| Abstract: | This specification defines a Jingle application type for transferring a file from one entity to another. The protocol provides a modular framework that enables the exchange of information about the file to be transferred as well as the negotiation of parameters such as the transport to be used. |
| Author: | Peter Saint-Andre |
| Copyright: | © 1999 - 2015 XMPP Standards Foundation. SEE LEGAL NOTICES. |
| Status: | Experimental |
| Type: | Standards Track |
| Version: | 0.16 |
| Last Updated: | 2014-08-11 |
WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.
1. Introduction
2. How It Works
3. Communicating the Checksum or Hashing Algorithm
4. Ranged Transfers
5. Use of Jingle Actions
6. Implementation Notes
6.1. Mandatory to Implement Technologies
6.2. Preference Order of Transport Methods
6.3. Migration from XEP-0096
7. Determining Support
8. Security Considerations
9. IANA Considerations
10. XMPP Registrar Considerations
10.1. Protocol Namespaces
10.2. Namespace Versioning
10.3. Service Discovery Features
10.4. Jingle Application Formats
11. XML Schema
12. Acknowledgements
Appendices
A: Document Information
B: Author Information
C: Legal Notices
D: Relation to XMPP
E: Discussion Venue
F: Requirements Conformance
G: Notes
H: Revision History
SI File Transfer (XEP-0096) [1] was the original XMPP protocol extension for file transfer negotiation. However, that protocol has several drawbacks, most related to the Stream Initiation (XEP-0095) [2] protocol on which it depends:
To overcome these drawbacks, this specification defines a file transfer negotiation method that meets the following requirements:
Jingle file transfer is only as reliable as the transports on which it depends. In particular, SOCKS5 Bytestreams ("S5B") does not always result in NAT or firewall traversal. To work around that problem, this specification requires all implementations to support as a fallback mechanism In-Band Bytestreams ("IBB"), which usually results in a successful (if slow) file transfer.
Note: It is likely that a future version of this specification will also recommend implementation of a Jingle transport method that emulates the IETF's ICE-TCP technology, as specified in TCP Candidates with Interactive Connectivity Establishment (ICE) [7].
This section provides a friendly introduction to Jingle file transfer.
First, the party that wishes to initiate the file transfer determines the responder's capabilities (via Service Discovery (XEP-0030) [8] or Entity Capabilities (XEP-0115) [9]). Here we assume that the responder supports the following service discovery features:
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
The initiator then sends a Jingle session-initiation request to a potential responder. The content-type of the request specifies two things:
In this example, the initiator is <romeo@montague.lit>, the responder is <juliet@capulet.lit>, the application type is a file offer, and the transport method is jingle-s5b.
The flow is as follows.
Initiator Responder | | | session-initiate | | (with <file/> and S5B) | |---------------------------->| | ack | |<----------------------------| | session-accept | |<----------------------------| | ack | |---------------------------->| | [ file transfer ] | |============================>| | session-terminate | |<----------------------------| | ack | |---------------------------->| | |
First the initiator sends a Jingle session-initiate.
Example 1. Initiator sends session-initiate
<iq from='romeo@montague.lit/orchard'
id='nzu25s8'
to='juliet@capulet.lit/balcony'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='session-initiate'
initiator='romeo@montague.lit/orchard'
sid='851ba2'>
<content creator='initiator' name='a-file-offer'>
<description xmlns='urn:xmpp:jingle:apps:file-transfer:4'>
<file>
<date>1969-07-21T02:56:15Z</date>
<desc>This is a test. If this were a real file...</desc>
<media-type>text/plain</media-type>
<name>test.txt</name>
<range/>
<size>1022</size>
<hash xmlns='urn:xmpp:hashes:1' algo='sha-1'>552da749930852c69ae5d2141d3766b1</hash>
</file>
</description>
<transport xmlns='urn:xmpp:jingle:transports:s5b:1'
mode='tcp'
sid='vj3hs98y'>
<candidate cid='hft54dqy'
host='192.168.4.1'
jid='romeo@montague.lit/orchard'
port='5086'
priority='8257636'
type='direct'/>
<candidate cid='hutr46fe'
host='24.24.24.1'
jid='romeo@montague.lit/orchard'
port='5087'
priority='8258636'
type='direct'/>
</transport>
</content>
</jingle>
</iq>
Note: As in XEP-0096, inclusion of the <range/> child of the <file/> element indicates that the initiatior supports ranged transfers as described below under Ranged Transfers.
