From 4bfd864f10b68b71482b35c818559068ef8d5797 Mon Sep 17 00:00:00 2001
From: Thomas Voss <mail@thomasvoss.com>
Date: Wed, 27 Nov 2024 20:54:24 +0100
Subject: doc: Add RFC documents

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+Network Working Group                                   M. Garcia-Martin
+Request for Comments: 5547                                      Ericsson
+Category: Standards Track                                     M. Isomaki
+                                                                   Nokia
+                                                            G. Camarillo
+                                                               S. Loreto
+                                                                Ericsson
+                                                              P. Kyzivat
+                                                           Cisco Systems
+                                                                May 2009
+
+
+      A Session Description Protocol (SDP) Offer/Answer Mechanism
+                        to Enable File Transfer
+
+Status of This Memo
+
+   This document specifies an Internet standards track protocol for the
+   Internet community, and requests discussion and suggestions for
+   improvements.  Please refer to the current edition of the "Internet
+   Official Protocol Standards" (STD 1) for the standardization state
+   and status of this protocol.  Distribution of this memo is unlimited.
+
+Copyright Notice
+
+   Copyright (c) 2009 IETF Trust and the persons identified as the
+   document authors.  All rights reserved.
+
+   This document is subject to BCP 78 and the IETF Trust's Legal
+   Provisions Relating to IETF Documents in effect on the date of
+   publication of this document (http://trustee.ietf.org/license-info).
+   Please review these documents carefully, as they describe your rights
+   and restrictions with respect to this document.
+
+Abstract
+
+   This document provides a mechanism to negotiate the transfer of one
+   or more files between two endpoints by using the Session Description
+   Protocol (SDP) offer/answer model specified in RFC 3264.  SDP is
+   extended to describe the attributes of the files to be transferred.
+   The offerer can describe either the files it wants to send or the
+   files it would like to receive.  The answerer can either accept or
+   reject the offer separately for each individual file.  The transfer
+   of one or more files is initiated after a successful negotiation.
+   The Message Session Relay Protocol (MSRP) is defined as the default
+   mechanism to actually carry the files between the endpoints.  The
+   conventions on how to use MSRP for file transfer are also provided in
+   this document.
+
+
+
+Garcia-Martin, et al.       Standards Track                     [Page 1]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+Table of Contents
+
+   1. Introduction ....................................................3
+   2. Terminology .....................................................4
+   3. Definitions .....................................................4
+   4. Overview of Operation ...........................................5
+   5. File Selector ...................................................6
+   6. Extensions to SDP ...............................................7
+   7. File Disposition Types .........................................13
+   8. Protocol Operation .............................................13
+      8.1. The 'file-transfer-id' Attribute ..........................14
+      8.2. Offerer's Behavior ........................................17
+           8.2.1. The Offerer Is a File Sender .......................17
+           8.2.2. The Offerer Is a File Receiver .....................18
+           8.2.3. SDP Offer for Several Files ........................18
+      8.3. Answerer's Behavior .......................................19
+           8.3.1. The Answerer Is a File Receiver ....................19
+           8.3.2. The Answerer Is a File Sender ......................20
+      8.4. Aborting an Ongoing File Transfer Operation ...............22
+      8.5. Indicating File Transfer Offer/Answer Capability ..........25
+      8.6. Reusage of Existing "m=" Lines in SDP .....................26
+      8.7. MSRP Usage ................................................26
+      8.8. Considerations about the 'file-icon' Attribute ............28
+   9. Examples .......................................................28
+      9.1. Offerer Sends a File to the Answerer ......................28
+      9.2. Offerer Requests a File from the Answerer and
+           Second File Transfer ......................................33
+      9.3. Example of a Capability Indication ........................40
+   10. Security Considerations .......................................41
+   11. IANA Considerations ...........................................42
+      11.1. Registration of New SDP Attributes .......................42
+           11.1.1. Registration of the file-selector Attribute .......43
+           11.1.2. Registration of the file-transfer-id Attribute ....43
+           11.1.3. Registration of the file-disposition Attribute ....43
+           11.1.4. Registration of the file-date Attribute ...........44
+           11.1.5. Registration of the file-icon Attribute ...........44
+           11.1.6. Registration of the file-range Attribute ..........45
+   12. Acknowledgments ...............................................45
+   13. References ....................................................45
+      13.1. Normative References .....................................45
+      13.2. Informative References ...................................46
+   Appendix A.  Alternatives Considered ..............................48
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                     [Page 2]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+1.  Introduction
+
+   The Session Description Protocol (SDP) offer/answer [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.
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                     [Page 3]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   o  It must be possible for the recipient to request a file from the
+      sender, providing meta information about the file.  The sender
+      must be able to decide whether to send a file matching the
+      request.
+
+   The rest of this document is organized as follows.  Section 3 defines
+   a few terms used in this document.  Section 4 provides the overview
+   of operation.  Section 5 introduces the concept of the file selector.
+   The detailed syntax and semantics of the new SDP attributes and
+   conventions on how the existing ones are used are defined in
+   Section 6.  Section 7 discusses the file disposition types.
+   Section 8 describes the protocol operation involving SDP and MSRP.
+   Finally, some examples are given in Section 9.
+
+2.  Terminology
+
+   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+   document are to be interpreted as described in BCP 14, RFC 2119
+   [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.
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                     [Page 4]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   Push operation:  A file transfer operation where the SDP offerer
+      takes the role of the file sender and the SDP answerer takes the
+      role of the file receiver.
+
+   Pull operation:  A file transfer operation where the SDP offerer
+      takes the role of the file receiver and the SDP answerer takes the
+      role of the file sender.
+
+4.  Overview of Operation
+
+   An SDP offerer creates an SDP body that contains the description of
+   one or more files that the offerer wants to send or receive.  The
+   offerer sends the SDP offer to the remote endpoint.  The SDP answerer
+   can accept or reject the transfer of each of those files separately.
+
+   The actual file transfer is carried out using the Message Session
+   Relay Protocol (MSRP) [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
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                     [Page 5]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   place with ordinary MSRP.  Note that the 'sendonly' and 'recvonly'
+   attributes refer to the direction of MSRP SEND requests and do not
+   preclude other protocol elements (such as 200 responses, REPORT
+   requests, etc.).
+
+      In principle the file transfer can work even with an endpoint
+      supporting only regular MSRP without understanding the extensions
+      defined herein, in a particular case where that endpoint is both
+      the SDP answerer and the file receiver.  The regular MSRP endpoint
+      answers the offer as it would answer any ordinary MSRP offer
+      without paying attention to the extension attributes.  In such a
+      scenario, the user experience would, however, be reduced, since
+      the recipient would not know (by any protocol means) the reason
+      for the session and would not be able to accept/reject it based on
+      the file attributes.
+
+5.  File Selector
+
+   When the file receiver generates the SDP offer, this SDP offer needs
+   to unambiguously identify the requested file at the file sender.  For
+   this purpose, we introduce the notion of a file selector, which is a
+   tuple composed of one or more of the following individual selectors:
+   the name, size, type, and hash of the file.  The file selector can
+   include any number of selectors, so all four of them do not always
+   need to be present.
+
+   The purpose of the file selector is to provide enough information
+   about the file to the remote entity, so that both the local and the
+   remote entity can refer to the same file.  The file selector is
+   encoded in a 'file-selector' media attribute in SDP.  The formal
+   syntax of the 'file-selector' media attribute is described in
+   Figure 1.
