SIMPLE Working Group B. Campbell
Internet-Draft J. Rosenberg
Expires: October 11, 2004 R. Sparks
dynamicsoft
P. Kyzivat
Cisco Systems
C. Boulton
Ubiquity Software Corporation
April 12, 2004
The Message Session Relay Protocol
draft-ietf-simple-message-sessions-05
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes the Message Session Relay Protocol (MSRP), a
mechanism for transmitting a series of Instant Messages within a
session. MSRP sessions are managed using the Session Description
Protocol (SDP) offer/answer model carried by a signaling protocol
such as the Session Initiation Protocol (SIP).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 4
2. Motivation for Session-mode Messaging . . . . . . . . . . 4
3. Scope of this Document . . . . . . . . . . . . . . . . . . 5
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . 6
5. Architectural Considerations . . . . . . . . . . . . . . . 7
6. SDP Offer-Answer Exchanges for MSRP Sessions . . . . . . . 7
6.1 Use of the SDP M-line . . . . . . . . . . . . . . . . . . 7
6.2 The Accept Types Attribute . . . . . . . . . . . . . . . . 8
6.3 MIME Wrappers . . . . . . . . . . . . . . . . . . . . . . 8
6.4 URL Negotiations . . . . . . . . . . . . . . . . . . . . . 9
6.5 Path Attributes with Multiple URLs . . . . . . . . . . . . 10
6.6 Updated SDP Offers . . . . . . . . . . . . . . . . . . . . 11
6.7 Example SDP Exchange . . . . . . . . . . . . . . . . . . . 11
6.8 Connection Negotiation . . . . . . . . . . . . . . . . . . 12
7. The Message Session Relay Protocol . . . . . . . . . . . . 12
7.1 MSRP URLs . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1.1 MSRP URL Comparison . . . . . . . . . . . . . . . . . . . 13
7.1.2 Resolving MSRP Host Device . . . . . . . . . . . . . . . . 13
7.1.3 The msrps URL Scheme . . . . . . . . . . . . . . . . . . . 14
7.2 Connection Managment . . . . . . . . . . . . . . . . . . . 14
7.3 MSRP messages . . . . . . . . . . . . . . . . . . . . . . 15
7.4 MSRP Transactions . . . . . . . . . . . . . . . . . . . . 16
7.5 MSRP Sessions . . . . . . . . . . . . . . . . . . . . . . 17
7.5.1 Initiating an MSRP session . . . . . . . . . . . . . . . . 17
7.5.2 Handling VISIT requests . . . . . . . . . . . . . . . . . 19
7.5.3 Sending Instant Messages on a Session . . . . . . . . . . 19
7.5.4 Ending a Session . . . . . . . . . . . . . . . . . . . . . 20
7.5.5 Managing Session State and Connections . . . . . . . . . . 21
7.6 Delivery Status Notification . . . . . . . . . . . . . . . 22
7.6.1 Endpoint DSN Request . . . . . . . . . . . . . . . . . . . 22
7.6.2 DSN generation . . . . . . . . . . . . . . . . . . . . . . 23
7.6.3 Receiving positive DSN . . . . . . . . . . . . . . . . . . 24
7.6.4 Receiving negative DSN . . . . . . . . . . . . . . . . . . 24
7.6.5 DSN headers in MSRP . . . . . . . . . . . . . . . . . . . 24
7.7 Message Fragmentation . . . . . . . . . . . . . . . . . . 24
7.7.1 MSRP Usage of message/byteranges . . . . . . . . . . . . . 24
7.8 Method Descriptions . . . . . . . . . . . . . . . . . . . 25
7.8.1 SEND . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7.8.2 VISIT . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7.8.3 REPORT . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.9 Response Code Descriptions . . . . . . . . . . . . . . . . 26
7.9.1 200 . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.9.2 400 . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.9.3 415 . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.9.4 426 . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.9.5 481 . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
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7.9.6 506 . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.10 Header Field Descriptions . . . . . . . . . . . . . . . . 26
7.10.1 TR-ID . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.10.2 To . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.10.3 From . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.10.4 Content-Type . . . . . . . . . . . . . . . . . . . . . . . 27
8. Example . . . . . . . . . . . . . . . . . . . . . . . . . 27
9. IANA Considerations . . . . . . . . . . . . . . . . . . . 30
9.1 MSRP Port . . . . . . . . . . . . . . . . . . . . . . . . 30
9.2 MSRP URL Schemes . . . . . . . . . . . . . . . . . . . . . 30
9.2.1 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.2.2 Character Encoding . . . . . . . . . . . . . . . . . . . . 30
9.2.3 Intended Usage . . . . . . . . . . . . . . . . . . . . . . 30
9.2.4 Protocols . . . . . . . . . . . . . . . . . . . . . . . . 30
9.2.5 Security Considerations . . . . . . . . . . . . . . . . . 30
9.2.6 Relevant Publications . . . . . . . . . . . . . . . . . . 30
9.3 SDP Parameters . . . . . . . . . . . . . . . . . . . . . . 31
9.3.1 Accept Types . . . . . . . . . . . . . . . . . . . . . . . 31
9.3.2 Wrapped Types . . . . . . . . . . . . . . . . . . . . . . 31
9.3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
10. Security Considerations . . . . . . . . . . . . . . . . . 31
10.1 TLS and the MSRPS Scheme . . . . . . . . . . . . . . . . . 31
10.1.1 Sensitivity of the Session URL . . . . . . . . . . . . . . 32
10.1.2 End to End Protection of IMs . . . . . . . . . . . . . . . 33
10.1.3 CPIM compatibility . . . . . . . . . . . . . . . . . . . . 33
10.1.4 PKI Considerations . . . . . . . . . . . . . . . . . . . . 34
11. Changes from Previous Draft Versions . . . . . . . . . . . 34
11.1 draft-ietf-simple-message-sessions-04 . . . . . . . . . . 34
11.2 draft-ietf-simple-message-sessions-03 . . . . . . . . . . 35
11.3 draft-ietf-simple-message-sessions-02 . . . . . . . . . . 35
11.4 draft-ietf-simple-message-sessions-01 . . . . . . . . . . 35
11.5 draft-ietf-simple-message-sessions-00 . . . . . . . . . . 36
11.6 draft-campbell-simple-im-sessions-01 . . . . . . . . . . . 37
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . 37
Normative References . . . . . . . . . . . . . . . . . . . 37
Informational References . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . 39
Intellectual Property and Copyright Statements . . . . . . 40
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1. Introduction
The MESSAGE [12] extension to SIP [2] allows SIP to be used to
transmit instant messages. Instant messages sent using the MESSAGE
method are normally independent of each other. This approach is often
called page-mode messaging, since it follows a model similar to that
used by many two-way pager devices. Page-mode messaging makes sense
for instant message exchanges where a small number of messages occur.
Endpoints may treat page-mode messages as if they took place in an
imaginative session, but there is no formal relationship between one
message and another.
There are also applications in which it is useful for instant
messages to be formally associated in a session. For example, a user
may wish to join a text conference, participate in the conference for
some period of time, then leave the conference. This usage is
analogous to regular media sessions that are typically initiated,
managed, and terminated using SIP. We commonly refer to this model as
session-mode messaging.
One of the primary purposes of SIP and SDP (Section 6) is the
management of media sessions. Session-mode messaging can be thought
of as a media session like any other. This document describes the
motivations for session-mode messaging, the Message Session Relay
Protocol, and the use of the SDP offer/answer mechanism for managing
MSRP session.
2. Motivation for Session-mode Messaging
Message sessions offer several advantages over page-mode messages.
