Multi-party Chat
Using the Message Session Relay Protocol (MSRP)NokiaP.O. Box 407NOKIA GROUPFIN00045Finland+358 50 389 1644aki.niemi@nokia.comEricssonCalle Via de los Poblados 13MadridES28033Spainmiguel.a.garcia@ericsson.comCisco SystemsPhilip Pedersens vei 20N-1366 LysakerNorway+47 67 125 125geirsand@cisco.comhttp://www.cisco.com
General
I-DInternet-Draftmessagingmessage sessionsmulti-partyThe Message Session Relay Protocol (MSRP) defines a
mechanism for sending instant messages within a peer-to-peer
session, negotiated using the Session Initiation Protocol (SIP)
and the Session Description Protocol (SDP). This document
defines the necessary tools for establishing multi-party
chat sessions, or chat rooms, using MSRP.
The Message Session Relay Protocol (MSRP)
defines a mechanism for sending a series of instant
messages within a session. The
Session Initiation Protocol (SIP) in
combination with the Session Description
Protocol (SDP) allows for two peers to establish and manage
such sessions.
In another application of SIP, a user agent can join in a
multi-party conversation called a conference that is hosted
by a specialized user agent called a focus
. Such a
conference can naturally involve MSRP sessions.
It is the responsibility of an
entity handling the media to relay instant messages received from
one participant to the rest of the participants in the
conference.
Several such systems already exist in the
Internet. Participants in a chat room can be identified with a
pseudonym or nickname, and decide whether their real identifier is
disclosed to other participants. Participants can also use a rich
set of features such as the ability to send private instant
messages to other participants.
Similar conferences supporting chat rooms are already
available today. For example, Internet
Relay Chat (IRC), Extensible
Messaging and Presence Protocol (XMPP): Core based chat
rooms, and many other proprietary systems provide chat room
functionality. Specifying equivalent functionality for
MSRP-based systems provides competitive features and enables
interworking between the systems.
This document defines requirements, conventions, and
extensions for providing private messages and nickname
management in centralized conferences with MSRP. Participants
in a chat room can be identified by a pseudonym, and decide if
their real identifier is disclosed to other participants. This
memo uses the SIP Conferencing
Framework as a design basis. It also aims to be
compatible with the A Framework for
Centralized Conferencing. Should requirements arise,
future mechanisms for providing similar functionality in
generic conferences might be developed, for example, where the
media is not only restricted to MSRP. The mechanisms described
in this document provide a future compatible short-term
solution for MSRP centralized conferences. 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, BCP 14, and indicate
requirement levels for compliant implementations.This memo deals with tightly coupled SIP
conferences defined in
SIP Conferencing Framework and adopts the terminology
from that document. In addition to that terminology,
we introduce some new terms:
a pseudonym or descriptive name associated to a
participant. See for details
an instance of a tightly coupled conference, in which the media
exchanged between the participants consist of MSRP
based instant messages. Also known as a chat room. a synonym for a multi-party chat. a URI that identifies a particular
chat room, and is a synonym of a Conference URI defined in
RFC 4353 . the conference participant that
originally created an instant message and sent it to the chat room for
delivery. the destination conference
participant(s). This defaults to the full conference participant
list, minus the IM Sender. a media level entity that is a
MSRP endpoint. It is a special MSRP endpoint that receives
MSRP messages, and delivers them to the other conference
participants. The MSRP switch has a similar role to a conference
mixer with the exception that the MSRP switch does not actually
"mix" together different input media streams; it merely relays the
messages between participants.
an instant message sent in a chat room intended for a
single participant. A private IM is usually rendered
distinctly from the rest of the IMs, indicating that the
message was a private communication.
a URI concealing the
participant's SIP AOR from the other participants in the
conference. The allocation of such a URI is out of scope of
this specification. An anonymous URI must be valid for the
length of the conference, and will be utilized by the MSRP
switch to forward messages to and from anonymous
participants.
a notification
mechanism that allows conference participants to learn
conference information including roster and state changes in
a conference. This would typically be A Session Initiation Protocol (SIP) Event
Package for Conference State or
Conference Event Package Data Format Extension for Centralized Conferencing.
Although conference frameworks describing many types of
conferencing applications already exist, such as the
Framework for Centralized Conferencing
and the
SIP Conferencing
Framework, the exact details of session-based instant
messaging conferences are not well-defined at the moment.
