
and MUST NOT send a transaction response. If the value is "partial",
it MUST NOT send a 200 transaction response to the request, but
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SHOULD send an appropriate non-200 class response if a failure
occurs.
As stated above, if no Failure-Report header field is present, it
MUST be treated the same as a Failure-Report header field with value
of "yes".
Construction of failure REPORT requests is identical to that for
success REPORT requests, except the Status header field code and
reason fields MUST contain appropriate error codes. Any error
response code defined in this specification MAY also be used in
failure reports.
If a failure REPORT request is sent in response to a SEND request
that contained a chunk, it MUST include a Byte-Range header field
indicating the actual range being reported on. It can take the
range-start and total values from the original SEND request, but MUST
calculate the range-end field from the actual body data.
Endpoints SHOULD NOT send REPORT requests if they have reason to
believe the request will not be delivered. For example, they SHOULD
NOT send a REPORT request on a session that is no longer valid.
This section only describes failure report generation behavior for
MSRP endpoints. Relay behavior is beyond the scope of this
document, and will be considered in a separate document [22]. We
expect failure reports to be more commonly generated by relays
than by endpoints.
7.2. Constructing Responses
If an MSRP endpoint receives a request that either contains a
Failure-Report header field value of "yes", or does not contain a
Failure-Report header field at all, it MUST immediately generate a
response. Likewise, if an MSRP endpoint receives a request that
contains a Failure-Report header field value of "partial", and the
receiver is unable to process the request, it SHOULD immediately
generate a response.
To construct the response, the endpoint first creates the response
start-line, inserting appropriate response code and reason fields.
The transaction identifier in the response start line MUST match the
transaction identifier from the original request.
The endpoint then inserts an appropriate To-Path header field. If
the request triggering the response was a SEND request, the To-Path
header field is formed by copying the last (right-most) URL in the
From-Path header field of the request. (Responses to SEND requests
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are returned only to the previous hop.) For responses to all other
request methods, the To-Path header field contains the full path back
to the original sender. This full path is generated by taking the
list of URLs from the From-Path of the original request, reversing
the list, and writing the reversed list into the To-Path of the
response. (Legal REPORT requests do not request responses, so this
specification doesn't exercise the behavior described above, however
we expect that extensions for gateways and relays will need such
behavior.)
Finally, the endpoint inserts a From-Path header field containing the
URL that identifies it in the context of the session, followed by the
end-line after the last header field. The response MUST be
transmitted back on the same connection on which the original request
arrived.
7.3. Receiving Requests
The receiving endpoint must first check the URL in the To-Path to
make sure the request belongs to an existing session. When the
request is received, the To-Path will have exactly one URL, which
MUST map to an existing session that is associated with the
connection on which the request arrived. If this is not true, then
the receiver MUST generate a 481 error and ignore the request. Note
that if the Failure-Report header field had a value of "no", then no
error report would be sent.
Further request processing by the receiver is method specific.
7.3.1. Receiving SEND Requests
When the receiving endpoint receives a SEND request, it first
determines if it contains a complete message, or a chunk from a
larger message. If the request contains no Byte-Range header field,
or contains one with a range-start value of "1", and the closing line
continuation flag has a value of "$", then the request contained the
entire message. Otherwise, the receiver looks at the Message-ID
value to associate chunks together into the original message. It
forms a virtual buffer to receive the message, keeping track of which
bytes have been received and which are missing. The receiver takes
the data from the request and places it in the appropriate place in
the buffer. The receiver SHOULD determine the actual length of each
chunk by inspecting the payload itself; it is possible the body is
shorter than the range-end field indicates. This can occur if the
sender interrupted a SEND request unexpectedly. It is worth noting
that the chunk that has a termination character of "$" defines the
total length of the message.
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It is technically illegal for the sender to prematurely interrupt
a request that had anything other than "*" in the last-byte
position of the Byte-Range header field. But having the receiver
calculate a chunk length based on actual content adds resilience
in the face of sender errors. Since this should never happen with
compliant senders, this only has a SHOULD strength.
Receivers MUST not assume the chunks will be delivered in order or
that they will receive all the chunks with "+" flags before they
receive the chunk with the "$" flag. In certain cases of connection
failure, it is possible for information to be duplicated. If chunk
data is received that overlaps already received data for the same
message, the last chunk received SHOULD take precedence (even though
this may not have been the last chunk transmitted). For example, if
bytes 1 to 100 were received and a chunk arrives that contains bytes
50 to 150, this second chunk will overwrite bytes 50 to 100 of the
data that had already been received. Although other schemes work,
this is the easiest for the receiver and results in consistent
behavior between clients.
There are situations in which the receiver may not be able to give
precedence to the last chunk received when chunks overlap. For
example, the recipient might incrementally render chunks as they
arrive. If a new chunk arrives that overlaps with a previously
rendered chunk, it would be too late to "take back" any
conflicting data from the first chunk. Therefore, the requirement
to give precedence to the most recent chunk is specified at a
"SHOULD" strength. This requirement is not intended to disallow
applications where this behavior does not make sense.
The seven "-" in the end-line are used so that the receiver can
search for the value "----", 32 bits at a time to find the probable
location of the end-line. This allows most processors to locate the
boundaries and copy the memory at the same rate that a normal memory
copy could be done. This approach results in a system that is as
fast as framing based on specifying the body length in the header
fields of the request, but also allows for the interruption of
messages.
What is done with the body is outside the scope of MSRP and largely
determined by the MIME Content-Type and Content-Disposition. The
body MAY be rendered after the whole message is received or partially
rendered as it is being received.
If the SEND request contained a Content-Type header field indicating
an unsupported MIME type, and the Failure-Report value is not "no",
the receiver MUST generate a response with a status code of 415. All
MSRP endpoints MUST be able to receive the multipart/mixed [15] and
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multipart/alternative [15] MIME types.
If the Success-Report header field was set to "yes", then when a
complete message has been received, the receiver MUST send a success
REPORT with a byte range covering the whole message. If the Success-
Report header field is set to "yes", then the receiver MAY generate
incremental success REPORTs as the chunks are received. These can be
sent periodically and cover all the bytes that have been received so
far, or they can be sent after a chunk arrives and cover just the
part from that chunk.
It is helpful to think of a success REPORT as reporting on a
particular range of bytes, rather than on a particular chunk sent
by a client. The sending client cannot depend on the Byte-Range
header field in a given success report matching that of a
particular SEND request. For example, an intervening MSRP relay
may break chunks into smaller chunks, or aggregate multiple chunks
into larger ones.
A side effect of this is, even if no relay is used, the receiving
client may report on byte ranges that do not exactly match those
in the original chunks sent by the sender. It can wait until all
bytes in a message are received and report on the whole, it can
report as it receives each chunk, or it can report on any other
received range.
Reporting on ranges smaller than the entire message contents
allows certain improved user experiences for the sender. For
example, a sending client could display incremental status
information showing which ranges of bytes have been acknowledged
by the receiver.
However, the choice on whether to report incrementally is entirely
up to the receiving client. There is no mechanism for the sender
to assert its desire to receive incremental reports or not. Since
the presence of a relay can cause the receiver to see a very
different chunk allocation than the sender, such a mechanism would
be of questionable value.
