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RFC 3920

Extensible Messaging and Presence Protocol (XMPP): Core

Pages: 30
Obsoleted by:  6120
Updated by:  6122
Part 3 of 4 – Pages 48 to 74
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9. XML Stanzas

After TLS negotiation (Section 5) if desired, SASL negotiation (Section 6), and Resource Binding (Section 7) if necessary, XML stanzas can be sent over the streams. Three kinds of XML stanza are defined for the 'jabber:client' and 'jabber:server' namespaces: <message/>, <presence/>, and <iq/>. In addition, there are five common attributes for these kinds of stanza. These common attributes, as well as the basic semantics of the three stanza kinds, are defined herein; more detailed information regarding the syntax of XML stanzas in relation to instant messaging and presence applications is provided in [XMPP-IM].

9.1. Common Attributes

The following five attributes are common to message, presence, and IQ stanzas:

9.1.1. to

The 'to' attribute specifies the JID of the intended recipient for the stanza. In the 'jabber:client' namespace, a stanza SHOULD possess a 'to' attribute, although a stanza sent from a client to a server for handling by that server (e.g., presence sent to the server for broadcasting to other entities) SHOULD NOT possess a 'to' attribute. In the 'jabber:server' namespace, a stanza MUST possess a 'to' attribute; if a server receives a stanza that does not meet this restriction, it MUST generate an <improper-addressing/> stream error condition and terminate both the XML stream and the underlying TCP connection with the offending server.
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   If the value of the 'to' attribute is invalid or cannot be contacted,
   the entity discovering that fact (usually the sender's or recipient's
   server) MUST return an appropriate error to the sender, setting the
   'from' attribute of the error stanza to the value provided in the
   'to' attribute of the offending stanza.

9.1.2. from

The 'from' attribute specifies the JID of the sender. When a server receives an XML stanza within the context of an authenticated stream qualified by the 'jabber:client' namespace, it MUST do one of the following: 1. validate that the value of the 'from' attribute provided by the client is that of a connected resource for the associated entity 2. add a 'from' address to the stanza whose value is the bare JID (<node@domain>) or the full JID (<node@domain/resource>) determined by the server for the connected resource that generated the stanza (see Determination of Addresses (Section 3.5)) If a client attempts to send an XML stanza for which the value of the 'from' attribute does not match one of the connected resources for that entity, the server SHOULD return an <invalid-from/> stream error to the client. If a client attempts to send an XML stanza over a stream that is not yet authenticated, the server SHOULD return a <not-authorized/> stream error to the client. If generated, both of these conditions MUST result in closure of the stream and termination of the underlying TCP connection; this helps to prevent a denial of service attack launched from a rogue client. When a server generates a stanza from the server itself for delivery to a connected client (e.g., in the context of data storage services provided by the server on behalf of the client), the stanza MUST either (1) not include a 'from' attribute or (2) include a 'from' attribute whose value is the account's bare JID (<node@domain>) or client's full JID (<node@domain/resource>). A server MUST NOT send to the client a stanza without a 'from' attribute if the stanza was not generated by the server itself. When a client receives a stanza that does not include a 'from' attribute, it MUST assume that the stanza is from the server to which the client is connected. In the 'jabber:server' namespace, a stanza MUST possess a 'from' attribute; if a server receives a stanza that does not meet this restriction, it MUST generate an <improper-addressing/> stream error condition. Furthermore, the domain identifier portion of the JID
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   contained in the 'from' attribute MUST match the hostname of the
   sending server (or any validated domain thereof, such as a validated
   subdomain of the sending server's hostname or another validated
   domain hosted by the sending server) as communicated in the SASL
   negotiation or dialback negotiation; if a server receives a stanza
   that does not meet this restriction, it MUST generate an
   <invalid-from/> stream error condition.  Both of these conditions
   MUST result in closing of the stream and termination of the
   underlying TCP connection; this helps to prevent a denial of service
   attack launched from a rogue server.

9.1.3. id

The optional 'id' attribute MAY be used by a sending entity for internal tracking of stanzas that it sends and receives (especially for tracking the request-response interaction inherent in the semantics of IQ stanzas). It is OPTIONAL for the value of the 'id' attribute to be unique globally, within a domain, or within a stream. The semantics of IQ stanzas impose additional restrictions; see IQ Semantics (Section 9.2.3).

9.1.4. type

The 'type' attribute specifies detailed information about the purpose or context of the message, presence, or IQ stanza. The particular allowable values for the 'type' attribute vary depending on whether the stanza is a message, presence, or IQ; the values for message and presence stanzas are specific to instant messaging and presence applications and therefore are defined in [XMPP-IM], whereas the values for IQ stanzas specify the role of an IQ stanza in a structured request-response "conversation" and thus are defined under IQ Semantics (Section 9.2.3) below. The only 'type' value common to all three stanzas is "error"; see Stanza Errors (Section 9.3).

9.1.5. xml:lang

A stanza SHOULD possess an 'xml:lang' attribute (as defined in Section 2.12 of [XML]) if the stanza contains XML character data that is intended to be presented to a human user (as explained in RFC 2277 [CHARSET], "internationalization is for humans"). The value of the 'xml:lang' attribute specifies the default language of any such human-readable XML character data, which MAY be overridden by the 'xml:lang' attribute of a specific child element. If a stanza does not possess an 'xml:lang' attribute, an implementation MUST assume that the default language is that specified for the stream as defined under Stream Attributes (Section 4.4) above. The value of the 'xml:lang' attribute MUST be an NMTOKEN and MUST conform to the format defined in RFC 3066 [LANGTAGS].
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9.2. Basic Semantics

9.2.1. Message Semantics

The <message/> stanza kind can be seen as a "push" mechanism whereby one entity pushes information to another entity, similar to the communications that occur in a system such as email. All message stanzas SHOULD possess a 'to' attribute that specifies the intended recipient of the message; upon receiving such a stanza, a server SHOULD route or deliver it to the intended recipient (see Server Rules for Handling XML Stanzas (Section 10) for general routing and delivery rules related to XML stanzas).

