Call: A call is an informal term that refers to some communication between peers, generally set up for the purposes of a multimedia conversation.
呼叫：一个呼叫是一个非正式的术语，它是指在端点之间一个一些通讯行为，通常建立它的目的是用于多媒体对话
Call Leg: Another name for a dialog [31]; no longer used in this specification.
呼叫别名：
呼叫的别名，在本文档中不再使用。
Call Stateful: A proxy is call stateful if it retains state for a dialog from the initiating INVITE to the terminating BYE request. A call stateful proxy is always transaction stateful, but the converse is not necessarily true.
Call Stateful:
如果一个代理服务器（proxy）保存一个对话的状态（从最开始的 INVITE
到对话终结的 BYE） ，那么这个代理服务器就是请求有状态的。一个请求有状态（call
stateful）的代理服务器也一定是事务有状态的，但是事务有状态的不一定是请求有状态
的。
Core: Core designates the functions specific to a particular type of SIP entity, i.e., specific to either a stateful or stateless
proxy, a user agent or registrar. All cores, except those for the stateless proxy, are transaction users.
核心：核心指的是一些操作，这些操作明确到一个特定类型的SIP实体，比如说，它明确到一个有状态或者无状态的代理、UA或者是注册服务器。所有的核心，除了无状态代理，都是事务的用户。
Dialog: A dialog is a peer-to-peer SIP relationship between two UAs that persists for some time. A dialog is established by
SIP messages, such as a 2xx response to an INVITE request. A dialog is identified by a call identifier, local tag, and a remote tag. A dialog was formerly known as a call leg in RFC2543.
对话：一个对话指的是两个UA之间端到端的SIP关系，并且这种关系能维持一段时间。对话是通过SIP消息创建的，对于INVITE请求的2xx应答就会创建一个对话。对话是通过 call identifier（呼叫ID）, local tag（本地tag）, and a remote tag（远程tag）区分的。在RFC2543中对话被看做是呼叫别名。
Location Service:
A location service is used by a SIP redirect or proxy server to obtain information about a callee’s possible
location(s). It contains a list of bindings of address-of-record keys to zero or more contact addresses. The bindings can be created and removed in many ways; this specification defines a REGISTER method that updates the bindings.
定位服务：
定位服务被SIP重定向服务器或者代理服务器用来获取被叫方可能的位置，这些信息来源于一张记录表，这张记录表记录着地址关键字到0个或者多个联系地址。这些绑定信息可以通过多种方法创建和删除，本规范中定义个一个REGISTER方法用来更新这些记录信息。
Message: Data sent between SIP elements as part of the protocol. SIP messages are either requests or responses.
消息：
sip要素之间传送的协议数据就是消息，消息既可以是请求也可以是应答。
Method: The method is the primary function that a request is meant to invoke on a server. The method is carried in the request message itself. Example methods are INVITE and BYE.
方法就是一个请求想要从服务器端调取的服务。方法是请求消息自身携带的。典型的方法就是 INVITE 和 BYE。
Proxy, Proxy Server: An intermediary entity that acts as both a server and a client for the purpose of making requests on
behalf of other clients. A proxy server primarily plays the role of routing, which means its job is to ensure that a request is sent to another entity "closer" to the targeted user. Proxies are also useful for enforcing policy (for example, making sure a user is allowed to make a call). A proxy interprets, and, if necessary, rewrites specific parts of a request message before forwarding it.
代理、代理服务器：
一个中间的实体。它本身即作为服务端也作为客户端。这个客户端的目的是代表其他客户端创建请求。代理服务器主要扮演一个路由的角色，也就意味着他的工作是保证请求被送到另外一个“最接近”目的用户。代理服务器还对某些强制策略有用（比如确认用户是否被允许创建一个呼叫）。一个代理解释器，而且有必要的话在转发之前，可以重写请求消息的特定部分。
Redirect Server: A redirect server is a user agent server that generates 3xx responses to requests it receives, directing the client to contact an alternate set of URIs.
重定向服务器：
用户代理服务器是一个用户代理服务器，它给收到的请求生成3xx响应，指给客户端一些可联系的URI集。
Registrar: A registrar is a server that accepts REGISTER requests and places the information it receives in those requests into
the location service for the domain it handles.
注册服务器：
注册服务器接收REGISTER请求，并且将其收到的这些请求里的注册信息登记在定位服务器中，方便定位服务器处理。
Session: From the SDP specification: "A multimedia session is a set of multimedia senders and receivers and the data streams
flowing from senders to receivers. A multimedia conference is an example of a multimedia session." (RFC 2327 [1]) (A session
as defined for SDP can comprise one or more RTP sessions.) As defined, a callee can be invited several times, by different calls, to the same session. If SDP is used, a session is defined by the concatenation of the SDP user name, session id,
network type, address type, and address elements in the origin field.