Note: Computing the hash of the file before sending it can slow down the process of file transfer, since the sending application needs to process the file twice. The sender might prefer to send the hash after the file transfer has begun, using a transport-info message as described under Communicating the Hash.
The responder immediately acknowledges receipt of the Jingle session-initiate.
Example 2. Responder acknowledges session-initiate
<iq from='juliet@capulet.lit/balcony'
id='nzu25s8'
to='romeo@montague.lit/orchard'
type='result'/>
The initiator then attempts to initiate a SOCKS5 Bytestream with the responder as described in XEP-0260 and XEP-0065. In the meantime, the responder returns a Jingle session-accept. In the session-accept message, the <file/> element MAY contain a <range/> element to indicate that the receiver also supports ranged transfers as described below under Ranged Transfers.
Example 3. Responder sends session-accept
<iq from='juliet@capulet.lit/balcony'
id='jn2vs71g'
to='romeo@montague.lit/orchard'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='session-accept'
initiator='romeo@montague.lit/orchard'
sid='851ba2'>
<content creator='initiator' name='a-file-offer'>
<description xmlns='urn:xmpp:jingle:apps:file-transfer:4'>
<file>
<date>1969-07-21T02:56:15Z</date>
<desc>This is a test. If this were a real file...</desc>
<media-type>text/plain</media-type>
<name>test.txt</name>
<range/>
<size>1022</size>
<hash xmlns='urn:xmpp:hashes:1' algo='sha-1'>552da749930852c69ae5d2141d3766b1</hash>
</file>
</description>
<transport xmlns='urn:xmpp:jingle:transports:s5b:1'
mode='tcp'
sid='vj3hs98y'>
<candidate cid='ht567dq'
host='192.169.1.10'
jid='juliet@capulet.lit/balcony'
port='6539'
priority='8257636'
type='direct'/>
<candidate cid='hr65dqyd'
host='134.102.201.180'
jid='juliet@capulet.lit/balcony'
port='16453'
priority='7929856'
type='assisted'/>
<candidate cid='grt654q2'
host='2001:638:708:30c9:219:d1ff:fea4:a17d'
jid='juliet@capulet.lit/balcony'
port='6539'
priority='8257606'
type='direct'/>
</transport>
</content>
</jingle>
</iq>
The initiator acknowledges the Jingle session-accept.
Example 4. Initiator acknowledges session-accept
<iq from='romeo@montague.lit/orchard'
id='jn2vs71g'
to='juliet@capulet.lit/balcony'
type='result'/>
Once one client has successfully created a connection, it sends a <remote-candidate/> element to the peer inside a Jingle transport-info message. If a client receives a remote-candidate notification it SHOULD continue trying to connect to candidates sent by its peer if it has not tried all candidates with a higher priority than the one successfully used by the peer.
Example 5. Initiator sends remote-candidate in Jingle transport-info
<iq from='romeo@montague.lit/orchard'
id='hjdi8'
to='juliet@capulet.lit/balcony'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='transport-info'
initiator='romeo@montague.lit/orchard'
sid='a73sjjvkla37jfea'>
<content creator='initiator' name='ex'>
<transport xmlns='urn:xmpp:jingle:transports:s5b:1'
sid='vj3hs98y'>
<remote-candidate cid='hr65dqyd'/>
</transport>
</content>
</jingle>
</iq>
The peer immediately acknowledges receipt.
Example 6. Responder acknowledges remote-candidate message
<iq from='juliet@capulet.lit/balcony'
id='hjdi8'
to='romeo@montague.lit/orchard'
type='result'/>
(See XEP-0260 for further details.)
Now the parties exchange the file using the negotiated transport (here, SOCKS5 Bytestreams).
Once the transfer is completed, either party can acknowledge completion or terminate the Jingle session; preferably this is done by the entity that receives the file to ensure that the complete file (up to the advertised size) has been received.
Example 7. Receiver sends session-terminate
<iq from='juliet@capulet.lit/balcony'
id='og61bvs98'
to='romeo@montague.lit/orchard'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='session-terminate'
sid='a73sjjvkla37jfea'>
<reason>
<success/>
</reason>
</jingle>
</iq>
After terminating the session, the parties would close the data transport as described in the relevant specification (e.g., XEP-0260 or XEP-0261).
For a description of the transport fallback scenario (from SOCKS5 Bytestreams to In-Band Bytestreams), refer to XEP-0260.
At any time during the lifetime of the file transfer session, the hosting entity (i.e., the entity where the file resides) can communicate the checksum of the file to the receiving entity.
This can be done in the session-initiate message if the sender already knows the checksum, as shown above in Example 1.
