Internet DRAFT - draft-garcia-mmusic-file-transfer-mech
draft-garcia-mmusic-file-transfer-mech
MMUSIC Working Group M. Garcia-Martin
Internet-Draft M. Isomaki
Intended status: Standards Track Nokia
Expires: June 7, 2007 G. Camarillo
S. Loreto
Ericsson
December 4, 2006
Session Description Protocol (SDP) Offer/Answer Mechanism to Enable File
Transfer
draft-garcia-mmusic-file-transfer-mech-02.txt
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Copyright Notice
Copyright (C) The IETF Trust (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 to be transferred.
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The offerer can 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 separately for each individual file . The transfer
of one or more files is initiated after a successful negotiation.
The Message Session Relay Protocol (MSRP) is defined as the default
mechanism to actually carry the files between the endpoints. The
conventions on how to use MSRP for file transfer are also provided in
this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Alternatives Considered . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 6
5. File selector . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Extensions to SDP . . . . . . . . . . . . . . . . . . . . . . 8
7. File Disposition Types . . . . . . . . . . . . . . . . . . . . 12
8. Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Offerer's Behavior . . . . . . . . . . . . . . . . . . . . 14
8.1.1. The Offerer is a File Sender . . . . . . . . . . . . . 14
8.1.2. The Offerer is a File Receiver . . . . . . . . . . . . 14
8.1.3. SDP Offer for Several Files . . . . . . . . . . . . . 15
8.2. Answerer's Behavior . . . . . . . . . . . . . . . . . . . 15
8.2.1. The Answerer is a File Receiver . . . . . . . . . . . 15
8.2.2. The Answerer is a File Sender . . . . . . . . . . . . 16
8.3. Re-usage of Existing m= Lines in SDP . . . . . . . . . . . 17
8.4. MSRP Usage . . . . . . . . . . . . . . . . . . . . . . . . 18
9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Offerer sends a file to the Answerer . . . . . . . . . . . 18
9.2. Offerer requests a file from the Answerer and second
file transfer . . . . . . . . . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 28
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
11.1. Registration of new SDP attributes . . . . . . . . . . . . 28
11.2. Registration of new Content Disposition value . . . . . . 29
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
13.1. Normative References . . . . . . . . . . . . . . . . . . . 29
13.2. Informational References . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
Intellectual Property and Copyright Statements . . . . . . . . . . 33
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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. 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) [12] 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 [16] 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 [15] 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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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 6. Section 8 describes the protocol operation
involving SDP and MSRP. Finally, some examples are given in
Section 9.
1.1. Alternatives Considered
The 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) [14]. 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.
This specification defines a set of SDP attributes that describe a
file to be transfered between two endpoits. The information needed
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to describe a file could be potentially encoded in a few different
ways. The MMUSIC working group considered a few alternative
approaches before deciding to use the encoding described in
Section 6. In particular, the working group looked at the MIME
'external-body' type and the use of a single SDP parameter.
A MIME 'external-body' could potentially be used to describe the file
to be transfered. In fact, many of the SDP parameters this
specification defines are also supported by 'external-body' body
parts. The MMUSIC working group decided not to use 'external-body'
body parts because a number of existing offer/answer implementations
do not support multipart bodies.
The information carried in the SDP attributes defined in Section 6
could potentially be encoded in a single SDP attribute. The MMUSIC
working group decided not to follow this approach because it is
expected that implementations support only a subset of the parameters
defined in Section 6. Those implementations will be able to use
regular SDP rules in order to ignore non-supported SDP parameters.
If all the information was encoded in a single SDP attribute, those
rules, which relate to backwards compatibility, would need to be
redefined specifically for that parameter.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"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 send a file to the
file receiver.
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File receiver: The endpoint that is willing to receive a file from
the file sender.
File selector: A tuple of file attributes that the SDP offerer
includes in the SDP in order to select a file at the SDP answerer.
This is described in more detail in Section 5.
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.
