MMUSIC Working Group M. Garcia-Martin Internet-Draft M. Isomaki Intended status: Standards Track Nokia Expires: June 21, 2007 G. Camarillo S. Loreto Ericsson December 18, 2006 A Session Description Protocol (SDP) Offer/Answer Mechanism to Enable File Transfer draft-ietf-mmusic-file-transfer-mech-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on June 21, 2007. Copyright Notice Copyright (C) The IETF Trust (2006). Abstract This document provides a mechanism to negotiate the transfer of one or more files between two endpoints by using the Session Description Protocol (SDP) offer/answer model specified in RFC 3264. SDP is extended to describe the attributes of the files to be transferred. Garcia-Martin, et al. Expires June 21, 2007 [Page 1] Internet-Draft File Transfer with SDP offer/answer December 2006 The offerer can either describe the files it wants to send, or the files it would like to receive. The answerer can either accept or reject the offer separately for each individual file . The transfer of one or more files is initiated after a successful negotiation. The Message Session Relay Protocol (MSRP) is defined as the default mechanism to actually carry the files between the endpoints. The conventions on how to use MSRP for file transfer are also provided in this document. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Alternatives Considered . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7 5. File selector . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Extensions to SDP . . . . . . . . . . . . . . . . . . . . . . 9 7. File Disposition Types . . . . . . . . . . . . . . . . . . . . 13 8. Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 14 8.1. Offerer's Behavior . . . . . . . . . . . . . . . . . . . . 15 8.1.1. The Offerer is a File Sender . . . . . . . . . . . . . 15 8.1.2. The Offerer is a File Receiver . . . . . . . . . . . . 15 8.1.3. SDP Offer for Several Files . . . . . . . . . . . . . 16 8.2. Answerer's Behavior . . . . . . . . . . . . . . . . . . . 16 8.2.1. The Answerer is a File Receiver . . . . . . . . . . . 16 8.2.2. The Answerer is a File Sender . . . . . . . . . . . . 17 8.3. Re-usage of Existing m= Lines in SDP . . . . . . . . . . . 18 8.4. MSRP Usage . . . . . . . . . . . . . . . . . . . . . . . . 19 9. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 9.1. Offerer sends a file to the Answerer . . . . . . . . . . . 19 9.2. Offerer requests a file from the Answerer and second file transfer . . . . . . . . . . . . . . . . . . . . . . 23 10. Security Considerations . . . . . . . . . . . . . . . . . . . 29 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 11.1. Registration of new SDP attributes . . . . . . . . . . . . 29 11.1.1. Registration of the file-selector attribute . . . . . 30 11.1.2. Registration of the disposition attribute . . . . . . 30 11.1.3. Registration of the file-date attribute . . . . . . . 30 11.1.4. Registration of the icon attribute . . . . . . . . . . 30 11.1.5. Registration of the file-range attribute . . . . . . . 31 11.2. Registration of new Content Disposition value . . . . . . 31 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32 13.1. Normative References . . . . . . . . . . . . . . . . . . . 32 13.2. Informational References . . . . . . . . . . . . . . . . . 32 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33 Garcia-Martin, et al. Expires June 21, 2007 [Page 2] Internet-Draft File Transfer with SDP offer/answer December 2006 Intellectual Property and Copyright Statements . . . . . . . . . . 35 Garcia-Martin, et al. Expires June 21, 2007 [Page 3] Internet-Draft File Transfer with SDP offer/answer December 2006 1. Introduction The Session Description Protocol (SDP) Offer/Answer [7] provides a mechanism for two endpoints to arrive at a common view of a multimedia session between them. These sessions often contain real- time media streams such as voice and video, but are not limited to that. Basically, any media component type can be supported, as long as there is a specification how to negotiate it within the SDP offer/ answer exchange. The Message Session Relay Protocol (MSRP) [12] is a protocol for transmitting instant messages (IM) in the context of a session. The protocol specification includes a description how to use it with SDP. In addition to plain text messages, MSRP is able to carry arbitrary (binary) Multipurpose Internet Mail Extensions (MIME) [2] compliant content, such as images or video clips. There are many cases where the endpoints involved in a multimedia session would like to exchange files within the context of that session. With MSRP it is possible to embed files as MIME objects inside the stream of instant messages. MSRP also has other features that are useful for file transfer. Message chunking enables the sharing of the same transport connection between the transfer of a large file and interactive IM exchange without blocking the IM. MSRP relays [15] provide a mechanism for Network Address Translator (NAT) traversal. Finally, Secure MIME (S/MIME) [8] can be used for ensuring the integrity and confidentiality of the transfered content. However, the baseline MSRP does not readily meet all the requirements for file transfer services within multimedia sessions. There are four main missing features: o The recipient MUST be able to distinguish "file transfer" from "file attached to IM", allowing the recipient to treat the cases differently. o It MUST be possible for the sender to send the request for a file transfer. It MUST be possible for the recipient to accept or decline it, using the meta information in the request. The actual transfer MUST take place only after acceptance by the recipient. o It MUST be possible for the sender to pass some meta information on the file before the actual transfer. This MUST be able to include at least content type, size, hash and name of the file, as well as a short (human readable) description. o It MUST be possible for the recipient to request a file from the sender, providing meta information about the file. The sender MUST be able to decide whether to send a file matching the request. Garcia-Martin, et al. Expires June 21, 2007 [Page 4] Internet-Draft File Transfer with SDP offer/answer December 2006 The rest of this document is organized as follows. Section 3 defines a few terms used in this document. Section 4 provides the overview of operation. The detailed syntax and semantics of the new SDP attributes and conventions on how the existing ones are used is defined in Section 6. Section 8 describes the protocol operation involving SDP and MSRP. Finally, some examples are given in Section 9. 