+
+   The file selection process is applied to all the available files at
+   the host.  The process selects those files that match each of the
+   selectors present in the 'file-selector' attribute.  The result can
+   be zero, one, or more files, depending on the presence of the
+   mentioned selectors in the SDP and depending on the available files
+   in a host.  The file transfer mechanism specified in this document
+   requires that a file selector eventually results at most in a single
+   file to be chosen.  Typically, if the hash selector is known, it is
+   enough to produce a file selector that points to exactly zero or one
+   file.  However, a file selector that selects a unique file is not
+   always known by the offerer.  Sometimes only the name, size, or type
+   of file is known, so the file selector may result in selecting more
+   than one file, which is an undesired case.  The opposite is also
+   true: if the file selector contains a hash selector and a name
+   selector, there is a risk that the remote host has renamed the file,
+
+
+
+Garcia-Martin, et al.       Standards Track                     [Page 6]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   in which case, although a file whose computed hash equals the hash
+   selector exists, the file name does not match that of the name
+   selector.  Thus, in this case, the file selection process will result
+   in the selection of zero files.
+
+   This specification uses the Secure Hash Algorithm 1, SHA-1 [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
+
+
+
+Garcia-Martin, et al.       Standards Track                     [Page 7]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   filesize-value       = integer        ;integer defined in RFC 4566
+
+   filetype-selector    = "type:" type "/" subtype *(";" ft-parameter)
+   ft-parameter         = attribute "=" DQUOTE value-string DQUOTE
+                                      ; attribute is defined in RFC 2045
+                        ; free insertion of linear-white-space is not
+                        ; permitted in this context.
+                        ; note: value-string has to be re-encoded
+                        ; when translating between this and a
+                        ; Content-Type header.
+   value-string         = filename-string
+
+   hash-selector        = "hash:" hash-algorithm ":" hash-value
+   hash-algorithm       = token     ; see IANA Hash Function
+                                    ; Textual Names registry
+                                    ; only "sha-1" currently supported
+   hash-value           = 2HEXDIG *(":" 2HEXDIG)
+                                ; Each byte in upper-case hex, separated
+                                ; by colons.
+                                ; HEXDIG defined in RFC 5234
+
+   file-tr-id-attr      = "file-transfer-id:" file-tr-id-value
+   file-tr-id-value     = token
+
+   file-disp-attr       = "file-disposition:" file-disp-value
+   file-disp-value      = token
+
+   file-date-attr       = "file-date:"  date-param *(SP date-param)
+
+   date-param           = c-date-param / m-date-param / r-date-param
+   c-date-param         = "creation:" DQUOTE date-time DQUOTE
+   m-date-param         = "modification:" DQUOTE date-time DQUOTE
+   r-date-param         = "read:" DQUOTE date-time DQUOTE
+                             ; date-time is defined in RFC 5322
+                             ; numeric timezones (+HHMM or -HHMM)
+                             ; must be used
+                             ; DQUOTE defined in RFC 5234 files.
+
+   file-icon-attr       = "file-icon:" file-icon-value
+   file-icon-value      = cid-url        ; cid-url defined in RFC 2392
+
+   file-range-attr      = "file-range:" start-offset "-" stop-offset
+   start-offset         = integer        ; integer defined in RFC 4566
+   stop-offset          = integer / "*"
+
+                   Figure 1: Syntax of the SDP extension
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                     [Page 8]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   When used for capability query (see Section 8.5), the 'file-selector'
+   attribute MUST NOT contain any selector, because its presence merely
+   indicates compliance to this specification.
+
+   When used in an SDP offer or answer, the 'file-selector' attribute
+   MUST contain at least one selector.  Selectors characterize the file
+   to be transferred.  There are four selectors in this attribute:
+   'name', 'size', 'type', and 'hash'.
+
+   The 'name' selector in the 'file-selector' attribute contains the
+   filename of the content enclosed in double quotes.  The filename is
+   encoded in UTF-8 [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
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                     [Page 9]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   translating parameters from a Content-Type header and a 'type'
+   selector, the parameter has to be re-encoded prior to its
+   accommodation as a parameter of the 'type' selector (see the ABNF
+   syntax of 'ft-parameter').
+
+   The 'hash' selector in the 'file-selector' attribute provides a hash
+   computation of the file to be transferred.  This is commonly used by
+   file transfer protocols.  For example, FLUTE [FLUTE-REV] uses hashes
+   (called message digests) to verify the contents of the transfer.  The
+   purpose of the 'hash' selector is two-fold: On one side, in pull
+   operations, it allows the file receiver to identify a remote file by
+   its hash rather than by its file name, providing that the file
+   receiver has learned the hash of the remote file by some out-of-band
+   mechanism.  On the other side, in either push or pull operations, it
+   allows the file receiver to verify the contents of the received file,
+   or even avoid unnecessary transmission of an existing file.
+
+      The address space of the SHA-1 algorithm is big enough to avoid
+      any collision in hash computations in between two endpoints.  When
+      transferring files, the actual file transfer protocol should
+      provide reliable transmission of data, so verifications of
+      received files should always succeed.  However, if endpoints need
+      to protect the integrity of a file, they should use some other
+      mechanism than the 'hash' selector specified in this memo.
+
+   The 'hash' selector includes the hash algorithm and its value.
+   Possible hash algorithms are those defined in the IANA registry of
+   Hash Function Textual Names [IANA].  Implementations according to
+   this specification MUST add a 160-bit string resulting from the
+   computation of US Secure Hash Algorithm 1 (SHA1) [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.
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 10]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   The 'file-disposition' attribute provides a suggestion to the other
+   endpoint about the intended disposition of the file.  Section 7
+   provides further discussion of the possible values.  The value of
+   this attribute SHOULD be the same as the disposition type parameter
+   of the Content-Disposition header field [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.
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 11]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   The 'file-range' attribute provides a mechanism to signal a chunk of
+   a file rather than the complete file.  This enables use cases where a
+   file transfer can be interrupted and resumed, even perhaps changing
+   one of the endpoints.  The 'file-range' attribute contains the "start
+   offset" and "stop offset" of the file, separated by a dash "-".  The
+   "start offset" value refers to the octet position of the file where
+   the file transfer should start.  The first octet of a file is denoted
+   by the ordinal number "1".  The "stop offset" value refers to the
+   octet position of the file where the file transfer should stop,
+   inclusive of this octet.  The "stop offset" value MAY contain a "*"
+   if the total size of the file is not known in advance.  The absence
+   of this attribute indicates a complete file, i.e., as if the 'file-
+   range' attribute would have been present with a value "1-*".  The
+   'file-range' attribute must not be confused with the Byte-Range
+   header in MSRP.  The former indicates the portion of a file that the
+   application would read and pass onto the MSRP stack for
+   transportation.  From the point of view of MSRP, the portion of the
+   file is viewed as a whole message.  The latter indicates the number
+   of bytes of that message that are carried in a chunk and the total
+   size of the message.  Therefore, MSRP starts counting the delivered
+   message at octet number 1, independently of the position of that
+   octet in the file.
+
+   The following is an example of an SDP body that contains the
+   extensions defined in this memo:
+
+   v=0
+   o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com
+   s=
+   c=IN IP4 host.atlanta.example.com
+   t=0 0
+   m=message 7654 TCP/MSRP *
+   i=This is my latest picture
+   a=sendonly
+   a=accept-types:message/cpim
+   a=accept-wrapped-types:*
+   a=path:msrp://atlanta.example.com:7654/jshA7we;tcp
+   a=file-selector:name:"My cool picture.jpg" type:image/jpeg
+     size:32349 hash:sha-1:
+     72:24:5F:E8:65:3D:DA:F3:71:36:2F:86:D4:71:91:3E:E4:A2:CE:2E
+   a=file-transfer-id:vBnG916bdberum2fFEABR1FR3ExZMUrd
+   a=file-disposition:attachment
+   a=file-date:creation:"Mon, 15 May 2006 15:01:31 +0300"
+   a=file-icon:cid:id2@alicepc.example.com
+   a=file-range:1-32349
+
+            Figure 2: Example of SDP describing a file transfer
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 12]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+      NOTE: The 'file-selector' attribute in the above figure is split
+      in three lines for formatting purposes.  Real implementations will
+      encode it in a single line.
