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Network Working Group                                P. Saint-Andre, Ed.
Request for Comments: 3921                    Jabber Software Foundation
Category: Standards Track                                   October 2004


          Extensible Messaging and Presence Protocol (XMPP):
                     Instant Messaging and Presence

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This memo describes extensions to and applications of the core
   features of the Extensible Messaging and Presence Protocol (XMPP)
   that provide the basic instant messaging (IM) and presence
   functionality defined in RFC 2779.



Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.   Syntax of XML Stanzas  . . . . . . . . . . . . . . . . . . .   4
   3.   Session Establishment  . . . . . . . . . . . . . . . . . . .  10
   4.   Exchanging Messages  . . . . . . . . . . . . . . . . . . . .  13
   5.   Exchanging Presence Information  . . . . . . . . . . . . . .  16
   6.   Managing Subscriptions . . . . . . . . . . . . . . . . . . .  26
   7.   Roster Management  . . . . . . . . . . . . . . . . . . . . .  27
   8.   Integration of Roster Items and Presence Subscriptions . . .  32
   9.   Subscription States  . . . . . . . . . . . . . . . . . . . .  56
   10.  Blocking Communication . . . . . . . . . . . . . . . . . . .  62
   11.  Server Rules for Handling XML Stanzas  . . . . . . . . . . .  85
   12.  IM and Presence Compliance Requirements  . . . . . . . . . .  88
   13.  Internationalization Considerations  . . . . . . . . . . . .  89
   14.  Security Considerations  . . . . . . . . . . . . . . . . . .  89
   15.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  90
   16.  References . . . . . . . . . . . . . . . . . . . . . . . . .  91
   A.   vCards . . . . . . . . . . . . . . . . . . . . . . . . . . .  93
   B.   XML Schemas. . . . . . . . . . . . . . . . . . . . . . . . .  93
   C.   Differences Between Jabber IM/Presence Protocols and XMPP. . 105
   Contributors . . . . . . . . . . . . . . . . . . . . . . . . . .  106
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . .  106
   Author's Address. . . . . . . . . . . . . . . . . . . . . . . . . 106
   Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 107

1.  Introduction

1.1.  Overview

   The Extensible Messaging and Presence Protocol (XMPP) is a protocol
   for streaming XML [XML] elements in order to exchange messages and
   presence information in close to real time.  The core features of
   XMPP are defined in Extensible Messaging and Presence Protocol
   (XMPP): Core [XMPP-CORE].  These features -- mainly XML streams, use
   of TLS and SASL, and the <message/>, <presence/>, and <iq/> children
   of the stream root -- provide the building blocks for many types of
   near-real-time applications, which may be layered on top of the core
   by sending application-specific data qualified by particular XML
   namespaces [XML-NAMES].  This memo describes extensions to and
   applications of the core features of XMPP that provide the basic
   functionality expected of an instant messaging (IM) and presence
   application as defined in RFC 2779 [IMP-REQS].

   XMPP是一种通过将XML elements流化以达到近乎实时地传递消息(message)和在线信息
   (presence information)的协议。XMPP的核心功能在(XMPP): Core [XMPP-CORE]中定义。
   这些功能——主要包括XML的流化，TLS和SASL的使用，以及像<message/>,<presence/>以及
   <iq/>这样的流的子元素——提供给我们一些素材，使我们能创建很多"半实时(near-real-time)"
   的应用程序，而这些半实时的应用程序或许会位于应用程序的核心层之上，专门负责发送由程序专用的
   XML数据。这份memo讲述的是怎样扩展和使用XMPP的核心功能，而XMPP的核心协议仅涵盖了由RFC 
   2779[IMP-REQS]定义的，基本的IM和presence application 所应该提供的功能。 




1.2.  Requirements

   For the purposes of this memo, the requirements of a basic instant
   messaging and presence application are defined by [IMP-REQS], which
   at a high level stipulates that a user must be able to complete the
   following use cases:
   
   作为本文档的基础，基本的IM和"在线应用程序presence application"的要求
   刊载在[IMP-REQS]里面。这份文档在大体上规定了用户必须能完成如下的功能。

   o  Exchange messages with other users
      与其它用户交换消息
   o  Exchange presence information with other users
      与其它用户交换在线信息
   o  Manage subscriptions to and from other users
      管理和限制自己或他人的在线信息的发布
   o  Manage items in a contact list (in XMPP this is called a "roster")
      管理通讯录里的资料(XMPP称之为roster)
   o  Block communications to or from specific other users
      切断与他人的通讯

   Detailed definitions of these functionality areas are contained in
   [IMP-REQS], and the interested reader is directed to that document
   regarding the requirements addressed herein.

   这些功能的详细定义刊载在[IMP-REQS]里，感兴趣的读者可以自己去看。

   [IMP-REQS] also stipulates that presence services must be separable
   from instant messaging services; i.e., it must be possible to use the
   protocol to provide a presence service, an instant messaging service,
   or both.  Although the text of this memo assumes that implementations
   and deployments will want to offer a unified instant messaging and
   presence service, there is no requirement that a service must offer
   both a presence service and an instant messaging service, and the
   protocol makes it possible to offer separate and distinct services
   for presence and for instant messaging.

