Link Efficiency

The category of link efficiency encompasses two real topics: compression and fragmentation. Rather than treat these topics in two separate chapters, I have included them in one chapter (Chapter 7, "Link-Efficiency Tools") to match the organization of the Cisco QoS courses (and the IOS documentation to some degree).

Compression reduces bandwidth utilization by making packets smaller before transmission. Two general types of compression tools exist in IOS—payload compression and header compression. Payload compression compresses the "packet"—the portion of the data link frame between the frame header and trailer. Header compression compresses just particular headers. Figure 2-6 shows the typical scope of the compressed portions of a frame over a PPP link.

Figure 2-6 Scope of Compression for Payload and Header Compression Types

Payload Compression

Payload Compression

PPP Header

IP

TCP

Data

PPP

Trailer

Header Compression

Header Compression

PPP Header

IP

UDP

RTP

Data

PPP

Trailer

Payload Compression

Payload Compression

Compression tools differ in how much CPU load they create and which parts of the frame they compress. Based on the CPU load and what is compressed, you can make good decisions about when to use each tool.

Payload compression can be applied to all packets, with some good results. Suppose that the compression algorithm manages to compress x bytes of payload into (1/2x)—a reasonable 2:1 compression ratio. The router saves a lot of bandwidth with the compression a 1500-byte packet into a 750-byte packet. Given the variation and unpredictable nature of the contents of the packets, compression ratios between 2:1 and 4:1 are reasonable with payload compression.

Header compression takes advantage of the fact that the headers being compressed are predictable. Much larger compression ratios can be achieved, many times with less CPU load than payload compression. However, header compression only operates on headers. For instance, compressed RTP compresses packets with IP/UDP/RTP headers, as shown in Figure 2-6. The 40 bytes of the IP/UDP/RTP headers compress to between 2 and 4 bytes. For a minimum packet size of 60 bytes, typical of G.729 VoIP calls, cRTP reduces the packet from 60 bytes to between 22 to 24 bytes, a significant improvement.

The other major category of link-efficiency tools is link fragmentation and interleaving (LFI), also just called fragmentation. The concept is simple: When a router starts sending a packet, it never just stops sending that packet in order to send a higher-priority packet—it finishes sending the first packet, and then sends the higher-priority packet. On slow links, the time it takes for one large packet to be serialized may cause too much delay, particularly for VoIP and video traffic. LFI tools fragment large packets into smaller packets, and then interleave the high-priority packet between the fragments. For instance, it takes 214 ms to serialize one 1500-byte packet over a 56-kbps link—which blows the VoIP one-way delay budget. (As described in Chapter 7, Cisco recommends that you considered LFI when the link speed is 768 kbps or less.) Figure 2-7 shows the process of fragmentation.

Figure 2-7 Link Fragmentation and Interleaving

1 X 1500 Byte Packet, Followed by 1 X 200 Byte Delay-Sensitive Packet

Output Queue 1:

3 Fragments of Packet #1 Shown

Output Queue 1:

3 Fragments of Packet #1 Shown

Without LFI, packet 2 has to wait 214 ms on the 1500-byte packet. With LFI fragmenting packet 1 into 3 parts, the serialization delay is reduced to about 72 ms.

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