7.3.2. Receiving REPORT Requests
When an endpoint receives a REPORT request, it correlates it to the
original SEND request using the Message-ID and the Byte-Range, if
present. If it requested success reports, then it SHOULD keep enough
state about each outstanding sent message so that it can correlate
REPORT requests to the original messages.
An endpoint that receives a REPORT request containing a Status header
field with a namespace field of "000" MUST interpret the report in
exactly the same way it would interpret an MSRP transaction response
with a response code matching the short-code field.
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It is possible to receive a failure report or a failure transaction
response for a chunk that is currently being delivered. In this
case, the entire message corresponding to that chunk SHOULD be
aborted, by including the "#" character in the continuation field of
the end-line.
It is possible that an endpoint will receive a REPORT request on a
session that is no longer valid. The endpoint's behavior if this
happens is a matter of local policy. The endpoint is not required to
take any steps to facilitate such late delivery, i.e. it is not
expected to keep a connection active in case late REPORTs might
arrive.
When an endpoint that sent a SEND request receives a failure REPORT
indicating that a particular byte range was not received, it MUST
treat the session as failed. If it wishes to recover, it MUST first
re-negotiate the URLs at the signaling level then resend that range
of bytes of the message on the resulting new session.
MSRP nodes MUST NOT send MSRP REPORT requests in responses to other
REPORT requests.
8. Using MSRP with SIP and SDP
MSRP sessions will typically be initiated using the Session
Description Protocol (SDP) [2] via the SIP offer/answer mechanism
[3].
This document defines a handful of new SDP parameters to set up MSRP
sessions. These are detailed below and in the IANA Considerations
section.
An MSRP media-line (that is, a media-line proposing MSRP) in the
session description is accompanied by a mandatory "path" attribute.
This attribute contains a space-separated list of URLs that must be
visited to contact the user agent advertising this session-
description. If more than one URL is present, the leftmost URL is
the first URL that must be visited to reach the target resource.
(The path list can contain multiple URLs to allow for the deployment
of gateways or relays in the future.) MSRP implementations that can
accept incoming connections without the need for relays will
typically only provide a single URL here.
An MSRP media line is also accompanied by an "accept-types"
attribute, and optionally an "accept-wrapped-types" attribute. These
attributes are used to specify the MIME types that are acceptable to
the endpoint.
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8.1. SDP Connection and Media Lines
The format of an SDP connection-line takes the following format:
c=<network type> <address type> <connection address>
The network type and address type fields are used as normal for SDP.
The connection address field MUST be set to the IP address or fully
qualified domain name from the MSRP URL identifying the endpoint in
its path attribute.
The general format of an SDP media-line is:
m=<media> <port> <protocol> <format list>
An offered or accepted media-line for MSRP over TCP MUST include a
protocol field value of "TCP/MSRP", or "TCP/TLS/MSRP" for TLS. The
media field value MUST be "message". The format list field MUST be
set to "*".
The port field value MUST match the port value used in the endpoint's
MSRP URL in the path attribute, except that, as described in [3], a
user agent that wishes to accept an offer, but not a specific media-
line MUST, set the port number of that media-line to zero (0) in the
response. Since MSRP allows multiple sessions to share the same TCP
connection, multiple m-lines in a single SDP document may share the
same port field value; MSRP devices MUST NOT assume any particular
relationship between m-lines on the sole basis that they have
matching port field values.
MSRP devices do not use the c-line address field, or the m-line
port and format list fields to determine where to connect.
Rather, they use the attributes defined in this specification.
The connection information is copied to the c-line and m-line for
purposes of backwards compatibility with conventional SDP usages.
While MSRP could theoretically carry any media type, "message" is
appropriate.
8.2. URL Negotiations
Each endpoint in an MSRP session is identified by a URL. These URLs
are negotiated in the SDP exchange. Each SDP offer or answer that
proposes MSRP MUST contain a path attribute containing one or more
MSRP URLs. The path attribute is used in an SDP a-line, and has the
following syntax:
path = path-label ":" path-list
path-label = "path"
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path-list= MSRP-URL *(SP MSRP-URL)
where MSRP-URL is an msrp: or msrps: URL as defined in Section 6.
MSRP URLs included in an SDP offer or answer MUST include explicit
port numbers.
An MSRP device uses the URL to determine a host address, port,
transport, and protection level when connecting, and to identify the
target when sending requests and responses.
The offerer and answerer each selects a URL to represent itself and
sends it to the peer device in the SDP document. Each device stores
the path value received from the peer and uses that value as the
target for requests inside the resulting session. If the path
attribute received from the peer contains more than one URL, then the
target URL is the rightmost, while the leftmost entry represents the
adjacent hop. If only one entry is present, then it is both the peer
and adjacent hop URL. The target path is the entire path attribute
value received from the peer.
The following example shows an SDP offer with a session URL of
"msrp://alice.example.com:7394/2s93i;tcp"
v=0
o=alice 2890844526 2890844527 IN IP4 alice.example.com
s= -
c=IN IP4 alice.example.com
t=0 0
m=message 7394 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://alice.example.com:7394/2s93i;tcp
The rightmost URL in the path attribute MUST identify the endpoint
that generated the SDP document, or some other location where that
endpoint wishes to receive requests associated with the session. It
MUST be assigned for this particular session, and MUST NOT duplicate
any URL in use for any other session in which the endpoint is
currently participating. It SHOULD be hard to guess, and protected
from eavesdroppers. This is discussed in more detail in Section 14.
8.3. Path Attributes with Multiple URLs
As mentioned previously, this document describes MSRP for peer-to-
peer scenarios, that is, when no relays are used. The use of relays
are described in a separate document [22]. In order to allow an MSRP
device that only implements the core specification to interoperate
with devices that use relays, this document must include a few
assumptions about how relays work.
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An endpoint that uses one or more relays will indicate that by
putting a URL for each device in the relay chain into the SDP path
attribute. The final entry will point to the endpoint itself. The
other entries will indicate each proposed relay, in order. The first
entry will point to the first relay in the chain from the perspective
of the peer; that is, the relay to which the peer device, or a relay
operating on its behalf, should connect.
Endpoints that do not wish to insert a relay, including those that do
not support relays at all, will put exactly one URL into the path
attribute. This URL represents both the endpoint for the session,
and the connection point.
Even though endpoints that implement only this specification will
never introduce a relay, they need to be able to interoperate with
other endpoints that do use relays. Therefore, they MUST be prepared
to receive more than one URL in the SDP path attribute. When an
endpoint receives more than one URL in a path attribute, only the
first entry is relevant for purposes of resolving the address and
port, and establishing the network connection, as it describes the
first adjacent hop.
If an endpoint puts more than one URL in a path attribute, the final
URL in the path attribute (the peer URL) identifies the session, and
must not duplicate the URL of any other session in which the endpoint
is currently participating. Uniqueness requirements for other
entries in the path attribute are out of scope for this document.
8.4. Updated SDP Offers
MSRP endpoints may sometimes need to send additional SDP exchanges
for an existing session. They may need to send periodic exchanges
with no change to refresh state in the network, for example, SIP
session timers or the SIP UPDATE[23] request. They may need to
change some other stream in a session without affecting the MSRP
stream, or they may need to change an MSRP stream without affecting
some other stream.