+
+7.  File Disposition Types
+
+   The SDP offer/answer for file transfer allows the file sender to
+   indicate a preferred disposition of the file to be transferred in a
+   new 'file-disposition' attribute.  In principle, any value listed in
+   the IANA registry for Mail Content Disposition Values [IANA] is
+   acceptable; however, most of them may not be applicable.
+
+   There are two content dispositions of interest for file transfer
+   operations.  On one hand, the file sender may just want the file to
+   be rendered immediately in the file receiver's device.  On the other
+   hand, the file sender may just want to indicate to the file receiver
+   that the file should not be rendered at the reception of the file.
+   The recipient's user agent may want to interact with the user
+   regarding the file disposition or it may save the file until the user
+   takes an action.  In any case, the exact actions are implementation
+   dependent.
+
+   To indicate that a file should be automatically rendered, this memo
+   uses the existing 'render' value of the Content Disposition type in
+   the new 'file-disposition' attribute in SDP.  To indicate that a file
+   should not be automatically rendered, this memo uses the existing
+   'attachment' value of the Content-Disposition type in the new 'file-
+   disposition' attribute in SDP.  The default value is 'render', i.e.,
+   the absence of a 'file-disposition' attribute in the SDP has the same
+   semantics as 'render'.
+
+      The disposition value 'attachment' is specified in RFC 2183
+      [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.
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 13]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   A file transfer session is initiated by the offerer sending an SDP
+   offer to the answerer.  The answerer either accepts or rejects the
+   file transfer session and sends an SDP answer to the offerer.
+
+   We can differentiate two use cases, depending on whether the offerer
+   is the file sender or file receiver:
+
+   1.  The offerer is the file sender, i.e., the offerer wants to
+       transmit a file to the answerer.  Consequently, the answerer is
+       the file receiver.  In this case, the SDP offer contains a
+       'sendonly' attribute, and accordingly the SDP answer contains a
+       'recvonly' attribute.
+
+   2.  The offerer is the file receiver, i.e., the offerer wants to
+       fetch a file from the answerer.  Consequently, the answerer is
+       the file sender.  In this case, the SDP offer contains a session
+       or media 'recvonly' attribute, and accordingly the SDP answer
+       contains a session or media 'sendonly' attribute.
+
+8.1.  The 'file-transfer-id' Attribute
+
+   This specification creates an extension to the SDP offer/answer model
+   [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.
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 14]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   In other cases, a file transfer was successfully completed.  Then, if
+   an endpoint resends the SDP offer with the media stream for the file
+   transfer, then the other endpoint wouldn't be able to determine
+   whether or not a new file transfer should start.
+
+   To address these scenarios, this specification defines the 'file-
+   transfer-id' attribute, which contains a globally unique random
+   identifier allocated to the file transfer operation.  The file
+   transfer identifier helps both endpoints to determine whether the SDP
+   offer is requesting a new file transfer or it is a repetition of the
+   SDP.  A new file transfer is one that, in case of acceptance, will
+   provoke the actual transfer of a file.  This is typically the case of
+   new offer/answer exchanges, or in cases where an endpoint wants to
+   abort the existing file transfer and restart the file transfer once
+   more.  On the other hand, the repetition of the SDP does not lead to
+   any actual file to be transferred, potentially because the file
+   transfer is still going on or because it has already finished.  This
+   is the case of repeated offer/answer exchanges, which can be due to a
+   number of reasons (session timer, addition/removal of other media
+   types in the SDP, update in SDP due to changes in other session
+   parameters, etc.).
+
+   Implementations according to this specification MUST include a 'file-
+   transfer-id' attribute in SDP offers and answers.  The SDP offerer
+   MUST select a file transfer identifier according to the syntax and
+   add it to the 'file-transfer-id' attribute.  The SDP answerer MUST
+   copy the value of the 'file-transfer-id' attribute in the SDP answer.
+
+   The file transfer identifier MUST be unique within the current
+   session (never used before in this session), and it is RECOMMENDED to
+   be unique across different sessions.  It is RECOMMENDED to select a
+   relatively big random identifier (e.g., 32 characters) to avoid
+   duplications.  The SDP answerer MUST keep track of the proposed file
+   transfer identifiers in each session and copy the value of the
+   received file transfer identifier in the SDP answer.
+
+   If a file transfer is suspended and resumed at a later time, the
+   resumption is considered a new file transfer (even when the file to
+   be transferred is the same); therefore, the SDP offerer MUST choose a
+   new file transfer identifier.
+
+   If an endpoint sets the port number to zero in the media description
+   of a file transfer, for example, because it wants to reject the file
+   transfer operation, then the SDP answer MUST mirror the value of the
+   'file-transfer-id' attribute included in the SDP offer.  This
+   effectively means that setting a media stream to zero has higher
+   precedence than any value that the 'file-transfer-id' attribute can
+   take.
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 15]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   As a side effect, the 'file-transfer-id' attribute can be used for
+   aborting and restarting again an ongoing file transfer.  Assume that
+   two endpoints agree on a file transfer and the actual transfer of the
+   file is taking place.  At some point in time in the middle of the
+   file transfer, one endpoint sends a new SDP offer, equal to the
+   initial one except for the value of the 'file-transfer-id' attribute,
+   which is a new globally unique random value.  This indicates that the
+   offerer wants to abort the existing transfer and start a new one,
+   according to the SDP parameters.  The SDP answerer SHOULD abort the
+   ongoing file transfer, according to the procedures of the file
+   transfer protocol (e.g., MSRP), and start sending file once more from
+   the initial requested octet.  Section 8.4 further discusses aborting
+   a file transfer.
+
+   If an endpoint creates an SDP offer where the 'file-transfer-id'
+   attribute value does not change with respect to a previously sent
+   one, but the file selector changes so that a new file is selected,
+   then this is considered an error, and the SDP answerer MUST abort the
+   file transfer operation (e.g., by setting the port number to zero in
+   the SDP answer).  Note that endpoints MAY change the 'file-selector'
+   attribute as long as the selected file does not change (e.g., by
+   adding a hash selector); however, it is RECOMMENDED that endpoints do
+   not change the value of the 'file-selector' attribute if it is
+   requested to transfer the same file described in a previous SDP
+   offer/answer exchange.
+
+   Figure 3 summarizes the relation of the 'file-transfer-id' attribute
+   with the file selector in subsequent SDP exchanges.
+
+                      \                |             |               |
+                       \ file selector |  different  |     same      |
+     'file-transfer-id' \              |    file     |     file      |
+     ==================================+=============+===============+
+                                       |  new file   |   new file    |
+      changed                          |  transfer   |   transfer    |
+                                       |  operation  |   operation   |
+     ----------------------------------+-------------+---------------+
+                                       |             | existing file |
+      unchanged                        |   error     |   transfer    |
+                                       |             |   operation   |
+     ----------------------------------+-------------+---------------+
+
+      Figure 3: Relation of the 'file-transfer-id' attribute with the
+             selector of the file in a subsequent SDP exchange
+
+   In another scenario, an endpoint that has successfully transferred a
+   file wants to send an SDP due to other reasons than the transfer of a
+   file.  The SDP offerer creates an SDP file description that maintains
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 16]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   the media description line corresponding to the file transfer.  The
+   SDP offerer MUST then set the port number to zero and MUST keep the
+   same value of the 'file-transfer-id' attribute that the initial file
+   transfer got.