To allow interoperable chat implementations, for both
conference-aware, and conference-unaware user agents, certain
conventions for MSRP conferences need to be defined. It also seems
beneficial to provide a set of features that enhance the baseline
multi-party MSRP in order to be able to create systems that have
functionality on par with existing chat systems, as well as enable
building interworking gateways to these existing chat systems.
We define the following requirements:
A basic requirement is the existence of a multi-party
conference, where participants can join and leave the conference
and get instant messages exchanged to the rest of the participants.
A conference participant must be able to determine the
identifiers of the sender and recipient of the received
IMs. Note that the actual identifiers depend no those
which were selected by the sender or recipient when he or she
joined the conference.
A conference participant must be able to determine the
recipient of the received message. For instance, the recipient of the
message might be the entire conference or a single participant
of the conference (i.e., a private message).
It must be possible to send a message to a single
participant within the conference (i.e., a private instant message).
A conference participant may have a nickname or pseudonym
associated with their real identifier.
It must be possible for a participant to change their nickname
during the progress of the conference.
It must be possible that a participant is only known by an
anonymous identifier and not their real identifier to the rest
of the conference.
It must be possible for the conference participants to
learn the chat room capabilities described in this document.
In order to set up a conference, one must first be
created. Users wishing to host a conference themselves can of
course do just that; their User Agent (UA) simply morphs from an
ordinary UA into a special purpose one called a Focus UA.
Another, commonly used setup is one where a dedicated
node in the network functions as a Focus UA.
Each chat room has an identifier of its own: a SIP URI that
participants use to join the conference, e.g. by sending an
INVITE request. The conference focus processes the invitations,
and as such, maintains SIP dialogs with each participant. In a
multi-party chat, or chat room, MSRP is one of the
established media streams. Each conference participant
establishes an MSRP session with the MSRP switch, which is a
special purpose MSRP application. The MSRP sessions can be relayed
by one or more MSRP relays, which are specified in RFC 4976.
This is illustrated in
The MSRP switch is similar to a
conference mixer in that it handles media sessions with each
of the participants and bridges these streams together. However,
unlike a conference mixer, the MSRP switch merely forwards
messages between participants but doesn't actually mix the
streams in any way. The system is illustrated in
.
Typically conference participants also subscribe to a
conference event package to gather information about the
conference roster in the form of conference state
notifications. For example, participants can learn about
other participants' identifiers, including their nicknames.
All messages in the chat room use the
'Message/CPIM' wrapper content type,
so that it is possible to distinguish between private and regular
messages. When a participant wants to send an instant message to
the conference, it constructs an MSRP SEND request and submits
it to the MSRP switch including a regular payload (e.g. a
Message/CPIM message that contains a text, HTML, an image,
etc.). The Message/CPIM To header is set to the chat room
URI. The switch then fans out the SEND request to all of the
other participants using their existing MSRP sessions.
A participant can also send a private instant message
addressed to a participant whose identifier has been learned,
e.g. via a conference event package. In this case the sender
creates an MSRP SEND request with a Message/CPIM wrapper whose
To header contains not the chat room URI but the recipient's
URI. The MSRP switch then forwards the SEND request to that
recipient. This specification supports the sending of private
messages to one and only one recipient. However, if the
recipient is logged from different endpoints, the MSRP switch
will distribute the private message to each endpoint the
recipient is logged.
We extend the current MSRP negotiation that takes place in
SDP to allow participants to
learn whether the chat room supports and is willing to accept
(e.g. due to local policy restrictions) certain MSRP functions
defined in this memo, such as nicknames or private
messaging.
Naturally, when a participant wishes to leave a chat room,
it sends a SIP BYE request to the Focus UA and
terminates the SIP dialog with the focus and MSRP sessions with
the MSRP switch.
This document assumes that each chat room is allocated its own
SIP URI. A user joining a chat room sends an INVITE request to
that SIP URI, and as a result, a new MSRP session is established
between the user and the MSRP switch. It is assumed that an
MSRP session is mapped to a chat room. If a user wants to join
a second chat room, he creates a different INVITE request,
through a different SIP dialog, which leads to the creation of
a second MSRP session between the user and the MSRP
switch. Notice that these two MSRP sessions can still be
multiplexed over the same TCP connection as per regular MSRP
procedures. However, each chat room is associated to a
unique MSRP session and a unique SIP dialog.
