| Rfc | 7701 |
|---|
| Title | Multi-party Chat Using the Message Session Relay Protocol (MSRP) |
|---|
| Author | A.
Niemi, M. Garcia-Martin, G. Sandbakken |
|---|
| Date | December 2015 |
|---|
| Format: | TXT,
HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Internet Engineering Task Force (IETF) A. Niemi
Request for Comments: 7701
Category: Standards Track M. Garcia-Martin
ISSN: 2070-1721 Ericsson
G. Sandbakken
Cisco Systems
December 2015
Multi-party Chat Using the Message Session Relay Protocol (MSRP)
Abstract
The Message Session Relay Protocol (MSRP) defines a mechanism for
sending instant messages (IMs) 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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7701.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Contributions published or made publicly available before November
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Without obtaining an adequate license from the person(s) controlling
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than English.
Table of Contents
1. Introduction ....................................................4
2. Terminology .....................................................5
3. Motivations and Requirements ....................................6
4. Overview of Operation ...........................................7
4.1. Policy Attributes of the Chat Room ........................10
5. Creating, Joining, and Deleting a Chat Room ....................12
5.1. Creating a Chat Room ......................................12
5.2. Joining a Chat Room .......................................12
5.3. Deleting a Chat Room ......................................14
6. Sending and Receiving Instant Messages .........................14
6.1. Regular Messages ..........................................14
6.2. Private Messages ..........................................17
6.3. MSRP Reports and Responses ................................19
6.4. Congestion Avoidance ......................................20
7. Nicknames ......................................................21
7.1. Using Nicknames within a Chat Room ........................22
7.2. Modifying a Nickname ......................................24
7.3. Removing a Nickname .......................................25
7.4. Nicknames in Conference Event Packages ....................25
8. The SDP 'chatroom' Attribute ...................................25
9. Examples .......................................................28
9.1. Joining a Chat Room .......................................28
9.2. Setting Up a Nickname .....................................30
9.3. Sending a Regular Message to the Chat Room ................31
9.4. Sending a Private Message to a Participant ................33
9.5. Chunked Private Message ...................................35
9.6. Nickname in a Conference Information Document .............35
10. IANA Considerations ...........................................37
10.1. New MSRP Method ..........................................37
10.2. New MSRP Header ..........................................37
10.3. New MSRP Status Codes ....................................37
10.4. New SDP Attribute ........................................38
11. Security Considerations .......................................38
12. References ....................................................40
12.1. Normative References .....................................40
12.2. Informative References ...................................43
Acknowledgments ...................................................43
Contributors ......................................................43
Authors' Addresses ................................................44
1. Introduction
The Message Session Relay Protocol (MSRP) [RFC4975] defines a
mechanism for sending a series of instant messages within a session.
The Session Initiation Protocol (SIP) [RFC3261] in combination with
the Session Description Protocol (SDP) [RFC4566] allows for two peers
to establish and manage such sessions.
In another application of SIP, a User Agent (UA) can join in a multi-
party conversation called a "conference" that is hosted by a
specialized UA called a "focus" [RFC4353]. Such a conference can
naturally involve MSRP sessions. It is the responsibility of an
entity handling the media to relay IMs 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 can
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 room functionality are already
available today. For example, Internet Relay Chat (IRC) [RFC2810],
Extensible Messaging and Presence Protocol (XMPP): Core [RFC6120], as
well as many other proprietary systems. Specifying equivalent
functionality for MSRP-based systems eases interworking between these
systems.
This document defines requirements, conventions, and extensions for
providing private messages and nickname management in centralized
chat rooms with MSRP. Participants in a chat room can be identified
by a pseudonym and decide if their real identifier should be
disclosed to other participants. This memo uses the SIP Conferencing
Framework [RFC4353] as a design basis. It also aims to be compatible
with "A Framework for Centralized Conferencing" [RFC5239]. 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 chat rooms.
2. Terminology
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 BCP 14 [RFC2119] and
indicate requirement levels for compliant implementations.
This memo deals with "Tightly Coupled SIP Conferences" as defined in
the SIP Conferencing Framework [RFC4353] and adopts the terminology
from that document. In addition, we introduce some new terms:
Nickname: a pseudonym or descriptive name associated with a
participant. See Section 7 for details.
Multi-party Chat: an instance of a tightly coupled conference, in
which the media exchanged between the participants consist of
MSRP-based IMs. Also known as a chat room.
Chat Room: a synonym for a multi-party chat.
Chat Room URI: a URI that identifies a particular chat room and
that is a synonym of a "Conference URI" as defined in RFC 4353
[RFC4353].
Sender: the chat room participant who originally created an IM and
sent it to the chat room server for further delivery.
Recipient: the destination chat room participant(s). This defaults
to the full conference participant list minus the IM Sender.
MSRP Switch: a media-level entity that is an MSRP endpoint. It is
a special MSRP endpoint that receives MSRP messages and delivers
them to the other chat room 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 chat room
participants.
Private IM: an IM sent in a chat room intended for a single
participant. Generally speaking, a private IM is seen by the MSRP
switch, in addition to the sender and recipient. A private IM is
usually rendered distinctly from the rest of the IMs, indicating
that the message was a private communication.
Anonymous URI: a URI concealing the participant's SIP address of
record (AOR) from the other participants in the chat room. The
allocation of such a URI is out of scope of this specification.
An anonymous URI must be valid for the length of the chat room
session and will be utilized by the MSRP switch to forward
messages to and from anonymous participants. Privacy and
anonymity are discussed in greater detail in RFC 3323 [RFC3323]
and RFC 3325 [RFC3325].
Conference Event Package: a notification mechanism that allows
conference participants to learn conference information including
roster and state changes in a conference. This would typically be
the mechanisms defined in "A Session Initiation Protocol (SIP)
Event Package for Conference State" [RFC4575] or "Conference Event
Package Data Format Extension for Centralized Conferencing (XCON)"
[RFC6502].
Identifier: a string used to recognize or establish as being a
particular user.
To log in: to enter identifying data, as a name or password, into a
chat room, so as to be able to do work with the chat room.
3. Motivations and Requirements
Although conference frameworks describing many types of conferencing
applications already exist, such as the one in "A Framework for
Centralized Conferencing" [RFC5239] and the SIP Conferencing
Framework [RFC4353], the exact details of session-based instant
messaging conferences (chat rooms) are not well-defined at the
moment.
To allow interoperable chat implementations, for both conference-
aware and conference-unaware UAs, certain conventions for MSRP chat
rooms 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 to enable the building of interworking
gateways to these existing chat systems.
We define the following requirements:
REQ-1: A basic requirement is the existence of a chat room, where
participants can join and leave the chat room and exchange
IMs with the rest of the participants.