   [IMP-REQS]同时还规定，在线信息的服务必须与及时信息的服务相分离。比方说，
   协议必须允许你只使用在线信息，或及时信息服务，当然也可以两个都用。虽然
   我们认为真正实现的时候，在线信息和及时信息服务肯定是合二为一的，但是它
   也没有强求你一定必须同时提供在线信息合及时信息服务。这个协议完全允许你
   提供单独的在线信息或及时信息服务。

   Note: While XMPP-based instant messaging and presence meets the
   requirements of [IMP-REQS], it was not designed explicitly with that
   specification in mind, since the base protocol evolved through an
   open development process within the Jabber open-source community
   before RFC 2779 was written.  Note also that although protocols
   addressing many other functionality areas have been defined in the
   Jabber community, such protocols are not included in this memo
   because they are not required by [IMP-REQS].

   注意：虽然基于XMPP的及时信息和在线服务符合[IMP-REQS]的要求，但它并
   不是刻意去实现的，因为Jabber开源社区早在RFC 2779发布之前就已经促成了
   基本的协议。同时还要指出的是，虽然Jabber社区还为协议设计了很多别的方面
   的功能，但由于[IMP-REQS]并没有要求，因此我们这里也就不涉及了。

1.3.  Terminology

   This memo inherits the terminology defined in [XMPP-CORE].

   The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14, RFC 2119 [TERMS].




2.  Syntax of XML Stanzas

    XML Stanzas的语法

   The basic semantics and common attributes of XML stanzas qualified by
   the 'jabber:client' and 'jabber:server' namespaces are defined in
   [XMPP-CORE].  However, these namespaces also define various child
   elements, as well as values for the common 'type' attribute, that are
   specific to instant messaging and presence applications.  Thus,
   before addressing particular "use cases" for such applications, we
   here further describe the syntax of XML stanzas, thereby
   supplementing the discussion in [XMPP-CORE].

   XML stanzas都归在'jabber:client'和'jabber:server'名字空间之下，其基本语义
   和常用属性由[XMPP-CORE]定义。除了常用的'type'属性的值之外，这两个名字空间
   还定义了很多专供及时信息或在线信息服务的child elements。因此在描述此类应用
   程序的“use case”之前，我们先探讨一下XML stanzas的语法，也借此补充一下
   [XMPP-CORE]。

2.1.  Message Syntax
      
   Message stanzas qualified by the 'jabber:client' or 'jabber:server'
   namespace are used to "push" information to another entity.  Common
   uses in instant messaging applications include single messages,
   messages sent in the context of a chat conversation, messages sent in
   the context of a multi-user chat room, headlines and other alerts,
   and errors.

   归在'jabber:client'或'jabber:server'名字空间之下的Message stanzas是用来
   向另一个entity"推"信息用的。在IM应用程序里，这种stanzas常用被用于发送“单条
   消息(single message)”，在chat环境下发送消息，在多用户的chat room的环境下
   发送消息，以及发送headline，警告和错误消息。

2.1.1.  Types of Message

   The 'type' attribute of a message stanza is RECOMMENDED; if included,
   it specifies the conversational context of the message, thus
   providing a hint regarding presentation (e.g., in a GUI).  If
   included, the 'type' attribute MUST have one of the following values:

   message stanza的'type'属性是RECOMMENDED的；如果有，它表示这个消息是在怎样
   的交谈环境下发出的，因此它也暗示了该怎样显示这条消息(比方说在GUI环境下)。
   'type'属性如果有必须(MUST)是下面值里的一个。

   o  chat -- The message is sent in the context of a one-to-one chat
      conversation.  A compliant client SHOULD present the message in an
      interface enabling one-to-one chat between the two parties,
      including an appropriate conversation history.
      
      chat -- 消息是在一对一的chat谈话中发出的。客户端应当(SHOULD)在一个
      能让用户进行一对一谈话的界面里提示这条消息，包括交谈的记录。

   o  error -- An error has occurred related to a previous message sent
      by the sender (for details regarding stanza error syntax, refer to
      [XMPP-CORE]).  A compliant client SHOULD present an appropriate
      interface informing the sender of the nature of the error.
      
      error -- 表示先前发送的消息发生了错误(关于stanza的错误的语法，
      请参阅[XMPP-CORE])。客户端应当(SHOULD)能告诉发送发错误的性质。

   o  groupchat -- The message is sent in the context of a multi-user
      chat environment (similar to that of [IRC]).  A compliant client
      SHOULD present the message in an interface enabling many-to-many
      chat between the parties, including a roster of parties in the
      chatroom and an appropriate conversation history.  Full definition
      of XMPP-based groupchat protocols is out of scope for this memo.

      groupchat -- 这条消息是在多用户交谈环境下发出的(类似[IRC])。客户端
      应当(SHOULD)在能允许用户进行多对多谈话的界面里显示这条消息，这其中
      应该包括chat room的参与者的roster(名单)，以及适当的谈话记录。基于XMPP
      的群组交谈协议超出了本文档的范围。

   o  headline -- The message is probably generated by an automated
      service that delivers or broadcasts content (news, sports, market
      information, RSS feeds, etc.).  No reply to the message is
      expected, and a compliant client SHOULD present the message in an
      interface that appropriately differentiates the message from
      standalone messages, chat sessions, or groupchat sessions (e.g.,
      by not providing the recipient with the ability to reply).
      
