# XEP-0047: In-Band Bytestreams

Abstract: | This specification defines an XMPP protocol extension that enables any two entities to establish a one-to-one bytestream between themselves, where the data is broken down into smaller chunks and transported in-band over XMPP.
Authors: | Justin Karneges, Peter Saint-Andre
Copyright: | © 1999 - 2010 XMPP Standards Foundation. SEE LEGAL NOTICES.
Status: | Draft
Type: | Standards Track
Version: | 1.2
Last Updated: | 2009-03-17

NOTICE: The protocol defined herein is a Draft Standard of the XMPP Standards Foundation. Implementations are encouraged and the protocol is appropriate for deployment in production systems, but some changes to the protocol are possible before it becomes a Final Standard.

## Table of Contents

1. Introduction
2. Protocol
2.1. Creating a Bytestream
2.2. Sending Data
2.3. Closing the Bytestream
3. Bidirectionality
4. Use of Message Stanzas
5. Usage Guidelines
6. Security Considerations
7. IANA Considerations
8. XMPP Registrar Considerations
8.1. Protocol Namespaces
9. XML Schema

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

## 1. Introduction

This document describes In-Band Bytestreams (IBB), a simple XMPP protocol extension that enables two entities to establish a virtual bytestream over which they can exchange Base64-encoded chunks of data over XMPP itself. Because IBB provides a generic bytestream, its usage is open-ended. To date it has been used as a fallback method for sending files when out-of-band methods such as SOCKS5 Bytestreams [1] are not available. However, IBB could also be useful for any kind of relatively low-bandwidth activity, such as games, shell sessions, or encrypted communication.

## 2. Protocol

IBB as specified in this document defines two protocol aspects:

1. How to set up and tear down an IBB session using <open/> and <close/> elements sent within IQ stanzas.
2. How to send chunks of IBB data using IQ (or message) stanzas containing <data/> elements.

Other methods can be used for setup and teardown, such as Jingle [2] as specified in Jingle In-Band Bytestreams Transport Method [3].

### 2.1 Creating a Bytestream

In order to set up an in-band bytestream, the initiator sends an IQ stanza of type "set" containing an <open/> element qualified by the 'http://jabber.org/protocol/ibb' namespace. This element possesses three attributes:

- The REQUIRED 'block-size' attribute defines the maximum size in bytes of each data chunk (which SHOULD NOT be greater than 65535).
- The REQUIRED 'sid' attribute defines a unique session ID for this IBB session (which SHOULD match the NMTOKEN datatype).
- The OPTIONAL 'stanza' attribute defines whether the data will be sent using IQ stanzas or message stanzas (the default value is "iq").

Example 1. Initiator requests session

    <iq from='romeo@montague.net/orchard'
        id='jn3h8g65'
        to='juliet@capulet.com/balcony'
        type='set'>
      <open xmlns='http://jabber.org/protocol/ibb'
            block-size='4096'
            sid='i781hf64'
            stanza='iq'/>
    </iq>

If the responder wishes to proceed with the IBB session, it returns an IQ-result to the initiator.

Example 2. Responder accepts session

    <iq from='juliet@capulet.com/balcony'
        id='jn3h8g65'
        to='romeo@montague.net/orchard'
        type='result'/>

If the responder does not support IBB, it returns a <service-unavailable/> or <feature-not-implemented/> error.

Example 3. Responder does not support IBB

    <iq from='juliet@capulet.com/balcony'
        id='jn3h8g65'
        to='romeo@montague.net/orchard'
        type='error'/>
      <error type='cancel'>
        <service-unavailable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
      </error>
    </iq>

If the responder supports IBB but would prefer a smaller block-size, it returns a <resource-constraint/> error.

Example 4. Responder prefers smaller chunks

    <iq from='juliet@capulet.com/balcony'
        id='jn3h8g65'
        to='romeo@montague.net/orchard'
        type='error'/>
      <error type='modify'>
        <resource-constraint xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
      </error>
    </iq>

If the responder supports IBB but does not wish to proceed with the session, it returns a <not-acceptable/> error.

Example 5. Responder does not wish to proceed

    <iq from='juliet@capulet.com/balcony'
        id='jn3h8g65'
        to='romeo@montague.net/orchard'
        type='error'/>
      <error type='cancel'>
        <not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
      </error>
    </iq>

### 2.2 Sending Data

If the responder informs the initiator that it wishes to proceed with the session, the initiator can begin to send data over the bytestream (in addition, because the bytestream is bidirectional, the responder can also send data; see the Bidirectionality section of this document for details).