REQ-2: A recipient of an IM in a chat room must be able to determine
the identifier of the sender of the message. Note that the
actual identifier depends on the one that was used by the
sender when joining the chat room.
REQ-3: A recipient of an IM in a chat room must be able to determine
the identifier of the recipient of received messages. For
instance, the recipient of the message might be the entire
chat room or a single participant (i.e., a private message).
Note that the actual identifier may depend on the one that
was used by the recipient when he or she joined the chat
room.
REQ-4: It must be possible to send a message to a single participant
within the chat room (i.e., a private IM).
REQ-5: A chat room participant may have a nickname or pseudonym
associated with their real identifier.
REQ-6: It must be possible for a participant to change their
nickname during the progress of the chat room session.
REQ-7: It must be possible for a participant to be known only by an
anonymous identifier and not their real identifier by the
rest of the chat room.
REQ-8: It must be possible for chat room participants to learn the
chat room capabilities described in this document.
4. Overview of Operation
Before a chat room can be entered, it must be created. Users wishing
to host a chat room themselves can, of course, do just that; their 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 chat room, e.g., by sending an INVITE
request to it. 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 chat room 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 [RFC4976]. This is illustrated in Figure 1.
MSRP Sessions
+--------------------------+
| |
+---+--+ +---+--+ |
| SIP | | SIP | |
| MSRP | | MSRP | +-----+-----+
|Client| |Client| | MSRP |
+---+--+ ++--+--+ | Relay |
| | \ +-----+-----+
SIP Dialogs | / +----+ |
| | \ | MSRP Sessions
+----+------+--+ | |
| | +-+-----+-----+
| Conference | | MSRP |
| Focus UA |........| Switch |
| | | |
+----+-------+-+ +-+-----+-----+
| \ | |
SIP Dialogs | | +------+ | MSRP Sessions
| \ / |
+---+--+ +-+--+-+ +-----+-----+
| SIP | | SIP | | MSRP |
| MSRP | | MSRP | | Relay |
|Client| |Client| +-----+-----+
+---+--+ +------+ |
| |
+--------------------------+
MSRP Sessions
Figure 1: Multi-party Chat Overview Shown with MSRP Relays
and a Conference Focus UA
The MSRP switch is similar to a conference mixer in that it both
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: it doesn't
actually mix the streams in any way. The system is illustrated in
Figure 2.
+------+
| MSRP |
|Client|
+------+ +--.---+ +------+
| MSRP | | | MSRP |
|Client| | _|Client|
+------._ | ,' +------+
`._ | ,'
`.. +----------+ ,'
`| |'
| MSRP |
| Switch |
,| |_
_,-'' +----------+ ``-._
+------.-' | `--+------+
| MSRP | | | MSRP |
|Client| | |Client|
+------+ | +------+
+---'--+
| MSRP |
|Client|
+------+
Figure 2: Multi-party Chat in a Centralized Chat Room
Typically, chat room 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 [RFC3862], to distinguish between private and regular messages.
When a participant wants to send an instant message to the chat room,
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 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 IM 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 in from different endpoints, the MSRP switch will
distribute the private message to each endpoint at which the
recipient is logged in.
We extend the current MSRP negotiation that takes place in SDP
[RFC4566] 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. This is achieved by a new 'chatroom'
attribute in SDP (please refer to Section 8 for a detailed
description).
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 with a unique
MSRP session and a unique SIP dialog.
4.1. Policy Attributes of the Chat Room
The Conference Framework with SIP [RFC4353] introduces the notion of
a Conference Policy as "The complete set of rules governing a
particular conference." A chat room is a specialized type of
conference, and the conference policy is sometimes extended with new
chat-specific rules. This section lists all the Conference Policy
attributes used by the present document and refers to sections in the
document where the usage of these attributes are described in greater
detail.
Nicknames: Whether the chat room accepts users to be recognized with
a nickname. See Sections 7, 7.1, and 8 for details. Also, the
scope of uniqueness of the nickname: the chat room (conference
instance), a realm or domain, a server, etc.
Nickname quarantine: The quarantine to be imposed on a nickname once
it is not currently in use (e.g., because the participant holding
this nickname abandons the chat room), prior to the wide
availability of this nickname to other users. This allows the
initial holder of the nickname to join the chat room during the
quarantine period and claim the same nickname they were previously
using. See Section 11 for details.
Private messaging: Whether the chat room allows users to send
private messages to other users of the chat room through the MSRP
switch. See Sections 6.2 and 8 for details.
Deletion of the chat room: Whether the chat room can be deleted when
the creator leaves the chat room or through an out-of-band
mechanism. See Section 5.3 for details.
Simultaneous access: Whether a user can log in from different
endpoints using the same identity. See Sections 6.1 and 6.2 for
details.
Force TLS transport: Whether the MSRP switch accepts only Transport
Layer Security (TLS) as an MSRP transport, in an effort to
guarantee confidentiality and privacy. See Section 11 for
details.
Maximum message size in congested MSRP sessions: The maximum size of
messages that can be distributed to a user over a congested MSRP
session. See Section 6.4 for details.
Chunk reception timer: The value of a time that controls the maximum
time that the MSRP switch is waiting for the reception of
different chunks belonging to the same message. If the timer
expires, the MSRP switch will discard the associated message
state. See Section 6.1 for details.
Supported wrapped media types: The list of media types that the MSRP
switch accepts in Message/CPIM wrappers sent from participants.
This list is included in the 'accept-wrapped-types' attribute of
the MSRP message media line in SDP. If the MSRP switch accepts
additional media types to those explicitly listed, a "*" is added
to the list. A single "*" indicates that the chat room accepts
any wrapped media type.
5. Creating, Joining, and Deleting a Chat Room
5.1. Creating a Chat Room
Since we consider a chat room a particular type of conference having
MSRP media, the methods defined by the SIP Conference Framework
[RFC4353] 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.
5.2. Joining a Chat Room
Participants usually join the chat room by sending an INVITE request
to the chat room URI. The chat room then uses regular SIP mechanisms
to authenticate the participant. This may include, e.g., client
certificates, SIP Digest authentication [RFC3261], asserted network
identity [RFC3325], SIP Identity header field [RFC4474], etc. As
long as the user is authenticated, the INVITE request is accepted by
the focus and the user is brought into the actual chat room.
This specification requires all IMs to be wrapped in a Message/CPIM
wrapper [RFC3862]. Therefore, the 'accept-types' attribute for the
MSRP message media in both the SDP offer and answer need to include
at least the value 'Message/CPIM' (notice that RFC 4975 [RFC4975]
mandates this 'accept-types' attribute in SDP). If the 'accept-
types' attribute does not contain the value 'Message/CPIM', the
conference focus will reject the request. The actual instant message
payload type is negotiated in the 'accept-wrapped-types' attribute in
SDP (see RFC 4975 [RFC4975] for details). There is no default
wrapped type. Typical wrapped type values can include text/plain,
text/html, image/jpeg, image/png, audio/mp3, etc. It is RECOMMENDED
that participant endpoints add an 'accept-wrapped-types' attribute to
the MSRP 'message' media line in SDP, where the supported wrapped
types are declared, as per RFC 4975 procedures [RFC4975].
