2.11 Summary

This chapter has examined the lowest layer in the Internet protocol suite, the link layer. We looked at the difference between Ethernet and IEEE 802.2/802.3 encapsulation, and the encapsulation used by SLIP and PPP. Since both SLIP and PPP are often used on slow links, both provide a way to compress the common fields that don't often change. This provides better interactive response.

The loopback interface is provided by most implementations . Access to this interface is either through the special loopback address, normally 127.0.0.1, or by sending IP datagrams to one of the host's own IP addresses. Loopback data has been completely processed by the transport layer and by IP when it loops around to go up the protocol stack.

We described an important feature of many link layers , the MTU, and the related concept of a path MTU. Using the typical MTUs for serial lines, we calculated the latency involved in SLIP and CSLIP links.

This chapter has covered only a few of the common data-link technologies used with TCP/IP today. One reason for the success of TCP/IP is its ability to work on top of almost any data-link technology.