Information Technology Reference
In-Depth Information
Dynamic routing protocols, such as RIP and OSPF, provide a means by which routers can communicate
and share information about routes that they have learned or are connected to. This contrasts with static
routing, in which routes are established by the network administrator and do not change unless they are
manually altered. An IP routing table consists of destination address/next-hop pairs. A sample entry,
shown in Figure 7-5, is interpreted as meaning, “To get to network 34.1.0.0 (subnet 1 on network 34),
the next stop is the node at address 54.34.23.12.”
Figure7-5
An Example of an IP Routing Table
Destination
address
Next
hop
34.1.0.0
78.2.0.0
147.9.5.0
17.12.0.0
54.34.23.12
54.34.23.12
54.32.12.10
54.32.12.10
IP routing specifies that IP datagrams travel through internetworks one hop at a time; the entire route is
not known at the outset of the journey. Instead, at each stop, the next destination is calculated by
matching the destination address within the datagram with an entry in the current node's routing table.
Each node's involvement in the routing process consists only of forwarding packets based on internal
information, regardless of whether the packets get to their final destination. In other words, IP does not
provide for error reporting back to the source when routing anomalies occur. This task is left to other
Internet protocols, such as the Internet Control Message Protocol (ICMP) and TCP protocol.
ICMP
ICMP performs a number of tasks within an IP internetwork, the principal of which is reporting routing
failures back to the source of a datagram. In addition, ICMP provides helpful messages such as the
following:
•
Echo and reply messages to test node reachability across an internetwork
•
Redirect messages to stimulate more efficient routing
•
Time exceeded messages to inform sources that a datagram has exceeded its allocated time to exist
within the internetwork
•
Router advertisement and router solicitation messages to determine the addresses of routers on
directly attached subnetworks
The Transport Layer
The Internet transport layer is implemented by Transport Control Protocol (TCP) and the User Datagram
Protocol (UDP). TCP provides connection-oriented data transport, whereas UDP operation is
connectionless.