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 [RFC3840] 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
cannot 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
[RFC4566].
The conference focus of a chat room MUST only use a Message/CPIM
[RFC3862] top-level wrapper as a payload of MSRP messages, and the
focus MUST declare it in the SDP offer or answer as per regular
procedures in RFC 4975 [RFC4975]. This implies that if the
conference focus receives, from a participant's endpoint, an SDP
offer that does not include the value 'Message/CPIM' in the 'accept-
types' attribute for the MSRP message media line, the conference
focus SHOULD either reject the MSRP message media stream or reject
the complete SDP offer by using regular SIP or SDP procedures (e.g.,
creating an SDP answer that sets to zero the port of the MSRP message
media line, responding the INVITE with a 488 response, etc.).
If the conference focus accepts the participant's SDP offer, when the
conference focus generates the SDP answer, it MUST set the 'accept-
types' attribute for the MSRP message media line to a value of
'Message/CPIM'. This specification requires all IMs to be wrapped in
a Message/CPIM wrapper, therefore, the 'accept-types' attribute in
this SDP body contains a single value of 'Message/CPIM'. The actual
IM payload type is negotiated in the 'accept-wrapped-types' attribute
in SDP (see RFC 4975 [RFC4975] for details). The conference focus
MAY also add an 'accept-wrapped-types' attribute to the MSRP message
media line in SDP containing the supported wrapped types, according
to the supported wrapped media types policy.
Note that the Message/CPIM wrapper is used to carry the sender
information that, otherwise, it will not be available to the
recipient. Additionally, the Message/CPIM wrapper carries the
recipient information (e.g., To and Cc headers).
If the UA supports anonymous participation and the user chooses to
use it, the participant's UA SHOULD do at least one of these options:
(a) provide an anonymous URI in SIP headers that otherwise reveal
identifiers. Please refer to RFC 3323 [RFC3323] for a detailed
description of which headers are subject to reveal identifiers
and how to populate them; or
(b) trust the conference focus and request privacy of their URI,
e.g., by means of the SIP Privacy header field [RFC3323],
network asserted identity [RFC3325], or a similar privacy
mechanism.
If the participant has requested privacy, the conference focus MUST
expose a participant's anonymous URI through the conference event
package [RFC4575].
The conference focus of a chat room learns the supported chat room
capabilities in the endpoint by means of the 'chatroom' attribute
exchanged in the SDP offer/answer (please refer to Section 8 for a
detailed description). The conference focus MUST inform the MSRP
switch of the chat room capabilities of each participant that joins
the chat room (note that the interface defined between the conference
focus and the MSRP switch is outside the scope of this
specification). This information allows the MSRP switch, e.g., to
avoid the distribution of private messages to participants whose
endpoints do not support private messaging.
5.3. Deleting a Chat Room
As with creating a conference, the methods defined by the SIP
Conference Framework [RFC4353] 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. For example, the policy can determine whether the
chat room is deleted when the creator leaves the room or whether an
out-of-band mechanism is responsible for the deletion.
6. Sending and Receiving Instant Messages
6.1. Regular Messages
This section describes the conventions used to send and receive IMs
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 [RFC3862] 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 [RFC4975].
The sender MAY choose the desired MSRP report model (e.g., populate
the Success-Report and Failure-Report MSRP header fields).
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 MUST also populate the From header of the Message/CPIM wrapper
with a proper identifier by which the user is recognized in the chat
room. Identifiers that can be used (among others) are:
o A SIP URI [RFC3261] representing the participant's address-of-
record
o A tel URI [RFC3966] representing the participant's telephone
number
o An IM URI [RFC3860] representing the participant's instant
messaging address
o An anonymous URI representing the participant's anonymous address
If the participant wants to remain anonymous, the participant's
endpoint MUST populate an anonymous URI in the From header of the
Message/CPIM wrapper. Other participants of the chat room will use
this anonymous URI in the To header of the Message/CPIM wrapper when
sending private messages. 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.
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. Section 6.3 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 [RFC4975]. 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 chat room 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.
When generating a copy of the SEND request to each participant in the
chat room, the MSRP switch MUST evaluate the wrapped media types that
the recipient is able to accept. This was learned through the
'accept-wrapped-types' attribute of the MSRP message media line in
SDP. If the MSRP switch is aware that the media type of the wrapped
content is not acceptable to the recipient, the MSRP switch SHOULD
NOT forward this message to that endpoint. Note that this version of
the specification does not require the MSRP switch to notify the
sender about this failure. Extensions to this specification may
improve handling of unknown media types.
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. But, bear in mind that the
MSRP switch SHOULD still implement some sanity checking. Please
refer to the security considerations in Section 11 for further
details.
When forwarding chunked messages as soon as they are received, the
Message/CPIM wrapper is only present at the beginning of the message,
typically within the first chunk. Subsequent chunks will contain the
rest of the message, but not the Message/CPIM headers. Therefore, an
MSRP switch that receives a subsequent message may face challenges in
determining the correct list of recipients of the message. An MSRP
switch that uses this fast forwarding procedure MUST temporarily
store the Message-ID of the MSRP message to correlate the different
chunks; it MUST also temporarily store the list of recipients to
which the initial chunks were delivered. The MSRP switch SHOULD
forward subsequent chunks only to those recipients who were sent the
initial chunks, except if the MSRP switch has knowledge that one of
the recipients of the initial chunks has dropped from the chat room.
This behavior also avoids new participants who had joined the chat
room when the first chunk was distributed from receiving subsequent
chunks that would otherwise need to be discarded.
Once the MSRP switch receives the last chunk of a message, and that
chunk is successfully sent to each of the recipients, the MSRP switch
discards the temporary storage of MSRP Message-ID and the associated
list of recipients.
In some occasions, a sender might suffer a transport error condition
(such as loss of connectivity or depletion of battery) that makes the
sending of a message incomplete, e.g., some chunks were received by
the MSRP switch, but not all of them. This is a behavior already
considered in the core MSRP specification (see RFC 4975 [RFC4975]
Section 5.4). The problem in the context of a chat room lies with
the use of temporary storage for fast forwarding. In order to
prevent attacks related to the exhaustion of temporary storage of
chunked messages, on receiving a first chunk of a message, where the
MSRP switch is using the fast forward method, the MSRP switch MUST
set a chunk reception timer for controlling the reception of the
remaining chunks.