The actual file transfer is carried out using the Message Session
Relay Protocol (MSRP) [12]. Each SDP "m=" line describes an MSRP
media stream used to transfer a single file. That is, the transfer
of multiple simultaneous files requires multiple "m=" lines and
corresponding MSRP media streams. It should be noted that multiple
MSRP media streams can share a single transport layer connection, so
this mechanism will not lead to excessive use of transport resources.
Each "m=" line for an MSRP media stream is accompanied with a few
attributes describing the file to be transferred. If the file sender
generates the SDP offer, the attributes describe a local file to be
sent (push), and the file receiver can use this information to either
accept or reject the transfer. However, if the SDP offer is
generated by the file receiver, the attributes are intended to
characterize a particular file that the file receiver is willing to
get (pull) from the file sender. It is possible that the file sender
does not have a matching file or does not want to send the file, in
which case the offer is rejected.
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,
hash, icon (e.g., if the file is a picture), etc.
SDP direction attributes (e.g., 'sendonly', '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.
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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. File selector
Specially in case the SDP offer is generated by the file receiver,
the offer needs a mechanism to unambiguously identify the requested
file. For this purpose, the file transfer mechanism introduces the
concept of a file selector, which is defined as the combination of
the 4-tuple composed of the name, size, type, and hash of the file.
We call each of these individual items a selector.
The purpose of the file selector is to provide enough information
that characterizes a file to the remote entity, so that both the
local and the remote entity can refer to the same file. The file
selector is encoded in a 'file-selector' media attribute in SDP. The
formal syntax of the 'file-selector' media attribute is described in
Figure 1.
The file selection process is applied to all the available files at
the host. The process selects those file that match each of the
4-tuple selectors present in the 'file-selector' attribute. Thus, a
file selector can point to zero, one, or more files, depending on the
presence of the mentioned selectors in the SDP and depending on the
available files in a host. The file transfer mechanism specified in
this document requires that a file selector eventually results at
most in a single file to be chosen. Typically, if the hash selector
is known, it is enough to produce a file selector that points to
exactly zero or one file. However, a file selector that selects a
unique file is not always known by the offerer. Sometimes only the
name, size or type of file are known, so the file selector may result
in selecting more than one file, which is an undesired case. The
opposite is also true: if the file selector contains a hash and a
name selectors, there is a risk that 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.
Since there are several hashing algorithms, such as SHA-1 [6], SHA-
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256, SHA-384, SHA-512 [11], etc., a file selector MAY contain several
hashes, each one describing the hash of the file with a different
hashing algorithm. Implementations that make use of the hash SHOULD
select one among the supported ones before selecting a file. So,
when several hashes are present in the SDP, the file selector
consists of the union of the name, size, type, and any of the
supported hash algorithms.
Implementations according to this specification MUST implement the
'file-selector' attribute and MAY implement any of the other
attributes specified in this specification. SDP offers and answers
for file transfer MUST contain a 'file-selector' media attribute that
selects the file to be transferred and MAY contain any of the other
attributes specified in this specification.
6. Extensions to SDP
We define a number of new SDP [10] attributes 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 = file-selector-attr / disposition-attr /
file-date-attr / icon-attr /
file-range-attr
;attribute is defined in RFC 4566
file-selector-attr = "file-selector:" (selector) *(SP selector)
selector = filename-selector / filesize-selector /
filetype-selector / hash-selector
filename-selector = "name:" DQUOTE filename-string DQUOTE
; DQUOTE defined in RFC 4234
filename-string = byte-string ;byte-string defined in RFC 4566
filesize-selector = "size:" filesize-value
filesize-value = integer ;integer defined in RFC 4566
filetype-selector = "type:" type "/" subtype *(";"parameter)
; parameter defined in RFC 2045
type = token
subtype = token
hash-selector = "hash:" hash-algorithm ":" hash-value
hash-algorithm = token ;see IANA Hash Function
;Textual Names registry
hash-value = hex-val ;hex-val defined in RFC 4234
disposition-attr = "disposition:" disposition-value
disposition-value = token
file-date = "file-date:" date-param *(SP date-param)
date-param = c-date-param / m-date-param / r-date-param
c-date-param = "creation:" DQUOTE date-time DQUOTE
m-date-param = "modification:" DQUOTE date-time DQUOTE
r-date-param = "read:" DQUOTE date-time DQUOTE
; date-time is defined in RFC 2822
; numeric timezones (+HHMM or -HHMM)
; must be used
; DQUOTE defined in RFC 4234
icon-attr = "icon:" icon-value
icon-value = cid-url ;cid-url defined in RFC 2392
file-range-attr = "file-range:" integer "-" integer
;integer defined in RFC 4566
Figure 1: Syntax of the SDP extension
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The 'file-selector' attribute is composed of one or more selectors
which parametrize the file to be transferred. There are four
selectors in this attribute: 'name', 'size', 'type', and 'hash'.
