Address-of-Record: 記錄地址。一個(gè)address-of-record(AOR)是一個(gè)SIP或者SIPS URI它指向了一個(gè)具有定位服務(wù)的主機(jī)，這個(gè)主機(jī)可以把URI映射成為用戶真正物理位置的URI。通常情況下，定位服務(wù)器是通過(guò)登記服務(wù)來(lái)建立的。一個(gè) AOR經(jīng)常被認(rèn)為是一個(gè)用戶的”公共地址”
Back-to-Back UserAgent:背對(duì)背的用戶代理（B2BUA）是一個(gè)邏輯實(shí)體，它就像用戶代理服務(wù)器（UAS）一樣接收和處理請(qǐng)求。為了決定該如何應(yīng)答一個(gè)請(qǐng)求， B2BUA就像UAC一樣工作，并且發(fā)出請(qǐng)求。但是它不像代理服務(wù)器（proxy），它維持對(duì)話狀態(tài)，并且參與已經(jīng)建立的對(duì)話中的每一個(gè)請(qǐng)求。由于它是直接的UAC和UAS的串連，所以，不需要對(duì)他有額外的定義。
Call：呼叫，一個(gè)呼叫是一個(gè)非正式的術(shù)語(yǔ)，它是指在端點(diǎn)之間一個(gè)一些通訊行為，通常用于建立多媒體對(duì)話。
Call Leg: 對(duì)話的別名；在本規(guī)范中沒有使用。
Call Stateful: 如果一個(gè)代理服務(wù)器（proxy）保存一個(gè)對(duì)話的狀態(tài)（從最開始的INVITE到對(duì)話終結(jié)的BYE），那么這個(gè)代理服務(wù)器就是請(qǐng)求有狀態(tài)的。一個(gè)請(qǐng)求有狀態(tài)（call stateful）的代理服務(wù)器也一定是事務(wù)有狀態(tài)的，但是事務(wù)有狀態(tài)的不一定是請(qǐng)求有狀態(tài)的。
Client：客戶端。一個(gè)客戶端是一個(gè)任意的網(wǎng)絡(luò)元素，它發(fā)出SIP請(qǐng)求和接收SIP應(yīng)答?？蛻舳丝赡軙?huì)也可能不會(huì)和人交互。用戶代理客戶端（UAC）和代理服務(wù)器都是客戶端。
Conference: 一個(gè)包含多個(gè)參與方的多媒體會(huì)話（見后）。
Core：核心。核心定義了SIP實(shí)體的特定類別。比如定義了一個(gè)有狀態(tài)和無(wú)狀態(tài)的代理服務(wù)器，一個(gè)用戶代理或者注冊(cè)服務(wù)器（registrar）。所有的核心，除了無(wú)狀態(tài)代理服務(wù)器，都是事務(wù)用戶。
Dialog:對(duì)話，一個(gè)對(duì)話是持續(xù)一段時(shí)間的兩個(gè)UA之間的端到端的SIP關(guān)系。一個(gè)對(duì)話由SIP消息建立，就像用2xx響應(yīng)INVITE請(qǐng)求。我們用Call identifier，local tag（本地tag），remote tag（對(duì)方tag）來(lái)標(biāo)志一個(gè)對(duì)話，一個(gè)對(duì)話在RFC 2543中被正式叫做CALL LEG.