9.2.2. Presence Semantics

The <presence/> element can be seen as a basic broadcast or "publish-subscribe" mechanism, whereby multiple entities receive information about an entity to which they have subscribed (in this case, network availability information). In general, a publishing entity SHOULD send a presence stanza with no 'to' attribute, in which case the server to which the entity is connected SHOULD broadcast or multiplex that stanza to all subscribing entities. However, a publishing entity MAY also send a presence stanza with a 'to' attribute, in which case the server SHOULD route or deliver that stanza to the intended recipient. See Server Rules for Handling XML Stanzas (Section 10) for general routing and delivery rules related to XML stanzas, and [XMPP-IM] for presence-specific rules in the context of an instant messaging and presence application.

9.2.3. IQ Semantics

Info/Query, or IQ, is a request-response mechanism, similar in some ways to [HTTP]. The semantics of IQ enable an entity to make a request of, and receive a response from, another entity. The data content of the request and response is defined by the namespace declaration of a direct child element of the IQ element, and the interaction is tracked by the requesting entity through use of the 'id' attribute. Thus, IQ interactions follow a common pattern of structured data exchange such as get/result or set/result (although an error may be returned in reply to a request if appropriate):
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   Requesting                 Responding
     Entity                     Entity
   ----------                 ----------
       |                           |
       | <iq type='get' id='1'>    |
       | ------------------------> |
       |                           |
       | <iq type='result' id='1'> |
       | <------------------------ |
       |                           |
       | <iq type='set' id='2'>    |
       | ------------------------> |
       |                           |
       | <iq type='error' id='2'>  |
       | <------------------------ |
       |                           |

   In order to enforce these semantics, the following rules apply:

   1.  The 'id' attribute is REQUIRED for IQ stanzas.

   2.  The 'type' attribute is REQUIRED for IQ stanzas.  The value MUST
       be one of the following:

       *  get -- The stanza is a request for information or
          requirements.

       *  set -- The stanza provides required data, sets new values, or
          replaces existing values.

       *  result -- The stanza is a response to a successful get or set
          request.

       *  error -- An error has occurred regarding processing or
          delivery of a previously-sent get or set (see Stanza Errors
          (Section 9.3)).

   3.  An entity that receives an IQ request of type "get" or "set" MUST
       reply with an IQ response of type "result" or "error" (the
       response MUST preserve the 'id' attribute of the request).

   4.  An entity that receives a stanza of type "result" or "error" MUST
       NOT respond to the stanza by sending a further IQ response of
       type "result" or "error"; however, as shown above, the requesting
       entity MAY send another request (e.g., an IQ of type "set" in
       order to provide required information discovered through a
       get/result pair).
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   5.  An IQ stanza of type "get" or "set" MUST contain one and only one
       child element that specifies the semantics of the particular
       request or response.

   6.  An IQ stanza of type "result" MUST include zero or one child
       elements.

   7.  An IQ stanza of type "error" SHOULD include the child element
       contained in the associated "get" or "set" and MUST include an
       <error/> child; for details, see Stanza Errors (Section 9.3).

9.3. Stanza Errors

Stanza-related errors are handled in a manner similar to stream errors (Section 4.7). However, unlike stream errors, stanza errors are recoverable; therefore error stanzas include hints regarding actions that the original sender can take in order to remedy the error.

9.3.1. Rules

The following rules apply to stanza-related errors: o The receiving or processing entity that detects an error condition in relation to a stanza MUST return to the sending entity a stanza of the same kind (message, presence, or IQ), whose 'type' attribute is set to a value of "error" (such a stanza is called an "error stanza" herein). o The entity that generates an error stanza SHOULD include the original XML sent so that the sender can inspect and, if necessary, correct the XML before attempting to resend. o An error stanza MUST contain an <error/> child element. o An <error/> child MUST NOT be included if the 'type' attribute has a value other than "error" (or if there is no 'type' attribute). o An entity that receives an error stanza MUST NOT respond to the stanza with a further error stanza; this helps to prevent looping.
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9.3.2. Syntax

The syntax for stanza-related errors is as follows: <stanza-kind to='sender' type='error'> [RECOMMENDED to include sender XML here] <error type='error-type'> <defined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> <text xmlns='urn:ietf:params:xml:ns:xmpp-stanzas' xml:lang='langcode'> OPTIONAL descriptive text </text> [OPTIONAL application-specific condition element] </error> </stanza-kind> The stanza-kind is one of message, presence, or iq. The value of the <error/> element's 'type' attribute MUST be one of the following: o cancel -- do not retry (the error is unrecoverable) o continue -- proceed (the condition was only a warning) o modify -- retry after changing the data sent o auth -- retry after providing credentials o wait -- retry after waiting (the error is temporary) The <error/> element: o MUST contain a child element corresponding to one of the defined stanza error conditions specified below; this element MUST be qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace. o MAY contain a <text/> child containing XML character data that describes the error in more detail; this element MUST be qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace and SHOULD possess an 'xml:lang' attribute. o MAY contain a child element for an application-specific error condition; this element MUST be qualified by an application-defined namespace, and its structure is defined by that namespace. The <text/> element is OPTIONAL. If included, it SHOULD be used only to provide descriptive or diagnostic information that supplements the meaning of a defined condition or application-specific condition. It SHOULD NOT be interpreted programmatically by an application. It
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   SHOULD NOT be used as the error message presented to a user, but MAY
   be shown in addition to the error message associated with the
   included condition element (or elements).