1.1. Alternatives Considered The requirements are related to the description and negotiation of the session, not to the actual file transfer mechanism. Thus, it is natural that in order to meet them it is enough to define attribute extensions and usage conventions to SDP, while MSRP itself needs no extensions and can be used as it is. This is effectively the approach taken in this specification. Another goal has been to specify the SDP extensions in such a way that a regular MSRP endpoint which does not support them could still in some cases act as an endpoint in a file transfer session, albeit with a somewhat reduced functionality. In some ways the aim of this specification is similar to the aim of content indirection mechanism in the Session Initiation Protocol (SIP) [14]. Both mechanisms allow a user agent to decide whether or not to download a file based on information about the file. However, there are some differences. With content indirection, it is not possible for the other endpoint to explicitly accpet or reject the file transfer. Also, it is not possible for an endpoint to request a file from another endpoint. Furthermore, content indirection is not tied to the context of a media session, which is sometimes a desirable property. Finally, content indirection typically requires some server infrastructure, which may not always be available. (It is possible to use content indirection directly between the endpoints too, but in that case there is no definition for how it works for endpoints behind NATs.) Based on the argumentation above, this document defines the SDP attribute extensions and usage conventions needed for meeting the requirements on file transfer services with the SDP offer/answer model, using MSRP as the transfer protocol within the session. In principle it is possible to use the SDP extensions defined here and replace MSRP with any other similar protocol that can carry MIME objects. This kind of specification can be written as a separate document if the need arises. This specification defines a set of SDP attributes that describe a file to be transfered between two endpoits. The information needed Garcia-Martin, et al. Expires June 21, 2007 [Page 5] Internet-Draft File Transfer with SDP offer/answer December 2006 to describe a file could be potentially encoded in a few different ways. The MMUSIC working group considered a few alternative approaches before deciding to use the encoding described in Section 6. In particular, the working group looked at the MIME 'external-body' type and the use of a single SDP parameter. A MIME 'external-body' could potentially be used to describe the file to be transfered. In fact, many of the SDP parameters this specification defines are also supported by 'external-body' body parts. The MMUSIC working group decided not to use 'external-body' body parts because a number of existing offer/answer implementations do not support multipart bodies. The information carried in the SDP attributes defined in Section 6 could potentially be encoded in a single SDP attribute. The MMUSIC working group decided not to follow this approach because it is expected that implementations support only a subset of the parameters defined in Section 6. Those implementations will be able to use regular SDP rules in order to ignore non-supported SDP parameters. If all the information was encoded in a single SDP attribute, those rules, which relate to backwards compatibility, would need to be redefined specifically for that parameter. 2. Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 [1] and indicate requirement levels for compliant implementations. 3. Definitions For the purpose of this document, the following definitions specified in RFC 3264 [7] apply: o Answerer o Offerer Additionally, we define the following terms: File sender: The endpoint that is willing to send a file to the file receiver. Garcia-Martin, et al. Expires June 21, 2007 [Page 6] Internet-Draft File Transfer with SDP offer/answer December 2006 File receiver: The endpoint that is willing to receive a file from the file sender. File selector: A tuple of file attributes that the SDP offerer includes in the SDP in order to select a file at the SDP answerer. This is described in more detail in Section 5. 4. Overview of Operation An SDP offerer creates an SDP body that contains the description of one or more files that the offerer wants to send or receive. The offerer sends the SDP offer to the remote endpoint. The SDP answerer can accept or reject the transfer of each of those files. The actual file transfer is carried out using the Message Session Relay Protocol (MSRP) [12]. Each SDP "m=" line describes an MSRP media stream used to transfer a single file. That is, the transfer of multiple simultaneous files requires multiple "m=" lines and corresponding MSRP media streams. It should be noted that multiple MSRP media streams can share a single transport layer connection, so this mechanism will not lead to excessive use of transport resources. Each "m=" line for an MSRP media stream is accompanied with a few attributes describing the file to be transferred. If the file sender generates the SDP offer, the attributes describe a local file to be sent (push), and the file receiver can use this information to either accept or reject the transfer. However, if the SDP offer is generated by the file receiver, the attributes are intended to characterize a particular file that the file receiver is willing to get (pull) from the file sender. It is possible that the file sender does not have a matching file or does not want to send the file, in which case the offer is rejected. The attributes describing each file are provided in SDP by a set of new SDP attributes, most of which have been directly borrowed from MIME. This way, user agents can decide whether or not to accept a given file transfer based on the file's name, size, description, hash, icon (e.g., if the file is a picture), etc. SDP direction attributes (e.g., 'sendonly', 'recvonly') are used to indicate the direction of the transfer, i.e., whether the SDP offerer is willing to send of receive the file. Assuming that the answerer accepts the file transfer, the actual transfer of the files takes place with ordinary MSRP. Garcia-Martin, et al. Expires June 21, 2007 [Page 7] Internet-Draft File Transfer with SDP offer/answer December 2006 In principle the file transfer can work even with an endpoint supporting only regular MSRP without understanding the extensions defined herein, in a special case where that endpoint is the recipient of the file. The regular MSRP endpoint answers the offer as it would answer any ordinary MSRP offer without paying attention to the extension attributes. In such a scenario the user experience would however be reduced, as the recipient would not know (by any protocol means) the reason for the session and would not be able to accept/reject it based on the file attributes. 5. File selector Specially in case the SDP offer is generated by the file receiver, the offer needs a mechanism to unambiguously identify the requested file. For this purpose, the file transfer mechanism introduces the concept of a file selector, which is defined as the combination of the 4-tuple composed of the name, size, type, and hash of the file. We call each of these individual items a selector. The purpose of the file selector is to provide enough information that characterizes a file to the remote entity, so that both the local and the remote entity can refer to the same file. The file selector is encoded in a 'file-selector' media attribute in SDP. The formal syntax of the 'file-selector' media attribute is described in Figure 1. The file selection process is applied to all the available files at the host. The process selects those file that match each of the 4-tuple selectors present in the 'file-selector' attribute. Thus, a file selector can point to zero, one, or more files, depending on the presence of the mentioned selectors in the SDP and depending on the available files in a host. The file transfer mechanism specified in this document requires that a file selector eventually results at most in a single file to be chosen. Typically, if the hash selector is known, it is enough to produce a file selector that points to exactly zero or one file. However, a file