Either peer may initiate an updated exchange at any time. The
endpoint that sends the new offer assumes the role of offerer for all
purposes. The answerer MUST respond with a path attribute that
represents a valid path to itself at the time of the updated
exchange. This new path may be the same as its previous path, but
may be different. The new offerer MUST NOT assume that the peer will
answer with the same path it used previously.
If either party wishes to send an SDP document that changes nothing
at all, then it MUST have the same o-line as in the previous
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exchange.
8.5. Connection Negotiation
Previous versions of this document included a mechanism to negotiate
the direction for any required TCP connection. The mechanism was
loosely based on the COMEDIA [25] work being done in the MMUSIC
working group. The primary motivation was to allow MSRP sessions to
succeed in situations where the offerer could not accept connections
but the answerer could. For example, the offerer might be behind a
NAT, while the answerer might have a globally routable address.
The SIMPLE working group chose to remove that mechanism from MSRP, as
it added a great deal of complexity to connection management.
Instead, MSRP now specifies a default connection direction. The
party that sent the original offer is responsible for connecting to
its peer.
8.6. Content Type Negotiation
An SDP media-line proposing MSRP MUST be accompanied by an accept-
types attribute.
An entry of "*" in the accept-types attribute indicates that the
sender may attempt to send content with media types that have not
been explicitly listed. Likewise, an entry with an explicit type and
a "*" character as the subtype indicates that the sender may attempt
to send content with any subtype of that type. If the receiver
receives an MSRP request and is able to process the media type, it
does so. If not, it will respond with a 415 response. Note that all
explicit entries SHOULD be considered preferred over any non-listed
types. This feature is needed as, otherwise, the list of formats for
rich IM devices may be prohibitively large.
The accept-types attribute may include container types, that is, MIME
formats that contain other types internally. If compound types are
used, the types listed in the accept-types attribute may be used both
as the root payload, or may be wrapped in a listed container type.
Any container types MUST also be listed in the accept-types
attribute.
Occasionally an endpoint will need to specify a MIME body type that
can only be used if wrapped inside a listed container type.
Endpoints MAY specify MIME types that are only allowed when wrapped
inside compound types using the "accept-wrapped-types" attribute in
an SDP a-line.
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The semantics for accept-wrapped-types are identical to those of the
accept-types attribute, with the exception that the specified types
may only be used when wrapped inside container types listed in
accept-types attribute. Only types listed in the accept-types
attribute may be used as the "root" type for the entire body. Since
any type listed in accept-types may be used both as a root body, and
wrapped in other bodies, format entries from accept-types SHOULD NOT
be repeated in this attribute.
This approach does not allow for specifying distinct lists of
acceptable wrapped types for different types of containers. If an
endpoint understands a MIME type in the context of one wrapper, it is
assumed to understand it in the context of any other acceptable
wrappers, subject to any constraints defined by the wrapper types
themselves.
The approach of specifying types that are only allowed inside of
containers separately from the primary payload types allows an
endpoint to force the use of certain wrappers. For example, a
CPIM [12] gateway device may require all messages to be wrapped
inside message/cpim bodies, but may allow several content types
inside the wrapper. If the gateway were to specify the wrapped
types in the accept-types attribute, its peer might attempt to use
those types without the wrapper.
If the recipient of an offer does not understand any of the payload
types indicated in the offered SDP, it SHOULD indicate that using the
appropriate mechanism of the rendezvous protocol. For example, in
SIP, it SHOULD return a SIP 488 response.
An endpoint MAY indicate the maximum size message they wish to
receive using the max-size a-line attribute. Max-size refers to the
complete message in octets, not the size of any one chunk. Senders
SHOULD NOT exceed the max-size limit for any message sent in the
resulting session. However, the receiver should consider max-size
value as a hint.
The formal syntax for these attributes are as follows:
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accept-types = accept-types-label ":" format-list
accept-types-label = "accept-types"
accept-wrapped-types = wrapped-types-label ":" format-list
wrapped-types-label = "accept-wrapped-types"
format-list = format-entry *( SP format-entry)
format-entry = (type "/" subtype) / (type "/" "*") / ("*")
type = token
subtype = token
max-size = max-size-label ":" max-size-value
max-size-label = "max-size"
max-size-value = 1*(DIGIT) ;max size in octets
8.7. Example SDP Exchange
Endpoint A wishes to invite Endpoint B to an MSRP session. A offers
the following session description:
v=0
o=usera 2890844526 2890844527 IN IP4 alice.example.com
s= -
c=IN IP4 alice.example.com
t=0 0
m=message 7394 TCP/MSRP *
a=accept-types: message/cpim text/plain text/html
a=path:msrp://alice.example.com:7394/2s93i9;tcp
B responds with its own URL:
v=0
o=userb 2890844530 2890844532 IN IP4 bob.example.com
s= -
c=IN IP4 bob.example.com
t=0 0
m=message 8493 TCP/MSRP *
a=accept-types:message/cpim text/plain
a=path:msrp://bob.example.com:8493/si438ds;tcp
8.8. MSRP User Experience with SIP
In typical SIP applications, when an endpoint receives an INVITE
request, it alerts the user, and waits for user input before
responding. This is analogous to the typical telephone user
experience, where the callee "answers" the call.
In contrast, the typical user experience for instant messaging
applications is that the initial received message is immediately
displayed to the user, without waiting for the user to "join" the
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conversation. Therefore, the principle of least surprise would
suggest that MSRP endpoints using SIP signaling SHOULD allow a mode
where the endpoint quietly accepts the session, and begins displaying
messages.
This guideline may not make sense for all situations, such as for
mixed media applications, where both MSRP and audio sessions are
offered in the same INVITE. In general, good application design
should take precedence.
SIP INVITE requests may be forked by a SIP proxy, resulting in more
than one endpoint receiving the same INVITE. SIP early media [28]
techniques can be used to establish a preliminary session with each
endpoint so the initial message(s) are displayed on each endpoint,
and canceling the INVITE transaction for any endpoints that do not
send MSRP traffic after some period of time, so that they cease
receiving MSRP traffic from the inviter.
9. Formal Syntax
MSRP is a text protocol that uses the UTF-8 [14] transformation
format.
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC-2234 [6].
msrp-req-or-resp = msrp-request / msrp-response
msrp-request = req-start headers [content-stuff] end-line
msrp-response = resp-start headers end-line
req-start = pMSRP SP transact-id SP method CRLF
resp-start = pMSRP SP transact-id SP status-code [SP comment] CRLF
comment = utf8text
pMSRP = %x4D.53.52.50 ; MSRP in caps
transact-id = ident
method = mSEND / mREPORT / other-method
mSEND = %x53.45.4e.44 ; SEND in caps
mREPORT = %x52.45.50.4f.52.54; REPORT in caps
other-method = 1*UPALPHA
status-code = 3DIGIT ; any code defined in this document
; or an extension document
MSRP-URL = msrp-scheme "://" [userinfo "@"] hostport
["/" session-id] ";" transport *( ";" url-parameter)
; userinfo as defined in RFC3986, except
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; limited to unreserved.