This chunk reception timer can be reset every time a new chunk of the
same message is received. When this timer expires, the MSRP switch
MUST consider that the sending of the message was aborted, and it MAY
discard all the message state associated with it, including the
Message-ID and the list of recipients. Additionally, if this chunk
reception timer expires, the MSRP switch MAY choose to send an abort
chunk (i.e., one with the "#" flag set) to each to the recipients.
This is just an optimization, since MSRP endpoints need to be able to
handle incomplete messages as per regular MSRP.
The specific value of this chunk reception timer is not standardized;
it is subject of local policy. However, it is recommended not to be
a short value. For example, a time interval on the order of a normal
TCP timeout (i.e., around 540 seconds) would be reasonable. A value
on the order of a few seconds would not.
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 chat room. 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.
It is possible that a participant, identified by a SIP AoR or other
valid URI, joins a chat room simultaneously from two or more
different SIP UAs. It is recommended that the MSRP switch implements
means to map a URI to two or more MSRP sessions. If the policy of
the chat room allows simultaneous access, the MSRP switch MUST copy
all regular messages intended to the recipient through each MSRP
session mapped to the recipient's URI.
6.2. Private Messages
This section describes the conventions used to send and receive
private IMs, i.e., IMs that are addressed to one participant of the
chat room rather than to all of them. The chat room has a local
policy that determines whether or not private messages are supported.
A chat room can signal support for private messages using the
'chatroom' attribute in SDP (please refer to Section 8 for a detailed
description).
When a chat room participant wishes to send a private IM to a
participant in the chat room, it follows the same procedures to
create a SEND request as for regular messages (Section 6.1). 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.
This version of the specification does not support sending a
private message to multiple recipients, i.e., the presence of
multiple To headers in the Message/CPIM wrapper of the MSRP SEND
request. This is due to added complexity, for example, with the
need to determine whether a message was not delivered to some of
the intended recipients. Implementations that still want to
recreate this function can send a series of single private
messages, one private message per intended recipient. The
endpoint can correlate this series of messages and create the
effect of a private message addressed to multiple recipients.
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). Section 6.3
provides further information regarding the actions to be taken in
case this validation fails.
Then, the MSRP switch inspects 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 [RFC4975]. Then, the MSRP switch verifies
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 verifies 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
428 response 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 toward 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 (Section 6.1). If
the To header field is different from the chat room URI, the MSRP
endpoints know 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
the sender of the message, instead of using the sender's actual URI.
As with regular messages, if the policy of the chat room allows
simultaneous access, the MSRP switch MUST copy all private messages
intended to the recipient through each MSRP session mapped to the
recipient's URI.
6.3. MSRP Reports and Responses
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 sections (Sections 6.1 or 6.2,
respectively).
MSRP switches MUST follow the success report and failure report
handling described in Section 7 of RFC 4975 [RFC4975], 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 [RFC4975].
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 [RFC4975]).
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 cases without
error, the MSRP switch MUST construct responses according to
Section 7.2 of RFC 4975 [RFC4975].
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.
6.4. Congestion Avoidance
Congestion can occur when multiple heterogeneous interfaces are used
by a number of users who are participating in a chat room, and, in
particular, when paths become overloaded by any application. Some of
these users might have fast paths capable of high throughputs while
other users might be slow paths with constrained throughputs. Some
paths might become congested only by the chat application; other
paths gets congested by other applications. Therefore, it is
possible that a subset of the participants of the chat room are able
to send and receive a large number of messages in a short time or
with large contents (e.g., pictures), whereas others are not able to
keep up the pace.
Additionally, since MSRP uses a connection-oriented transport
protocol such as TCP, it is expected that TCP congestion control
mechanisms be activated if congestion occurs. Details on congestion
control are specified in RFC 5681 [RFC5681].
While this document does not mandate a particular MSRP-specific
mechanism to avoid congestion in any of the paths, something that is
deemed outside the scope of this document, this document provides
some recommendations for implementors to consider.
It is RECOMMENDED that MSRP switches implement one or more MSRP-
specific strategies to detect and avoid congestion. Possible
strategies (but definitely not a comprehensive list) include:
o If the MSRP switch is writing data to a send buffer and detects
that the send buffer associated with that TCP connection is
getting full (e.g., close to 80% of its capacity), the MSRP switch
marks the associated MSRP sessions making use of that TCP
connection as "congested".
o Prior to sending a new MSRP message to a user, the MSRP switch
verifies the congested flag associated to that MSRP session. If
the MSRP session is marked as congested, the MSRP switch can apply
a congestion avoidance mechanism, such as:
* The MSRP switch MAY discard regular MSRP messages sent to that
user while the congestion flag is raised for the user's TCP
connection. In order to inform the user of the congestion, the
MSRP switch MAY send a regular MSRP message to the user whose
congestion flag is raised. This message indicates that some
other messages are being discarded due to network congestion.
However, it should be noted that this message can get stuck at
MSRP switch, if the path is fully congested, i.e., it may not
be delivered anyhow.
* The MSRP can implement a temporary policy to disallow the
distribution of messages larger than a certain size to MSRP
sessions marked as congested. Similarly, the user should be
informed of this fact by the MSRP switch sending a regular MSRP
message indicating this condition.
o If the MSRP switch determines that the congestion flag associated
with a given TCP connection has been raised for quite some time
(on the order of a few minutes), or if the interface is down, this
may be considered an indication that the TCP connection has not
been able to recover from a congestion state. The MSRP switch MAY
close this congested TCP connection as well as the MSRP session
and SIP session.
7. Nicknames
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 chat room. 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. A nickname is associated with a URI; the
focus knows the participant by its association to this URI.
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 moniker by 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 they 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 unambiguously be associated with a single user in the scope of
the chat room (conference instance). This scope is similar to having
a nickname unique per user inside a chat room from "Extensible
Messaging and Presence Protocol (XMPP): Core" [RFC6120]. The chat
room may have policies associated with nicknames. It may not accept
nickname strings at all, or it may provide a wider unambiguous scope
like a domain or server, similar to IRC [RFC2810].
7.1. Using Nicknames within a Chat Room
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 chat room 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-Nickname header
that contains the nickname string that the participant wants to
reserve. This nickname string MUST NOT be zero octets in length and
MUST NOT be more than 1023 octets in length. Finally, MSRP NICKNAME
requests MUST NOT include Success-Report or Failure-Report header
fields.
Bear in mind that nickname strings, like the rest of the MSRP
message, use the UTF-8 transformation format [RFC3629].
Therefore, a character may be encoded in more than one octet.
An MSRP switch that receives a NICKNAME request containing a
Use-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 403 response, as per RFC
4975 [RFC4975].
