As we have seen many times in this book, because FreeSWITCH is a B2BUA (Back to Back User Agent), when a user makes a call via FS, FS actually originates a completely independent new call (to callee), and bridges the two calls' audio streams. The streams then flow from caller to FreeSWITCH to callee (and back). In this context, the first call (from caller to FS) is named "A-leg", while the second (from FS to callee) is named "B-leg".

From the point of view of the user, what she experiences as a call is what for us VoIP geeks is two calls, or a bridged call, or A-leg and B-leg.

It's obviously very useful to be able to visualize and debug a complete bridged call, made by the two legs. This gives us the complete picture of what was experienced by the end user.

We have at least two sides to configure: We need FreeSWITCH to introduce into SIP packets a correlation ID that will tag two calls as two legs of a bridged call, and we need to instruct the Capture Server to use...