Real Time Protocol (Rtp) Header Compression - Enterasys Security Router X-PeditionTM User Manual

Leaving the Router ID unconfigured or allowing it to be assigned by default to a physical IP
interface can be risky because physical interfaces are impermanent and their IP addresses can be
re-configured. A change in an IP address or the state of a physical interface that has been selected
as the Router ID will cause the XSR to drop and recreate its neighbor adjacencies, leading to
unnecessary instability.
The XSR permits you to configure the Router ID, using the
following conditions:
• The configured Router ID IP address need not correspond to a valid interface (loopback or
physical).
• When the Router ID is changed, dependant protocols (i.e., OSPF) are informed and the
protocol restarted with the new Router ID.
• Precedence for Router ID selection is: user-configured Router ID, then highest IP address
loopback port, and highest IP address physical port.

Real Time Protocol (RTP) Header Compression

RTP header compression is useful for VoIP traffic over low speed PPP wan links . For VoIP
packets, the 40 bytes of header (IP header + UDP + RTP header) is almost as large as the VoIP
payload itself. The ability to reduce the header size enables more voice traffic to be passed over the
PPP wan link.
RFC 2508 describes a scheme where the 40 byte combined header of RTP (VoIP) traffic can be
reduced to 2 to 4 bytes under most circumstances.
With release 7.6.0.0, the XSR now supports both the VJ Header Compression (for TCP and UDP
header) and the new IP Header Compression (for TCP, UDP and RTP header compression).
XSR cannot be configured to initiate VJ header compression, but it does response to VJ Header
compression configuration option from the remote peer with a NAK or REJ.
In release 7.6.0.0, the behavior is changed slightly. If RTP is not enabled, then upon receiving a VJ
header compression negotiation option, the XSR sends back a NAK or REJ, same as in current
release.
XSR uses the following criteria to select packets for RTP compression:
Must be UDP packet,
UDP payload must be less than 500 bytes,
Packet must not be fragmented, and
The destination port of the packet must be within user configurable port range (there is no
restriction on the source port). The XSR does not impose any restriction on RTP de-compression.
The following new alarms are added for RTP header compression:
High Severity alarm:
Out of memory, RTP compression disabled
Low Severity alarms:
RTP_compression RX failed allocate memory
ip router-id command, with the
XSR User's Guide 5-25
IP Routing Protocols