The 'name' selector in the 'file-selector' attribute contains the
filename of the content enclosed in double quotes. The filename is
encoded in UTF-8. Its value SHOULD be the same as the 'filename'
parameter of Content-Disposition header field [3] that could be
signalled by the actual file transfer.
The 'size' selector in the 'file-selector' 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. Note that
the 'size' selector merely includes the file size, and does not
include any potential overhead added by a wrapper, such as message/
cpim.
The 'type' selector in the 'file-selector' 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 'type' selectors. Possible MIME Media Type
values are the ones listed in the IANA registry for MIME Media
Types [18]. Zero or more parameters can follow. The syntax of
'parameter' is specified in RFC 2045 [2] .
The 'hash' selector in the 'file-selector' attribute provides a hash
of the file to be transferred. This is commonly used by file
transfer protocols. For example, FLUTE [17] uses hashes (called
message digests) to verify the contents of the transfer. The purpose
of the 'hash' selector 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' selector includes the hash algorithm and its value. In
fact, since there are several hashing algorithms, the SDP MAY contain
several 'hash' selectors with different algorithms. Possible hash
algorithms are those defined in the IANA registry of Hash Function
Textual Names [19]. Implementations according to this specification
MUST support the US Secure Hash Algorithm 1 (SHA1) [6] and MAY
support other hashing algorithms. The value is the byte string
resulting of applying the hash algorithm to the content of the whole
file.
The 'disposition' attribute provides a suggestion to the other
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endpoint about the intended disposition of the file. Section 7
provides further discussion of the possible values. 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 'file-date' attribute indicates the dates at which the file was
created, modified, or last read. This attribute MAY contain a
combination of the 'creation', 'modification', and 'read' parameters.
Only one parameter of each type (creation, modification, or read)
MUST be present in a 'file-date' attribute.
The 'creation' parameter indicates the date at which the file was
created. The value MUST be a quoted 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 parameter 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' parameter indicates the date at which the file was
last modified. The value MUST be a quoted string which contains a
representation of the last modification date to the file in RFC 2822
[5] 'date-time' format. Numeric timezones (+HHMM or -HHMM) MUST be
used. The value of this parameter 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' parameter indicates the date at which the file was last
read. The value MUST be a quoted string which contains a
representation of the last date the file was read in RFC 2822 [5]
'date-time' format. Numeric timezones (+HHMM or -HHMM) MUST be used.
The value of this parameter 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 a small preview icon representing the contents of the file
to be 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 to 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 'file-range' attribute provides a mechanism to signal a chunk of
a file rather than the complete file. This enable use cases where a
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file transfer can be interrupted, resumed, even perhaps changing one
of the endpoints. The 'file-range' attribute is composed to two
integer values separated by a dash "-". The first integer value
refers to the first byte of the file to be transferred. The second
integer value refers to the last byte of the file to be transferred.