After the session-initiate message, this can also be done by sending a session-info message containing a <checksum/> element qualified by the 'urn:xmpp:jingle:apps:file-transfer:4' namespace, which in turn contains a <file/> element that MUST at least contain a child element of <hash/> qualified by the 'urn:xmpp:hashes:1' namespace and MAY contain other elements qualified by the 'urn:xmpp:jingle:apps:file-transfer:4' namespace (e.g. <name/> and <date/>). Each <hash/> element contains a checksum of the file contents produced in accordance with the hashing function specified by the 'algo' attribute, which MUST be one of the functions listed in the IANA Hash Function Textual Names Registry [10].
Example 8. Initiator sends checksum in session-info
<iq from='romeo@montague.lit/orchard'
id='kqh401b5'
to='juliet@capulet.lit/balcony'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='session-info'
initiator='romeo@montague.lit/orchard'
sid='a73sjjvkla37jfea'>
<checksum xmlns='urn:xmpp:jingle:apps:file-transfer:4'>
<file>
<hash xmlns='urn:xmpp:hashes:1' algo='sha-1'>552da749930852c69ae5d2141d3766b1</hash>
</file>
</checksum>
</jingle>
</iq>
If the initiator wishes to communicate only the hashing algorithm as the beginning of the session (e.g., because it has not yet calculated the checksum), it can send an empty <hash/> element (without a checksum in the XML character data as shown in the previous examples) in the session-initiate message; this enables the recipient to check the file during the transfer session (which can be helpful in the case of transfers that are truncated or fail mid-stream).
Example 9. Initiator communicates hashing algorithm in session-initiate
<iq from='romeo@montague.lit/orchard'
id='nzu25s8'
to='juliet@capulet.lit/balcony'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='session-initiate'
initiator='romeo@montague.lit/orchard'
sid='851ba2'>
<content creator='initiator' name='a-file-offer'>
<description xmlns='urn:xmpp:jingle:apps:file-transfer:4'>
<file>
<date>1969-07-21T02:56:15Z</date>
<desc>This is a test. If this were a real file...</desc>
<media-type>text/plain</media-type>
<name>test.txt</name>
<range/>
<size>1022</size>
<hash xmlns='urn:xmpp:hashes:1' algo='sha-1'/>
</file>
</description>
<transport xmlns='urn:xmpp:jingle:transports:s5b:1'
mode='tcp'
sid='vj3hs98y'>
<candidate cid='hft54dqy'
host='192.168.4.1'
jid='romeo@montague.lit/orchard'
port='5086'
priority='8257636'
type='direct'/>
<candidate cid='hutr46fe'
host='24.24.24.1'
jid='romeo@montague.lit/orchard'
port='5087'
priority='8258636'
type='direct'/>
</transport>
</content>
</jingle>
</iq>
As in XEP-0096, a transfer can include only part of a file (e.g., to restart delivery of a truncated transfer session at a point other than the start of the file). This is done using the <range/> element from XEP-0096. The usage is illustrated in the following examples.
Let us imagine that the parties negotiate a file transfer session using, say, In-Band Bytestreams. During the transfer, the recipient goes offline unexpectedly and IBB stanzas from the sender to the recipient begin to bounce. When the recipient comes back online, the sender could initiate a new Jingle session and specify that it wants to send all chunks after byte 270336 (which might be the 66th chunk of size 4096).
Example 10. Sender requests session to send file, with range
<iq from='romeo@montague.lit/orchard'
id='wsn361c3'
to='juliet@capulet.lit/balcony'
type='set'>
<jingle xmlns='urn:xmpp:jingle:1'
action='session-initiate'
initiator='romeo@montague.lit/orchard'
sid='uj3b2'>
<content creator='initiator' name='restart'>
<description xmlns='urn:xmpp:jingle:apps:file-transfer:4'>
<file>
<range offset='270336'/>
<hash xmlns='urn:xmpp:hashes:1' algo='sha-1'>552da749930852c69ae5d2141d3766b1</hash>
</file>
</description>
<transport xmlns='urn:xmpp:jingle:transports:s5b:1'
mode='tcp'
sid='vj3hs98y'>
<candidate cid='hft54dqy'
host='192.168.4.1'
jid='romeo@montague.lit/orchard'
port='5086'
priority='8257636'
type='direct'/>
<candidate cid='hutr46fe'
host='24.24.24.1'
jid='romeo@montague.lit/orchard'
port='5087'
priority='8258636'
type='direct'/>
</transport>
</content>
</jingle>
</iq>
Jingle file transfer uses only a few of the actions defined in XEP-0166. Jingle usage is summarized in the following table.