For message exchanges that include more than a small number of
message transactions, message sessions offer a way to remove
messaging load from intervening SIP proxies. For example, a minimal
session setup and tear-down requires one INVITE/ACK transaction, and
one BYE transaction, for a total of 5 SIP messages. Normal SIP
request routing allows for all but the initial INVITE transaction to
bypass any intervening proxies that do not specifically request to be
in the path for future requests. Session-mode messages never cross
the SIP proxies themselves.
Each page-mode message involves a complete SIP transaction, that is,
a request and a response. Any page-mode message exchange that
involves more than 2 MESSAGE requests will generate more SIP requests
than a minimal session initiation sequence. Since MESSAGE is normally
used outside of a SIP dialog, these requests will typically traverse
the entire proxy network between the endpoints.
Due to network congestion concerns, the MESSAGE method has
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significant limitations in message size, a prohibition against
overlapping requests, etc. Much of this has been required because of
perceived limitations in the congestion-avoidance features of SIP
itself. Work is in progress to mitigate these concerns.
However, session-mode messages are always sent over reliable,
congestion-safe transports. Therefore, there are no restrictions on
message sizes. There is no requirement to wait for acknowledgement
before sending another message, so that message transactions can be
overlapped.
Message sessions allow greater efficiency for secure message
exchanges. The SIP MESSAGE request inherits the S/MIME features of
SIP, allowing a message to be signed and/or encrypted. However, this
approach requires public key operations for each message. With
session-mode messaging, a session key can be established at the time
of session initiation. This key can be used to protect each message
that is part of the session. This requires only symmetric key
operations for each subsequent IM, and no additional certificate
exchanges are required after the initial exchange. The establishment
of the session key can be done using standard techniques that apply
to voice and video, in addition to instant messaging.
Finally, SIP devices can treat message sessions like any other media
sessions. Any SIP feature that can be applied to other sorts of media
sessions can equally apply to message sessions. For example,
conferencing [14], third party call control [15], call transfer [16],
QoS integration [17], and privacy [18] can all be applied to message
sessions.
Messaging sessions can also reduce the overhead in each individual
message. In page-mode, each message needs to include all of the SIP
headers that are mandated by RFC 3261 [2]. However, many of these
headers are not needed once a context is established for exchanging
messages. As a result, messaging session mechanisms can be designed
with significantly less overhead.
3. Scope of this Document
This document describes the use of MSRP between endpoints. It does
not specify the use of intermediaries, nor does it prohibit such use.
We expect an extension to this specification to define MSRP
intermediaries and their use.
This document describes the use of MSRP over TCP. MSRP may be used
over other congestion-controlled protocols such as SCTP. However, the
specific bindings for other such protocols are outside the scope of
this document.
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4. Protocol Overview
The Message Session Relay Protocol (MSRP) provides a mechanism for
transporting session-mode messages between endpoints. MSRP uses
connection oriented, reliable network transport protocols only. It
can operate in the presence of many NAT and firewall environments, as
it allows participants to positively associate message sessions with
specific connections, and does not depend upon connection source
address, which may be obscured by NATs.
MSRP uses the following primitives:
SEND: Used to send message content from one endpoint to another.
VISIT: Used by an endpoint to establish a session association to the
host endpoint.
Assume A is an endpoint that wishes to establish a message session,
and B is the endpoint invited by A. A invites B to participate in a
message session by sending a URL. This URL is temporary, and must not
duplicate any URL that A has offered for other active sessions.
B "visits" A by connecting to A and sending a VISIT request
containing the URL that A provided. This associates the connection
from B with the session. B then responds to the invitation with a URL
of its own. This informs A that B has accepted the session, and will
accept messages at that URL. A and B may now exchange messages using
SEND requests on the connection. Each party targets such requests to
the peer's URL.
When either party wishes to end the session, it informs its peer
using the appropriate mechanism of the chosen signaling protocol,
such as a SIP BYE request.
The end to end case looks something like the following. (Note that
the example shows a logical flow only; syntax will come later in this
document.)
A->B (SDP): offer (msrp://A/123)
B->A (MSRP): VISIT (msrp://A/123, msrp://B/456)
A->B (MSRP): 200 OK
B->A (SDP): answer(msrp://B/456)
A->B (MSRP): SEND (msrp://B/456)
B->A (MSRP): 200 OK
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B->A (MSRP): SEND (msrp://A/123)
A->B (MSRP): 200 OK
5. Architectural Considerations
There are a number of considerations that, if handled in a reasonable
fashion, will allow more effective use of the protocols described in
this document.
6. SDP Offer-Answer Exchanges for MSRP Sessions
MSRP sessions will typically be initiated using the Session
Description Protocol (SDP) [1] offer-answer mechanism, carried in the
Session Initiation Protocol (SIP) [2] or any other protocol
supporting it.
6.1 Use of the SDP M-line
The SDP "m"-line takes the following form:
m=<media> <port> <protocol> <format list>
For non-RTP media sessions, The media field specifies the top level
MIME media type for the session. For MSRP sessions, the media field
MUST have the value of "message". The port field is normally not
used, and MAY be set to any value chosen by the endpoint. A port
field value of zero has the standard SDP meaning. Non-zero values
MUST not be repeated in other MSRP m-lines in the same SDP document.
The proto field MUST designate the message session mechanism and
transport protocol, separated by a "/" character. For MSRP, left part
of this value MUST be "msrp". For MSRP over TCP, the right part of
this field MUST take the value "tcp". For MSRP over other transport
protocols, the field value MUST be defined by the specification for
that protocol binding.
The format list list is ignored for MSRP. This is because MSRP
formats are specified as MIME content types, which are not convenient
to encode in the SDP format list syntax. Instead, the allowed formats
are negotiated using "a"-line attributes. For MSRP sessions, the
format list SHOULD contain a "*" character, and nothing else.
The port field in the M-line is not used to determine the port to
which to connect. Rather, the actual port is determined by the the
MSRP URL (Section 7.1) in the path attribute. However, a port value
of zero has the normal SDP meaning.
The following example illustrates an m-line for a message session,
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where the endpoint is willing to accept root payloads of message/
cpim, plain text or HTML. The second two types could either be
presented as the root body, or could be contained within message/cpim
bodies.
m=message 9999 msrp/tcp *
6.2 The Accept Types Attribute
MSRP can carry any MIME encoded payload. Endpoints specify MIME
content types that they are willing to receive in the accept types
"a"-line attribute. This attribute has the following syntax:
accept-types = accept-types-label ":" format-list
accept-types-label = "accept-types"
format-list = format-entry *( SP
format-entry) format-entry = (type "/" subtype) / ("*")
type = token
subtype = token
SDP offers for MSRP sessions MUST include an accept-types attribute.
SDP answers MUST also include the attribute, which MUST contain
either the same list as in the offer or a subset of that list.
A "*" entry in the accept-types attribute indicates that the sender
may attempt to send messages with media types that have not been
explicitly listed. If the receiver is able to process the media type,
it does so. If not, it will respond with a 415. 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.
(Note that the container type MUST also be listed in the accept-types
attribute.)
6.3 MIME Wrappers
The MIME content-types in the accept-types attribute will often
include container types; that is, types that contain other types. For
example, "message/cpim" or "multipart/mixed." 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 to be wrapped
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inside compound types using the "accept-wrapped-types" attribute in
an SDP a-line. This attribute has the following syntax:
accept-wrapped-types = wrapped-types-label ":" format-list
wrapped-types-label = "accept-wrapped-types" `
The format-list element has the identical syntax as defined for the
accept-types attribute. The semantics for this attribute are
identical to those of the accept-types attribute, with the exception
that the specified types may only be used when wrapped inside
containers. Only types listed in accept-types 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 the m-line 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
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 could choose to use those
types without the wrapper.
