SIMPLE WG C. Jennings
Internet-Draft R. Mahy
Expires: October 19, 2004 Cisco Systems, Inc.
April 20, 2004
Relay Extensions for Message Sessions Relay Protocol (MSRP)
draft-ietf-simple-msrp-relays-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This Internet-Draft will expire on October 19, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The SIMPLE Working Group uses two separate models for conveying
instant messages. Pager-mode messages stand alone, whereas
Session-mode messages are setup as part of a session using the SIP
protocol. MSRP (Message Sessions Relay Protocol) is a protocol for
near-real-time, peer-to-peer exchange of binary content without
intermediaries, which is designed to be signaled using a separate
rendezvous protocol such as SIP. This document introduces the notion
of message relay intermediaries to MSRP and describes the extensions
necessary to use them.
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Table of Contents
1. Conventions and Definitions . . . . . . . . . . . . . . . . 3
2. Introduction and Requirements . . . . . . . . . . . . . . . 3
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . 4
4. New Protocol Elements . . . . . . . . . . . . . . . . . . . 10
4.1 The AUTH Method . . . . . . . . . . . . . . . . . . . . . . 10
4.2 The Use-Path header . . . . . . . . . . . . . . . . . . . . 10
4.3 Authentication headers . . . . . . . . . . . . . . . . . . . 10
4.4 Time-related headers . . . . . . . . . . . . . . . . . . . . 11
5. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1 Client behavior . . . . . . . . . . . . . . . . . . . . . . 11
5.1.1 Connecting to relays acting on your behalf . . . . . . . . . 11
5.1.2 Sending requests . . . . . . . . . . . . . . . . . . . . . . 12
5.1.3 Receiving Requests . . . . . . . . . . . . . . . . . . . . . 13
5.1.4 Managing Connections . . . . . . . . . . . . . . . . . . . . 13
5.2 Relay behavior . . . . . . . . . . . . . . . . . . . . . . . 13
5.2.1 Handling Incoming Connections . . . . . . . . . . . . . . . 13
5.2.2 Generic request behavior . . . . . . . . . . . . . . . . . . 13
5.2.3 Receiving AUTH requests . . . . . . . . . . . . . . . . . . 13
5.2.4 Forwarding SEND requests . . . . . . . . . . . . . . . . . . 15
5.2.5 Forwarding non-SEND requests . . . . . . . . . . . . . . . . 16
5.2.6 Forwarding Responses . . . . . . . . . . . . . . . . . . . . 16
5.2.7 Managing Connections . . . . . . . . . . . . . . . . . . . . 17
5.2.8 Forwarding unknown requests . . . . . . . . . . . . . . . . 17
5.3 Acting as a Message Taker . . . . . . . . . . . . . . . . . 17
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . 17
7. Finding MSRP Servers . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . 20
8.1 Using HTTP Authentication . . . . . . . . . . . . . . . . . 20
8.2 Using TLS . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.3 Threat Model . . . . . . . . . . . . . . . . . . . . . . . . 20
8.4 Security Mechanism . . . . . . . . . . . . . . . . . . . . . 21
8.5 Preventing Spam and Denial of Service Attacks . . . . . . . 22
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 23
10. Example SDP with multiple hops . . . . . . . . . . . . . . . 23
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 23
Normative References . . . . . . . . . . . . . . . . . . . . 24
Informative References . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . 27
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1. Conventions and Definitions
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 [1].
Below we list several definitions important to MSRP:
'MSRP node:' A host that implements the MSRP protocols as a Client
or a Relay
'MSRP Client:' A MSRP role which is the initial sender or final
target of messages and delivery status.
'MSRP Relay:' A MSRP role which forwards messages and delivery
status and may provide policy enforcement. Relays MAY fragment
and reassemble portions of messages.
'Message-Taker:' A MSRP Client which persistently stores messages
on behalf of specific users or resources
'message:' arbitrary MIME content which one client wishes to send
to another. For the purposes of this specification, a complete
MIME body as opposed to a portion of a complete message.
'message fragment:' a portion of a complete message carried in
(for example) a message/byteranges MIME type.
'message:' binary MIME content of an arbitrary type. Each message
has a unique message-id. In MSRP, messages may be broken up into
pieces and sent in separate SEND requests.
'end-to-end:' delivery of data from the initiating client to the
final target client
'hop:' delivery of data between one MSRP node and an adjacent
node.
'transaction:' a request and response as seen from a single MSRP
node. Each transaction has a locally significant transaction
identifier.
2. Introduction and Requirements
The IETF SIMPLE Working Group has identified a number of scenarios
where using a separate protocol for bulk messaging is desirable. In
particular, the SIMPLE WG will use this facility to handle a sequence
of messages as a session of media initiated using SIP [2], just like
any other media type. (The benefits of the session-mode approach are
further discussed in [19].) The SIMPLE Working Group has also
developed MSRP (the Message Sessions Relay Protocol) to convey
sessions of messages directly between two end systems with no
intermediaries. With MSRP, messages can be arbitrarily large and all
traffic is sent over reliable, congestion-safe transports.
This document describes extensions to the core MSRP protocol to
introduce intermediaries called Relays. With these extensions MSRP
clients can communicate directly, or through an arbitrary number of
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relays. Each client is responsible for identifying any relays acting
on its behalf and providing appropriate credentials. Clients which
can receive new TCP connections directly do not have to implement any
new functionality to work with these relays.
This document is far from complete, but was submitted to allow the
SIMPLE WG to understand the proposed concept and bring up issues with
the general approach.