The first byte of a file is indicated with "1". The absence of this
attribute indicates a complete file, i.e., like if the 'file-range'
attribute would have been present with values 1-<size_of_file>. The
'file-range' attribute must not be confused with the Byte-Range
header in MSRP. The former indicates the portion of a file that the
application would read and pass onto the MSRP stack for
transportation. From the point of view of MSRP, the portion of the
file is viewed as a whole message. The latter indicates the number
of bytes of that message that are carried in a chunk and the total
size of the message. Therefore, MSRP starts counting the delivered
message at byte number 1, independently of position of that byte in
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=file-selector:name:"My cool picture.jpg" type:image/jpeg
size:32349 hash:sha-1:72245FE8653DDAF371362F86D471913EE4A2CE2E
a=disposition:not-render
a=file-date:creation:"Mon, 15 May 2006 15:01:31 +03:00"
a=icon:cid:id2@alicepc.example.com
a=file-range:1-32349
Figure 2: Example of SDP describing a file transfer
NOTE: The 'file-selector' attribute in the above figure is split in
two lines for formatting purposes. Real implementations will encode
it in a single line.
7. File Disposition Types
The SDP Offer/Answer for file transfer allows the file sender to
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indicate a preferred disposition of the file to be transferred in a
new 'disposition' attribute. In principle, any value listed in the
IANA registry for Mail Content Disposition Values [20] is acceptable,
however, most of them may not be applicable.
There are two content dispositions of interest for file transfer
operations. On one hand, the file sender may just want the file to
be rendered immediately in the file receiver's device. On the other
hand, the file sender may just want to indicate the file recipient
that the file should not be rendered at the reception of the file.
The recipient's user agent may want to interact with the user
regarding the file disposition or it may save the file until the user
takes an action. In any case, the exact actions are implementation
dependent.
To indicate that a file should be automatically rendered, this memo
uses the existing 'render' value of the Content Disposition type in
the new 'disposition' attribute in SDP. To indicate that a file
should not be automatically rendered, this memo defines a new value
'not-render' of the Content Disposition type. The default value is
'render', i.e., the absence of a 'disposition' attribute in the SDP
has the same semantics as 'render'.
8. Protocol Operation
This Section discusses how to use the parameters defined in Section 6
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.
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8.1. 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 [12] procedures. All the media line attributes specified and
required by MSRP [12] (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.
8.1.1. The Offerer is a File Sender
In a push operation, the file sender creates and SDP offer describing
the file to be sent. Then it sends the SDP offer to the file
receiver. The file sender MUST add a 'file-selector' attribute media
line containing at least one of the 'type', 'size', 'hash',
parameters in indicating the type, size, or hash of the file,
respectively. If the file sender is able to compute the hash of the
file with different hashing algorithms, it MAY add several 'hash'
parameters, each one referring to a different hashing algorithm.
Additionally, the file sender MUST add a session or media 'sendonly'
attribute to the SDP offer.
Not all the selectors in the 'file-selector' attribute might be
known when the file sender creates the SDP offer, for example,
because the host is still processing the file.
The 'hash' parameter in the 'file-selector' attribute contains
valuable information to the file receiver to identify whether the
file is already available and need not be transmitted. If the
sender supports several hashing algorithms, then several 'hash'
parameters can be included.
The file sender MAY also add a 'name' parameter in the 'file-
selector' attribute, and an 'icon', 'disposition', and 'file-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 the
file or not). The three date attributes provide the answerer with an
indication of the age of the file. The file sender MAY also add a
'file-range' attribute indicating the start and stop offset of the
file transfer.
8.1.2. The Offerer is a File Receiver
In a pull operation, the file receiver creates the SDP offer and
sends it to the file sender. The file receiver MUST include a 'file-
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selector' attribute and SHOULD add, at least, one of the parameters
defined for such attribute (i.e., 'name', 'type', 'size', or 'hash').
Several 'hash' parameters MAY be included if each 'hash' parameter is
computed with a different hashing algorithm. 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. 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). Additionally, the
file receiver MUST create an SDP offer that contains a session or
media 'recvonly' attribute.
The file receiver MAY also add a 'file-range' attribute indicating
the start and stop offset of the file transfer.
8.1.3. 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 [12].
8.2. 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 [12] and SDP [10]
procedures.
