定义

RFC3261在第14节指明：

An INVITE request sent within an existing dialog is known as a re-INVITE.

意思是re-INVITE是对话（Dialog）内发出的INVITE，其本质为INVITE，即re-INVITE不是新的SIP method（方法）。

功能

我们知道INVITE是用来初始化一个会话（Session），RFC3261在第13节中的原话如下：

When a user agent client desires to initiate a session (for example, audio, video, or a game), it formulates an INVITE request. The INVITE request asks a server to establish a session.
……
A 2xx response to an INVITE establishes a session, and it also creates a dialog between the UA that issued the INVITE and the UA that generated the 2xx response.

且第14节又简洁地复述了一遍：

A successful INVITE request establishes both a dialog between two user agents and a session using the offer-answer model.

一个会话建立后还可以修改吗？回答是肯定的，如何修改？这就需要re-INVITE，第14节中的原话如下：

This modification can involve changing addresses or ports, adding a media stream, deleting a media stream, and so on. This is accomplished by sending a new INVITE request within the same dialog that established the session.
……
Note that a single re-INVITE can modify the dialog and the parameters of the session at the same time.

简而言之，INVITE用来建立会话的，而re-INVITE是用来修改会话的。

识别re-INVITE

既然re-INVITE的本质就是INVITE，那对于一个SIP报文，我们怎么辨别它是INVITE，还是re-INVITE呢？

如果您对对话和会话的建立过程了解的话，区分INVITE和re-INVITE其实是非常容易的。

您知道什么叫SIP对话吗？RFC3261在第4节的概述中就对对话下了个定义：

The combination of the To tag, From tag, and Call-ID completely defines a peer-to-peer SIP relationship between Alice and Bob and is referred to as a dialog.

对！请牢记在心：对话的三要素就是To tag，From tag和Call-ID。

显然，用来初始化会话的INVITE发出时，UAC只知道To头，而不知道To tag。也就是说，这时INVITE中有From，From tag，To和Call-ID，就是没有To tag，这很正常，此时对话尚未建立，UAS还没有参与进来，而To tag正是UAS产生的，正如From tag产生于UAC。

这一点，RFC3261在第8.1.1.2小节描述To头明确指出：

A request outside of a dialog MUST NOT contain a To tag; the tag in the To field of a request identifies the peer of the dialog. Since no dialog is established, no tag is present.

那有人可能会问：什么时候会出现To tag，是不是UAS刚开始回应时就携带To tag呢？答案是不一定，RFC3261在第8.2.6.2小节指明：

However, if the To header field in the request did not contain a tag, the URI in the To header field in the response MUST equal the URI in the To header field; additionally, the UAS MUST add a tag to the To header field in the response (with the exception of the 100 (Trying) response, in which a tag MAY be present). This serves to identify the UAS that is responding, possibly resulting in a component of a dialog ID. The same tag MUST be used for all responses to that request, both final and provisional (again excepting the 100 (Trying)).

读到这里，您肯定知道，既然re-INVITE是用来修改会话的，在对话内发送，其中肯定含有To tag。

结论

以上罗嗦了很多，总结起来就是一句话，那就是：

INVITE报文中肯定没有To tag，而re-INVITE中肯定含有To tag。