+
+8.2.  Offerer's Behavior
+
+   An offerer who wishes to send or receive one or more files to or from
+   an answerer MUST build an SDP [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.
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 17]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   When the file sender receives the SDP answer, if the port number of
+   the answer for a file request is non-zero, the file sender starts the
+   transfer of the file according to the negotiated parameters in SDP.
+
+8.2.2.  The Offerer Is a File Receiver
+
+   In a pull operation, the file receiver creates the SDP offer and
+   sends it to the file sender.  The file receiver MUST include a 'file-
+   selector' attribute and MUST include, at least, one of the selectors
+   defined for such attribute (i.e., 'name', 'type', 'size', or 'hash').
+   In many cases, if the hash of the file is known, that is enough to
+   identify the file; therefore, the offerer can include only a 'hash'
+   selector.  However, particularly in cases where the hash of the file
+   is unknown, the file name, size, and type can provide a description
+   of the file to be fetched.  If the file receiver wishes to start a
+   new file transfer, it MUST add a 'file-transfer-id' attribute
+   containing a new globally unique random value.  The file receiver MAY
+   also add a 'file-range' attribute indicating the start and stop
+   offsets of the file.  There is no need for the file receiver to
+   include further file attributes in the SDP offer; thus, it is
+   RECOMMENDED that SDP offerers do not include any other file attribute
+   defined by this specification (other than the mandatory ones).
+   Additionally, the file receiver MUST add a session or media
+   'recvonly' attribute in the SDP offer.  Then, the file receiver sends
+   the SDP offer to the file sender.
+
+   When the file receiver receives the SDP answer, if the port number of
+   the answer for a file request is non-zero, then the file receiver
+   should receive the file using the protocol indicated in the "m="
+   line.  If the SDP answer contains a supported hashing algorithm in
+   the 'hash' selectors of the 'file-selector' attribute, then the file
+   receiver SHOULD compute the hash of the file after its reception and
+   check it against the hash received in the answer.  In case the
+   computed hash does not match the one contained in the SDP answer, the
+   file receiver SHOULD consider that the file transfer failed and
+   SHOULD inform the user.  Similarly, the file receiver SHOULD also
+   verify that the other selectors declared in the SDP match the file
+   properties, otherwise, the file receiver SHOULD consider that the
+   file transfer failed and SHOULD inform the user.
+
+8.2.3.  SDP Offer for Several Files
+
+   An offerer that wishes to send or receive more than one file
+   generates an "m=" line per file along with the file attributes
+   described in this specification.  This way, the answerer can reject
+   individual files by setting the port number of their associated "m="
+   lines to zero, as per regular SDP [RFC4566] procedures.  Similarly,
+   the answerer can accept each individual file separately by setting
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 18]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   the port number of their associated "m=" lines to non-zero value.
+   Each file has its own file transfer identifier, which uniquely
+   identifies each file transfer.
+
+   Using an "m=" line per file implies that different files are
+   transferred using different MSRP sessions.  However, all those MSRP
+   sessions can be set up to run over a single TCP connection, as
+   described in Section 8.1 of RFC 4975 [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
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 19]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   to the procedures specified in RFC 3264 [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
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 20]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   present).  The file selector identifies zero or more candidate files
+   to be sent.  If the file selector is unable to identify any file,
+   then the answerer MUST reject the MSRP stream for file transfer by
+   setting the port number to zero, and then the answerer SHOULD also
+   reject the SDP as per procedures in RFC 3264 [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
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 21]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   with the same range of values.  If the file sender does not accept
+   sending that range of octets, it SHOULD reject the transfer of the
+   file.
+
+8.4.  Aborting an Ongoing File Transfer Operation
+
+   Either the file sender or the file receiver can abort an ongoing file
+   transfer at any time.  Unless otherwise noted, the entity that aborts
+   an ongoing file transfer operation MUST follow the procedures at the
+   media level (e.g., MSRP) and at the signaling level (SDP offer/
+   answer), as described below.
+
+   Assume the scenario depicted in Figure 4 where a file sender wishes
+   to abort an ongoing file transfer without initiating an alternative
+   file transfer.  Assume that an ongoing MSRP SEND request is being
+   transmitted.  The file sender aborts the MSRP message by including
+   the '#' character in the continuation field of the end-line of a SEND
+   request, according to the MSRP procedures (see Section 7.1 of RFC
+   4975 [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.
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 22]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+                        File sender            File receiver
+                            |                        |
+                            |\                       |
+                            | \                      |
+                            |  \                     |
+                            |   \                    |
+                            |    \                   |
+                            |     \                  |
+                     abort->|      \  MSRP SEND (#)  |
+                            |       +--------------->|
+                            | MSRP 200               |
+                            |<-----------------------|
+                            | re-INVITE (SDP offer)  |
+                            |----------------------->|
+                            | SIP 200 OK (SDP answer)|
+                            |<-----------------------|
+                            | SIP ACK                |
+                            |----------------------->|
+                            |                        |
+
+           Figure 4: File sender aborts an ongoing file transfer
+
+   When the file receiver wants to abort the file transfer, there are
+   two possible scenarios, depending on the value of the Failure-Report
+   header in the ongoing MSRP SEND request.  Assume now the scenario
+   depicted in Figure 5 where the MSRP SEND request includes a Failure-
+   Report header set to a value different than "no".  When the file
+   receiver wishes to abort the ongoing file transfer, the file receiver
+   generates an MSRP 413 response to the current MSRP SEND request (see
+   Section 10.5 of RFC 4975 [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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 23]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+                     File sender            File receiver
+                         |                        |
+                         |\                       |
+                         | \  MSRP SEND           |
+                         |  \ Failure-Report: yes |
+                         |   \                    |
+                         |    \                   |
+                         |     \                  |
+                         |      \                 |
+                         |       \                |
+                         |        \               |
+                         | MSRP 413               |<-abort
+                         |<-----------------------|
+                         |          \   (#)       |
+                         |           +----------->|
+                         | re-INVITE (SDP offer)  |
+                         |<-----------------------|
+                         | SIP 200 OK (SDP answer)|
+                         |----------------------->|
+                         | SIP ACK                |
+                         |<-----------------------|
+                         |                        |
+
+    Figure 5: File receiver aborts an ongoing file transfer.  Failure-
+             Report set to a value different than "no" in MSRP
+
+   In another scenario, depicted in Figure 6, an ongoing file transfer
+   is taking place, where the MSRP SEND request contains a Failure-
+   Report header set to the value "no".  When the file receiver wants to
+   abort the ongoing transfer, it MUST close the MSRP session by
+   generating a new SDP offer that sets the port number to zero in the
+   related "m=" line that describes the file transfer (see Section 8.2
+   of RFC 3264 [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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 24]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+                     File sender            File receiver
+                         |                        |
+                         |\                       |
+                         | \  MSRP SEND           |
+                         |  \ Failure-Report: no  |
+                         |   \                    |
+                         |    \                   |
+                         |     \                  |
+                         |      \                 |
+                         |       \                |
+                         |        \               |
+                         | re-INVITE (SDP offer)  |<-abort
+                         |<-----------------------|
+                         |          \   (#)       |
+                         |           +----------->|
+                         | MSRP 200               |
+                         |<-----------------------|
+                         | SIP 200 OK (SDP answer)|
+                         |----------------------->|
+                         | SIP ACK                |
+                         |<-----------------------|
+                         |                        |
+
+    Figure 6: File receiver aborts an ongoing file transfer.  Failure-
+                        Report set to "no" in MSRP
+
+   A file sender that receives an SDP offer setting the port number to
+   zero in the related "m=" line for file transfer, first, if an ongoing
+   MSRP SEND request is being transmitted, aborts the MSRP message by
+   including the '#' character in the continuation field of the end-line
+   of a SEND request, according to the MSRP procedures (see Section 7.1
+   of RFC 4975 [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
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 25]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   an "m=" line containing the media type (message, for MSRP).  An
+   endpoint that implements the procedures specified in this document
+   SHOULD also add a 'file-selector' media attribute for the "m=message"
+   line.  The 'file-selector' media attribute MUST be empty, i.e., it
+   MUST NOT contain any selector.  The endpoint MUST NOT add any of the
+   other file attributes defined in this specification.