   Finally, to maintain backward compatibility, the schema (specified in
   [XMPP-IM]) allows the optional inclusion of a 'code' attribute on the
   <error/> element.

9.3.3. Defined Conditions

The following conditions are defined for use in stanza errors. o <bad-request/> -- the sender has sent XML that is malformed or that cannot be processed (e.g., an IQ stanza that includes an unrecognized value of the 'type' attribute); the associated error type SHOULD be "modify". o <conflict/> -- access cannot be granted because an existing resource or session exists with the same name or address; the associated error type SHOULD be "cancel". o <feature-not-implemented/> -- the feature requested is not implemented by the recipient or server and therefore cannot be processed; the associated error type SHOULD be "cancel". o <forbidden/> -- the requesting entity does not possess the required permissions to perform the action; the associated error type SHOULD be "auth". o <gone/> -- the recipient or server can no longer be contacted at this address (the error stanza MAY contain a new address in the XML character data of the <gone/> element); the associated error type SHOULD be "modify". o <internal-server-error/> -- the server could not process the stanza because of a misconfiguration or an otherwise-undefined internal server error; the associated error type SHOULD be "wait". o <item-not-found/> -- the addressed JID or item requested cannot be found; the associated error type SHOULD be "cancel". o <jid-malformed/> -- the sending entity has provided or communicated an XMPP address (e.g., a value of the 'to' attribute) or aspect thereof (e.g., a resource identifier) that does not adhere to the syntax defined in Addressing Scheme (Section 3); the associated error type SHOULD be "modify".
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   o  <not-acceptable/> -- the recipient or server understands the
      request but is refusing to process it because it does not meet
      criteria defined by the recipient or server (e.g., a local policy
      regarding acceptable words in messages); the associated error type
      SHOULD be "modify".

   o  <not-allowed/> -- the recipient or server does not allow any
      entity to perform the action; the associated error type SHOULD be
      "cancel".

   o  <not-authorized/> -- the sender must provide proper credentials
      before being allowed to perform the action, or has provided
      improper credentials; the associated error type SHOULD be "auth".

   o  <payment-required/> -- the requesting entity is not authorized to
      access the requested service because payment is required; the
      associated error type SHOULD be "auth".

   o  <recipient-unavailable/> -- the intended recipient is temporarily
      unavailable; the associated error type SHOULD be "wait" (note: an
      application MUST NOT return this error if doing so would provide
      information about the intended recipient's network availability to
      an entity that is not authorized to know such information).

   o  <redirect/> -- the recipient or server is redirecting requests for
      this information to another entity, usually temporarily (the error
      stanza SHOULD contain the alternate address, which MUST be a valid
      JID, in the XML character data of the <redirect/> element); the
      associated error type SHOULD be "modify".

   o  <registration-required/> -- the requesting entity is not
      authorized to access the requested service because registration is
      required; the associated error type SHOULD be "auth".

   o  <remote-server-not-found/> -- a remote server or service specified
      as part or all of the JID of the intended recipient does not
      exist; the associated error type SHOULD be "cancel".

   o  <remote-server-timeout/> -- a remote server or service specified
      as part or all of the JID of the intended recipient (or required
      to fulfill a request) could not be contacted within a reasonable
      amount of time; the associated error type SHOULD be "wait".

   o  <resource-constraint/> -- the server or recipient lacks the system
      resources necessary to service the request; the associated error
      type SHOULD be "wait".
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   o  <service-unavailable/> -- the server or recipient does not
      currently provide the requested service; the associated error type
      SHOULD be "cancel".

   o  <subscription-required/> -- the requesting entity is not
      authorized to access the requested service because a subscription
      is required; the associated error type SHOULD be "auth".

   o  <undefined-condition/> -- the error condition is not one of those
      defined by the other conditions in this list; any error type may
      be associated with this condition, and it SHOULD be used only in
      conjunction with an application-specific condition.

   o  <unexpected-request/> -- the recipient or server understood the
      request but was not expecting it at this time (e.g., the request
      was out of order); the associated error type SHOULD be "wait".

9.3.4. Application-Specific Conditions

As noted, an application MAY provide application-specific stanza error information by including a properly-namespaced child in the error element. The application-specific element SHOULD supplement or further qualify a defined element. Thus, the <error/> element will contain two or three child elements: <iq type='error' id='some-id'> <error type='modify'> <bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> <too-many-parameters xmlns='application-ns'/> </error> </iq> <message type='error' id='another-id'> <error type='modify'> <undefined-condition xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> <text xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'> Some special application diagnostic information... </text> <special-application-condition xmlns='application-ns'/> </error> </message>
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10. Server Rules for Handling XML Stanzas

Compliant server implementations MUST ensure in-order processing of XML stanzas between any two entities. Beyond the requirement for in-order processing, each server implementation will contain its own "delivery tree" for handling stanzas it receives. Such a tree determines whether a stanza needs to be routed to another domain, processed internally, or delivered to a resource associated with a connected node. The following rules apply:

10.1. No 'to' Address

If the stanza possesses no 'to' attribute, the server SHOULD process it on behalf of the entity that sent it. Because all stanzas received from other servers MUST possess a 'to' attribute, this rule applies only to stanzas received from a registered entity (such as a client) that is connected to the server. If the server receives a presence stanza with no 'to' attribute, the server SHOULD broadcast it to the entities that are subscribed to the sending entity's presence, if applicable (the semantics of presence broadcast for instant messaging and presence applications are defined in [XMPP-IM]). If the server receives an IQ stanza of type "get" or "set" with no 'to' attribute and it understands the namespace that qualifies the content of the stanza, it MUST either process the stanza on behalf of the sending entity (where the meaning of "process" is determined by the semantics of the qualifying namespace) or return an error to the sending entity.