8.2.1. The Answerer is a File Receiver
In a push operation the answerer is the file receiver. When the file
receiver gets the SDP answer, it extracts the attributes and
parameters that describe the file and typically requests permission
to the user to accept or reject the reception of the file. If the
file transfer operation is accepted, the file receiver MUST create an
SDP answer according to the procedures specified in RFC 3264 [7]. If
the offer contains 'name', 'type', 'size', parameters in the 'file-
selector' attribute, the answerer MUST copy them into the answer. If
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the offer contains one ore more 'hash' parameters in the 'file-
selector' attribute, the answerer discards those with non-supported
hashing algorithms and MUST copy the remaining (if any) to the 'file-
selector' attribute of the answer. This informs the offerer that the
answerer supports this specification. Then the file receiver MUST
add a 'recvonly' attribute according to the procedures specified in
RFC 3264 [7]. The file receiver MUST NOT include 'icon',
'disposition', or 'file-date' attributes in the SDP answer.
If the SDP offer contains one or more 'hash' parameters in the 'file-
selector' attribute, the answerer discards those with unsupported
hashing algorithms. The file receiver can use the remaining hashes
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.
If the SDP offer contains a 'file-range' attribute and the file
receiver accepts to receive the range of bytes declared in there, the
file receiver MUST include a 'file-range' attribute in the SDP answer
with the same range of values. If the file receiver does not accept
the reception of that range of bytes, it SHOULD reject the transfer
of the file.
8.2.2. The Answerer is a File Sender
In a pull operation the answerer is a file sender. In this case, the
file sender MUST first inspect the received SDP offer and apply the
file selector. The file selector is encoded in the 'file-selector'
media attribute line in SDP. First, if the file selector contains
several hashes, the file sender MUST select one of them which
contains a supported hashing algorithm and discard the rest. Then
the file sender applies the file selector, which implies selecting
those files that match one by one with the 'name', 'type', 'size',
and 'hash' parameters of the 'file-selector' attribute line (if they
are present). 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 then the file sender SHOULD
also reject the SDP as per procedures in RFC 3264 [7], if this is the
only stream described in the SDP offer.
If the file selector points to a single file and the file sender
decides to accept the file transfer, the file sender MUST create an
SDP answer that contains a 'sendonly' attribute, according to the
procedures described RFC 3264 [7]. If the SDP offer included several
'hash' parameters, the file sender SHOULD include at least one in the
SDP answer, selected among those present in the offer and, at the
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same time, supported by the file sender. If the SDP offer did not
include a 'hash' parameter, the file sender SHOULD add one or more
'hash' parameters, according to the supported hashing algorithms.
The file sender MAY also include a 'type' parameter in the 'file-
selector' attribute line of the SDP answer. The answerer MAY also
include an 'icon' and 'disposition' attributes to further describe
the file. Although the answerer MAY also include a 'name' and 'size'
parameters in the 'file-selector' attribute, and a 'file-date'
attribute, it is RECOMMENDED not to include them in the SDP answer if
the actual file transfer protocol (e.g., MSRP [12]) 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.
If the SDP offer contains a 'file-range' attribute and the file
sender accepts to send the range of bytes declared in there, the file
sender MUST include a 'file-range' attribute in the SDP answer with
the same range of values. If the file sender does not accept sending
that range of bytes, it SHOULD reject the transfer of the file.
8.3. 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
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.
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8.4. 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 8.1
and Section 8.2 is only used to transfer a single file. So, senders
MUST only use the dedicated 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.
In the absence of a 'file-range' attribute in the SDP, the MSRP file
transfer MUST start with the first byte of the file and end with the
last byte (i.e., the whole file is transferred). If a 'file-range'
attribute is present in SDP, the file sender application MUST extract
the indicated range of bytes from the file (start and stop bytes).
Then the file sender application SHOULD wrap those bytes in an
appropriate wrapper, such as message/cpim. Last, the file sender
application delivers the message/cpim to MSRP for transportation.
MSRP will consider the message/cpim as a whole message, and will
start numbering bytes at number 1.
Note that the default content disposition of MSRP bodies is 'render'.
When MSRP is used to transfer files, the MSRP Content-Disposition
header can also take the value 'not-render' defined by this memo.
