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What are Call leg & Call ID in SIP?

A call leg is a one to one signaling relationship between two user agents. The call ID is an identifier carried in SIP message that pertains to the call. A call is a collection of call legs.

A UAC starts by sending an INVITE. Due to forking, it may receive multiple 200 OK from different UAs. Each represents to a different call leg within the same call.

A call is thus a group of call legs. A call leg is an end-to-end connection between UAs.

The CSeq spaces in the two directions of a call leg are autonomous. Within a single direction, the sequence number is increased for each transaction.

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