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IETF RFC 5547
A Session Description Protocol (SDP) Offer/Answer Mechanism to Enable File Transfer
Last modified on Tuesday, May 5th, 2009
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Network Working Group M. Garcia-Martin
Request for Comments: 5547 Ericsson
Category: Standards Track M. Isomaki
Nokia
G. Camarillo
S. Loreto
Ericsson
P. Kyzivat
Cisco Systems
May 2009
A Session Description Protocol (SDP) Offer/Answer Mechanism
to Enable File Transfer
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
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.
The offerer can describe either 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.
Garcia-Martin, et al. Standards Track PAGE 1 
RFC 5547 SDP Offer/Answer for File Transfer May 2009
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
3. Definitions .....................................................4
4. Overview of Operation ...........................................5
5. File Selector ...................................................6
6. Extensions to SDP ...............................................7
7. File Disposition Types .........................................13
8. Protocol Operation .............................................13
8.1. The 'file-transfer-id' Attribute ..........................14
8.2. Offerer's Behavior ........................................17
8.2.1. The Offerer Is a File Sender .......................17
8.2.2. The Offerer Is a File Receiver .....................18
8.2.3. SDP Offer for Several Files ........................18
8.3. Answerer's Behavior .......................................19
8.3.1. The Answerer Is a File Receiver ....................19
8.3.2. The Answerer Is a File Sender ......................20
8.4. Aborting an Ongoing File Transfer Operation ...............22
8.5. Indicating File Transfer Offer/Answer Capability ..........25
8.6. Reusage of Existing "m=" Lines in SDP .....................26
8.7. MSRP Usage ................................................26
8.8. Considerations about the 'file-icon' Attribute ............28
9. Examples .......................................................28
9.1. Offerer Sends a File to the Answerer ......................28
9.2. Offerer Requests a File from the Answerer and
Second File Transfer ......................................33
9.3. Example of a Capability Indication ........................40
10. Security Considerations .......................................41
11. IANA Considerations ...........................................42
11.1. Registration of New SDP Attributes .......................42
11.1.1. Registration of the file-selector Attribute .......43
11.1.2. Registration of the file-transfer-id Attribute ....43
11.1.3. Registration of the file-disposition Attribute ....43
11.1.4. Registration of the file-date Attribute ...........44
11.1.5. Registration of the file-icon Attribute ...........44
11.1.6. Registration of the file-range Attribute ..........45
12. Acknowledgments ...............................................45
13. References ....................................................45
13.1. Normative References .....................................45
13.2. Informative References ...................................46
Appendix A. Alternatives Considered ..............................48
Garcia-Martin, et al. Standards Track PAGE 2
RFC 5547 SDP Offer/Answer for File Transfer May 2009
1. Introduction
The Session Description Protocol (SDP) offer/answer [RFC 3264]
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) [RFC 4975] is a protocol for
transmitting instant messages (IMs) in the context of a session. The
protocol specification describes the usage of SDP for establishing an
MSRP session. In addition to plain text messages, MSRP is able to
carry arbitrary (binary) Multipurpose Internet Mail Extensions (MIME)
[RFC 2045] 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 [RFC 4976] provide a mechanism for Network Address Translator
(NAT) traversal. Finally, Secure MIME (S/MIME) [RFC 3851] can be used
for ensuring the integrity and confidentiality of the transferred
content.
However, the baseline MSRP does not readily meet all the requirements
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.
Garcia-Martin, et al. Standards Track PAGE 3
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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.
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. Section 5 introduces the concept of the file selector.
The detailed syntax and semantics of the new SDP attributes and
conventions on how the existing ones are used are defined in
Section 6. Section 7 discusses the file disposition types.
Section 8 describes the protocol operation involving SDP and MSRP.
Finally, some examples are given in Section 9.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC 2119].
3. Definitions
For the purpose of this document, the following definitions specified
in RFC 3264 [RFC 3264] apply:
o Answer
o Answerer
o Offer
o Offerer
Additionally, we define the following terms:
File sender: The endpoint that is willing to send a file to the file
receiver.
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.
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RFC 5547 SDP Offer/Answer for File Transfer May 2009
Push operation: A file transfer operation where the SDP offerer
takes the role of the file sender and the SDP answerer takes the
role of the file receiver.
Pull operation: A file transfer operation where the SDP offerer
takes the role of the file receiver and the SDP answerer takes the
role of the file sender.
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 separately.
The actual file transfer is carried out using the Message Session
Relay Protocol (MSRP) [RFC 4975]. Each SDP "m=" line describes an
MSRP media stream used to transfer a single file at a time. 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 or receive the file. Assuming that the answerer
accepts the file transfer, the actual transfer of the files takes
Garcia-Martin, et al. Standards Track PAGE 5
RFC 5547 SDP Offer/Answer for File Transfer May 2009
place with ordinary MSRP. Note that the 'sendonly' and 'recvonly'
attributes refer to the direction of MSRP SEND requests and do not
preclude other protocol elements (such as 200 responses, REPORT
requests, etc.).
In principle the file transfer can work even with an endpoint
supporting only regular MSRP without understanding the extensions
defined herein, in a particular case where that endpoint is both
the SDP answerer and the file receiver. 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, since
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
When the file receiver generates the SDP offer, this SDP offer needs
to unambiguously identify the requested file at the file sender. For
this purpose, we introduce the notion of a file selector, which is a
tuple composed of one or more of the following individual selectors:
the name, size, type, and hash of the file. The file selector can
include any number of selectors, so all four of them do not always
need to be present.
The purpose of the file selector is to provide enough information
about the 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 files that match each of the
selectors present in the 'file-selector' attribute. The result can
be 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 is 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 selector and a name
selector, there is a risk that the remote host has renamed the file,
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RFC 5547 SDP Offer/Answer for File Transfer May 2009
in which case, although a file whose computed hash equals the hash
selector exists, the file name does not match that of the name
selector. Thus, in this case, the file selection process will result
in the selection of zero files.
This specification uses the Secure Hash Algorithm 1, SHA-1 [RFC 3174].
If future needs require adding support for different hashing
algorithms, they will be specified as extensions to this document.
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.
The 'file-selector' media attribute is also useful when learning the
support of the file transfer offer/answer capability that this
document specifies. This is further explained in Section 8.5.
6. Extensions to SDP
We define a number of new SDP [RFC 4566] attributes that provide the
required information to describe the transfer of a file with MSRP.
These are all media-level-only attributes in SDP. The following is
the formal ABNF syntax [RFC 5234] of these new attributes. It is
built above the SDP [RFC 4566] grammar, RFC 2045 [RFC 2045], RFC 2183
[RFC 2183], RFC 2392 [RFC 2392], and RFC 5322 [RFC 5322].
attribute =/ file-selector-attr / file-disp-attr /
file-tr-id-attr / file-date-attr /
file-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 5234
filename-string = 1*(filename-char/percent-encoded)
filename-char = %x01-09/%x0B-0C/%x0E-21/%x23-24/%x26-FF
; any byte except NUL, CR, LF,
; double quotes, or percent
percent-encoded = "%" HEXDIG HEXDIG
; HEXDIG defined in RFC 5234
filesize-selector = "size:" filesize-value
Garcia-Martin, et al. Standards Track PAGE 7
RFC 5547 SDP Offer/Answer for File Transfer May 2009
filesize-value = integer ;integer defined in RFC 4566
filetype-selector = "type:" type "/" subtype *(";" ft-parameter)
ft-parameter = attribute "=" DQUOTE value-string DQUOTE
; attribute is defined in RFC 2045
; free insertion of linear-white-space is not
; permitted in this context.
; note: value-string has to be re-encoded
; when translating between this and a
; Content-Type header.
value-string = filename-string
hash-selector = "hash:" hash-algorithm ":" hash-value
hash-algorithm = token ; see IANA Hash Function
; Textual Names registry
; only "sha-1" currently supported
hash-value = 2HEXDIG *(":" 2HEXDIG)
; Each byte in upper-case hex, separated
; by colons.
