One document matched: draft-garcia-mmusic-file-transfer-mech-00.txt
MMUSIC Working Group M. Garcia-Martin
Internet-Draft M. Isomaki
Intended status: Standards Track Nokia
Expires: December 15, 2006 G. Camarillo
S. Loreto
Ericsson
June 13, 2006
Session Description Protocol (SDP) Offer/Answer Mechanism to Enable File
Transfer
draft-garcia-mmusic-file-transfer-mech-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document provides a mechanism to negotiate the transfer of one
or more files between two endpoints by using the Session Description
Protocol (SDP) offer/answer model specified in RFC 3264. SDP is
extended to describe the attributes of the files. The offerer can
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either describe the files it wants to send, or the files it would
like to receive. The answerer can either accept or reject the offer.
The transfer of files is initiated after a successful negotiation.
The Message Session Relay Protocol (MSRP) is defined to as the
default mechanism to actually carry the files between the endpoints.
The conventions on how to use MSRP are provided in the document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5
5. Extensions to SDP . . . . . . . . . . . . . . . . . . . . . . 6
6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 10
6.1. File selector . . . . . . . . . . . . . . . . . . . . . . 10
6.2. Offerer's Behavior . . . . . . . . . . . . . . . . . . . . 11
6.2.1. The Offerer is a File Sender . . . . . . . . . . . . . 11
6.2.2. The Offerer is a File Receiver . . . . . . . . . . . . 11
6.2.3. SDP Offer for Several Files . . . . . . . . . . . . . 12
6.3. Answerer's Behavior . . . . . . . . . . . . . . . . . . . 12
6.3.1. The Answerer is a File Receiver . . . . . . . . . . . 12
6.3.2. The Answerer is a File Sender . . . . . . . . . . . . 12
6.4. Re-usage of Existing m= Lines in SDP . . . . . . . . . . . 13
6.5. MSRP Usage . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Offerer sends a file to the Answerer . . . . . . . . . . . 14
7.2. Offerer requests a file from the Answerer and second
file transfer . . . . . . . . . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Normative References . . . . . . . . . . . . . . . . . . . 24
11.2. Informational References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . . . 27
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1. Introduction
The Session Description Protocol (SDP) Offer/Answer [7] provides a
mechanism for two endpoints to arrive at a common view of a
multimedia session between them, the session described with SDP [10].
These sessions often contain real-time media streams such as voice
and video, but are not limited to that. Basically, any media
component type can be supported, as long as there is a specification
how to negotiate it within the SDP offer/answer exchange.
The Message Session Relay Protocol (MSRP) [11] is a protocol for
transmitting instant messages (IM) in the context of a session. The
protocol specification includes a description how to use it with SDP.
In addition to plain text messages, MSRP is able to carry arbitrary
(binary) Multipurpose Internet Mail Extensions (MIME) [2] compliant
content, such as images or video clips.
There are many cases where the endpoints involved in a multimedia
session would like to exchange files within the context of that
session. With MSRP it is possible to embed files as MIME objects
inside the stream of instant messages. MSRP also has other features
that are useful for file transfer. Message chunking enables the
sharing of the same transport connection between the transfer of a
large file and interactive IM exchange without blocking the IM. MSRP
relays [15] provide a mechanism for Network Address Translator (NAT)
traversal. Finally, Secure MIME (S/MIME) [8] can be used for
ensuring the integrity and confidentiality of the transfered content.
However, the baseline MSRP does not readily meet all the requirements
expressed in [14] for file transfer services within multimedia
sessions. There are four main missing features:
o The recipient MUST be able to distinguish "file transfer" from
"file attached to IM", allowing the recipient to treat the cases
differently.
o It MUST be possible for the sender to send the request for a file
transfer. It MUST be possible for the recipient to accept or
decline it, using the meta information in the request. The actual
transfer MUST take place only after acceptance by the recipient.
o It MUST be possible for the sender to pass some meta information
on the file before the actual transfer. This MUST be able to
include at least content type, size, hash and name of the file, as
well as a short (human readable) description.
o It MUST be possible for the recipient to request a file from the
sender, providing meta information about the file. The sender
MUST be able to decide whether to send a file matching the
request.
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All these requirements are related to the description and negotiation
of the session, not to the actual file transfer mechanism. Thus, it
is natural that in order to meet them it is enough to define
attribute extensions and usage conventions to SDP, while MSRP itself
needs no extensions and can be used as it is. This is effectively
the approach taken in this specification. Another goal has been to
specify the SDP extensions in such a way that a regular MSRP endpoint
which does not support them could still in some cases act as an
endpoint in a file transfer session, albeit with a somewhat reduced
functionality.