Downstream: 它是事務(wù)中的消息傳遞方向。它特指從UAC到UAS的請(qǐng)求流的方向，
Final Response：終結(jié)響應(yīng)。一個(gè)響應(yīng)終端SIP事務(wù)的應(yīng)答，和事務(wù)中間的臨時(shí)響應(yīng)相反。所有的2xx，3xx，4xx，5xx，6xx響應(yīng)都是終結(jié)響應(yīng)。
Header：頭。頭域是在SIP消息頭部用來(lái)描述這個(gè)SIP消息信息的部分。它由一堆頭域字段組成。
Header Field:頭域字段。頭域字段是在SIP消息頭域的字段。一個(gè)頭域字段可以由多個(gè)頭域字段行組成。一個(gè)頭域字段由一個(gè)頭域名和（零個(gè)或多個(gè)）頭域值組成。多個(gè)頭域值用’,’分割。某些頭域字段只能有單個(gè)值，比如結(jié)果域（result）就只能有一個(gè)值。
Header Field Value:頭域值。一個(gè)頭域值是一個(gè)單個(gè)的值，一個(gè)頭域字段可以有0個(gè)或者多個(gè)頭域值。
Home Domain：宿主機(jī)。一個(gè)提供SIP服務(wù)的主機(jī)。一般指的是在登記服務(wù)中指明的記錄地址中的URI的主機(jī)。
Informational Response:提示應(yīng)答。和臨時(shí)應(yīng)答一樣。
Initiator, Calling Party, Caller: 用INVITE初始一個(gè)會(huì)話（和對(duì)話）的那方。一個(gè)caller從發(fā)出INVITE請(qǐng)求建立對(duì)話開始，到對(duì)話終止都一直是這個(gè)角色。
Invitation: 一個(gè)INVITE請(qǐng)求。
Invitee,Invited User,Called Party, Callee:被叫方。收到INVITE請(qǐng)求并且建立會(huì)話的那方。一個(gè)被叫方從收到INVITE請(qǐng)求起，到終止INVITE建立的對(duì)話結(jié)束，都稱作被叫方。
Location Service: 定位服務(wù)。定位服務(wù)是用來(lái)給SIP轉(zhuǎn)發(fā)或者代理服務(wù)器確定被叫方可能的位置使用的。它包含一張綁定了address-of-record的表，被叫方可能有0到多個(gè)記錄。綁定的記錄可以通過(guò)多種渠道添加和刪除；本規(guī)范定義了REGISTER方法來(lái)更新綁定表。
Loop：環(huán)路。當(dāng)請(qǐng)求抵達(dá)一個(gè)代理服務(wù)器，代理服務(wù)器轉(zhuǎn)發(fā)這個(gè)請(qǐng)求，當(dāng)這個(gè)請(qǐng)求再次來(lái)到同一個(gè)代理服務(wù)器，就稱之為環(huán)路。當(dāng)?shù)诙蔚诌_(dá)的時(shí)候， Request－URI中包含了上次抵達(dá)的資料，并且由于并沒有什么東西可以改變轉(zhuǎn)發(fā)的策略，這樣就導(dǎo)致這個(gè)請(qǐng)求還會(huì)再次被轉(zhuǎn)發(fā)回來(lái)。環(huán)路請(qǐng)求是錯(cuò)誤的，所以，處理程序需要檢測(cè)和防止協(xié)議中出現(xiàn)的環(huán)路請(qǐng)求。
Loose Routing:丟失路由。代理服務(wù)器在下述情況下會(huì)丟失路由。
A proxy is said to be loose routing if it follows the procedures defined in this specification for processing of the Route header field. These procedures separate the destination of the request (present in the Request-URI) from the set of proxies that need to be visited along the way (present in the Route header field). A proxy compliant to these mechanisms is also known as a loose router.