; HEXDIG defined in RFC 5234
file-tr-id-attr = "file-transfer-id:" file-tr-id-value
file-tr-id-value = token
file-disp-attr = "file-disposition:" file-disp-value
file-disp-value = token
file-date-attr = "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 5322
; numeric timezones (+HHMM or -HHMM)
; must be used
; DQUOTE defined in RFC 5234 files.
file-icon-attr = "file-icon:" file-icon-value
file-icon-value = cid-url ; cid-url defined in RFC 2392
file-range-attr = "file-range:" start-offset "-" stop-offset
start-offset = integer ; integer defined in RFC 4566
stop-offset = integer / "*"
Figure 1: Syntax of the SDP extension
Garcia-Martin, et al. Standards Track PAGE 8
RFC 5547 SDP Offer/Answer for File Transfer May 2009
When used for capability query (see Section 8.5), the 'file-selector'
attribute MUST NOT contain any selector, because its presence merely
indicates compliance to this specification.
When used in an SDP offer or answer, the 'file-selector' attribute
MUST contain at least one selector. Selectors characterize 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 [RFC 3629]. Its value SHOULD be the same as the
'filename' parameter of the Content-Disposition header field
[RFC 2183] that would be signaled by the actual file transfer. If a
file name contains double quotes or any other character that the
syntax does not allow in the 'name' selector, they MUST be percent-
encoded. The 'name' selector MUST NOT contain characters that can be
interpreted as directory structure by the local operating system. If
such characters are present in the file name, they MUST be percent-
encoded.
Note that the 'name' selector might still contain characters that,
although not meaningful for the local operating system, might
still be meaningful to the remote operating system (e.g., '\',
'/', ':'). Therefore, implementations are responsible for
sanitizing the input received from the remote endpoint before
doing a local operation in the local file system, such as the
creation of a local file. Among other things, implementations can
percent-encode characters that are meaningful to the local
operating system before doing file system local calls.
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 as the 'size' parameter of the Content-Disposition header
field [RFC 2183] that would be signaled 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 [RFC 3862].
The 'type' selector in the 'file-selector' attribute contains the
MIME media and submedia types of the content. In general, anything
that can be expressed in a Content-Type header field (see RFC 2045
[RFC 2045]) 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 [IANA]. Zero or more parameters can follow. When
Garcia-Martin, et al. Standards Track PAGE 9
RFC 5547 SDP Offer/Answer for File Transfer May 2009
translating parameters from a Content-Type header and a 'type'
selector, the parameter has to be re-encoded prior to its
accommodation as a parameter of the 'type' selector (see the ABNF
syntax of 'ft-parameter').
The 'hash' selector in the 'file-selector' attribute provides a hash
computation of the file to be transferred. This is commonly used by
file transfer protocols. For example, FLUTE [FLUTE-REV] uses hashes
(called message digests) to verify the contents of the transfer. The
purpose of the 'hash' selector is two-fold: On one side, in pull
operations, it allows the file receiver to identify a remote file by
its hash rather than by its file name, providing that the file
receiver has learned the hash of the remote file by some out-of-band
mechanism. On the other side, in either push or pull operations, it
allows the file receiver to verify the contents of the received file,
or even avoid unnecessary transmission of an existing file.
The address space of the SHA-1 algorithm is big enough to avoid
any collision in hash computations in between two endpoints. When
transferring files, the actual file transfer protocol should
provide reliable transmission of data, so verifications of
received files should always succeed. However, if endpoints need
to protect the integrity of a file, they should use some other
mechanism than the 'hash' selector specified in this memo.
The 'hash' selector includes the hash algorithm and its value.
Possible hash algorithms are those defined in the IANA registry of
Hash Function Textual Names [IANA]. Implementations according to
this specification MUST add a 160-bit string resulting from the
computation of US Secure Hash Algorithm 1 (SHA1) [RFC 3174] if the
'hash' selector is present. If need arises, extensions can be
drafted to support several hashing algorithms. Therefore,
implementations according to this specification MUST be prepared to
receive SDP containing more than a single 'hash' selector in the
'file-selector' attribute.
The value of the 'hash' selector is the byte string resulting from
applying the hash algorithm to the content of the whole file, even
when the file transfer is limited to a number of octets (i.e., the
'file-range' attribute is indicated).
The 'file-transfer-id' attribute provides a randomly chosen globally
unique identification to the actual file transfer. It is used to
distinguish a new file transfer request from a repetition of the SDP
(or the fraction of the SDP that deals with the file description).
This attribute is described in much greater detail in Section 8.1.
Garcia-Martin, et al. Standards Track PAGE 10
RFC 5547 SDP Offer/Answer for File Transfer May 2009
The 'file-disposition' attribute provides a suggestion to the other
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 as the disposition type parameter
of the Content-Disposition header field [RFC 2183] that would be
signaled by the actual file transfer protocol.
The 'file-date' attribute indicates the dates on which the file was
created, modified, or last read. This attribute MAY contain a
combination of the 'creation', 'modification', and 'read' parameters,
but MUST NOT contain more than one of each type .
The 'creation' parameter indicates the date on which the file was
created. The value MUST be a quoted string that contains a
representation of the creation date of the file in RFC 5322 [RFC 5322]
'date-time' format. Numeric timezones (+HHMM or -HHMM) MUST be used.
The value of this parameter SHOULD be the same as the 'creation-date'
parameter of the Content-Disposition header field [RFC 2183] that
would be signaled by the actual file transfer protocol.
The 'modification' parameter indicates the date on which the file was
last modified. The value MUST be a quoted string that contains a
representation of the last modification date to the file in RFC 5322
[RFC 5322] 'date-time' format. Numeric timezones (+HHMM or -HHMM)
MUST be used. The value of this parameter SHOULD be the same as the
'modification-date' parameter of the Content-Disposition header field
[RFC 2183] that would be signaled by the actual file transfer
protocol.
The 'read' parameter indicates the date on which the file was last
read. The value MUST be a quoted string that contains a
representation of the last date the file was read in RFC 5322
[RFC 5322] 'date-time' format. Numeric timezones (+HHMM or -HHMM)
MUST be used. The value of this parameter SHOULD be the same as the
'read-date' parameter of the Content-Disposition header field
[RFC 2183] that would be signaled by the actual file transfer
protocol.
The 'file-icon' attribute can be useful with certain file types such
as images. It allows the file sender to include a pointer to a body
that includes a small preview icon representing the contents of the
file to be transferred, which the file receiver can use to determine
whether it wants to receive such file. The 'file-icon' attribute
contains a Content-ID URL, which is specified in RFC 2392 [RFC 2392].
Section 8.8 contains further considerations about the 'file-icon'
attribute.
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RFC 5547 SDP Offer/Answer for File Transfer May 2009
The 'file-range' attribute provides a mechanism to signal a chunk of
a file rather than the complete file. This enables use cases where a
file transfer can be interrupted and resumed, even perhaps changing
one of the endpoints. The 'file-range' attribute contains the "start
offset" and "stop offset" of the file, separated by a dash "-". The
"start offset" value refers to the octet position of the file where
the file transfer should start. The first octet of a file is denoted
by the ordinal number "1". The "stop offset" value refers to the
octet position of the file where the file transfer should stop,
inclusive of this octet. The "stop offset" value MAY contain a "*"
if the total size of the file is not known in advance. The absence
of this attribute indicates a complete file, i.e., as if the 'file-
range' attribute would have been present with a value "1-*". 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 octet number 1, independently of the position of that
octet 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:message/cpim
a=accept-wrapped-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:
72:24:5F:E8:65:3D:DA:F3:71:36:2F:86:D4:71:91:3E:E4:A2:CE:2E
a=file-transfer-id:vBnG916bdberum2fFEABR1FR3ExZMUrd
a=file-disposition:attachment
a=file-date:creation:"Mon, 15 May 2006 15:01:31 +0300"
a=file-icon:cid:id2@alicepc.example.com
a=file-range:1-32349
Figure 2: Example of SDP describing a file transfer
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RFC 5547 SDP Offer/Answer for File Transfer May 2009
NOTE: The 'file-selector' attribute in the above figure is split
in three 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
indicate a preferred disposition of the file to be transferred in a
new 'file-disposition' attribute. In principle, any value listed in
the IANA registry for Mail Content Disposition Values [IANA] 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 to the file receiver
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 'file-disposition' attribute in SDP. To indicate that a file
should not be automatically rendered, this memo uses the existing
'attachment' value of the Content-Disposition type in the new 'file-
disposition' attribute in SDP. The default value is 'render', i.e.,
the absence of a 'file-disposition' attribute in the SDP has the same
semantics as 'render'.