6.4 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 MUST
contain one or more MSRP URL in a path attribute. This attribute has
the following syntax:
a=path ":" MSRP_URL *(SP MSRP_URL)
where MSRP_URL is an MSRP or MSRPS URL as defined in Section 7.1.
The answerer will use the offered URL(s) to resolve the host address
and port when connecting, and to identify the target when sending
messages. For MSRP sessions, the address field in the C-line is not
relevant, and MUST be ignored. The port field in the M-line MUST be
ignored if non-zero. Zero values have the usual meaning for SDP.
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Both offerer and answerer store the path values received from the
peer. For a given endpoint, the local URL is the URL that the
endpoint put into a path attribute value to send to its peer. The
peer URL is the URL received from the peer. If the path attribute
received from the peer contains more than one URL, then the peer URL
is the last entry, while the first entry is the connection URL. If
only one entry is present, then it is both the peer and connection
URL. The remote path is the entire path attribute value received from
the peer.
The following example shows an SDP offer with a session URL of
"msrp://a.example.com:7394/2s93i"
v=0
o=someuser 2890844526 2890844527 IN IP4 alice.example.com
s=
c=IN IP4 alice.example.com m=message 9999 msrp/tcp *
a=accept-types:text/plain
a=path:msrp://a.example.com:7394/2s93i
The first URI in the path attribute MUST identify the endpoint that
generated the SDP document, or some other location where that
endpoint wishes to receive messages associated with the session. If
the URL identifies the endpoint, it MUST MUST be a temporary URL
assigned just for this particular session, and MUST NOT duplicate any
URL in use for any other session in which the endpoint is currently
participating. Further, it SHOULD be hard to guess, and protected
from eavesdroppers. This will be discussed in more detail in Section
10.
6.5 Path Attributes with Multiple URLs
As mentioned previously, this document describes MSRP for
peer-to-peer scenarios, that is, when no relays are used. However, we
expect a separate document to describe the use of relays in the near
future. The path attribute supports lists of URLs in order to
facilitate that work. For peer-to-peer session, a path attribute will
contain exactly one URL, describing an endpoint. This means that
endpoints that only implement this specification will never send more
than one URL in a path attribute, but MUST be prepared to receive
more than one. When an endpoint receives more than one URL in a path
header, only the first entry is relevant for purposes of resolving
the address and port, and establishing the network connection, thus
the term connection URL.
If an endpoint puts more than one URL in a path attribute, final URL
in the path (the peer URL) attribute MUST exhibit the uniqueness
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properties described above. Uniqueness requirements for other entries
in the attribute are out of scope for this document.
6.6 Updated SDP Offers
To do: Revisit this section based on new connection management rules
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
timers. 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.
If either party wish to send an SDP document that changes nothing at
all, then it MUST have the same o-line version as in the previous
exchange.
6.7 Example SDP Exchange
Endpoint A wishes to invite Endpoint B to a 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 9999 msrp/tcp *
a=accept-types: message/cpim text/plain text/html
a=path:msrp://alice.example.com:7394/2s93i9
Endpoint B performs a VISIT transaction passing the URL of msrp://
alice.example.com:7394/2s93i9. B indicates that it has accomplished
this by answering with:
v=0
o=userb 2890844530 2890844532 IN IP4 bob.example.com
s=
c=IN IP4 dontlookhere
t=0 0
m=message 9999 msrp/tcp *
a=accept-types:message/cpim text/plain
a=path:msrp://bob.example.com:8493/si438ds
A may now send IMs to B by executing SEND transactions.
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6.8 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 COMEDIA [20]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 accpet 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 for
a number of reasons:
It added a great deal of complexity to session creation.
The work in MSRP had begun to diverge from the work in MMUSIC.
There was a lack of successful implementation experience of the
COMEDIA work.
7. The Message Session Relay Protocol
The Message Session Relay Protocol (MSRP) is a text based, message
oriented protocol for the transfer of instant messages in the context
of a session. MSRP uses the UTF8 character set.
MSRP messages MUST be sent over a reliable, congestion-controlled,
connection-oriented transport protocol. This document specifies the
use of MSRP over TCP. Other documents may specify bindings for other
such protocols.
7.1 MSRP URLs
An MSRP URL follows a subset of the URL syntax in Appendix A of
RFC2396 [4], with a scheme of "msrp":
msrp_url = "msrp://" [userinfo "@"] hostport ["/" resource]
resource = 1*unreserved
The constructions for "userinfo", "hostport", and "unreserved" are
detailed in RFC2396 [4].
An MSRP URL server part identifies a participant in an MSRP session.
If the server part contains a numeric IP address, it MUST also
contain a port. The resource part identifies a particular session the
participant. The absence of the resource part indicates a reference
to an MSRP host device, but does not specifically refer to a
particular session resource.
MSRP has an IANA registered recommended port defined in Section 9.1.
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This value is not a default, as the URL process described herein will
always explicitly resolve a port number. 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 server part 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, 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.
The following is an example of a typical MSRP URL:
msrp://host.example.com:8493/asfd34
7.1.1 MSRP URL Comparison
MSRP URL comparisons MUST be performed according to the following
rules:
1. The host part is compared as case insensitive.
2. If the port exists explicitly in either URL, then it must match
exactly. An URL with an explicit port is never equivalent to
another with no port specified.
3. The resource part is compared as case insensitive. A URL without
a resource part is never equivalent to one that includes a
resource part.
4. Userinfo parts are not considered for URL comparison.
Path normalization is not relevant for MSRP URLs. Escape
normalization is not required, since the relevant parts are limited
to unreserved characters.
7.1.2 Resolving MSRP Host Device
An MSRP host device is identified by the server part of an MSRP URL.
If the server part contains a numeric IP address and port, they MUST
be used as listed.
If the server part contains a host name and a port, the connecting
device MUST determine a host address by doing an A or AAAA DNS query,
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and use the port as listed.
If the server part contains a host name but no port, the connecting
device MUST perform the following steps:
1. Construct an SRV [6] query string by prefixing the host name with
the service field "_msrp" and the protocol field ("_tcp" for
TCP). For example, "_msrp._tcp.host.example.com".
2. Perform a DNS SRV query using this query string.
3. Select a resulting record according to the rules in RFC2782 [6].
Determine the port from the chosen record.
4. If necessary, determine a host device address by performing an A
or AAAA query on the host name field in the selected SRV result
record. If multiple A or AAAA records are returned, the first
entry SHOULD be chosen for the initial connection attempt. This
allows any ordering created in the DNS to be preserved.
5. If the 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. If all of these
fail, the device SHOULD attempt to use any additional SRV records
that may have been returned, following the normal rules for SRV
record selection.
In most cases, the transport protocol will be determined separately
from the resolution process. For example, if the MSRP URL was
communicated in an SDP offer or answer, the SDP M-line will contain
the transport protocol. When an MSRP URL is communicated outside of
SDP, the protocol SHOULD also be communicated. If a device needs to
resolve an MSRP URL and does not know the protocol, it SHOULD assume
TCP.
7.1.3 The msrps URL Scheme
The "msrps" URL Scheme indicates that each hop MUST be secured with
TLS. Otherwise, it is used identically as an MSRP URL, except that a
MSRPS URL MUST NOT be considered equivalent to an MSRP URL. The MSRPS
scheme is further discussed in Section 10.
