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Unformatted text preview: curate in Figure 1.7 we should say that the unit of data passed between IP and the network
interface is a packet. This packet can be either an IP datagram or a fragment of an IP datagram. We discuss
fragmentation in detail in Section 11.5. We could draw a nearly identical picture for UDP data. The only changes are that the unit of
information that UDP passes to IP is called a UDP datagram, and the size of the UDP header is
8 bytes. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/introduc.htm (9 of 20) [12/09/2001 14.46.31] Chapter 1. Introduction Figure 1.7 Encapsulation of data as it goes down the protocol stack.
Recall from Figure 1.4 that TCP, UDP, ICMP, and IGMP all send data to IP. IP must add some
type of identifier to the IP header that it generates, to indicate the layer to which the data
belongs. IP handles this by storing an 8-bit value in its header called the protocol field. A value
of 1 is for ICMP, 2 is for IGMP, 6 indicates TCP, and 17 is for UDP.
Similarly, many different applications can be using TCP or UDP at any one time. The transport
layer protocols store an identifier in the headers they generate to identify the application. Both
TCP and UDP use 16-bit port numbers to identify applications. TCP and UDP store the source
port number and the destination port number in their respective headers.
The network interface sends and receives frames on behalf of IP, ARP, and RARP. There must
be some form of identification in the Ethernet header indicating which network layer protocol
generated the data. To handle this there is a 16-bit frame type field in the Ethernet header. 1.7 Demultiplexing
When an Ethernet frame is received at the destination host it starts its way up the protocol stack
and all the headers are removed by the appropriate protocol box. Each protocol box looks at
certain identifiers in its header to determine which box in the next upper layer receives the data.
This is called demultiplexing. Figure 1.8 shows how this takes place. file:///D|/Documents%20and%20Settings/bigini/Docu...h...
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