Since we consider a chat room a particular type of conference
having MSRP media, the methods
defined by the SIP Conference
Framework for creating conferences are directly
applicable to a chat room.
Once a chat room is created, it is identified by a SIP URI,
like any other conference.
Participants usually join the conference by sending an
INVITE request to the conference URI. As long as the
conference policy allows, the INVITE request is accepted by
the focus and the user is brought into the conference.
The MSRP switch needs to be aware of the URIs of the
participant (SIP, Tel, or IM URIs) in order to validate
messages sent from this participant prior to their
forwarding. This information is known to the focus of the
conference. Therefore an interface between the focus and the
MSRP switch is assumed. However, the interface between the
focus and the MSRP switch is outside the scope of this
document.
Conference aware participants will detect that the peer is a
focus due to the presence of the
"isfocus" feature tag in the Contact header field of
the 200-class response to the INVITE request. Conference
unaware participants will not notice it is a focus, and can
not apply the additional mechanisms defined in this
document. Participants are also aware that the mixer is an
MSRP switch due to the presence of a 'message' media type
and either TCP/MSRP or TCP/TLS/MSRP as the protocol field in
the media line of SDP.
The conference focus of a chat room MUST include support for
a Message/CPIM top-level wrapper
for the MSRP messages by setting the 'accept-types' MSRP
media line attribute in the SDP offer or answer to include
'Message/CPIM'.
Note that the 'Message/CPIM' wrapper is used to carry the sender
information that, otherwise, it will not be available to the
recipient. Additionally, 'Message/CPIM' wrapper carries the
recipient information (e.g. To and Cc: headers).
If a participant wants to remain anonymous to the rest of the
participants in the conference, the participant's UA must provide
an anonymous URI to the conference focus. The URI will be used in
the From and To headers in the 'Message/CPIM' wrapper, and can
be learned by the other participants of the
conference. Notice that in order for the anonymity mechanism
to work, the anonymous URI must not reveal the participant's
SIP AOR. The mechanism for acquiring an anonymous URI is
outside the scope of this specification.
The conference focus of a chat room MUST learn the chat room
capabilities of each participant that joins the chat
room. The conference focus MUST inform the MSRP switch of
such support in order to prevent the MSRP switch from
distributing private messages to participants who do not
support private messaging. The recipient would not be able
to render the message as private, and any potential reply
would be sent to the whole chat room.
As with creating a conference, the methods defined by the
SIP Conference Framework
for deleting a conference are directly applicable to a chat
room. The MSRP switch will terminate the MSRP sessions with
all the participants.
Deleting a chat room is an action that heavily depends on the
policy of the chat room. The policy can determine that the chat
room is deleted when the creator leaves the conference, or with
any out of band mechanism.
This section describes the conventions used to send and receive
instant messages that are addressed to all the participants in the
chat room. These are sent over a regular MSRP SEND request that
contains a Message/CPIM wrapper
that in turn contains the desired payload (e.g. text, image,
video-clip, etc.).
When a chat room participant wishes to send an instant
message to all the other participants in the chat room,
it constructs an MSRP SEND request according to the
procedures specified in RFC
4975. The sender MAY choose the desired MSRP report
model (e.g., populate the Success-Report and Failure-Report
MSRP header fields).
The SEND request MUST contain a top-level wrapper of type
'Message/CPIM' according to RFC
3862. The actual instant message payload MUST be
included as payload of the 'Message/CPIM' wrapper and MAY be
of any type negotiated in the SDP 'accept-types' attribute
according to the MSRP rules.
On sending a regular message the sender MUST populate the To
header of the Message/CPIM wrapper with the URI of the chat
room. The sender SHOULD populate the From header of the
Message/CPIM wrapper with a proper identifier by which the
user is recognized in the conference. Identifiers that can be
used (among others) are:
A SIP URI representing the
participant's address-of-recordA tel URI representing the
participant's telephone numberAn IM URI representing the
participant's instant messaging addressAn Anonymous URI representing the participant's
anonymous address
An MSRP switch that receives a SEND request from a
participant SHOULD first verify that the From header field
of the Message/CPIM wrapper is correctly populated with a
valid URI of a participant. This imposes a requirement for
the focus of the conference to inform the MSRP switch of the
URIs by which the participant is known, in order for the
MSRP switch to validate messages.
provides further information with the actions to be taken in
case this validation fails.