Once the file transfer is completed, the file sender SHOULD close the
MSRP session, and MUST behave according to the MSRP [12] procedures
with respect closing MSRP sessions.
9. Examples
9.1. Offerer sends a file to the Answerer
This section shows an example flow for a file transfer scenario. The
example assumes that SIP [13] 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=file-selector:name:"My cool picture.jpg" type:image/jpeg
size:4092 hash:sha-1:72245FE8653DDAF371362F86D471913EE4A2CE2E
a=disposition:render
a=file-date:creation:"Mon, 15 May 2006 15:01:31 +03:00"
a=icon:cid:id2@alicepc.example.com
--boundary71
Content-Type: image/jpeg
Content-Transfer-Encoding: binary
Content-ID: <id2@alicepc.example.com>
Content-Length: [length of image]
Content-Disposition: icon
...small preview icon of the file...
--boundary71--
Figure 4: INVITE request containing an SDP offer for file transfer
NOTE: The 'file-selector' attribute in the above figure is split in
two lines for formatting purposes. Real implementations will encode
it in a single line.
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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
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=file-selector:name:"My cool picture.jpg" type:image/jpeg
size:4092 hash:sha-1:72245FE8653DDAF371362F86D471913EE4A2CE2E
Figure 5: SDP answer accepting the SDP offer for file transfer
NOTE: The 'file-selector' attribute in the above figure is split in
two lines for formatting purposes. Real implementations will encode
it in a single line.
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: 1-2048/4385
Content-Type: message/cpim
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@example.com>
DateTime: 2006-05-15T15:02:31-03:00
Content-Disposition: render; filename="My cool picture.jpg";
creation-date="Mon, 15 May 2006 15:01:31 +03:00";
size=4092
Content-Type: image/jpeg
... first set of bytes of the JPEG image ...
-------d93kswow+
Figure 6: MSRP SEND request containing the first chunk of actual file
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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: 1-2048/4385
-------d93kswow$
Figure 7: MSRP 200 OK response
F6: Alice sends the second and last chunk.
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: 2049-4385/4385
Content-Type: message/cpim
... second set of bytes of the JPEG image ...
-------op2nc9a$
Figure 8: MSRP SEND request containing the second chunk of actual
file
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: 2049-4385/4385
-------op2nc9a$
Figure 9: MSRP 200 OK response
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.
9.2. 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
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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.
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=file-selector:hash:sha-1:72245FE8653DDAF371362F86D471913EE4A2CE2E
Figure 11: INVITE request containing an SDP offer for file transfer
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=file-selector:hash:sha-1:72245FE8653DDAF371362F86D471913EE4A2CE2E
type:image/jpeg
Figure 12: SDP answer accepting the SDP offer for file transfer
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: message/cpim
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@example.com>
DateTime: 2006-05-15T15:02:31-03:00
Content-Disposition: render; 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=1931
Content-Type: image/jpeg
...binary JPEG image...
-------d93kswow$
Figure 13: MSRP SEND request containing the actual file
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$
Figure 14: MSRP 200 OK response
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: multipart/mixed; boundary="boundary71"
Content-Length: [length of multipart]
--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=file-selector:name:"sunset.jpg" type:image/jpeg
size:4096 hash:sha-1:58231FE8653BBCF371362F86D471913EE4B1DF2F
a=disposition:render
a=file-date:creation:"Sun, 21 May 2006 13:02:15 +03:00"
a=icon:cid:id3@alicepc.example.com
--boundary71
Content-Type: image/jpeg
Content-Transfer-Encoding: binary
Content-ID: <id3@alicepc.example.com>
Content-Length: [length of image]
Content-Disposition: icon
...small preview icon...
--boundary71--
Figure 15: Reuse of the SDP in a second file transfer
NOTE: The 'file-selector' attribute in the above figure is split in
two lines for formatting purposes. Real implementations will encode
it in a single line.