selector that selects a unique file is not always known by the offerer. Sometimes only the name, size or type of file are known, so the file selector may result in selecting more than one file, which is an undesired case. The opposite is also true: if the file selector contains a hash and a name selectors, there is a risk that the remote host has renamed the file, although there is a file with the indicated hash, the file name does not match, thus, the file selector will result in the selection of zero files. Since there are several hashing algorithms, such as SHA-1 [6], SHA- Garcia-Martin, et al. Expires June 21, 2007 [Page 8] Internet-Draft File Transfer with SDP offer/answer December 2006 256, SHA-384, SHA-512 [11], etc., a file selector MAY contain several hashes, each one describing the hash of the file with a different hashing algorithm. Implementations that make use of the hash SHOULD select one among the supported ones before selecting a file. So, when several hashes are present in the SDP, the file selector consists of the union of the name, size, type, and any of the supported hash algorithms. Implementations according to this specification MUST implement the 'file-selector' attribute and MAY implement any of the other attributes specified in this specification. SDP offers and answers for file transfer MUST contain a 'file-selector' media attribute that selects the file to be transferred and MAY contain any of the other attributes specified in this specification. 6. Extensions to SDP We define a number of new SDP [10] attributes that provide the required information to describe the transfer of a file with MSRP. These are all media level only attributes in SDP. The following is the formal ABNF syntax [9] of these new attributes. It is built above the SDP [10] grammar, RFC 2045 [2], RFC 2183 [3], and RFC 2392 [4]. Garcia-Martin, et al. Expires June 21, 2007 [Page 9] Internet-Draft File Transfer with SDP offer/answer December 2006 attribute = file-selector-attr / disposition-attr / file-date-attr / icon-attr / file-range-attr ;attribute is defined in RFC 4566 file-selector-attr = "file-selector:" (selector) *(SP selector) selector = filename-selector / filesize-selector / filetype-selector / hash-selector filename-selector = "name:" DQUOTE filename-string DQUOTE ; DQUOTE defined in RFC 4234 filename-string = byte-string ;byte-string defined in RFC 4566 filesize-selector = "size:" filesize-value filesize-value = integer ;integer defined in RFC 4566 filetype-selector = "type:" type "/" subtype *(";"parameter) ; parameter defined in RFC 2045 type = token subtype = token hash-selector = "hash:" hash-algorithm ":" hash-value hash-algorithm = token ;see IANA Hash Function ;Textual Names registry hash-value = hex-val ;hex-val defined in RFC 4234 disposition-attr = "disposition:" disposition-value disposition-value = token file-date = "file-date:" date-param *(SP date-param) date-param = c-date-param / m-date-param / r-date-param c-date-param = "creation:" DQUOTE date-time DQUOTE m-date-param = "modification:" DQUOTE date-time DQUOTE r-date-param = "read:" DQUOTE date-time DQUOTE ; date-time is defined in RFC 2822 ; numeric timezones (+HHMM or -HHMM) ; must be used ; DQUOTE defined in RFC 4234 icon-attr = "icon:" icon-value icon-value = cid-url ;cid-url defined in RFC 2392 file-range-attr = "file-range:" integer "-" integer ;integer defined in RFC 4566 Figure 1: Syntax of the SDP extension Garcia-Martin, et al. Expires June 21, 2007 [Page 10] Internet-Draft File Transfer with SDP offer/answer December 2006 The 'file-selector' attribute is composed of one or more selectors which parametrize the file to be transferred. There are four selectors in this attribute: 'name', 'size', 'type', and 'hash'. The 'name' selector in the 'file-selector' attribute contains the filename of the content enclosed in double quotes. The filename is encoded in UTF-8. Its value SHOULD be the same as the 'filename' parameter of Content-Disposition header field [3] that could be signalled by the actual file transfer. The 'size' selector in the 'file-selector' attribute indicates the size of the file in octets. The value of this attribute SHOULD be the same of the 'size' parameter of Content-Disposition header field [3] that could be signalled by the actual file transfer. Note that the 'size' selector merely includes the file size, and does not include any potential overhead added by a wrapper, such as message/ cpim. The 'type' selector in the 'file-selector' attribute contains the MIME media type of the content. In general, anything that can be expressed in a Content-Type header field (see RFC 2045 [2]) can also be expressed with the 'type' selectors. Possible MIME Media Type values are the ones listed in the IANA registry for MIME Media Types [17]. Zero or more parameters can follow. The syntax of 'parameter' is specified in RFC 2045 [2] . The 'hash' selector in the 'file-selector' attribute provides a hash of the file to be transferred. This is commonly used by file transfer protocols. For example, FLUTE [16] uses hashes (called message digests) to verify the contents of the transfer. The purpose of the 'hash' selector is two-fold: On one side, it allows the file receiver to identify a file by its hash rather than by its file name, providing that the file receiver has learned the hash of the file by some out-of-band mechanism. On the other side, it allows the file sender to provide the hash of the file to be transmitted, which can be used by the file receiver for verification of its contents or to avoid the unnecessary transmission of a file that already exists. The 'hash' selector includes the hash algorithm and its value. In fact, since there are several hashing algorithms, the SDP MAY contain several 'hash' selectors with different algorithms. Possible hash algorithms are those defined in the IANA registry of Hash Function Textual Names [18]. Implementations according to this specification MUST support the US Secure Hash Algorithm 1 (SHA1) [6] and MAY support other hashing algorithms. The value is the byte string resulting of applying the hash algorithm to the content of the whole file. The 'disposition' attribute provides a suggestion to the other Garcia-Martin, et al. Expires June 21, 2007 [Page 11] Internet-Draft File Transfer with SDP offer/answer December 2006 endpoint about the intended disposition of the file. Section 7 provides further discussion of the possible values. The value of this attribute SHOULD be the same of the disposition type parameter of the Content-Disposition header field [3] that could be signalled by the actual file transfer. The 'file-date' attribute indicates the dates at which the file was created, modified, or last read. This attribute MAY contain a combination of the 'creation', 'modification', and 'read' parameters. Only one parameter of each type (creation, modification, or read) MUST be present in a 'file-date' attribute. The 'creation' parameter indicates the date at which the file was created. The value MUST be a quoted string which contains a representation of the creation date of the file in RFC 2822 [5] 'date-time' format. Numeric timezones (+HHMM or -HHMM) MUST be used. The value of this parameter SHOULD be the same of the 'creation-date' parameter of Content-Disposition header field [3] that could be signalled by the actual file transfer. The 'modification' parameter indicates the date at which the file was last modified. The value MUST be a quoted string which