      headline -- 这条消息或许是由某个后台服务生成并发送或广播的(新闻，体育，
      市场信息，RSS服务等到)。它不需要用户的回应。客户端应当(SHOULD)在
      一个合适的界面下提示这条消息以将其与"standalone message"，"chat session"
      和"groupchat session"相区分(比方说用户只能看不能reply)。

   o  normal -- The message is a single message that is sent outside the
      context of a one-to-one conversation or groupchat, and to which it
      is expected that the recipient will reply.  A compliant client
      SHOULD present the message in an interface enabling the recipient
      to reply, but without a conversation history.

      normal -- 这是一个single message，既不是一对一的交谈，也不是群组交谈，
      而且允许接受方回应。客户端应当(SHOULD)在一个能让用户reply的界面里显示
      这条消息，但是不要有谈话记录。

   An IM application SHOULD support all of the foregoing message types;
   if an application receives a message with no 'type' attribute or the
   application does not understand the value of the 'type' attribute
   provided, it MUST consider the message to be of type "normal" (i.e.,
   "normal" is the default).  The "error" type MUST be generated only in
   response to an error related to a message received from another
   entity.
    
   IM application应当支持所有上述message type，如果它收到了一个没有'type'属性的message
   或者它看不懂这个'type'属性，那它必须(MUST)将其认做是"normal"的message(也就是说，
   "normal"是默认的属性)。"error"只限于(MUST)用来回应与对方发出的message相关的错误。

   Although the 'type' attribute is OPTIONAL, it is considered polite to
   mirror the type in any replies to a message; furthermore, some
   specialized applications (e.g., a multi-user chat service) MAY at
   their discretion enforce the use of a particular message type (e.g.,
   type='groupchat').
   
   虽然'type'属性是OPTIONAL的，但是最好在回应信息里面附上相同的'type'信息；此外有些专用程序
   (比方说多用户的chat service)可以(MAY)根据他们自己的需要，强制使用特定的message type
   (比方说type='groupchat')。

2.1.2.  Child Elements

   As described under extended namespaces (Section 2.4), a message
   stanza MAY contain any properly-namespaced child element.

   正如我们将在"扩展名字空间"(Section2.4)所讲的，message stanza可以(MAY)包含任意的
   "属于适当的名字空间"的child element。

   In accordance with the default namespace declaration, by default a
   message stanza is qualified by the 'jabber:client' or 'jabber:server'
   namespace, which defines certain allowable children of message
   stanzas.  If the message stanza is of type "error", it MUST include
   an <error/> child; for details, see [XMPP-CORE].  Otherwise, the
   message stanza MAY contain any of the following child elements
   without an explicit namespace declaration:
   
   为了与默认的名字空间的声明相一致，默认情况下，message stanza被归到'jabber:client'或
   'jabber:server'名字空间下。这两个名字空间定义了一些被认可的message stanza的child。
   'error'类型的message stanza必须(MUST)包含<error/> child；具体细节请看[XMPP-CORE]。
   除此之外，message stanza可以(MAY)不经声明就使用下列的child elements。

   1.  <subject/>
   2.  <body/>
   3.  <thread/>

2.1.2.1.  Subject

   The <subject/> element contains human-readable XML character data
   that specifies the topic of the message.  The <subject/> element MUST
   NOT possess any attributes, with the exception of the 'xml:lang'
   attribute.  Multiple instances of the <subject/> element MAY be
   included for the purpose of providing alternate versions of the same
   subject, but only if each instance possesses an 'xml:lang' attribute
   with a distinct language value.  The <subject/> element MUST NOT
   contain mixed content (as defined in Section 3.2.2 of [XML]).

   <subject/> element包含human-readable的，表示message的标题的XML字符数据。
   <subject/>绝对不能(MUST NOT)包含除'xml:lang'之外的任何属性。为了能提供同一个
   subject的多个版本，一个message里面可以(MAY)包含多个<body/> element。但这仅限于
   每个<body/>都包含'xml:lang'属性，并且其language值都不相同的情形。<subject/>
   绝对不能(MUST NOT)包含mixed content(见Section 3.2.2)


2.1.2.2.  Body

   The <body/> element contains human-readable XML character data that
   specifies the textual contents of the message; this child element is
   normally included but is OPTIONAL.  The <body/> element MUST NOT
   possess any attributes, with the exception of the 'xml:lang'
   attribute.  Multiple instances of the <body/> element MAY be included
   but only if each instance possesses an 'xml:lang' attribute with a
   distinct language value.  The <body/> element MUST NOT contain mixed
   content (as defined in Section 3.2.2 of [XML]).