In some ways the aim of this specification is similar to the aim of
content indirection mechanism in the Session Initiation Protocol
(SIP) [13]. Both mechanisms allow a user agent to decide whether or
not to download a file based on information about the file. However,
there are some differences. With content indirection, it is not
possible for the other endpoint to explicitly accpet or reject the
file transfer. Also, it is not possible for an endpoint to request a
file from another endpoint. Furthermore, content indirection is not
tied to the context of a media session, which is sometimes a
desirable property. Finally, content indirection typically requires
some server infrastructure, which may not always be available. (It
is possible to use content indirection directly between the endpoints
too, but in that case there is no definition for how it works for
endpoints behind NATs.)
Based on the argumentation above, this document defines the SDP
attribute extensions and usage conventions needed for meeting the
requirements on file transfer services with the SDP offer/answer
model, using MSRP as the transfer protocol within the session.
In principle it is possible to use the SDP extensions defined here
and replace MSRP with any other similar protocol that can carry
MIME objects. This kind of specification can be written as a
separate document if the need arises.
The rest of this document is organized as follows. Section 3 defines
a few terms used in this document. Section 4 provides the overview
of operation. The detailed syntax and semantics of the new SDP
attributes and conventions on how the existing ones are used is
defined in Section 5. Section 6 describes the protocol operation
involving SDP and MSRP. Finally, some examples are given in
Section 7.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
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"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations.
3. Definitions
For the purpose of this document, the following definitions specified
in RFC 3264 [7] apply:
o Answerer
o Offerer
Additionally, we define the following terms:
File sender: The endpoint that is willing to transmit a file to the
file receiver.
File receiver: The endpoint that is willing to receive a file from
the file sender.
File selector: The tuple composed of file attributes that are used
in SDP offer in order to delimit the set of files that matches the
offer. This is described in more detail in Section 6.1.
4. Overview of Operation
An SDP offerer creates an SDP body that contains the description of
one or more files that the offerer wants to send or receive. The
offerer sends the SDP offer to the remote endpoint. The SDP answerer
can accept or reject the transfer of each of those files.
File transfer is modelled on top of the Message Session Relay
Protocol (MSRP) [11]. Each SDP "m=" line describes an MSRP-based
media stream used to transfer a single file. That is, the transfer
of multiple files requires multiple "m=" lines. Each "m=" line
describing an MSRP media stream for file transfer is complemented
with a few attributes describing the file to be transferred. SDP
direction attibutes "a=sendonly" or "a=recvonly" are used to indicate
the direction of the transfer, i.e. whether the SDP offerer is
willing to send of receive the file. Assuming that the answerer
accepts the file transfer, the actual transfer of the files takes
place with ordinary MSRP.
The attributes describing each file are provided in SDP by a set of
new SDP attributes, most of which have been directly borrowed from
MIME. This way, user agents can decide whether or not to accept a
given file transfer based on the file's name, size, description,
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hash, icon (e.g., if the file is a picture), etc.
In principle the file transfer can work even with an endpoint
supporting only regular MSRP without understanding the extensions
defined herein, in a special case where that endpoint is the
recipient of the file. The regular MSRP endpoint answers the
offer as it would answer any ordinary MSRP offer without paying
attention to the extension attributes. In such a scenario the
user experience would however be reduced, as the recipient would
not know (by any protocol means) the reason for the session and
would not be able to accept/reject it based on the file
attributes.
5. Extensions to SDP
We define a number of attributes for SDP [10] that provide the
required information to describe the transfer of a file with MSRP.
The following is the formal ABNF syntax [9] of these new attributes.
It is built above the SDP [10] grammar, RFC 2045 [2], RFC 2183 [3],
and RFC 2392 [4].
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attribute = filename-attr / filetype-attr /
disposition-attr / filesize-attr /
creation-date-attr /
modification-date-attr / read-date-attr /
icon-attr / hash-attr /
;attribute is defined in sdp-new
filename-attr = "filename:" filename-string
filename-string = byte-string ;byte-string defined in sdp-new
filetype-attr = "filetype:" type "/" subtype *(";"parameter)
; parameter defined in RFC 2045
type = token
subtype = token
disposition-attr = "disposition:" disposition-value
disposition-value = token
filesize-attr = "filesize:" filesize-value
filesize-value = integer ;integer defined in sdp-new
creation-date-attr = "creation-date:" date-time
modification-date-attr = "modification-date:" date-time
read-date-attr = "read-date:" date-time
; date-time is defined in RFC 2822
; numeric timezones (+HHMM or -HHMM)
; must be used
icon-attr = "icon:" icon-value
icon-value = cid-url ;cid-url defined in RFC 2392
hash-attr = "hash:" hash-algorithm WSP hash-value
hash-algorithm = token ;see IANA Hash Algorithm
;section in the IPSEC
;registry
hash-value = hex-val ;hex-val defined in RFC 4234
Figure 1: Syntax of the SDP extension
The 'filename' attribute contains the filename of the content, and
its value is a byte string (specified in SDP [10]). The value of
this attribute SHOULD be the same of the 'filename' parameter of
Content-Disposition header field [3] that could be signalled by the
actual file transfer.