; hostport as defined in RFC3261
msrp-scheme = "msrp" / "msrps"
session-id = 1*( unreserved / "+" / "=" / "/" )
; unreserved as defined in RFC3986
transport = "tcp" / ALPHANUM
url-parameter = token ["=" token]
headers = To-Path CRLF From-Path CRLF 1*( header CRLF )
header = Message-ID
/ Success-Report
/ Failure-Report
/ Byte-Range
/ Status
/ ext-header
To-Path = "To-Path:" SP MSRP-URL *( SP MSRP-URL )
From-Path = "From-Path:" SP MSRP-URL *( SP MSRP-URL )
Message-ID = "Message-ID:" SP ident
Success-Report = "Success-Report:" SP ("yes" / "no" )
Failure-Report = "Failure-Report:" SP ("yes" / "no" / "partial" )
Byte-Range = "Byte-Range:" SP range-start "-" range-end "/" total
range-start = 1*DIGIT
range-end = 1*DIGIT / "*"
total = 1*DIGIT / "*"
Status = "Status:" SP namespace SP status-code [SP text-reason]
namespace = 3(DIGIT); "000" for all codes defined in this document.
text-reason = utf8text
ident = alphanum 3*31ident-char
ident-char = alphanum / "." / "-" / "+" / "%" / "="
content-stuff = *(Other-Mime-header CRLF)
Content-Type 2CRLF data CRLF
Content-Type = "Content-Type:" SP media-type
media-type = type "/" subtype *( ";" gen-param )
type = token
subtype = token
gen-param = pname [ "=" pval ]
pname = token
pval = token / quoted-string
token = 1*(%x21 / %x23-27 / %x2A-2B / %x2D-2E
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/ %x30-39 / %x41-5A / %x5E-7E)
; token is compared case-insensitive
quoted-string = DQUOTE *(qdtext / qd-esc) DQUOTE
qdtext = SP / HTAB / %x21 / %x23-5B / %x5D-7E
/ UTF8-NONASCII
qd-esc = (BACKSLASH BACKSLASH) / (BACKSLASH DQUOTE)
BACKSLASH = "\"
UPALPHA = %x41-5A
ALPHANUM = ALPHA / DIGIT
Other-Mime-header = (Content-ID
/ Content-Description
/ Content-Disposition
/ mime-extension-field);
; Content-ID, and Content-Description are defined in RFC2045.
; Content-Disposition is defined in RFC2183
; MIME-extension-field indicates additional MIME extension
; header fields as described in RFC2045
data = *OCTET
end-line = "-------" transact-id continuation-flag CRLF
continuation-flag = "+" / "$" / "#"
ext-header = hname ":" SP hval CRLF
hname = ALPHA *token
hval = utf8text
utf8text = *(HTAB / %x20-7E / UTF8-NONASCII)
UTF8-NONASCII = %xC0-DF 1UTF8-CONT
/ %xE0-EF 2UTF8-CONT
/ %xF0-F7 3UTF8-CONT
/ %xF8-Fb 4UTF8-CONT
/ %xFC-FD 5UTF8-CONT
UTF8-CONT = %x80-BF
10. Response Code Descriptions
This section summarizes the semantics of various response codes that
may be used in MSRP transaction responses. These codes may also be
used in the Status header field in REPORT requests.
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10.1. 200
The 200 response code indicates a successful transaction.
10.2. 400
A 400 response indicates a request was unintelligible. The sender
may retry the request after correcting the error.
10.3. 403
A 403 response indicates the attempted action is not allowed. The
sender should not try the request again.
10.4. 408
A 408 response indicates that a downstream transaction did not
complete in the alloted time. It is never sent by any elements
described in this specification. However, 408 is used in the MSRP
Relay extension; therefore MSRP endpoints may receive it. An
endpoint MUST treat a 408 response in the same manner as it would
treat a local timeout.
10.5. 413
A 413 response indicates that the receiver wishes the sender to stop
sending the particular message. Typically, a 413 is sent in response
to a chunk of an undesired message.
If a message sender receives a 413 in a response, or in a REPORT
request, it MUST NOT send any further chunks in the message, that is,
any further chunks with the same Message-ID value. If the sender
receives the 413 while in the process of sending a chunk, and the
chunk is interruptible, the sender MUST interrupt it.
10.6. 415
A 415 response indicates the SEND request contained a MIME content-
type that is not understood by the receiver. The sender should not
send any further messages with the same content-type for the duration
of the session.
10.7. 423
A 423 response indicates that one of the requested parameters is out
of bounds. It is used by the relay extensions to this document.
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10.8. 481
A 481 response indicates that the indicated session does not exist.
The sender should terminate the session.
10.9. 501
A 501 response indicates that the recipient does not understand the
request method.
The 501 response code exists to allow some degree of method
extensibility. It is not intended as a license to ignore methods
defined in this document; rather it is a mechanism to report lack
of support of extension methods.
10.10. 506
A 506 response indicates that a request arrived on a session which is
already bound to another network connection. The sender should cease
sending messages for that session on this connection.
11. Examples
11.1. Basic IM Session
This section shows an example flow for the most common scenario. The
example assumes SIP is used to transport the SDP exchange. Details
of the SIP messages and SIP proxy infrastructure are omitted for the
sake of brevity. In the example, assume the offerer is
sip:alice@example.com and the answerer is sip:bob@example.com.
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Alice Bob
| |
| |
|(1) (SIP) INVITE |
|----------------------->|
|(2) (SIP) 200 OK |
|<-----------------------|
|(3) (SIP) ACK |
|----------------------->|
|(4) (MSRP) SEND |
|----------------------->|
|(5) (MSRP) 200 OK |
|<-----------------------|
|(6) (MSRP) SEND |
|<-----------------------|
|(7) (MSRP) 200 OK |
|----------------------->|
|(8) (SIP) BYE |
|----------------------->|
|(9) (SIP) 200 OK |
|<-----------------------|
| |
| |
1. Alice constructs a local URL of
msrp://alicepc.example.com:7777/iau39;tcp .
Alice->Bob (SIP): INVITE sip:bob@example.com
v=0
o=alice 2890844557 2890844559 IN IP4 alicepc.example.com
s= -
c=IN IP4 alicepc.example.com
t=0 0
m=message 7777 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://alicepc.example.com:7777/iau39;tcp
2. Bob listens on port 8888, and sends the following response:
Bob->Alice (SIP): 200 OK
v=0
o=bob 2890844612 2890844616 IN IP4 bob.example.com
s= -
c=IN IP4 bob.example.com
t=0 0
m=message 8888 TCP/MSRP *
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a=accept-types:text/plain
a=path:msrp://bob.example.com:8888/9di4ea;tcp
3. Alice->Bob (SIP): ACK sip:bob@example.com
4. (Alice opens connection to Bob.) Alice->Bob (MSRP):
MSRP d93kswow SEND
To-Path: msrp://bob.example.com:8888/9di4ea;tcp
From-Path: msrp://alicepc.example.com:7777/iau39;tcp
Message-ID: 12339sdqwer
Content-Type: text/plain
Hi, I'm Alice!
-------d93kswow$
5. Bob->Alice (MSRP):
MSRP d93kswow 200 OK
To-Path: msrp://alicepc.example.com:7777/iau39;tcp
From-Path: msrp://bob.example.com:8888/9di4ea;tcp
-------d93kswow$
6. Bob->Alice (MSRP):
MSRP dkei38sd SEND
To-Path: msrp://alicepc.example.com:7777/iau39;tcp
From-Path: msrp://bob.example.com:8888/9di4ea;tcp
Message-ID: 456
Content-Type: text/plain
Hi, Alice! I'm Bob!