10.2. Foreign Domain

If the hostname of the domain identifier portion of the JID contained in the 'to' attribute does not match one of the configured hostnames of the server itself or a subdomain thereof, the server SHOULD route the stanza to the foreign domain (subject to local service provisioning and security policies regarding inter-domain communication). There are two possible cases: A server-to-server stream already exists between the two domains: The sender's server routes the stanza to the authoritative server for the foreign domain over the existing stream There exists no server-to-server stream between the two domains: The sender's server (1) resolves the hostname of the foreign domain (as defined under Server-to-Server Communications (Section 14.4)), (2) negotiates a server-to-server stream between the two domains (as defined under Use of TLS (Section 5) and Use of SASL (Section
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      6)), and (3) routes the stanza to the authoritative server for the
      foreign domain over the newly-established stream

   If routing to the recipient's server is unsuccessful, the sender's
   server MUST return an error to the sender; if the recipient's server
   can be contacted but delivery by the recipient's server to the
   recipient is unsuccessful, the recipient's server MUST return an
   error to the sender by way of the sender's server.

10.3. Subdomain

If the hostname of the domain identifier portion of the JID contained in the 'to' attribute matches a subdomain of one of the configured hostnames of the server itself, the server MUST either process the stanza itself or route the stanza to a specialized service that is responsible for that subdomain (if the subdomain is configured), or return an error to the sender (if the subdomain is not configured).

10.4. Mere Domain or Specific Resource

If the hostname of the domain identifier portion of the JID contained in the 'to' attribute matches a configured hostname of the server itself and the JID contained in the 'to' attribute is of the form <domain> or <domain/resource>, the server (or a defined resource thereof) MUST either process the stanza as appropriate for the stanza kind or return an error stanza to the sender.

10.5. Node in Same Domain

If the hostname of the domain identifier portion of the JID contained in the 'to' attribute matches a configured hostname of the server itself and the JID contained in the 'to' attribute is of the form <node@domain> or <node@domain/resource>, the server SHOULD deliver the stanza to the intended recipient of the stanza as represented by the JID contained in the 'to' attribute. The following rules apply: 1. If the JID contains a resource identifier (i.e., is of the form <node@domain/resource>) and there exists a connected resource that matches the full JID, the recipient's server SHOULD deliver the stanza to the stream or session that exactly matches the resource identifier. 2. If the JID contains a resource identifier and there exists no connected resource that matches the full JID, the recipient's server SHOULD return a <service-unavailable/> stanza error to the sender.
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   3.  If the JID is of the form <node@domain> and there exists at least
       one connected resource for the node, the recipient's server
       SHOULD deliver the stanza to at least one of the connected
       resources, according to application-specific rules (a set of
       delivery rules for instant messaging and presence applications is
       defined in [XMPP-IM]).

11. XML Usage within XMPP

11.1. Restrictions

XMPP is a simplified and specialized protocol for streaming XML elements in order to exchange structured information in close to real time. Because XMPP does not require the parsing of arbitrary and complete XML documents, there is no requirement that XMPP needs to support the full feature set of [XML]. In particular, the following restrictions apply. With regard to XML generation, an XMPP implementation MUST NOT inject into an XML stream any of the following: o comments (as defined in Section 2.5 of [XML]) o processing instructions (Section 2.6 therein) o internal or external DTD subsets (Section 2.8 therein) o internal or external entity references (Section 4.2 therein) with the exception of predefined entities (Section 4.6 therein) o character data or attribute values containing unescaped characters that map to the predefined entities (Section 4.6 therein); such characters MUST be escaped With regard to XML processing, if an XMPP implementation receives such restricted XML data, it MUST ignore the data.

11.2. XML Namespace Names and Prefixes

XML Namespaces [XML-NAMES] are used within all XMPP-compliant XML to create strict boundaries of data ownership. The basic function of namespaces is to separate different vocabularies of XML elements that are structurally mixed together. Ensuring that XMPP-compliant XML is namespace-aware enables any allowable XML to be structurally mixed with any data element within XMPP. Rules for XML namespace names and prefixes are defined in the following subsections.
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11.2.1. Streams Namespace

A streams namespace declaration is REQUIRED in all XML stream headers. The name of the streams namespace MUST be 'http://etherx.jabber.org/streams'. The element names of the <stream/> element and its <features/> and <error/> children MUST be qualified by the streams namespace prefix in all instances. An implementation SHOULD generate only the 'stream:' prefix for these elements, and for historical reasons MAY accept only the 'stream:' prefix.