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The 'filetype' attribute contains the MIME media type of the content.
In general, anything that can be expressed in a Content-Type header
field (see RFC 2045 [2]) can also be expressed with the 'filetype'
attribute. Possible MIME Media Type values are the ones listed in
the IANA registry for MIME Media Types. Zero or more parameters can
follow. The syntax of 'parameter' is specified in RFC 2045 [2] .
The 'disposition' attribute provides a suggestion to the other
endpoint about the intended disposition of the file. Possible values
are the one listed in the IANA registry for Mail Content Disposition
Values, although most likely only the "inline" and "attachment"
values are significant for file transfer applications. The value of
this attribute SHOULD be the same of the disposition type parameter
of the Content-Disposition header field [3] that could be signalled
by the actual file transfer.
The 'filesize' attribute indicates the size of the file in octets.
The value of this attribute SHOULD be the same of the 'size'
parameter of Content-Disposition header field [3] that could be
signalled by the actual file transfer.
The 'creation-date' attribute indicates the date at which the file
was created. The value MUST be a string which contains a
representation of the creation date of the file in RFC 2822 [5]
'date-time' format. Numeric timezones (+HHMM or -HHMM) MUST be used.
The value of this attribute SHOULD be the same of the 'creation-date'
parameter of Content-Disposition header field [3] that could be
signalled by the actual file transfer.
The 'modification-date' attribute indicates the date at which the
file was last modified. The value MUST be a string which contains a
representation of the creation date of the file in RFC 2822 [5]
'date-time' format. Numeric timezones (+HHMM or -HHMM) MUST be used.
The value of this attribute SHOULD be the same of the 'modification-
date' parameter of Content-Disposition header field [3] that could be
signalled by the actual file transfer.
The 'read-date' attribute indicates the date at which the file was
last read. The value MUST be a string which contains a
representation of the creation date of the file in RFC 2822 [5]
'date-time' format. Numeric timezones (+HHMM or -HHMM) MUST be used.
The value of this attribute SHOULD be the same of the 'read-date'
parameter of Content-Disposition header field [3] that could be
signalled by the actual file transfer.
The 'icon' attribute can be useful with certain file types such as
images. It allows the sender to include a pointer to a body that
includes an icon representing the contents of the file to be
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transferred. This allows the sender to include the icon as another
body accompanying the SDP, and to the recipient to get the icon of
the file that can potentially be transferred. It is recommended to
keep icons restricted to the minimum number of bytes that provide
significance. The 'icon' attribute contains a Content-ID URL, which
is specified in RFC 2392 [4].
The 'hash' attribute provides a hash of the file to be transferred.
This is commonly used by file transfer protocols. For example, FLUTE
[16] uses hashes (called message digests) to verify the contents of
the transfer. The purpose of the 'hash' attribute is two-fold: On
one side, it allows the file receiver to identify a file by its hash
rather than by its file name, providing that the file receiver has
learned the hash of the file by some out-of-band mechanism. On the
other side, it allows the file sender to provide the hash of the file
to be transmitted, which can be used by the file receiver for
verification of its contents or to avoid the unnecessary transmission
of a file that already exists. The 'hash' attribute includes the
type of hash and its value. Possible types of hash are the ones
defined in the Hash Algorithm Section of the IANA registry of the
IPSec registry. Implementations according to this specification MUST
implement the US Secure Hash Algorithm 1 (SHA1) [6] and MAY implement
other hashing algorithms. The creator of the SDP MAY also add more
than one 'hash' attribute (presumably with different types of hash)
to the same file. The value is the byte string resulting of applying
the hash algorithm to the content of the file.
The following is an example of an SDP body that contains the
extensions defined in this memo:
v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
s=
c=IN IP4 host.atlanta.example.com
t=0 0
m=message 7654 TCP/MSRP *
i=This is my latest picture
a=sendonly
a=accept-types:*
a=path:msrp://atlanta.example.com:7654/jshA7we;tcp
a=filename:My cool picture.jpg
a=filetype:image/jpeg
a=disposition:inline
a=filesize:32349
a=creation-date:Mon, 15 May 2006 15:01:31 +03:00
a=icon:cid:id2@alicepc.example.com
a=hash:SHA 72245FE8653DDAF371362F86D471913EE4A2CE2E
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Figure 2: Example of SDP describing a file transfer
6. Protocol Operation
This Section discusses how to use the parameters defined in Section 5
in the context of an offer/answer [7] exchange. Additionally, this
section also discusses the behavior of the endpoints using MSRP.