The disposition value 'attachment' is specified in RFC 2183
[RFC 2183] with the following definition:
"Body parts can be designated 'attachment' to indicate that
they are separate from the main body of the mail message, and
that their display should not be automatic, but contingent upon
some further action of the user."
In the case of this specification, the 'attachment' disposition
type is used to indicate that the display of the file should not
be automatic, but contingent upon some further action of the user.
8. Protocol Operation
This section discusses how to use the parameters defined in Section 6
in the context of an offer/answer [RFC 3264] exchange. Additionally,
this section also discusses the behavior of the endpoints using MSRP.
Garcia-Martin, et al. Standards Track PAGE 13
RFC 5547 SDP Offer/Answer for File Transfer May 2009
A file transfer session is initiated by the offerer sending 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 session
or media 'recvonly' attribute, and accordingly the SDP answer
contains a session or media 'sendonly' attribute.
8.1. The 'file-transfer-id' Attribute
This specification creates an extension to the SDP offer/answer model
[RFC 3264], and because of that, it is assumed that the existing SDP
behavior is kept intact. The SDP behavior requires, for example,
that SDP is sent again to the remote party in situations where the
media description or perhaps other SDP parameters have not changed
with respect to a previous offer/answer exchange. Let's consider the
SIP Session Timer (RFC 4028) [RFC 4028], which uses re-INVITE requests
to refresh sessions. RFC 4028 recommends to send unmodified SDP in a
re-INVITE to refresh the session. Should this re-INVITE contain SDP
describing a file transfer operation and occur while the file
transfer was still going on, there would be no means to detect
whether the SDP creator wanted to abort the current file transfer
operation and initiate a new one or the SDP file description was
included in the SDP due to other reasons (e.g., session timer
refresh).
A similar scenario occurs when two endpoints have successfully agreed
on a file transfer, which is currently taking place when one of the
endpoints wants to add additional media streams to the existing
session. In this case, the endpoint sends a re-INVITE request that
contains the SDP. The SDP needs to maintain the media descriptions
for the current ongoing file transfer and add the new media
descriptions. The problem is that the other endpoint is not able to
determine whether or not a new file transfer is requested.
Garcia-Martin, et al. Standards Track PAGE 14
RFC 5547 SDP Offer/Answer for File Transfer May 2009
In other cases, a file transfer was successfully completed. Then, if
an endpoint resends the SDP offer with the media stream for the file
transfer, then the other endpoint wouldn't be able to determine
whether or not a new file transfer should start.
To address these scenarios, this specification defines the 'file-
transfer-id' attribute, which contains a globally unique random
identifier allocated to the file transfer operation. The file
transfer identifier helps both endpoints to determine whether the SDP
offer is requesting a new file transfer or it is a repetition of the
SDP. A new file transfer is one that, in case of acceptance, will
provoke the actual transfer of a file. This is typically the case of
new offer/answer exchanges, or in cases where an endpoint wants to
abort the existing file transfer and restart the file transfer once
more. On the other hand, the repetition of the SDP does not lead to
any actual file to be transferred, potentially because the file
transfer is still going on or because it has already finished. This
is the case of repeated offer/answer exchanges, which can be due to a
number of reasons (session timer, addition/removal of other media
types in the SDP, update in SDP due to changes in other session
parameters, etc.).
Implementations according to this specification MUST include a 'file-
transfer-id' attribute in SDP offers and answers. The SDP offerer
MUST select a file transfer identifier according to the syntax and
add it to the 'file-transfer-id' attribute. The SDP answerer MUST
copy the value of the 'file-transfer-id' attribute in the SDP answer.
The file transfer identifier MUST be unique within the current
session (never used before in this session), and it is RECOMMENDED to
be unique across different sessions. It is RECOMMENDED to select a
relatively big random identifier (e.g., 32 characters) to avoid
duplications. The SDP answerer MUST keep track of the proposed file
transfer identifiers in each session and copy the value of the
received file transfer identifier in the SDP answer.
If a file transfer is suspended and resumed at a later time, the
resumption is considered a new file transfer (even when the file to
be transferred is the same); therefore, the SDP offerer MUST choose a
new file transfer identifier.
If an endpoint sets the port number to zero in the media description
of a file transfer, for example, because it wants to reject the file
transfer operation, then the SDP answer MUST mirror the value of the
'file-transfer-id' attribute included in the SDP offer. This
effectively means that setting a media stream to zero has higher
precedence than any value that the 'file-transfer-id' attribute can
take.
Garcia-Martin, et al. Standards Track PAGE 15
RFC 5547 SDP Offer/Answer for File Transfer May 2009
As a side effect, the 'file-transfer-id' attribute can be used for
aborting and restarting again an ongoing file transfer. Assume that
two endpoints agree on a file transfer and the actual transfer of the
file is taking place. At some point in time in the middle of the
file transfer, one endpoint sends a new SDP offer, equal to the
initial one except for the value of the 'file-transfer-id' attribute,
which is a new globally unique random value. This indicates that the
offerer wants to abort the existing transfer and start a new one,
according to the SDP parameters. The SDP answerer SHOULD abort the
ongoing file transfer, according to the procedures of the file
transfer protocol (e.g., MSRP), and start sending file once more from
the initial requested octet. Section 8.4 further discusses aborting
a file transfer.
If an endpoint creates an SDP offer where the 'file-transfer-id'
attribute value does not change with respect to a previously sent
one, but the file selector changes so that a new file is selected,
then this is considered an error, and the SDP answerer MUST abort the
file transfer operation (e.g., by setting the port number to zero in
the SDP answer). Note that endpoints MAY change the 'file-selector'
attribute as long as the selected file does not change (e.g., by
adding a hash selector); however, it is RECOMMENDED that endpoints do
not change the value of the 'file-selector' attribute if it is
requested to transfer the same file described in a previous SDP
offer/answer exchange.
Figure 3 summarizes the relation of the 'file-transfer-id' attribute
with the file selector in subsequent SDP exchanges.
\ | | |
\ file selector | different | same |
'file-transfer-id' \ | file | file |
==================================+=============+===============+
| new file | new file |
changed | transfer | transfer |
| operation | operation |
----------------------------------+-------------+---------------+
| | existing file |
unchanged | error | transfer |
| | operation |
----------------------------------+-------------+---------------+
Figure 3: Relation of the 'file-transfer-id' attribute with the
selector of the file in a subsequent SDP exchange
In another scenario, an endpoint that has successfully transferred a
file wants to send an SDP due to other reasons than the transfer of a
file. The SDP offerer creates an SDP file description that maintains
Garcia-Martin, et al. Standards Track PAGE 16
RFC 5547 SDP Offer/Answer for File Transfer May 2009
the media description line corresponding to the file transfer. The
SDP offerer MUST then set the port number to zero and MUST keep the
same value of the 'file-transfer-id' attribute that the initial file
transfer got.
