TCP/IP Tutorial and Technical Overview - IBM Redbooks

► Ease of implementation: Distance vector protocols are the simplest routing
protocol to configure and maintain. Because of this, these protocols have the
largest implementation base. Limited training is required to perform problem
resolution in these environments.
In small, non-changing environments, static routes are also simple to
implement. These definitions change only when sites are added or removed
from the network. The administrator must assess the importance of each of
these requirements when determining the appropriate routing protocol for an
environment.
5.11 Additional functions performed by the router
The main functions performed by a router relate to managing the IP routing table
and forwarding data. However, the router should be able to provide information
alerting other devices to potential network problems.
This information is provided by the ICMP protocol described in 3.2, “Internet
Control Message Protocol (ICMP)” on page 109. The information includes:
► ICMP Destination Unreachable: The destination address specified in the IP
packet references an unknown IP network.
► ICMP Redirect: Redirect forwarding of traffic to a more suitable router along
the path to the destination.
► ICMP Source Quench: Congestion problems (for example, too many
incoming datagrams for the available buffer space) have been encountered in
a device along the path to the destination.
► ICMP Time Exceeded: The Time-to-Live field of an IP datagram has reached
zero. The packet is not able to be delivered to the final destination.
In addition, each IP router should support the following base ICMP operations
and messages:
► Parameter problem: This message is returned to the packet’s source if a
problem with the IP header is found. The message indicates the type and
location of the problem. The router discards the errored packet.
► Address mask request/reply: A router must implement support for receiving
ICMP Address Mask Request messages and responding with ICMP Address
Mask Reply messages.
► Timestamp: The router must return a Timestamp Reply to every Timestamp
message that is received. It should be designed for minimum variability in
delay. To synchronize the clock on the router, the UDP Time Server Protocol
or the Network Time Protocol (NTP) can be used.
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