3Com 3C13636 Configuration Manual page 313

3Com Router 3000 Ethernet Family
Configuration Guide
implementations of network audio/video applications. However, there is also concern
that 40-byte IP/UDP/RTP header containing a 20-byte IP header, 8-byte UDP header
and 12-byte RTP header, is too large an overhead for 20-byte or 160-byte payloads.
To reduce overhead, you can use IPHC to compress headers. In many cases, all
three headers can be compressed to 2 to 5 bytes. The effect of the header
compression proves considerable that a payload of 40 bytes can be compressed to 5
bytes through the process with the compression ratio as (40+40) / (40+5), about 1.78.
The process of IPHC is illustrated in the following figure.
incoming packets
Traffic classifying
Figure 1-3 IP header compression
II. Stac LZS compression
Stac LZS compression is a link-layer data compression standard developed by Stac
Electronics. Stac LZS is a Lempel-Ziv-based algorithm that compresses only packet
payloads. It replaces a continuous data flow with binary code that can accommodate
to the change of data. While allowing for more flexibility, this requires more CPU
resources.
III. VJ TCP header compression
VJ TCP header compression was defined in RFC 1144 for use on low-speed links.
Each TCP/IP packet transmitted over a TCP connection contains a typical 40-byte
TCP/IP header containing an IP header and a TCP header that are 20-byte long each.
The information in some fields of these headers, however, is unchanged through the
lifetime of the connection and needs sending only once, while the information in some
other fields changes but regularly and within a definite range. Based on this idea, VJ
TCP header compression may compress a 40-byte TCP/IP header to 3 to 5 bytes. It
can significantly improve the transmission speed of some applications, such as FTP,
on a low-speed serial link like PPP.
IV. Link Fragmentation and Interleaving
On the low speed serial link, real-time interactive communication (such as Telnet and
VoIP) is performed, and block and delay may occur when large packets are
transmitted. For example, if a voice packet arrives when large packets are being
RTP header
cpmpression
queue
Non RTP traffic
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1-21
Chapter 1 PPP and MP Configuration
Sending queue