Snom Technology Ag; Media Relay; Controlling Routing - Snom 4S NAT Filter Admin Manual

NAT Filter. This is a setting that you might have to coordinate with your
firewall.

4.3.5 Media Relay

If you set the Always Relay flag, the filter will always relay media
via the filter and does not allow bypassing it by ICE contacts. That means
it will remove ICE contacts from the SDP and not insert an additional
address for it self. This flag is useful when you want to make sure that all
media flows through the filter, e.g. for measurement purposes or because
you want to be able to record all calls. However, it will not be possible to
do local media path optimization if you turn this flag on.

4.3.6 Controlling Routing

The Loose Routing flag influences the way that the NAT Filter
inserts routing headers into SIP packets. Loose routing is the routing
mechanism that is proposed in the latest SIP document; however there
are devices which are not able to deal properly with these routing headers
(the new standard is not backward compatible with the old standard).
The Hide Routing flag will replace route sets with a unique route
index when requests or responses are sent to a registered user agent. Via
headers are also replaced with one Via header. This feature has several
advantages. First, it will reduce the packet size significantly, especially
when your core network uses several proxies or loops requests through
the proxy several times. Usually, UDP packets will have a size significantly
below the MTU size of 1492 bytes for Ethernet. This is a tremendous
advantage that solves many problems with equipment that does not
support UDP fragmentation.
Second, it hides important information about your network
topology from the user agents. For example, when you are terminating
calls with a PSTN gateway, the users are not able to see the IP address
of the PSTN gateway in the routing path (if you turn always relay on, this
address will also not occur in the SDP). Users will only "see" the filter as
the only window to the outside world. This makes attacks much more
difficult. It is much easier to protect only the filter against attacks than
you whole SIP network.
The third big advantage is that is solves many problems with poor
SIP implementations. Typically, immature SIP implementations can not
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