The Goals of the MSRP Relay extensions are listed below:
o convey arbitrary binary MIME data without modification or transfer
encoding
o continue to support client to client operation (no relay servers
required)
o operate through an arbitrary number of relays for policy
enforcement
o allow each client to control which relays are traversed on its
behalf
o prevent unsolicited messages (spam), "open relays", and denial of
service amplification
o allow relays to use one or a small number of TCP or TLS [3]
connections to carry messages for multiple sessions, recipients,
and senders
o allow large messages to be sent over a slow connection without
causing head-of-line blocking problems
o allow transmission of a large message to be interrupted and
resumed in place when network connectivity is lost and later
reestablished
o offer notification of message failure at any intermediary
o provide notification of message storage (desirable)
o allow relays to delete state after a short amount of time
3. Protocol Overview
With the introduction of this extension, MSRP has the concept of both
clients and relays. Clients send messages to relays and/or other
clients. Relays forward messages and message delivery status to
clients and other relays. Clients which can open TCP connections to
each other without intervening policy restrictions, can communicate
directly with each other. Clients who are behind a firewall or who
need to use an intermediary for policy reasons can use the services
of a relay. Each client is responsible for enlisting the assistance
of one or more relays for its half of the communication.
We also define the special role of a Message-Taker, which is a client
that can receive messages and store them persistently on behalf of a
user. Note that these roles can be co-resident.
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Clients which use a relay operate by first opening a connection with
a relay, authenticating, and retreiving a URI on the relay the client
can provide to its peers to receive messages later. When a client
uses a relay, it first opens a TLS connection to its first relay and
authenticates using an AUTH request which can contain HTTP Digest or
Basic [4] Authentication credentials. In a successful AUTH response,
the relay provides an MSRP URI associated with the path back to the
client that the client can give to other clients for end-to-end
message delivery.
When clients wish to send a short message, they send a SEND request
with the entire contents of the message. If any relays are required,
they are included in the To-Path header. The leftmost URI in the
To-Path header is the next hop to deliver a request or response. The
rightmost URI in the To-Path header is the final target.
MSRP SEND
Tr-ID: 892341
To-Path: msrp:example.org:9000/kjfjan \
msrp:magic-coookie@example.net:9000/aeiug \
msrp:bob.example.net:8145/foo
From-Path: msrp:alice.example.com:7965/bar
Boundary: 6aef
Content-Type: text/plain
Hi Bob, I'm about to send you LoTR.mpeg
-------6aef$
MSRP 200 OK
Tr-ID: 892341
To-Path: msrp:alice.example.com:7965/bar
From-Path: msrp:example.org:9000/kjfjan
MSRP SEND
Tr-ID: 132452
To-Path: msrp:magic-coookie@example.net:9000/aeiug \
msrp:bob.example.net:8145/foo
From-Path: msrp:example.org:9000/kjfjan \
msrp:alice.example.com:7965/bar
Boundary: 6aef
Content-Type: text/plain
Hi Bob, I'm about to send you LoTR.mpeg
-------6aef$
MSRP 200 OK
Tr-ID: 132452
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To-Path: msrp:example.org:9000/kjfjan
From-Path: msrp:magic-coookie@example.net:9000/aeiug
MSRP SEND
Tr-ID: 0987231
To-Path: msrp:bob.example.net:8145/foo
From-Path: msrp:magic-coookie@example.net:9000/aeiug \
msrp:example.org:9000/kjfjan \
msrp:alice.example.com:7965/bar
Boundary: 6aef
Content-Type: text/plain
Hi Bob, I'm about to send you LoTR.mpeg
-------6aef$
MSRP 200 OK
Tr-ID: 0987231
To-Path: msrp:magic-coookie@example.net:9000/aeiug
From-Path: msrp:bob.example.net:8145/foo
MSRP REPORT
Tr-ID: 784333
To-Path: msrp:magic-coookie@example.net:9000/aeiug \
msrp:example.org:9000/kjfjan \
msrp:alice.example.com:7965/bar
From-Path: msrp:bob.example.net:8145/foo
Receipt: success
MSRP REPORT
Tr-ID: 784333
To-Path: msrp:example.org:9000/kjfjan \
msrp:alice.example.com:7965/bar
From-Path: msrp:magic-coookie@example.net:9000/aeiug \
msrp:bob.example.net:8145/foo
Receipt: success
MSRP REPORT
Tr-ID: 784333
To-Path: msrp:alice.example.com:7965/bar
From-Path: msrp:example.org:9000/kjfjan \
msrp:magic-coookie@example.net:9000/aeiug \
msrp:bob.example.net:8145/foo
Receipt: success
MSRP 200 OK
Tr-ID: 784333
To-Path: msrp:example.org:9000/kjfjan \
msrp:magic-coookie@example.net:9000/aeiug \
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msrp:bob.example.net:8145/foo
From-Path: msrp:alice.example.com:7965/bar
Receipt: success
MSRP 200 OK
Tr-ID: 784333
To-Path: msrp:magic-coookie@example.net:9000/aeiug \
msrp:bob.example.net:8145/foo
From-Path: msrp:example.org:9000/kjfjan \
msrp:alice.example.com:7965/bar
Receipt: success
MSRP 200 OK
Tr-ID: 784333
To-Path: msrp:bob.example.net:8145/foo
From-Path: msrp:example.org:9000/kjfjan \
msrp:magic-coookie@example.net:9000/aeiug \
msrp:alice.example.com:7965/bar
Receipt: success
Typical flow involving two relays
Alice a.example.org b.example.net Bob
| | | |
|::::::::::::::::::::>| connection opened |<::::::::::::::::::::|
|--- AUTH ----------->| |<-- AUTH ------------|
|<-- 401 Auth---------| |--- 401 Auth-------->|
|--- AUTH ----------->| |<-- AUTH ------------|
|<-- 200 OK-----------| |--- 200 OK---------->|
| | | |
.... time passes ....
