path: root/doc/rfc/rfc5547.txt

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.



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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









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1.  Introduction

   The Session Description Protocol (SDP) offer/answer [RFC3264]
   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) [RFC4975] 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)
   [RFC2045] 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 [RFC4976] provide a mechanism for Network Address Translator
   (NAT) traversal.  Finally, Secure MIME (S/MIME) [RFC3851] 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.





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   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
   [RFC2119].

3.  Definitions

   For the purpose of this document, the following definitions specified
   in RFC 3264 [RFC3264] 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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   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) [RFC4975].  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






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   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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   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 [RFC3174].
   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 [RFC4566] 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 [RFC5234] of these new attributes.  It is
   built above the SDP [RFC4566] grammar, RFC 2045 [RFC2045], RFC 2183
   [RFC2183], RFC 2392 [RFC2392], and RFC 5322 [RFC5322].

   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



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   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





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   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 [RFC3629].  Its value SHOULD be the same as the
   'filename' parameter of the Content-Disposition header field
   [RFC2183] 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 [RFC2183] 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 [RFC3862].

   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
   [RFC2045]) 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






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   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) [RFC3174] 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.





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   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 [RFC2183] 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 [RFC5322]
   '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 [RFC2183] 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
   [RFC5322] '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
   [RFC2183] 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
   [RFC5322] '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
   [RFC2183] 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 [RFC2392].
   Section 8.8 contains further considerations about the 'file-icon'
   attribute.





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   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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      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
      [RFC2183] 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 [RFC3264] exchange.  Additionally,
   this section also discusses the behavior of the endpoints using MSRP.



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   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
   [RFC3264], 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) [RFC4028], 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.






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   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.



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   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



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   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 [RFC4566] 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 [RFC4975] 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 [RFC4975] (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.



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   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 [RFC4566] procedures.  Similarly,
   the answerer can accept each individual file separately by setting



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   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 [RFC4975].  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 [RFC4566] 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 [RFC4975] and SDP [RFC4566] 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 [RFC3264].  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



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   to the procedures specified in RFC 3264 [RFC3264].  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 [RFC3264].  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



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   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 [RFC3264], 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 [RFC3264].  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 [RFC4975]) can
   accommodate a Content-Disposition header field [RFC2183] 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



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   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 [RFC4975]).  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
   [RFC3264]).  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
   [RFC3264]).  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.







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                        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 [RFC4975]).  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 [RFC3264]).  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.














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                     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 [RFC3264]).  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.














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                     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 [RFC4975]).  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 [RFC3264]).  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 [RFC3264] provides provisions for
   indicating a capability to another endpoint (see Section 9 of RFC
   3264 [RFC3264]).  The mechanism assumes a high-level protocol, such
   as SIP [RFC3261], that provides a capability query (such as a SIP
   OPTIONS request).  RFC 3264 [RFC3264] 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




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   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 [RFC3264] 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
   [RFC3264], 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.



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   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 [RFC4975] 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 [RFC3862].
   Usage of a wrapper is negotiated in the SDP (see Section 8.6 in RFC
   4975 [RFC4975]).  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 [RFC4975]
   procedures with respect to closing MSRP sessions.  Note that MSRP





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   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 [RFC2392] 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 [RFC2387].  When creating a multipart/
   related MIME wrapper, the SDP body MUST be the root body, which
   according to RFC 2387 [RFC2387] is identified as the first body in
   the multipart/related MIME wrapper or explicitly identified by the
   'start' parameter.  According to RFC 2387 [RFC2387], 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 [RFC3261] is used to transport the SDP
   offer/answer exchange, although the SIP details are briefly shown for
   the sake of brevity.





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   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.















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   INVITE sip:bob@example.com SIP/2.0
   To: Bob <sip:bob@example.com>
   From: Alice <sip:alice@example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 1 INVITE
   Max-Forwards: 70
   Date: Sun, 21 May 2006 13:02:03 GMT
   Contact: <sip:alice@alicepc.example.com>
   Content-Type: multipart/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




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      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.
















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   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




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   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.





