Then the MSRP switch should inspect the To header field of
the Message/CPIM wrapper. If the MSRP switch receives a
message containing several To header fields in the
Message/CPIM wrapper the MSRP switch MUST reject the MSRP
SEND request with a 403 response, as per procedures in RFC 4975. Then, if the To header
field of the Message/CPIM wrapper contains the chat room URI
and there are no other To header fields, the MSRP switch can
generate a copy of the SEND request to each of the
participants in the conference except the sender. The MSRP
switch MUST NOT modify the content received in the SEND
request. However, the MSRP switch MAY re-chunk any of the
outbound MSRP SEND requests.
Note that the MSRP switch does not need to wait for the
reception of the complete MSRP chunk or MSRP message before
it starts the distribution to the rest of the
participants. Instead, once the MSRP switch has received the
headers of the Message/CPIM wrapper it SHOULD start the
distribution process. Having the header of the Message/CPIM
wrapper only in the first chunk, the MSRP switch MUST track
the Message-Id until the last chunk of the message has been
distributed.
An MSRP endpoint that receives a SEND request from the MSRP
switch containing a Message/CPIM wrapper SHOULD first
inspect the To header field of the Message/CPIM wrapper. If
the To header field is set to the chat room URI, it should
render it as a regular message that has been distributed to
all the participants in the conference. Then the MSRP
endpoint SHOULD inspect the From header field of the
Message/CPIM wrapper to identify the sender. The From header
field will include a URI that identifies the sender. The
endpoint might have also received further identifier
information through a subscription to a conference event
package.
This section describes the conventions used to send and
receive private instant messages, i.e., instant messages
that are addressed to one participant of the chat room
rather to all of them. A chat room can signal support for
private messages using the 'chatroom' attribute in SDP (see
for details).
When a chat room participant wishes to send a private
instant message to a participant in the chat room, it
follows the same procedures for creating a SEND request as
for regular messages . The
only difference is that the MSRP endpoint MUST populate a
single To header of the Message/CPIM wrapper with the
identifier of the intended recipient. The identifier can be
SIP, TEL, and IM URIs typically learned from the information
received in notifications of a conference event package.
As for regular messages, an MSRP switch that receives a SEND
request from a participant SHOULD first verify that the From
header field of the Message/CPIM wrapper is correctly
populated with a valid URI (i.e., the URI is a participant
of this chat room). provides further
information with the actions to be taken in case this
validation fails.
Then the MSRP switch MUST inspect the To header field of the
Message/CPIM wrapper. If the MSRP switch receives a message
containing several To header fields in the Message/CPIM
wrapper the MSRP switch MUST reject the MSRP SEND request
with a 403 response, as per procedures in RFC 4975. Then the MSRP switch MUST
verify that the To header of the Message/CPIM wrapper
matches the URI of a participant of the chat room. If this
To header field does not contain the URI of a participant of
the chat room or if the To header field cannot be resolved
(e.g., caused by a mistyped URI), the MSRP switch MUST
reject the request with a 404 response. This new 404 status
code indicates a failure to resolve the recipient URI in the
To header field of the Message/CPIM wrapper.
Notice the importance of the From and To headers in the
Message/CPIM wrapper. If an intermediary modifies these
values, the MSRP switch might not be able to identify the
source or intended destination of the message, resulting
in a rejection of the message.
Finally, the MSRP switch MUST verify that the recipient
supports private messages. If the recipient does not support
private messages, the MSRP switch MUST reject the request
with a 428 response. This new response 428 indicates that
the recipient does not support private messages. Any
potential REPORT request that the MSRP switch sends to the
sender MUST include a Message/CPIM wrapper containing the
original From header field included in the SEND request and
the To header field of the original Message/CPIM wrapper.
The MSRP switch MUST NOT forward private messages to a
recipient that does not support private messaging.
If successful, the MSRP switch should search its mapping
table to find the MSRP sessions established towards the
recipient. If a match is found the MSRP switch MUST create
a SEND request and MUST copy the contents of the sender's
message to it.
An MSRP endpoint that receives a SEND request from the MSRP
switch does the same validations as for regular messages . If the To
header field is different from the chat room URI, the MSRP
endpoints knows that this is a private message. The
endpoint should render who it is from based on the value of
the From header of the Message/CPIM wrapper. The endpoint
can also use the sender’s nickname, possibly learned via a
conference event package, to render such nickname rather
than the sender’s actual URI.