| | | |
|--- SEND ----------->| | |
|<-- 200 OK ----------|:::::::::::::::::::>| (slow link) |
| |--- SEND ---------->| |
| |<-- 200 OK ---------|--- SEND ----------->|
| | | ....>|
| | | ..>|
| | |<-- 200 OK ----------|
| | |<-- REPORT ----------|
| |<-- REPORT ---------| |
|<-- REPORT ----------| | |
|--- 200 OK --------->| | |
| |--- 200 OK -------->| |
| | |--- 200 OK --------->|
| | | |
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SEND requests are sent hop-by-hop. (Each relay that receives a SEND
request acknowledges receipt of the request before forwarding the
content in other SEND requests.) All other requests are sent
end-to-end.
With the introduction of relays, the subtle semantics of the To-Path
and From-Path header becomes more relevant. The To-Path in both
requests and responses is the list of URIs that need to be visited in
order to reach the final target of the request. The From-Path is the
list of URIs that indicate how to get back to the original sender of
the request or response (Note these semantics are slightly different
for SEND requests). This differs from the To and =46rom headers in =
SIP,
which do not "swap" from request to response. (Note that sometimes a
request is sent to or from an intermediary directly.)
When a relay forwards a request, it removes its address from the
To-Path header and inserts it at as the first URI in the From-Path
header. For example if the path from Alice to Bob is through relays
A and B, when B receives the request it contains path headers that
look like this:
To-Path: msrp:B msrp:Bob
From-Path: msrp:A msrp:Alice
after forwarding the request, the path headers look like this:
To-Path: msrp:Bob
From-Path: msrp:B msrp:A msrp:Alice
MSRP Nodes respond to SEND requests by taking the first URI form the
From-Path and placing that in a To-Path header in the response, and
placing their URI in the From-Path of the response. MSRP Nodes
response to all other requests addressed to them, by swapping the
To-Path and From-Path headers.
When sending large content the client may split up a messsage into
smaller pieces; each SEND request might contain only a portion of the
complete message. For example, when Alice sends Bob a 4GB file
called "LoTR.mpeg", she sends several SEND requests each with a
portion of the complete message. Relays can repack message fragments
en-route. As individual parts of the complete message arrive at the
final destination client, the receiving client can optionally send
REPORT requests indicating delivery status.
MSRP nodes can send individual portions of a complete message in
multiple SEND requests. Each parcel uses the message/byteranges MIME
type defined in RFC 2616 [5] to correlate that part to the complete
message. As each SEND request is received, the next hop acknowledges
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the request. As relays receive parcels they can reassemble or
re-fragment them as long as each chunk is sent in order. Once a chunk
or complete message arrives at the destination client, the
destination can optionally send a REPORT request indicating that a
chunk arrived end-to-end. This request travels back along the reverse
path of the SEND request. Unlike the SEND request which is
acknowledged along every hop, only the sender of the REPORT request
responds to an REPORT. Relays then forward the REPORT response back
to the recipient of the original SEND.
Flow involving re-chunking through two relays
Alice a.example.org b.example.net Bob
| | | |
| | | |
|--- AUTH ----------->| |<-- AUTH ------------|
|<-- 401 Auth---------| |--- 401 Auth-------->|
|--- AUTH ----------->| |<-- AUTH ------------|
|<-- 200 OK-----------| |--- 200 OK---------->|
| | | |
.... time passes ....
| | | |
|--- SEND 0-3 ------->| | |
|<-- 200 OK ----------| | (slow link) |
|--- SEND 4-7 ------->|--- SEND 0-5 ------>| |
|<-- 200 OK ----------|<-- 200 OK ---------|--- SEND 0-3 ------->|
|--- SEND 8-10 ------>|--- SEND 6-10 ----->| ....>|
|<-- 200 OK ----------|<-- 200 OK ---------| ..>|
| | |<-- 200 OK ----------|
| | |<-- REPORT 0-3 ------|
| |<-- REPORT 0-3 -----|--- SEND 4-7 ------->|
|<-- REPORT 0-3 ------| | ...>|
|--- 200 OK --------->| | ..>|
| |--- 200 OK -------->| |
| | |--- 200 OK --------->|
| | |<-- REPORT 4-7 ----->|
| |<-- REPORT 4-7 -----|--- SEND 8-10 ------>|
|<-- REPORT 4-7 ------| | ..>|
|--- 200 OK --------->| |<-- 200 OK ----------|
| |<-- REPORT done-----|<-- REPORT done -----|
|<-- REPORT done -----|--- 200 OK -------->| |
|--- 200 OK --------->| |--- 200 OK --------->|
| |--- 200 OK -------->| |
| | |--- 200 OK --------->|
| | | |
Relays only keep transaction state for a short period of time for
each chunk. Delivery over each hop should take no more than 32
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seconds after the last byte of data is sent. Clients applications
define their own implementation-dependent timers for end-to-end
message delivery.
In some cases the end user node may not have its own client or that
client or node may be unavailable. In this case, a message-taker can
take receipt of the message or fragment and deliver a REPORT back to
the sender indicating that the message or fragment was successfully
stored.
For client to client communication, the sender of a message typically
opens a new TCP connection (with or without TLS) if one is needed.