If the policy of the chat room allows the usage of nicknames, any new
nickname requested MUST be prepared and compared with nicknames
already in use or reserved following the rules defined in
"Preparation, Enforcement, and Comparison of Internationalized
Strings Representing Nicknames" [RFC7700].
This mitigates the problem of nickname duplication, but it does not
solve a problem whereby users can choose similar (but different)
characters to represent two different nicknames. For example, "BOY"
and "B0Y" are different nicknames that can mislead users. The former
uses the capital letter "O" while the latter uses the number zero
"0". In many fonts, the letter "O" and the number zero "0" might be
quite similar and difficult to perceive as different characters.
Chat rooms MAY provide a mechanism to mitigate confusable nicknames.
In addition to preparing and comparing following the rules above, the
MSRP switch SHOULD only allow the reservation of an already-used
nickname if the same user (e.g., identified by the SIP AOR) that is
currently using the nickname is making this subsequent request. This
may include, e.g., allowing the participant's URI to 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 [RFC3261], included in a trusted SIP P-Asserted-
Identity header field [RFC3325], included in a valid SIP Identity
header field [RFC4474], or 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 verifies if the suggested nickname can be accepted
(see below).
The reservation of a nickname can fail in several cases. If the
NICKNAME request contains a malformed value in the Use-Nickname
header field, the MSRP switch MUST answer the NICKNAME request with a
424 response code. This can be the case when the value of the
Use-Nickname header field does not conform to the syntax.
The reservation of a nickname can also fail if the value of the
Use-Nickname header field of the NICKNAME request is a reserved word
(not to be used as a nickname by any user) or that particular value
is already in use by another user. In these cases, the MSRP switch
MUST answer the NICKNAME request with a 425 response code.
In both error conditions (receiving a 424 or 425 response code), the
nickname usage is considered failed; the nickname is not allocated to
this user. The user can select a different nickname and retry
another NICKNAME request.
If the MSRP switch is able to accept the suggested nickname to be
used by this user, the MSRP switch MUST answer the NICKNAME request
with a 200 response as per regular MSRP procedures.
As indicated earlier, this specification defines a new MSRP header
field: Use-Nickname. The Use-Nickname header field carries a
nickname string. This specification defines the usage of the
Use-Nickname header field in NICKNAME requests. If need arises,
usages of the Use-Nickname header field in other MSRP methods should
be specified separately.
According to RFC 4975 [RFC4975], MSRP uses the UTF-8 transformation
format [RFC3629]. The syntax of the MSRP NICKNAME method and the
Use-Nickname header field is built upon the MSRP formal syntax
[RFC4975] using the Augmented Backus-Naur Form (ABNF) [RFC5234].
other-method =/ NICKNAMEm
; other-method defined in RFC 4975
NICKNAMEm = %x4E.49.43.4B.4E.41.4D.45 ; NICKNAME in caps
ext-header =/ Use-Nickname
; ext-header defined in RFC 4975
Use-Nickname = "Use-Nickname:" SP nickname
nickname = DQUOTE 1*1023(qdtext / qd-esc) DQUOTE
; qdtext and qd-esc defined in RFC 4975
Note that, according to RFC 4975 [RFC4975], "quoted-string" admits a
subset of UTF-8 characters [RFC3629]. Please refer to Section 9 of
RFC 4975 [RFC4975] for more details.
Once the MSRP switch has reserved a nickname and has bound it to a
URI (e.g., a SIP AoR), 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 UA. In this case,
the user MAY request a nickname that is the same or different 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.
7.2. Modifying a Nickname
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 Section 7.1.
If a NICKNAME request that attempts to modify the current nickname of
the user fails for some reason, 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.
7.3. Removing a Nickname
If the participant no longer wants to be known by a nickname in the
chat room, the participant can follow the method described in
Section 7.2. The nickname element of the Use-Nickname header MUST be
set to an empty quoted string.
7.4. Nicknames in Conference Event Packages
Typically the conference focus acts as a notifier of the conference
event package, RFC 4575 [RFC4575]. It is RECOMMENDED that conference
foci and endpoints support RFC 6502 [RFC6502] for providing
information regarding the conference and, in particular, supplying
information of the roster of the conference. It is also RECOMMENDED
that conference foci and endpoints support RFC 6501 [RFC6501], which
extends the <user> element originally specified in RFC 4575 [RFC4575]
with a new 'nickname' attribute. This allows endpoints to learn the
nicknames of participants of the chat room.
8. The SDP 'chatroom' Attribute
There are a handful of use cases where a participant would like to
learn the chat room capabilities supported by the local policy of 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 IM addressed
to a participant, but since the MSRP switch does not support the
functions specified in this memo, the message would eventually be
distributed to all the participants of the chat room.
The reverse case also exists. A participant, say Alice, whose UA
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 forbidden.
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 [RFC4566] 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 chat room. The 'chatroom'
attribute can be complemented with additional modifiers that further
indicate the intersection of support and local policy allowance for a
number of functions specified in this document. Specifically, we
provide the means to indicate support for the use of nicknames and
private messaging.
The 'chatroom' attribute merely indicates the capabilities supported
and allowed by the local policy. This attribute is not a negotiation
subject to the SDP offer/answer model [RFC3264], but instead a
declaration. Therefore, a 'chatroom' attribute included in an SDP
answer does not need to be a subset of the values included in the
'chatroom' attribute of its corresponding SDP offer. Consequently,
an SDP answer MAY contain a 'chatroom' attribute even if its
corresponding SDP offer did not include it.
In subsequent SDP offer/answer [RFC3264] exchanges pertaining to the
same session, the 'chatroom' attribute MAY be modified with respect
to an earlier SDP offer/answer exchange. The new value of this
attribute indicates the current support and local policy, meaning
that some restrictions can apply now or might have been removed. If
the 'chatroom' attribute is not included in a subsequent SDP offer/
answer, but a corresponding MSRP stream is still in place, it
indicates that support for the procedures indicated in this document
are disabled.
The 'chatroom' SDP attribute has the following ABNF [RFC5234] syntax:
attribute =/ chatroom-attr
; attribute defined in RFC 4566
chatroom-attr = chatroom-label [":" chat-token
*(SP chat-token)]
chatroom-label = "chatroom"
chat-token = (nicknames-token / private-msg-token /
ext-token)
nicknames-token = "nickname"
private-msg-token = "private-messages"
ext-token = private-token / standard-token
private-token = toplabel "." *(domainlabel ".") token
; toplabel defined in RFC 3261
; domainlabel defined in RFC 3261
; token defined in RFC 3261
standard-token = token
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 the use of the procedures specified in Section 7. A
conference focus that includes the 'private-messages' in the SDP
description is signaling that the MSRP switch supports and the chat
room allows the use of the procedures specified in Section 6.2.
