Chapter 2. Link Layer

from figure 1.4 we see that the purpose of the link layer in the TCP/IP protocol suite is to send and receive

a. IP datagrams for the IP module

b. ARP requests and replies for the ARP module

c. RARP request and replies for the RARP module

The term Ethernet generally refers to a standard published in 1982 by Digital Equipment Corp . It is the predominant form of local area network technology used with TCP/IP today. It uses an access method called CSMA/CD, which stands for Carrier Sense, Multiple Acess with Collision Detection. It Operates at 10Mbits/sec and uses 48-bit addresses.

RFC 893 describes another form of encapsulation used on Ethernets , called trailer encapsulation

It was an experiment with early BSD systems on DEC VAXes that improved performance by rearranging the order of the fields in the IP datagram.

The variable-length fields at the beginning of the data portion of the Ethernet frame were moved to the end.

SLIP stands for Serial Line IP . It is a simple form of encapsulation for IP datagrames on serial lines, and is specified in RFC 1055.

SLIP has become popular for connecting home systems to the Internet, through the ubiquitous RS-232 serial port found on almost every compurter and high-speed modems

Since SLIP lines are often slow and frequently used for interactive traffic , there tend to be many small TCP packets exchanged across a SLIP line

To carry 1 byte of data requires a 20-byte IP header and a 20-byte TCP header, an overhead of 40 bytes.

PPP , the Point-to-Point Protocol, corrects all the deficiencies SLIP

Most implementations support a loopback interface that allows a client and server on the same host to communicate with each other using TCP/IP.

The class A network ID 127 is reserved for the loopback interface

As we can see from Figure 2.1 , there is a limit on the size of the frame for both Ethernet encapsulation and 802.3 encapsulation.

This limits the number of bytes of data to 1500 and 1492, respectively.

When two hosts on the same network are communicating with each other , it is the MTU of the network that is important

If the line speed is 9600 bits/sec , with 8 bits per byte, plus 1 start bit and 1 stop bit, the line speed is 960bytes/sec.