8.2. Offerer's Behavior
An offerer who wishes to send or receive one or more files to or from
an answerer MUST build an SDP [RFC 4566] description of a session
containing one "m=" line per file. When MSRP is used as the transfer
mechanism, each "m=" line also describes a single MSRP session,
according to the MSRP [RFC 4975] procedures. Any "m=" lines that may
have already been present in a previous SDP exchange are normally
kept unmodified; the new "m=" lines are added afterwards (Section 8.6
describes cases when "m=" lines are reused). All the media line
attributes specified and required by MSRP [RFC 4975] (e.g., "a=path",
"a=accept-types", etc.) MUST be included as well.
8.2.1. The Offerer Is a File Sender
In a push operation, the file sender creates an SDP offer describing
the file to be sent. The file sender MUST add a 'file-selector'
attribute media line containing at least one of the 'type', 'size',
or 'hash' selectors in indicating the type, size, or hash of the
file, respectively. If the file sender wishes to start a new file
transfer, the file sender MUST add a 'file-transfer-id' attribute
containing a new globally unique random identifier value.
Additionally, the file sender MUST add a session or media 'sendonly'
attribute to the SDP offer. Then the file sender sends the SDP offer
to the file receiver.
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' selector in the 'file-selector' attribute contains
valuable information for the file receiver to identify whether the
file is already available and need not be transmitted.
The file sender MAY also add a 'name' selector in the 'file-selector'
attribute, and 'file-icon', 'file-disposition', and 'file-date'
attributes further describing the file to be transferred. The 'file-
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 offsets of the
file.
Garcia-Martin, et al. Standards Track PAGE 17
RFC 5547 SDP Offer/Answer for File Transfer May 2009
When the file sender receives the SDP answer, if the port number of
the answer for a file request is non-zero, the file sender starts the
transfer of the file according to the negotiated parameters in SDP.
8.2.2. 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-
selector' attribute and MUST include, at least, one of the selectors
defined for such attribute (i.e., 'name', 'type', 'size', or 'hash').
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'
selector. However, particularly 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. If the file receiver wishes to start a
new file transfer, it MUST add a 'file-transfer-id' attribute
containing a new globally unique random value. The file receiver MAY
also add a 'file-range' attribute indicating the start and stop
offsets of the file. There is no need for the file receiver 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 add a session or media
'recvonly' attribute in the SDP offer. Then, the file receiver sends
the SDP offer to the file sender.
When the file receiver receives the SDP answer, if the port number of
the answer for a file request is non-zero, then the file receiver
should receive the file using the protocol indicated in the "m="
line. If the SDP answer contains a supported hashing algorithm in
the 'hash' selectors of the 'file-selector' attribute, then the file
receiver SHOULD compute the hash of the file after its reception and
check it against the hash received in the answer. In case the
computed hash does not match the one contained in the SDP answer, the
file receiver SHOULD consider that the file transfer failed and
SHOULD inform the user. Similarly, the file receiver SHOULD also
verify that the other selectors declared in the SDP match the file
properties, otherwise, the file receiver SHOULD consider that the
file transfer failed and SHOULD inform the user.
8.2.3. SDP Offer for Several Files
An offerer that wishes to send or receive more than one file
generates an "m=" line per file along with the file attributes
described in this specification. This way, the answerer can reject
individual files by setting the port number of their associated "m="
lines to zero, as per regular SDP [RFC 4566] procedures. Similarly,
the answerer can accept each individual file separately by setting
Garcia-Martin, et al. Standards Track PAGE 18
RFC 5547 SDP Offer/Answer for File Transfer May 2009
the port number of their associated "m=" lines to non-zero value.
Each file has its own file transfer identifier, which uniquely
identifies each file transfer.
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 RFC 4975 [RFC 4975]. The same TCP
connection can also be reused for sequential file transfers.
8.3. 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 [RFC 4566] procedures. The rejected answer
MUST contained a 'file-selector' and 'file-transfer-id' attributes
whose values mirror the corresponding values of the SDP offer.
If the answerer decides to accept the file, it proceeds as per
regular MSRP [RFC 4975] and SDP [RFC 4566] procedures.
8.3.1. The Answerer Is a File Receiver
In a push operation, the SDP answerer is the file receiver. When the
file receiver gets the SDP offer, it first examines the port number.
If the port number is set to zero, the file transfer operation is
closed, and no more data is expected over the media stream. Then, if
the port number is different than zero, the file receiver inspects
the 'file-transfer-id' attribute. If the value of the 'file-
transfer-id' attribute has been previously used, then the existing
session remains without changes; perhaps the file transfer is still
in progress, or perhaps it has concluded, but there are no changes
with respect to the current status. In any case, independently of
the port number, the SDP answerer creates a regular SDP answer and
sends it to the offerer.
If the port number is different than zero and the SDP offer contains
a new 'file-transfer-id' attribute, then it is signaling a request
for a new file transfer. The SDP answerer extracts the attributes
and parameters that describe the file and typically requests
permission from 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 [RFC 3264]. If the offer contains 'name', 'type', or 'size'
selectors in the 'file-selector' attribute, the answerer MUST copy
them into the answer. The file receiver copies the value of the
'file-transfer-id' attribute to the SDP answer. Then the file
receiver MUST add a session or media 'recvonly' attribute according
Garcia-Martin, et al. Standards Track PAGE 19
RFC 5547 SDP Offer/Answer for File Transfer May 2009
to the procedures specified in RFC 3264 [RFC 3264]. The file receiver
MUST NOT include 'file-icon', 'file-disposition', or 'file-date'
attributes in the SDP answer.
The file receiver can use the hash 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 or not the file transfer is accepted 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 octets 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 octets, it SHOULD reject the
transfer of the file.
When the file transfer operation is complete, the file receiver
computes the hash of the file and SHOULD verify that it matches the
hash declared in the SDP. If they do not match, the file receiver
SHOULD consider that the file transfer failed and SHOULD inform the
user. Similarly, the file receiver SHOULD also verify that the other
selectors declared in the SDP match the file properties; otherwise,
the file receiver SHOULD consider that the file transfer failed and
SHOULD inform the user.
8.3.2. The Answerer Is a File Sender
In a pull operation the answerer is the file sender. In this case,
the SDP answerer MUST first inspect the value of the
'file-transfer-id' attribute. If it has not been previously used
throughout the session, then acceptance of the file MUST provoke the
transfer of the file over the negotiated protocol. However, if the
value has been previously used by another file transfer operation
within the session, then the file sender MUST NOT alert the user and
MUST NOT start a new transfer of the file. No matter whether or not
an actual file transfer is initiated, the file sender MUST create a
proper SDP answer that contains the 'file-transfer-id' attribute with
the same value received in the SDP offer, and then it MUST continue
processing the SDP answer.
The file sender MUST always create an SDP answer according to the SDP
offer/answer procedures specified in RFC 3264 [RFC 3264]. The file
sender inspects the file selector of the received SDP offer, which is
encoded in the 'file-selector' media attribute line. 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'
selectors of the 'file-selector' attribute line (if they are
Garcia-Martin, et al. Standards Track PAGE 20
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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 answerer SHOULD also
reject the SDP as per procedures in RFC 3264 [RFC 3264], 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 in RFC 3264 [RFC 3264]. The file sender SHOULD
add a 'hash' selector in the answer with the locally computed SHA-1
hash over the complete file. If a hash value computed by the file
sender differs from that specified by the file receiver, the file
sender can either send the file without that hash value or reject the
request by setting the port in the media stream to zero. The file
sender MAY also include a 'type' selector in the 'file-selector'
attribute line of the SDP answer. The answerer MAY also include
'file-icon' and 'file-disposition' attributes to further describe the
file. Although the answerer MAY also include a 'name' and 'size'
selectors 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 [RFC 4975]) can
accommodate a Content-Disposition header field [RFC 2183] 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 signaled
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 choice
cannot be done, the answerer 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 octets declared in there, the
file sender MUST include a 'file-range' attribute in the SDP answer
Garcia-Martin, et al. Standards Track PAGE 21
RFC 5547 SDP Offer/Answer for File Transfer May 2009
with the same range of values. If the file sender does not accept
sending that range of octets, it SHOULD reject the transfer of the
file.