An example of the 'chatroom' attribute for an MSRP media stream that
indicates the acceptance of nicknames and private messages:
a=chatroom:nickname private-messages
An example of a 'chatroom' attribute for an MSRP media stream where
the endpoint, e.g., an MSRP switch, does not allow nicknames or
private messages.
a=chatroom
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 an IETF document MUST follow the 'standard-
token' ABNF previously defined. In this type of extension, care 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 be included in the DNS name of the
vendor. Then, the token is reversed in order to avoid clashes of
tokens. The following is an example of an extension named "foo.chat"
by a vendor "example.com"
a=chatroom:nickname private-messages com.example.chat.foo
Note that feature names created by different organizations are not
intended to have the same semantics or even interoperate.
9. Examples
9.1. Joining a Chat Room
Figure 3 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 chat room extensions defined in this
document.
Alice Conference Focus
| |
|F1: (SIP) INVITE |
|----------------------->|
|F2: (SIP) 200 OK |
|<-----------------------|
|F3: (SIP) ACK |
|----------------------->|
| |
Figure 3: Flow Diagram of a User Joining a Chat Room
F1: Alice constructs an SDP description that includes an MSRP media
stream. She also indicates her support for the chat room extensions
defined in this document. She sends the INVITE request to the chat
room server.
INVITE sip:chatroom22@chat.example.com SIP/2.0
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
Max-Forwards: 70
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Chatroom 22 <sip:chatroom22@chat.example.com>
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:alice@client.atlanta.example.com;transport=tcp>
Content-Type: application/sdp
Content-Length: 290
v=0
o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
s=-
c=IN IP4 client.atlanta.example.com
m=message 7654 TCP/MSRP *
a=accept-types:message/cpim text/plain text/html
a=path:msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
a=chatroom:nickname private-messages
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.
SIP/2.0 200 OK
Via: SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;received=192.0.2.101
From: Alice <sip:alice@atlanta.example.com>;tag=9fxced76sl
To: Chatroom 22 <sip:chatroom22@chat.example.com>;tag=8321234356
Call-ID: 3848276298220188511@atlanta.example.com
CSeq: 1 INVITE
Contact: <sip:chatroom22@chat.example.com;transport=tcp> \
;methods="INVITE,BYE,OPTIONS,ACK,CANCEL,SUBSCRIBE,NOTIFY" \
;automata;isfocus;message;event="conference"
Content-Type: application/sdp
Content-Length: 290
v=0
o=chat 2890844527 2890844527 IN IP4 chat.example.com
s=-
c=IN IP4 chat.example.com
m=message 12763 TCP/MSRP *
a=accept-types:message/cpim
a=accept-wrapped-types:text/plain text/html *
a=path:msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
a=chatroom:nickname private-messages
F3: The session established is acknowledged (details not shown).
9.2. Setting Up a Nickname
Figure 4 shows an example of Alice setting up a nickname using the
chat room 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.
Alice MSRP Switch
| |
|F1: (MSRP) NICKNAME |
|----------------------->|
|F2: (MSRP) 425 |
|<-----------------------|
|F3: (MSRP) NICKNAME |
|----------------------->|
|F4: (MSRP) 200 |
|<-----------------------|
| |
Figure 4: Flow Diagram of a User Setting up Her Nickname
F1: Alice sends an MSRP NICKNAME request that contains her proposed
nicknames in the Use-Nickname header field.
MSRP d93kswow NICKNAME
To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
Use-Nickname: "Alice the great"
-------d93kswow$
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
425 response.
MSRP d93kswow 425 Nickname reserved or already in use
To-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
From-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
-------d93kswow$
F3: Alice receives the response. She proposes a new nickname in a
second NICKNAME request.
MSRP 09swk2d NICKNAME
To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
Use-Nickname: "Alice in Wonderland"
-------09swk2d$
F4: The MSRP switch accepts the nickname proposal and answers with a
200 response.
MSRP 09swk2d 200 OK
To-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
From-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
-------09swk2d$
9.3. Sending a Regular Message to the Chat Room
Figure 5 is 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.
Alice MSRP Switch Bob Charlie
| | | |
| F1: (MSRP) SEND | | |
|--------------------->| F3: (MSRP) SEND | |
| F2: (MSRP) 200 |----------------------->| |
|<---------------------| F4: (MSRP) SEND | |
| |------------------------------->|
| | F5: (MSRP) 200 OK | |
| |<-----------------------| |
| | F6: (MSRP) 200 OK | |
| |<------------------------------ |
| | | |
| | | |
Figure 5: Sending a Regular Message to the Chat Room
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.
MSRP 3490visdm SEND
To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
Message-ID: 99s9s2
Byte-Range: 1-*/*
Content-Type: message/cpim
To: <sip:chatroom22@chat.example.com;transport=tcp>
From: <sip:alice@atlanta.example.com>
DateTime: 2009-03-02T15:02:31-03:00
Content-Type: text/plain
Hello guys, how are you today?
-------3490visdm$
F2: The MSRP switch acknowledges the reception of the SEND request
with a 200 (OK) response.
MSRP 3490visdm 200 OK
To-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
From-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
Message-ID: 99s9s2
-------3490visdm$
F3: The MSRP switch creates a new MSRP SEND request that contains the
received Message/CPIM wrapper and sends it to Bob.
MSRP 490ej23 SEND
To-Path: msrp://client.biloxi.example.com:4923/49dufdje2;tcp
From-Path: msrp://chat.example.com:5678/jofofo3;tcp
Message-ID: 304sse2
Byte-Range: 1-*/*
Content-Type: message/cpim
To: <sip:chatroom22@chat.example.com;transport=tcp>
From: <sip:alice@atlanta.example.com>
DateTime: 2009-03-02T15:02:31-03:00
Content-Type: text/plain
Hello guys, how are you today?
-------490ej23$
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 to all participants in the chat room
rather than a private message addressed to him.
The rest of the message flows are analogous to the previous. They
are not shown here.
9.4. Sending a Private Message to a Participant
Figure 6 is 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.
Alice MSRP Switch Bob
| | |
| F1: (MSRP) SEND | |
|--------------------->| F3: (MSRP) SEND |
| F2: (MSRP) 200 |----------------------->|
|<---------------------| F4: (MSRP) 200 |
| |<-----------------------|
| | |
Figure 6: Sending a Private Message 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.
MSRP 6959ssdf SEND
To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
Message-ID: okj3kw
Byte-Range: 1-*/*
Content-Type: message/cpim
To: <sip:bob@example.com>
From: <sip:alice@example.com>
DateTime: 2009-03-02T15:02:31-03:00
Content-Type: text/plain
Hello Bob.
-------6959ssdf$
F2: The MSRP switch acknowledges the reception of the SEND request
with a 200 (OK) response.