   <body/> element包含human-readable的，用来表示消息的正文内容的XML字符数据；通常
   message都会包含这个child，但它是可选的(OPTIONAL)。<body/> element绝对不能(MUST NOT)
   包含除'xml:lang'之外的任何属性。一个message里面可以(MAY)包含多个<subject/> element。
   但这仅限于每个<subject/>都包含'xml:lang'属性，并且其language值都不相同的情形。
   <body/>绝对不能(MUST NOT)包含mixed content(见Section 3.2.2)

2.1.2.3.  Thread

   The <thread/> element contains non-human-readable XML character data
   specifying an identifier that is used for tracking a conversation
   thread (sometimes referred to as an "instant messaging session")
   between two entities.  The value of the <thread/> element is
   generated by the sender and SHOULD be copied back in any replies.  If
   used, it MUST be unique to that conversation thread within the stream
   and MUST be consistent throughout that conversation (a client that
   receives a message from the same full JID but with a different thread
   ID MUST assume that the message in question exists outside the
   context of the existing conversation thread).  The use of the
   <thread/> element is OPTIONAL and is not used to identify individual
   messages, only conversations.  A message stanza MUST NOT contain more
   than one <thread/> element.  The <thread/> element MUST NOT possess
   any attributes.  The value of the <thread/> element MUST be treated
   as opaque by entities; no semantic meaning may be derived from it,
   and only exact comparisons may be made against it.  The <thread/>
   element MUST NOT contain mixed content (as defined in Section 3.2.2
   of [XML]).

   <thread/> element包含非human-readable的XML字符数据。它是一个用于表示两个entity
   之间的conversion thread的(有时也被称作"instance messaging session")标识符。
   <thread/> element的值由sender生成，而且每次回复的时候必须(SHOULD)包含在内。如果
   用到了它，那么它必须(MUST)与stream里的conversation thread一一对应，而且整个谈话期间
   必须保持一致(如果client端收到了同一个JID发出的，但是thread ID不同的message，必须(MUST)
   认定这个消息与当前的conversation thread无关。)<thread/> element是OPTIONAL的，
   且不用于标识单个message，它是用来标识conversation的。一个message stanza MUST NOT
   包含二个或二个以上的<thread/> element。<thread> element MUST NOT包含任何属性。
   对entities来说<thread/>的值必须(MUST)是不透明的；不能用它来获取任何语义信息，而且只能
   对它进行精确地比较。<thread/> MUST NOT包含mixed content(Section 3.2.2)

2.2.  Presence Syntax

   Presence stanzas are used qualified by the 'jabber:client' or
   'jabber:server' namespace to express an entity's current network
   availability (offline or online, along with various sub-states of the
   latter and optional user-defined descriptive text), and to notify
   other entities of that availability.  Presence stanzas are also used
   to negotiate and manage subscriptions to the presence of other
   entities.

   Presence stanza属于'jabber:client'或'jabber:server'名字空间，它们是用来表示
   entity的当前网络可得性(offline还是online，此外后者还有很多sub-state和可选的用户
   自定义的叙述性文字)，并且将其通知给其它entities的。Persence stanzas也被用于协商
   和管理订阅其它entities的presence信息。

2.2.1.  Types of Presence

   The 'type' attribute of a presence stanza is OPTIONAL.  A presence
   stanza that does not possess a 'type' attribute is used to signal to
   the server that the sender is online and available for communication.
   If included, the 'type' attribute specifies a lack of availability, a
   request to manage a subscription to another entity's presence, a
   request for another entity's current presence, or an error related to
   a previously-sent presence stanza.  If included, the 'type' attribute
   MUST have one of the following values:

   presence stanza的'type'属性是OPTIONAL的。不带'type'属性的presence stanza
   是用来通知服务器sender已经上线，并且available for communication。如果有，那么这个
   'type'或者用来表示可得性方面的限制，或者用来管理订阅其它用户的presence信息，或者用于
   查询其它用户的当前可得性，或者是先前发送的presence stanza出了错。如果有，'type'属性
   MUST是下列值中的一个:

   o  unavailable -- Signals that the entity is no longer available for
      communication.
      
      unavailable -- 表示entity不再available for communication了

   o  subscribe -- The sender wishes to subscribe to the recipient's
      presence.
      
      subscribe -- sender希望能订阅recipent的presence信息

   o  subscribed -- The sender has allowed the recipient to receive
      their presence.
      
      subscribed -- 发送方同意让接受方订阅自己的在线信息

   o  unsubscribe -- The sender is unsubscribing from another entity's
      presence.
      
      unsubscribe -- 发送方取消订阅另一方的在线信息

   o  unsubscribed -- The subscription request has been denied or a
      previously-granted subscription has been cancelled.

      unsubscribed -- 订阅请求被拒绝或者先前授予的订阅权限被撤销了

   o  probe -- A request for an entity's current presence; SHOULD be
      generated only by a server on behalf of a user.

      probe -- 请求entity的当前在线信息；只有(SHOULD)server才能代表用户去生成此类信息。

   o  error -- An error has occurred regarding processing or delivery of
      a previously-sent presence stanza.

      error -- 先前发送的presence stanza在处理或投送过程中出现了错误。

   For detailed information regarding presence semantics and the
   subscription model used in the context of XMPP-based instant
   messaging and presence applications, refer to Exchanging Presence
   Information (Section 5) and Managing Subscriptions (Section 6).