It is possible that a participant, identified by a SIP
Address of Record or other valid URI, joins a conference of
instant messages from two or more different SIP UAs. It is
RECOMMENDED that the MSRP switch can map a URI to two or
more MSRP sessions. If the policy of the server allows for
this, the MSRP switch MUST copy all messages intended to the
recipient through each MSRP session mapped to the
recipient's URI.
This section discusses the common procedures for regular and
private messages with respect to MSRP reports and
responses. Any particular procedure affecting only regular
messages or only private messages is discussed in the
previous or , respectively.
MSRP switches MUST follow the success report and failure
report handling described in section 7 of RFC 4975, complemented with the
procedures described in this section. The MSRP switch MUST
act as an MSRP endpoint receiver of the request according to
section 5.3 of RFC 4975.
If the MSRP switch receives an MSRP SEND request that does
not contain a Message/CPIM wrapper, the MSRP switch MUST
reject the request with a 415 response (specified in RFC 4975).
If the MSRP switch receives an MSRP SEND request where the
URI included in the From header field of the Message/CPIM
wrapper is not valid, (e.g, because it does not "belong" to
the sender of the message or is not a valid participant of
the chat room), the MSRP switch MUST reject the request with
a 403 response. In non-error cases, the MSRP switch MUST
construct responses according to section 7.2 of RFC 4975.
When the MSRP switch forwards a SEND request, it MAY use any
report model in the copies intended for the recipients.
The receiver reports from the recipients MUST NOT be
forwarded to the originator of the original SEND request.
This could lead to having the sender receiving multiple
reports for a single MSRP request.
A common characteristic of existing chat room services is
that participants have the ability to present themselves with
a nickname to the rest of the participants of the
conference. It is used for easy reference of participants
in the chat room, and can also provide anonymous participants
with a meaningful descriptive name.
A nickname is a useful construct in many use cases, of which
MSRP chat is but one example. It is associated with a URI of
which the participant is known to the focus. Therefore, if a
user joins the chat room under the same URI from multiple
devices, he or she may request the same nickname across all
these devices.
A nickname is a user selectable appearance of which the
participant wants to be known to the other participants. It is
not a 'display-name', but it is used somewhat like a display
name. A main difference is that a nickname is unique inside a
chat room to allow an unambiguous reference to a participant
in the chat. Nicknames may be long lived, or may be
temporary. Users also need to reserve a nickname prior to its
utilization.
This memo specifies the nickname as a string. The nickname
string MUST be unambiguous within the scope of the chat room
(conference instance). This scope is similar to having a
nickname unique inside a chat room from Extensible Messaging and Presence
Protocol. The chat room may have policies associated
with nicknames. It may not accept nickname strings at all, or
a it may provide a wider unambiguous scope like a domain or
server, similar to Internet Relay Chat
(IRC).
This memo provides a mechanism to reserve a nickname for a
participant for as long as the participant is logged into
the chat room. The mechanism is based on a NICKNAME MSRP
method (see below) and a new "Use-Nickname" header. Note
that other mechanisms may exist (for example, a web page
reservation system), although they are outside the scope of
this document.
A conference participant who has established an MSRP session
with the MSRP switch, where the MSRP switch has indicated
the support and availability of nicknames with the
'nicknames' token in the 'chatroom' SDP attribute, MAY send
a NICKNAME request to the MSRP switch. The NICKNAME request
MUST include a new Use&nbhy;Nickname header that contains
the nickname string that the participant wants to
reserve. MSRP NICKNAME requests MUST NOT include
Success-Report or Failure-Report header fields.
An MSRP switch that receives a NICKNAME request containing
a nickname in the Use&nbhy;Nickname header field SHOULD
first verify whether the policy of the chat room allows the
nickname functionality. If not allowed, the MSRP switch
MUST reject the request with a 501 response, as per RFC 4975.
If the policy of the chat room allows the usage of
nicknames, the MSRP switch SHOULD validate that the SIP AOR
is entitled to reserve the nickname. This may include, e.g.,
allowing that the participant's URI may use the same
nickname when the participant has joined the chat room from
different devices under the same URI. The participant's
authenticated identifier can be derived after a successful
SIP Digest Authentication , be
included in a trusted SIP
P-Asserted-Identity header field , be included in a
valid SIP Identity header field
, or be derived from any other present or future SIP
authentication mechanism. Once the MSRP switch has
validated that the participant is entitled to reserve the
requested nickname, the MSRP switch MUST answer the NICKNAME
request with a 200 response as per regular MSRP procedures.