Table 1: Jingle Actions Used in File Transfer
| Action | Use |
|---|---|
| content-accept | Unused |
| content-add | Unused |
| content-modify | Unused |
| content-reject | Unused |
| content-remove | Unused |
| description-info | Unused |
| security-info | Unused |
| session-accept | Accepting a file offer or request |
| session-info | Communicating the file hash |
| session-initiate | Initiating a file offer or request |
| session-terminate | Ending a file transfer session |
| transport-accept | Accepting fallback from S5B to IBB |
| transport-info | Used in SOCKS5 Bytestreams |
| transport-reject | Rejecting fallback from S5B to IBB |
| transport-replace | Fallback from S5B to IBB |
All implementations MUST support the Jingle In-Band Bytestreams Transport Method (XEP-0261) as a reliable method of last resort. An implementation SHOULD support other transport methods as well, especially the Jingle SOCKS5 Bytestreams Transport Method (XEP-0260).
An application MAY present transport methods in any order, except that the Jingle In-Band Bytestreams Transport Method MUST be the lowest preference.
Support for Jingle file transfer can be determined through discovery of the 'urn:xmpp:jingle:apps:file-transfer:4' namespace (see Namespace Versioning regarding the possibility of incrementing the version number), via either service discovery (XEP-0030) or entity capabilities (XEP-0115). If the initiator knows that the responder supports Jingle file transfer, it SHOULD first attempt negotiation using Jingle rather than Stream Initiation.
To advertise its support for the Jingle File Transfer, when replying to service discovery information ("disco#info") requests an entity MUST return URNs for any version of this protocol that the entity supports -- e.g., "urn:xmpp:jingle:apps:file-transfer:4" for this version (see Namespace Versioning regarding the possibility of incrementing the version number).
Example 11. Service discovery information request
<iq from='romeo@montague.lit/orchard'
id='uw72g176'
to='juliet@capulet.lit/balcony'
type='get'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
Example 12. Service discovery information response
<iq from='juliet@capulet.lit/balcony'
id='uw72g176'
to='romeo@montague.lit/orchard'
type='result'>
<query xmlns='http://jabber.org/protocol/disco#info'>
<feature var='urn:xmpp:jingle:1'/>
<feature var='urn:xmpp:jingle:apps:file-transfer:4'/>
<feature var='urn:xmpp:jingle:transports:s5b:1'/>
<feature var='urn:xmpp:jingle:transports:ibb:1'/>
</query>
</iq>
As noted, if an application supports exchange of multiple files, it MUST advertise a service discovery feature of "urn:xmpp:jingle:apps:file-transfer:multi".
In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in Entity Capabilities (XEP-0115) [11]. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.
It is RECOMMENDED for implementations to use the strongest hashing algorithm available to both parties. See XEP-0300 for further discussion.
In order to secure the data stream, implementations SHOULD use encryption methods appropriate to the transport method being used. For example, end-to-end encryption can be negotiated over either SOCKS5 Bytestreams or In-Band Bytestreams as described in XEP-0260 and XEP-0261.
Refer to XEP-0047, XEP-0065, XEP-0096, XEP-0260, and XEP-0261 for related security considerations.
No interaction with the Internet Assigned Numbers Authority (IANA) [12] is required as a result of this document.
The XML character data of the <media-type/> element SHOULD be a value registered with the IANA in the IANA MIME Media Types Registry [13].
This specification defines the following XML namespace:
Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar [14] shall add the foregoing namespace to the registry located at <http://xmpp.org/registrar/namespaces.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) [15].
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
The service discovery feature for advertising support for exchange of multiple files is "urn:xmpp:jingle:apps:file-transfer:multi".
The registry submission is as follows.