7.2 Connection Managment
When an MSRP endpoint receives an SDP offer, and intends to accept
it, it MUST establish a connection device described by the connection
URL, if a connection does not already exist. If it already has a
connection associated with another session for which the connection
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URL host part matches the host part of the connection URL for this
session, it SHOULD use the that connection, instead. Once connected,
the answerer MUST send a VISIT request to associate the new session
with the connection, prior to sending the SDP answer.
Either endpoint MAY tear down a connection when it no longer has any
active or proposed sessions associated with the connection.
7.3 MSRP messages
MSRP messages are either requests or responses. Requests and
responses are distinguished from one another by the first line. The
first line of a Request takes the form of the request-start entry
below. Likewise, the first line of a response takes the form of
response-start. The syntax for an MSRP message is as follows:
msrp-message = request-start/response-start *(header CRLF)
[CRLF body]
request-start = "MSRP" SP length SP Method CRLF
response-start = "MSRP" SP length SP Status-Code SP
Reason CRLF
length = 1*DIGIT ; the length of the message,
; exclusive of the start line.
Method = SEND / VISIT / other-method
other-method = token
header = Tran-ID / Session-URL / Content-Types /
From / To / Message-Receipt / Receipt-ID /
Byte-Range
Status-Code = 200 ;Success
/ 400 ;Bad Request
/ 403 ;Forbidden
/ 415 ;Unsupported Content Type
/ 426 ;Upgrade Required
/ 481 ;No session
/ 506 ;duplicate session
Reason = token ; Human readable text describing status
Tran-ID = "Tr-ID" ":" token
Content-Type = "Content-Type" ":" media-type
media-type = type "/" subtype *( ";" parameter )
type = token
subtype = token
parameter = attribute "=" value
attribute = token
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value = token | quoted-string
To = "To" ":" msrp_url *(SP msrp_url)
From = "From" ":" msrp_url
Message-Receipt = "Message-Receipt" ":" message-receipt-spec
( SEMI receipt-type )
message-receipt-spec = "negative" / "none" / "all"
receipt-type = "receipt-type" "=" alt-receipt-type
alt-receipt-type = r-type SLASH r-subtype *(SEMI r-parameter)
r-type = discrete-type / composite-type
discrete-type = "text" / "image" / "audio" / "video"
/ "application" / extension-token
composite-type = "message" / "multipart" / extension-token
extension-token = ietf-token / x-token
ietf-token = token
x-token = "x-" token
r-subtype = extension-token / iana-token
iana-token = token
r-parameter = r-attribute "=" r-value
r-attribute = token
r-value = token / quoted-string
Receipt-ID = "Receipt-ID" ":" token
Byte-Range = "Byte-Range" ":" byte-range-spec
byte-range-spec = first-byte "-" last-byte
first-byte = 1*DIGIT
last-byte = 1*DIGIT
All requests and responses MUST contain at least a TR-ID header
field. All requests must also contain the To and From header fields.
Messages MAY contain other fields, depending on the method or
response code.
7.4 MSRP Transactions
An MSRP transaction consists of exactly one request and one response.
A response matches a transaction if it share the same TR-ID value,
and arrives on the same connection on which the transaction was sent.
Endpoints MUST select TR-ID header field values in requests so that
they are not repeated by the same endpoint in scope of the given
session. TR-ID values SHOULD be globally unique. The TR-ID space of
each endpoint is independent of that of its peer. Endpoints MUST NOT
infer any semantics from the TR-ID header field beyond what is stated
above. In particular, TR-ID values are not required to follow any
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sequence.
MSRP Transactions complete when a response is received, or after a
timeout interval expires with no response. Endpoints MUST treat such
timeouts in exactly the same way they would treat a 500 response. The
timeout interval SHOULD be 30 seconds, but other values may be
established as a matter of local policy.
7.5 MSRP Sessions
AN MSRP session is a context in which a series of instant messages
are exchanged, using SEND requests. A session has two endpoints,
identified by MSRP URLs.
7.5.1 Initiating an MSRP session
When an endpoint wishes to engage a peer in a message session, it
invites the peer to communicate using an SDP offer, carried over SIP
or some other protocol supporting the SDP offer/answer model. For the
purpose of this document, we will refer to the endpoint choosing to
initiate communication as the offerer, and the peer being invited as
the answerer.
The offerer MUST be prepared to accept a connection from the
answerer.
The offerer MUST perform the following steps:
1. Construct a MSRP URL to serve as the local URL. This URL MUST
resolve to the location that the offerer wishes to host the
connection.
2. Listen for a connection from the peer.
3. Construct an SDP offer as described in Section 6, including the
list of allowed IM payload formats in the accept-types attribute.
The offerer puts its local URL into the path attribute, as
described in Section 6.4. This URL becomes the offerer's local
path.
4. Send the SDP offer using the normal processing for the signaling
protocol.
If the answerer chooses to participate, it MUST perform the following
steps:
1. Parse the first URL from the offered path attribute, to be the
connection URL. The full path attribute value will be the
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answerer's remote path. If the path only contained a single URL
entry, then the connection URL and the remote path are identical.
2. Determine if it has any existing connection that is associated
with a connection URL host part that matches that of the
connection URL for this session, and with a transport protocol
matching that from the M-line. If one exists, the answerer SHOULD
use it for the new session rather than establishing a new
connection.
[Open Issue: Should we discuss situations when an endpoint may
want to intentially not share a connection?]
3. If no appropriate connection already exists, determine the host
address and port from the peer URL, following the procedures in
section Section 7.1, and connect using the transport protocol
from the M-line.
4. Construct a MSRP URL . This URL MUST resolve to the the answerer.
This URL becomes the answerer's local URL.
5. Construct a VISIT request, which MUST contain the following
information:
1. An To header field containing the remote URL.
2. A From containing the answerer's local URL.
3. A TR-ID header field containing a unique transaction ID.
4. A size field containing size of the message subsequent to the
start-line.
6. Send the request and wait for a response
7. If the VISIT transaction succeeds, send a SDP answer via the
signaling protocol, according to the following rules:
1. The C-line is copied unmodified from the offer.
2. The M-Line contains a dummy port value, the protocol field
from the original offer.
3. The accept-types attribute contains the SEND payload media
types that the answerer is willing to accept. The
accept-types attribute in the answer MUST be either the same
as that of the offer, or it MUST be a subset.
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4. The path attribute contains the answerer's local URL.
8. If the VISIT transaction fails, the answerer MUST reject the
offer.
7.5.2 Handling VISIT requests
An MSRP endpoint that is hosting a session will receive a VISIT
request from the visiting endpoint. When an endpoint receives a VISIT
request, it MUST perform the following procedures:
1. Check if state exists for a session with a local URL that matches
the To header field value of the VISIT request. If so, and if no
previous VISIT request has been received for that URL, then
return a 200 response, and save state associating the URL in the
From header field with the connection on which the request was
received .
2. If the state exists, and a matching VISIT transaction has already
occured, return a 506 response and do not change session state in
any way.
3. If no matching state exists, return a 481 response, and do not
change session state in any way.
7.5.3 Sending Instant Messages on a Session
Once a MSRP session has been established, either endpoint may send
instant messages to its peer using the SEND method. When an endpoint
wishes to do so, it MUST construct a SEND request according to the
following process:
1. Insert a To header field containing the remote path. Note that
this is the full remote path, not just the connection or peer
URL.
2. Insert a From header field containing the local URL.
3. Insert the message payload in the body, and the media type in the
Content-Type header field. The media type MUST match one of the
types in the format list negotiated in the SDP exchange. If a "*"
was present in the accept-types attribute, then the media type
SHOULD match one of the explicitly listed entries, but MAY be any
other arbitrary value.