The reservation of a nickname can fail, e.g. if the NICKNAME
request contains a malformed or non-existent
Use&nbhy;Nickname header field, or if the same nickname has
already been reserved by another participant (i.e., by
another URI) in the chat room. The validation can also fail
where the sender of the message is not entitled to reserve
the nickname. In any of these cases the MSRP switch MUST
answer the NICKNAME request with a 423 response. The
semantics of the 423 response are: "Nickname usage failed;
the nickname is not allocated to this user".
As indicated earlier, this specification defines a new MSRP
header field: "Use-Nickname". The Use&nbhy;Nickname header
field carries a nickname string, and
SHOULD be included in the NICKNAME requests.
The syntax of the NICKNAME method and the "Use-Nickname"
header field is built upon the
MSRP formal syntax
Once the MSRP switch has reserved a nickname and has bound
it to a URI (e.g., a SIP Address-of-Record), the MSRP server
MAY allow the usage of the same nickname by the same user
(identified by the same URI, such as a SIP AoR) over a
second MSRP session. This might be the case if the user
joins the same chat room from a different SIP User Agent. In
this case, the user MAY request the same or a different
nickname than that used in conjunction with the first MSRP
session; the MSRP server MAY accept the usage of the same
nickname by the same user. The MSRP switch MUST NOT
automatically assign the same nickname to more than one MSRP
session established from the same URI, because this can
create confusion to the user as whether the same nickname is
bound to the second MSRP session.
Typically a participant will reserve a nickname as soon as the participant
joins the chat room. But it is also
possible for a participant to modify his/her own nickname and
replace it with a new one at any time during the duration of the
MSRP session. Modification of the nickname is not different from
the initial reservation and usage of a nickname, thus the
NICKNAME method is used as described in
.
If a NICKNAME request that attempts to modify the current
nickname of the user for some reason fails, the current nickname
stays in effect. A new nickname comes into effect and the old
one is released only after a NICKNAME request is accepted with a
200 response.
If the participant no longer wants to be known by a nickname
in the conference, the participant can follow the method
described in .
The nickname element of the Use&nbhy;Nickname header MUST be
set to an empty quoted string.
Typically the conference focus acts as a notifier of the
conference event package. To notify subscribers of the
nickname reserved for a given participant, it is RECOMMENDED
that conference focus and endpoints support Conference Event
Package Data Format Extension for Centralized Conferencing
. The Conference
Information Data Model for Centralized Conferencing
extends the user element from RFC
4575 with a 'nickname' attribute.
There are a handful of use cases where a participant would
like to learn the chat room capabilities supported by the MSRP
switch and the chat room. For example, a participant would
like to learn if the MSRP switch supports private messaging,
otherwise, the participant may send what he believes is a
private instant message addressed to a participant, but since
the MSRP switch does not support the functions specified in
this memo, the message gets eventually distributed to all the
participants of the chat room.
The reverse case also exists. A participant, say Alice, whose
user agent does not support the extensions defined by this
document joins the chat room. The MSRP switch learns that Alice's
application does not support private messaging nor nicknames. If
another participant, say Bob, sends a private message to Alice,
the MSRP switch does not distribute it to Alice, because Alice is
not able to differentiate it from a regular message sent to the
whole roster. Furthermore, if Alice replied to this message, she
would do it to the whole roster. Because of this, the MSRP switch
also keeps track of users who do not support the extensions
defined in this document.
In another scenario, the policy of a chat room may indicate
that certain functions are not allowed. For example, the policy
may indicate that nicknames or private messages are not
allowed.
In order to provide the user with a good chat room experience,
we define a new 'chatroom' SDP attribute. The 'chatroom'
attribute is a media-level value
attribute that MAY be included in conjunction with an
MSRP media stream (i.e., when an m= line in SDP indicates
"TCP/MSRP" or "TCP/TLS/MSRP"). The 'chatroom' attribute
without further modifiers (e.g., chat-tokens) indicates that the
endpoint supports the procedures described in this document
for transferring MSRP messages to/from a multi-party
conference. The 'chatroom' attribute can be complemented with
additional modifiers that further indicate the intersection
of support and chat room local policy allowance for a number
of functions specified in this document. Specifically, we
provide the means for indicating support to use nicknames and
private messaging.