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                   Alice's UAC                 Bob's UAS
                         |                        |
                         |(1) (SIP) INVITE        |
                         |----------------------->|
                         |(2) (SIP) 200 OK        |
                         |<-----------------------|
                         |(3) (SIP) ACK           |
                         |----------------------->|
                         |                        |
                         |(4) (MSRP) SEND (file)  |
                         |<-----------------------|
                         |(5) (MSRP) 200 OK       |
                         |----------------------->|
                         |                        |
                         |(6) (SIP) INVITE        |
                         |----------------------->|
                         |(7) (SIP) 200 OK        |
                         |<-----------------------|
                         |(8) (SIP) ACK           |
                         |----------------------->|
                         |                        |
                         |(9) (MSRP) SEND (file)  |
                         |----------------------->|
                         |(10) (MSRP) 200 OK      |
                         |<-----------------------|
                         |                        |
                         |(11) (SIP) BYE          |
                         |<-----------------------|
                         |(12) (SIP) 200 OK       |
                         |----------------------->|
                         |                        |
                         |                        |

     Figure 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.












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   INVITE sip:bob@example.com SIP/2.0
   To: Bob <sip:bob@example.com>
   From: Alice <sip:alice@example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 1 INVITE
   Max-Forwards: 70
   Date: Sun, 21 May 2006 13:02:03 GMT
   Contact: <sip:alice@alicepc.example.com>
   Content-Type: application/sdp
   Content-Length: [length of SDP]

   v=0
   o=alice 2890844526 2890844526 IN IP4 alicepc.example.com
   s=
   c=IN IP4 alicepc.example.com
   t=0 0
   m=message 7654 TCP/MSRP *
   a=recvonly
   a=accept-types: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:














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   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.




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   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.






































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   INVITE sip:bob@example.com SIP/2.0
   To: Bob <sip:bob@example.com>;tag=1928323431
   From: Alice <sip:alice@example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 2 INVITE
   Max-Forwards: 70
   Date: Sun, 21 May 2006 13:02:33 GMT
   Contact: <sip:alice@alicepc.example.com>
   Content-Type: multipart/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




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      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.















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   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.






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                   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.




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   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 [RFC3261] and
   MSRP [RFC4975] 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
   [RFC3261] and MSRP [RFC4975].

   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 [RFC4566], follows.





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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



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      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








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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

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2045]    Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part One: Format of Internet Message
                Bodies", RFC 2045, November 1996.

   [RFC2183]    Troost, R., Dorner, S., and K. Moore, "Communicating
                Presentation Information in Internet Messages: The
                Content-Disposition Header Field", RFC 2183,
                August 1997.






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   [RFC2387]    Levinson, E., "The MIME Multipart/Related Content-type",
                RFC 2387, August 1998.

   [RFC2392]    Levinson, E., "Content-ID and Message-ID Uniform
                Resource Locators", RFC 2392, August 1998.

   [RFC3174]    Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
                (SHA1)", RFC 3174, September 2001.

   [RFC3264]    Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
                with Session Description Protocol (SDP)", RFC 3264,
                June 2002.

   [RFC3629]    Yergeau, F., "UTF-8, a transformation format of ISO
                10646", STD 63, RFC 3629, November 2003.

   [RFC3851]    Ramsdell, B., "Secure/Multipurpose Internet Mail
                Extensions (S/MIME) Version 3.1 Message Specification",
                RFC 3851, July 2004.

   [RFC3862]    Klyne, G. and D. Atkins, "Common Presence and Instant
                Messaging (CPIM): Message Format", RFC 3862,
                August 2004.

   [RFC4566]    Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
                Description Protocol", RFC 4566, July 2006.

   [RFC4975]    Campbell, B., Mahy, R., and C. Jennings, "The Message
                Session Relay Protocol (MSRP)", RFC 4975,
                September 2007.

   [RFC5234]    Crocker, D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5322]    Resnick, P., Ed., "Internet Message Format", RFC 5322,
                October 2008.

13.2.  Informative References

   [RFC3261]    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.

   [RFC4028]    Donovan, S. and J. Rosenberg, "Session Timers in the
                Session Initiation Protocol (SIP)", RFC 4028,
                April 2005.




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   [RFC4483]    Burger, E., "A Mechanism for Content Indirection in
                Session Initiation Protocol (SIP) Messages", RFC 4483,
                May 2006.

   [RFC4976]    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.





































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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) [RFC4483].  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 [RFC3264]), 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



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   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.





























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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






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