11.2.2. Default Namespace

A default namespace declaration is REQUIRED and is used in all XML streams in order to define the allowable first-level children of the root stream element. This namespace declaration MUST be the same for the initial stream and the response stream so that both streams are qualified consistently. The default namespace declaration applies to the stream and all stanzas sent within a stream (unless explicitly qualified by another namespace, or by the prefix of the streams namespace or the dialback namespace). A server implementation MUST support the following two default namespaces (for historical reasons, some implementations MAY support only these two default namespaces): o jabber:client -- this default namespace is declared when the stream is used for communications between a client and a server o jabber:server -- this default namespace is declared when the stream is used for communications between two servers A client implementation MUST support the 'jabber:client' default namespace, and for historical reasons MAY support only that default namespace. An implementation MUST NOT generate namespace prefixes for elements in the default namespace if the default namespace is 'jabber:client' or 'jabber:server'. An implementation SHOULD NOT generate namespace prefixes for elements qualified by content (as opposed to stream) namespaces other than 'jabber:client' and 'jabber:server'. Note: The 'jabber:client' and 'jabber:server' namespaces are nearly identical but are used in different contexts (client-to-server communications for 'jabber:client' and server-to-server communications for 'jabber:server'). The only difference between the two is that the 'to' and 'from' attributes are OPTIONAL on stanzas sent within 'jabber:client', whereas they are REQUIRED on stanzas
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   sent within 'jabber:server'.  If a compliant implementation accepts a
   stream that is qualified by the 'jabber:client' or 'jabber:server'
   namespace, it MUST support the common attributes (Section 9.1) and
   basic semantics (Section 9.2) of all three core stanza kinds
   (message, presence, and IQ).

11.2.3. Dialback Namespace

A dialback namespace declaration is REQUIRED for all elements used in server dialback (Section 8). The name of the dialback namespace MUST be 'jabber:server:dialback'. All elements qualified by this namespace MUST be prefixed. An implementation SHOULD generate only the 'db:' prefix for such elements and MAY accept only the 'db:' prefix.

11.3. Validation

Except as noted with regard to 'to' and 'from' addresses for stanzas within the 'jabber:server' namespace, a server is not responsible for validating the XML elements forwarded to a client or another server; an implementation MAY choose to provide only validated data elements but this is OPTIONAL (although an implementation MUST NOT accept XML that is not well-formed). Clients SHOULD NOT rely on the ability to send data which does not conform to the schemas, and SHOULD ignore any non-conformant elements or attributes on the incoming XML stream. Validation of XML streams and stanzas is OPTIONAL, and schemas are included herein for descriptive purposes only.

11.4. Inclusion of Text Declaration

Implementations SHOULD send a text declaration before sending a stream header. Applications MUST follow the rules in [XML] regarding the circumstances under which a text declaration is included.

11.5. Character Encoding

Implementations MUST support the UTF-8 (RFC 3629 [UTF-8]) transformation of Universal Character Set (ISO/IEC 10646-1 [UCS2]) characters, as required by RFC 2277 [CHARSET]. Implementations MUST NOT attempt to use any other encoding.

12. Core Compliance Requirements

This section summarizes the specific aspects of the Extensible Messaging and Presence Protocol that MUST be supported by servers and clients in order to be considered compliant implementations, as well as additional protocol aspects that SHOULD be supported. For compliance purposes, we draw a distinction between core protocols
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   (which MUST be supported by any server or client, regardless of the
   specific application) and instant messaging protocols (which MUST be
   supported only by instant messaging and presence applications built
   on top of the core protocols).  Compliance requirements that apply to
   all servers and clients are specified in this section; compliance
   requirements for instant messaging servers and clients are specified
   in the corresponding section of [XMPP-IM].

12.1. Servers

In addition to all defined requirements with regard to security, XML usage, and internationalization, a server MUST support the following core protocols in order to be considered compliant: o Application of the [NAMEPREP], Nodeprep (Appendix A), and Resourceprep (Appendix B) profiles of [STRINGPREP] to addresses (including ensuring that domain identifiers are internationalized domain names as defined in [IDNA]) o XML streams (Section 4), including Use of TLS (Section 5), Use of SASL (Section 6), and Resource Binding (Section 7) o The basic semantics of the three defined stanza kinds (i.e., <message/>, <presence/>, and <iq/>) as specified in stanza semantics (Section 9.2) o Generation (and, where appropriate, handling) of error syntax and semantics related to streams, TLS, SASL, and XML stanzas In addition, a server MAY support the following core protocol: o Server dialback (Section 8)

12.2. Clients

A client MUST support the following core protocols in order to be considered compliant: o XML streams (Section 4), including Use of TLS (Section 5), Use of SASL (Section 6), and Resource Binding (Section 7) o The basic semantics of the three defined stanza kinds (i.e., <message/>, <presence/>, and <iq/>) as specified in stanza semantics (Section 9.2) o Handling (and, where appropriate, generation) of error syntax and semantics related to streams, TLS, SASL, and XML stanzas
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   In addition, a client SHOULD support the following core protocols:

   o  Generation of addresses to which the [NAMEPREP], Nodeprep
      (Appendix A), and Resourceprep (Appendix B) profiles of
      [STRINGPREP] can be applied without failing

13. Internationalization Considerations

XML streams MUST be encoded in UTF-8 as specified under Character Encoding (Section 11.5). As specified under Stream Attributes (Section 4.4), an XML stream SHOULD include an 'xml:lang' attribute that is treated as the default language for any XML character data sent over the stream that is intended to be presented to a human user. As specified under xml:lang (Section 9.1.5), an XML stanza SHOULD include an 'xml:lang' attribute if the stanza contains XML character data that is intended to be presented to a human user. A server SHOULD apply the default 'xml:lang' attribute to stanzas it routes or delivers on behalf of connected entities, and MUST NOT modify or delete 'xml:lang' attributes from stanzas it receives from other entities.