8.4. Aborting an Ongoing File Transfer Operation
Either the file sender or the file receiver can abort an ongoing file
transfer at any time. Unless otherwise noted, the entity that aborts
an ongoing file transfer operation MUST follow the procedures at the
media level (e.g., MSRP) and at the signaling level (SDP offer/
answer), as described below.
Assume the scenario depicted in Figure 4 where a file sender wishes
to abort an ongoing file transfer without initiating an alternative
file transfer. Assume that an ongoing MSRP SEND request is being
transmitted. The file sender aborts the MSRP message by including
the '#' character in the continuation field of the end-line of a SEND
request, according to the MSRP procedures (see Section 7.1 of RFC
4975 [RFC 4975]). Since a file is transmitted as one MSRP message,
aborting the MSRP message effectively aborts the file transfer. The
file receiver acknowledges the MSRP SEND request with a 200 response.
Then the file sender SHOULD close the MSRP session by creating a new
SDP offer that sets the port number to zero in the related "m=" line
that describes the file transfer (see Section 8.2 of RFC 3264
[RFC 3264]). This SDP offer MUST conform with the requirements of
Section 8.2.1. The 'file-transfer-id' attribute MUST be the same
attribute that identifies the ongoing transfer. Then the file sender
sends this SDP offer to the file receiver.
Rather than close the MSRP session by setting the port number to
zero in the related "m=" line, the file sender could also tear
down the whole session, e.g., by sending a SIP BYE request.
Note that it is the responsibility of the file sender to tear down
the MSRP session. Implementations should be prepared for
misbehaviors and implement measures to avoid hang states. For
example, upon expiration of a timer the file receiver can close the
aborted MSRP session by using regular MSRP procedures.
A file receiver that receives the above SDP offer creates an SDP
answer according to the procedures of the SDP offer/answer (RFC 3264
[RFC 3264]). This SDP answer MUST conform with the requirements of
Section 8.3.1. Then the file receiver sends this SDP answer to the
file sender.
Garcia-Martin, et al. Standards Track PAGE 22
RFC 5547 SDP Offer/Answer for File Transfer May 2009
File sender File receiver
| |
|\ |
| \ |
| \ |
| \ |
| \ |
| \ |
abort->| \ MSRP SEND (#) |
| +--------------->|
| MSRP 200 |
|<-----------------------|
| re-INVITE (SDP offer) |
|----------------------->|
| SIP 200 OK (SDP answer)|
|<-----------------------|
| SIP ACK |
|----------------------->|
| |
Figure 4: File sender aborts an ongoing file transfer
When the file receiver wants to abort the file transfer, there are
two possible scenarios, depending on the value of the Failure-Report
header in the ongoing MSRP SEND request. Assume now the scenario
depicted in Figure 5 where the MSRP SEND request includes a Failure-
Report header set to a value different than "no". When the file
receiver wishes to abort the ongoing file transfer, the file receiver
generates an MSRP 413 response to the current MSRP SEND request (see
Section 10.5 of RFC 4975 [RFC 4975]). Then the file receiver MUST
close the MSRP session by generating a new SDP offer that sets the
port number to zero in the related "m=" line that describes the file
transfer (see Section 8.2 of RFC 3264 [RFC 3264]). This SDP offer
MUST conform with the requirements expressed in Section 8.2.2. The
'file-transfer-id' attribute MUST be the same attribute that
identifies the ongoing transfer. Then the file receiver sends this
SDP offer to the file sender.
Garcia-Martin, et al. Standards Track PAGE 23
RFC 5547 SDP Offer/Answer for File Transfer May 2009
File sender File receiver
| |
|\ |
| \ MSRP SEND |
| \ Failure-Report: yes |
| \ |
| \ |
| \ |
| \ |
| \ |
| \ |
| MSRP 413 |<-abort
|<-----------------------|
| \ (#) |
| +----------->|
| re-INVITE (SDP offer) |
|<-----------------------|
| SIP 200 OK (SDP answer)|
|----------------------->|
| SIP ACK |
|<-----------------------|
| |
Figure 5: File receiver aborts an ongoing file transfer. Failure-
Report set to a value different than "no" in MSRP
In another scenario, depicted in Figure 6, an ongoing file transfer
is taking place, where the MSRP SEND request contains a Failure-
Report header set to the value "no". When the file receiver wants to
abort the ongoing transfer, it MUST close the MSRP session by
generating a new SDP offer that sets the port number to zero in the
related "m=" line that describes the file transfer (see Section 8.2
of RFC 3264 [RFC 3264]). This SDP offer MUST conform with the
requirements expressed in Section 8.2.2. The 'file-transfer-id'
attribute MUST be the same attribute that identifies the ongoing
transfer. Then the file receiver sends this SDP offer to the file
sender.
Garcia-Martin, et al. Standards Track PAGE 24
RFC 5547 SDP Offer/Answer for File Transfer May 2009
File sender File receiver
| |
|\ |
| \ MSRP SEND |
| \ Failure-Report: no |
| \ |
| \ |
| \ |
| \ |
| \ |
| \ |
| re-INVITE (SDP offer) |<-abort
|<-----------------------|
| \ (#) |
| +----------->|
| MSRP 200 |
|<-----------------------|
| SIP 200 OK (SDP answer)|
|----------------------->|
| SIP ACK |
|<-----------------------|
| |
Figure 6: File receiver aborts an ongoing file transfer. Failure-
Report set to "no" in MSRP
A file sender that receives an SDP offer setting the port number to
zero in the related "m=" line for file transfer, first, if an ongoing
MSRP SEND request is being transmitted, aborts the MSRP message by
including the '#' character in the continuation field of the end-line
of a SEND request, according to the MSRP procedures (see Section 7.1
of RFC 4975 [RFC 4975]). Since a file is transmitted as one MSRP
message, aborting the MSRP message effectively aborts the file
transfer. Then the file sender creates an SDP answer according to
the procedures of the SDP offer/answer (RFC 3264 [RFC 3264]). This
SDP answer MUST conform with the requirements of Section 8.3.2. Then
the file sender sends this SDP answer to the file receiver.
8.5. Indicating File Transfer Offer/Answer Capability
The SDP offer/answer model [RFC 3264] provides provisions for
indicating a capability to another endpoint (see Section 9 of RFC
3264 [RFC 3264]). The mechanism assumes a high-level protocol, such
as SIP [RFC 3261], that provides a capability query (such as a SIP
OPTIONS request). RFC 3264 [RFC 3264] indicates how to build the SDP
that is included in the response to such capability query. As such,
RFC 3264 indicates that an endpoint builds an SDP body that contains
Garcia-Martin, et al. Standards Track PAGE 25
RFC 5547 SDP Offer/Answer for File Transfer May 2009
an "m=" line containing the media type (message, for MSRP). An
endpoint that implements the procedures specified in this document
SHOULD also add a 'file-selector' media attribute for the "m=message"
line. The 'file-selector' media attribute MUST be empty, i.e., it
MUST NOT contain any selector. The endpoint MUST NOT add any of the
other file attributes defined in this specification.
8.6. Reusage of Existing "m=" Lines in SDP
The SDP offer/answer model [RFC 3264] provides rules that allow SDP
offerers and answerers to modify an existing media line, i.e., reuse
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
[RFC 3264], in particular those defined in Section 8.3, MUST apply for
file transfer operations. An endpoint that wants to reuse an
existing "m=" line to start the file transfer of another file creates
a different 'file-selector' attribute and selects a new globally
unique random value of the 'file-transfer-id' attribute.