The 'chatroom' SDP attribute has the following Augmented BNF (ABNF) syntax:
A given 'chat-token' value MUST NOT appear more than once in a
'chatroom' attribute.
A conference focus that includes the 'nicknames' token in the
session description is signaling that the MSRP switch supports and
the chat room allows to use the procedures specified in
. A conference focus that includes the
'private-messages' in the SDP description is signaling that the
MSRP switch supports and the chat room allows to use the procedures
specified in .
Example of the 'chatroom' attribute for an MSRP media stream
that indicates the acceptance of nicknames and private
messages:
An example of a 'chatroom' attribute for an MSRP media stream
where the endpoint, e.g., an MSRP switch, does not allow
either nicknames nor private messages.
The 'chatroom' attribute allows extensibility with the
addition of new tokens. No IANA registry is provided at this
time, since no extensions are expected at the time of this
writing. Extensions to the 'chatroom' attribute can be defined
in IETF documents or as private vendor extensions.
Extensions defined in IETF document MUST follow the
'standard-token' ABNF previously defined. In this type of
extensions, are must be taken in the selection of the token to
avoid a clash with any of the tokens previously defined.
Private extensions MUST follow the 'private-token' ABNF
previously defined. The 'private-token' MUST include the DNS
name of the vendor in reverse order in the token, in order to
avoid clashes of tokens. The following is an example of a
"chat.foo" extension by vendor "example.com"
presents a flow diagram where
Alice joins a chat room by sending an INVITE request. This
INVITE request contains a session description that includes the
chatroom extensions defined in this document.F1: Alice constructs an SDP description that includes an
MSRP media stream. She also indicates her support for the
chatroom extensions defined in this document. She sends the
INVITE request to the chat room server.
F2: The chat room server accepts the session
establishment. It includes the 'isfocus' and other relevant
feature tags in the Contact header field of the
response. The chat room server also builds an SDP answer
that forces the reception of messages wrapped in
Message/CPIM wrappers. It also includes the 'chatroom'
attribute with the allowed extensions.
F3: The session established is acknowledged (details not
shown).
shows an example of Alice
setting up a nickname using the conference as provider. Her
first proposal is not accepted because that proposed nickname
is already in use. Then, she makes a second proposal with a
new nickname. This second proposal is accepted.F1: Alice sends an MSRP NICKNAME request that contains her
proposed nicknames in the Use-Nickname header field.F2: The MSRP switch analyzes the existing allocation of
nicknames and detects that the nickname "Alice the great"
is already provided to another participant in the chat room.
The MSRP switch answers with a 423 response.F3: Alice receives the response. She proposes a new
nickname in a second NICKNAME request.F4: The MSRP switch accepts the nickname proposal and
answers with a 200 response. depicts a flow
diagram where Alice is sending a regular message addressed to
the chat room. The MSRP switch distributes the message to the
rest of the participants.F1: Alice builds a text message and wraps it in a
Message/CPIM wrapper. She addresses the message to the chat
room. She encloses the resulting Message/CPIM wrapper in an
MSRP SEND request and sends it to the MSRP switch via the
existing TCP connection.F2: The MSRP switch acknowledges the reception of the SEND
request with a 200 (OK) response.F3: The MSRP switch creates a new MSRP SEND request that
contains the received Message/CPIM wrapper and sends it to Bob.
Since the received message is addressed to the chat room URI
in the From header of the Message/CPIM header, Bob knows
that this is a regular message distributed all participants
in the chat room, rather that a private message addressed to him.
The rest of the message flows are analogous to the
previous. They are not shown here. depicts a flow
diagram where Alice is sending a private message addressed to
Bob's SIP AOR. The MSRP switch distributes the message only to
Bob.F1: Alice builds a text message and wraps it in a
Message/CPIM wrapper. She addresses the message to Bob's URI,
which she learned from a notification in the conference event
package. She encloses the resulting Message/CPIM wrapper in an
MSRP SEND request and sends it to the MSRP switch via the
existing TCP connection.F2: The MSRP switch acknowledges the reception of the SEND
request with a 200 (OK) response.F3: The MSRP switch creates a new MSRP SEND request that
contains the received Message/CPIM wrapper and sends it only to
Bob. Bob can distinguish the sender in the From header of the
Message/CPIM wrapper. He also identifies this as a private
message due to the presence of his own SIP AOR in the To
header field of the Message/CPIM wrapper.F4: Bob acknowledges the reception of the SEND
request with a 200 (OK) response.