contains a representation of the last modification date to the file in RFC 2822 [5] 'date-time' format. Numeric timezones (+HHMM or -HHMM) MUST be used. The value of this parameter SHOULD be the same of the 'modification-date' parameter of Content-Disposition header field [3] that could be signalled by the actual file transfer. The 'read' parameter indicates the date at which the file was last read. The value MUST be a quoted string which contains a representation of the last date the file was read in RFC 2822 [5] 'date-time' format. Numeric timezones (+HHMM or -HHMM) MUST be used. The value of this parameter SHOULD be the same of the 'read-date' parameter of Content-Disposition header field [3] that could be signalled by the actual file transfer. The 'icon' attribute can be useful with certain file types such as images. It allows the sender to include a pointer to a body that includes a small preview icon representing the contents of the file to be transferred. This allows the sender to include the icon as another body accompanying the SDP, and to the recipient to get the icon of the file to be transferred. It is recommended to keep icons restricted to the minimum number of bytes that provide significance. The 'icon' attribute contains a Content-ID URL, which is specified in RFC 2392 [4]. The 'file-range' attribute provides a mechanism to signal a chunk of a file rather than the complete file. This enable use cases where a Garcia-Martin, et al. Expires June 21, 2007 [Page 12] Internet-Draft File Transfer with SDP offer/answer December 2006 file transfer can be interrupted, resumed, even perhaps changing one of the endpoints. The 'file-range' attribute is composed to two integer values separated by a dash "-". The first integer value refers to the first byte of the file to be transferred. The second integer value refers to the last byte of the file to be transferred. The first byte of a file is indicated with "1". The absence of this attribute indicates a complete file, i.e., like if the 'file-range' attribute would have been present with values 1-. The 'file-range' attribute must not be confused with the Byte-Range header in MSRP. The former indicates the portion of a file that the application would read and pass onto the MSRP stack for transportation. From the point of view of MSRP, the portion of the file is viewed as a whole message. The latter indicates the number of bytes of that message that are carried in a chunk and the total size of the message. Therefore, MSRP starts counting the delivered message at byte number 1, independently of position of that byte in the file. The following is an example of an SDP body that contains the extensions defined in this memo: v=0 o=alice 2890844526 2890844526 IN IP4 host.atlanta.example.com s= c=IN IP4 host.atlanta.example.com t=0 0 m=message 7654 TCP/MSRP * i=This is my latest picture a=sendonly a=accept-types:* a=path:msrp://atlanta.example.com:7654/jshA7we;tcp a=file-selector:name:"My cool picture.jpg" type:image/jpeg size:32349 hash:sha-1:72245FE8653DDAF371362F86D471913EE4A2CE2E a=disposition:not-render a=file-date:creation:"Mon, 15 May 2006 15:01:31 +03:00" a=icon:cid:id2@alicepc.example.com a=file-range:1-32349 Figure 2: Example of SDP describing a file transfer NOTE: The 'file-selector' attribute in the above figure is split in two lines for formatting purposes. Real implementations will encode it in a single line. 7. File Disposition Types The SDP Offer/Answer for file transfer allows the file sender to Garcia-Martin, et al. Expires June 21, 2007 [Page 13] Internet-Draft File Transfer with SDP offer/answer December 2006 indicate a preferred disposition of the file to be transferred in a new 'disposition' attribute. In principle, any value listed in the IANA registry for Mail Content Disposition Values [19] is acceptable, however, most of them may not be applicable. There are two content dispositions of interest for file transfer operations. On one hand, the file sender may just want the file to be rendered immediately in the file receiver's device. On the other hand, the file sender may just want to indicate the file recipient that the file should not be rendered at the reception of the file. The recipient's user agent may want to interact with the user regarding the file disposition or it may save the file until the user takes an action. In any case, the exact actions are implementation dependent. To indicate that a file should be automatically rendered, this memo uses the existing 'render' value of the Content Disposition type in the new 'disposition' attribute in SDP. To indicate that a file should not be automatically rendered, this memo defines a new value 'not-render' of the Content Disposition type. The default value is 'render', i.e., the absence of a 'disposition' attribute in the SDP has the same semantics as 'render'. 8. Protocol Operation This Section discusses how to use the parameters defined in Section 6 in the context of an offer/answer [7] exchange. Additionally, this section also discusses the behavior of the endpoints using MSRP. Usually the file transfer session is initiated when the offerer sends an SDP offer to the answerer. The answerer either accepts or rejects the file transfer session and sends an SDP answer to the offerer. We can differentiate two use cases, depending on whether the offerer is the file sender or file receiver: 1. The offerer is the file sender, i.e., the offerer wants to transmit a file to the answerer. Consequently the answerer is the file receiver. In this case the SDP offer contains a 'sendonly' attribute, and accordingly the SDP answer contains a 'recvonly' attribute. 2. The offerer is the file receiver, i.e., the offerer wants to fetch a file from the answerer. Consequently the answerer is the file sender. In this case the SDP offer contains a 'recvonly' attribute, and accordingly the SDP answer contains a 'sendonly' attribute. Garcia-Martin, et al. Expires June 21, 2007 [Page 14] Internet-Draft File Transfer with SDP offer/answer December 2006 8.1. Offerer's Behavior An offerer that wishes to send or receive one or more files to or from an answerer MUST build an SDP [10] description of a session containing one or more "m=" lines, each one describing an MSRP session (and thus, one file transfer operation), according to the MSRP [12] procedures. All the media line attributes specified and required by MSRP [12] (e.g., "a=path", "a=accept-types", etc.) MUST be included as well. For each file to be transferred there MUST be a separate "m=" line. 8.1.1. The Offerer is a File Sender In a push operation, the file sender creates and SDP offer describing the file to be sent. Then it sends the SDP offer to the file receiver. The file sender MUST add a 'file-selector' attribute media line containing at least one of the 'type', 'size', 'hash', parameters in indicating the type, size, or hash of the file, respectively. If the file sender is able to compute the hash of the file with different hashing algorithms, it MAY add several 'hash' parameters, each one referring to a different hashing algorithm. Additionally, the file sender MUST add a session or media 'sendonly' attribute to the SDP offer. Not all the selectors in the 'file-selector' attribute might be known when the file sender creates the SDP offer, for example, because the host is still processing the file. The 'hash' parameter in the 'file-selector' attribute contains valuable information to the file receiver to identify whether the file is already available and need not be transmitted. If the sender supports several hashing algorithms, then several 'hash' parameters can be included. The file sender MAY also add a 'name' parameter in the 'file- selector' attribute, and an 'icon', 'disposition', and 'file-date' attributes further describing the file to be transferred. The 'disposition' attribute provides a presentation suggestion, (for example: the file sender would like the file receiver to render the file or not). The three date attributes provide the answerer with an indication of the age of the file. The file sender MAY also add a 'file-range' attribute indicating the start and stop offset of the file transfer. 