If the file offerer resends an SDP offer with a port different than
zero, then the 'file-transfer-id' attribute determines whether a new
file transfer will start or whether the file transfer does not need
to start. If the SDP answerer accepts the SDP, then file transfer
starts from the indicated octet (if a 'file-range' attribute is
present).
8.7. MSRP Usage
The file transfer service specified in this document uses "m=" lines
in SDP to describe the unidirectional transfer of a file.
Consequently, each MSRP session established following the procedures
in Section 8.2 and Section 8.3 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.
File transfer may be accomplished using a new multimedia session
established for the purpose. Alternatively, a file transfer may be
conducted within an existing multimedia session, without regard for
the media in use within that session. Of particular note, file
transfer may be done within a multimedia session containing an MSRP
session used for regular instant messaging. If file transfer is
initiated within an existing multimedia session, the SDP offerer MUST
NOT reuse an existing "m=" line that is still being used by MSRP
(either regular MSRP for instant messaging or an ongoing file
transfer). Rather, it MUST add an additional "m=" line or else reuse
an "m=" line that is no longer being used.
Garcia-Martin, et al. Standards Track PAGE 26
RFC 5547 SDP Offer/Answer for File Transfer May 2009
Additionally, implementations according to this specification MUST
include a single file in a single MSRP message. Notice that the MSRP
specification defines "MSRP message" as a complete unit of MIME or
text content, which can be split and delivered in more than one MSRP
request; each of these portions of the complete message is called a
"chunk". So, it is still valid to send a file in several chunks, but
from the MSRP point of view, all the chunks together form an MSRP
message: the Common Presence and Instant Messaging (CPIM) message
that wraps the file. When chunking is used, it should be noticed
that MSRP does not require to wait for a 200-class response for a
chunk before sending the following one. Therefore, it is valid to
send pipelined MSRP SEND requests containing chunks of the same MSRP
message (the file). Section 9.1 contains an example of a file
transfer using pipelined MSRP requests.
The MSRP specification [RFC 4975] defines a 'max-size' SDP attribute.
This attribute specifies the maximum number of octets of an MSRP
message that the creator of the SDP is willing to receive (notice
once more the definition of "MSRP message"). File receivers MAY add
a 'max-size' attribute to the MSRP "m=" line that specifies the file,
indicating the maximum number of octets of an MSRP message. File
senders MUST NOT exceed the 'max-size' limit for any message sent in
the resulting session.
In the absence of a 'file-range' attribute in the SDP, the MSRP file
transfer MUST start with the first octet of the file and end with the
last octet (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 octets from the file (start and stop offset
octets, both inclusive). Then the file sender application MAY wrap
those octets in an appropriate wrapper. MSRP mandates
implementations to implement the message/cpim wrapper [RFC 3862].
Usage of a wrapper is negotiated in the SDP (see Section 8.6 in RFC
4975 [RFC 4975]). Last, the file sender application delivers the
content (e.g., the message/cpim body) to MSRP for transportation.
MSRP will consider the delivered content as a whole message, and will
start numbering bytes with the 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 'attachment' as indicated in
Section 7.
Once the file transfer is completed, the file sender SHOULD close the
MSRP session and MUST behave according to the MSRP [RFC 4975]
procedures with respect to closing MSRP sessions. Note that MSRP
Garcia-Martin, et al. Standards Track PAGE 27
RFC 5547 SDP Offer/Answer for File Transfer May 2009
session management is not related to TCP connection management. As a
matter of fact, MSRP allows multiple MSRP sessions to share the same
TCP connection.
8.8. Considerations about the 'file-icon' Attribute
This specification allows a file sender to include a small preview of
an image file: an icon. A 'file-icon' attribute contains a
Content-ID (CID) URL [RFC 2392] pointing to an additional body that
contains the actual icon. Since the icon is sent as a separate body
along the SDP body, the file sender MUST wrap the SDP body and the
icon bodies in a MIME multipart/related body. Therefore,
implementations according to this specification MUST implement the
multipart/related MIME type [RFC 2387]. When creating a multipart/
related MIME wrapper, the SDP body MUST be the root body, which
according to RFC 2387 [RFC 2387] is identified as the first body in
the multipart/related MIME wrapper or explicitly identified by the
'start' parameter. According to RFC 2387 [RFC 2387], the 'type'
parameter MUST be present and point to the root body, i.e., the SDP
body.
Assume that an endpoint behaving according to this specification
tries to send a file to a remote endpoint that neither implements
this specification nor implements multipart MIME bodies. The file
sender sends an SDP offer that contains a multipart/related MIME body
that includes an SDP body part and an icon body part. The file
receiver, not supporting multipart MIME types, will reject the SDP
offer via a higher protocol mechanism (e.g., SIP). In this case, it
is RECOMMENDED that the file sender removes the icon body part,
creates a single SDP body (i.e., without multipart MIME), and resends
the SDP offer. This provides some backwards compatibility with file
receives that do not implement this specification and increases the
chances of getting the SDP accepted at the file receiver.
Since the icon is sent as part of the signaling, it is RECOMMENDED to
keep the size of icons restricted to the minimum number of octets
that provide significance.
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 [RFC 3261] is used to transport the SDP
offer/answer exchange, although the SIP details are briefly shown for
the sake of brevity.
Garcia-Martin, et al. Standards Track PAGE 28
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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 7.
Alice's UAC Bob's UAS
| |
|(1) (SIP) INVITE |
|----------------------->|
|(2) (SIP) 200 OK |
|<-----------------------|
|(3) (SIP) ACK |
|----------------------->|
| |
|(4) (MSRP) SEND (chunk) |
|----------------------->|
|(5) (MSRP) SEND (chunk) |
|----------------------->|
|(6) (MSRP) 200 OK |
|<-----------------------|
|(7) (MSRP) 200 OK |
|<-----------------------|
| |
|(8) (SIP) BYE |
|----------------------->|
|(9) (SIP) 200 OK |
|<-----------------------|
| |
| |
Figure 7: 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.
Garcia-Martin, et al. Standards Track PAGE 29
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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/related; type="application/sdp";
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:message/cpim
a=accept-wrapped-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:
72:24:5F:E8:65:3D:DA:F3:71:36:2F:86:D4:71:91:3E:E4:A2:CE:2E
a=file-transfer-id:Q6LMoGymJdh0IKIgD6wD0jkcfgva4xvE
a=file-disposition:render
a=file-date:creation:"Mon, 15 May 2006 15:01:31 +0300"
a=file-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 8: INVITE request containing an SDP offer for file transfer
Garcia-Martin, et al. Standards Track PAGE 30
RFC 5547 SDP Offer/Answer for File Transfer May 2009
NOTE: The Content-Type header field and the 'file-selector'
attribute in the above figure are split in several lines for
formatting purposes. Real implementations will encode it in a
single line.
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:message/cpim
a=accept-wrapped-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:
72:24:5F:E8:65:3D:DA:F3:71:36:2F:86:D4:71:91:3E:E4:A2:CE:2E
a=file-transfer-id:Q6LMoGymJdh0IKIgD6wD0jkcfgva4xvE
Figure 9: SDP answer accepting the SDP offer for file transfer
NOTE: The 'file-selector' attribute in the above figure is split
in three 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.
Garcia-Martin, et al. Standards Track PAGE 31
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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 +0300";
size=4092
Content-Type: image/jpeg
... first set of bytes of the JPEG image ...
-------d93kswow+
Figure 10: MSRP SEND request containing the first chunk of actual
file
F5: Alice sends the second and last chunk. Note that MSRP allows to
send pipelined chunks, so there is no need to wait for the 200 (OK)
response from the previous 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 11: MSRP SEND request containing the second chunk of actual
file
F6: 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 12: MSRP 200 OK response
Garcia-Martin, et al. Standards Track PAGE 32
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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 13: 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 selector 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 file
transfer 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 reuses 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). In particular, Alice creates
a new file transfer identifier since this is a new file transfer
operation. Figure 14 shows the sequence flow.