MSRP 6959ssdfm 200 OK
To-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
From-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
Message-ID: okj3kw
-------6959ssdfm$
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.
MSRP 9v9s2 SEND
To-Path: msrp://client.biloxi.example.com:4923/49dufdje2;tcp
From-Path: msrp://chat.example.com:5678/jofofo3;tcp
Message-ID: d9fghe982
Byte-Range: 1-*/*
Content-Type: message/cpim
To: <sip:bob@example.com>
From: <sip:alice@atlanta.example.com>
DateTime: 2009-03-02T15:02:31-03:00
Content-Type: text/plain
Hello Bob.
-------9v9s2$
F4: Bob acknowledges the reception of the SEND request with a 200
(OK) response.
MSRP 9v9s2 200 OK
To-Path: msrp://chat.example.com:5678/jofofo3;tcp
From-Path: msrp://client.biloxi.example.com:4923/49dufdje2;tcp
Message-ID: d9fghe982
-------9v9s2$
9.5. Chunked Private Message
The MSRP message below is a depiction of the same private message
described in Section 9.4, 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.
MSRP 7443ruls SEND
To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
Message-ID: aft4to
Byte-Range: 1-*/174
Content-Type: message/cpim
To: <sip:bob@example.com>
From: <sip:alice@example.com>
-------7443ruls$
MSRP 7443ruls SEND
To-Path: msrp://chat.example.com:12763/kjhd37s2s20w2a;tcp
From-Path: msrp://client.atlanta.example.com:7654/jshA7weztas;tcp
Message-ID: aft4to
Byte-Range: 68-174/174
Content-Type: message/cpim
DateTime: 2009-03-02T15:02:31-03:00
Content-Type: text/plain
Hello Bob
-------7443ruls$
9.6. Nickname in a Conference Information Document
Figure 7 is a depiction of an XML conference information document
received in a SIP NOTIFY request as a notification to the XCON
Conference Event Package, RFC 6502 [RFC6502]. The conference
information document follows the XCON Data Model specified in RFC
6501 [RFC6501].
The conference information document of Figure 7 presents information
of two users who are participating in the conference (see each of the
<user> elements). Each participant is bound to a nickname, shown in
the 'nickname' attribute of the <user> element.
NOTE: The purpose of Figure 7 is to show the user-to-nickname
relationship. It is believed that the example is correct,
according to RFC 6501 [RFC6501]. In case of contradictions
between this specification and RFC 6501 [RFC6501], the latter has
precedence.
<?xml version="1.0" encoding="UTF-8"?>
<conference-info
xmlns="urn:ietf:params:xml:ns:conference-info"
xmlns:xcon="urn:ietf:params:xml:ns:xcon-conference-info"
entity="sip:chatroom22@chat.example.com"
state="full" version="1">
<!--
CONFERENCE INFO
-->
<conference-description>
<subject>MSRP nickname example</subject>
</conference-description>
<!--
CONFERENCE STATE
-->
<conference-state>
<user-count>2</user-count>
</conference-state>
<!--
USERS
-->
<users>
<user entity="sip:bob@example.com"
state="full"
xcon:nickname="Dopey Donkey">
<display-text>Bob Hoskins</display-text>
</user>
<!--
USER
-->
<user entity="sip:alice@atlanta.example.com"
state="full"
xcon:nickname="Alice the great">
<display-text>Alice Kay</display-text>
</user>
</users>
</conference-info>
Figure 7: Nickname in a Conference Information Document
10. IANA Considerations
10.1. New MSRP Method
This specification defines a new MSRP method that has been added to
the "Methods" subregistry of the "Message Session Relay Protocol
(MSRP) Parameters" registry:
NICKNAME
See Section 7 for details.
10.2. New MSRP Header
This specification defines a new MSRP header that has been added to
the "Header Fields" subregistry of the "Message Session Relay
Protocol (MSRP) Parameters" registry:
Use-Nickname
See Section 7 for details.
10.3. New MSRP Status Codes
This specification defines four new MSRP status codes that have been
added to the "Status Codes" subregistry of the "Message Session Relay
Protocol (MSRP) parameters" registry.
The 404 status code indicates the failure to resolve the recipient's
URI in the To header field of the Message/CPIM wrapper in the SEND
request, e.g., due to an unknown recipient. See Section 6.2 for
details.
The 424 status code indicates a failure in allocating the requested
nickname due to a malformed syntax in the Use-Nickname header field.
See Section 7 for details.
The 425 status code indicates a failure in allocating the requested
nickname because the requested nickname in the Use-Nickname header
field is reserved or is already in use by another user. See
Section 7 for details.
The 428 status code indicates that the recipient of a SEND request
does not support private messages. See Section 6.2 for details.
Table 1 summarizes the IANA registration data with respect to new
MSRP status codes:
+-------+-------------------------------------+-----------+
| Value | Description | Reference |
+-------+-------------------------------------+-----------+
| 404 | Failure to resolve recipient's URI | RFC 7701 |
| 424 | Malformed nickname | RFC 7701 |
| 425 | Nickname reserved or already in use | RFC 7701 |
| 428 | Private messages not supported | RFC 7701 |
+-------+-------------------------------------+-----------+
Table 1: New Status Codes
10.4. New SDP Attribute
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 1000
Attribute name: chatroom
Long-form attribute name: Chat Room
Type of attribute: media level only
This attribute is not subject to the charset attribute
Description: This attribute identifies support and local policy
allowance for a number of chat room related functions
Specification: RFC 7701 (this document)
See Section 8 for details.
11. Security Considerations
This document proposes extensions to the Message Session Relay
Protocol [RFC4975]. Therefore, the security considerations of that
document apply to this document as well.
A chat room is, by its nature, a potential Denial-of-Service (DoS)
accelerator as it takes a message from one entity and sends it to
many. Implementers of both UAs and switches need to carefully
consider the set of anti-DoS measures that are appropriate for this
application, and switch implementations, in particular, ought to
include appropriate anti-DoS features. The details of what is
appropriate will vary over time and will also depend on the specific
needs of the implementation; thus, they cannot be specified here.
If the participant's SIP UA does not understand the "isfocus" feature
tag [RFC3840], it will not know that it is connected to a conference
instance. The participant might not be notified that its MSRP client
will try to send messages having potential multiple recipients to the
MSRP switch. If the participant's MSRP client does not support the
extensions of this specification, it is unlikely that it will try to
send a message using the Message/CPIM wrapper content type [RFC3862],
and the MSRP switch will reject the request with a 415 response
[RFC4975]. Still, if a participant's MSRP client does create a
message with a valid Message/CPIM wrapper content type [RFC3862]
having the To header set to the URI of the chat room and the From
header set to the URI of which the participant that is known to the
chat room, 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 chat room, the message can
be forwarded as a private message without the participant knowing.