Each chunk of data is contained in a <data/> element qualified by the 'http://jabber.org/protocol/ibb' namespace. The data element SHOULD be sent in an IQ stanza to enable proper tracking and throttling, but MAY be sent in a message stanza. The data to be sent, prior to any wrapping in the <data/> element and IQ or message stanza, MUST NOT be larger than the 'block-size' determined in the bytestream negotiation.

Example 6. Sending data in an IQ stanza

    <iq from='romeo@montague.net/orchard'
        id='kr91n475'
        to='juliet@capulet.com/balcony'
        type='set'>
      <data xmlns='http://jabber.org/protocol/ibb' seq='0' sid='i781hf64'>
        qANQR1DBwU4DX7jmYZnncmUQB/9KuKBddzQH+tZ1ZywKK0yHKnq57kWq+RFtQdCJ
        WpdWpR0uQsuJe7+vh3NWn59/gTc5MDlX8dS9p0ovStmNcyLhxVgmqS8ZKhsblVeu
        IpQ0JgavABqibJolc3BKrVtVV1igKiX/N7Pi8RtY1K18toaMDhdEfhBRzO/XB0+P
        AQhYlRjNacGcslkhXqNjK5Va4tuOAPy2n1Q8UUrHbUd0g+xJ9Bm0G0LZXyvCWyKH
        kuNEHFQiLuCY6Iv0myq6iX6tjuHehZlFSh80b5BVV9tNLwNR5Eqz1klxMhoghJOA
      </data>
    </iq>

Each chunk of data is included as the XML character data of the <data/> element after being encoded as Base64 as specified in Section 4 of RFC 4648 [4].

The <data/> element MUST possess a 'seq' attribute; this is a 16-bit unsigned integer that acts as a counter for data chunks sent within this session. The 'seq' value starts at 0 (zero) and MUST be incremented for each packet sent. Thus, the second chunk sent has a 'seq' value of 1, the third chunk has a 'seq' value of 2, and so on. The counter loops at maximum, so that after value 65535 the 'seq' MUST start again at 0.

The <data/> element MUST also possess a 'sid' attribute that ties the data chunk to this particular IBB session.

In the case of IQ stanzas, if the packet can be processed then the recipient MUST reply with an IQ stanza of type "result".

Example 7. Acknowledging data using IQ

    <iq from='juliet@capulet.com/balcony'
        id='kr91n475'
        to='romeo@montague.net/orchard'
        type='result'/>

The sender need not wait for these acknowledgements before sending further stanzas. However, it is RECOMMENDED that the sender does wait in order to minimize possible rate-limiting penalties.

It is possible that delivery of the stanza might fail, e.g. because the recipient has gone offline or because a server-to-server link has gone down). In this case the entity that detects the error shall return an appropriate XMPP stanza eror, such as <recipient-unavailable/> or <remote-server-timeout/>. Upon receiving notice that delivery of a data packet has failed, the sender MUST consider the bytestream to be closed and invalid.

It is also possible that the recipient might detect an error with the data packet, e.g. because the session ID is unknown, because the sequence number has already been used, or because the data is not formatted in accordance with Section 4 of RFC 4648. In this case the recipient shall return an appropriate XMPP stanza error, such as <item-not-found/>, <unexpected-request/> or <bad-request/>. Upon receiving notice that a data packet cannot be processed by the recipient, the sender SHOULD close the bytestream as described under Closing the Bytestream but MAY attempt to correct the error and re-send the offending data packet using the same sequence number (the recipient MUST NOT consider a sequence number to have been used until the data packet has been successfully processed).

Data packets MUST be processed in the order they are received. If an out-of-sequence packet is received for a particular bytestream (determined by checking the 'seq' attribute), then this indicates that a packet has been lost. The recipient MUST NOT process the data of such an out-of-sequence packet, nor any that follow it within the same bytestream; instead, the recipient MUST close the bytestream as described in the next section.

### 2.3 Closing the Bytestream

To close the bytestream, either party sends an IQ-set containing a <close/> element.

Example 8. Closing the bytestream

    <iq from='romeo@montague.net/orchard'
        id='us71g45j'
        to='juliet@capulet.com/balcony'
        type='set'>
      <close xmlns='http://jabber.org/protocol/ibb' sid='i781hf64'/>
    </iq>

The receiving party then acknowledges that the bytestream has been closed by returning an IQ-result.

Example 9. Success response

    <iq from='juliet@capulet.com/balcony'
        id='us71g45j'
        to='romeo@montague.net/orchard'
        type='result'/>

It is possible that the recipient of the close notification does not know about the bytestream, in which case it would return an <item-not-found/> error.