   要想详细了解相关信息，请参阅Section 5 Exchanging Presence Information
   和Section 6 Managing Subscriptions。

2.2.2.  Child Elements

   As described under extended namespaces (Section 2.4), a presence
   stanza MAY contain any properly-namespaced child element.

   就像Section 2.4的扩展名字空间所说的，presence stanza 可以包含任何
   properly-namespaced的child element。

   In accordance with the default namespace declaration, by default a
   presence stanza is qualified by the 'jabber:client' or
   'jabber:server' namespace, which defines certain allowable children
   of presence stanzas.  If the presence stanza is of type "error", it
   MUST include an <error/> child; for details, see [XMPP-CORE].  If the
   presence stanza possesses no 'type' attribute, it MAY contain any of
   the following child elements (note that the <status/> child MAY be
   sent in a presence stanza of type "unavailable" or, for historical
   reasons, "subscribe"):

   为了与默认的名字空间的声明保持一致，默认的presence stanza被归在'jabber:client'
   或'jabber:server'名字空间之下，这两个名字空间还定义了一些被认可的presence stanza
   的children。如果presence stanza是'error'类型的，那么它MUST包含<error/>；细节请看
   [XMPP-CORE]。如果presence stanza没有'type'属性，那么它MAY包含下列child element
   (注意，<status/> child可以随'unavailable' type的presence stanza或，出于历史原因，
   'subscribe' type的presence stanza发出。)

   1.  <show/>
   2.  <status/>
   3.  <priority/>
2.2.2.1.  Show

   The OPTIONAL <show/> element contains non-human-readable XML
   character data that specifies the particular availability status of
   an entity or specific resource.  A presence stanza MUST NOT contain
   more than one <show/> element.  The <show/> element MUST NOT possess
   any attributes.  If provided, the XML character data value MUST be
   one of the following (additional availability types could be defined
   through a properly-namespaced child element of the presence stanza):
   
   OPTIONAL的<show/> element包含非human-readable的XML字符数据。它表示某个entity
   或资源的可得性。presence stanza绝对不能(MUST NOT)包含两个或两个以上的<show/>。
   <show/> MUST NOT包含任何属性。如果(presence stanza)里有(<show/>)，那么它的XML
   字符数据必须(MUST)是下列值中的一个(你还可以通过presence stanza的child element，
   在适当的名字空间里定义更多的类型。)

   o  away -- The entity or resource is temporarily away.
   
      away -- 这个entity或资源暂时不可得。

   o  chat -- The entity or resource is actively interested in chatting.

      chat -- 这个entity或资源正在chatting。

   o  dnd -- The entity or resource is busy (dnd = "Do Not Disturb").
      
      dnd -- 这个entity或资源正忙(dnd = 'Do Not Disturb')。

   o  xa -- The entity or resource is away for an extended period (xa =
      "eXtended Away").
      
      xa -- 这个entity或资源要离线一段时间 (xa = eXtended Away)

   If no <show/> element is provided, the entity is assumed to be online
   and available.

   如果没有<show/> element，那么这个entity就将被认为是可得的。

2.2.2.2.  Status

   The OPTIONAL <status/> element contains XML character data specifying
   a natural-language description of availability status.  It is
   normally used in conjunction with the show element to provide a
   detailed description of an availability state (e.g., "In a meeting").
   The <status/> element MUST NOT possess any attributes, with the
   exception of the 'xml:lang' attribute.  Multiple instances of the
   <status/> element MAY be included but only if each instance possesses
   an 'xml:lang' attribute with a distinct language value.

   OPTIONAL的<status/> element包含了用人类语言描述的可得性状态的XML字符数据。通常它会
   和<show/> element一起使用，以提供可得性状态的详细描述(比方说"在开会")。<status/> 
   MUST NOT包含除xml:lang之外的任何属性。presence stanza里面可以(MAY)包含多个<status/>，
   只是每个<status/>都必须包含一个xml:lang属性并且其language的值不同。

2.2.2.3.  Priority

   The OPTIONAL <priority/> element contains non-human-readable XML
   character data that specifies the priority level of the resource. The
   value MUST be an integer between -128 and +127.  A presence stanza
   MUST NOT contain more than one <priority/> element.  The <priority/>
   element MUST NOT possess any attributes.  If no priority is provided,
   a server SHOULD consider the priority to be zero.  For information
   regarding the semantics of priority values in stanza routing within
   instant messaging and presence applications, refer to Server Rules
   for Handling XML Stanzas (Section 11).

