XEP-0153: vCard-Based Avatars

Abstract:This document provides historical documentation of a vCard-based protocol for exchanging user avatars.
Author:Peter Saint-Andre
Copyright:© 1999 – 2017 XMPP Standards Foundation. SEE LEGAL NOTICES.
Last Updated:2018-02-26

NOTICE: This Historical specification provides canonical documentation of a protocol that is in use within the Jabber/XMPP community. This document is not a standards-track specification within the XMPP Standards Foundation's standards process; however, it might be converted to standards-track in the future or might be obsoleted by a more modern protocol.

Table of Contents

1. Introduction
2. Requirements
3. Use Cases
    3.1. User Publishes Avatar
    3.2. Contact Retrieves Avatar
4. Business Rules
    4.1. Inclusion of Update Data in Presence
    4.2. Downloading and Uploading the vCard
    4.3. Multiple Resources
    4.4. Resetting the Image Hash
    4.5. XML Syntax
    4.6. Image Restrictions
5. Implementation Notes
6. Security Considerations
7. IANA Considerations
8. XMPP Registrar Considerations
    8.1. Protocol Namespaces
9. XML Schema
10. Acknowledgements

    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

There exist several proposed protocols for communicating user avatar information over Jabber/XMPP (see IQ-Based Avatars (XEP-0008) [1] and User Avatar (XEP-0084) [2]). This document describes another such protocol that is in use today on the Jabber/XMPP network. This document is historical and does not purport to propose a standards-track protocol. However, a future protocol may improve on the approach documented herein.

2. Requirements

The protocol described herein seems to have been designed with the following requirements in mind:

3. Use Cases

3.1 User Publishes Avatar

Before informing contacts of the user's avatar, the user's client first publishes the avatar data to the user's public vCard using the protocol defined in vcard-temp (XEP-0054) [3].

Example 1. User's Client Publishes Avatar Data to vCard

<iq from='juliet@capulet.com'
  <vCard xmlns='vcard-temp'>

Example 2. User's Server Acknowledges Publish

<iq to='juliet@capulet.com' type='result' id='vc1'/>

Next, the user's client computes the SHA1 hash of the avatar image data itself (not the base64-encoded version) in accordance with RFC 3174 [4]. This hash is then included in the user's presence information. This is done by putting the hash encoded as hexadecimal digits as the XML character data of the <photo/> child of an <x/> element qualified by the 'vcard-temp:x:update' namespace, as shown in the following example:

Example 3. User's Client Includes Avatar Hash in Presence Broadcast

<presence from='juliet@capulet.com/balcony'>
  <x xmlns='vcard-temp:x:update'>

Note that while XML Schema defines the canonical representation of hexadecimal values to be upper-case, the historical use throughout the XMPP ecosystem has established lower-case use. Entities need to be able to process both and may prefer to emit lower-case for compatibility.

The user's server then broadcasts that presence information to all contacts who are subscribed to the user's presence information.

3.2 Contact Retrieves Avatar

When the recipient's client receives the hash of the avatar image, it SHOULD check the hash to determine if it already has a cached copy of that avatar image. If not, it retrieves the sender's full vCard in accordance with the protocol flow described in XEP-0054 (note that this request is sent to the user's bare JID, not full JID):

Example 4. Contact's Client Requests User's vCard

<iq from='romeo@montague.net/orchard'
  <vCard xmlns='vcard-temp'/>

Example 5. Server Returns vCard on Behalf of User

<iq from='juliet@capulet.com'
  <vCard xmlns='vcard-temp'>

4. Business Rules

4.1 Inclusion of Update Data in Presence

The following rules apply to inclusion of the update child element (<x xmlns='vcard-temp:x:update'/>) in presence broadcasts:

  1. If a client supports the protocol defined herein, it MUST include the update child element in every presence broadcast it sends and SHOULD also include the update child in directed presence stanzas (e.g., directed presence sent when joining Multi-User Chat (XEP-0045) [5] rooms).

  2. If a client is not yet ready to advertise an image, it MUST send an empty update child element, i.e.:

    Example 6. User Is Not Ready to Advertise an Image

      <x xmlns='vcard-temp:x:update'/>
  3. If there is no avatar image to be advertised, the photo element MUST be empty, i.e.:

    Example 7. No Image to be Advertised

      <x xmlns='vcard-temp:x:update'>

    If the client subsequently obtains an avatar image (e.g., by updating or retrieving the vCard), it SHOULD then publish a new <presence/> stanza with character data in the <photo/> element.