Registry Submission
<var>
<name>urn:xmpp:jingle:apps:file-transfer:multi</name>
<desc>Signals support for exchange of multiple files.</desc>
<doc>XEP-0234</doc>
</var>
The XMPP Registrar shall include "file-transfer" in its registry of Jingle application formats. The registry submission is as follows:
<application>
<name>file-transfer</name>
<desc>Jingle sessions for the transfer of a file</desc>
<transport>streaming</transport>
<doc>XEP-0234</doc>
</application>
<?xml version='1.0' encoding='UTF-8'?>
<xs:schema
xmlns:xs='http://www.w3.org/2001/XMLSchema'
targetNamespace='urn:xmpp:jingle:apps:file-transfer:4'
xmlns='urn:xmpp:jingle:apps:file-transfer:4'
elementFormDefault='qualified'>
<xs:import namespace='urn:xmpp:hashes:1'/>
<xs:element name='description'>
<xs:complexType>
<xs:choice>
<xs:element name='file' type='fileTransferElementType'/>
</xs:choice>
</xs:complexType>
</xs:element>
<xs:element name='checksum' type='fileTransferElementType'/>
<xs:complexType name='fileTransferElementType'>
<xs:sequence>
<xs:element name='file'/>
</xs:sequence>
</xs:complexType>
<xs:element name='file'>
<xs:complexType>
<xs:sequence xmlns:h='urn:xmpp:hashes:1'>
<xs:element name='date' type='xs:date'/>
<xs:element name='media-type' type='xs:string'/>
<xs:element name='name' type='xs:string'/>
<xs:element name='size' type='xs:positiveInteger'/>
<xs:element ref='h:hash' minOccurs='0' maxOccurs='unbounded'/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
Thanks to Diana Cionoiu, Olivier Crête, Viktor Fast, Philipp Hancke, Waqas Hussain, Justin Karneges, Steffen Larsen, Yann Leboulanger, Marcus Lundblad, Robert McQueen, Joe Maissel, Glenn Maynard, Ali Sabil, Sjoerd Simons, Lance Stout, Will Thompson, Matthew Wild, and Jiří Zárevúcky for their feedback.
Series: XEP
Number: 0234
Publisher: XMPP Standards Foundation
Status:
Experimental
Type:
Standards Track
Version: 0.16
Last Updated: 2014-08-11
Approving Body: XMPP Council
Dependencies: XMPP Core, XEP-0047, XEP-0065, XEP-0096, XEP-0166
Supersedes: None
Superseded By: None
Short Name: NOT_YET_ASSIGNED
Source Control:
HTML
This document in other formats:
XML
PDF
Email:
peter@andyet.net
JabberID:
stpeter@stpeter.im
URI:
https://stpeter.im/
The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
Errata can be sent to <editor@xmpp.org>.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
1. XEP-0096: SI File Transfer <http://xmpp.org/extensions/xep-0096.html>.
2. XEP-0095: Stream Initiation <http://xmpp.org/extensions/xep-0095.html>.
3. XEP-0166: Jingle <http://xmpp.org/extensions/xep-0166.html>.
4. XEP-0260: Jingle SOCKS5 Bytestreams Transport Method <http://xmpp.org/extensions/xep-0260.html>.
5. XEP-0261: Jingle In-Band Bytestreams Transport Method <http://xmpp.org/extensions/xep-0261.html>.
6. XEP-0300: Use of Cryptographic Hash Functions in XMPP <http://xmpp.org/extensions/xep-0300.html>.
7. TCP Candidates with Interactive Connectivity Establishment (ICE) <http://tools.ietf.org/html/rfc6544>. Work in progress.
8. XEP-0030: Service Discovery <http://xmpp.org/extensions/xep-0030.html>.
9. XEP-0115: Entity Capabilities <http://xmpp.org/extensions/xep-0115.html>.
10. IANA registry of Hash Function Textual Names <http://www.iana.org/assignments/hash-function-text-names>.
11. XEP-0115: Entity Capabilities <http://xmpp.org/extensions/xep-0115.html>.
12. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
13. IANA registry of MIME media types <http://www.iana.org/assignments/media-types>.
14. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <http://xmpp.org/registrar/>.
15. XEP-0053: XMPP Registrar Function <http://xmpp.org/extensions/xep-0053.html>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Updated to track revisions to XEP-0300.
(psa)Added multi-file use case; updated spec to reflect XEP-0260 and XEP-0261; added algorithm attribute from XEP-0096; increased namespace versions from 1 to 2.
(psa)Clarified usage of Jingle actions as well as several ambiguous points in the text, including use of the range feature from XEP-0096.
(psa)Added session-info message and namespace for communicating the file hash.
(psa)Described the file retrieval case; updated referenced namespaces.
(psa)Corrected fallback scenario to use transport-replace and transport-accept.
(psa)Harmonized with XEP-0166; modified fallback to use transport-replace and transport-accept.
(psa)Modified fallback scenario to use content-replace action during pending state.
(psa)Harmonized negotiation flows with other Jingle application types.
(psa)Corrected and more clearly explained negotiation flows for consistency with XEP-0166 and other Jingle specifications.
(psa)Added transport negotiation scenario.
(psa)Initial published version.
(psa)Corrected use of content-replace action; specified that the In-Band Bytestreams transport method is mandatory-to-implement but must have the lowest preference order.
(psa)END