   OPTIONAL的<priority/> element包含非human-readable的，表示资源优先级的XML字符数据。
   它的值MUST是一个介于-128到+127之间的整数。presence stanza MUST NOT包含两个或
   两个以上的<priority/>元素。<priority/>元素MUST NOT包含任何属性。如果(presence
   stanza)没有提供<priority/>，那么服务器SHOULD认为它的优先级为0。至于优先级的值是怎样
   影响stanza的路由的，请参阅Server Rules for Handling XML Stanzas (Section 11)。
   
2.3.  IQ Syntax
      
      IQ 的语法

   IQ stanzas provide a structured request-response mechanism.  The
   basic semantics of that mechanism (e.g., that the 'id' attribute is
   REQUIRED) are defined in [XMPP-CORE], whereas the specific semantics
   required to complete particular use cases are defined in all cases by
   an extended namespace (Section 2.4) (note that the 'jabber:client'
   and 'jabber:server' namespaces do not define any children of IQ
   stanzas other than the common <error/>).  This memo defines two such
   extended namespaces, one for Roster Management (Section 7) and the
   other for Blocking Communication (Section 10); however, an IQ stanza
   MAY contain structured information qualified by any extended
   namespace.

   IQ stanza提供了一个结构化的"请求-回答"(request-response)机制。这一机制的基本语义
   (比方说'id'属性是REQUIRED的)由[XMPP-CORE]定义，而用于某项use case的专属语义则
   通过"扩展的名字空间(extended namespace)"由这个case定义(注意，'jabber:client'和
   'jabber:server'名字空间除了常用的<error/>之外没有再定义别的IQ stanza的child)。
   本文档定义了两个这类extended namespace，一个是Roster Management (Section 7) 
   另一个是Blocking Communication (Section 10)。但是IQ stanza MAY包含任何扩展
   名字空间之下的结构化信息。

2.4.  Extended Namespaces

   While the three XML stanza kinds defined in the "jabber:client" or
   "jabber:server" namespace (along with their attributes and child
   elements) provide a basic level of functionality for messaging and
   presence, XMPP uses XML namespaces to extend the stanzas for the
   purpose of providing additional functionality.  Thus a message or
   presence stanza MAY contain one or more optional child elements
   specifying content that extends the meaning of the message (e.g., an
   XHTML-formatted version of the message body), and an IQ stanza MAY
   contain one such child element.  This child element MAY have any name
   and MUST possess an 'xmlns' namespace declaration (other than
   "jabber:client", "jabber:server", or
   "http://etherx.jabber.org/streams") that defines all data contained
   within the child element.

   虽然'jabber:client'和'jabber:server'名字空间所定义的这三个XML stanza(及其属性和
   child element)提供了基本的IM和presence的功能，但为了能提供更多的功能，XMPP还允许你
   用XML的名字空间来扩展stanza。这样一来，message和presence stanza MAY包含一个
   或多个child elements，并以此来扩展消息的内容(比方说XHTML的消息正文)，而IQ stanza
   MAY包含一个这样的child element。这个child element可以'MAY'是任何名字，但是MUST包含
   定义了这些child element的数据的'xmlns'名字空间的声明(不能是'jabber:client',
   'jabber:server'或'http://etherx.jabber.org/streams')。

   Support for any given extended namespace is OPTIONAL on the part of
   any implementation (aside from the extended namespaces defined
   herein).  If an entity does not understand such a namespace, the
   entity's expected behavior depends on whether the entity is (1) the
   recipient or (2) an entity that is routing the stanza to the
   recipient:
   
   implementation可以自行决定(OPTIONAL)是否支持这些扩展名字空间(这里定义的名字空间除外)。
   如果entity不知道这个名字空间，那么其行为将由(1)接受方或(2)将stanza路由到接受方的entity
   决定。
   
   
   Recipient: If a recipient receives a stanza that contains a child
      element it does not understand, it SHOULD ignore that specific XML
      data, i.e., it SHOULD not process it or present it to a user or
      associated application (if any).  In particular:
      
   接受方：如果接受方收到一个包含它不认识的child element的stanza，那么它SHOULD忽略
         这个XML数据。比方说，它SHOULD不处理，或者把它交给用户，或相关的程序(如果有的话)。
         特别是：

      *  If an entity receives a message or presence stanza that
         contains XML data qualified by a namespace it does not
         understand, the portion of the stanza that is in the unknown
         namespace SHOULD be ignored.

         如果一个entity收到的message或presence stanza里面包含了它所不认识的namespace
         的XML数据的话，那么它SHOULD忽略这部分未知的namespace的stanza。

      *  If an entity receives a message stanza whose only child element
         is qualified by a namespace it does not understand, it MUST
         ignore the entire stanza.

         如果一个entity收到的message stanza里面只有一个child element，且这个element
         属于一个它不认识的namespace，那么它MUST忽略这整个stanza。

      *  If an entity receives an IQ stanza of type "get" or "set"
         containing a child element qualified by a namespace it does not
         understand, the entity SHOULD return an IQ stanza of type
         "error" with an error condition of <service-unavailable/>.
         
         如果entity收到的'get'或'set'类型的IQ stanza里包含一个它所不认识的namespace
         的child element的话，那么这个entity SHOULD回一个error condition为
         <service-unavailable/>的'error'类型的IQ stanza。

   Router: If a routing entity (usually a server) handles a stanza that
      contains a child element it does not understand, it SHOULD ignore
      the associated XML data by passing it on untouched to the
      recipient.
     