-------dkei38sd$
7. Alice->Bob (MSRP):
MSRP dkei38sd 200 OK
To-Path: msrp://alicepc.example.com:7777/iau39;tcp
From-Path: msrp://bob.example.com:8888/9di4ea;tcp
-------dkei38sd$
8. Alice->Bob (SIP): BYE sip:bob@example.com
Alice invalidates local session state.
9. Bob invalidates local state for the session.
Bob->Alice (SIP): 200 OK
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11.2. Message with XHTML Content
MSRP dsdfoe38sd SEND
To-Path: msrp://alice.atlanta.com:7777/iau39;tcp
From-Path: msrp://bob.atlanta.com:8888/9di4ea;tcp
Message-ID: 456
Content-Type: application/xhtml+xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"_http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd_">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<title>FY2005 Results</title>
</head>
<body>
<p>See the results at <a
href="http://example.org/">example.org</a>.</p>
</body>
</html>
-------dsdfoe38sd$
11.3. Chunked Message
For an example of a chunked message, see the example in Section 5.1.
11.4. System Message
Sysadmin->Alice (MSRP):
MSRP d93kswow SEND
To-Path: msrp://alicepc.example.com:8888/9di4ea;tcp
From-Path: msrp://example.com:7777/iau39;tcp
Message-ID: 12339sdqwer
Failure-Report: no
Success-Report: no
Content-Type: text/plain
This conference will end in 5 minutes
-------d93kswow$
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11.5. Positive Report
Alice->Bob (MSRP):
MSRP d93kswow SEND
To-Path: msrp://bob.example.com:8888/9di4ea;tcp
From-Path: msrp://alicepc.example.com:7777/iau39;tcp
Message-ID: 12339sdqwer
Success-Report: yes
Failure-Report: no
Content-Type: text/html
<html><body>
<p>Here is that important link...
<a href="www.example.com/foobar">foobar</a>
</p>
</body></html>
-------d93kswow$
Bob->Alice (MSRP):
MSRP dkei38sd REPORT
To-Path: msrp://alicepc.example.com:7777/iau39;tcp
From-Path: msrp://bob.example.com:8888/9di4ea;tcp
Message-ID: 12339sdqwer
Status: 000 200 OK
-------dkei38sd$
11.6. Forked IM
Traditional IM systems generally do a poor job of handling multiple
simultaneous IM clients online for the same person. While some do a
better job than many existing systems, handling of multiple clients
is fairly crude. This becomes a much more significant issue when
always-on mobile devices are available, but it is desirable to use
them only if another IM client is not available.
Using SIP makes rendezvous decisions explicit, deterministic, and
very flexible. In contrast, "page-mode" IM systems use implicit
implementation-specific decisions which IM clients cannot influence.
With SIP session mode messaging, rendezvous decisions can be under
control of the client in a predictable, interoperable way for any
host that implements callee capabilities [30]. As a result,
rendezvous policy is managed consistently for each address of record.
The following example shows Juliet with several IM clients where she
can be reached. Each of these has a unique SIP Contact and MSRP
session. The example takes advantage of SIP's capability to "fork"
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an invitation to several Contacts in parallel, in sequence, or in
combination. Juliet has registered from her chamber, the balcony,
her PDA, and as a last resort, you can leave a message with her
Nurse. Juliet's contacts are listed below. The q-values express
relative preference (q=1.0 is the highest preference).
The example uses REGISTER to learn of Juliet's registered
contacts. This does not constitute an endorsement of that
approach; it is used here to avoid cluttering the example with too
many SIP details. A more realistic application would be the use a
SIP proxy or redirect server for this purpose.
We query for a list of Juliet's contacts by sending a REGISTER:
REGISTER sip:thecapulets.example.com SIP/2.0
To: Juliet <sip:juliet@thecapulets.example.com>
From: Romeo <sip:romeo@montague.example.com>;tag=12345
Via: SIP/2.0/UDP romeospc.example.com:5060;branch=z9hG4bKnashds7
Call-ID: 09887877
Max-Forwards=70
CSeq: 772 REGISTER
The Response contains her Contacts:
SIP/2.0 200 OK
To: Juliet <sip:juliet@thecapulets.example.com>
From: Juliet <sip:juliet@thecapulets.example.com>;tag=12345
Via: SIP/2.0/UDP romeospc.example.com:5060;branch=z9hG4bKnashds7
Call-ID: 09887877
CSeq: 772 REGISTER
Contact: <sip:juliet@balcony.thecapulets.example.com>
;q=0.9;expires=3600
Contact: <sip:juliet@chamber.thecapulets.example.com>
;q=1.0;expires=3600
Contact: <sip:jcapulet@veronamobile.example.net>;q=0.4;expires=3600
Contact: <sip:nurse@thecapulets.example.com>;q=0.1;expires=3600
When Romeo opens his IM program, he selects Juliet and types the
message "art thou hither?" (instead of "you there?"). His client
sends a SIP invitation to sip:juliet@thecapulets.example.com. The
proxy there tries first the balcony and the chamber simultaneously.
A client is running on both those systems, both of which set up early
sessions of MSRP with Romeo's client. The client automatically sends
the message over MSRPS to the two MSRP URIs involved. After a delay
of a several seconds with no reply or activity from Juliet, the proxy
cancels the invitation at her first two contacts, and forwards the
invitation on to Juliet's PDA. Since her father is talking to her
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about her wedding, she selects "Do Not Disturb" on her PDA, which
sends a "Busy Here" response. The proxy then tries the Nurse, who
answers and tells Romeo what is going on.
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Romeo Juliet's Juliet/ Juliet/ Juliet/ Nurse
Proxy balcony chamber PDA
| | | | | |
|--INVITE--->| | | | |
| |--INVITE--->| | | |
| |<----180----| | | |
|<----180----| | | | |
|---PRACK---------------->| | | |
|<----200-----------------| | | |
|<===Early MSRP Session==>| art thou hither? | |
| | | | | |
| |--INVITE---------------->| | |
| |<----180-----------------| | |
|<----180----| | | | |
|---PRACK----------------------------->| | |
|<----200------------------------------| | |
|<========Early MSRP Session==========>| art thou hither? |
| | | | | |
| | | | | |
| | .... Time Passes .... | | |
| | | | | |
| | | | | |
| |--CANCEL--->| | | |
| |<---200-----| | | |
| |<---487-----| | | |
| |----ACK---->| | | |
| |--CANCEL---------------->| | |
| |<---200------------------| | |
| |<---487------------------| | |
| |----ACK----------------->| | |
| |--INVITE---------------------------->| romeo wants
| | | | | to IM w/ you
| |<---486 Busy Here--------------------| |
| |----ACK----------------------------->| |
| | | | | |
| |--INVITE---------------------------------------->|
| |<---200 OK---------------------------------------|
|<--200 OK---| | | | |
|---ACK------------------------------------------------------->|
|<================MSRP Session================================>|
| | | | | |
| Hi Romeo, Juliet is |
| with her father now |
| can I take a message?|
| |
| Tell her to go to confession tomorrow.... |
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12. Extensibility
MSRP was designed to be only minimally extensible. New MSRP Methods,
header fields, and status codes can be defined in standards track
RFCs. MSRP does not contain a version number or any negotiation
mechanism to require or discover new features. If an extension is
specified in the future that requires negotiation, the specification
will need to describe how the extension is to be negotiated in the
encapsulating signaling protocol. If a non-interoperable update or
extension occurs in the future, it will be treated as a new protocol,
and must describe how its use will be signaled.