Usually the file transfer session is initiated when the offerer sends
an SDP offer to the answerer. The answerer either accepts or rejects
the file transfer session and sends an SDP answer to the offerer.
We can differentiate two use cases, depending on whether the offerer
is the file sender or file receiver:
1. The offerer is the file sender, i.e., the offerer wants to
transmit a file to the answerer. Consequently the answerer is
the file receiver. In this case the SDP offer contains a
'sendonly' attribute, and accordingly the SDP answer contains a
'recvonly' attribute.
2. The offerer is the file receiver, i.e., the offerer wants to
fetch a file from the answerer. Consequently the answerer is the
file sender. In this case the SDP offer contains a 'recvonly'
attribute, and accordingly the SDP answer contains a 'sendonly'
attribute.
6.1. File selector
The protocol specified in this document requires a mechanism to
identify files in a remote entitly. We introduce the concept of a
file selector, which is defined as the tuple composed of the 'hash',
'filename', 'filesize', and 'filetype' attributes in SDP.
The file selector selects all the files where each of the attributes
matches with the attributes present in the selector, i.e. the set of
files which is the intersection of the files macthing each of the
attributes separately. Thus, a file selector can point to zero, one,
or more files, depending on the presence of the mentioned attributes
in the SDP and depending on the available files in a host. The file
transfer mechanism that we specify in this document requires that a
file selector eventually results at most in a single file to be
chosen. Typically, if the 'hash' attribute is known, the 'hash'
attribute is enough to produce a file selector that points to zero or
one file. However, a file selector selecting a unique file is not
always known by the offerer. Sometimes only the 'filename',
'filesize', or 'filetype' attributes are known, so the file selector
may result in more than one file, an undesired case. The opposite is
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also true, if the file selector contains a 'hash' and a 'filename'
attributes, but the user at the remote host has renamed the file,
although there is a file with the indicated hash, the file name does
not match, thus, the file selector will result in the selection of
zero files.
6.2. Offerer's Behavior
An offerer that wishes to send or receive one or more files to or
from an answerer MUST build an SDP [10] description of a session
containing one or more "m=" lines, each one describing an MSRP
session (and thus, one file transfer operation), according to the
MSRP [11] procedures. All the media line attributes specified and
required by MSRP [11] (e.g., "a=path", "a=accept-types", etc.) MUST
be included as well. For each file to be transferred there MUST be a
separate "m=" line.
6.2.1. The Offerer is a File Sender
If the offerer is a file sender, it MUST add a session or media
'sendonly' attribute to the SDP offer. Additionally, the offerer
SHOULD also add a 'filetype', 'filesize', and 'hash' attributes
indicating the type, size, and hash of the file, respectively.
These attributes might not be known when the offerer creates the
SDP offer, for example, because the host is still processing the
file.
The 'hash' attribute contains valuable information to the file
receiver to identify whether the file is already available and
need not be transmitted.
The offerer MAY also add a 'filename', 'icon', 'disposition',
'creation-date', 'modification-date', and 'read-date' attributes
further describing the file to be transferred. The 'disposition'
attribute provides a presentation suggestion, (for example: the file
sender would like the file receiver to render file "inline", or save
it as an "attachment"). The three date attributes provide the
answerer with an indication of the age of the file.
6.2.2. The Offerer is a File Receiver
If the offerer is a file receiver, it MUST create an SDP offer that
contains a session or media 'recvonly' attribute. Then the offerer
SHOULD add at least one of the attributes that constitute the file
selector ('hash', 'filename', 'filesize', or 'filetype'). In many
cases, if the hash of the file is known, that is enough to identify
the file, therefore, the offerer can include only a 'hash' attribute.
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However, specially in cases where the hash of the file is unknown,
the file name, size, and type can provide a description of the file
to be fetched. There is no need to for the file offerer to include
further file attributes in the SDP offer, thus it is RECOMMENDED that
SDP offerers do not include any other file attribute defined by this
specification (other than the mandatory ones).
6.2.3. SDP Offer for Several Files
An offerer that wishes to send or receive more than one file
generates an "m=" line per file. This way, the answerer can reject
individual files by setting the port number of their associated "m="
lines to zero, as per regular SDP [10] procedures.
Using an "m=" line per file implies that different files are
transferred using different MSRP sessions. However, all those MSRP
sessions can be set up to run over a single TCP connection, as
described in Section 8.1 of [11].