Relays reuse existing connections first, but can open new connections
(typically to another relay) to deliver requests such as SEND or
REPORT. Responses can only be sent over existing connections.
4. New Protocol Elements
4.1 The AUTH Method
AUTH requests are used by clients with ephemeral addresses to create
a handle they can use to receive incoming requests. AUTH requests
can also contain credentials used to authenticate a client, and
authorization policy used to block Denial of Service attacks. AUTH
requests are discussed in more detail in Section XXX TODO.
In response to an AUTH request, a successful response contains a Path
header with a list of URIs that the Client can give to its peers to
route responses back to the Client.
4.2 The Use-Path header
The Use-Path header is a list of URIs provided by an MSRP Relay in
response to a successful AUTH request. This list of URIs can be used
by the MSRP Client that sent the AUTH request to receive MSRP
requests, and can advertise this list of URIs, for example in a
session description.
4.3 Authentication headers
The Authentication-Info header provides optional information for HTTP
Digest authentication. This header MAY be included in the response
to an AUTH request. Semantics of the header are described in RFC
2617
The Authorization header contains authentication credentials for HTTP
Digest authentication in an AUTH request. Section [x.y] . Note that
the parameters of this header are separated by commas instead of
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semicolons. The presence of commas in this header does not imply
that there is more than one header field value for this header field
(only one header field value is allowed). Semantics of the header are
described in RFC 2617. This header MUST NOT appear in any parcel
other than an AUTH request.
The WWW-Authenticate header [more]
4.4 Time-related headers
The Expires header in a provides a relative time after which the
action implied by the method of the request is no longer of interest.
In a request, the Expires header indicates how long the sender would
like to . In a response, the Expires header indicates how long the
responder considers this information relevant (if the responder
[more]. Specifically an Expires header in an AUTH request indicates
how long the provided URIs will be valid.
The Min-Expires header contains the minimum duration a server will
permit in an Expires header. It is sent only in 423 "Interval
Out-of-Bounds" responses. Likewise the Max-Expires header contains
the maximum duration a server will permit in an Expires header.
423 Interval Out-of-bounds. Max-Expires header
5. Procedures
5.1 Client behavior
5.1.1 Connecting to relays acting on your behalf
Clients which want to use the services of a relay or list of relays,
need to send an AUTH request to each relay which will act on their
behalf. For example, some organizations could deploy an "intra-org"
relay and an "extra-org" relay. A client using these relays opens a
connection to the intra-org relay and sends an AUTH request.
response
Clients can be configured (typically through discovery or manual
provisioning) with a list of relays they need to use. They MUST be
able to form a connection to each relay and send an AUTH command to
get a URI that can be used in route headers. The client can
authenticate the relay by looking at the relay's TLS certificate. The
relay MUST authenticate the client using digest authentication.
The relay will return a URI, or list of URIs, in the Use-Path header
of the response. When using a session-protocol such as SIP, these
URIs are used by the client in the path attribute that is sent in the
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SDP to setup the session. The same URI can be used for multiple
sessions to send to the client.
When sending an AUTH request, the client MAY add an Expires header to
request a MSRP URI that is valid for no longer that the provided
interval. If an AUTH request returns a 401 Unauthorized request, the
client SHOULD fetch the Digest challenge from the WWW-Authenticate
header in the response and retry the AUTH request, including an
Authorization header with the Digest response. Unlike in HTTP and
SIP, Digest authentication in MSRP is only permitted for AUTH
requests. Example with two relays on one side. Need to AUTH to
first, then use the supplied route header to AUTH to second thought
the first.
NOTE - only auth not auth-int is needed because TLS provides
integrity
When a client wishes to use more than one relay, they must AUTH to
each relay they wish to use. Consider a client A, that whishes
messages to flow from A to the first relays, R1, then on to a second
relays, R2. This client with do a normal AUTH with R1. It will then
do an AUTH transaction with R2 that is routed through R1. The client
will form this AUTH messages by setting the request URI to R2 and
adding a route header with the URI learned from R1 then sending this
message to R1. R1 will forward this like a REPORT request is
forwarded to R2.
When the client sends an AUTH request, it may set the Expires header
a relative time. The relay will return a URI that is only valid for
that periods of time.
auth to-path: intra-org
from-path: alice@a
200 to-path: alice@a
from-path: intra-org
use-path: alice@intra-org/abcd alice@a
auth to-path: alice@intra-org/abcd extra-org
auth to-path: extra-org
200 use-path: extra-org/xyzpdq alice@intra-org/abcd alice@a
200 use-path: extra-org/xyzpdq alice@intra-org/abcd alice@a
5.1.2 Sending requests
The procedure for sending SEND, VISIT, and REPORT requests is
identical for clients whether relays are involved or not. The
specific procedures are described in section TODO of [MSRP].
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As usual, once the next-hop URI is determined, the client MUST find
the appropriate address, port, and transport to use and then check if
there is already an existing suitable connection to the next-hop
target. If so, the client MUST send the request over the most
suitable connection. Suitability MAY be determined by a variety of
factors such as measured load and local policy, however in most
simple implementations a connection will be suitable if it exists and
is in an active state.
5.1.3 Receiving Requests
The procedure for receiving requests is identical for clients whether
relays are involved or not.
5.1.4 Managing Connections
Clients should open connection whenever they wish to deliver a
request and no suitable connection exists. For client to client
connections, a client should close a connection when there are no
longer any sessions associated with the connection. For connections
to relays, the client should leave a connection up until no sessions
are using the connection for a locally defined period of time, which
defaults to 5 minutes for foreign relays and one hour for the
client's relays.