Message:消息。SIP元素之間傳送的協(xié)議數(shù)據(jù)就是消息。SIP消息既可以是請(qǐng)求也可以是應(yīng)答。
Method:方法。方法是在服務(wù)器請(qǐng)求處理的主要功能。方法是請(qǐng)求消息自身攜帶的。典型的方法就是INVITE和BYE。
Outbound Proxy:對(duì)外代理服務(wù)器。一個(gè)代理服務(wù)器接收到客戶的請(qǐng)求，即使它不是由Request_URI所決定的服務(wù)器。通常一個(gè)UA會(huì)手工配置一個(gè)對(duì)外的代理服務(wù)器，或者可以通過(guò)一個(gè)自動(dòng)配置的協(xié)議自動(dòng)配置一個(gè)。
Parallel Search: 并行搜索。并行搜索情況下，代理服務(wù)器會(huì)向多個(gè)用戶可能存在的地方發(fā)起請(qǐng)求，并且等待應(yīng)答。同串行搜索不同的地方是，并行搜索不會(huì)等待上一個(gè)請(qǐng)求應(yīng)答回來(lái)之后再發(fā)起下一個(gè)搜索，而是一個(gè)接一個(gè)的發(fā)起搜索請(qǐng)求。
Provisional Response: 臨時(shí)應(yīng)答。服務(wù)器用來(lái)標(biāo)志自己正在處理的應(yīng)答，但是本應(yīng)答并不結(jié)束一個(gè)SIP事務(wù)。1xx應(yīng)答就是臨時(shí)的，其他應(yīng)答標(biāo)志著事務(wù)的結(jié)束。
Proxy,Proxy Server:代理、代理服務(wù)器。一個(gè)中間的實(shí)體。它本身即作為客戶端也作為服務(wù)端，為其他客戶端提供請(qǐng)求的轉(zhuǎn)發(fā)服務(wù)。一個(gè)代理服務(wù)器首先提供的是路由服務(wù)，也就是說(shuō)保證請(qǐng)求被發(fā)到更加”靠近”目標(biāo)用戶的地方。代理服務(wù)器對(duì)某些強(qiáng)制政策有用（比如，確認(rèn)一個(gè)用戶是否允許建立一個(gè)呼叫等）。一個(gè)代理服務(wù)器翻譯，并且，如果有需要的話，再轉(zhuǎn)發(fā)前會(huì)重寫請(qǐng)求消息。
Recursion：回路、遞歸。一個(gè)客戶端，在響應(yīng)請(qǐng)求的時(shí)候產(chǎn)生新的到Contract包頭域的URI請(qǐng)求的時(shí)候，會(huì)在3xx響應(yīng)中陷入遞歸。 A client recurses on a 3xx response when it generates a new request to one or more of the URIs in the Contact header field in the response.
Redirect Server:重定向服務(wù)器。一個(gè)重定向服務(wù)器是一個(gè)產(chǎn)生3xx應(yīng)答的UAS服務(wù)器，指示客戶端連接別的URI。
Registrar: 登記員。一個(gè)登記員（登記服務(wù)器）是一個(gè)接收REGISTER請(qǐng)求得服務(wù)器。他把請(qǐng)求得信息放到定位服務(wù)器中，這樣可以讓定位服務(wù)器很方便得查找位置信息。
Regular Transaction:常規(guī)事務(wù)。凡不包含INVITE,ACK,或者CANCEL方法得事務(wù)就是常規(guī)事務(wù)。
Request: 請(qǐng)求。 一個(gè)由客戶端發(fā)到服務(wù)端得SIP信息，用于執(zhí)行特定得功能。
Response：應(yīng)答。一個(gè)由服務(wù)端發(fā)到客戶端得SIP信息。用來(lái)標(biāo)志從客戶端發(fā)往服務(wù)端得請(qǐng)求處理得情況得。
Ringback: 回鈴音?；剽徱羰且粋€(gè)信號(hào)音。是給呼叫方得一個(gè)信號(hào)表示被叫方正在振鈴（Ringing）。
Route Set: 路由集。路由集合是一個(gè)順序得SIP或者SIPS URI。這些URI描述了傳遞一個(gè)請(qǐng)求所必須經(jīng)歷得代理列表。一個(gè)路由集可以是自適應(yīng)得，因?yàn)榘^中包含了Record-Route(記錄路由)，也可以是依賴配置得到得。