+
+8.6.  Reusage of Existing "m=" Lines in SDP
+
+   The SDP offer/answer model [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.
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 26]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   Additionally, implementations according to this specification MUST
+   include a single file in a single MSRP message.  Notice that the MSRP
+   specification defines "MSRP message" as a complete unit of MIME or
+   text content, which can be split and delivered in more than one MSRP
+   request; each of these portions of the complete message is called a
+   "chunk".  So, it is still valid to send a file in several chunks, but
+   from the MSRP point of view, all the chunks together form an MSRP
+   message: the Common Presence and Instant Messaging (CPIM) message
+   that wraps the file.  When chunking is used, it should be noticed
+   that MSRP does not require to wait for a 200-class response for a
+   chunk before sending the following one.  Therefore, it is valid to
+   send pipelined MSRP SEND requests containing chunks of the same MSRP
+   message (the file).  Section 9.1 contains an example of a file
+   transfer using pipelined MSRP requests.
+
+   The MSRP specification [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
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 27]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   session management is not related to TCP connection management.  As a
+   matter of fact, MSRP allows multiple MSRP sessions to share the same
+   TCP connection.
+
+8.8.  Considerations about the 'file-icon' Attribute
+
+   This specification allows a file sender to include a small preview of
+   an image file: an icon.  A 'file-icon' attribute contains a
+   Content-ID (CID) URL [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.
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 28]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   Alice, the SDP offerer, wishes to send an image file to Bob (the
+   answerer).  Alice's User Agent Client (UAC) creates a unidirectional
+   SDP offer that contains the description of the file that she wants to
+   send to Bob's User Agent Server (UAS).  The description also includes
+   an icon representing the contents of the file to be transferred.  The
+   sequence flow is shown in Figure 7.
+
+                   Alice's UAC                 Bob's UAS
+                         |                        |
+                         |(1) (SIP) INVITE        |
+                         |----------------------->|
+                         |(2) (SIP) 200 OK        |
+                         |<-----------------------|
+                         |(3) (SIP) ACK           |
+                         |----------------------->|
+                         |                        |
+                         |(4) (MSRP) SEND (chunk) |
+                         |----------------------->|
+                         |(5) (MSRP) SEND (chunk) |
+                         |----------------------->|
+                         |(6) (MSRP) 200 OK       |
+                         |<-----------------------|
+                         |(7) (MSRP) 200 OK       |
+                         |<-----------------------|
+                         |                        |
+                         |(8) (SIP) BYE           |
+                         |----------------------->|
+                         |(9) (SIP) 200 OK        |
+                         |<-----------------------|
+                         |                        |
+                         |                        |
+
+    Figure 7: Flow diagram of an offerer sending a file to an answerer
+
+   F1: Alice constructs an SDP description of the file to be sent and
+   attaches it to a SIP INVITE request addressed to Bob.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 29]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   INVITE sip:bob@example.com SIP/2.0
+   To: Bob <sip:bob@example.com>
+   From: Alice <sip:alice@example.com>;tag=1928301774
+   Call-ID: a84b4c76e66710
+   CSeq: 1 INVITE
+   Max-Forwards: 70
+   Date: Sun, 21 May 2006 13:02:03 GMT
+   Contact: <sip:alice@alicepc.example.com>
+   Content-Type: multipart/related; type="application/sdp";
+                 boundary="boundary71"
+   Content-Length: [length]
+
+   --boundary71
+   Content-Type: application/sdp
+   Content-Length: [length of SDP]
+
+   v=0
+   o=alice 2890844526 2890844526 IN IP4 alicepc.example.com
+   s=
+   c=IN IP4 alicepc.example.com
+   t=0 0
+   m=message 7654 TCP/MSRP *
+   i=This is my latest picture
+   a=sendonly
+   a=accept-types:message/cpim
+   a=accept-wrapped-types:*
+   a=path:msrp://alicepc.example.com:7654/jshA7we;tcp
+   a=file-selector:name:"My cool picture.jpg" type:image/jpeg
+     size:4092 hash:sha-1:
+     72:24:5F:E8:65:3D:DA:F3:71:36:2F:86:D4:71:91:3E:E4:A2:CE:2E
+   a=file-transfer-id:Q6LMoGymJdh0IKIgD6wD0jkcfgva4xvE
+   a=file-disposition:render
+   a=file-date:creation:"Mon, 15 May 2006 15:01:31 +0300"
+   a=file-icon:cid:id2@alicepc.example.com
+
+   --boundary71
+   Content-Type: image/jpeg
+   Content-Transfer-Encoding: binary
+   Content-ID: <id2@alicepc.example.com>
+   Content-Length: [length of image]
+   Content-Disposition: icon
+
+   [...small preview icon of the file...]
+
+   --boundary71--
+
+    Figure 8: INVITE request containing an SDP offer for file transfer
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 30]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+      NOTE: The Content-Type header field and the 'file-selector'
+      attribute in the above figure are split in several lines for
+      formatting purposes.  Real implementations will encode it in a
+      single line.
+
+   From now on we omit the SIP details for the sake of brevity.
+
+   F2: Bob receives the INVITE request, inspects the SDP offer and
+   extracts the icon body, checks the creation date and file size, and
+   decides to accept the file transfer.  So Bob creates the following
+   SDP answer:
+
+   v=0
+   o=bob 2890844656 2890844656 IN IP4 bobpc.example.com
+   s=
+   c=IN IP4 bobpc.example.com
+   t=0 0
+   m=message 8888 TCP/MSRP *
+   a=recvonly
+   a=accept-types:message/cpim
+   a=accept-wrapped-types:*
+   a=path:msrp://bobpc.example.com:8888/9di4ea;tcp
+   a=file-selector:name:"My cool picture.jpg" type:image/jpeg
+     size:4092 hash:sha-1:
+     72:24:5F:E8:65:3D:DA:F3:71:36:2F:86:D4:71:91:3E:E4:A2:CE:2E
+   a=file-transfer-id:Q6LMoGymJdh0IKIgD6wD0jkcfgva4xvE
+
+      Figure 9: SDP answer accepting the SDP offer for file transfer
+
+      NOTE: The 'file-selector' attribute in the above figure is split
+      in three lines for formatting purposes.  Real implementations will
+      encode it in a single line.
+
+   F4: Alice opens a TCP connection to Bob and creates an MSRP SEND
+   request.  This SEND request contains the first chunk of the file.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 31]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   MSRP d93kswow SEND
+   To-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
+   From-Path: msrp://alicepc.example.com:7654/iau39;tcp
+   Message-ID: 12339sdqwer
+   Byte-Range: 1-2048/4385
+   Content-Type: message/cpim
+
+   To: Bob <sip:bob@example.com>
+   From: Alice <sip:alice@example.com>
+   DateTime: 2006-05-15T15:02:31-03:00
+   Content-Disposition: render; filename="My cool picture.jpg";
+                      creation-date="Mon, 15 May 2006 15:01:31 +0300";
+                      size=4092
+   Content-Type: image/jpeg
+
+   ... first set of bytes of the JPEG image ...