4. Set the TR-ID header field to a unique value.
5. Send the request on the connection associated with the session.
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6. If a 2xx response code is received, the transaction was
successful.
7. If a 415 response is received, this indicates the recipient is
unable or unwilling to process the media type. The sender SHOULD
NOT attempt to send that particular media type again in the
context of this session.
8. If any other response code is received, or if the transaction
times out, the endpoint SHOULD assume the session has failed,
either tear down the session, or attempt to re-establish the
session by sending an updated SDP offer proposing a new
connection. If a new connection is established, the endpoint MAY
choose to resend the content on the new connection.
Open Issue: Do we need to create a duplicate mechanism to suppress
duplicate messages when a new connection is established in this
fashion? mechanism? List consensus seems to indicate we do. We may
need to specify that the tr-id space spans a sequence of
connections if they are associated with same stream, and of
course, specify what it means for a stream to span connections.
When an endpoint receives a SEND request, it MUST perform the
following steps.
1. Check that it has state for a session with a local URL matching
the To value. If no matching session exists, return a 481
response.
2. Determine that it understands the media type in the body, if any
exists.
3. If it does, return a 200 response and render the message to the
user. The method of rendering is a matter of local policy. If the
message contained no body at all, the endpoint should quietly
ingore it.
4. If it does not understand the media type, return a 415 response.
The endpoint MUST NOT return a 415 response for any media type
for which it indicated support in the SDP exchange.
7.5.4 Ending a Session
When either endpoint in an MSRP session wishes to end the session, it
first signals its intent using the normal processing for the
signaling protocol. For example, in SIP, it would send a BYE request
to the peer. After agreeing to end the session, the host endpoint
MUST release any resources acquired as part of the session.
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Each peer MUST destroy all local state for the session. This involves
completely removing the state entry for the session and invalidating
the session URL.
If no other sessions are using the connection, the endpoint that
opened it SHOULD tear it down. However, the passive party MAY tear
down an unused connection after a locally configured timeout period.
When an endpoint chooses to close a session, it may have SEND
transactions outstanding. For example, it may have send SEND requests
to which it has not yet received a response, or it may have received
SEND requests that to which it has not responded. Once an endpoint
has decided to close the connection, it SHOULD wait for such
outstanding transactions to complete. It SHOULD NOT generate any new
SEND transactions, and it MAY choose not to respond to any new SEND
requests that are received after it decides to close the session. It
SHOULD not respond to any new messages that arrive after it signals
its intent to close the session.
When an endpoint is signaled of its peer's intent to close a session,
it SHOULD NOT initiate any more SEND requests. It SHOULD wait for any
outstanding transactions that it initiated to complete, and it SHOULD
attempt respond to any open SEND transactions received prior to being
signaled.
It is not possible to completely eliminate the chance of a session
terminating with incomplete SEND transactions. When this occurs, the
endpoint SHOULD clearly inform the user that the messages may not
have been delivered.
7.5.5 Managing Session State and Connections
A MSRP session is represented by state at each endpoint, identified
by the local URL and remote path. An active session also has an
associated network connection.
If the connection fails for any reason, the session hosting device
MUST invalidate the session state for all sessions using the
connection. Once a connection is dropped, any associated session
state MUST NOT be reused. If an endpoint wishes to continue to
communicate after detecting a connection failure, it MAY initiate a
new SDP exchange to negotiate a new session URL. Otherwise, it SHOULD
attempt to tear down the session using the rules of the signaling
protocol.
It would be nice to allow sessions to be recovered after a
connection failure, perhaps by allowing the active endpoint to
reconnect, and send a new VISIT request. However, this approach
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creates a race condition between the time that the hosting device
notices the failed connection, and the time that the endpoint
tries to recover the session. If the endpoint attempts to
reconnect prior to the hosting device noticing the failure, the
hosting device will interpret the recovery attempt as a conflict.
The only way around this would be to force the hosting device to
do a liveness check on the original connection, which would create
a lot of complexity and overhead that do not seem to be worth the
trouble.
Open Issue: Is this still an issue with shared connections?
7.6 Delivery Status Notification
Delivery Status Notification (DSN)[10] provides an extensible MIME
content-type that is used to convey both positive and negative status
of messages in the network. This functionality is extremely useful
for MSRP sessions that traverse a relay device. Relay support is
considered out of scope for this specification and will be included
in a separate specification. This section will only cover
functionality required by non-relay aware endpoints for basic MSRP
operation. An MSRP endpoint MUST be capable of performing the DSN
operations described in this specification and SHOULD support the DSN
MIME type outlined. An MSRP endpoint MAY use an alternative payload
for reporting message status using the procedures outlined in this
specification which MUST be negotiated during the SDP offer/answer
exchange.
7.6.1 Endpoint DSN Request
An endpoint that wishes to be informed of message delivery/failure
needs to request such information. This is achieved by including an
MSRP Receipt-Request header in the request. The header can equal one
of three values:
negative: Indicates the client only requires failure delivery report.
none: Indicates the client requires no delivery reports.
all: Indicates the client requires both positive and negative
delivery reports.
Within the scope of this specification the Receipt-Request header is
only used in MSRP SEND requests. Future extensions to this
specification MAY use the mechanism described in this document for
delivery/failure status notification of other MSRP requests.
The default value for this header if not present in a request is
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'negative'. An example of this header would be:
Message-Receipt: negative
The default DSN MIME type is detailed in RFC 1894[10]. It is
possible for MSRP endpoints to use a different format if required.
This can be achieved by including a 'receipt-type' parameter in the
Message-Receipt header. This parameter contains the alternative MIME
type that SHOULD be used for this particular receipt transaction.
The value included in this header MUST equal a value negotiated
during the SDP offer/answer exchange.
Open Issue: If we negotiate this in the SDP, that also means the
format would be legal for normal messages. Is this okay? Also, I
assume that if we negotiated "*" in the sdp, then any format would be
legal? Do we even need this to be extensible?
Open Issue: Is the RFC1894 the right thing to use? Do we need to add
further verbiage on the format beyond the reference to the RFC?
7.6.2 DSN generation
An MSRP endpoint implementing this specification SHOULD be able to
generate positive delivery status of MSRP requests. On receiving an
MSRP request containing a Message-Receipt header with a value of
TRUE, the endpoint will carry out normal MSRP response generation and
MUST generate an MSRP REPORT request using the following procedures:
1. Insert a To header containing the From value from the original
request.
2. Insert a From header containing the To value from the original
request.
3. Insert the message status payload in the body of the request. If
the default DSN MIME type from DSN[10] is used then the MSRP
Content-Type header MUST use the value multipart/report. The
relevance of DSN headers in MSRP can be found in section 7.6.5.
An alternative MIME type MAY be used but MUST be specified in the
Content-Type header. Negative DSN generation is considered out
of the scope of this document and will be covered in a separate
MSRP relay document.
4. Insert a new transaction ID (TR-ID).
5. Insert the TR-ID value that appeared in the original MSRP request
into the Receipt-ID header. This allows a requesting client to
explicitly correlate a REPORT request with the original request.
This correlation is implementation specific and makes no
requirements on clients to hold state for transactions ID's.
Information regarding the original request can be obtained from
the DSN MIME type outlined in [10].
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6. If the associated SEND request contained a chunk, that is, used
the "message/byteranges" fromat, insert an MSRP Byte-Range header
containing the value from the Content-range header field. It is
possible that an intermediary device may have broken the MSRP
SEND request into chunks without the knowledge of the sending
client.