Server：服務(wù)器。一個(gè)server是一個(gè)網(wǎng)絡(luò)元素接收請(qǐng)求并且處理請(qǐng)求并且發(fā)送回應(yīng)給請(qǐng)求方。典型得服務(wù)器就是代理服務(wù)器（proxies），用戶代理服務(wù)器（user agent servers），重定向服務(wù)器，登記服務(wù)器。
Sequential Search：順序查找。在順序查找中，代理服務(wù)器順序嘗試聯(lián)系地址，在處理下一個(gè)之前必須等待上一個(gè)請(qǐng)求已經(jīng)有一個(gè)結(jié)束應(yīng)答。一個(gè)2xx或者6xx系列得最終應(yīng)答總是結(jié)束一個(gè)順序查找。
Session：會(huì)話。根據(jù)SDP得描述：”一個(gè)多媒體會(huì)話是一個(gè)由多媒體發(fā)送方和接受方組成得集合，并且包括在發(fā)送方和接受方之間得數(shù)據(jù)流。一個(gè)多媒體會(huì)議是一個(gè)典型得多媒體會(huì)話。”(RFC 2327[1])(一個(gè)session在SDP訂一下可以是一個(gè)或者多個(gè)RTP sessino)。在定義中，一個(gè)被叫方可以被多次邀請(qǐng)，被不同得呼叫方邀請(qǐng)，到同一個(gè)會(huì)話。在SDP中，一個(gè)會(huì)話可以被SDP用戶名，session id,網(wǎng)絡(luò)類型，地址類型，地址元素得一個(gè)集合串所規(guī)定。
SIP 事務(wù)：一個(gè)SIP事務(wù)是在客戶端和服務(wù)端得事件，包括了從第一個(gè)由客戶端發(fā)送到服務(wù)端得請(qǐng)求，到最后一個(gè)（非1xx）服務(wù)端向客戶端發(fā)出得終結(jié)應(yīng)答。如果請(qǐng)求是一個(gè)INVITE請(qǐng)求，并且終結(jié)應(yīng)答是一個(gè)非2xx得應(yīng)答，那么事務(wù)還包括一個(gè)ACK給服務(wù)器做應(yīng)答。給INVITE請(qǐng)求的2xx應(yīng)答的ACK回應(yīng)，是一個(gè)獨(dú)立的事務(wù)。
Spiral：回溯。一個(gè)回溯是指一個(gè)SIP請(qǐng)求，路由給一個(gè)proxy，并且轉(zhuǎn)發(fā)，但是又被路由回這個(gè)proxy，但是不同于回路（遞歸）的是，這次路由回來(lái)的請(qǐng)求包的包頭中，包含了不同于原請(qǐng)求的請(qǐng)求包部分，使得本次proxy決定的路由轉(zhuǎn)發(fā)與上次不同。通常，這是說(shuō)，請(qǐng)求的Request- URI不同于上次的Request_URI。一個(gè)回溯不是一個(gè)錯(cuò)誤，不同于回路（環(huán)路loop）。通常導(dǎo)致這樣的現(xiàn)象是呼叫轉(zhuǎn)發(fā)（call forwarding）。一個(gè)用戶呼叫joe@example.com。example.com代理服務(wù)器轉(zhuǎn)發(fā)請(qǐng)求到Joe的PC,并且Joe的pc呼叫轉(zhuǎn)移到bob@example.com。這個(gè)請(qǐng)求被轉(zhuǎn)發(fā)回example.com代理服務(wù)器。可是這個(gè)并不是一個(gè)環(huán)路（loop）。因?yàn)檎?qǐng)求的目的地址變成了另一個(gè)用戶，這就是回溯，是一個(gè)合法的情況。
Stateful Proxy:有狀態(tài)的代理服務(wù)器。在邏輯上，有狀態(tài)的代理服務(wù)器就是處理一個(gè)請(qǐng)求的過(guò)程中，維持的一個(gè)本規(guī)范所定義的客戶端和服務(wù)端的事務(wù)狀態(tài)機(jī)。也是一個(gè)事務(wù)又狀態(tài)代理服務(wù)器(transaction stateful proxy)。具體的stateful proxy在第16節(jié)定義。一個(gè)（事務(wù)）有狀態(tài)代理服務(wù)器和一個(gè)call stateful proxy不是一回事。