+   -------d93kswow+
+
+   Figure 10: MSRP SEND request containing the first chunk of actual
+   file
+
+   F5: Alice sends the second and last chunk.  Note that MSRP allows to
+   send pipelined chunks, so there is no need to wait for the 200 (OK)
+   response from the previous chunk.
+
+   MSRP op2nc9a SEND
+   To-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
+   From-Path: msrp://alicepc.example.com:7654/iau39;tcp
+   Message-ID: 12339sdqwer
+   Byte-Range: 2049-4385/4385
+   Content-Type: message/cpim
+
+   ... second set of bytes of the JPEG image ...
+   -------op2nc9a$
+
+   Figure 11: MSRP SEND request containing the second chunk of actual
+   file
+
+   F6: Bob acknowledges the reception of the first chunk.
+
+   MSRP d93kswow 200 OK
+   To-Path: msrp://alicepc.example.com:7654/iau39;tcp
+   From-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
+   Byte-Range: 1-2048/4385
+   -------d93kswow$
+
+                      Figure 12: MSRP 200 OK response
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 32]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   F7: Bob acknowledges the reception of the second chunk.
+
+   MSRP op2nc9a 200 OK
+   To-Path: msrp://alicepc.example.com:7654/iau39;tcp
+   From-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
+   Byte-Range: 2049-4385/4385
+   -------op2nc9a$
+
+                      Figure 13: MSRP 200 OK response
+
+   F8: Alice terminates the SIP session by sending a SIP BYE request.
+
+   F9: Bob acknowledges the reception of the BYE request and sends a 200
+   (OK) response.
+
+9.2.  Offerer Requests a File from the Answerer and Second File Transfer
+
+   In this example, Alice, the SDP offerer, first wishes to fetch a file
+   from Bob, the SDP answerer.  Alice knows that Bob has a specific file
+   she wants to download.  She has learned the hash of the file by some
+   out-of-band mechanism.  The hash selector is enough to produce a file
+   selector that points to the specific file.  So, Alice creates an SDP
+   offer that contains the file descriptor.  Bob accepts the file
+   transfer and sends the file to Alice.  When Alice has completely
+   received Bob's file, she intends to send a new image file to Bob.
+   Therefore, Alice reuses the existing SDP media line with different
+   attributes and updates the description of the new file she wants to
+   send to Bob's User Agent Server (UAS).  In particular, Alice creates
+   a new file transfer identifier since this is a new file transfer
+   operation.  Figure 14 shows the sequence flow.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 33]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+                   Alice's UAC                 Bob's UAS
+                         |                        |
+                         |(1) (SIP) INVITE        |
+                         |----------------------->|
+                         |(2) (SIP) 200 OK        |
+                         |<-----------------------|
+                         |(3) (SIP) ACK           |
+                         |----------------------->|
+                         |                        |
+                         |(4) (MSRP) SEND (file)  |
+                         |<-----------------------|
+                         |(5) (MSRP) 200 OK       |
+                         |----------------------->|
+                         |                        |
+                         |(6) (SIP) INVITE        |
+                         |----------------------->|
+                         |(7) (SIP) 200 OK        |
+                         |<-----------------------|
+                         |(8) (SIP) ACK           |
+                         |----------------------->|
+                         |                        |
+                         |(9) (MSRP) SEND (file)  |
+                         |----------------------->|
+                         |(10) (MSRP) 200 OK      |
+                         |<-----------------------|
+                         |                        |
+                         |(11) (SIP) BYE          |
+                         |<-----------------------|
+                         |(12) (SIP) 200 OK       |
+                         |----------------------->|
+                         |                        |
+                         |                        |
+
+     Figure 14: Flow diagram of an offerer requesting a file from the
+              answerer and then sending a file to the answer
+
+   F1: Alice constructs an SDP description of the file she wants to
+   receive and attaches the SDP offer to a SIP INVITE request addressed
+   to Bob.
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 34]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   INVITE sip:bob@example.com SIP/2.0
+   To: Bob <sip:bob@example.com>
+   From: Alice <sip:alice@example.com>;tag=1928301774
+   Call-ID: a84b4c76e66710
+   CSeq: 1 INVITE
+   Max-Forwards: 70
+   Date: Sun, 21 May 2006 13:02:03 GMT
+   Contact: <sip:alice@alicepc.example.com>
+   Content-Type: application/sdp
+   Content-Length: [length of SDP]
+
+   v=0
+   o=alice 2890844526 2890844526 IN IP4 alicepc.example.com
+   s=
+   c=IN IP4 alicepc.example.com
+   t=0 0
+   m=message 7654 TCP/MSRP *
+   a=recvonly
+   a=accept-types:message/cpim
+   a=accept-wrapped-types:*
+   a=path:msrp://alicepc.example.com:7654/jshA7we;tcp
+   a=file-selector:hash:sha-1:
+     72:24:5F:E8:65:3D:DA:F3:71:36:2F:86:D4:71:91:3E:E4:A2:CE:2E
+   a=file-transfer-id:aCQYuBRVoUPGVsFZkCK98vzcX2FXDIk2
+
+    Figure 15: INVITE request containing an SDP offer for file transfer
+
+      NOTE: The 'file-selector' attribute in the above figure is split
+      in two lines for formatting purposes.  Real implementations will
+      encode it in a single line.
+
+   From now on we omit the SIP details for the sake of brevity.
+
+   F2: Bob receives the INVITE request, inspects the SDP offer, computes
+   the file descriptor, and finds a local file whose hash equals the one
+   indicated in the SDP.  Bob accepts the file transfer and creates an
+   SDP answer as follows:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 35]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   v=0
+   o=bob 2890844656 2890855439 IN IP4 bobpc.example.com
+   s=
+   c=IN IP4 bobpc.example.com
+   t=0 0
+   m=message 8888 TCP/MSRP *
+   a=sendonly
+   a=accept-types:message/cpim
+   a=accept-wrapped-types:*
+   a=path:msrp://bobpc.example.com:8888/9di4ea;tcp
+   a=file-selector:type:image/jpeg hash:sha-1:
+     72:24:5F:E8:65:3D:DA:F3:71:36:2F:86:D4:71:91:3E:E4:A2:CE:2E
+   a=file-transfer-id:aCQYuBRVoUPGVsFZkCK98vzcX2FXDIk2
+
+      Figure 16: SDP answer accepting the SDP offer for file transfer
+
+      NOTE: The 'file-selector' attribute in the above figure is split
+      in two lines for formatting purposes.  Real implementations will
+      encode it in a single line.
+
+   F4: Alice opens a TCP connection to Bob.  Bob then creates an MSRP
+   SEND request that contains the file.
+
+
+   MSRP d93kswow SEND
+   To-Path: msrp://alicepc.example.com:7654/jshA7we;tcp
+   From-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
+   Message-ID: 12339sdqwer
+   Byte-Range: 1-2027/2027
+   Content-Type: message/cpim
+
+   To: Bob <sip:bob@example.com>
+   From: Alice <sip:alice@example.com>
+   DateTime: 2006-05-15T15:02:31-03:00
+   Content-Disposition: render; filename="My cool photo.jpg";
+                  creation-date="Mon, 15 May 2006 15:01:31 +0300";
+                  modification-date="Mon, 15 May 2006 16:04:53 +0300";
+                  read-date="Mon, 16 May 2006 09:12:27 +0300";
+                  size=1931
+   Content-Type: image/jpeg
+
+   ...binary JPEG image...
+   -------d93kswow$
+
+          Figure 17: MSRP SEND request containing the actual file
+
+   F5: Alice acknowledges the reception of the SEND request.