6.3. Answerer's Behavior
If the answerer wishes to reject a file offered by the offerer, it
sets the port number of the "m=" line associated with the file to
zero, as per regular SDP [10] procedures. If the answerer decides to
accept the file, it proceeds as per regular MSRP [11] and SDP [10]
procedures.
6.3.1. The Answerer is a File Receiver
If the answerer is a file receiver and decides to accept the file
transfer it MUST create an SDP answer (per RFC 3264 [7]) containing a
'recvonly' attribute. If the offer contains 'filetype', 'filesize',
'filename' or 'hash' attributes, the answerer MUST copy them into the
answer. This informs the offerer that the answerer supports this
specification. If the answerer is a file receiver, it MUST NOT
include 'icon', 'disposition', 'creation-date', 'modification-date',
or 'read-date' attributes in the SDP answer.
If the received offer contains a 'hash' attribute, the answerer can
use it to find out if a local file with the same hash is already
available, in which case, this could imply the reception of a
duplicated file. It is up to the answerer to determine whether the
file transfer is accepted or not in case of a duplicated file.
6.3.2. The Answerer is a File Sender
If the answerer is a file sender, it MUST first inspect the received
SDP offer and apply the file selector. The file selector is the set
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of files that results from the intersection of the files whose
attributes individually match with the 'filetype', 'filesize',
'filename', and 'hash' attributes (if they are present) that modify
the same "m=" line in the SDP offer (i.e., the four mentioned
attributes are located under the same "m=" line in SDP). The file
selector identifies zero or more candidate files to be sent. If the
file selector is unable to identify any file, then the answerer MUST
reject the MSRP stream for file transfer by setting the port number
to zero (and if it is the only stream in the SDP offer, then if
SHOULD reject the SDP as per procedures in RFC 3264 [7]).
If the file selector points to a single file and the answerer decides
to accept the file transfer, the answerer MUST create an SDP answer
(per RFC 3264 [7]) that contains a 'sendonly' attribute. The
answerer SHOULD add a 'hash' attribute containing the hash of the
file to be sent and MAY include, 'filetype', and 'icon' 'disposition'
attributes to further describe the file. Although the answerer MAY
also include 'filename' 'disposition', 'creation-date',
'modification-date', 'read-date', and 'filesize' attributes, it is
RECOMMENDED not to include them if the actual file transfer protocol
(e.g., MSRP [11]) can accommodate a Content-Disposition header field
[3] with the equivalent parameters.
The whole idea of adding file descriptors to SDP is to provide a
mechanism where a file transfer can be accepted prior to its
start. Adding any SDP attributes that are otherwise signalled
later in the file transfer protocol would just duplicate the
information, but will not provide any information to the offerer
to accept or reject the file transfer (note that the offerer is
requesting a file).
Last, if the file selector points to multiple candidate files, the
answerer MAY use some local policy, e.g. consulting the user, to
choose one of them to be defined in the SDP answer. If that choise
cannot be done, the answere SHOULD reject the MSRP media stream for
file transfer (by setting the port number to zero).
If the need arises, future specifications can provide a suitable
mechanism that allows to either select multiple files or, e.g.,
resolve ambiguities by returning a list of files that match the
file selector.
6.4. Re-usage of Existing m= Lines in SDP
The SDP Offer/Answer Model [7] provides rules that allow SDP offerers
and answerers to modify an existing media line, i.e., re-use an
existing media line with different attributes. The same is also
possible when SDP signals a file transfer operation according to the
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rules of this memo. Therefore, the procedures defined in RFC 3264
[7], in particular those defined in Section 8.3, MUST apply for file
transfer operations.
6.5. MSRP Usage
The file transfer service specified in this document uses "m=" lines
to describe the unidirectional transfer of a file. Consequently,
each MSRP session established following the procedures in Section 6.2
and Section 6.3 is only used to transfer a single file. So, senders
MUST only use a given MSRP session to send the file described in the
SDP offer or answer. That is, senders MUST NOT send additional files
over the same MSRP session.
Once the file transfer is completed, the file sender SHOULD close the
MSRP session, and MUST behave according to the MSRP [11] procedures
with respect closing MSRP sessions.
7. Examples
7.1. Offerer sends a file to the Answerer
This section shows an example flow for a file transfer scenario. The
example assumes that SIP [12] is used to transport the SDP offer/
answer exchange, although the SIP details are briefly shown in the
sake of brevity.
Alice, the SDP offerer, wishes to send an image file to Bob (the
answerer). Alice's User Agent Client (UAC) creates a unidirectional
SDP offer that contains the description of the file that she wants to
send to Bob's User Agent Server (UAS). The description also includes
an icon representing the contents of the file to be transferred. The
sequence flow is shown in Figure 3.