8.1.2. The Offerer is a File Receiver In a pull operation, the file receiver creates the SDP offer and sends it to the file sender. The file receiver MUST include a 'file- Garcia-Martin, et al. Expires June 21, 2007 [Page 15] Internet-Draft File Transfer with SDP offer/answer December 2006 selector' attribute and SHOULD add, at least, one of the parameters defined for such attribute (i.e., 'name', 'type', 'size', or 'hash'). Several 'hash' parameters MAY be included if each 'hash' parameter is computed with a different hashing algorithm. In many cases, if the hash of the file is known, that is enough to identify the file, therefore, the offerer can include only a 'hash' attribute. However, specially in cases where the hash of the file is unknown, the file name, size, and type can provide a description of the file to be fetched. There is no need to for the file offerer to include further file attributes in the SDP offer, thus it is RECOMMENDED that SDP offerers do not include any other file attribute defined by this specification (other than the mandatory ones). Additionally, the file receiver MUST create an SDP offer that contains a session or media 'recvonly' attribute. The file receiver MAY also add a 'file-range' attribute indicating the start and stop offset of the file transfer. 8.1.3. SDP Offer for Several Files An offerer that wishes to send or receive more than one file generates an "m=" line per file. This way, the answerer can reject individual files by setting the port number of their associated "m=" lines to zero, as per regular SDP [10] procedures. Using an "m=" line per file implies that different files are transferred using different MSRP sessions. However, all those MSRP sessions can be set up to run over a single TCP connection, as described in Section 8.1 of [12]. 8.2. Answerer's Behavior If the answerer wishes to reject a file offered by the offerer, it sets the port number of the "m=" line associated with the file to zero, as per regular SDP [10] procedures. If the answerer decides to accept the file, it proceeds as per regular MSRP [12] and SDP [10] procedures. 8.2.1. The Answerer is a File Receiver In a push operation the answerer is the file receiver. When the file receiver gets the SDP answer, it extracts the attributes and parameters that describe the file and typically requests permission to the user to accept or reject the reception of the file. If the file transfer operation is accepted, the file receiver MUST create an SDP answer according to the procedures specified in RFC 3264 [7]. If the offer contains 'name', 'type', 'size', parameters in the 'file- selector' attribute, the answerer MUST copy them into the answer. If Garcia-Martin, et al. Expires June 21, 2007 [Page 16] Internet-Draft File Transfer with SDP offer/answer December 2006 the offer contains one ore more 'hash' parameters in the 'file- selector' attribute, the answerer discards those with non-supported hashing algorithms and MUST copy the remaining (if any) to the 'file- selector' attribute of the answer. This informs the offerer that the answerer supports this specification. Then the file receiver MUST add a 'recvonly' attribute according to the procedures specified in RFC 3264 [7]. The file receiver MUST NOT include 'icon', 'disposition', or 'file-date' attributes in the SDP answer. If the SDP offer contains one or more 'hash' parameters in the 'file- selector' attribute, the answerer discards those with unsupported hashing algorithms. The file receiver can use the remaining hashes to find out if a local file with the same hash is already available, in which case, this could imply the reception of a duplicated file. It is up to the answerer to determine whether the file transfer is accepted or not in case of a duplicated file. If the SDP offer contains a 'file-range' attribute and the file receiver accepts to receive the range of bytes declared in there, the file receiver MUST include a 'file-range' attribute in the SDP answer with the same range of values. If the file receiver does not accept the reception of that range of bytes, it SHOULD reject the transfer of the file. 8.2.2. The Answerer is a File Sender In a pull operation the answerer is a file sender. In this case, the file sender MUST first inspect the received SDP offer and apply the file selector. The file selector is encoded in the 'file-selector' media attribute line in SDP. First, if the file selector contains several hashes, the file sender MUST select one of them which contains a supported hashing algorithm and discard the rest. Then the file sender applies the file selector, which implies selecting those files that match one by one with the 'name', 'type', 'size', and 'hash' parameters of the 'file-selector' attribute line (if they are present). The file selector identifies zero or more candidate files to be sent. If the file selector is unable to identify any file, then the answerer MUST reject the MSRP stream for file transfer by setting the port number to zero, and then the file sender SHOULD also reject the SDP as per procedures in RFC 3264 [7], if this is the only stream described in the SDP offer. If the file selector points to a single file and the file sender decides to accept the file transfer, the file sender MUST create an SDP answer that contains a 'sendonly' attribute, according to the procedures described RFC 3264 [7]. If the SDP offer included several 'hash' parameters, the file sender SHOULD include at least one in the SDP answer, selected among those present in the offer and, at the Garcia-Martin, et al. Expires June 21, 2007 [Page 17] Internet-Draft File Transfer with SDP offer/answer December 2006 same time, supported by the file sender. If the SDP offer did not include a 'hash' parameter, the file sender SHOULD add one or more 'hash' parameters, according to the supported hashing algorithms. The file sender MAY also include a 'type' parameter in the 'file- selector' attribute line of the SDP answer. The answerer MAY also include an 'icon' and 'disposition' attributes to further describe the file. Although the answerer MAY also include a 'name' and 'size' parameters in the 'file-selector' attribute, and a 'file-date' attribute, it is RECOMMENDED not to include them in the SDP answer if the actual file transfer protocol (e.g., MSRP [12]) can accommodate a Content-Disposition header field [3] with the equivalent parameters. The whole idea of adding file descriptors to SDP is to provide a mechanism where a file transfer can be accepted prior to its start. Adding any SDP attributes that are otherwise signalled later in the file transfer protocol would just duplicate the information, but will not provide any information to the offerer to accept or reject the file transfer (note that the offerer is requesting