Information Technology Reference
In-Depth Information
The descriptions of the messages and replies are:
•
Source quench message (4) - sent by a gateway or a destination host when it discards a
datagram (possibly through lack of buffer memory), and identifies that the sender should
reduce the flow of traffic transmission. The host should then reduce the flow, and gradu-
ally increase it, as long as it does not receive any more source quench messages.
•
Time exceeded message (11) - this is sent either by a gateway when a datagram has a
Time-to-Live field which is zero and has been deleted, or when a host cannot reassemble
a fragmented datagram due to missing fragments, within a certain time limit.
•
Parameter problem message (12) - sent by a gateway or a host when they encounter a
problem with one of the parameters in an IP header.
•
Destination unreachable message (3) - sent by a gateway to identify that a host cannot be
reached or a TCP port process does not exist.
•
Redirected message (5) - sent by a gateway to inform other gateways that there is a better
route to a given network destination address.
•
Information reply message (15)- sent in reply to an information request.( see information
request (16) for a typical usage).
•
Information request (16) - this request can be sent with a fully specified source IP ad-
dress, and a zero destination IP address. The replying IP gateway then replies with an in-
formation reply message with its fully specified IP address. In this way the host can de-
termine the network address that it is connected to.
•
Echo message (8) - requests an echo. (see echo reply message (0)).
•
Echo reply message (0) - the data received in the echo message (8) must be returned in
this message.
23.6 TCP/IP internets
Figure 23.5 illustrates a sample TCP/IP implementation. A gateway
MERCURY
provides a link
between a Token Ring network (NETWORK A) and the Ethernet network (ETHER C). An-
other gateway
PLUTO
connects NETWORK B to ETHER C. The TCP/IP protocol allows a
host on NETWORK A to communicate with
VAX01
.
23.6.1 Selecting internet addresses
Each node using TCP/IP communications requires an IP address which is then matched to its
Token Ring or Ethernet MAC address. The MAC address allows nodes on the same segment
to communicate with each other. In order for nodes on a different network to communicate,
each must be configured with an IP address.
Nodes on a TCP/IP network are either hosts or gateways. Any nodes that run application
software or are terminals are hosts. Any node that routes TCP/IP packets between networks
is called a TCP/IP gateway node. This node must have the necessary network controller
boards to physically interface to other networks it connects with.
23.6.2 Format of the IP address
A typical IP address consists of two fields: the left field (or the network number) identifies
the network, and the right number (or the host number) identifies the particular host within
that network. Figure 23.6 illustrates this.