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 36]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   MSRP d93kswow 200 OK
+   To-Path: msrp://bobpc.example.com:8888/9di4ea;tcp
+   From-Path: msrp://alicepc.example.com:7654/jshA7we;tcp
+   Byte-Range: 1-2027/2027
+   -------d93kswow$
+
+                      Figure 18: MSRP 200 OK response
+
+   F6: Alice reuses the existing SDP media line inserting the
+   description of the file to be sent and attaches it to a SIP re-INVITE
+   request addressed to Bob.  Alice reuses the TCP port number for the
+   MSRP stream, but changes the MSRP session and the 'file-transfer-id'
+   value according to this specification.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 37]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   INVITE sip:bob@example.com SIP/2.0
+   To: Bob <sip:bob@example.com>;tag=1928323431
+   From: Alice <sip:alice@example.com>;tag=1928301774
+   Call-ID: a84b4c76e66710
+   CSeq: 2 INVITE
+   Max-Forwards: 70
+   Date: Sun, 21 May 2006 13:02:33 GMT
+   Contact: <sip:alice@alicepc.example.com>
+   Content-Type: multipart/related; type="application/sdp";
+                 boundary="boundary71"
+   Content-Length: [length of multipart]
+
+   --boundary71
+   Content-Type: application/sdp
+   Content-Length: [length of SDP]
+
+   v=0
+   o=alice 2890844526 2890844527 IN IP4 alicepc.example.com
+   s=
+   c=IN IP4 alicepc.example.com
+   t=0 0
+   m=message 7654 TCP/MSRP *
+   i=This is my latest picture
+   a=sendonly
+   a=accept-types:message/cpim
+   a=accept-wrapped-types:*
+   a=path:msrp://alicepc.example.com:7654/iau39;tcp
+   a=file-selector:name:"sunset.jpg" type:image/jpeg
+     size:4096 hash:sha-1:
+     58:23:1F:E8:65:3B:BC:F3:71:36:2F:86:D4:71:91:3E:E4:B1:DF:2F
+   a=file-transfer-id:ZVE8MfI9mhAdZ8GyiNMzNN5dpqgzQlCO
+   a=file-disposition:render
+   a=file-date:creation:"Sun, 21 May 2006 13:02:15 +0300"
+   a=file-icon:cid:id3@alicepc.example.com
+
+   --boundary71
+   Content-Type: image/jpeg
+   Content-Transfer-Encoding: binary
+   Content-ID: <id3@alicepc.example.com>
+   Content-Length: [length of image]
+   Content-Disposition: icon
+
+   [..small preview icon...]
+
+   --boundary71--
+
+           Figure 19: Reuse of the SDP in a second file transfer
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 38]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+      NOTE: The Content-Type header field and the 'file-selector'
+      attribute in the above figure are split in several lines for
+      formatting purposes.  Real implementations will encode it in a
+      single line.
+
+   F7: Bob receives the re-INVITE request, inspects the SDP offer and
+   extracts the icon body, checks the creation date and the file size,
+   and decides to accept the file transfer.  So Bob creates an SDP
+   answer where he reuses the same TCP port number, but changes his MSRP
+   session, according to the procedures of this specification.
+
+   v=0
+   o=bob 2890844656 2890855440 IN IP4 bobpc.example.com
+   s=
+   c=IN IP4 bobpc.example.com
+   t=0 0
+   m=message 8888 TCP/MSRP *
+   a=recvonly
+   a=accept-types:message/cpim
+   a=accept-wrapped-types:*
+   a=path:msrp://bobpc.example.com:8888/eh10dsk;tcp
+   a=file-selector:name:"sunset.jpg" type:image/jpeg
+     size:4096 hash:sha-1:
+     58:23:1F:E8:65:3B:BC:F3:71:36:2F:86:D4:71:91:3E:E4:B1:DF:2F
+   a=file-transfer-id:ZVE8MfI9mhAdZ8GyiNMzNN5dpqgzQlCO
+   a=file-disposition:render
+
+      Figure 20: SDP answer accepting the SDP offer for file transfer
+
+      NOTE: The 'file-selector' attribute in the above figure is split
+      in three lines for formatting purposes.  Real implementations will
+      encode it in a single line.
+
+   F9: If a TCP connection towards Bob is already open, Alice reuses
+   that TCP connection to send an MSRP SEND request that contains the
+   file.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 39]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   MSRP d95ksxox SEND
+   To-Path: msrp://bobpc.example.com:8888/eh10dsk;tcp
+   From-Path: msrp://alicepc.example.com:7654/iau39;tcp
+   Message-ID: 13449sdqwer
+   Byte-Range: 1-2027/2027
+   Content-Type: message/cpim
+
+   To: Bob <sip:bob@example.com>
+   From: Alice <sip:alice@example.com>
+   DateTime: 2006-05-21T13:02:15-03:00
+   Content-Disposition: render; filename="Sunset.jpg";
+                      creation-date="Sun, 21 May 2006 13:02:15 -0300";
+                      size=1931
+   Content-Type: image/jpeg
+
+   ...binary JPEG image...
+   -------d95ksxox+
+
+          Figure 21: MSRP SEND request containing the actual file
+
+   F10: Bob acknowledges the reception of the SEND request.
+
+   MSRP d95ksxox 200 OK
+   To-Path: msrp://alicepc.example.com:7654/iau39;tcp
+   From-Path: msrp://bobpc.example.com:8888/eh10dsk;tcp
+   Byte-Range: 1-2027/2027
+   -------d95ksxox$
+
+                      Figure 22: MSRP 200 OK response
+
+   F11: Then Bob terminates the SIP session by sending a SIP BYE
+   request.
+
+   F12: Alice acknowledges the reception of the BYE request and sends a
+   200 (OK) response.
+
+9.3.  Example of a Capability Indication
+
+   Alice sends an OPTIONS request to Bob (this request does not contain
+   SDP).  Bob answers with a 200 (OK) response that contain the SDP
+   shown in Figure 24.  The SDP indicates support for CPIM messages that
+   can contain other MIME types.  The maximum MSRP message size that the
+   endpoint can receive is 20000 octets.  The presence of the 'file-
+   selector' attribute indicates support for the file transfer offer/
+   answer mechanism.
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 40]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+                   Alice's UAC                 Bob's UAS
+                         |                        |
+                         |(1) (SIP) OPTIONS       |
+                         |----------------------->|
+                         |(2) (SIP) 200 OK        |
+                         |          with SDP      |
+                         |<-----------------------|
+                         |                        |
+                         |                        |
+
+              Figure 23: Flow diagram of a capability request
+
+   v=0
+   o=bob 2890844656 2890855439 IN IP4 bobpc.example.com
+   s=-
+   c=IN IP4 bobpc.example.com
+   t=0 0
+   m=message 0 TCP/MSRP *
+   a=accept-types:message/cpim
+   a=accept-wrapped-types:*
+   a=max-size:20000
+   a=file-selector
+
+       Figure 24: SDP of the 200 (OK) response to an OPTIONS request
+
+10.  Security Considerations
+
+   The SDP attributes defined in this specification identify a file to
+   be transferred between two endpoints.  An endpoint can offer to send
+   the file to the other endpoint or request to receive the file from
+   the other endpoint.  In the former case, an attacker modifying those
+   SDP attributes could cheat the receiver making it think that the file
+   to be transferred was a different one.  In the latter case, the
+   attacker could make the sender send a different file than the one
+   requested by the receiver.  Consequently, it is RECOMMENDED that
+   integrity protection be applied to the SDP session descriptions
+   carrying the attributes specified in this specification.
+   Additionally, it is RECOMMENDED that senders verify the properties of
+   the file against the selectors that describe it.
+
+   The descriptions of the files being transferred between endpoints may
+   reveal information the endpoints may consider confidential.