In order to allow extension header fields without breaking
interoperability, if an MSRP device receives a request or response
containing a header field that it does not understand, it MUST ignore
the header field and process the request or response as if the header
field was not present. If an MSRP device receives a request with an
unknown method, it MUST return a 501 response.
MSRP was designed to use lists of URLs instead of a single URL in the
To-Path and From-Path header fields in anticipation of relay or
gateway functionality being added. In addition, msrp: and msrps:
URLs can contain parameters that are extensible.
13. CPIM Compatibility
MSRP sessions may go to a gateway to other CPIM [26] compatible
protocols. If this occurs, the gateway MUST maintain session state,
and MUST translate between the MSRP session semantics and CPIM
semantics, which do not include a concept of sessions. Furthermore,
when one endpoint of the session is a CPIM gateway, instant messages
SHOULD be wrapped in "message/cpim" [12] bodies. Such a gateway MUST
include "message/cpim" as the first entry in its SDP accept-types
attribute. MSRP endpoints sending instant messages to a peer that
has included "message/cpim" as the first entry in the accept-types
attribute SHOULD encapsulate all instant message bodies in "message/
cpim" wrappers. All MSRP endpoints MUST support the message/cpim
type, and SHOULD support the S/MIME[7] features of that format.
If a message is to be wrapped in a message/cpim envelope, the
wrapping MUST be done prior to breaking the message into chunks, if
needed.
All MSRP endpoints MUST recognize the From, To, DateTime, and Require
header fields as defined in RFC3862. Such applications SHOULD
recognize the CC header field, and MAY recognize the Subject header
field. Any MSRP application that recognizes any message/cpim header
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field MUST understand the NS (name space) header field.
All message/cpim body parts sent by an MSRP endpoint MUST include the
From and To header fields. If the message/cpim body part is
protected using S/MIME, then it MUST also include the DateTime header
field.
The NS, To, and CC header fields may occur multiple times. Other
header fields defined in RFC3862 MUST NOT occur more than once in a
given message/cpim body part in an MSRP message. The Require header
field MAY include multiple values. The NS header field MAY occur
zero or more times, depending on how many name spaces are being
referenced.
Extension header fields MAY occur more than once, depending on the
definition of such header fields.
Using message/cpim envelopes are also useful if an MSRP device
wishes to send a message on behalf of some other identity. The
device may add a message/cpim envelope with the appropriate From
header field value.
14. Security Considerations
Instant Messaging systems are used to exchange a variety of sensitive
information ranging from personal conversations, to corporate
confidential information, to account numbers and other financial
trading information. IM is used by individuals, corporations, and
governments for communicating important information. IM systems need
to provide the properties of integrity and confidentiality for the
exchanged information, the knowledge that you are communicating with
the correct party, and allow the possibility of anonymous
communication. MSRP pushes many of the hard problems to SIP when SIP
sets up the session, but some of the problems remain. Spam and DoS
attacks are also very relevant to IM systems.
MSRP needs to provide confidentiality and integrity for the messages
it transfers. It also needs to provide assurances that the connected
host is the host that it meant to connect to and that the connection
has not been hijacked.
14.1. Transport Level Protection
When using only TCP connections, MSRP security is fairly weak. If
host A is contacting host B, B passes its hostname and a secret to A
using a rendezvous protocol. Although MSRP requires the use of a
rendezvous protocol with the ability to protect this exchange, there
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is no guarantee that the protection will be used all the time. If
such protection is not used, anyone can see this secret. Host A then
connects to the provided host name and passes the secret in the clear
across the connection to B. Host A assumes that it is talking to B
based on where it sent the SYN packet and then delivers the secret in
plain text across the connections. Host B assumes it is talking to A
because the host on the other end of the connection delivered the
secret. An attacker that could ACK the SYN packet could insert
itself as a man in the middle in the connection.
When using TLS connections, the security is significantly improved.
We assume that the host accepting the connection has a certificate
from a well-known certificate authority. Furthermore, we assume that
the signaling to set up the session is protected by the rendezvous
protocol. In this case, when host A contacts host B, the secret is
passed through a confidential channel to A. A connects with TLS to B.
B presents a valid certificate, so A knows it really is connected to
B. A then delivers the secret provided by B, so that B can verify it
is connected to A. In this case, a rogue SIP Proxy can see the secret
in the SIP signaling traffic and could potentially insert itself as a
man-in-the-middle.
Realistically, using TLS with certificates from well known
certificate authorities is difficult for peer-to-peer connections, as
the types of hosts that end clients use for sending instant messages
are unlikely to have long-term stable IP addresses or DNS names that
certificates can bind to. In addition, the cost of server
certificates from well-known certificate authorities is currently
expensive enough to discourage their use for each client. Using TLS
in a peer-to-peer mode without well known certificate is discussed in
Section 14.3.
TLS becomes much more practical when some form of relay is
introduced. Clients can then form TLS connections to relays, which
are much more likely to have TLS certificates. While this
specification does not address such relays, they are described by a
companion document [22]. That document makes extensive use of TLS to
protect traffic between clients and relays, and between one relay and
another.
TLS is used to authenticate devices and to provide integrity and
confidentiality for the header fields being transported. MSRP
elements MUST implement TLS and MUST also implement the TLS
ClientExtendedHello extended hello information for server name
indication as described in [11]. A TLS cipher-suite of
TLS_RSA_WITH_AES_128_CBC_SHA [13] MUST be supported (other cipher-
suites MAY also be supported).
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14.2. S/MIME
The only strong security for non-TLS connections is achieved using
S/MIME.
Since MSRP carries arbitrary MIME content, it can trivially carry
S/MIME protected messages as well. All MSRP implementations MUST
support the multipart/signed MIME type even if they do not support
S/MIME. Since SIP can carry a session key, S/MIME messages in the
context of a session could also be protected using a key-wrapped
shared secret [27] provided in the session setup. MSRP can carry
unencoded binary payloads. Therefore MIME bodies MUST be transferred
with a transfer encoding of binary. If a message is both signed and
encrypted, it SHOULD be signed first, then encrypted. If S/MIME is
supported, SHA-1, RSA, and AES-128 MUST be supported.
This does not actually require the endpoint to have certificates from
a well-known certificate authority. When MSRP is used with SIP, the
Identity [16] and Certificates [24] mechanisms provide S/MIME based
delivery of a secret between A and B. No SIP intermediary except the
explicitly trusted authentication service (one per user) can see the
secret. The S/MIME encryption of the SDP can also be used by SIP to
exchange keying material that can be used in MSRP. The MSRP session
can then use S/MIME with this keying material to encrypt and sign
messages sent over MSRP. The connection can still be hijacked since
the secret is sent in clear text to the other end of the TCP
connection, but the consequences are mitigated if all the MSRP
content is encrypted and signed with S/MIME. Although out of scope
for this document, the SIP negotiation of MSRP session can negotiate
symmetric keying material to be used with S/MIME for integrity and
privacy.