7.6.3 Receiving positive DSN
An MSRP endpoint implementing this specification MUST be able to
receive positive delivery status of MSRP requests.
7.6.4 Receiving negative DSN
An MSRP endpoint implementing this specification MUST be able to
receive negative delivery status of MSRP requests.
7.6.5 DSN headers in MSRP
To Do - Define meaning + relevance of DSN headers.
7.7 Message Fragmentation
MSRP devices MAY break large content into fragments, and send each
fragment in a separate SEND request. Each fragment is encapsulated
using the "message/byteranges" MIME type, defined in RFC2616 [11], to
correlate parts of the message. The definition of large is
determined by local policy. MSRP endpoints MUST be capable of
receiving and processing fragmented messages.
Open Issue: Do we want to negotiate the use of message/byterange like
any other MIME type? I assume no, as we want to allow relays to
fragment messages, and relays are not privy to the content-types
negotiated for a session.
Although relays are not in scope for this document, we expect that
relays will be able to introduce fragmentation, as well as change the
fragmentation of previously fragemented messages. Therefore, all MSRP
endpoints MUST be able to receive fragmented messages.
7.7.1 MSRP Usage of message/byteranges
The "message/byteranges" type allows multiple ranges in a single
document. However, MSRP devices MUST NOT include more than one byte
range in a single request. Although the HTTP usage for a document
containing a single byte range indicates putting the "Content-Range"
header in a header field, rather than in the body itself,
"Content-Range" MUST NOT appear as an MSRP header field.
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[Open Issue: How much of the message/byteranges specification should
we explain or copy forward? Copying too much obscures the fact that
rfc2616 is the normative definition, but it may be helpful to have
more context here.]
If the MSRP device has a priori knowledge of the overall content
length, it SHOULD put that length into instance-length. The device
MAY place a "*" in instance-length if it does not have such
knowledge.
Similarly, if the device has a priori knowledge of the number of
bytes in a byte range, it SHOULD place the last bype position in
last-byte-pos. Otherwise, it MAY use a "*". If "*" is present, the
recipient MUST determine the last-byte-position through normal
request framing and body processing. An MSRP device MUST put the
initial byte position in first-byte-pos.
7.8 Method Descriptions
This section summarizes the purpose of each MSRP method. All MSRP
messages MUST contain the TR-ID, From, and To header fields. All
messages MUST contain a length field in the start line that indicates
the overall length of the request, including any body, but not
including the start line itself. Additional requirements exist
depending on the individual method.
7.8.1 SEND
The SEND method is used by both the host and visitor endpoints to
send instant messages to its peer endpoint. A SEND request MUST
contain a To header field containing the sender's remote path, and a
From header field containing the sender's local URL. SEND requests
SHOULD contain a MIME body part. The body MUST be of a media type
included in the format list negotiated in the SDP exchange. If a body
is present, the request MUST contain a Content-Type header field
identifying the media type of the body.
To Do: We plan to expand the use of MIME headers to allow any
standard MIME header in a SEND request. This is not included in
this version for the sake of getting the draft out as soon as
possible, but will follow soon.
7.8.2 VISIT
The visiting endpoint uses the VISIT method to associate a network
connection with the session state at the hosting device. A VISIT
request MUST include a To header including the sender's connection
URL, and a From header field containing the sender's local URL.
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7.8.3 REPORT
Report is used by an endpoint/relay to convey message delivery status
7.9 Response Code Descriptions
This section summarizes the various response codes. Except where
noted, all responses MUST contain a TR-ID header field matching the
TR-ID header field of the original request, and To and From headers
matching those of the original request.
7.9.1 200
The 200 response code indicates a successful transaction.
7.9.2 400
A 400 response indicates a request was unintelligible.
7.9.3 415
A 415 response indicates the SEND request contained a MIME
content-type that is not understood by the receiver.
7.9.4 426
A 426 response indicates that the request is only allowed over TLS
protected connections.
7.9.5 481
A 481 response indicates that no session exists for the connection.
7.9.6 506
A 506 response indicates that a VISIT request occurred in which the
To header indicates a local path that is already associated with
another connection. A 506 response MUST NOT be returned in response
to any method other than VISIT.
7.10 Header Field Descriptions
This section summarizes the various header fields. MSRP header fields
are single valued; that is, they MUST NOT occur more than once in a
particular request or response.
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7.10.1 TR-ID
The TR-ID header field contains a transaction identifier used to map
a response to the corresponding request. A TR-ID value MUST be unique
among all values used by a given endpoint inside a given session.
MSRP elements MUST NOT assume any additional semantics for TR-ID.
7.10.2 To
The To header field is used to indicate the sender's remote path. All
MSRP requests MUST contain a To header field.
7.10.3 From
The From header field is used to indicate the sender's local URL. All
MSRP requests MUST contain a From header field.
7.10.4 Content-Type
The Content-Type header field is used to indicate the MIME media type
of the body. Content-Type MUST be present if a body is present.
To Do: The work group has agreed to allow the use of any standard
MIME header. This is not reflected in this version, but will be in
a shortly forthcoming one.
8. Example
This section shows an example message 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@atlanta.com and the answerer is
sip:bob@biloxi.com. In any given MSRP message, an "xx" in the length
field indicates the actual length of the rest of the message.
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Alice Bob
| |
| |
|(1) (SIP) INVITE |
|----------------------->|
|(2) (MSRP) VISIT |
|<-----------------------|
|(3) (MSRP) 200 OK |
|----------------------->|
|(4) (SIP) 200 OK |
|<-----------------------|
|(5) (SIP) ACK |
|----------------------->|
|(6) (MSRP) SEND |
|----------------------->|
|(7) (MSRP) 200 OK |
|<-----------------------|
|(8) (MSRP) SEND |
|<-----------------------|
|(9) (MSRP) 200 OK |
|----------------------->|
|(10) (SIP) BYE |
|----------------------->|
|(11) (SIP) 200 OK |
|<-----------------------|
| |
| |
1. Alice constructs a local URL of msrp://alicepc.atlanta.com:7777/
iau39 and listens for a connection on TCP port 7777.
Alice->Bob (SIP): INVITE sip:bob@biloxi.com
v=0
o=alice 2890844557 2890844559 IN IP4 host.anywhere.com
s=
c=IN IP4 fillername
t=0 0
m=message 9999 msrp/tcp *
a=accept-types:text/plain
a=path:msrp://alicepc.atlanta.com:7777/iau39
2. Bob opens a TCP connection to alicepc.atlanta.com:7777:
Bob->Alice (MSRP):
MSRP xx VISIT
To:msrp://alicepc.atlanta.com:7777/iau39
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From:msrp://bob.atlanta.com:8888/9di4ea
Tr-ID:sie09s
3. Alice->Bob (MSRP):
MSRP xx 200 OK
To:msrp://alicepc.atlanta.com:7777/iau39
From:msrp://bob.atlanta.com:8888/9di4ea
Tr-ID:sie09s
4. Bob->Alice (SIP): 200 OK
v=0
o=bob 2890844612 2890844616 IN IP4 host.anywhere.com
s=
c=IN IP4 ignorefield
t=0 0
m=message 9999 msrp/tcp *
a=accept-types:text/plain
a=path:msrp://bob.atlanta.com:8888/9di4ea
5. Alice->Bob (SIP): ACK
6. Alice->Bob (MSRP):
MSRP xx SEND
To:msrp://bob.atlanta.com:8888/9di4ea
From:msrp://alicepc.atlanta.com:7777/iau39
TR-ID: 123
Content-Type: "text/plain"
Hi, I'm Alice!