5.2 Relay behavior
5.2.1 Handling Incoming Connections
5.2.2 Generic request behavior
Upon receiving a new request, relays first verify the validity of the
request. [NO: Relays then tag valid requests with a
locally-significant connection identifier which they add to the last
URI in the Back-Path header. This is used to insure that responses
can be routed over an existing connection. ???] Relays then examine
the first URI in the To-Path header and remove this URI if it matches
a URI corresponding to the relay. Authorization -- determine if the
final target is a URI under its control or from a URI under its
control.
5.2.3 Receiving AUTH requests
When a relay receives an AUTH request, it must digest challenge the
request. Once the challenge is complete, it MUST provide a URI that
can be used in future route headers. When the route URI is received
in future messages. It MUST verify that this URI was issues by this
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relay. It MUST ensure that the message is either being forwarded from
an entity that did the AUTH request that resulted in this URI or it
is being forwarded to the the entity that did the AUTH request that
resulted in this URI.
Discuss forwarding of AUTH requests for another relay
The relay does not necessarily needs to save state to meet these
requirements. One way that a relay could implement this is the
following. When an AUTH request arrives, the relay concatenates the
current time, the identity of the sender of the AUTH request, the
identity of the previous hop the request came from. It then takes the
concatenates string and encrypts it with a key only the relay knows
and uses this for form the user portion of the sims URI that it
returns. Later when it receives a URI, it can decrypt this
information and use it to decide if the request should be forwarded
or not. If the relay is actually several servers that share a DNS
name, the URI may also encrypt which server actually has the
connection to the client.
When a relay receive an AUTH request, it must authenticate the client
that sent it with digest, it must also authenticate the previous hop
that send the message to it. When previous hop was a relay this is
done with the mutual TLS while when the previous hop was a client
mutual TLS MAY be used it is available or the client authorization
from the digest is used. The relay will generate the base URI of a
family of URIs, each of which allows messages to be forwarded to and
from this client. If the previous hop was authenticated by mutual
TLS, then the URI MUST be valid to route across any connection the
relay has to the previous hop relay. If the previous hop was not
authenticated by mutual TLS, then the URI MUST only be valid to route
across the same connection that the AUTH was received on. If this
connection is closed then reopened, the URI MUST NOT be valid. Valid
to route means that when the relay receives a messages that contains
this URI, if the message it going to element that was the previous
hop in the AUTH, then the relay can forward it and if the messages is
coming from previous hop in the AUTH, then the relay can forward it
to any location, otherwise the RELAY must discard the message and MAY
send a REPORT indicating the auth URI was bad. If the AUTH request
contains an Expires header, then the relay MUST ensure that the URI
is not valid to route after the expiry time.
[*** NOTE: Consider moving to another section ***]
It is possible to implement all of the above requirements without the
relay saving any state. When a relay starts up it could pick a crypto
random 128 bit password (K) and 128 bit initialization vector (IV).
If the relay was actually a NDS farm, all the machines in the farm
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would need to share the same K. When an ATUH request was received the
relay form a string that contains: the expiry time of the URI, an
indication if the previous hop was mutual TLS authenticated or not
and it it was, the name of the previous hop, if it was not the
identifier for the connection which received the AUTH request. This
string would be padded by appending a byte with the value 0x80 then
adding zero or more bytes with the value of 0x00 until the string
length is a multiple of 16 bytes long. A new random IV vector would
be selected (it needs to change because it forms the salt) and the
padded string would be encrypted using AES-CBC with a key of K. The
IV and encrypted data and an SPI (security parameter index) that
changed each time K changed would be base 64 encoded and form the
user portion of the request URI. The SPI allows the key to be changed
and for the system to know which K should be used. Later when the
relay received this URI, it could decrypt it and check the current
time was before the expiry time and check that the messages was
coming from or going to the connection or location specified in the
URI. Integrity protection is not required because it is extremely
unlikely that random data that was decrypted would result in a valid
location that was the same as the messages was routing to or from.
When implementing something like this, implementers should be careful
not to use a scheme like EBE that would allows portion of encrypted
tokens to be cut and paste into others.
Note: A successful AUTH response returns a Route header which
contains a base MSRP URI that the client can use to create a number
of different URIs which are all associated with the current
connection.
5.2.4 Forwarding SEND requests
A MSRP relay that receives a SEND request MUST respond with a final
response immediately. A 200-class response indicates the successful
receipt of a message fragment, but does not mean that the message has
been forwarded on to its next hop.
The final response to the SEND MUST be sent to the previous hop,
which could be a MSRP relay or the original sender of the SEND
request.
The 2xx response to the SEND MUST NOT contain a body. A 4xx or 5xx
response indicates that the message was not delivered successfully.
A 6xx response means it was delivered successfully, but refused.
The MSRP relay MAY further break up the message fragment received in
the SEND request into smaller fragments and forward them to the next
hop in separate SEND requests. It MAY also combine message fragments
received before or after this SEND request, and forward them out in a
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single SEND request to the next hop identified in the Hops header.
The MSRP relay MUST NOT combine message fragments from SEND requests
with different values in the Message-ID header.
The MSRP relay MAY choose whether to further fragment the message, or
combine message fragments, or send the message as is, based on some
policy which is administered, or based on the network speed to the
next hop, or any other mechanism.
If the MSRP relay has knowledge of the byte range that it will
transmit to the next hop, it SHOULD update the message/byteranges
parameter in the SEND request appropriately.