    Note: This enables recipients to distinguish between the absence of an image (empty photo element) and mere support for the protocol (empty update child).

4.2 Downloading and Uploading the vCard

The following rules apply to downloading and uploading the vCard:

  1. A client MUST NOT advertise an avatar image without first downloading the current vCard. Once it has done this, it MAY advertise an image. However, a client MUST advertise an image if it has just uploaded the vCard with a new avatar image. In this case, the client MAY choose not to redownload the vCard to verify its contents.

  2. Within a given session, a client MUST NOT attempt to upload a given avatar image more than once. The client MAY upload the avatar image to the vCard on login and after that MUST NOT upload the vCard again unless the user actively changes the avatar image.

  3. The client MUST NOT poll for new versions of the user's vCard in order to determine whether to update the avatar image hash.

4.3 Multiple Resources

Jabber/XMPP allows multiple resources to authenticate for the same JID simultaneously. This introduces the potential of conflict between the resources regarding the user's avatar image. The following rules apply when a client receives a presence broadcast from another resource of its own JID:

  1. If the presence stanza received from the other resource does not contain the update child element, then the other resource does not support vCard-based avatars. That resource could modify the contents of the vCard (including the photo element); because polling for vCard updates is not allowed, the client MUST stop advertising the avatar image hash. However, the client MAY reset its hash if all instances of non-conforming resources have gone offline.

  2. If the presence stanza received from the other resource contains the update child element, then the other resource conforms to the protocol for vCard-based avatars. There are three possible scenarios:

    • If the update child element is empty, then the other resource supports the protocol but does not have its own avatar image. Therefore the client can ignore the other resource and continue to broadcast the existing image hash.

    • If the update child element contains an empty photo element, then the other resource has updated the vCard with an empty BINVAL. Therefore the client MUST retrieve the vCard. If the retrieved vCard contains a photo element with an empty BINVAL, then the client MUST stop advertising the old image.

    • If the update child element contains a non-empty photo element, then the client MUST compare the image hashes. If the hashes are identical, then the client can ignore the other resource and continue to broadcast the existing image hash. If the hashes are different, then the client MUST NOT attempt to resolve the conflict by uploading its avatar image again. Instead, it MUST defer to the content of the retrieved vCard by resetting its image hash (see below) and providing that hash in future presence broadcasts.

4.4 Resetting the Image Hash

Resetting the image hash consists of the following steps:

  1. Immediately send out a presence element with an empty update child element (containing no photo element).

  2. Download the vCard from the server.

  3. If the BINVAL is empty or missing, advertise an empty photo element in future presence broadcasts.

  4. If the BINVAL contains image data, calculate the hash of image and advertise that hash in future presence broadcasts.

4.5 XML Syntax

The following rules apply to the XML syntax:

  1. The <PHOTO/> element SHOULD contain a <BINVAL/> child whose XML character data is Base64-encoded data for the avatar image.

  2. The <PHOTO/> element SHOULD NOT contain an <EXTVAL/> that points to a URI for the image file.

  3. The <PHOTO/> element MUST NOT contain the avatar image itself.

  4. The <PHOTO/> element SHOULD contain a <TYPE/> child whose XML character data specifies the content-type of the image data. The XML character data SHOULD be "image/gif", "image/jpeg", or "image/png".

  5. The <PHOTO/> element MUST NOT possess a 'mime-type' attribute.

4.6 Image Restrictions

The following rules apply to images:

  1. The image SHOULD use less than eight kilobytes (8k) of data; this restriction is to be enforced by the publishing client.

  2. The image height and width SHOULD be between thirty-two (32) and ninety-six (96) pixels; the recommended size is sixty-four (64) pixels high and sixty-four (64) pixels wide.

  3. The image SHOULD be square.

  4. The image content type [6] SHOULD be image/gif, image/jpeg, or image/png; support for the "image/png" content type is REQUIRED, support for the "image/gif" and "image/jpeg" content types is RECOMMENDED, and support for any other content type is OPTIONAL.

