Internet Control Message Protocol

(ICMP)

Internet Control Message Protocol (ICMP)




ICMP must be included in every TCP/IP implementation. ICMP is defined in RFC 792. It provides
feedback to the sender on problems, as well as internet settings such as the subnet mask. The
most frequently used ICMP messages are the ECHO REQUEST and ECHO REPLY. PING uses
these messages to find a remote host. The frames in figure 11 include the ICMP
messages ECHO REQUEST (frame 1) and ECHO REPLY (frame 2).

ICMP messages are contained within IP datagrams. This ensures the ICMP message will be
able to find its way to the appropriate host within an internet. Figure 12 lists the
ICMP message types.


Type Field - Message Type

0 - Echo Reply
3 - Destination Unreachable
4 - Source Quence
5 - Redirect
8 - Echo Request
11 - Time Exceeded for a Datagram
12 - Parameter Problem on a Datagram
13 - Timestamp Request
14 - Timestamp Reply
15 - Information Request
16 - Information Reply
17 - Address Mask Request
18 - Address Mask Reply

Figure 12: ICMP Message Types

The other two important ICMP messages after ECHO REQUEST and ECHO REPLY are the
REDIRECT and SOURCE QUENCH messages. The REDIRECT message is sent by a gateway to
the host instructing the host to use a different route when the router detects that its route is
not as optimal as that of another router. In the [tcp_xif] section there is the
defaultgateway0 parameter. The defaultgateway0 parameter instructs IP which
gateway to send the IP datagram when it needs to be delivered to a different subnet. If the
gateway detects a better route for the IP datagram, it will send the host the REDIRECT message
with the prefered gateway. LM TCP/IP will then use this new IP address to send all traffic for
another subnet.

The SOURCE QUENCH message informs the host that the gateway cannot keep up with the
traffic and requests the host to throttle back. The host lowers the rate at which it sends
datagrams to the host until it stops receiving SOURCE QUENCH messages, at which time it
gradually increases sending datagrams to the normal amount.

IP Parameters in PROTOCOL.INI

The following is an excerpt from PROTOCOL.INI that give example settings for parameters for IP:

[TCP_XIF]
IPADDRESS0 = 11 1 1 69
SUBNETMASK0 = 255 255 0 0
DEFAULTGATEWY0 = 11 1 1 90
SNAP = NO

The three parameters, ipaddress0, subnetmask0, and defaultgateway0
were discussed in detail above. The snap parameter affects the data link level, but will
be discussed here.

The snap parameter is used to encapsulate IP and ARP on IEEE 802 networks. SNAP
stands for Subnetwork Access Protocol. It is defined in RFC 1042. Figure 13 illustrates
what a frame with the SNAP header would look like.



Figure 13: Ethernet Frame with SNAP Header

Although it could be used to encapsulate 802.3 frames, SNAP is most commonly used on 802.5
(Token Ring) networks. When TCPTSR.EXE or TCPDRV.OS2 installs, it detects whether the
driver is an Ethernet or Token Ring driver. If Ethernet, the default for snap is no. If
Token Ring, the snap parameter defaults to yes.

When the snap parameter is set to no, LM TCP/IP traffic goes out as Ethernet frames. The
type value for IP is 0800 hex. However, since the code originally came from HP and they have
HP 3000's that might only speak 802.3, the LM TCP/IP will be able to send and receive 802.3
frames with the additional required software needed to communicate with HP 3000's.



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