14. Security Considerations

14.1. High Security

For the purposes of XMPP communications (client-to-server and server-to-server), the term "high security" refers to the use of security technologies that provide both mutual authentication and integrity-checking; in particular, when using certificate-based authentication to provide high security, a chain-of-trust SHOULD be established out-of-band, although a shared certificate authority signing certificates could allow a previously unknown certificate to establish trust in-band. See Section 14.2 below regarding certificate validation procedures. Implementations MUST support high security. Service provisioning SHOULD use high security, subject to local security policies.

14.2. Certificate Validation

When an XMPP peer communicates with another peer securely, it MUST validate the peer's certificate. There are three possible cases: Case #1: The peer contains an End Entity certificate which appears to be certified by a chain of certificates terminating in a trust anchor (as described in Section 6.1 of [X509]).
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   Case #2: The peer certificate is certified by a Certificate Authority
      not known to the validating peer.

   Case #3: The peer certificate is self-signed.

   In Case #1, the validating peer MUST do one of two things:

   1.  Verify the peer certificate according to the rules of [X509].
       The certificate SHOULD then be checked against the expected
       identity of the peer following the rules described in [HTTP-TLS],
       except that a subjectAltName extension of type "xmpp" MUST be
       used as the identity if present.  If one of these checks fails,
       user-oriented clients MUST either notify the user (clients MAY
       give the user the opportunity to continue with the connection in
       any case) or terminate the connection with a bad certificate
       error.  Automated clients SHOULD terminate the connection (with a
       bad certificate error) and log the error to an appropriate audit
       log.  Automated clients MAY provide a configuration setting that
       disables this check, but MUST provide a setting that enables it.

   2.  The peer SHOULD show the certificate to a user for approval,
       including the entire certificate chain.  The peer MUST cache the
       certificate (or some non-forgeable representation such as a
       hash).  In future connections, the peer MUST verify that the same
       certificate was presented and MUST notify the user if it has
       changed.

   In Case #2 and Case #3, implementations SHOULD act as in (2) above.

14.3. Client-to-Server Communications

A compliant client implementation MUST support both TLS and SASL for connections to a server. The TLS protocol for encrypting XML streams (defined under Use of TLS (Section 5)) provides a reliable mechanism for helping to ensure the confidentiality and data integrity of data exchanged between two entities. The SASL protocol for authenticating XML streams (defined under Use of SASL (Section 6)) provides a reliable mechanism for validating that a client connecting to a server is who it claims to be. Client-to-server communications MUST NOT proceed until the DNS hostname asserted by the server has been resolved. Such resolutions SHOULD first attempt to resolve the hostname using an [SRV] Service of "xmpp-client" and Proto of "tcp", resulting in resource records such as "_xmpp-client._tcp.example.com." (the use of the string
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   "xmpp-client" for the service identifier is consistent with the IANA
   registration).  If the SRV lookup fails, the fallback is a normal
   IPv4/IPv6 address record resolution to determine the IP address,
   using the "xmpp-client" port of 5222, registered with the IANA.

   The IP address and method of access of clients MUST NOT be made
   public by a server, nor are any connections other than the original
   server connection required.  This helps to protect the client's
   server from direct attack or identification by third parties.

14.4. Server-to-Server Communications

A compliant server implementation MUST support both TLS and SASL for inter-domain communications. For historical reasons, a compliant implementation SHOULD also support Server Dialback (Section 8). Because service provisioning is a matter of policy, it is OPTIONAL for any given domain to communicate with other domains, and server-to-server communications MAY be disabled by the administrator of any given deployment. If a particular domain enables inter-domain communications, it SHOULD enable high security. Administrators may want to require use of SASL for server-to-server communications in order to ensure both authentication and confidentiality (e.g., on an organization's private network). Compliant implementations SHOULD support SASL for this purpose. Inter-domain connections MUST NOT proceed until the DNS hostnames asserted by the servers have been resolved. Such resolutions MUST first attempt to resolve the hostname using an [SRV] Service of "xmpp-server" and Proto of "tcp", resulting in resource records such as "_xmpp-server._tcp.example.com." (the use of the string "xmpp-server" for the service identifier is consistent with the IANA registration; note well that the "xmpp-server" service identifier supersedes the earlier use of a "jabber" service identifier, since the earlier usage did not conform to [SRV]; implementations desiring to be backward compatible should continue to look for or answer to the "jabber" service identifier as well). If the SRV lookup fails, the fallback is a normal IPv4/IPv6 address record resolution to determine the IP address, using the "xmpp-server" port 5269, registered with the IANA. Server dialback helps protect against domain spoofing, thus making it more difficult to spoof XML stanzas. It is not a mechanism for authenticating, securing, or encrypting streams between servers as is done via SASL and TLS, and results in weak verification of server identities only. Furthermore, it is susceptible to DNS poisoning attacks unless DNSSec [DNSSEC] is used, and even if the DNS
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   information is accurate, dialback cannot protect from attacks where
   the attacker is capable of hijacking the IP address of the remote
   domain.  Domains requiring robust security SHOULD use TLS and SASL.
   If SASL is used for server-to-server authentication, dialback SHOULD
   NOT be used since it is unnecessary.

14.5. Order of Layers

The order of layers in which protocols MUST be stacked is as follows: 1. TCP 2. TLS 3. SASL 4. XMPP The rationale for this order is that [TCP] is the base connection layer used by all of the protocols stacked on top of TCP, [TLS] is often provided at the operating system layer, [SASL] is often provided at the application layer, and XMPP is the application itself.

