Data Communications Networking Devices - 4th Ed.pdf

206 __________________________________________________________ WIDE AREA NETWORKSSimilar to an X.25 network, on a TCP/IP network datagrams are routed via thebest path available to the destination as the datagram is placed onto the network.An alternative to datagram transmission is the use of a virtual circuit, wherenetwork nodes establish a ®xed path when a connection is initiated and subsequentdata exchanges occur on that path. TCP implements transmission via the use of avirtual circuit, while IP provides a datagram oriented gateway transmission servicebetween networks.The routing of datagrams through a network can occur over different paths, withsome datagrams arriving out of sequence from the order in which they weretransmitted. In addition, as datagrams ¯ow between networks they encounterphysical limitations imposed upon the amount of data that can be transported basedupon the transport mechanism used to move data on the network. For example, theInformation ®eld in an Ethernet frame is limited to 1500 bytes while a 4 MbpsToken-Ring can transport 4500 bytes in its Information ®eld. Thus, as datagrams¯ow between networks, they may have to be fragmented into two or more datagramsto be transported through different networks to their ultimate destination.For example, consider the transfer of a 20 000 byte ®le from a ®le server connectedto a Token-Ring network to a workstation connected to an Ethernet LAN via a pairof routers providing a connection between the two local area networks. The 4 MbpsToken-Ring network supports a maximum Information ®eld of 4500 bytes in eachframe transmitted on that network, while the maximum size of the Information®eld in an Ethernet frame is 1500 bytes. In addition, depending upon the protocolused on the wide area network connection between routers, the WAN protocol'sInformation ®eld could be limited to 512 or 1024 bytes. Thus, the IP protocol mustbreak up the ®le transfer into a series of datagrams whose size is acceptable fortransmission between networks. As an alternative, IP can transmit data using asmall maximum datagram size, commonly 576 bytes, to prevent fragmentation. Iffragmentation is necessary, the source host can transmit using the maximumdatagram size available on its network. When the datagram arrives at the router, IPoperating on that communications device will then fragment each datagram into aseries of smaller datagrams. Upon receipt at the destination, each datagram mustthen be put back into its correct sequence so that the ®le can be correctly reformed,a responsibility of IP residing on the destination host.Figure 2.23 illustrates the routing of two datagrams from workstation 1 on aToken-Ring network to server 2 connected to an Ethernet LAN. As the routing ofdatagrams is a connectionless service, no call setup is required, which enhancestransmission ef®ciency. In comparison, when TCP is used, it provides aconnection-oriented service regardless of the lower layer delivery system e.g. IP).TCP requires the establishment of a virtual circuit in which a temporary path isdeveloped between source and destination. This path is ®xed and the ¯ow ofdatagrams is restricted to the established path. When the User Datagram ProtocolUDP), a different layer 4 protocol in the TCP/IP protocol suite, is used in placeof TCP, the ¯ow of data at the Transport layer continues to be connectionless andresults in the transport of datagrams over available paths rather than a ®xed pathresulting from the establishment of a virtual circuit.The actual division of a message into datagrams is the responsibility of the layer 4protocol, either TCP or UDP, while fragmentation is the responsibility of IP. In