GPRS help

[Ext User(juriguen)]

I'm new to this group, and I'm not really sure if you usually post
technical problems, but lets go:

I'm trying to understand, deeply, how the GPRS' GTP protocol works. My
doubt is regarding the Update PDP Context, when the procedure occurs
due to an Inter SGSN Routing Area Update.

Concretely, I don't understand how does the New SGSN inform the GGSN
about the changing of the open PDP contexts for the user who has moved
from the Old SGSN.

I'll try to explain a bit more in detail. As far as I know, a PDP
Context is characterized by the originator and destinator IPs and the
SGSN's and GGSN's control and data TEIds.
How do all these parameters update when the New SGSN wants to indicate
the GGSN that it's going to deal with all the open Contexts for a user?

Thanks in advance for any help :)