14.6. Lack of SASL Channel Binding to TLS

The SASL framework does not provide a mechanism to bind SASL authentication to a security layer providing confidentiality and integrity protection that was negotiated at a lower layer. This lack of a "channel binding" prevents SASL from being able to verify that the source and destination end points to which the lower layer's security is bound are equivalent to the end points that SASL is authenticating. If the end points are not identical, the lower layer's security cannot be trusted to protect data transmitted between the SASL authenticated entities. In such a situation, a SASL security layer should be negotiated that effectively ignores the presence of the lower layer security.

14.7. Mandatory-to-Implement Technologies

At a minimum, all implementations MUST support the following mechanisms: for authentication: the SASL [DIGEST-MD5] mechanism for confidentiality: TLS (using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher) for both: TLS plus SASL EXTERNAL(using the TLS_RSA_WITH_3DES_EDE_CBC_SHA cipher supporting client-side certificates)
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14.8. Firewalls

Communications using XMPP normally occur over [TCP] connections on port 5222 (client-to-server) or port 5269 (server-to-server), as registered with the IANA (see IANA Considerations (Section 15)). Use of these well-known ports allows administrators to easily enable or disable XMPP activity through existing and commonly-deployed firewalls.

14.9. Use of base64 in SASL

Both the client and the server MUST verify any [BASE64] data received during SASL negotiation. 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 64 encoding visually hides otherwise easily recognized information, such as passwords, but does not provide any computational confidentiality. Base 64 encoding MUST follow the definition in Section 3 of RFC 3548 [BASE64].

14.10. Stringprep Profiles

XMPP makes use of the [NAMEPREP] profile of [STRINGPREP] for the processing of domain identifiers; for security considerations related to Nameprep, refer to the appropriate section of [NAMEPREP]. In addition, XMPP defines two profiles of [STRINGPREP]: Nodeprep (Appendix A) for node identifiers and Resourceprep (Appendix B) for resource identifiers. The Unicode and ISO/IEC 10646 repertoires have many characters that look similar. In many cases, users of security protocols might do visual matching, such as when comparing the names of trusted third parties. Because it is impossible to map similar-looking characters without a great deal of context, such as knowing the fonts used, stringprep does nothing to map similar-looking characters together, nor to prohibit some characters because they look like others. A node identifier can be employed as one part of an entity's address in XMPP. One common usage is as the username of an instant messaging user; another is as the name of a multi-user chat room; many other kinds of entities could use node identifiers as part of their
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   addresses.  The security of such services could be compromised based
   on different interpretations of the internationalized node
   identifier; for example, a user entering a single internationalized
   node identifier could access another user's account information, or a
   user could gain access to an otherwise restricted chat room or
   service.

   A resource identifier can be employed as one part of an entity's
   address in XMPP.  One common usage is as the name for an instant
   messaging user's connected resource (active session); another is as
   the nickname of a user in a multi-user chat room; many other kinds of
   entities could use resource identifiers as part of their addresses.
   The security of such services could be compromised based on different
   interpretations of the internationalized resource identifier; for
   example, a user could attempt to initiate multiple sessions with the
   same name, or a user could send a message to someone other than the
   intended recipient in a multi-user chat room.

15. IANA Considerations

15.1. XML Namespace Name for TLS Data

A URN sub-namespace for TLS-related data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in The IETF XML Registry [XML-REG].) URI: urn:ietf:params:xml:ns:xmpp-tls Specification: RFC 3920 Description: This is the XML namespace name for TLS-related data in the Extensible Messaging and Presence Protocol (XMPP) as defined by RFC 3920. Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>

15.2. XML Namespace Name for SASL Data

A URN sub-namespace for SASL-related data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].) URI: urn:ietf:params:xml:ns:xmpp-sasl Specification: RFC 3920 Description: This is the XML namespace name for SASL-related data in the Extensible Messaging and Presence Protocol (XMPP) as defined by RFC 3920. Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>
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15.3. XML Namespace Name for Stream Errors

A URN sub-namespace for stream-related error data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].) URI: urn:ietf:params:xml:ns:xmpp-streams Specification: RFC 3920 Description: This is the XML namespace name for stream-related error data in the Extensible Messaging and Presence Protocol (XMPP) as defined by RFC 3920. Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>

15.4. XML Namespace Name for Resource Binding

A URN sub-namespace for resource binding in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].) URI: urn:ietf:params:xml:ns:xmpp-bind Specification: RFC 3920 Description: This is the XML namespace name for resource binding in the Extensible Messaging and Presence Protocol (XMPP) as defined by RFC 3920. Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>

15.5. XML Namespace Name for Stanza Errors

A URN sub-namespace for stanza-related error data in the Extensible Messaging and Presence Protocol (XMPP) is defined as follows. (This namespace name adheres to the format defined in [XML-REG].) URI: urn:ietf:params:xml:ns:xmpp-stanzas Specification: RFC 3920 Description: This is the XML namespace name for stanza-related error data in the Extensible Messaging and Presence Protocol (XMPP) as defined by RFC 3920. Registrant Contact: IETF, XMPP Working Group, <xmppwg@jabber.org>

15.6. Nodeprep Profile of Stringprep

The Nodeprep profile of stringprep is defined under Nodeprep (Appendix A). The IANA has registered Nodeprep in the stringprep profile registry. Name of this profile: Nodeprep
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   RFC in which the profile is defined:

      RFC 3920

   Indicator whether or not this is the newest version of the profile:

      This is the first version of Nodeprep

15.7. Resourceprep Profile of Stringprep

The Resourceprep profile of stringprep is defined under Resourceprep (Appendix B). The IANA has registered Resourceprep in the stringprep profile registry. Name of this profile: Resourceprep RFC in which the profile is defined: RFC 3920 Indicator whether or not this is the newest version of the profile: This is the first version of Resourceprep

15.8. GSSAPI Service Name

The IANA has registered "xmpp" as a GSSAPI [GSS-API] service name, as defined under SASL Definition (Section 6.3).