Before forwarding the SEND request to the next hop, the MSRP relay
MUST inspect the first URI in the To-Path header. If it indicates
this relay, the relay removes this URI from the To-Path header and
inserts this URI in the From-Path header before any other URIs.
If the MSRP relay fails to forward the SEND on to the next hop, it
SHOULD return a REPORT back to the sender of the SEND indicating the
reason for failure using the list of URIs in the From-Path header.
[how? example. see section]
5.2.5 Forwarding non-SEND requests
An MSRP relay that receives any request other than a SEND request
(including new methods unknown to the relay), first follows the
validation and authorization rules for all requests in Section x.y.
Then the relay moves its URI from the beginning of the To-Path
header, to the beginning of the From-Path header and forwards the
request on to the next hop. It MUST use the most suitable conection,
etc, etc.. If no suitable connection exists, the relay opens a new
connection.
5.2.6 Forwarding Responses
Relays receiving a response, first check the Tr-ID of the response.
If the relay is unaware of this transaction, the response MUST be
dropped. Likewise if the message is unparsable, the relay MUST drop
the response. If the response matches an existing transaction, the
transaction state MUST be deleted. The relay MUST verify that the
first URI in the To-Path corresponds to it. If not, the response
SHOULD be dropped. If there are additional URIs in the To-Path
header, the relay can then move its URI from the list To-Path header,
insert its URI in front of any other URIs in the From-Path header,
and forward the response to the next URI in the To-Path header. The
relay sends the request over the best connection which corresponds to
the next URI in the To-Path header. If this connection has closed,
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then the response is silently discarded.
5.2.7 Managing Connections
Relays should keep connection open as long as possible. If a
connection has not been used in a significant time (many minutes) it
could be closed. If the relay runs out of resource and must close
connections, it should first stop accepting new connections from
clients then start closing connections on a least recently used
basis.
5.2.8 Forwarding unknown requests
Requests with an unknown method are forwarded as if they were REPORT
requests.
5.3 Acting as a Message Taker
A Message Taker merely acts like a Client which returns different
REPORT responses.
TODO - how do I let the message taker know to send all the requests
it saved for me to me. I assume I still send REPORTs to the original
sender as well as the message take to let them know I got the
message.
6. Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in RFC-2234 [6].
AUTHm =3D %x41.55.54.48 ; AUTH in caps
Method =3D SENDm / VISITm / REPORTm / AUTHm
/ extension-method
/ "401" ; Authentication Required
/ "423" ; Interval Out-of-Bounds
Authentication-Info =3D "Authentication-Info" HCOLON ainfo
*(COMMA ainfo)
ainfo =3D nextnonce / message-qop
/ response-auth / cnonce
/ nonce-count
nextnonce =3D "nextnonce" EQUAL nonce-value
response-auth =3D "rspauth" EQUAL response-digest
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response-digest =3D LDQUOT *LHEX RDQUOT
Authorization =3D "Authorization" HCOLON credentials
credentials =3D ("Digest" LWS digest-response)
/ other-response
digest-response =3D dig-resp *(COMMA dig-resp)
dig-resp =3D username / realm / nonce / digest-uri
/ dresponse / algorithm / cnonce
/ opaque / message-qop
/ nonce-count / auth-param
username =3D "username" EQUAL username-value
username-value =3D quoted-string
digest-uri =3D "uri" EQUAL LDQUOT digest-uri-value RDQUOT
digest-uri-value =3D rquest-uri ; Equal to request-uri as specified
by HTTP/1.1
message-qop =3D "qop" EQUAL qop-value
cnonce =3D "cnonce" EQUAL cnonce-value
cnonce-value =3D nonce-value
nonce-count =3D "nc" EQUAL nc-value
nc-value =3D 8LHEX
dresponse =3D "response" EQUAL request-digest
request-digest =3D LDQUOT 32LHEX RDQUOT
auth-param =3D auth-param-name EQUAL
( token / quoted-string )
auth-param-name =3D token
other-response =3D auth-scheme LWS auth-param
*(COMMA auth-param)
auth-scheme =3D token
LHEX =3D DIGIT / %x61-66 ;lowercase a-f
; Some elements (authentication) force hex alphas to be lower case.
WWW-Authenticate =3D "WWW-Authenticate" HCOLON challenge
challenge =3D ("Digest" LWS digest-cln *(COMMA =
digest-cln))
/ other-challenge
other-challenge =3D auth-scheme LWS auth-param
*(COMMA auth-param)
digest-cln =3D realm / domain / nonce
/ opaque / stale / algorithm
/ qop-options / auth-param
realm =3D "realm" EQUAL realm-value
realm-value =3D quoted-string
domain =3D "domain" EQUAL LDQUOT URI
*( 1*SP URI ) RDQUOT
URI =3D MSRP-URI / anyURI
nonce =3D "nonce" EQUAL nonce-value
nonce-value =3D quoted-string
opaque =3D "opaque" EQUAL quoted-string
stale =3D "stale" EQUAL ( "true" / "false" )
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algorithm =3D "algorithm" EQUAL ( "MD5" / "MD5-sess"
/ token )
qop-options =3D "qop" EQUAL LDQUOT qop-value
*("," qop-value) RDQUOT
qop-value =3D "auth" / token
7. Finding MSRP Servers
### FIX ENTIRE SECTION ###
When sending a response, the response is always forwarded over an
existing connection using the connection handle set in the receiver
parameter in the topmost Via header field value and the sent-by
transport in that Via header field value to determine the correct
connection.
When resolving a URI (for example from a Route header field, or from
the Request-URI), examine the hostport portion of the URI and the
transport URI parameter to decide how to proceed.