The MSRP message below depicts the example of the same
private message described in , but now the message is
split in two chunks. The MSRP switch must wait for the
complete set of Message/CPIM headers before distributing the
messages.
depicts two user elements in a conference information
document both having the nickname element with a nickname string.
This specification defines a new MSRP method to be added to
the Methods sub-registry of the Message Session Relay
Protocol (MSRP) Parameters registry:
NICKNAME
See section for details.
This specification defines a new MSRP header to be added to
the Header Field sub-registry of the Message Session
Relay Protocol (MSRP) Parameters registry:
Use&nbhy;Nickname
See for details.
This specification defines three new MSRP status codes to be
added to the Status-Code sub-registry of the Message Session
Relay Protocol (MSRP) parameters registry.
The 404 status code indicates the failure to resolve the
recipient URI in the To header field of the Message/CPIM
wrapper in the SEND request, e.g, due to an unknown
recipient. See for
details.
The 423 response indicates a failure in allocating the
requested NICKNAME. This can be caused by a malformed
NICKNAME request (e.g., no Use&nbhy;Nickname header field),
an already allocated nickname, or a policy that prevents the
sender to use nicknames. See for details.
The 428 status code indicates that the recipient of a SEND
request does not support private messages. See for details.
summarizes the IANA
registration data with respect to new MSRP status codes:
ValueDescriptionReference404Failure to resolve recipient's URIRFC
XXXX423Unable to allocate requested nicknameRFC
XXXX428Private messages not supportedRFC XXXX This specification defines a new media-level attribute in the
Session Description Protocol (SDP) Parameters registry. The
registration data is as follows:
Contact: Miguel Garcia <miguel.a.garcia@ericsson.com>Phone: +34 91 339 1000Attribute name: chatroomLong-form attribute name: Chat RoomType of attribute: media level only This attribute is not subject to the charset attributeDescription: This attribute identifies support and
local policy allowance for a number of chat room related
functionsSpecification: RFC XXXX
See section for details.
This document proposes extensions to the Message Session Relay Protocol
. Therefore, the security considerations of that document
apply to this document as well.
If the participant's SIP user agent doesn't understand the
"isfocus" feature tag , it will
not know that it is connected to a conference instance. The
participant might not be notified that the participant's MSRP
client will try to send messages to the MSRP switch having
potentially multiple recipients. If the participant's MSRP
client doesn't support the extensions of this specification, it
is unlikely that it will try to send a message using 'Message/CPIM' wrapper content type, and
the MSRP switch will reject the request with a 415 response. Still if a participant's
MSRP client does create a message with a valid 'Message/CPIM' wrapper content type
having the To header set to the URI of the chat room and the
From header set to the URI of which the participant is known to
the conference, the participant might be unaware that the
message can be forwarded to multiple recipients. Equally if the
To header is set to a valid URI of a recipient known to the
conference, the message can be forwarded as a private message
without the participant knowing.
If a participant wants to avoid eavesdropping, the participant's
MSRP client can send the messages over a
TLS transport connection,
as allowed by MSRP. It's up to the policy of the MSRP switch
if the messages are forwarded to the other participant's
in the chat room using TLS transport.
Nicknames will be used to show the appearances of the
participants of the conference. A successful take over of a
nickname from a participant might lead to private messages to
be sent to the wrong destination. The recipient's URI will be
different from the URI associated to the original owner of the
nickname, but the sender might not notice this. To avoid
takeovers the MSRP switch MUST make sure that a nickname is
unique inside a chat room. Also the security consideration for
any authenticated identity mechanisms used to validate the SIP
AOR will apply to this document as well. If a nickname can be
reserved if it previously has been used by another participant
in the chat room, is up to the policy of the chat room.
This work would have never been possible without the fruitful
discussions in the SIMPLE WG mailing list, specially with
Brian Rosen (Neustar) and Paul Kyzivat (Cisco), who provided
extensive review and improvements throughout the document.
The authors want to thank Eva Leppanen, Adamu Haruna,
Adam Roach, Matt Lepinski, Mary Barnes, Ben Campbell, Paul
Kyzivat, Adrian Georgescu, Nancy Greene, and Flemming Andreasen
for providing comments.