15.9. Port Numbers

The IANA has registered "xmpp-client" and "xmpp-server" as keywords for [TCP] ports 5222 and 5269 respectively. These ports SHOULD be used for client-to-server and server-to-server communications respectively, but their use is OPTIONAL.

16. References

16.1. Normative References

[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 3548, July 2003.
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   [CHARSET]    Alvestrand, H., "IETF Policy on Character Sets and
                Languages", BCP 18, RFC 2277, January 1998.

   [DIGEST-MD5] Leach, P. and C. Newman, "Using Digest Authentication as
                a SASL Mechanism", RFC 2831, May 2000.

   [DNS]        Mockapetris, P., "Domain names - implementation and
                specification", STD 13, RFC 1035, November 1987.

   [GSS-API]    Linn, J., "Generic Security Service Application Program
                Interface Version 2, Update 1", RFC 2743, January 2000.

   [HTTP-TLS]   Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [IDNA]       Faltstrom, P., Hoffman, P., and A. Costello,
                "Internationalizing Domain Names in Applications
                (IDNA)", RFC 3490, March 2003.

   [IPv6]       Hinden, R. and S. Deering, "Internet Protocol Version 6
                (IPv6) Addressing Architecture", RFC 3513, April 2003.

   [LANGTAGS]   Alvestrand, H., "Tags for the Identification of
                Languages", BCP 47, RFC 3066, January 2001.

   [NAMEPREP]   Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
                Profile for Internationalized Domain Names (IDN)", RFC
                3491, March 2003.

   [RANDOM]     Eastlake 3rd, D., Crocker, S., and J. Schiller,
                "Randomness Recommendations for Security", RFC 1750,
                December 1994.

   [SASL]       Myers, J., "Simple Authentication and Security Layer
                (SASL)", RFC 2222, October 1997.

   [SRV]        Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
                specifying the location of services (DNS SRV)", RFC
                2782, February 2000.

   [STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
                Internationalized Strings ("stringprep")", RFC 3454,
                December 2002.

   [TCP]        Postel, J., "Transmission Control Protocol", STD 7, RFC
                793, September 1981.
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   [TERMS]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [TLS]        Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
                RFC 2246, January 1999.

   [UCS2]       International Organization for Standardization,
                "Information Technology - Universal Multiple-octet coded
                Character Set (UCS) - Amendment 2: UCS Transformation
                Format 8 (UTF-8)", ISO Standard 10646-1 Addendum 2,
                October 1996.

   [UTF-8]      Yergeau, F., "UTF-8, a transformation format of ISO
                10646", STD 63, RFC 3629, November 2003.

   [X509]       Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
                X.509 Public Key Infrastructure Certificate and
                Certificate Revocation List (CRL) Profile", RFC 3280,
                April 2002.

   [XML]        Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler,
                "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C
                REC-xml, October 2000, <http://www.w3.org/TR/REC-xml>.

   [XML-NAMES]  Bray, T., Hollander, D., and A. Layman, "Namespaces in
                XML", W3C REC-xml-names, January 1999,
                <http://www.w3.org/TR/REC-xml-names>.

16.2. Informative References

[ACAP] Newman, C. and J. Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, November 1997. [ASN.1] CCITT, "Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1)", 1988. [DNSSEC] Eastlake 3rd, D., "Domain Name System Security Extensions", RFC 2535, March 1999. [HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003.
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   [IMP-REQS]   Day, M., Aggarwal, S., Mohr, G., and J. Vincent,
                "Instant Messaging / Presence Protocol Requirements",
                RFC 2779, February 2000.

   [IRC]        Oikarinen, J. and D. Reed, "Internet Relay Chat
                Protocol", RFC 1459, May 1993.

   [JEP-0029]   Kaes, C., "Definition of Jabber Identifiers (JIDs)", JSF
                JEP 0029, October 2003.

   [JEP-0078]   Saint-Andre, P., "Non-SASL Authentication", JSF JEP
                0078, July 2004.

   [JEP-0086]   Norris, R. and P. Saint-Andre, "Error Condition
                Mappings", JSF JEP 0086, February 2004.

   [JSF]        Jabber Software Foundation, "Jabber Software
                Foundation", <http://www.jabber.org/>.

   [POP3]       Myers, J. and M. Rose, "Post Office Protocol - Version
                3", STD 53, RFC 1939, May 1996.

   [SIMPLE]     SIMPLE Working Group, "SIMPLE WG",
                <http://www.ietf.org/html.charters/simple-charter.html>.

   [SMTP]       Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
                April 2001.

   [URI]        Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
                Resource Identifiers (URI): Generic Syntax", RFC 2396,
                August 1998.

   [USINGTLS]   Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
                2595, June 1999.

   [XML-REG]    Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
                January 2004.

   [XMPP-IM]    Saint-Andre, P., Ed., "Extensible Messaging and Presence
                Protocol (XMPP): Instant Messaging and Presence", RFC
                3921, October 2004.