a file). Last, if the file selector points to multiple candidate files, the answerer MAY use some local policy, e.g. consulting the user, to choose one of them to be defined in the SDP answer. If that choise cannot be done, the answere SHOULD reject the MSRP media stream for file transfer (by setting the port number to zero). If the need arises, future specifications can provide a suitable mechanism that allows to either select multiple files or, e.g., resolve ambiguities by returning a list of files that match the file selector. If the SDP offer contains a 'file-range' attribute and the file sender accepts to send the range of bytes declared in there, the file sender MUST include a 'file-range' attribute in the SDP answer with the same range of values. If the file sender does not accept sending that range of bytes, it SHOULD reject the transfer of the file. 8.3. Re-usage of Existing m= Lines in SDP The SDP Offer/Answer Model [7] provides rules that allow SDP offerers and answerers to modify an existing media line, i.e., re-use an existing media line with different attributes. The same is also possible when SDP signals a file transfer operation according to the rules of this memo. Therefore, the procedures defined in RFC 3264 [7], in particular those defined in Section 8.3, MUST apply for file transfer operations. Garcia-Martin, et al. Expires June 21, 2007 [Page 18] Internet-Draft File Transfer with SDP offer/answer December 2006 8.4. MSRP Usage The file transfer service specified in this document uses "m=" lines to describe the unidirectional transfer of a file. Consequently, each MSRP session established following the procedures in Section 8.1 and Section 8.2 is only used to transfer a single file. So, senders MUST only use the dedicated MSRP session to send the file described in the SDP offer or answer. That is, senders MUST NOT send additional files over the same MSRP session. In the absence of a 'file-range' attribute in the SDP, the MSRP file transfer MUST start with the first byte of the file and end with the last byte (i.e., the whole file is transferred). If a 'file-range' attribute is present in SDP, the file sender application MUST extract the indicated range of bytes from the file (start and stop bytes). Then the file sender application SHOULD wrap those bytes in an appropriate wrapper, such as message/cpim. Last, the file sender application delivers the message/cpim to MSRP for transportation. MSRP will consider the message/cpim as a whole message, and will start numbering bytes at number 1. Note that the default content disposition of MSRP bodies is 'render'. When MSRP is used to transfer files, the MSRP Content-Disposition header can also take the value 'not-render' defined by this memo. Once the file transfer is completed, the file sender SHOULD close the MSRP session, and MUST behave according to the MSRP [12] procedures with respect closing MSRP sessions. 9. Examples 9.1. Offerer sends a file to the Answerer This section shows an example flow for a file transfer scenario. The example assumes that SIP [13] is used to transport the SDP offer/ answer exchange, although the SIP details are briefly shown in the sake of brevity. Alice, the SDP offerer, wishes to send an image file to Bob (the answerer). Alice's User Agent Client (UAC) creates a unidirectional SDP offer that contains the description of the file that she wants to send to Bob's User Agent Server (UAS). The description also includes an icon representing the contents of the file to be transferred. The sequence flow is shown in Figure 3. Garcia-Martin, et al. Expires June 21, 2007 [Page 19] Internet-Draft File Transfer with SDP offer/answer December 2006 Alice's UAC Bob's UAS | | |(1) (SIP) INVITE | |----------------------->| |(2) (SIP) 200 OK | |<-----------------------| |(3) (SIP) ACK | |----------------------->| | | |(4) (MSRP) SEND (chunk) | |----------------------->| |(5) (MSRP) 200 OK | |<-----------------------| |(6) (MSRP) SEND (chunk) | |----------------------->| |(7) (MSRP) 200 OK | |<-----------------------| | | |(8) (SIP) BYE | |----------------------->| |(9) (SIP) 200 OK | |<-----------------------| | | | | Figure 3: Flow diagram of an offerer sending a file to an answerer F1: Alice constructs an SDP description of the file to be sent and attaches it to a SIP INVITE request addressed to Bob. Garcia-Martin, et al. Expires June 21, 2007 [Page 20] Internet-Draft File Transfer with SDP offer/answer December 2006 INVITE sip:bob@example.com SIP/2.0 To: Bob From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 1 INVITE Max-Forwards: 70 Date: Sun, 21 May 2006 13:02:03 GMT Contact: Content-Type: multipart/mixed; boundary="boundary71" Content-Length: [length] --boundary71 Content-Type: application/sdp Content-Length: [length of SDP] v=0 o=alice 2890844526 2890844526 IN IP4 alicepc.example.com s= c=IN IP4 alicepc.example.com t=0 0 m=message 7654 TCP/MSRP * i=This is my latest picture a=sendonly a=accept-types: * a=path:msrp://alicepc.example.com:7654/jshA7we;tcp a=file-selector:name:"My cool picture.jpg" type:image/jpeg size:4092 hash:sha-1:72245FE8653DDAF371362F86D471913EE4A2CE2E a=disposition:render a=file-date:creation:"Mon, 15 May 2006 15:01:31 +03:00" a=icon:cid:id2@alicepc.example.com --boundary71 Content-Type: image/jpeg Content-Transfer-Encoding: binary Content-ID: Content-Length: [length of image] Content-Disposition: icon ...small preview icon of the file... --boundary71-- Figure 4: INVITE request containing an SDP offer for file transfer NOTE: The 'file-selector' attribute in the above figure is split in two lines for formatting purposes. Real implementations will encode it in a single line. Garcia-Martin, et al. Expires June 21, 2007 [Page 21] Internet-Draft File Transfer with SDP offer/answer December 2006 From now on we omit the SIP details for the sake of brevity. F2: Bob receives the INVITE request, inspects the SDP offer and extracts the icon body, checks the creation date and file size, and decides to accept the file transfer. So Bob creates the following SDP answer: v=0 o=bob 2890844656 2890844656 IN IP4 bobpc.example.com s= c=IN IP4 bobpc.example.com t=0 0 m=message 8888 TCP/MSRP * a=recvonly a=accept-types: * a=path:msrp://bobpc.example.com:8888/9di4ea;tcp a=file-selector:name:"My cool picture.jpg" type:image/jpeg size:4092 hash:sha-1:72245FE8653DDAF371362F86D471913EE4A2CE2E Figure 5: SDP answer accepting the SDP offer for file transfer NOTE: The 'file-selector' attribute in the above figure is split in two lines for formatting purposes. Real implementations will encode it in a single line. F4: Alice opens a TCP connection to Bob and creates an MSRP SEND request. This SEND request contains the first chunk of the file. MSRP d93kswow SEND To-Path: msrp://bobpc.example.com:8888/9di4ea;tcp From-Path: msrp://alicepc.example.com:7654/iau39;tcp Message-ID: 12339sdqwer Byte-Range: 1-2048/4385 Content-Type: message/cpim To: Bob From: Alice DateTime: 2006-05-15T15:02:31-03:00 Content-Disposition: render; filename="My cool picture.jpg"; creation-date="Mon, 15 May 2006 15:01:31 +03:00"; size=4092 Content-Type: image/jpeg ... first set of bytes of the JPEG image ... -------d93kswow+ Figure 6: MSRP SEND request containing the first chunk of actual file Garcia-Martin, et al. Expires June 21, 2007 [Page 22] Internet-Draft File Transfer with SDP offer/answer December 2006 F5: Bob acknowledges the reception of the first chunk. MSRP d93kswow 200 OK To-Path: msrp://alicepc.example.com:7654/iau39;tcp From-Path: msrp://bobpc.example.com:8888/9di4ea;tcp Byte-Range: 1-2048/4385 -------d93kswow$ Figure 7: MSRP 200 OK response F6: Alice sends the second and last chunk. MSRP op2nc9a SEND To-Path: msrp://bobpc.example.com:8888/9di4ea;tcp From-Path: msrp://alicepc.example.com:7654/iau39;tcp Message-ID: 12339sdqwer Byte-Range: 2049-4385/4385 Content-Type: message/cpim ... second set of bytes of the JPEG image ... -------op2nc9a$ Figure 8: MSRP SEND request containing the second chunk of actual file F7: Bob acknowledges the reception of the second chunk. MSRP op2nc9a 200 OK To-Path: msrp://alicepc.example.com:7654/iau39;tcp From-Path: msrp://bobpc.example.com:8888/9di4ea;tcp Byte-Range: 2049-4385/4385 -------op2nc9a$ Figure 9: MSRP 200 OK response F8: Alice terminates the SIP session by sending a SIP BYE request. F9: Bob acknowledges the reception of the BYE request and sends a 200 (OK) response. 