Stateless Proxy：無(wú)狀態(tài)的代理服務(wù)器。在邏輯上，無(wú)狀態(tài)代理服務(wù)器在處理請(qǐng)求中，并不維持客戶和服務(wù)端的事務(wù)狀態(tài)機(jī)。一個(gè)無(wú)狀態(tài)的代理服務(wù)器直接轉(zhuǎn)發(fā)每一個(gè)接收到的請(qǐng)求和每一個(gè)接收到的響應(yīng)。
Strict Routing:嚴(yán)格路由。路由處理規(guī)則如果復(fù)核RFC2543協(xié)議（and many prior work in progress versions of this RFC）就是一個(gè)嚴(yán)格路由。在這個(gè)規(guī)則下，如果在包頭中包含Route域，那么代理服務(wù)器就會(huì)刪除Request_URI域內(nèi)容。本文檔并不要求一定要有嚴(yán)格路由，本文檔只要求松散路由就可以了。支持嚴(yán)格路由的代理服務(wù)器也叫嚴(yán)格路由器。
Target Refresh Request: 目標(biāo)刷新請(qǐng)求。一個(gè)Target Refresh Request是一個(gè)在對(duì)話中發(fā)出的請(qǐng)求，用來(lái)更改對(duì)話目標(biāo)的請(qǐng)求。
Transaction User(TU):事務(wù)用戶。在transaction 層之上的協(xié)議層。TU包括了UAC 核心，UAS core,和proxy core。
Upstream:上行流。一個(gè)在事務(wù)中的消息流向方向。它是指由用戶代理服務(wù)器（UAS）發(fā)出應(yīng)答到用戶代理客戶端（UAC）的消息流向方向。
URL-encoded:一串根據(jù)RFC2396－2.4節(jié)編碼的字符。
User Agent Client(UAC):用戶代理客戶端。用戶代理客戶端是一個(gè)邏輯的概念，他創(chuàng)建一個(gè)新請(qǐng)求，并且用客戶事務(wù)狀態(tài)機(jī)發(fā)送這個(gè)請(qǐng)求。UAC角色只在事務(wù)中存在。換句話說(shuō)，UAC就是一小段代碼初始化一個(gè)請(qǐng)求，并且在事務(wù)中遵循UAC的規(guī)則。如果它接下來(lái)收到一個(gè)請(qǐng)求，那么在那個(gè)事務(wù)中，它就是作為UAS來(lái)處理請(qǐng)求。
UAC Core:UAC核心。在transaction和transport層之上得UAC實(shí)現(xiàn)的功能集合。
User Agent Server(UAS): 用戶代理服務(wù)器.UAS是一個(gè)邏輯的實(shí)體，對(duì)SIP請(qǐng)求做響應(yīng)的。應(yīng)答接受、拒絕、或者轉(zhuǎn)發(fā)對(duì)應(yīng)的請(qǐng)求。UAS角色在事務(wù)中存在。換句話說(shuō)，是響應(yīng)請(qǐng)求的一小段軟件，在事務(wù)中作為UAS存在。如果他發(fā)出請(qǐng)求，那么他就在事務(wù)中作為UAC存在。
UAS Core：UAS核心。在transaction和transport層智商的UAS實(shí)現(xiàn)的功能集合。
User Agent（UA）。一個(gè)邏輯實(shí)體的概念，包含UAC和UAS。
UAC和UAS，就像代理服務(wù)器和轉(zhuǎn)發(fā)服務(wù)器，是在事務(wù)by事務(wù)的原理（串行事務(wù)處理）上定義的。例如，當(dāng)發(fā)出一個(gè)初始化INVITE請(qǐng)求的時(shí)候，UA作為UAC初始化一個(gè)呼叫動(dòng)作，當(dāng)從被叫方接收到一個(gè)BYE請(qǐng)求的時(shí)候，UA作為UAS響應(yīng)。類似的，同樣的代碼可以對(duì)一個(gè)請(qǐng)求做為proxy服務(wù)器處理，對(duì)另一個(gè)請(qǐng)求作為重定向服務(wù)器。
proxy，location，registrar服務(wù)器都是邏輯實(shí)體，在它們的實(shí)現(xiàn)中，可能是作為單個(gè)應(yīng)用實(shí)現(xiàn)的。
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The following terms have special significance for SIP.