  5. The image data MUST conform to the base64Binary datatype [7] and thus be encoded in accordance with Section 6.8 of RFC 2045 [8], which recommends that base64 data should have lines limited to at most 76 characters in length. However, any whitespace characters (e.g., '\r' and '\n') MUST be ignored.

5. Implementation Notes

The XML character data of the <TYPE/> element is a hint. If the XML character data of the <TYPE/> specifies a content type that does not match the data provided in the <BINVAL/> element, the processing application MUST adhere to the content type of the actual image data and MUST ignore the <TYPE/>. If the <TYPE/> is something other than image/gif, image/jpeg, or image/png, it SHOULD be ignored.

If the image data exceeds the 8 KB restriction, the processing application SHOULD process the data.

6. Security Considerations

This document introduces no security considerations above and beyond those described in XMPP Core [9], XMPP IM [10], and vcard-temp (XEP-0054) [3].

7. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [11].

8. XMPP Registrar Considerations

8.1 Protocol Namespaces

The XMPP Registrar [12] includes 'vcard-temp:x:update' in its registry of protocol namespaces (see <https://xmpp.org/registrar/namespaces.html>).

9. XML Schema

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


      The protocol documented by this schema is defined in
      XEP-0153: http://www.xmpp.org/extensions/xep-0153.html

  <xs:element name='x'>
        <xs:element name='photo' minOccurs='0' type='xs:hexBinary'/>


10. Acknowledgements

The author wishes to thank the helpful developers who have implemented this protocol and provided feedback regarding its documentation.


Appendix A: Document Information

Series: XEP
Number: 0153
Publisher: XMPP Standards Foundation
Status: Active
Type: Historical
Version: 1.1
Last Updated: 2018-02-26
Approving Body: XMPP Council
Dependencies: XMPP Core, XMPP IM, XEP-0054
Supersedes: None
Superseded By: None
Short Name: vcard-avatar
Schema: <http://www.xmpp.org/schemas/vcard-avatar.xsd>
Source Control: HTML
This document in other formats: XML  PDF

Appendix B: Author Information

Peter Saint-Andre

Email: xsf@stpeter.im
JabberID: peter@jabber.org
URI: http://stpeter.im/

Appendix C: Legal Notices


This XMPP Extension Protocol is copyright © 1999 – 2018 by the XMPP Standards Foundation (XSF).


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 <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).

Appendix D: Relation to XMPP

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.

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-0008: IQ-Based Avatars <https://xmpp.org/extensions/xep-0008.html>.

2. XEP-0084: User Avatar <https://xmpp.org/extensions/xep-0084.html>.

3. XEP-0054: vcard-temp <https://xmpp.org/extensions/xep-0054.html>.

4. RFC 3174: US Secure Hash Algorithm 1 (SHA1) <http://tools.ietf.org/html/rfc3174>.

5. XEP-0045: Multi-User Chat <https://xmpp.org/extensions/xep-0045.html>.

6. The IANA registry of content types is located at <http://www.iana.org/assignments/media-types/>.

7. See <http://www.w3.org/TR/xmlschema-2/#base64Binary>.

8. RFC 2045: Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies <http://tools.ietf.org/html/rfc2045>.

9. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.

10. RFC 6121: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence <http://tools.ietf.org/html/rfc6121>.

11. 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/>.

12. 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 <https://xmpp.org/registrar/>.

Appendix H: Revision History

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

Version 1.1 (2018-02-26)

Clarify encoding of the photo hash in the presence update. (jwi)

Version 1.0 (2006-08-16)

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

Version 0.3 (2006-01-12)

Collected all syntax rules into dedicated section; incorporated feedback from implementation experience; adjusted text regarding base64 encoding. (psa)

Version 0.2 (2005-10-18)

Changed 8k limit from MUST NOT to SHOULD NOT; specified that client should publish new presence stanza if it obtains an avatar image after sending an empty photo element; specified that the update child should be included in directed presence stanzas; more clearly specified Base64 rules. (psa)

Version 0.1 (2005-06-16)

Initial version. (psa)

Version 0.0.3 (2005-06-14)

Changed type from Informational to Historical, adjusted text accordingly. (psa)

Version 0.0.2 (2005-06-13)

Specified that the image data is actually Base64 encoded. (psa)

Version 0.0.1 (2005-06-09)

First draft. (psa)