L2 Payload Compression Results

Compression increases throughput and decreases delay. Use hardware compression when possible. Examples: Stacker, Predictor, MPPC.

If no compression is used, the throughput is limited by the link bandwidth, and the average delay is influenced by the forwarding or buffering delay, the serialization, and the propagation delay.

If compression is enabled—even if the serialization delay is now shorter because the frame is smaller—the compression or decompression delay may increase the overall latency between the two hops. The perceived throughput is generally increased because the size of the Layer 2 payload is reduced, therefore allowing more Layer 2 frames to be sent through a transmission resource in a given time period. The throughput is limited by the effectiveness of the Layer 2 payload compression algorithm and may be significantly higher than the link bandwidth limit.

If hardware-assisted Layer 2 payload compression is used, the compression or decompression delays may become insignificant compared to forwarding and serialization delays, and overall latency may decrease. The throughput is again limited by the effectiveness of the Layer 2 payload compression method and may be significantly higher than the link bandwidth limit.

Note Layer 2 payload compression configuration will not be covered in this module. Refer to the latest Cisco IOS documentation for configuration details.

Header Compression

This topic describes the purpose of header compression and how header compression affects throughput and delay.

Header Compression

IP and higher-layer headers are compressed.

60 Bytes VoIP (G.729) Packet 67% headers overhead

• Header compression reduces the size of the packet headers.

• The payload size is not changed.

• Example: (class-based) TCP and (class-based) RTP header compression.

IP and higher-layer headers are compressed.

60 Bytes VoIP (G.729) Packet 67% headers overhead

Header compression increases the throughput and reduces the delay by compressing the protocol headers. Header compression is most useful for applications that generate small payloads because the protocol headers of such applications use a significant percentage of bandwidth on a link relative to their payload. Real-time applications typically generate small payloads. Target applications for header compression include Telnet and RTP applications.

TCP and RTP header compression applies to all TCP and RTP flows. For example, if TCP compression is enabled on a link, there is no mechanism to restrict its function to specific application types. TCP header compression for bulk data transfer yields little bandwidth savings. Class-based TCP header compression can be performed only on a certain traffic class, such as the Telnet traffic class.

The header compression algorithm tracks active transport-layer connections over an interface. After the packet has been forwarded, the header compression algorithm compresses the Layer 3 and Layer 4 headers within the frame, and replaces the headers with a session index from the session dictionary (table). Only the non-constant parameters in the headers will be sent along with the session index. The packet is then sent to the output queue, and transmitted to the remote peer. When the remote peer receives the packet, the header is decompressed using the local session table, and passed to the forwarding process.

For example, without RTP header compression, the IP/UDP/RTP header overhead of the voice packet shown in the figure is about 67 percent (40 / 60 x 100 percent). With RTP header compression, the IP/UDP/RTP header can be reduced to 2 or 4 bytes (without and with checksum, respectively) for most packets. Therefore, the IP/UDP/RTP header overhead can be reduced to about 9 percent (2 / 22 x 100 percent) or 17 percent (4 / 24 x 100 percent).

Example: Header Compression Results

The figure compares two throughput and latency scenarios on a PPP link.

Link bandwidth

Link bandwidth

Throughput

Throughput

Delay=1 ms Delay=8 ms Total Delay=9 ms r

HEADER COMPRESSION Link bandwidth Throughput

Delay=2 ms Delay=4 ms Total Delay=6 ms y

• Header compression increases compression delay and reduces serialization delay.

If header compression is not used, the throughput is limited by the link bandwidth, and the average delay is influenced only by the forwarding or buffering delay, the serialization, and the propagation delay.

If header compression is enabled, compressing the protocol headers causes the packet to become smaller, therefore allowing more packets to be sent through a transmission resource in a given time period to increase the throughput. Because the packet size is smaller, the serialization delay also becomes smaller, thus reducing the overall delay. Header compression has a low compression delay and a relatively low CPU overhead and is recommended on links slower than 2 Mbps.

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