TCP/IP Configuration and Management Manual

NonStop TCP/IP Processes and Protocols
TCP/IP Configuration and Management Manual—427132-004
B-22
Internet Control Message Protocol (ICMP)
protocol. Applications can build their own transport-layer protocols directly on this
protocol by using raw sockets.
Message Routing
In the IP, messages called datagrams pass from a source (host) to a destination. An IP
datagram may consist of one or several network-layer messages, or packets.
Datagrams in the Internet often pass through a dozen different networks before
reaching their final destination. The message routing is performed by the IP and is
transparent to the application. The application only needs to specify its own local
Internet address and the remote Internet address of the destination.
The IP uses routes, which are Internet paths, to transmit datagrams. A route specifies
both a final destination and the gateway or host to which a datagram must be sent next
to reach that destination. The IP layer at each host maintains a table of routes. The
table lists the Internet address of each accessible network, or of a particular accessible
host on a network, and the gateway used to get to that network or host.
To keep their routing tables simple, hosts often specify a default gateway or router to
which datagrams can be sent if the routing table contains no entry for a particular
network. The hosts thus rely on the gateways, which contain comprehensive routing
tables, to determine the best path for the datagram.
Raw Sockets
As for TCP and UDP, described above, application processes call a socket routine to
request the NonStop TCP/IP software to create a raw socket when needed. A raw
socket provides direct access to the IP. Raw sockets allow applications to take
advantage of protocol features not directly accessible through the TCP or UDP
interfaces. Applications also can develop new protocols by using raw sockets.
Programming at the IP level by using raw sockets requires the application to provide
support for the transport protocol used above IP. If your application program refers to a
transport protocol by name, the program must provide an entry, including the protocol
number and name, in the file $SYSTEM.ZTCPIP.PROTOCOLS, as described in
Section 1, Configuration Quick Start
.
For a detailed description of the socket library routines available, refer to the
TCP/IP
Programming Manual. For more detailed technical information on the IP, refer to RFC
791 (
DDN Protocol Handbook, Volume 2, DDN Network Information Center,
December, 1985, pp. 2-99 through 2-150). Also, refer to the book
TCP/IP Illustrated by
W. Richard Stevens, Prentice Hall, 1994.
Internet Control Message Protocol (ICMP)
The NonStop TCP/IP software uses the Internet Control Message Protocol (ICMP) with
the IP to report errors and other control information from destination systems or
gateways.