Address-of-Record: An address-of-record (AOR) is a SIP or SIPS URI that points to a domain with a location service that can map the URI to another URI where the user might be available.
Typically, the location service is populated through
registrations. An AOR is frequently thought of as the “public address” of the user.
Back-to-Back User Agent: A back-to-back user agent (B2BUA) is a logical entity that receives a request and processes it as a user agent server (UAS). In order to determine how the request should be answered, it acts as a user agent client (UAC) and generates requests. Unlike a proxy server, it maintains dialog state and must participate in all requests sent on the dialogs it has established. Since it is a concatenation of a UAC and UAS, no explicit definitions are needed for its behavior.
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Call: A call is an informal term that refers to some communication between peers, generally set up for the purposes of a multimedia conversation.
Call Leg: Another name for a dialog [31]; no longer used in this specification.
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.
Client: A client is any network element that sends SIP requests and receives SIP responses. Clients may or may not interact directly with a human user. User agent clients and proxies are clients.
Conference: A multimedia session (see below) that contains multiple participants.
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.
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 RFC 2543.
Downstream: A direction of message forwarding within a transaction that refers to the direction that requests flow from the user agent client to user agent server.
Final Response: A response that terminates a SIP transaction, as opposed to a provisional response that does not. All 2xx, 3xx, 4xx, 5xx and 6xx responses are final.
Header: A header is a component of a SIP message that conveys information about the message. It is structured as a sequence of header fields.
Header Field: A header field is a component of the SIP message header. A header field can appear as one or more header field rows. Header field rows consist of a header field name and zero or more header field values. Multiple header field values on a Rosenberg, et. al. Standards Track [Page 21]
RFC 3261 SIP: Session Initiation Protocol June 2002
given header field row are separated by commas. Some header
fields can only have a single header field value, and as a
result, always appear as a single header field row.
Header Field Value: A header field value is a single value; a
header field consists of zero or more header field values.
Home Domain: The domain providing service to a SIP user.
Typically, this is the domain present in the URI in the
address-of-record of a registration.
Informational Response: Same as a provisional response.
Initiator, Calling Party, Caller: The party initiating a session
(and dialog) with an INVITE request. A caller retains this
role from the time it sends the initial INVITE that established
a dialog until the termination of that dialog.
Invitation: An INVITE request.
Invitee, Invited User, Called Party, Callee: The party that
receives an INVITE request for the purpose of establishing a
new session. A callee retains this role from the time it
receives the INVITE until the termination of the dialog
established by that INVITE.
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.
Loop: A request that arrives at a proxy, is forwarded, and later
arrives back at the same proxy. When it arrives the second
time, its Request-URI is identical to the first time, and other
header fields that affect proxy operation are unchanged, so
that the proxy would make the same processing decision on the
request it made the first time. Looped requests are errors,
and the procedures for detecting them and handling them are
described by the protocol.
Loose Routing: A proxy is said to be loose routing if it follows
the procedures defined in this specification for processing of
the Route header field. These procedures separate the
destination of the request (present in the Request-URI) from
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the set of proxies that need to be visited along the way
(present in the Route header field). A proxy compliant to
these mechanisms is also known as a loose router.