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Alice's UAC Bob's UAS
| |
|(1) (SIP) INVITE |
|----------------------->|
|(2) (SIP) 200 OK |
|<-----------------------|
|(3) (SIP) ACK |
|----------------------->|
| |
|(4) (MSRP) SEND (chunk) |
|----------------------->|
|(5) (MSRP) 200 OK |
|<-----------------------|
|(6) (MSRP) SEND (chunk) |
|----------------------->|
|(7) (MSRP) 200 OK |
|<-----------------------|
| |
|(8) (SIP) BYE |
|----------------------->|
|(9) (SIP) 200 OK |
|<-----------------------|
| |
| |
Figure 3: Flow diagram of an offerer sending a file to an answerer
F1: Alice constructs an SDP description of the file to be sent and
attaches it to a SIP INVITE request addressed to Bob.
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INVITE sip:bob@example.com SIP/2.0
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 1 INVITE
Max-Forwards: 70
Date: Sun, 21 May 2006 13:02:03 GMT
Contact: <sip:alice@alicepc.example.com>
Content-Type: multipart/mixed; boundary="boundary71"
Content-Length: [length]
--boundary71
Content-Type: application/sdp
Content-Length: [length of SDP]
v=0
o=alice 2890844526 2890844526 IN IP4 alicepc.example.com
s=
c=IN IP4 alicepc.example.com
t=0 0
m=message 7654 TCP/MSRP *
i=This is my latest picture
a=sendonly
a=accept-types:*
a=path:msrp://alicepc.example.com:7654/jshA7we;tcp
a=filename:My cool picture.jpg
a=filetype:image/jpeg
a=disposition:inline
a=filesize:4096
a=creation-date:Mon, 15 May 2006 15:01:31 +03:00
a=icon:cid:id2@alicepc.example.com
a=hash:SHA 72245FE8653DDAF371362F86D471913EE4A2CE2E
--boundary71
Content-Type: image/jpeg
Content-Transfer-Encoding: binary
Content-ID: <id2@alicepc.example.com>
Content-Length: [length of image]
Content-Disposition: icon
...binary JPEG image...
--boundary71--
From now on we omit the SIP details for the sake of brevity.
F2: Bob receives the INVITE request, inspects the SDP offer and
extracts the icon body, checks the creation date and file size, and
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decides to accept the file transfer. So Bob creates the following
SDP answer:
v=0
o=bob 2890844656 2890844656 IN IP4 bobpc.example.com
s=
c=IN IP4 bobpc.example.com
t=0 0
m=message 8888 TCP/MSRP *
a=recvonly
a=accept-types:*
a=path:msrp://bobpc.example.com:8888/9di4ea;tcp
a=filename:My cool picture.jpg
a=filetype:image/jpeg
a=filesize:4096
a=hash:SHA 72245FE8653DDAF371362F86D471913EE4A2CE2E
F4: Alice opens a TCP connection to Bob and creates an MSRP SEND
request. This SEND request contains the first chunk of the file.
MSRP d93kswow SEND
To-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
From-Path: msrp://alicepc.example.com:7654/iau39;tcp
Message-ID: 12339sdqwer
Byte-Range: 1-2048/4096
Content-Type: image/jpeg
Content-Disposition: inline; filename="My cool picture.jpg";
creation-date="Mon, 15 May 2006 15:01:31 +03:00";
size=4096
... first chunk of the JPEG image ...
-------d93kswow+
F5: Bob acknowledges the reception of the first chunk.
MSRP d93kswow 200 OK
To-Path: msrp://alicepc.example.com:7654/iau39;tcp
From-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
Byte-Range: 1-2048/4096
-------d93kswow$
F6: Alice sends the second and last chunk.
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MSRP op2nc9a SEND
To-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
From-Path: msrp://alicepc.example.com:7654/iau39;tcp
Message-ID: 12339sdqwer
Byte-Range: 2049-4096/4096
Content-Type: image/jpeg
Content-Disposition: inline; filename="My cool picture.jpg";
creation-date="Mon, 15 May 2006 15:01:31 +03:00";
size=4096
... second (and last) chunk of the JPEG image ...
-------op2nc9a$
F7: Bob acknowledges the reception of the second chunk.
MSRP op2nc9a 200 OK
To-Path: msrp://alicepc.example.com:7654/iau39;tcp
From-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
Byte-Range: 2049-4096/4096
-------op2nc9a$
F8: Alice terminates the SIP session by sending a SIP BYE request.
F9: Bob acknowledges the reception of the BYE request and sends a 200
(OK) response.
