Header compression configuration
Header compression reduces the size of packet headers, thus reducing the amount of
bandwidth needed for data. The header compression method is based on the fact that most
of the header fields remain constant or change in predictable ways throughout the session.
Thus, instead of constantly retransmitting the header, each side keeps a context table of the
sessions (the normal headers), and while sending and receiving packets it replaces the full-
length headers with one or two bytes CID (context-id) plus unpredictable deltas from the last
packet.
The Branch Gateway offers both RTP header compression, for reducing the amount of
bandwidth needed for voice traffic, and TCP and UDP header compression, for reducing the
amount of bandwidth needed for non-voice traffic.
For header compression purposes, any UDP packet with an even destination port within a
user-configurable range of ports, is considered an RTP packet.
The Branch Gateway enables decompression whenever compression is enabled. However,
when enabling header compression on a Frame Relay interface, you must first verify that the
remote host is also employing header compression. Header compression on a Frame Relay
interface does not check what the remote host is employing. Thus, it may compress headers
even when the remote host is not configured to decompress headers.
You can configure how often a full header is transmitted, either as a function of time or of
transmitted compressed packets.
Related topics:
Header compression configuration options
Header compression support by interface
on page 235
Summary of IPHC header compression CLI commands
on page 236
Configuring VJ header compression
on page 238
Commands used to display and clear header compression statistics
on page 240
Header compression configuration options
The Branch Gateway offers two options for configuring header compression:
• IP Header compression (IPHC) method, as defined by RFC 2507. IPHC-type
compression applies to RTP, TCP, and UDP headers.
• Van Jacobson (VJ) method, as defined in RFC 1144. VJ compression applies to TCP
headers only.
VoIP QoS
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Summary of Contents for G430
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