> Hi, RFC 3261 says:
> 
> ------------------------
>  17.2.3 Matching Requests to Server Transactions
>    The  request matches a transaction if:
> 
>       1. the branch parameter in the request is equal to the 
> one in the
>          top Via header field of the request that created the
>          transaction, and
> 
>       2. the sent-by value in the top Via of the request is 
> equal to the
>          one in the request that created the transaction, and
> 
>       3. the method of the request matches the one that created the
>          transaction, except for ACK, where the method of the request
>          that created the transaction is INVITE.
> ------------------------
> 
> Does someone really implement the point 2?

Yes.

<snip>

> Well, IMHO it's not neccesary *at all* to match 
> the point 2, it's completely enough with 
> points 1 and 3. What do you thing?

The reason for 2 is subsequently discussed within the same section: "The 
sent-by value is used as part of the matching process because there could be 
accidental or malicious duplication of branch parameters from different 
clients."

Because the Via sent-by can be spoofed as easily as branch, the benefit is 
mainly associated with accidental duplication.

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