会话：根据SDP 的描述：“一个多媒体会话是多媒体的发送者和接收者以及传输与发送者与接收者之间的多媒体流，一个多媒体会议是一个多媒体会话的例子”（在SDP定义下的会话是可以包含一个或者多个RTP会话）。一个被叫方可以被邀请多次，被不同的呼叫方呼叫至同一个会话，如果使用了SDP，一个会话被SDP开头处的一些相关的SDP用户名、会话id、网络类型、地址类型元素定义。
SIP Transaction: A SIP transaction occurs between a client and a server and comprises all messages from the first request sent from the client to the server up to a final (non-1xx) response sent from the server to the client. If the request is INVITE
and the final response is a non-2xx, the transaction also includes an ACK to the response. The ACK for a 2xx response to an INVITE request is a separate transaction.
sip事务：一个sip事务指的是一个客户端、一个服务器以及从第一个由客户端发往服务器的请求一直到服务器发往客户端的非1xx最终响应。如果请求是INVITE并且最终的响应是非2xx，这个事务包含客户端对服务器响应的ack确认。如果INVITE请求有2xx响应，那么最终的ACK属于一个独立的事务。
Transaction User (TU): The layer of protocol processing that resides above the transaction layer. Transaction users include
the UAC core, UAS core, and proxy core.
事务用户：事务用户是协议处理层，位于传输层之上。事务用户包括UAC核心、UAS核心、和代理核心。
User Agent Client (UAC): A user agent client is a logical entity that creates a new request, and then uses the client transaction state machinery to send it. The role of UAC lasts only for the duration of that transaction. In other words, if
a piece of software initiates a request, it acts as a UAC for the duration of that transaction. If it receives a request later, it assumes the role of a user agent server for the processing of that transaction.
用户代理客户端：用户代理客户端是一个逻辑实体，它创建一个请求，然后使用事务状态机将其发送。UAC的只存在于事务过程中。话句话说，如果一小段代码创建了一个请求，那么在那段事务执行期间，它就是一个UAC，如果他之后收到一个请求，那么在那个事务执行期间就把他当做用户代理服务器来处理。
UAC Core: The set of processing functions required of a UAC that reside above the transaction and transport layers.
用户代理核心：UAC所必须的一些处理方法集，位于事务和传输层之上。
User Agent Server (UAS): A user agent server is a logical entity that generates a response to a SIP request. The response
accepts, rejects, or redirects the request. This role lasts only for the duration of that transaction. In other words, if a piece of software responds to a request, it acts as a UAS for the duration of that transaction. If it generates a request later, it assumes the role of a user agent client for the processing of that transaction.
用户代理服务器:用户代理服务器也是一个逻辑实体，它产生一个SIP请求的响应，这个响应接受、拒绝或者重定向请求。这个角色也只存在于事务期间，话句话说，如果一小段代码响应一个请求，那么在那段事务期间他就作为用户代理服务器，如果后续它产生了一个请求，那么在这段事务处理期间他就又被当做为用户代理客户端。
UAS Core: The set of processing functions required at a UAS that resides above the transaction and transport layers.
用户代理服务器核心：指的是用户代理服务器所需要的一些操作函数集。位于事务和传输层之上。
User Agent (UA): A logical entity that can act as both a user agent client and user agent server.
用户代理：一个逻辑实体，包含UAC和UAS。
The role of UAC and UAS, as well as proxy and redirect servers, are defined on a transaction-by-transaction basis. For example, the user agent initiating a call acts as a UAC when sending the initial INVITE request and as a UAS when receiving a BYE request from the callee. Similarly, the same software can act as a proxy server for one request and as a redirect server for the next request. Proxy, location, and registrar servers defined above are logical entities; implementations MAY combine them into a single application.
UAC 和 UAS的角色 就像代理服务器和转发服务器， 是在事务 by 事务的基础 （串行事务处理）上定义的。例如，当发出一个初始化 INVITE 请求的时候，UA 作为 UAC 初始化一个呼叫动作，当从被叫方接收到一个 BYE 请求的时候，UA 作为 UAS 响应。类似的，同样的代码可以对一个请求做为 proxy 服务器处理，对另一个请求作为重定向服务器。
上面定义的proxy，location，registrar 服务器都是逻辑实体，在具体的实现中可能是作为单个应
用实现的。