   Router: 如果路由方(通常是服务器)碰到一个含有它不认识的child element的stanza的话，
           那么它SHOULD不管这些XML数据，把它们原封不动地传给接受方。

3.  Session Establishment

   Most instant messaging and presence applications based on XMPP are
   implemented via a client-server architecture that requires a client
   to establish a session on a server in order to engage in the expected
   instant messaging and presence activities.  However, there are
   several pre-conditions that MUST be met before a client can establish
   an instant messaging and presence session.  These are:

   绝大多数基于XMPP的IM和presence应用程序都是以client-server形式实现的，这种模式要求
   客户与服务器建立会话以进行IM和presence活动。但是在客户端建立IM和presence会话之前，还
   必须(MUST)满足几个前提条件。它们是：
   

   1.  Stream Authentication -- a client MUST complete stream
       authentication as documented in [XMPP-CORE] before attempting to
       establish a session or send any XML stanzas.
       
       Stream Authentication(流认证) -- 正如[XMPP-CORE]所刊载的，在试图建立会话
       并发送XML stanza之前，客户MUST完成流认证。

   2.  Resource Binding -- after completing stream authentication, a
       client MUST bind a resource to the stream so that the client's
       address is of the form <user@domain/resource>, after which the
       entity is now said to be a "connected resource" in the
       terminology of [XMPP-CORE].
       
       资源绑定 -- 流认证完毕之后，客户MUST在这个流上绑定资源，这样客户的地址才会符合
       <user@domain/resouce>的格式。只有在做完这些之后，entity才能被视作是[XMPP-CORE]
       所称的'在线资源(connected resource)'。

   If a server supports sessions, it MUST include a <session/> element
   qualified by the 'urn:ietf:params:xml:ns:xmpp-session' namespace in
   the stream features it advertises to a client after the completion of
   stream authentication as defined in [XMPP-CORE]:
   
   如果服务器支持会话，那么在完成[XMPP-CORE]所定义的stream authentication之后，它MUST
   在向client公告的stream feature里面包含一个'urn:ietf:params:xml:ns:xmpp-session'
   名字空间下的<session/> element。

   Server advertises session establishment feature to client:

   服务器向客户公告会话建立的功能:

   <stream:stream
       xmlns='jabber:client'
       xmlns:stream='http://etherx.jabber.org/streams'
       id='c2s_345'
       from='example.com'
       version='1.0'>
   <stream:features>
     <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
   </stream:features>

   Upon being so informed that session establishment is required (and
   after completing resource binding), the client MUST establish a
   session if it desires to engage in instant messaging and presence
   functionality; it completes this step by sending to the server an IQ
   stanza of type "set" containing an empty <session/> child element
   qualified by the 'urn:ietf:params:xml:ns:xmpp-session' namespace:

   客户端会如此这般地被告知必须先建立session(并且在绑定资源)，如果它想使用IM和presence
   功能，就MUST先建立session。这一步是通过向sever发一个'set'类型的，包含一个空的
   <session/> child element的，归在'urn:ietf:params:xml:ns:xmpp-session'名字空间
   之下的IQ stanza来完成的。

   Step 1: Client requests session with server:
   
   第一步：客户向server请求会话：

   <iq to='example.com'
       type='set'
       id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
   </iq>

   Step 2: Server informs client that session has been created:
   
   第二步：服务器通知客户，会话已经创建完毕：

   <iq from='example.com'
       type='result'
       id='sess_1'/>

   Upon establishing a session, a connected resource (in the terminology
   of [XMPP-CORE]) is said to be an "active resource".

   创建完会话之后，'在线资源'(具体术语的含义，请参阅[XMPP-CORE])就会被认为是active的了。

   Several error conditions are possible.  For example, the server may
   encounter an internal condition that prevents it from creating the
   session, the username or authorization identity may lack permissions
   to create a session, or there may already be an active resource
   associated with a resource identifier of the same name.

   这里有可能会出现几种错误。比方说，服务器端可能会出现妨碍它创建会话的情况，像用户名或提请认证
   的身份未获许可，或者这个active resource已经同某个同名的resource identifier(资源标识符)
   联系在一起了。

   If the server encounters an internal condition that prevents it from
   creating the session, it MUST return an error.

   如果服务器端碰到了妨碍它创建会话的情况，那它MUST返回一个error。

   Step 2 (alt): Server responds with error (internal server error):

   第二步(亦有可能出现)：服务器端以error回答(internal server error):

   <iq from='example.com' type='error' id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
     <error type='wait'>
       <internal-server-error
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   If the username or resource is not allowed to create a session, the
   server MUST return an error (e.g., forbidden).
   
   如果用户名或资源未被允许创建会话，那么服务器MUST返回一个error(也就是，禁止)

   Step 2 (alt): Server responds with error (username or resource not
   allowed to create session):

   第二步(亦有可能出现)：服务器端以error回答(此用户名或资源不允许创建会话):

   <iq from='example.com' type='error' id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
     <error type='auth'>
       <forbidden
           xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   If there is already an active resource of the same name, the server
   MUST either (1) terminate the active resource and allow the
   newly-requested session, or (2) disallow the newly-requested session
   and maintain the active resource.  Which of these the server does is
   up to the implementation, although it is RECOMMENDED to implement
   case #1.  In case #1, the server SHOULD send a <conflict/> stream
   error to the active resource, terminate the XML stream and underlying
   TCP connection for the active resource, and return a IQ stanza of
   type "result" (indicating success) to the newly-requested session. In
   case #2, the server SHOULD send a <conflict/> stanza error to the
   newly-requested session but maintain the XML stream for that
   connection so that the newly-requested session has an opportunity to
   negotiate a non-conflicting resource identifier before sending
   another request for session establishment.