If the hostport is an IPv4 address or an IPv6 reference, send the
request to that address using the port and transport specified in the
URI. If no transport is provided, use the default (tls+tcp). If no
port number is provided, use the default for the selected protocol
(port 8999 for tcp, and port 9000 for tls over tcp).
If the hostport is a domain name and an explicit port number is
provided, attempt to lookup a valid address record (A, AAAA, or A6)
for the domain name. Connect using the specified protocol (or the
default of tls+tcp if none is specified) and port number.
If a domain name is provided, but no port number, perform a DNS SRV
[7] lookup for all transports supported by the client and select the
entry with the highest weight. If no SRV records are found, try an
address lookup using the default port number procedures described in
the previous paragraph. Note that AUTH requests MUST only be sent
over a TLS-protected channel. An SRV lookup in the example.com
domain might return:
;; in example.com. Pri Wght Port Target
_sims+tls._tcp IN SRV 0 1 9000 server1.example.com.
_sims+tls._tcp IN SRV 0 2 9000 server2.example.com.
_sims._tcp IN SRV 1 1 8999 server1.example.com.
_sims._tcp IN SRV 1 2 8999 server2.example.com.
If implementing a relay farm, it is RECOMMENDED that each member of
the relay farm have an SRV entry. If any members of the farm have
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multiple IP addresses (for example an IPv4 and an IPv6 address), each
of these addresses SHOULD be registered in DNS as separate A, AAAA,
or A6 records corresponding to a single target.
8. Security Considerations
This section first describes the security mechanisms available for
use in MSRP. Then the threat model is presented. Finally we list
implementation requirements related to security.
8.1 Using HTTP Authentication
AUTH requests SHOULD be authenticated using HTTP authentication.
HTTP authentication is done as described in [RFC 2617], with the
following exceptions. Basic authentication MUST NOT be used. A qop
value of auth-int MUST NOT be used as the AUTH requests are integrity
protected by TLS and there is no body to protect. Note that unlike in
some usages of HTTP Authentication (for example, SIP), the uri
parameter in the Authorize header is the same as the Request-URI in
the request line of the MSRP parcel of the AUTH request. Note the
BNF in RFC-2617 has an error--the value of the uri parameter MUST be
in quotes. The BNF in this document is correct, as are the examples
in RFC 2617.
8.2 Using TLS
TLS is used to authenticate relays to senders and to provide
integrity and confidentiality for the headers being transported. MSRP
client and relays MUST support TLS. Clients and relays MUST support
the TLS ClientExtendedHello extended hello information for server
name indication as described in RFC 3546 [8]. A TLS cipher-suite of
TLS_RSA_WITH_AES_128_CBC_SHA [9] MUST be supported (other
cipher-suites MAY also be suported). Relays must act as TLS servers
and present a certificate with their identity in the SubjectAltName
using the choice type of dnsName. Relay to relay connections MUST use
TLS and client to relay communications MUST use TLS for AUTH requests
and responses.
8.3 Threat Model
This section discuses the threat model and the broad mechanism that
must come into place to secure the protocol. The next section
describes the details of how the protocol mechanism meet the broad
requirements.
MSRP allows two peer to peer clients to exchange messages. Each peer
can select a set of relays to perform certain policy operation for
them. This combined set of relays is referred to as the route set.
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There often exists a channel outside of MSRP, such as out-of-band
provisioning or an explicit rendezvous protocol such as SIP, that can
securely negotiate setting up the MSRP session and communicate the
route set to both clients. A client may trust a relay with certain
types of routing and policy decisions but it might or might not trust
the relay with all the contents of the session. For example, a relay
being trusted to look for viruses would probably need to be allowed
to see all the contents of the session. A relay that helped deal with
firewall traversal of the ISPs firewall would likely not be trusted
with the contents of the session but would be trusted to correctly
forward information.
Clients need to be able to authenticate that the relay they are
communicating with is the one they trust. Likewise, relays need to be
able to authenticate the client is the authorized client for them to
forward information to. Clients need the option of ensuring
information between the relay and the client is integrity protected
and confidential to elements other than the relays and clients. To
simplify the number of options, traffic between relays must always be
integrity protected and encrypted regardless of if the client request
it or not. There is no way for the clients to tell the relays what
strength of crypto to use between relays other than the clients to
choose to use relays that are operated by people requiring an
adequate level of security.
The system also need to stop the messages from being directed to
relays that are not supposed to see them. To keep the relays from
being used in DDoS attacks, the relays must not forward messages
unless they have a trust relationship with either the client sending
or receiving the message and that they only forward that message if
it is coming from or going to the client they have the trust
relationship with. If a relay has a trust relationship with the
client that is the destination of the message, it should not send the
message anywhere except the client that is the destination.
Some terminology used in this discussion is SClient is the client
sending a message and RClient is the client receiving a message.
SRelay is a relay the sender trusts and RRelay is a relay the
receiver trusts. The message will go from SClient to SRelay1 to
SRelay2 to RRelay2 to RRelay1 to RClient.
8.4 Security Mechanism
Confidentiality and Privacy from elements not in the route set is
provided by using TLS on all the transports. If a client decided to
not use TLS that is it's choice but relays must use TLS. Clients must
implement TLS.
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The relays authenticate to the clients using TLS (but don't have to
do mutual TLS). The clients authenticate to the relays using HTTP
Digest inside of TLS. Relays authenticate to each other using mutual
TLS.
The clients can protect the contents so that the relays can not see
them by using S/MIME encryption. End to end signing is also possible
with S/MIME.
