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Unformatted text preview: that compressing the headers from 40 bytes
to 5 bytes reduces the round-trip time for the I byte of data from 85 to 12.5 ms.
Unfortunately these types of calculations are harder to make when newer error correcting,
compressing modems are being used. The compression employed by these modems reduces the
number of bytes sent across the wire, but the error correction may increase the amount of time to
transfer these bytes. Nevertheless, these calculations give us a starting point to make reasonable
In later chapters we'll use these serial line calculations to verify some of the timings that we see
when watching packets go across a serial link. 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
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
2.1 If your system supports the netstat(1) command (see Section 3.9 also), use it to determine
the interfaces on your system and their MTUs. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12