   如果这个名字已经同某个active resource绑在一起了，服务器必须要么(1)中止active resource
   然后让新请求的session进来，要么(2)拒绝新请求的session，同时保持这个active resource.
   虽然我们RECOMMEND(推荐)采用＃1，但具体情况还视实现决定。在case #1下，server SHOULD
   给active resource发一个<conflict/> stream error，并且中止连接这个active resource  
   的XML流和TCP连接，然后给新会话发一个'result' type(表示成功)的IQ stanza。在case #2
   下，server SHOULD发送一个<conflict/> stanza错误给新的会话请求，但它必须保持这条XML
   stream，这样再再次发送建立会话的请求之前，新请求的会话就有机会来协商一个不会引起冲突的
   资源表示符了。

   Step 2 (alt): Server informs existing active resource of resource
   conflict (case #1):

   第二步(亦有可能出现)：服务器端告诉已连接的active resource，资源冲突(case #1):

   <stream:error>
     <conflict xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
   </stream:error>
   </stream:stream>

   Step 2 (alt): Server informs newly-requested session of resource
   conflict (case #2):

   第二步(亦有可能出现)：服务器端告诉新申请的会话，出现资源冲突(#case 2)：

   <iq from='example.com' type='error' id='sess_1'>
     <session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
     <error type='cancel'>
       <conflict xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
     </error>
   </iq>

   After establishing a session, a client SHOULD send initial presence
   and request its roster as described below, although these actions are
   OPTIONAL.

   建完会话之后，客户端应当(SHOULD)如下面所说的，发送初始在线信息并请求roster，不过这些
   活动都是OPTIONAL的。

   Note: Before allowing the creation of instant messaging and presence
   sessions, a server MAY require prior account provisioning.  Possible
   methods for account provisioning include account creation by a server
   administrator as well as in-band account registration using the
   'jabber:iq:register' namespace; the latter method is out of scope for
   this memo, but is documented in [JEP-0077], published by the Jabber
   Software Foundation [JSF].

   注意：在接纳IM和presence session之前，服务器 MAY 要求客户端事先提供帐号。
   提供帐号的方法包括，由服务器的管理员创建帐号，或者用'jabber:iq:register'名字空间
   的stanza注册'in-band account';后一种方法超出了本文档的范围，不过它刊载在JSF发布的
   [JEP-0077]里面。

4.  Exchanging Messages

   Exchanging messages is a basic use of XMPP and is brought about when
   a user generates a message stanza that is addressed to another
   entity.  As defined under Server Rules for Handling XML Stanzas
   (Section 11), the sender's server is responsible for delivering the
   message to the intended recipient (if the recipient is on the same
   server) or for routing the message to the recipient's server (if the
   recipient is on a different server).

   传递消息是XMPP的基本用途，当用户生成一个message stanza并将它发送给另一个entity的时候
   这事就发生了。正如Server Rules for Handling XML Stanzas(Section 11)所定义的，
   (如果接受发和发送方使用的是同一个服务器)发送方的服务器要负责将消息发给接受方，(如果发送方和
   接收发使用的是不同的服务器)，那发送方的服务器要负责将消息路由至接受方分服务器。

   For information regarding the syntax of message stanzas as well as
   their defined attributes and child elements, refer to Message Syntax
   (Section 2.1).

   要想知道message stanza的语法及其定义的属性和child element，请参阅Message Syntax
   (Secion 2.1)

4.1.  Specifying an Intended Recipient

      指定接受方

   An instant messaging client SHOULD specify an intended recipient for
   a message by providing the JID of an entity other than the sender in
   the 'to' attribute of the <message/> stanza.  If the message is being
   sent in reply to a message previously received from an address of the
   form <user@domain/resource> (e.g., within the context of a chat
   session), the value of the 'to' address SHOULD be of the form
   <user@domain/resource> rather than of the form <user@domain> unless
   the sender has knowledge (via presence) that the intended recipient's
   resource is no longer available.  If the message is being sent
   outside the context of any existing chat session or received message,
   the value of the 'to' address SHOULD be of the form <user@domain>
   rather than of the form <user@domain/resource>.

   IM的客户SHOULD在<message/> stanza的'to'属性里提供一个与sender不同的JID，并以此
   指定这条消息的接受方。如果这条消息是回应先前收到的，来自<user@domain/resource>的
   消息的(比方说在一个chat session里)，那么'to'地址仍然SHOULD是<user@domain/resource>
   而不是<user@domain>，除非发送方知道(通过presence)，接受方的资源已经不可得了。如果
   发送的消息不在任何现存的chat session或received message环境下，'to'地址的值SHOULD
   是<user@domain>而不是<user@domain/resource>形式的。


4.2.  Specifying a Message Type