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Network Working Group                                   M. Garcia-MartinRequest for Comments: 5547                                      EricssonCategory: Standards Track                                     M. Isomaki                                                                   Nokia                                                            G. Camarillo                                                               S. Loreto                                                                Ericsson                                                              P. Kyzivat                                                           Cisco Systems                                                                May 2009A Session Description Protocol (SDP) Offer/Answer Mechanismto Enable File TransferStatus 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 toBCP 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 inRFC 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 2009Table of Contents1. Introduction ....................................................32. Terminology .....................................................43. Definitions .....................................................44. Overview of Operation ...........................................55. File Selector ...................................................66. Extensions to SDP ...............................................77. File Disposition Types .........................................138. Protocol Operation .............................................138.1. The 'file-transfer-id' Attribute ..........................148.2. Offerer's Behavior ........................................178.2.1. The Offerer Is a File Sender .......................178.2.2. The Offerer Is a File Receiver .....................188.2.3. SDP Offer for Several Files ........................188.3. Answerer's Behavior .......................................198.3.1. The Answerer Is a File Receiver ....................198.3.2. The Answerer Is a File Sender ......................208.4. Aborting an Ongoing File Transfer Operation ...............228.5. Indicating File Transfer Offer/Answer Capability ..........258.6. Reusage of Existing "m=" Lines in SDP .....................268.7. MSRP Usage ................................................268.8. Considerations about the 'file-icon' Attribute ............289. Examples .......................................................289.1. Offerer Sends a File to the Answerer ......................28      9.2. Offerer Requests a File from the Answerer and           Second File Transfer ......................................339.3. Example of a Capability Indication ........................4010. Security Considerations .......................................4111. IANA Considerations ...........................................4211.1. Registration of New SDP Attributes .......................4211.1.1. Registration of the file-selector Attribute .......4311.1.2. Registration of the file-transfer-id Attribute ....4311.1.3. Registration of the file-disposition Attribute ....4311.1.4. Registration of the file-date Attribute ...........4411.1.5. Registration of the file-icon Attribute ...........4411.1.6. Registration of the file-range Attribute ..........4512. Acknowledgments ...............................................4513. References ....................................................4513.1. Normative References .....................................4513.2. Informative References ...................................46Appendix A.  Alternatives Considered ..............................48Garcia-Martin, et al.       Standards Track                     [Page 2]

RFC 5547           SDP Offer/Answer for File Transfer           May 20091.  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 inSection 6.Section 7 discusses the file disposition types.Section 8 describes the protocol operation involving SDP and MSRP.   Finally, some examples are given inSection 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 inBCP 14,RFC 2119   [RFC2119].3.  Definitions   For the purpose of this document, the following definitions specified   inRFC 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 inSection 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 takesGarcia-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 inSection 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], andRFC 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 inRFC 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 inRFC 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 inRFC 5234   filesize-selector    = "size:" filesize-valueGarcia-Martin, et al.       Standards Track                     [Page 7]

RFC 5547           SDP Offer/Answer for File Transfer           May 2009   filesize-value       = integer        ;integer defined inRFC 4566   filetype-selector    = "type:" type "/" subtype *(";" ft-parameter)   ft-parameter         = attribute "=" DQUOTE value-string DQUOTE                                      ; attribute is defined inRFC 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 inRFC 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 inRFC 5322                             ; numeric timezones (+HHMM or -HHMM)                             ; must be used                             ; DQUOTE defined inRFC 5234 files.   file-icon-attr       = "file-icon:" file-icon-value   file-icon-value      = cid-url        ; cid-url defined inRFC 2392   file-range-attr      = "file-range:" start-offset "-" stop-offset   start-offset         = integer        ; integer defined inRFC 4566   stop-offset          = integer / "*"                   Figure 1: Syntax of the SDP extensionGarcia-Martin, et al.       Standards Track                     [Page 8]

RFC 5547           SDP Offer/Answer for File Transfer           May 2009   When used for capability query (seeSection 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 (seeRFC 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.  WhenGarcia-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 inSection 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 inRFC 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 inRFC 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 inRFC 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 inRFC 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 transferGarcia-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 inRFC 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 inSection 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 maintainsGarcia-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 settingGarcia-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 inSection 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 inRFC 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 accordingGarcia-Martin, et al.       Standards Track                    [Page 19]

RFC 5547           SDP Offer/Answer for File Transfer           May 2009   to the procedures specified inRFC 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 inRFC 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 areGarcia-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 inRFC 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 inRFC 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 answerGarcia-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 ofRFC4975 [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 (seeSection 8.2 of RFC 3264   [RFC3264]).  This SDP offer MUST conform with the requirements ofSection 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 ofSection 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 (seeSection 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 (seeSection 8.2 of RFC 3264 [RFC3264]).  This SDP offer   MUST conform with the requirements expressed inSection 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 (seeSection 8.2   of RFC 3264 [RFC3264]).  This SDP offer MUST conform with the   requirements expressed inSection 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 (seeSection 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 ofSection 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 ofRFC3264 [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 containsGarcia-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 inRFC 3264   [RFC3264], in particular those defined inSection 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   inSection 8.2 andSection 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 inRFC4975 [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 inSection 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 MSRPGarcia-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 toRFC 2387 [RFC2387] is identified as the first body in   the multipart/related MIME wrapper or explicitly identified by the   'start' parameter.  According toRFC 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.  Examples9.1.  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 transferGarcia-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 responseGarcia-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 transferGarcia-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 request10.  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 toRFC 4566 [RFC4566], follows.Garcia-Martin, et al.       Standards Track                    [Page 42]

RFC 5547           SDP Offer/Answer for File Transfer           May 200911.1.1.  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 554711.1.2.  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 554711.1.3.  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 onlyGarcia-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 554711.1.4.  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 554711.1.5.  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 5547Garcia-Martin, et al.       Standards Track                    [Page 44]

RFC 5547           SDP Offer/Answer for File Transfer           May 200911.1.6.  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 554712.  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.  References13.1.  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 2009Appendix 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 differentGarcia-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 inSection 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 inSection 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 inSection 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 2009Authors' 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.comGarcia-Martin, et al.       Standards Track                    [Page 50]