9.2. Offerer requests a file from the Answerer and second file transfer In this example Alice, the SDP offerer, first wishes to fetch a file from Bob, the SDP answerer. Alice knows that Bob has a specific file she wants to download. She has learned the hash of the file by some out-of-band mechanism. The hash attribute is enough to produce a file selector that points to the specific file. So, Alice creates an SDP offer that contains the file descriptor. Bob accepts the Garcia-Martin, et al. Expires June 21, 2007 [Page 23] Internet-Draft File Transfer with SDP offer/answer December 2006 transmission and sends the file to Alice. When Alice has completely received Bob's file, she intends to send a new image file to Bob. Therefore Alice re-uses the existing SDP media line with different attributes and updates the description of the new file she wants to send to Bob's User Agent Server (UAS). Figure 10 shows the sequence flow. Alice's UAC Bob's UAS | | |(1) (SIP) INVITE | |----------------------->| |(2) (SIP) 200 OK | |<-----------------------| |(3) (SIP) ACK | |----------------------->| | | |(4) (MSRP) SEND (file) | |<-----------------------| |(5) (MSRP) 200 OK | |----------------------->| | | |(6) (SIP) INVITE | |----------------------->| |(7) (SIP) 200 OK | |<-----------------------| |(8) (SIP) ACK | |----------------------->| | | |(9) (MSRP) SEND (file) | |----------------------->| |(10) (MSRP) 200 OK | |<-----------------------| | | |(11) (SIP) BYE | |<-----------------------| |(12) (SIP) 200 OK | |----------------------->| | | | | Figure 10: Flow diagram of an offerer requesting a file from the answerer and then sending a file to the answer F1: Alice constructs an SDP description of the file she wants to receive and attaches the SDP offer to a SIP INVITE request addressed to Bob. Garcia-Martin, et al. Expires June 21, 2007 [Page 24] Internet-Draft File Transfer with SDP offer/answer December 2006 INVITE sip:bob@example.com SIP/2.0 To: Bob From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 1 INVITE Max-Forwards: 70 Date: Sun, 21 May 2006 13:02:03 GMT Contact: Content-Type: application/sdp Content-Length: [length of SDP] v=0 o=alice 2890844526 2890844526 IN IP4 alicepc.example.com s= c=IN IP4 alicepc.example.com t=0 0 m=message 7654 TCP/MSRP * a=recvonly a=accept-types:image/jpeg a=path:msrp://alicepc.example.com:7654/jshA7we;tcp a=file-selector:hash:sha-1:72245FE8653DDAF371362F86D471913EE4A2CE2E Figure 11: INVITE request containing an SDP offer for file transfer From now on we omit the SIP details for the sake of brevity. F2: Bob receives the INVITE request, inspects the SDP offer, computes the file descriptor and finds a local file whose hash equals the one indicated in the SDP. Bob accepts the file transmission and creates an SDP answer as follows: v=0 o=bob 2890844656 2890855439 IN IP4 bobpc.example.com s= c=IN IP4 bobpc.example.com t=0 0 m=message 8888 TCP/MSRP * a=sendonly a=accept-types:* a=path:msrp://bobpc.example.com:8888/9di4ea;tcp a=file-selector:hash:sha-1:72245FE8653DDAF371362F86D471913EE4A2CE2E type:image/jpeg Figure 12: SDP answer accepting the SDP offer for file transfer F4: Alice opens a TCP connection to Bob. Bob then creates an MSRP SEND request that contains the file. Garcia-Martin, et al. Expires June 21, 2007 [Page 25] Internet-Draft File Transfer with SDP offer/answer December 2006 MSRP d93kswow SEND To-Path: msrp://alicepc.example.com:7654/iau39;tcp From-Path: msrp://bobpc.example.com:8888/9di4ea;tcp Message-ID: 12339sdqwer Byte-Range: 1-2027/2027 Content-Type: message/cpim To: Bob From: Alice DateTime: 2006-05-15T15:02:31-03:00 Content-Disposition: render; filename="My cool photo.jpg"; creation-date="Mon, 15 May 2006 15:01:31 +03:00"; modification-date="Mon, 15 May 2006 16:04:53 +03:00"; read-date="Mon, 16 May 2006 09:12:27 +03:00"; size=1931 Content-Type: image/jpeg ...binary JPEG image... -------d93kswow$ Figure 13: MSRP SEND request containing the actual file F5: Alice acknowledges the reception of the SEND request. MSRP d93kswow 200 OK To-Path: msrp://bobpc.example.com:8888/9di4ea;tcp From-Path: msrp://alicepc.example.com:7654/iau39;tcp Byte-Range: 1-2027/2027 -------d93kswow$ Figure 14: MSRP 200 OK response F6: Alice re-uses the existing SDP media line inserting the description of the file to be sent and attaches it to a SIP re-INVITE request addressed to Bob. Garcia-Martin, et al. Expires June 21, 2007 [Page 26] Internet-Draft File Transfer with SDP offer/answer December 2006 INVITE sip:bob@example.com SIP/2.0 To: Bob ;tag=1928323431 From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 2 INVITE Max-Forwards: 70 Date: Sun, 21 May 2006 13:02:33 GMT Contact: Content-Type: multipart/mixed; boundary="boundary71" Content-Length: [length of multipart] --boundary71 Content-Type: application/sdp Content-Length: [length of SDP] v=0 o=alice 2890844526 2890844527 IN IP4 alicepc.example.com s= c=IN IP4 alicepc.example.com t=0 0 m=message 5670 TCP/MSRP * i=This is my latest picture a=sendonly a=accept-types:* a=path:msrp://alicepc.example.com:5670/iau39;tcp a=file-selector:name:"sunset.jpg" type:image/jpeg size:4096 hash:sha-1:58231FE8653BBCF371362F86D471913EE4B1DF2F a=disposition:render a=file-date:creation:"Sun, 21 May 2006 13:02:15 +03:00" a=icon:cid:id3@alicepc.example.com --boundary71 Content-Type: image/jpeg Content-Transfer-Encoding: binary Content-ID: Content-Length: [length of image] Content-Disposition: icon ...small preview icon... --boundary71-- Figure 15: Reuse of the SDP in a second file transfer NOTE: The 'file-selector' attribute in the above figure is split in two lines for formatting purposes. Real implementations will encode it in a single line. Garcia-Martin, et al. Expires June 21, 2007 [Page 27] Internet-Draft File Transfer with SDP offer/answer December 2006 F7: Bob receives the re-INVITE request, inspects the SDP offer and extracts the icon body, checks the creation date and file size, and decides to accept the file transfer. So Bob creates the following SDP answer: v=0 o=bob 2890844656 2890855440 IN IP4 bobpc.example.com s= c=IN IP4 bobpc.example.com t=0 0 m=message 9999 TCP/MSRP * a=recvonly a=accept-types:* a=path:msrp://bobpc.example.com:9999/9an4ea;tcp a=file-selector:name:"sunset.jpg" type:image/jpeg size:4096 hash:sha-1:58231FE8653BBCF371362F86D471913EE4B1DF2F a=disposition:render Figure 16: SDP answer accepting the SDP offer for file transfer NOTE: The 'file-selector' attribute in the above figure is split in two lines for formatting purposes. Real implementations will encode it in a single line. F9: Alice opens a new TCP connection to Bob and creates an MSRP SEND request that contains the file. MSRP d95ksxox SEND To-Path: msrp://bobpc.example.com:9999/9an4ea;tcp From-Path: msrp://alicepc.example.com:5670/iau39;tcp Message-ID: 13449sdqwer Byte-Range: 1-2027/2027 Content-Type: message/cpim To: Bob From: Alice DateTime: 2006-05-21T13:02:15-03:00 Content-Disposition: render; filename="Sunset.jpg"; creation-date="Sun, 21 May 2006 13:02:15"; size=1931 Content-Type: image/jpeg ...binary JPEG image... -------d95ksxox+ Figure 17: MSRP SEND request containing the actual file F10: Bob acknowledges the reception of the SEND request. Garcia-Martin, et al. Expires June 21, 2007 [Page 28] Internet-Draft File Transfer with SDP offer/answer December 2006 MSRP d95ksxox 200 OK To-Path: msrp://alicepc.example.com:5670/iau39;tcp From-Path: msrp://bobpc.example.com:9999/9an4ea;tcp Byte-Range: 1-2027/2027 -------d95ksxox$ Figure 18: MSRP 200 OK response F11: Then Bob terminates the SIP session by sending a SIP BYE request. F12: Alice acknowledges the reception of the BYE request and sends a 200 (OK) response. 