Message: Data sent between SIP elements as part of the protocol.
SIP messages are either requests or responses.
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.
Outbound Proxy: A proxy that receives requests from a client, even
though it may not be the server resolved by the Request-URI.
Typically, a UA is manually configured with an outbound proxy,
or can learn about one through auto-configuration protocols.
Parallel Search: In a parallel search, a proxy issues several
requests to possible user locations upon receiving an incoming
request. Rather than issuing one request and then waiting for
the final response before issuing the next request as in a
sequential search, a parallel search issues requests without
waiting for the result of previous requests.
Provisional Response: A response used by the server to indicate
progress, but that does not terminate a SIP transaction. 1xx
responses are provisional, other responses are considered
final.
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.
Recursion: A client recurses on a 3xx response when it generates a
new request to one or more of the URIs in the Contact header
field in the response.
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.
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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.
Regular Transaction: A regular transaction is any transaction with
a method other than INVITE, ACK, or CANCEL.
Request: A SIP message sent from a client to a server, for the
purpose of invoking a particular operation.
Response: A SIP message sent from a server to a client, for
indicating the status of a request sent from the client to the
server.
Ringback: Ringback is the signaling tone produced by the calling
party’s application indicating that a called party is being
alerted (ringing).
Route Set: A route set is a collection of ordered SIP or SIPS URI
which represent a list of proxies that must be traversed when
sending a particular request. A route set can be learned,
through headers like Record-Route, or it can be configured.
Server: A server is a network element that receives requests in
order to service them and sends back responses to those
requests. Examples of servers are proxies, user agent servers,
redirect servers, and registrars.
Sequential Search: In a sequential search, a proxy server attempts
each contact address in sequence, proceeding to the next one
only after the previous has generated a final response. A 2xx
or 6xx class final response always terminates a sequential
search.
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.
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
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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.
Spiral: A spiral is a SIP request that is routed to a proxy,
forwarded onwards, and arrives once again at that proxy, but
this time differs in a way that will result in a different
processing decision than the original request. Typically, this
means that the request’s Request-URI differs from its previous
arrival. A spiral is not an error condition, unlike a loop. A
typical cause for this is call forwarding. A user calls
joe@example.com. The example.com proxy forwards it to Joe’s
PC, which in turn, forwards it to bob@example.com. This
request is proxied back to the example.com proxy. However,
this is not a loop. Since the request is targeted at a
different user, it is considered a spiral, and is a valid
condition.
Stateful Proxy: A logical entity that maintains the client and
server transaction state machines defined by this specification
during the processing of a request, also known as a transaction
stateful proxy. The behavior of a stateful proxy is further
defined in Section 16. A (transaction) stateful proxy is not
the same as a call stateful proxy.
Stateless Proxy: A logical entity that does not maintain the
client or server transaction state machines defined in this
specification when it processes requests. A stateless proxy
forwards every request it receives downstream and every
response it receives upstream.
Strict Routing: A proxy is said to be strict routing if it follows
the Route processing rules of RFC 2543 and many prior work in
progress versions of this RFC. That rule caused proxies to
destroy the contents of the Request-URI when a Route header
field was present. Strict routing behavior is not used in this
specification, in favor of a loose routing behavior. Proxies
that perform strict routing are also known as strict routers.
Target Refresh Request: A target refresh request sent within a
dialog is defined as a request that can modify the remote
target of the dialog.
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.
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Upstream: A direction of message forwarding within a transaction
that refers to the direction that responses flow from the user
agent server back to the user agent client.
URL-encoded: A character string encoded according to RFC 2396,
Section 2.4 [5].
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.
UAC Core: The set of processing functions required of a UAC that
reside above the transaction and transport layers.
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.
UAS Core: The set of processing functions required at a UAS that
resides above the transaction and transport layers.
User Agent (UA): A logical entity that can act as both a user
agent client and user agent server.
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