The complex part is making sure that relays do not send messages
place where they should not. This is done by having the client
authenticate to the relay and having the relay return a token.
Messages that contain this token can be relayed if they come from the
client that got the token or if they are being forwarded towards the
client that got the token. The tokens must only ever be seen by
things in the route set or other elements that at least one of the
parties trusts. If some 3rd party discovers the token that RRelay2
uses to forward messages to RClient, then that 3rd party can send as
many messages as they want to RRelay2 and it will forward them to
RClient. The 3rd party can not cause them to be forwarded anywhere
except to RClient eliminating the open relay problems. SRelay1 will
not forward the message unless it contains a valid token.
When SClient goes to get a token from SRelay2, this request is
relayed through SRelay1. SRelay authenticates that it really is
SClient requesting the token but it generates a token that is only
valid for forwarding messages to or from SRelay1. SRelay two knows it
is connected to SRelay1 because of the mutual TLS.
The tokens are carried in the user portion of the MSRP URLs.
Issues: How to tokens expire - rekeying. Will probably use Expire
header on AUTH response. Token MAY be valid for between 10 minutes
and 24 hours with 1 hour recommended. Both sides need to do a SIP
re-invite to set up new tokens before the old one expires.
Issues: Token good for single session or for all session
Note: tokens are only required for relays, not clients or note
takers.
TODO talk about example from client to client and from Client A, then
to a relay that A uses, RA, then on to client B.
8.5 Preventing Spam and Denial of Service Attacks
While this specification already implements a number of significant
improvements to prevent unsolicited messaging and Denial of Service,
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additional mechanisms are envisioned being useful in the future. The
402 Payment Required and 409 Puzzle Required response codes are
reserved for future use and may be useful to further discourage
unsolicited messages.
9. IANA Considerations
This document introduces no requirements for IANA.
10. Example SDP with multiple hops
A sample SDP offer for a MSRP session could look like:
c=3DIN IP4 invalid.none
m=3Dmessage 1234 msrp/tcp alice@alice.example.com
a=3Daccept: message/cpim text/plain text/html
a=3Dhop:msrp:magic456@a.example.com:1234;transport=3Dtcp
In this offer Alice wishes to receive MSRP messages at
alice@alice.example.com. She wants to use TCP as the transport for
the MSRP session. She can accept message/cpim, text/plain and text/
html message boldies in SEND requests. She wishes to use the relay
msrp:magic456@a.example.com for the MSRP session.
To this offer, Bob's answer could look like:
c=3DIN IP4 invalid.none
m=3Dmessage 1234 msrp/tcp bob@bob.example.com
a=3Daccept: message/cpim text/plain
a=3Dhop:msrp:magic789@b1.example.com:1234;transport=3Dtcp
a=3Dhop:msrp:magic012@b2.example.com:1234;transport=3Dtcp
Here Bob has agreed to use tcp as the transport, and wishes to
receive the MSRP messages at bob@bob.example.com. He can accept only
message/cpim and text/plain message bodies in SEND requests and has
rejected text/html offer made by Alice. He wishes to use two relays
for the MSRP session - msrp:magic789@b1.example.com and
msrp:magic012@b2.example.com.
11. Acknowledgments
Many thanks to the following members of the SIMPLE WG for spirited
discussions on session mode: Ben Campbell, Jonathan Rosenberg,
Robert Sparks, Paul Kyzivat, Allison Mankin, Jon Peterson, Brian
Rosen, Dean Willis, Adam Roach, Aki Niemi, Hisham Khartabil, Juhee
Garg, Pekka Pessi, and Chris Boulton
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Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[3] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
1999.
[4] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
Basic and Digest Access Authentication", RFC 2617, June 1999.
[5] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[6] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[7] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[8] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and
T. Wright, "Transport Layer Security (TLS) Extensions", RFC
3546, June 2003.
[9] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
Transport Layer Security (TLS)", RFC 3268, June 2002.
[10] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[11] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
[12] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
2633, June 1999.
[13] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
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1998.
[14] Braden, R., "Requirements for Internet Hosts - Application and
Support", STD 3, RFC 1123, October 1989.
[15] Troost, R., Dorner, S. and K. Moore, "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August 1997.
[16] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[17] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[18] Burger, E., Candell, E., Eliot, C. and G. Klyne, "Message
Context for Internet Mail", RFC 3458, January 2003.
Informative References
[19] Mahy, R., "Benefits of Session-Mode Instant Messaging",
draft-mahy-simple-why-session-mode-00.txt (work in progress),
February 2004.
[20] Campbell, B., "Instant Message Sessions in SIMPLE",
draft-ietf-simple-message-sessions-02 (work in progress), Oct
2003.
[21] Atkins, D. and G. Klyne, "Common Presence and Instant
Messaging: Message Format", draft-ietf-impp-cpim-msgfmt-08
(work in progress), January 2003.
[22] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[23] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[24] Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging /
Presence Protocol Requirements", RFC 2779, February 2000.
[25] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[26] Mahy, R., "Relay Requirements for Session-Mode Instant
Messaging", draft-mahy-simple-session-relay-reqs-00.txt (work
in progress), February 2004.
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Authors' Addresses
Cullen Jennings
Cisco Systems, Inc.
170 West Tasman Dr.
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 527-9132
EMail: fluffy@cisco.com
Rohan Mahy
Cisco Systems, Inc.
5617 Scotts Valley Drive, Suite 200
Scotts Valley, CA 95066
USA
EMail: rohan@cisco.com
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Internet-Draft MSRP Relays April 2004
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