10. Security Considerations The SDP attributed defined in this specification identify a file to be transfered between two endpoints. An endpoint can offer to send the file to the other endpoint or request to receive the file from the other endpoint. In the former case, an attacker modifying those SDP attributes could cheat the receiver making it think that the file to be transfered was a different one. In the latter case, the attacker could make the sender send a different file than the one requested by the receiver. Consequently, it is RECOMMENDED that integrity protection be applied to the SDP session descriptions carrying the attributes specified in this specification. The descriptions of the files being transfered between endpoints may reveal information the endpoints may consider confidential. Therefore, it is RECOMMENDED that SDP session descriptions carrying the attributes specified in this specification be encrypted. TLS and S/MIME are the natural choices to provide offer/answer exchanges with integrity protection and confidentiality. 11. IANA Considerations This document instructs IANA to register a number of SDP attributes an a new Content Disposition value, according to the following: 11.1. Registration of new SDP attributes This memo provides instructions to IANA to register a number of media level only attributes in the Session Description Protocol Parameters registry [20]. The registration data, according to RFC 4566 [10] follows. Garcia-Martin, et al. Expires June 21, 2007 [Page 29] Internet-Draft File Transfer with SDP offer/answer December 2006 Note to the RFC Editor: replace "RFC XXXX" with the RFC number of this specification. 11.1.1. Registration of the file-selector attribute Contact: Miguel Garcia Phone: +358 71800 8000 Attribute name: file-sector Long-form attribute name: Type of attribute: media level only This attribute is subject to the charset attribute Description: This attribute unambiguously identify a file by indicating a combination of the 4-tuple composed of the name, size, type, and hash of the file. Specification: RFC XXXX 11.1.2. Registration of the disposition attribute Contact: Miguel Garcia Phone: +358 71800 8000 Attribute name: disposition Long-form attribute name: Type of attribute: media level only 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 XXXX 11.1.3. Registration of the file-date attribute Contact: Miguel Garcia Phone: +358 71800 8000 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 at which the file was created, modified, or last read. Specification: RFC XXXX 11.1.4. Registration of the icon attribute Contact: Miguel Garcia Phone: +358 71800 8000 Attribute name: icon Long-form attribute name: Garcia-Martin, et al. Expires June 21, 2007 [Page 30] Internet-Draft File Transfer with SDP offer/answer December 2006 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 XXXX 11.1.5. Registration of the file-range attribute Contact: Miguel Garcia Phone: +358 71800 8000 Attribute name: file-range Long-form attribute name: Type of attribute: media level only This attribute is not subject to the charset attribute Description: it contains the range of transferred bytes of the file Specification: RFC XXXX 11.2. Registration of new Content Disposition value IANA acts on Mail Content Disposition Values and Parameters registry [19] and registers a new Mail Content Disposition Value according to the following data: Name Description Reference ----- ------------ --------- not-render the body should not be rendered to the user [RFCXXXX] Note to the RFC Editor: Please replace RFCXXXX with the RFC number of this specification. 12. Acknowledgements The authors would like to thank Mats Stille, Nancy Greene, Adamu Haruna, and Arto Leppisaari for discussing initial concepts described in this memo. Thanks to Pekka Kuure for reviewing initial versions this document and providing helpful comments. Joerg Ott, Jiwey Wang, Amitkumar Goel, Sudha Vs, Dan Wing, Juuso Lehtinen, Remi Denis- Courmont discussed and provided comments and improvements to this document. 13. References Garcia-Martin, et al. Expires June 21, 2007 [Page 31] Internet-Draft File Transfer with SDP offer/answer December 2006 13.1. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [3] Troost, R., Dorner, S., and K. Moore, "Communicating Presentation Information in Internet Messages: The Content- Disposition Header Field", RFC 2183, August 1997. [4] Levinson, E., "Content-ID and Message-ID Uniform Resource Locators", RFC 2392, August 1998. [5] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [6] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001. [7] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [8] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification", RFC 3851, July 2004. [9] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [10] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [11] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and HMAC-SHA)", RFC 4634, August 2006. [12] Campbell, B., "The Message Session Relay Protocol", draft-ietf-simple-message-sessions-18 (work in progress), December 2006. 13.2. Informational References [13] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [14] Burger, E., "A Mechanism for Content Indirection in Session Garcia-Martin, et al. Expires June 21, 2007 [Page 32] Internet-Draft File Transfer with SDP offer/answer December 2006 Initiation Protocol (SIP) Messages", RFC 4483, May 2006. [15] Jennings, C., "Relay Extensions for the Message Sessions Relay Protocol (MSRP)", draft-ietf-simple-msrp-relays-08 (work in progress), July 2006. [16] Paila, T., "FLUTE - File Delivery over Unidirectional Transport", draft-ietf-rmt-flute-revised-02 (work in progress), August 2006. URIs [17] [18] [19] [20] Authors' Addresses Miguel A. Garcia-Martin Nokia P.O.Box 407 NOKIA GROUP, FIN 00045 Finland Email: miguel.an.garcia@nokia.com Markus Isomaki Nokia Keilalahdentie 2-4 Espoo 02150 Finland Email: markus.isomaki@nokia.com Garcia-Martin, et al. Expires June 21, 2007 [Page 33] Internet-Draft File Transfer with SDP offer/answer December 2006 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 Garcia-Martin, et al. Expires June 21, 2007 [Page 34] Internet-Draft File Transfer with SDP offer/answer December 2006 Full Copyright Statement Copyright (C) The IETF Trust (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Garcia-Martin, et al. Expires June 21, 2007 [Page 35]