Example 10. Recipient does not know about the IBB session

    <iq type='error'
        from='juliet@capulet.com/balcony'
        to='romeo@montague.net/orchard'
        id='us71g45j'>
      <error type='cancel'>
        <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
      </error>
    </iq>

In either case, the sender MUST consider the bytestream to be closed and invalid.

## 3. Bidirectionality

An in-band bytestream is bidirectional. Therefore, either party to the bytestream is allowed to send data. Each sender MUST initialize the 'seq' attribute to zero and increment the 'seq' value by one with each chunk of data it sends. Each recipient MUST track chunks based on the 'seq' values it receives. The 'seq' values in each direction are independent of the values in the other direction. Thus there are two data sequences for each SessionID.

## 4. Use of Message Stanzas

It is RECOMMENDED to use IQ stanzas when sending data packets. However, an application MAY use message stanzas instead. If message stanzas are used when sending data packets, the sender SHOULD also use Advanced Message Processing [5] or some other stanza flow-control method. For proper tracking of delivery and processing errors related to data packets, the 'id' attribute SHOULD be used with message stanzas.

Example 11. Sending data in a message stanza

    <message from='romeo@montague.net/orchard'
             id='dsw71gj3'
             to='juliet@capulet.com/balcony'>
      <data xmlns='http://jabber.org/protocol/ibb' seq='0' sid='i781hf64'>
        qANQR1DBwU4DX7jmYZnncmUQB/9KuKBddzQH+tZ1ZywKK0yHKnq57kWq+RFtQdCJ
        WpdWpR0uQsuJe7+vh3NWn59/gTc5MDlX8dS9p0ovStmNcyLhxVgmqS8ZKhsblVeu
        IpQ0JgavABqibJolc3BKrVtVV1igKiX/N7Pi8RtY1K18toaMDhdEfhBRzO/XB0+P
        AQhYlRjNacGcslkhXqNjK5Va4tuOAPy2n1Q8UUrHbUd0g+xJ9Bm0G0LZXyvCWyKH
        kuNEHFQiLuCY6Iv0myq6iX6tjuHehZlFSh80b5BVV9tNLwNR5Eqz1klxMhoghJOA
      </data>
    </message>

## 5. Usage Guidelines

- Generally, in-band bytestreams SHOULD be used only as a last resort. SOCKS5 Bytestreams will almost always be preferable.
- A server MAY rate limit a connection, depending on the size and frequency of data packets.
- A server MAY disconnect a connection that sends overly large packets as defined by server policy.
- It is RECOMMENDED to use a 'block-size' of 4096.

## 6. Security Considerations

The In-Band Bytestreams protocol is as secure as the underlying XMPP transport. The application that uses IBB could have its own security layer, but this is outside of the scope of IBB.

An entity MUST verify any Base64 data received. An implementation MUST reject (not ignore) any characters that are not explicitly allowed by the Base64 alphabet; this helps to guard against creation of a covert channel that could be used to "leak" information. An implementation MUST NOT break on invalid input and MUST reject any sequence of Base64 characters containing the pad ('=') character if that character is included as something other than the last character of the data (e.g., "=AAA" or "BBBB=CCC"); this helps to guard against buffer overflow attacks and other attacks on the implementation. Base encoding visually hides otherwise easily recognized information, such as passwords, but does not provide any computational confidentiality. Base64 encoding MUST follow the definition in Section 4 of RFC 4648.

## 7. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].

## 8. XMPP Registrar Considerations

### 8.1 Protocol Namespaces

The XMPP Registrar [7] includes 'http://jabber.org/protocol/ibb' in its registry of XML namespaces at <http://xmpp.org/registrar/namespaces.html>.

## 9. XML Schema

    <?xml version='1.0' encoding='UTF-8'?>

    <xs:schema
        xmlns:xs='http://www.w3.org/2001/XMLSchema'
        targetNamespace='http://jabber.org/protocol/ibb'
        xmlns='http://jabber.org/protocol/ibb'
        elementFormDefault='qualified'>

      <xs:annotation>
        <xs:documentation>
          The protocol documented by this schema is defined in
          XEP-0047: http://www.xmpp.org/extensions/xep-0047.html
        </xs:documentation>
      </xs:annotation>