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F7: Bob receives the re-INVITE request, inspects the SDP offer and
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=file-selector:name:"sunset.jpg" type:image/jpeg
size:4096 hash:sha-1:58231FE8653BBCF371362F86D471913EE4B1DF2F
a=disposition:render
Figure 16: SDP answer accepting the SDP offer for file transfer
NOTE: The 'file-selector' attribute in the above figure is split in
two lines for formatting purposes. Real implementations will encode
it in a single line.
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: message/cpim
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@example.com>
DateTime: 2006-05-21T13:02:15-03:00
Content-Disposition: render; filename="Sunset.jpg";
creation-date="Sun, 21 May 2006 13:02:15";
size=1931
Content-Type: image/jpeg
...binary JPEG image...
-------d95ksxox+
Figure 17: MSRP SEND request containing the actual file
F10: Bob acknowledges the reception of the SEND request.
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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$
Figure 18: MSRP 200 OK response
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.
10. Security Considerations
The SDP attributed defined in this specification identify a file to
be transfered between two endpoints. An endpoint can offer to send
the file to the other endpoint or request to receive the file from
the other endpoint. In the former case, an attacker modifying those
SDP attributes could cheat the receiver making it think that the file
to be transfered was a different one. In the latter case, the
attacker could make the sender send a different file than the one
requested by the receiver. Consequently, it is RECOMMENDED that
integrity protection be applied to the SDP session descriptions
carrying the attributes specified in this specification.
The descriptions of the files being transfered between endpoints may
reveal information the endpoints may consider confidential.
Therefore, it is RECOMMENDED that SDP session descriptions carrying
the attributes specified in this specification be encrypted.
TLS and S/MIME are the natural choices to provide offer/answer
exchanges with integrity protection and confidentiality.
11. IANA Considerations
This document instructs IANA to register a number of SDP attributes
an a new Content Disposition value, according to the following:
11.1. Registration of new SDP attributes
IANA acts on Session Description Protocol Parameters registry [21]
and ads the following attributes in the attribute field (both session
and media level):
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Type SDP Name Reference
---- ------------------ ---------
att-field (both session and media level)
file-selector [RFCXXXX]
disposition [RFCXXXX]
file-date [RFCXXXX]
icon [RFCXXXX]
file-range [RFCXXXX]
Note to the RFC Editor: Please replace RFCXXX with the RFC number of
this specification.
11.2. Registration of new Content Disposition value
IANA acts on Mail Content Disposition Values and Parameters
registry [20] and registers a new Mail Content Disposition Value
according to the following data:
Name Description Reference
----- ------------ ---------
not-render the body should not be rendered to the user [RFCXXXX]
Note to the RFC Editor: Please replace RFCXXX with the RFC number of
this specification.
12. 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, Sudha Vs, Dan Wing, Juuso Lehtinen, Remi Denis-
Courmont discussed and provided comments and improvements to this
document.
13. References
13.1. 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.
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[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,
July 2004.
[9] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[10] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[11] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and
HMAC-SHA)", RFC 4634, August 2006.
[12] Campbell, B., "The Message Session Relay Protocol",
draft-ietf-simple-message-sessions-15 (work in progress),
July 2006.
13.2. Informational References
[13] 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.
[14] Burger, E., "A Mechanism for Content Indirection in Session
Initiation Protocol (SIP) Messages", RFC 4483, May 2006.
[15] 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.
[16] Jennings, C., "Relay Extensions for the Message Sessions Relay
Protocol (MSRP)", draft-ietf-simple-msrp-relays-08 (work in
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progress), July 2006.
[17] Paila, T., "FLUTE - File Delivery over Unidirectional
Transport", draft-ietf-rmt-flute-revised-02 (work in progress),
August 2006.
URIs
[18] <http://www.iana.org/assignments/media-types/>
[19] <http://www.iana.org/assignments/hash-function-text-names>
[20] <http://www.iana.org/assignments/mail-cont-disp>
[21] <http://www.iana.org/assignments/sdp-parameters>
Authors' Addresses
Miguel A. Garcia-Martin
Nokia
P.O.Box 407
NOKIA GROUP, FIN 00045
Finland
Email: miguel.an.garcia@nokia.com
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
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Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Salvatore.Loreto@ericsson.com
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