If the SIP messages have been encrypted, this presents
a challenge to this method of collecting signaling messages and will then require an
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alternative solution. Lawful intercept is challenged here as well, as many times these
performance management solutions are also used to support lawful intercept.
When topology hiding is being used, the P-CSCF does not know how to decrypt the
messages (it does not have the cipher key). Only the I-CSCF has the cipher keys, and
therefore the receiving network must also have an I-CSCF providing topology hiding
in order to decrypt these messages as they are received. The receiving network is
considered in this case the terminating network. The terminating network must be
able to read the headers for routing responses back to the requestor in the originating
network. Transiting networks do not need to see these headers but rely solely on the
request-URI for routing of the message. Remember the request-URI is located in
the first header of the message, where the method is identified, and is used to route the
message to its final destination (or the next hop in the network).
Topology hiding is also used to screen SIP messages and strip other information that
is of local significance only, such as billing information and IP addresses. Hiding this
information is useful for a number of reasons. Charging information is never shared
between two networks, and therefore the I-CSCF must ensure that the headers providing
information regarding how the network was accessed (P-ACCESS-NETWORK-ID)
and all charging headers are removed prior to forwarding the SIP message to another
network.
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