+   Therefore, it is RECOMMENDED that SDP session descriptions carrying
+   the attributes specified in this specification are encrypted.
+
+   TLS and S/MIME are the natural choices to provide offer/answer
+   exchanges with integrity protection and confidentiality.
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 41]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   When an SDP offer contains the description of a file to be sent or
+   received, the SDP answerer MUST first authenticate the SDP offerer
+   and then it MUST authorize the file transfer operation, typically
+   according to a local policy.  Typically, these functions are
+   integrated in the high-level protocol that carries SDP (e.g., SIP),
+   and in the file transfer protocol (e.g., MSRP).  If SIP [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.
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 42]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+11.1.1.  Registration of the file-selector Attribute
+
+   Contact:  Miguel Garcia <miguel.a.garcia@ericsson.com>
+
+   Phone:  +34 91 339 1000
+
+   Attribute name:  file-selector
+
+   Long-form attribute name:  File Selector
+
+   Type of attribute:  media level only
+      This attribute is subject to the charset attribute
+
+   Description:  This attribute unambiguously identifies a file by
+      indicating a combination of the 4-tuple composed of the name,
+      size, type, and hash of the file.
+
+   Specification:  RFC 5547
+
+11.1.2.  Registration of the file-transfer-id Attribute
+
+   Contact:  Miguel Garcia <miguel.a.garcia@ericsson.com>
+
+   Phone:  +34 91 339 1000
+
+   Attribute name:  file-transfer-id
+
+   Long-form attribute name:  File Transfer Identifier
+
+   Type of attribute:  media level only
+      This attribute is subject to the charset attribute
+
+   Description:  This attribute contains a unique identifier of the file
+      transfer operation within the session.
+
+   Specification:  RFC 5547
+
+11.1.3.  Registration of the file-disposition Attribute
+
+   Contact:  Miguel Garcia <miguel.a.garcia@ericsson.com>
+
+   Phone:  +34 91 339 1000
+
+   Attribute name:  file-disposition
+
+   Long-form attribute name:  File Disposition
+
+   Type of attribute:  media level only
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 43]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+      This attribute is not subject to the charset attribute
+
+   Description:  This attribute provides a suggestion to the other
+      endpoint about the intended disposition of the file.
+
+   Specification:  RFC 5547
+
+11.1.4.  Registration of the file-date Attribute
+
+   Contact:  Miguel Garcia <miguel.a.garcia@ericsson.com>
+
+   Phone:  +34 91 339 1000
+
+   Attribute name:  file-date
+
+   Long-form attribute name:
+
+   Type of attribute:  media level only
+      This attribute is not subject to the charset attribute
+
+   Description:  This attribute indicates the dates on which the file
+      was created, modified, or last read.
+
+   Specification:  RFC 5547
+
+11.1.5.  Registration of the file-icon Attribute
+
+   Contact:  Miguel Garcia <miguel.a.garcia@ericsson.com>
+
+   Phone:  +34 91 339 1000
+
+   Attribute name:  file-icon
+
+   Long-form attribute name:  File Icon
+
+   Type of attribute:  media level only
+      This attribute is not subject to the charset attribute
+
+   Description:  For image files, this attribute contains a pointer to a
+      body that includes a small preview icon representing the contents
+      of the file to be transferred.
+
+   Specification:  RFC 5547
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 44]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+11.1.6.  Registration of the file-range Attribute
+
+   Contact:  Miguel Garcia <miguel.a.garcia@ericsson.com>
+
+   Phone:  +34 91 339 1000
+
+   Attribute name:  file-range
+
+   Long-form attribute name:  File Range
+
+   Type of attribute:  media level only
+      This attribute is not subject to the charset attribute
+
+   Description:  This attribute contains the range of transferred octets
+      of the file.
+
+   Specification:  RFC 5547
+
+12.  Acknowledgments
+
+   The authors would like to thank Mats Stille, Nancy Greene, Adamu
+   Haruna, and Arto Leppisaari for discussing initial concepts described
+   in this memo.  Thanks to Pekka Kuure for reviewing initial versions
+   of this document and providing helpful comments.  Joerg Ott, Jiwey
+   Wang, Amitkumar Goel, Sudha Vs, Dan Wing, Juuso Lehtinen, Remi Denis-
+   Courmont, Colin Perkins, Sudhakar An, Peter Saint-Andre, Jonathan
+   Rosenberg, Eric Rescorla, Vikram Chhibber, Ben Campbell, Richard
+   Barnes, and Chris Newman discussed and provided comments and
+   improvements to this document.
+
+13.  References
+
+13.1.  Normative References
+
+   [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.
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 45]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   [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.
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 46]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   [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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 47]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+Appendix A.  Alternatives Considered
+
+   The requirements are related to the description and negotiation of
+   the session, not to the actual file transfer mechanism.  Thus, it is
+   natural that in order to meet them it is enough to define attribute
+   extensions and usage conventions to SDP, while MSRP itself needs no
+   extensions and can be used as it is.  This is effectively the
+   approach taken in this specification.  Another goal has been to
+   specify the SDP extensions in such a way that a regular MSRP endpoint
+   that does not support them could still in some cases act as an
+   endpoint in a file transfer session, albeit with a somewhat reduced
+   functionality.
+
+   In some ways, the aim of this specification is similar to the aim of
+   content indirection mechanism in the Session Initiation Protocol
+   (SIP) [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
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 48]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+   ways.  The MMUSIC working group considered a few alternative
+   approaches before deciding to use the encoding described in
+   Section 6.  In particular, the working group looked at the MIME
+   'external-body' type and the use of a single SDP attribute or
+   parameter.
+
+   A MIME 'external-body' could potentially be used to describe the file
+   to be transferred.  In fact, many of the SDP parameters this
+   specification defines are also supported by 'external-body' body
+   parts.  The MMUSIC working group decided not to use 'external-body'
+   body parts because a number of existing offer/answer implementations
+   do not support multipart bodies.
+
+   The information carried in the SDP attributes defined in Section 6
+   could potentially be encoded in a single SDP attribute.  The MMUSIC
+   working group decided not to follow this approach because it is
+   expected that implementations support only a subset of the parameters
+   defined in Section 6.  Those implementations will be able to use
+   regular SDP rules in order to ignore non-supported SDP parameters.
+   If all the information was encoded in a single SDP attribute, those
+   rules, which relate to backwards compatibility, would need to be
+   redefined specifically for that parameter.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 49]
+
+RFC 5547           SDP Offer/Answer for File Transfer           May 2009
+
+
+Authors' Addresses
+
+   Miguel A. Garcia-Martin
+   Ericsson
+   Calle Via de los Poblados 13
+   Madrid, ES  28033
+   Spain
+
+   EMail: miguel.a.garcia@ericsson.com
+
+
+   Markus Isomaki
+   Nokia
+   Keilalahdentie 2-4
+   Espoo  02150
+   Finland
+
+   EMail: markus.isomaki@nokia.com
+
+
+   Gonzalo Camarillo
+   Ericsson
+   Hirsalantie 11
+   Jorvas  02420
+   Finland
+
+   EMail: Gonzalo.Camarillo@ericsson.com
+
+
+   Salvatore Loreto
+   Ericsson
+   Hirsalantie 11
+   Jorvas  02420
+   Finland
+
+   EMail: Salvatore.Loreto@ericsson.com
+
+
+   Paul H. Kyzivat
+   Cisco Systems
+   1414 Massachusetts Avenue
+   Boxborough, MA  01719
+   USA
+
+   EMail: pkyzivat@cisco.com
+
+
+
+
+
+
+Garcia-Martin, et al.       Standards Track                    [Page 50]
+
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