7.2. Offerer requests a file from the Answerer and second file transfer
In this example Alice, the SDP offerer, first wishes to fetch a file
from Bob, the SDP answerer. Alice knows that Bob has a specific file
she wants to download. She has learned the hash of the file by some
out-of-band mechanism. The hash attribute is enough to produce a
file selector that points to the specific file. So, Alice creates an
SDP offer that contains the file descriptor. Bob accepts the
transmission and sends the file to Alice. When Alice has completely
received Bob's file, she intends to send a new image file to Bob.
Therefore Alice re-uses the existing SDP media line with different
attributes and updates the description of the new file she wants to
send to Bob's User Agent Server (UAS). Figure 10 shows the sequence
flow.
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Alice's UAC Bob's UAS
| |
|(1) (SIP) INVITE |
|----------------------->|
|(2) (SIP) 200 OK |
|<-----------------------|
|(3) (SIP) ACK |
|----------------------->|
| |
|(4) (MSRP) SEND (file) |
|<-----------------------|
|(5) (MSRP) 200 OK |
|----------------------->|
| |
|(6) (SIP) INVITE |
|----------------------->|
|(7) (SIP) 200 OK |
|<-----------------------|
|(8) (SIP) ACK |
|----------------------->|
| |
|(9) (MSRP) SEND (file) |
|----------------------->|
|(10) (MSRP) 200 OK |
|<-----------------------|
| |
|(11) (SIP) BYE |
|<-----------------------|
|(12) (SIP) 200 OK |
|----------------------->|
| |
| |
Figure 10: Flow diagram of an offerer requesting a file from the
answerer and then sending a file to the answer
F1: Alice constructs an SDP description of the file she wants to
receive and attaches the SDP offer to a SIP INVITE request addressed
to Bob.
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INVITE sip:bob@example.com SIP/2.0
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 1 INVITE
Max-Forwards: 70
Date: Sun, 21 May 2006 13:02:03 GMT
Contact: <sip:alice@alicepc.example.com>
Content-Type: application/sdp
Content-Length: [length of SDP]
v=0
o=alice 2890844526 2890844526 IN IP4 alicepc.example.com
s=
c=IN IP4 alicepc.example.com
t=0 0
m=message 7654 TCP/MSRP *
a=recvonly
a=accept-types:image/jpeg
a=path:msrp://alicepc.example.com:7654/jshA7we;tcp
a=hash:SHA 72245FE8653DDAF371362F86D471913EE4A2CE2E
>From now on we omit the SIP details for the sake of brevity.
F2: Bob receives the INVITE request, inspects the SDP offer, computes
the file descriptor and finds a local file whose hash equals the one
indicated in the SDP. Bob accepts the file transmission and creates
an SDP answer as follows:
v=0
o=bob 2890844656 2890855439 IN IP4 bobpc.example.com
s=
c=IN IP4 bobpc.example.com
t=0 0
m=message 8888 TCP/MSRP *
a=sendonly
a=accept-types:*
a=path:msrp://bobpc.example.com:8888/9di4ea;tcp
a=filetype:image/jpeg
a=hash:SHA 72245FE8653DDAF371362F86D471913EE4A2CE2E
F4: Alice opens a TCP connection to Bob. Bob then creates an MSRP
SEND request that contains the file.
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MSRP d93kswow SEND
To-Path: msrp://alicepc.example.com:7654/iau39;tcp
From-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
Message-ID: 12339sdqwer
Byte-Range: 1-2027/2027
Content-Type: image/jpeg
Content-Disposition: inline; filename="My cool photo.jpg";
creation-date="Mon, 15 May 2006 15:01:31 +03:00";
modification-date="Mon, 15 May 2006 16:04:53 +03:00";
read-date="Mon, 16 May 2006 09:12:27 +03:00";
size=2027
...binary JPEG image...
-------d93kswow$
F5: Alice acknowledges the reception of the SEND request.
MSRP d93kswow 200 OK
To-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
From-Path: msrp://alicepc.example.com:7654/iau39;tcp
Byte-Range: 1-2027/2027
-------d93kswow$
F6: Alice re-uses the existing SDP media line inserting the
description of the file to be sent and attaches it to a SIP re-INVITE
request addressed to Bob.