Garcia-Martin, et al. Standards Track PAGE 33
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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 14: 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.
Garcia-Martin, et al. Standards Track PAGE 34
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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:message/cpim
a=accept-wrapped-types:*
a=path:msrp://alicepc.example.com:7654/jshA7we;tcp
a=file-selector:hash:sha-1:
72:24:5F:E8:65:3D:DA:F3:71:36:2F:86:D4:71:91:3E:E4:A2:CE:2E
a=file-transfer-id:aCQYuBRVoUPGVsFZkCK98vzcX2FXDIk2
Figure 15: 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.
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 transfer and creates an
SDP answer as follows:
Garcia-Martin, et al. Standards Track PAGE 35
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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:message/cpim
a=accept-wrapped-types:*
a=path:msrp://bobpc.example.com:8888/9di4ea;tcp
a=file-selector:type:image/jpeg hash:sha-1:
72:24:5F:E8:65:3D:DA:F3:71:36:2F:86:D4:71:91:3E:E4:A2:CE:2E
a=file-transfer-id:aCQYuBRVoUPGVsFZkCK98vzcX2FXDIk2
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.
F4: Alice opens a TCP connection to Bob. Bob then creates an MSRP
SEND request that contains the file.
MSRP d93kswow SEND
To-Path: msrp://alicepc.example.com:7654/jshA7we;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 +0300";
modification-date="Mon, 15 May 2006 16:04:53 +0300";
read-date="Mon, 16 May 2006 09:12:27 +0300";
size=1931
Content-Type: image/jpeg
...binary JPEG image...
-------d93kswow$
Figure 17: MSRP SEND request containing the actual file
F5: Alice acknowledges the reception of the SEND request.
Garcia-Martin, et al. Standards Track PAGE 36
RFC 5547 SDP Offer/Answer for File Transfer May 2009
MSRP d93kswow 200 OK
To-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
From-Path: msrp://alicepc.example.com:7654/jshA7we;tcp
Byte-Range: 1-2027/2027
-------d93kswow$
Figure 18: MSRP 200 OK response
F6: Alice reuses 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. Alice reuses the TCP port number for the
MSRP stream, but changes the MSRP session and the 'file-transfer-id'
value according to this specification.
Garcia-Martin, et al. Standards Track PAGE 37
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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/related; type="application/sdp";
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 7654 TCP/MSRP *
i=This is my latest picture
a=sendonly
a=accept-types:message/cpim
a=accept-wrapped-types:*
a=path:msrp://alicepc.example.com:7654/iau39;tcp
a=file-selector:name:"sunset.jpg" type:image/jpeg
size:4096 hash:sha-1:
58:23:1F:E8:65:3B:BC:F3:71:36:2F:86:D4:71:91:3E:E4:B1:DF:2F
a=file-transfer-id:ZVE8MfI9mhAdZ8GyiNMzNN5dpqgzQlCO
a=file-disposition:render
a=file-date:creation:"Sun, 21 May 2006 13:02:15 +0300"
a=file-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 19: Reuse of the SDP in a second file transfer
Garcia-Martin, et al. Standards Track PAGE 38
RFC 5547 SDP Offer/Answer for File Transfer May 2009
NOTE: The Content-Type header field and the 'file-selector'
attribute in the above figure are split in several lines for
formatting purposes. Real implementations will encode it in a
single line.
F7: Bob receives the re-INVITE request, inspects the SDP offer and
extracts the icon body, checks the creation date and the file size,
and decides to accept the file transfer. So Bob creates an SDP
answer where he reuses the same TCP port number, but changes his MSRP
session, according to the procedures of this specification.
v=0
o=bob 2890844656 2890855440 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:message/cpim
a=accept-wrapped-types:*
a=path:msrp://bobpc.example.com:8888/eh10dsk;tcp
a=file-selector:name:"sunset.jpg" type:image/jpeg
size:4096 hash:sha-1:
58:23:1F:E8:65:3B:BC:F3:71:36:2F:86:D4:71:91:3E:E4:B1:DF:2F
a=file-transfer-id:ZVE8MfI9mhAdZ8GyiNMzNN5dpqgzQlCO
a=file-disposition:render
Figure 20: SDP answer accepting the SDP offer for file transfer
NOTE: The 'file-selector' attribute in the above figure is split
in three lines for formatting purposes. Real implementations will
encode it in a single line.
F9: If a TCP connection towards Bob is already open, Alice reuses
that TCP connection to send an MSRP SEND request that contains the
file.
Garcia-Martin, et al. Standards Track PAGE 39
RFC 5547 SDP Offer/Answer for File Transfer May 2009
MSRP d95ksxox SEND
To-Path: msrp://bobpc.example.com:8888/eh10dsk;tcp
From-Path: msrp://alicepc.example.com:7654/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 -0300";
size=1931
Content-Type: image/jpeg
...binary JPEG image...
-------d95ksxox+
Figure 21: MSRP SEND request containing the actual file
F10: Bob acknowledges the reception of the SEND request.
MSRP d95ksxox 200 OK
To-Path: msrp://alicepc.example.com:7654/iau39;tcp
From-Path: msrp://bobpc.example.com:8888/eh10dsk;tcp
Byte-Range: 1-2027/2027
-------d95ksxox$
Figure 22: 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.
9.3. Example of a Capability Indication
Alice sends an OPTIONS request to Bob (this request does not contain
SDP). Bob answers with a 200 (OK) response that contain the SDP
shown in Figure 24. The SDP indicates support for CPIM messages that
can contain other MIME types. The maximum MSRP message size that the
endpoint can receive is 20000 octets. The presence of the 'file-
selector' attribute indicates support for the file transfer offer/
answer mechanism.
Garcia-Martin, et al. Standards Track PAGE 40
RFC 5547 SDP Offer/Answer for File Transfer May 2009
Alice's UAC Bob's UAS
| |
|(1) (SIP) OPTIONS |
|----------------------->|
|(2) (SIP) 200 OK |
| with SDP |
|<-----------------------|
| |
| |
Figure 23: Flow diagram of a capability request
v=0
o=bob 2890844656 2890855439 IN IP4 bobpc.example.com
s=-
c=IN IP4 bobpc.example.com
t=0 0
m=message 0 TCP/MSRP *
a=accept-types:message/cpim
a=accept-wrapped-types:*
a=max-size:20000
a=file-selector
Figure 24: SDP of the 200 (OK) response to an OPTIONS request
10. Security Considerations
The SDP attributes defined in this specification identify a file to
be transferred 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 transferred 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.
Additionally, it is RECOMMENDED that senders verify the properties of
the file against the selectors that describe it.
The descriptions of the files being transferred 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 are encrypted.
TLS and S/MIME are the natural choices to provide offer/answer
exchanges with integrity protection and confidentiality.
Garcia-Martin, et al. Standards Track PAGE 41
RFC 5547 SDP Offer/Answer for File Transfer May 2009
When an SDP offer contains the description of a file to be sent or
received, the SDP answerer MUST first authenticate the SDP offerer
and then it MUST authorize the file transfer operation, typically
according to a local policy. Typically, these functions are
integrated in the high-level protocol that carries SDP (e.g., SIP),
and in the file transfer protocol (e.g., MSRP). If SIP [RFC 3261] and
MSRP [RFC 4975] are used, the standard mechanisms for user
authentication and authorization are sufficient.
It is possible that a malicious or misbehaving implementation tries
to exhaust the resources of the remote endpoint, e.g., the internal
memory or the file system, by sending very large files. To protect
from this attack, an SDP answer SHOULD first verify the identity of
the SDP offerer, and perhaps only accept file transfers from trusted
sources. Mechanisms to verify the identity of the file sender depend
on the high-level protocol that carries the SDP, for example, SIP
[RFC 3261] and MSRP [RFC 4975].
