TCP/IP Configuration and Management Manual

NonStop TCP/IP Processes and Protocols
TCP/IP Configuration and Management Manual—427132-004
B-23
ICMP Router Discovery Protocol
ICMP uses the Address Resolution Protocol (ARP) to convert 32-bit internet (IP)
addresses into 48-bit Ethernet addresses. NonStop TCP/IP software uses the Internet
Control Message Protocol (ICMP, RFC 792) along with IP to report errors and other
control information from destination systems or gateways. The ICMP is considered a
required part of any IP implementation.
For more detailed technical information on the ICMP, refer to RFC 792 (
DDN Protocol
Handbook, Volume 2, DDN Network Information Center, December, 1985, pp. 2-151
through 2-172). Also, refer to the book
TCP/IP Illustrated by W. Richard Stevens,
Prentice Hall, 1994.
ICMP Router Discovery Protocol
RFC 1256 defines the ICMP Router Discovery Protocol (IRDP) which uses ICMP
messages on the host side to solicit default routes from routers on the local attached
LAN and on the router side to advertise the availability of default routes. If multiple
redundant routers are present, IRDP can detect secondary routes when a primary
route fails. This secondary use of RFC 1256 is described by HP as Alternate Route
Selection.
A default route is a path from a host to a router along which the host forwards IP
packets that are destined for some non-local LAN. The router must figure out the path
to the destination LAN. This method effectively delegates all routing responsibilities to
the router, which is the recommended procedure for NonStop TCP/IP in most LAN
configurations. Without IRDP, the NonStop TCP/IP administrator must configure
manually a static default route to the specific IP address of the router. If the router's IP
address changes, the configuration files on the NonStop machine must change also.
With IRDP, there is no need to change any configuration files other than initially
enabling the IRDP feature.
When route hold-down times and advertisement intervals are configured to
approximately 30 seconds on the router, and there is no backup router on the LAN,
IRDP can be used for dead-gateway detection.
When IRDP is enabled on a subnet, NonStop TCP/IP processes IRDP messages from
multiple routers on the LAN and switches the default route to the most-preferred router
that is currently advertising a default route. When one router stops advertising, another
can take over. Typically, an application times out while the routers reconfigure their
own routing tables to account for the lost router. If the application can tolerate a short
time out, the TCP connection is maintained. For example, most Telnet implementations
can tolerate time outs up to two minutes.
The NonStop TCP/IP process directly interprets incoming IRDP advertisements and
updates its routing table by adding a default route to the advertising router. Adhering to
RFC 1256, NonStop TCP/IP solicits default routes when new subnets are started.
NonStop TCP/IP also solicits default routes when a multi-hop TCP connection begins
to time-out. A time-out may be due to a router failure.
For more detailed technical information on the ICMP, refer to RFC 1256 (
DDN Protocol
Handbook, Volume 2, DDN Network Information Center, December, 1985, pp. 2-151