7. Bob->Alice (MSRP):
MSRP xx 200 OK
To:msrp://bob.atlanta.com:8888/9di4ea
From:msrp://alicepc.atlanta.com:7777/iau39
TR-ID: 123
8. Bob->Alice (MSRP):
MSRP xx SEND
To:msrp://alice.atlanta.com:7777/iau39
From:msrp://bob.atlanta.com:8888/9di4ea
TR-ID: 456
Content-Type: "text/plain"
Hi, Alice! I'm Bob!
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9. Alice->Bob (MSRP):
MSRP xx 200 OK
To:msrp://alice.atlanta.com:7777/iau39
From:msrp://bob.atlanta.com:8888/9di4ea
TR-ID: 456
10. Alice->Bob (SIP): BYE
Alice invalidates local session state.
11. Bob invalidates local state for the session.
Bob->Alice (SIP): 200 OK
9. IANA Considerations
9.1 MSRP Port
MSRP uses TCP port XYX, to be determined by IANA after this document
is approved for publication. Usage of this value is described in
Section 7.1
9.2 MSRP URL Schemes
This document defines the URL schemes of "msrp" and "msrps".
9.2.1 Syntax
See Section 7.1.
9.2.2 Character Encoding
See Section 7.1.
9.2.3 Intended Usage
See Section 7.1.
9.2.4 Protocols
The Message Session Relay Protocol (MSRP).
9.2.5 Security Considerations
See Section 10.
9.2.6 Relevant Publications
RFCXXXX
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[Note to RFC Editor: Please replace RFCXXXX in the above paragraph
with the actual number assigned to this document.
9.3 SDP Parameters
This document registers the following SDP parameters in the
sdp-parameters registry:
9.3.1 Accept Types
Attribute-name: accept-types
Long-form Attribute Name Acceptable MIME Types
Type: Media level
Subject to Charset Attribute No
Purpose and Appropriate Values See Section 6.2.
9.3.2 Wrapped Types
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 See Section 6.3.
9.3.3 Path
Attribute-name: path
Long-form Attribute Name MSRP URL Path
Type: Media level
Subject to Charset Attribute No
Purpose and Appropriate Values See Section 6.4.
10. Security Considerations
There are a number of security considerations for MSRP, some of which
are mentioned elsewhere in this document. This section discusses
those further, and introduces some new ones.
10.1 TLS and the MSRPS Scheme
All MSRP devices must support TLS, with at least the
TLS_RSA_WITH_AES_128_CBC_SHA [8] cipher suite. Other cipher suites
MAY be supported.
MSRP does not define a separate TCP port for TLS connections. This
means that all MSRP server devices, that is, all devices that listen
for TCP connections, MUST be prepared to handle both TLS and plain
text connections on the same port. When a device accepts a TCP
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connection, it MUST watch for the TLS handshake messages to determine
if a particular connection uses TLS. If the first data received is
not part of a start TLS request, the device ceases to watch for the
TLS handshake until it reads the entire message. Once the message has
been completely received, the device resumes watching for the start
TLS message.
Any MSRP device MAY refuse to accept a given request over a non-TLS
connection by returning a 426 response.
MSRP devices acting in the role of TCP client MAY perform a TLS
handshake at any time, as long as the request occurs between MSRP
messages. The endpoint MUST NOT send a start TLS request in the
middle of an MSRP message.
The working group considered only requiring the endpoint to watch
for a TLS handshake at the beginning of the session. However, the
endpoint should be able to determine if a new message is a start
TLS request or an MSRP request by only reading ahead three bytes.
Therefore, the working group chose to allow a session to switch to
TLS in mid-stream, as long as the switch occurs between MRSP
messages.
The MSRPS URI scheme indicates that all hops in an MSRP session MUST
be protected with TLS. Since this document does not specify the use
of intermidiary devices, then MSRPS support is trivially equivilant
to TLS support. However, if intermediaries do exist, either as
described in an MSRP extension document, or as some sort of
proprietary devices, they MUST ensure protection at all hops for an
MSRPS URL.
A VISIT request for an MSRPS URL MUST be sent over a TLS protected
connection. If a hosting device receives a VISIT request for an MSRPS
URL over an unprotected connection, it MUST reject the request with a
426 response.
10.1.1 Sensitivity of the Session URL
The URL sent in the SDP offer for a MSRP session is used by the
answerer to identify itself to the hosting device. If an attacker
were able to acquire the session URL, either by guessing it or by
eavesdropping, there is a window of opportunity in which the attacker
could hijack the session by sending a VISIT request to the host
device before the true visiting endpoint. Because of this
sensitivity, the session URL SHOULD be constructed in a way to make
it difficult to guess, and should be sufficiently random so that it
is unlikely to be reused. All mechanisms used to transport this URL
to the answerer SHOULD be protected from eavesdroppers and
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man-in-the-middle attacks.
Therefore a MSRP device MUST support the use of TLS for at least the
VISIT request, which by extension indicates the endpoint MUST support
the use of TLS for all MSRP messages. Further, MSRP connections
SHOULD actually be protected with TLS. Further, an MSRP endpoint MUST
be capable of using the security features of the signaling protocol
in order to protect the SDP exchange and SHOULD actually use them on
all such exchanges. End-to-end protection schemes SHOULD be preferred
over hop-by-hop schemes for protection of the SDP exchange.
10.1.2 End to End Protection of IMs
Instant messages can contain very sensitive information. As a result,
as specified in RFC 2779 [3], instant messaging protocols need to
provide for encryption, integrity and authentication of instant
messages. Therefore MSRP endpoints MUST support the end-to-end
encryption and integrity of bodies sent via SEND requests using
Secure MIME (S/MIME) [7].
Note that while each protected body could use separate keying
material, this is inefficient in that it requires an independent
public key operation for each message. Endpoints wishing to invoke
end-to-end protection of message sessions SHOULD exchange symmetric
keys in SDP k-lines, and use secret key encryption on for each MSRP
message. When symmetric keys are present in the SDP, the offer-answer
exchange MUST be protected from eavesdropping and tampering using the
appropriate facilities of the signaling protocol. For example, if the
signaling protocol is SIP, the SDP exchange MUST be protected using
S/MIME.
10.1.3 CPIM compatibility
MSRP sessions may be gatewayed to other CPIM [19]compatible
protocols. If this occurs, the gateway MUST maintain session state,
and MUST translate between the MSRP session semantics and CPIM
semantics that 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" [5] 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 features of that format.
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10.1.4 PKI Considerations
Several aspects of MSRP will benefit from being used in the context
of a public key infrastructure. For example, the MSRPS scheme allows,
and even encourages, TLS connections between endpoint devices. And
while MSRP allows for a symmetric session key to protect all messages
in a session, it is most likely that session key itself would be
exchanged in a signaling protocol such as SIP. Since that key is
extremely sensitive, its exchange would also need to be protected. In
SIP, the preferred mechanism for this would be S/MIME, which would
also benefit from a PKI.
However, all of these features may be used without PKI. Each endpoint
could instead use self signed certificates. This will, of course, be
less convenient than with a PKI, in that there would be no
certificate authority to act as a trusted introducer. Peers would be
required to exchange certificates prior to securely communicating.
Since, at least for the immediate future, any given MSRP
implementation is likely to communicate with at least some peers that
do not have a PKI available, MSRP implementations SHOULD support the
use of self-signed certificates, and SHOULD support the ability to
configure lists of trusted certificates.
To Do: Add text discussion the use of TLS certificates in more
detail.