It is also RECOMMENDED that implementations take measures to avoid
attacks on resource exhaustion, for example, by limiting the size of
received files, verifying that there is enough space in the file
system to store the file prior to its reception, or limiting the
number of simultaneous file transfers.
File receivers MUST also sanitize all input, such as the local file
name, prior to making calls to the local file system to store a file.
This is to prevent the existence of meaningful characters to the
local operating system that could damage it.
Once a file has been transferred, the file receiver must take care of
it. Typically, file transfer is a commonly used mechanism for
transmitting computer virus, spyware, and other types of malware.
File receivers should apply all possible security technologies (e.g.,
anti-virus, anti-spyware) to mitigate the risk of damage at their
host.
11. IANA Considerations
IANA has registered a number of SDP attributes according to the
following.
11.1. Registration of New SDP Attributes
IANA has registered a number of media-level-only attributes in the
Session Description Protocol Parameters registry [IANA]. The
registration data, according to RFC 4566 [RFC 4566], follows.
Garcia-Martin, et al. Standards Track PAGE 42
RFC 5547 SDP Offer/Answer for File Transfer May 2009
11.1.1. Registration of the file-selector Attribute
Contact: Miguel Garcia <miguel.a.garcia@ericsson.com>
Phone: +34 91 339 1000
Attribute name: file-selector
Long-form attribute name: File Selector
Type of attribute: media level only
This attribute is subject to the charset attribute
Description: This attribute unambiguously identifies a file by
indicating a combination of the 4-tuple composed of the name,
size, type, and hash of the file.
Specification: RFC 5547
11.1.2. Registration of the file-transfer-id Attribute
Contact: Miguel Garcia <miguel.a.garcia@ericsson.com>
Phone: +34 91 339 1000
Attribute name: file-transfer-id
Long-form attribute name: File Transfer Identifier
Type of attribute: media level only
This attribute is subject to the charset attribute
Description: This attribute contains a unique identifier of the file
transfer operation within the session.
Specification: RFC 5547
11.1.3. Registration of the file-disposition Attribute
Contact: Miguel Garcia <miguel.a.garcia@ericsson.com>
Phone: +34 91 339 1000
Attribute name: file-disposition
Long-form attribute name: File Disposition
Type of attribute: media level only
Garcia-Martin, et al. Standards Track PAGE 43
RFC 5547 SDP Offer/Answer for File Transfer May 2009
This attribute is not subject to the charset attribute
Description: This attribute provides a suggestion to the other
endpoint about the intended disposition of the file.
Specification: RFC 5547
11.1.4. Registration of the file-date Attribute
Contact: Miguel Garcia <miguel.a.garcia@ericsson.com>
Phone: +34 91 339 1000
Attribute name: file-date
Long-form attribute name:
Type of attribute: media level only
This attribute is not subject to the charset attribute
Description: This attribute indicates the dates on which the file
was created, modified, or last read.
Specification: RFC 5547
11.1.5. Registration of the file-icon Attribute
Contact: Miguel Garcia <miguel.a.garcia@ericsson.com>
Phone: +34 91 339 1000
Attribute name: file-icon
Long-form attribute name: File Icon
Type of attribute: media level only
This attribute is not subject to the charset attribute
Description: For image files, this attribute contains a pointer to a
body that includes a small preview icon representing the contents
of the file to be transferred.
Specification: RFC 5547
Garcia-Martin, et al. Standards Track PAGE 44
RFC 5547 SDP Offer/Answer for File Transfer May 2009
11.1.6. Registration of the file-range Attribute
Contact: Miguel Garcia <miguel.a.garcia@ericsson.com>
Phone: +34 91 339 1000
Attribute name: file-range
Long-form attribute name: File Range
Type of attribute: media level only
This attribute is not subject to the charset attribute
Description: This attribute contains the range of transferred octets
of the file.
Specification: RFC 5547
12. Acknowledgments
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
of this document and providing helpful comments. Joerg Ott, Jiwey
Wang, Amitkumar Goel, Sudha Vs, Dan Wing, Juuso Lehtinen, Remi Denis-
Courmont, Colin Perkins, Sudhakar An, Peter Saint-Andre, Jonathan
Rosenberg, Eric Rescorla, Vikram Chhibber, Ben Campbell, Richard
Barnes, and Chris Newman discussed and provided comments and
improvements to this document.
13. References
13.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC 2183] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183,
August 1997.
Garcia-Martin, et al. Standards Track PAGE 45
RFC 5547 SDP Offer/Answer for File Transfer May 2009
[RFC 2387] Levinson, E., "The MIME Multipart/Related Content-type",
RFC 2387, August 1998.
[RFC 2392] Levinson, E., "Content-ID and Message-ID Uniform
Resource Locators", RFC 2392, August 1998.
[RFC 3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, September 2001.
[RFC 3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC 3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC 3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[RFC 3862] Klyne, G. and D. Atkins, "Common Presence and Instant
Messaging (CPIM): Message Format", RFC 3862,
August 2004.
[RFC 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC 4975] Campbell, B., Mahy, R., and C. Jennings, "The Message
Session Relay Protocol (MSRP)", RFC 4975,
September 2007.
[RFC 5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC 5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
13.2. Informative References
[RFC 3261] 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.
[RFC 4028] Donovan, S. and J. Rosenberg, "Session Timers in the
Session Initiation Protocol (SIP)", RFC 4028,
April 2005.
Garcia-Martin, et al. Standards Track PAGE 46
RFC 5547 SDP Offer/Answer for File Transfer May 2009
[RFC 4483] Burger, E., "A Mechanism for Content Indirection in
Session Initiation Protocol (SIP) Messages", RFC 4483,
May 2006.
[RFC 4976] Jennings, C., Mahy, R., and A. Roach, "Relay Extensions
for the Message Sessions Relay Protocol (MSRP)",
RFC 4976, September 2007.
[IANA] IANA, "Internet Assigned Numbers Authority",
<http://www.iana.org>.
[FLUTE-REV] Luby, M., Lehtonen, R., Roca, V., and T. Paila, "FLUTE -
File Delivery over Unidirectional Transport", Work
in Progress, September 2008.
Garcia-Martin, et al. Standards Track PAGE 47
RFC 5547 SDP Offer/Answer for File Transfer May 2009
Appendix A. 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
that 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) [RFC 4483]. 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
accept 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. The level of
requirements in implementations decides which solution meets the
requirements.
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. Essentially, such a
protocol should be able to be negotiated on an SDP offer/answer
exchange (RFC 3264 [RFC 3264]), be able to describe the file to be
transferred in SDP offer/answer exchange, be able to carry MIME
objects between two endpoints, and use a reliable transport
protocol (e.g., TCP).
This specification defines a set of SDP attributes that describe a
file to be transferred between two endpoints. The information needed
to describe a file could be potentially encoded in a few different
Garcia-Martin, et al. Standards Track PAGE 48
RFC 5547 SDP Offer/Answer for File Transfer May 2009
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 attribute or
parameter.
A MIME 'external-body' could potentially be used to describe the file
to be transferred. 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.
Garcia-Martin, et al. Standards Track PAGE 49
RFC 5547 SDP Offer/Answer for File Transfer May 2009
Authors' Addresses
Miguel A. Garcia-Martin
Ericsson
Calle Via de los Poblados 13
Madrid, ES 28033
Spain
EMail: miguel.a.garcia@ericsson.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
Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Salvatore.Loreto@ericsson.com
Paul H. Kyzivat
Cisco Systems
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
EMail: pkyzivat@cisco.com
Garcia-Martin, et al. Standards Track PAGE 50
A Session Description Protocol (SDP) Offer/Answer Mechanism to Enable File Transfer
RFC TOTAL SIZE: 112625 bytes
PUBLICATION DATE: Tuesday, May 5th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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