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INVITE sip:bob@example.com SIP/2.0
To: Bob <sip:bob@example.com>;tag=1928323431
From: Alice <sip:alice@example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 2 INVITE
Max-Forwards: 70
Date: Sun, 21 May 2006 13:02:33 GMT
Contact: <sip:alice@alicepc.example.com>
Content-Type: application/sdp; boundary="boundary71"
Content-Length: [length of SDP]
--boundary71
Content-Type: application/sdp
Content-Length: [length of SDP]
v=0
o=alice 2890844526 2890844527 IN IP4 alicepc.example.com
s=
c=IN IP4 alicepc.example.com
t=0 0
m=message 5670 TCP/MSRP *
i=This is my latest picture
a=sendonly
a=accept-types:*
a=path:msrp://alicepc.example.com:5670/iau39;tcp
a=filename:Sunset.jpg
a=filetype:image/jpeg
a=disposition:inline
a=filesize:4096
a=creation-date:Sun, 21 May 2006 13:02:15
a=icon:cid:id3@alicepc.example.com
a=hash:SHA 58231FE8653BBCF371362F86D471913EE4B1DF2F
--boundary71
Content-Type: image/jpeg
Content-Transfer-Encoding: binary
Content-ID: <id3@alicepc.example.com>
Content-Length: [length of image]
Content-Disposition: icon
...binary JPEG image...
--boundary71--
F7: Bob receives the re-INVITE request, inspects the SDP offer and
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extracts the icon body, checks the creation date and file size, and
decides to accept the file transfer. So Bob creates the following
SDP answer:
v=0
o=bob 2890844656 2890855440 IN IP4 bobpc.example.com
s=
c=IN IP4 bobpc.example.com
t=0 0
m=message 9999 TCP/MSRP *
a=recvonly
a=accept-types:*
a=path:msrp://bobpc.example.com:9999/9an4ea;tcp
a=filename:Sunset.jpg
a=filetype:image/jpeg
a=disposition:inline
a=filesize:4096
a=hash:SHA 58231FE8653BBCF371362F86D471913EE4B1DF2F
F9: Alice opens a new TCP connection to Bob and creates an MSRP SEND
request that contains the file.
MSRP d95ksxox SEND
To-Path: msrp://bobpc.example.com:9999/9an4ea;tcp
From-Path: msrp://alicepc.example.com:5670/iau39;tcp
Message-ID: 13449sdqwer
Byte-Range: 1-2027/2027
Content-Type: image/jpeg
Content-Disposition: inline; filename="Sunset.jpg";
creation-date="Sun, 21 May 2006 13:02:15";
size=4096
... binary JPEG image ...
-------d95ksxox+
F10: Bob acknowledges the reception of the SEND request.
MSRP d95ksxox 200 OK
To-Path: msrp://alicepc.example.com:5670/iau39;tcp
From-Path: msrp://bobpc.example.com:9999/9an4ea;tcp
Byte-Range: 1-2027/2027
-------d95ksxox$
F11: Then Bob terminates the SIP session by sending a SIP BYE
request.
F12: Alice acknowledges the reception of the BYE request and sends a
200 (OK) response.
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8. Security Considerations
TBD
9. IANA Considerations
TBD
10. Acknowledgements
The authors would like to thank Mats Stille, Nancy Greene, Adamu
Haruna, and Arto Leppisaari for discussing initial concepts described
in this memo. Thanks to Pekka Kuure for reviewing initial versions
this document and providing helpful comments. Joerg Ott, Jiwey Wang,
Amitkumar Goel, and Sudha Vs discussed and provided comments and
improvements to this document.
11. References
11.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[3] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The Content-
Disposition Header Field", RFC 2183, August 1997.
[4] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[5] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[6] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)",
RFC 3174, September 2001.
[7] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[8] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851,
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July 2004.
[9] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[10] Handley, M., "SDP: Session Description Protocol",
draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.
[11] Campbell, B., "The Message Session Relay Protocol",
draft-ietf-simple-message-sessions-14 (work in progress),
February 2006.
11.2. Informational References
[12] 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.
[13] Burger, E., "A Mechanism for Content Indirection in Session
Initiation Protocol (SIP) Messages", RFC 4483, May 2006.
[14] Isomaki, M., "Requirements and Possible Mechanisms for File
Transfer Services Within the Context of SIP Based
Communication", draft-isomaki-sipping-file-transfer-01 (work in
progress), March 2006.
[15] Jennings, C., "Relay Extensions for the Message Sessions Relay
Protocol (MSRP)", draft-ietf-simple-msrp-relays-07 (work in
progress), February 2006.
[16] Paila, T., "FLUTE - File Delivery over Unidirectional
Transport", draft-ietf-rmt-flute-revised-01 (work in progress),
January 2006.
Authors' Addresses
Miguel A. Garcia-Martin
Nokia
P.O.Box 407
NOKIA GROUP, FIN 00045
Finland
Email: miguel.an.garcia@nokia.com
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Markus Isomaki
Nokia
Keilalahdentie 2-4
Espoo 02150
Finland
Email: markus.isomaki@nokia.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Salvatore.Loreto@ericsson.com
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Full Copyright Statement
Copyright (C) The Internet Society (2006).
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contained in BCP 78, and except as set forth therein, the authors
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Acknowledgment
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