       <xs:element name='open'>
         <xs:complexType>
          <xs:simpleContent>
            <xs:extension base='empty'>
              <xs:attribute name='block-size' type='xs:unsignedShort' use='required'/>
              <xs:attribute name='sid' type='xs:NMTOKEN' use='required'/>
              <xs:attribute name='stanza' use='optional' default='iq'>
                <xs:simpleType>
                  <xs:restriction base='xs:NCName'>
                    <xs:enumeration value='iq'>
                    <xs:enumeration value='message'/>
                  </xs:restriction>
                </xs:simpleType>
              </xs:attribute>
            </xs:extension>
          </xs:simpleContent>
         </xs:complexType>
       </xs:element>

       <xs:element name='close'>
         <xs:complexType>
          <xs:simpleContent>
            <xs:extension base='empty'>
              <xs:attribute name='sid' type='xs:NMTOKEN' use='required'/>
            </xs:extension>
          </xs:simpleContent>
         </xs:complexType>
       </xs:element>

       <xs:element name='data'>
         <xs:complexType>
          <xs:simpleContent>
            <xs:extension base='xs:string'>
              <xs:attribute name='seq' type='xs:unsignedShort' use='required'/>
              <xs:attribute name='sid' type='xs:NMTOKEN' use='required'/>
            </xs:extension>
          </xs:simpleContent>
         </xs:complexType>
       </xs:element>

      <xs:simpleType name='empty'>
        <xs:restriction base='xs:string'>
          <xs:enumeration value=''/>
        </xs:restriction>
      </xs:simpleType>

    </xs:schema>

## Appendices

### Appendix A: Document Information

Series: XEP
Number: 0047
Publisher: XMPP Standards Foundation
Status: Draft
Type: Standards Track
Version: 1.2
Last Updated: 2009-03-17
Approving Body: XMPP Council
Dependencies: XMPP Core
Supersedes: None
Superseded By: None
Short Name: ibb
Schema: <http://www.xmpp.org/schemas/ibb.xsd>
Source Control: HTML RSS

### Appendix B: Author Information

### Justin Karneges

Email: justin@affinix.com
JabberID: justin@andbit.net

### Peter Saint-Andre

Email: stpeter@jabber.org
JabberID: stpeter@jabber.org
URI: https://stpeter.im/

### Appendix C: Legal Notices

#### Copyright

This XMPP Extension Protocol is copyright © 1999 - 2010 by the XMPP Standards Foundation (XSF).

#### Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

#### Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

#### Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

#### IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <http://xmpp.org/extensions/ipr-policy.shtml> or obtained by writing to XMPP Standards Foundation, c/o Peter Saint-Andre, 1899 Wynkoop Street, Suite 600, Denver, CO 80202 USA).

### Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) 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.

### Appendix E: Discussion Venue

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>.

### Appendix F: Requirements Conformance

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".

### Appendix G: Notes

1. XEP-0065: SOCKS5 Bytestreams <http://xmpp.org/extensions/xep-0065.html>.

2. XEP-0166: Jingle <http://xmpp.org/extensions/xep-0166.html>.

3. XEP-0261: Jingle In-Band Bytestreams Transport Method <http://xmpp.org/extensions/xep-0261.html>.

4. RFC 4648: The Base16, Base32, and Base64 Data Encodings <http://tools.ietf.org/html/rfc4648>.

5. XEP-0079: Advanced Message Processing <http://xmpp.org/extensions/xep-0079.html>.

6. 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/>.

7. 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/>.

### Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

#### Version 1.2 (2009-03-17)

Encouraged use of IQ stanzas rather than message stanzas for sending data; clarified bidirectional nature of IBB; more clearly specified data handling, error conditions, XML syntax, and attribute datatypes; added optional stanza attribute as in XEP-0261. (psa/jk)

#### Version 1.1 (2006-11-21)

Allowed IQ for delivery. (jk)

#### Version 1.0 (2003-12-10)

Per a vote of the Jabber Council, advanced status to Draft. (psa)

#### Version 0.8 (2003-12-04)

Add 'block-size' attribute and usage guidelines (jk)

#### Version 0.7 (2003-05-23)

Use IQ for shutdown, remove XEP-0022 dependency, loop the counter (jk)

#### Version 0.6 (2003-05-14)

Use message for delivery (jk)

#### Version 0.5 (2003-04-06)

Changed newseq to prevseq, added acks, changed seq size to 32-bit (jk)

#### Version 0.4 (2003-03-22)

Changed protocol, added sequence id (jk)

#### Version 0.3 (2002-12-10)

Removed the 'comment' block, changed namespace (jk)

#### Version 0.2 (2002-10-10)

Revised the text (jk)

#### Version 0.1 (2002-09-29)

Initial version. (jk)

END