11. Changes from Previous Draft Versions
This section to be deleted prior to publication as an RFC
11.1 draft-ietf-simple-message-sessions-04
Removed the direction attribute. Rather than using a comedia
styled direction negotiation, we just state that the answerer
opens any needed connection.
Changed the use of session URLs. Instead of a single session URL,
each endpoint is identified by a distinct URL. MSRP requests will
put the destination URL in a To header, and the sender URL in a
From header.
Changed the SDP exchange of MSRP URLs to handle the URL for each
endpoint. Further, changed the SDP attribute to support a list of
URLs in each direction. This may be used with relays to exchange
paths, rather than single URLs. MSRP endpoints must be able to
intelligently process such a list if received. This document does
not, however, describe how to generate such a list.
Added section for Delivery Status Notification handling, and added
associated entries into the syntax definition.
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Added content fragmentation section.
Removed recommendation to start separate session for large
transfers.
Corrected some mistakes in the syntax definitions.
Added Chris Boulton as a co-author for his contribution of the DSN
text.
11.2 draft-ietf-simple-message-sessions-03
Removed all specification of relays, and all features specific to
the use of relays. The working group has chosen to move relay work
into a separate effort, in order to advance the base
specification. (The MSRP acronym is unchanged for the sake of
convenience.) This included removal of the BIND method, all
response codes specific to BIND, Digest Authentication, and the
inactivity timeout.
Removed text indicating that an endpoint could retry failed
requests on the same connection. Rather, the endpoint should
consider the connection dead, and either signal a reconnection or
end the session.
Added text describing subsequent SDP exchanges. Added mandatory
"count" parameter to the direction attribute to allow explicit
signaling of the need to reconnect.
Added text to describe the use of send and receive only indicators
in SDP for one-way transfer of large content.
Added text requiring unique port field values if multiple M-line's
exist.
Corrected a number of editorial mistakes.
11.3 draft-ietf-simple-message-sessions-02
Moved all content type negotiation from the "m"-line format list
into "a"-line attributes. Added the accept-types attribute. This
is due to the fact that the sdp format-list syntax is not
conducive to encoding MIME content types values.
Added "other-method" construction to the message syntax to allow
for extensible methods.
Consolidated all syntax definitions into the same section. Cleaned
up ABNF for digest challenge and response syntax.
Changed the session inactivity timeout to 12 minutes.
Required support for the SHA1 alogorithm.
Required support for the message/cpim format.
Fixed lots of editorial issues.
Documented a number of open issues from recent list discussions.
11.4 draft-ietf-simple-message-sessions-01
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Abstract rewritten.
Added architectural considerations section.
The m-line format list now only describes the root body part for a
request. Contained body part types may be described in the
"accept-wrapped-types" a-line attribute.
Added a standard dummy value for the m-line port field. Clarified
that a zero in this field has normal SDP meaning.
Clarified that an endpoint is globally configured as to whether or
not to use a relay. There is no relay discovery mechanism
intrinsic to MSRP.
Changed digest algorithm to SHA1. Added TR-ID and S-URI to the
hash for digest authentication.
CMS usage replaced with S/MIME.
TLS and MSRPS usage clarified.
Session state timeout is now based on SEND activity, rather than
BIND and VISIT refreshes.
Default port added.
Added sequence diagrams to the example message flows.
Added discussion of self-signed certificates in the security
considerations section.
11.5 draft-ietf-simple-message-sessions-00
Name changed to reflect status as a work group item.
This version no longer supports the use of multiple sessions
across a single TCP session. This has several related changes:
There is now a single session URI, rather than a separate one for
each endpoint. The session URI is not required to be in requests
other than BIND and VISIT, as the session can be determined based
on the connection on which it arrives.
BIND and VISIT now create soft state, eliminating the need for the
RELEASE and LEAVE methods.
The MSRP URL format was changed to better reflect generic URL
standards. URL comparison and resolution rules were added. SRV
usage added.
Determination of host and visitor roles now uses a direction
attribute much like the one used in COMEDIA.
Format list negotiation expanded to allow a "prefer these formats
but try anything" semantic
Clarified handling of direction notification failures.
Clarified signaling associated with session failure due to dropped
connections.
Clarified security related motivations for MSRP.
Removed MIKEY dependency for session key exchange. Simple usage of
k-lines in SDP, where the SDP exchange is protected end-to-end
seems sufficient.
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11.6 draft-campbell-simple-im-sessions-01
Version 01 is a significant re-write. References to COMEDIA were
removed, as it was determined that COMEDIA would not allow
connections to be used bidirectional in the presence of NATs.
Significantly more discussion of a concrete mechanism has been added
to make up for no longer using COMEDIA. Additionally, this draft and
draft-campbell-cpimmsg-sessions (which would have also changed
drastically) have now been combined into this single draft.
12. Contributors
The following people contributed substantially to this ongoing
effort:
Rohan Mahy Allison Mankin Jon Peterson Brian Rosen Dean Willis Adam Roach
Cullen Jennings Aki Niemi Hisham Khartabil Pekka Pessi Chris Boulton
Normative References
[1] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[2] 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.
[3] Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging /
Presence Protocol Requirements", RFC 2779, February 2000.
[4] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URL): Generic Syntax", RFC 2396, August
1998.
[5] Atkins, D. and G. Klyne, "Common Presence and Instant Messaging
Message Format", draft-ietf-impp-cpim-msgfmt-08 (work in
progress), January 2003.
[6] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[7] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
2633, June 1999.
[8] Chown, P., ""Advanced Encryption Standard (AES) Ciphersuites
for Transport Layer Security (TLS)", RFC 3268, June 2002.
[9] Eastlake, 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
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(SHA1)", RFC 3174, September 2001.
[10] Moore, K. and G. Vaudreuil, "An Extensible Message Format for
Delivery Status Notifications", RFC 1894, January 1996.
[11] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
Informational References
[12] Campbell, B. and J. Rosenberg, "Session Initiation Protocol
Extension for Instant Messaging", RFC 3428, September 2002.
[13] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[14] Mahy, R., Campbell, B., Sparks, R., Rosenberg, J., Petrie, D.
and A. Johnston, "A Multi-party Application Framework for SIP",
draft-ietf-sipping-cc-framework-02 (work in progress), May
2003.
[15] Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
"Best Current Practices for Third Party Call Control in the
Session Initiation Protocol", draft-ietf-sipping-3pcc-04 (work
in progress), June 2003.
[16] Sparks, R. and A. Johnston, "Session Initiation Protocol Call
Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in
progress), February 2003.
[17] Camarillo, G., Marshall, W. and J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC
3312, October 2002.
[18] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323 , November 2002.
[19] Peterson, J., "A Common Profile for Instant Messaging (CPIM)",
draft-ietf-impp-im-04 (work in progress), August 2003.
[20] Yon, D., "Connection-Oriented Media Transport in SDP",
draft-ietf-mmusic-sdp-comedia-05 (work in progress), March
2003.
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Authors' Addresses
Ben Campbell
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
EMail: bcampbell@dynamicsoft.com
Jonathan Rosenberg
dynamicsoft
600 Lanidex Plaza
Parsippany, NJ 07054
EMail: jdrosen@dynamicsoft.com
Robert Sparks
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
EMail: rsparks@dynamicsoft.com
Paul Kyzivat
Cisco Systems
Mail Stop LWL3/12/2
900 Chelmsford St.
Lowell, MA 01851
EMail: pkyzivat@cisco.com
Chris Boulton
Ubiquity Software Corporation
Langstone Park
Newport, South Wales NP18 2LH
EMail: cboulton@ubiquity.net
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