To mitigate these problems, when the chat room detects that a UA does
not support the procedures of this document (i.e., when the SIP UA is
not chat room aware), the MSRP switch SHOULD send a regular MSRP
message indicating that the SIP UA is actually part of a chat room
and that all the messages that the user sends correctly formatted
will be distributed to a number of participants. Additionally, the
MSRP switch SHOULD also send a regular MSRP text message including
the list of participants in the chat room so that the user becomes
aware of the roster.
If a participant wants to avoid security concerns on the path between
himself and the MSRP switch (e.g., eavesdropping, faked packet
injection, or packet corruption), the participant's UA can force the
usage of MSRP over a TLS [RFC5246] transport connection. This is
negotiated in the SDP offer/answer exchange as per the regular
procedures of RFC 4975 [RFC4975]. This negotiation will result in
both endpoints establishing a TLS [RFC5246] transport connection that
is used to exchange MSRP messages. The MSRP switch may also have
local policy that forces the usage of TLS transport for all MSRP
sessions, something that is also negotiated in SDP as per the regular
procedures of RFC 4975 [RFC4975].
Nicknames are used to show the appearance of the participants of the
chat room. A successful takeover of a nickname from a participant
might lead to private messages being sent to the wrong destination.
The recipient's URI will be different from the URI associated with
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. The chat
room has a policy that determines the time that a nickname is still
reserved for its holder, once it is no longer being used. This
allows, e.g., a user that accidentally loses its connectivity, to
reconnect to the chat room and keep on using the same nickname. It
depends on the policy of the chat room if a nickname that has been
previously used by another participant of the chat room can be
reserved or not.
Section 7.1 discusses the problem of similar but different nicknames
(e.g., thanks to the use of similar characters), and chat rooms MAY
provide a mechanism to mitigate confusable nicknames.
Recipients of IMs should be cautious with the rendering of content,
which can be malicious in nature. This includes, but is not limited
to, the reception of HTML and JavaScript scripts, executable code,
phishing attempts, etc. Endpoints SHOULD always request permission
from the user before executing one of these actions.
It must be noted that endpoints using a TLS client side certificate
with real names in the certificates will not be anonymous to the MSRP
switch to which they connect. While the name in the certificate
might not be used by MSRP, the server will have a certificate with
the actual name in it.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323,
DOI 10.17487/RFC3323, November 2002,
<http://www.rfc-editor.org/info/rfc3323>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840,
DOI 10.17487/RFC3840, August 2004,
<http://www.rfc-editor.org/info/rfc3840>.
[RFC3860] Peterson, J., "Common Profile for Instant Messaging
(CPIM)", RFC 3860, DOI 10.17487/RFC3860, August 2004,
<http://www.rfc-editor.org/info/rfc3860>.
[RFC3862] Klyne, G. and D. Atkins, "Common Presence and Instant
Messaging (CPIM): Message Format", RFC 3862,
DOI 10.17487/RFC3862, August 2004,
<http://www.rfc-editor.org/info/rfc3862>.
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
DOI 10.17487/RFC4353, February 2006,
<http://www.rfc-editor.org/info/rfc4353>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
Session Initiation Protocol (SIP) Event Package for
Conference State", RFC 4575, DOI 10.17487/RFC4575, August
2006, <http://www.rfc-editor.org/info/rfc4575>.
[RFC4975] Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
"The Message Session Relay Protocol (MSRP)", RFC 4975,
DOI 10.17487/RFC4975, September 2007,
<http://www.rfc-editor.org/info/rfc4975>.
[RFC4976] Jennings, C., Mahy, R., and A. Roach, "Relay Extensions
for the Message Sessions Relay Protocol (MSRP)", RFC 4976,
DOI 10.17487/RFC4976, September 2007,
<http://www.rfc-editor.org/info/rfc4976>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5239] Barnes, M., Boulton, C., and O. Levin, "A Framework for
Centralized Conferencing", RFC 5239, DOI 10.17487/RFC5239,
June 2008, <http://www.rfc-editor.org/info/rfc5239>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<http://www.rfc-editor.org/info/rfc5681>.
[RFC6501] Novo, O., Camarillo, G., Morgan, D., and J. Urpalainen,
"Conference Information Data Model for Centralized
Conferencing (XCON)", RFC 6501, DOI 10.17487/RFC6501,
March 2012, <http://www.rfc-editor.org/info/rfc6501>.
[RFC6502] Camarillo, G., Srinivasan, S., Even, R., and J.
Urpalainen, "Conference Event Package Data Format
Extension for Centralized Conferencing (XCON)", RFC 6502,
DOI 10.17487/RFC6502, March 2012,
<http://www.rfc-editor.org/info/rfc6502>.
[RFC7700] Saint-Andre, P., "Preparation, Enforcement, and Comparison
of Internationalized Strings Representing Nicknames",
RFC 7700, DOI 10.17487/RFC7700, December 2015,
<http://www.rfc-editor.org/info/rfc7700>.
12.2. Informative References
[RFC2810] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810,
DOI 10.17487/RFC2810, April 2000,
<http://www.rfc-editor.org/info/rfc2810>.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
DOI 10.17487/RFC3325, November 2002,
<http://www.rfc-editor.org/info/rfc3325>.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, DOI 10.17487/RFC3966, December 2004,
<http://www.rfc-editor.org/info/rfc3966>.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474,
DOI 10.17487/RFC4474, August 2006,
<http://www.rfc-editor.org/info/rfc4474>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <http://www.rfc-editor.org/info/rfc6120>.
Acknowledgments
The authors want to thank Eva Leppanen, Adamu Haruna, Adam Roach,
Matt Lepinski, Mary Barnes, Ben Campbell, Paul Kyzivat, Adrian
Georgescu, Nancy Greene, Cullen Jennings, Flemming Andreasen, Suresh
Krishnan, Christer Holmberg, Saul Ibarra, Enrico Marocco, Alexey
Melnikov, Peter Saint-Andre, Stephen Farrell, and Martin Stiemerling
for providing comments.
Contributors
This work would have never been possible without the fruitful
discussions on the SIMPLE WG mailing list, especially with Brian
Rosen (Neustar) and Paul Kyzivat (Huawei), who provided extensive
review and improvements throughout the document.
Authors' Addresses
Aki Niemi
Email: aki.niemi@iki.fi
Miguel A. Garcia-Martin
Ericsson
Calle Via de los Poblados 13
Madrid, ES 28033
Spain
Email: miguel.a.garcia@ericsson.com
Geir A. Sandbakken
Cisco Systems
Philip Pedersensvei 1
1366 Lysaker
Norway
Email: geirsand@cisco.com