14.3. Using TLS in Peer to Peer Mode
TLS can be used with a self signed certificate as long as there is a
mechanism for both sides to ascertain that the other side used the
correct certificate. When used with SDP and SIP, the correct
certificate can be verified by passing a fingerprint of the
certificate in the SDP and ensuring that the SDP has suitable
integrity protection. When SIP is used to transport the SDP, the
integrity can be provided by the SIP Identity mechanism[16]. The
rest of this section describes the details of this approach.
If self-signed certificates are used, the content of the
subjectAltName attribute inside the certificate MAY use the uniform
resource identifier (URI) of the user. In SIP, this URI of the user
is the User's Address of Record (AOR). This is useful for debugging
purposes only and is not required to bind the certificate to one of
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the communication endpoints. Unlike normal TLS operations in this
protocol, when doing peer to peer TLS, the subjectAltName is not an
important component of the certificate verification. If the endpoint
is also able to make anonymous sessions, a distinct, unique
certificate MUST be used for this purpose. For a client that works
with multiple users, each user SHOULD have its own certificate.
Because the generation of public/private key pairs is relatively
expensive, endpoints are not required to generate certificates for
each session.
A certificate fingerprint is the output of a one-way hash function
computed over the distinguished encoding rules (DER) form of the
certificate. The endpoint MUST use the certificate fingerprint
attribute as specified in [17] and MUST include this in the SDP. The
certificate presented during the TLS handshake needs to match the
fingerprint exchanged via the SDP and if the fingerprint does not
match the hashed certificate then the endpoint MUST tear down the
media session immediately.
When using SIP, the integrity of the fingerprint can be ensured
through the SIP Identity mechanism [16]. When a client wishes to use
SIP to set up a secure MSRP session with another endpoint it sends an
offer in a SIP message to the other endpoint. This offer includes,
as part of the SDP payload, the fingerprint of the certificate that
the endpoint wants to use. The SIP message containing the offer is
sent to the offerer's sip proxy which will add an identity header
according to the procedures outlined in [16]. When the far endpoint
receives the SIP message it can verify the identity of the sender
using the identity header. Since the identity header is a digital
signature across several SIP headers, in addition to the body or
bodies of the SIP message, the receiver can also be certain that the
message has not been tampered with after the digital signature was
added to the SIP message.
An example of SDP with a fingerprint attribute is shown in the
following figure. Note the fingerprint is shown spread over two
lines due to formatting consideration but should all be on one line.
c=IN IP4 atlanta.example.com
m=message 7654 TCP/TLS/MSRP *
a=accept-types:text/plain
a=path:msrp://atlanta.example.com:7654/jshA7we;tcp
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
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14.4. Other Security Concerns
MSRP cannot be used as an amplifier for DoS attacks, but it can be
used to form a distributed attack to consume TCP connection resource
on servers. The attacker, Mallory, sends a SIP INVITE with no offer
to Alice. Alice returns a 200 with an offer and Mallory returns an
answer with SDP indicating that his MSRP address is the address of
Tom. Since Alice sent the offer, Alice will initiate a connection to
Tom using up resources on Tom's server. Given the huge number of IM
clients, and the relatively few TCP connections that most servers
support, this is a fairly straightforward attack.
SIP is attempting to address issues in dealing with spam. The spam
issue is probably best dealt with at the SIP level when an MSRP
session is initiated and not at the MSRP level.
If a sender chooses to employ S/MIME to protect a message, all S/MIME
operations apply to the complete message, prior to any breaking of
the message into chunks.
The signaling will have set up the session to or from some specific
URLs that will often have "im:" or "sip:" URI schemes. When the
signaling has been set up to a specific end user, and S/MIME is
implemented, then the client needs to verify that the name in the
SubjectAltName of the certificate contains an entry that matches the
URI that was used for the other end in the signaling. There are some
cases, such as IM conferencing, where the S/MIME certificate name and
the signaled identity will not match. In these cases, the client
should ensure that the user is informed that the message came from
the user identified in the certificate and does not assume that the
message came from the party they signaled.
In some cases, a sending device may need to attribute a message to
some other identity, and may use different identities for different
messages in the same session. For example, a conference server may
send messages on behalf of multiple users on the same session.
Rather than add additional header fields to MSRP for this purpose,
MSRP relies on the message/cpim format for this purpose. The sender
may envelop such a message in a message/cpim body, and place the
actual sender identity in the From field. The trustworthiness of
such an attribution is affected by the security properties of the
session in the same way that the trustworthiness of the identity of
the actual peer is affected, with the additional issue of determining
whether the recipient trusts the sender to assert the identity.
This approach can result in nesting of message/cpim envelopes. For
example, a message originates from a CPIM gateway, and is then
forwarded by a conference server onto a new session. Both the
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gateway and the conference server introduce envelopes. In this case,
the recipient client SHOULD indicate the chain of identity assertions
to the user, rather than allow the user to assume that either the
gateway or the conference server originated the message.
It is possible that a recipient might receive messages that are
attributed to the same sender via different MSRP sessions. For
example, Alice might be in a conversation with Bob via an MSRP
session over a TLS protected channel. Alice might then receive a
different message from Bob over a different session, perhaps with a
conference server that asserts Bob's identity in a message/cpim
envelope signed by the server.
MSRP does not prohibit multiple simultaneous sessions between the
same pair of identities. Nor does it prohibit an endpoint sending a
message on behalf of another identity, such as may be the case for a
conference server. The recipient's endpoint should determine its
level of trust of the authenticity of the sender independently for
each session. The fact that an endpoint trusts the authenticity of
the sender on any given session should not affect the level of trust
it assigns for apparently the same sender on a different session.
When MSRP clients form or acquire a certificate, they SHOULD ensure
that the subjectAltName has a GeneralName entry of type
uniformResourceIdentifier for each URL corresponding to this client
and should always include an "im:" URI. It is fine if the
certificate contains other URIs such as "sip:" or "xmpp:" URIs.
MSRP implementors should be aware of a potential attack on MSRP
devices that involves placing very large values in the byte-range
header field, potentially causing the device to allocate very large
memory buffers to hold the message. Implementations SHOULD apply
some degree of sanity checking on byte-range values before allocating
such buffers.
15. IANA Considerations
This specification instructs IANA to create a new registry for MSRP
parameters. The MSRP Parameter registry is a container for sub-
registries. This section further introduces sub-registries for MSRP
method names, status codes, and header field names.
[NOTE TO IANA/RFC Editor: Please replace all occurrences of RFCXXXX
in this section with the actual number assigned to this document.]
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15.1. MSRP Method Names
This specification establishes the Method sub-registry under MSRP
Parameters and initiates its population as follows:
SEND - [RFCXXXX]
REPORT - [RFCXXXX]
The following information must be provided in an RFC publication in
order to register a new MSRP Method:
The method name.
The RFC number in which the method is registered.
15.2. MSRP Header Fields
This specification establishes the header field-Field sub-registry
under MSRP Parameters. Its initial population is defined as follows:
To-Path - [RFCXXXX]
From-Path - [RFCXXXX]
Success-Report - [RFCXXXX]
Failure-Report - [RFCXXXX]
Byte-Range - [RFCXXXX]
Status - [RFCXXXX]
The following information must be provided in an RFC publication in
order to register a new MSRP Method:
The header field name.
The RFC number in which the method is registered.
15.3. MSRP Status Codes
This specification establishes the Status-Code sub-registry under
MSRP Parameters. Its initial population is defined in Section 10.
It takes the following format:
Code [RFC Number]
The following information must be provided in an RFC publication in
order to register a new MSRP Method:
The status code number.
The RFC number in which the method is registered.
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15.4. MSRP Port
MSRP uses TCP port XYZ. Usage of this value is described in
Section 6.
[NOTE TO IANA/RFC Editor: Please replace XYZ in this section with the
assigned port number.]
15.5. MSRP URL Schemes
This document defines the URL schemes of "msrp" and "msrps".
Syntax: See Section 6.
Character Encoding: See Section 6.
Intended Usage: See Section 6.
Protocols: The Message Session Relay Protocol (MSRP).
Security Considerations: See Section 14.
Relevant Publications: RFCXXXX
15.6. SDP Transport Protocol
MSRP defines the a new SDP protocol field values "TCP/MSRP" and "TCP/
TLS/MSRP", which should be registered in the sdp-parameters registry
under "proto". This first value indicates the MSRP protocol when TCP
is used as an underlying transport. The second indicates that TLS is
used.
Specifications defining new protocol values must define the rules for
the associated media format namespace. The "TCP/MSRP" and "TCP/TLS/
MSRP" protocol values allow only one value in the format field (fmt),
which is a single occurrence of "*". Actual format determination is
made using the "accept-types" and "accept-wrapped-types" attributes.
15.7. SDP Attribute Names
This document registers the following SDP attribute parameter names
in the sdp-parameters registry. These names are to be used in the
SDP att-name field.
15.7.1. Accept Types
Contact Information: Ben Campbell (ben@estacado.net)
Attribute-name: accept-types
Long-form Attribute Name: Acceptable MIME Types
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Type: Media level
Subject to Charset Attribute: No
Purpose and Appropriate Values: The "accept-types" attribute contains
a list of MIME content-types that the endpoint is willing to
receive. It may contain zero or more registered MIME types, or
"*" in a space delimited string.
15.7.2. Wrapped Types
Contact Information: Ben Campbell (ben@estacado.net)
Attribute-name: accept-wrapped-types
Long-form Attribute Name: Acceptable MIME Types Inside Wrappers
Type: Media level
Subject to Charset Attribute: No
Purpose and Appropriate Values: The "accept-wrapped-types" attribute
contains a list of MIME content-types that the endpoint is willing
to receive in an MSRP message with multipart content, but may not
be used as the outermost type of the message. It may contain zero
or more registered MIME types, or "*" in a space delimited string.
15.7.3. Max Size
Contact Information: Ben Campbell (ben@estacado.net)
Attribute-name: max-size
Long-form Attribute Name: Maximum message size.
Type: Media level
Subject to Charset Attribute: No
Purpose and Appropriate Values: The "max-size" attribute indicates
the largest message an endpoint wishes to accept. It may take any
numeric value, specified in octets.
15.7.4. Path
Contact Information: Ben Campbell (ben@estacado.net)
Attribute-name: path
Long-form Attribute Name: MSRP URL Path
Type: Media level
Subject to Charset Attribute: No
Purpose and Appropriate Values: The "path" attribute indicates a
series of MSRP devices that must be visited by messages sent in
the session, including the final endpoint. The attribute contains
one or more MSRP URIs, delimited by the space character.
16. Contributors and Acknowledgments
In addition to the editors, the following people contributed
extensive work to this document: Chris Boulton, Paul Kyzivat, Orit
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Levin, Adam Roach, Jonathan Rosenberg, and Robert Sparks.
The following people contributed substantial discussion and feedback
to this ongoing effort: Eric Burger, Allison Mankin, Jon Peterson,
Brian Rosen, Dean Willis, Aki Niemi, Hisham Khartabil, Pekka Pessi,
Miguel Garcia, Peter Ridler, Sam Hartman, and Jean Mahoney.
17. References
17.1. Normative References
[1] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[2] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", draft-ietf-mmusic-sdp-new-26 (work in
progress), July 2006.
[3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[4] 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.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[7] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851,
July 2004.
[8] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[9] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The Content-
Disposition header field", RFC 2183, August 1997.
[10] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 3986,
January 2005.
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[11] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
T. Wright, "Transport Layer Security (TLS) Extensions",
RFC 3546, June 2003.
[12] Klyne, G. and D. Atkins, "Common Presence and Instant Messaging
(CPIM): Message Format", RFC 3862, August 2004.
[13] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
Transport Layer Security (TLS)", RFC 3268, June 2002.
[14] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 3629, November 2003.
[15] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", rfc 2046,
November 1996.
[16] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-06 (work in progress), October 2005.
[17] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", draft-ietf-mmusic-comedia-tls-05
(work in progress), September 2005.
17.2. Informational References
[18] Johnston, A. and O. Levin, "Session Initiation Protocol Call
Control - Conferencing for User Agents",
draft-ietf-sipping-cc-conferencing-07 (work in progress),
June 2005.
[19] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control in the
Session Initiation Protocol", RFC 3725, April 2004.
[20] Sparks, R., Johnston, A., and D. Petrie, "Session Initiation
Protocol Call Control - Transfer",
draft-ietf-sipping-cc-transfer-05 (work in progress),
July 2005.
[21] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and
D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[22] Jennings, C., Mahy, R., and A. , "Relay Extensions for Message
Sessions Relay Protocol (MSRP)",
Campbell, et al. Expires August 29, 2006 [Page 56]
Internet-Draft MSRP February 2006
draft-ietf-simple-msrp-relays-07 (work in progress),
February 2006.
[23] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.
[24] Jennings, C. and J. Peterson, "Certificate Management Service
for SIP", draft-ietf-sipping-certs-02 (work in progress),
July 2005.
[25] Yon, D. and G. Camarillo, "Connection-Oriented Media Transport
in SDP", rfc 4145, September 2005.
[26] Peterson, J., "A Common Profile for Instant Messaging (CPIM)",
rfc 3860, August 2004.
[27] Housley, R., "Triple-DES and RC2 Key Wrapping", RFC 3217,
December 2001.
[28] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone
Generation in the Session Initiation Protocol (SIP)", rfc 3960,
December 2004.
[29] Saint-Andre, P., "Extensible Messaging and Presence Protocol
(XMPP): Instant Messaging and Presence", RFC 3921,
October 2004.
[30] Rosenberg, J., "Indicating User Agent Capabilities in the
Session Initiation Protocol (SIP)", RFC 3840, August 2004.
[31] Peterson, J., "Address Resolution for Instant Messaging and
Presence", rfc 3861, August 2004.
Campbell, et al. Expires August 29, 2006 [Page 57]
Internet-Draft MSRP February 2006
Authors' Addresses
Ben Campbell (editor)
Estacado Systems
17210 Campbell Road
Suite 250
Dallas, TX 75252
USA
Email: ben@estacado.net
Rohan Mahy (editor)
SIP Edge, LLC
Email: rohan@ekabal.com
Cullen Jennings (editor)
Cisco Systems, Inc.
170 West Tasman Dr.
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 421-9990
Email: fluffy@cisco.com
Campbell, et al. Expires August 29, 2006 [Page 58]
Internet-Draft MSRP February 2006
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Campbell, et al. Expires August 29, 2006 [Page 59]