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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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3.2.1 ICMP messages<br />

ICMP messages are described in RFC 792 <strong>and</strong> RFC 950, belong to STD 5, <strong>and</strong><br />

are m<strong>and</strong>atory.<br />

ICMP messages are sent in <strong>IP</strong> datagrams. The <strong>IP</strong> header has a protocol number<br />

of 1 (ICMP) <strong>and</strong> a type of service of zero (routine). The <strong>IP</strong> data field contains the<br />

ICMP message shown in Figure 3-27.<br />

0 8 16 31<br />

identifier sequence number checksum<br />

ICMP data (depending on the type of message)<br />

Figure 3-27 ICMP: Message format<br />

The message contains the following components:<br />

Type Specifies the type of the message:<br />

0 Echo reply<br />

3 Destination unreachable<br />

4 Source quench<br />

5 Redirect<br />

8 Echo<br />

9 Router advertisement<br />

10 Router solicitation<br />

11 Time exceeded<br />

12 Parameter problem<br />

13 Time stamp request<br />

14 Time stamp reply<br />

17 Address mask request<br />

18 Address mask reply<br />

30 Traceroute<br />

37 Domain name request)<br />

38 Domain name reply)<br />

The following RFCs are required to be mentioned for some of the ICMP<br />

message types: RFC 1256, RFC 1393, <strong>and</strong> RFC 1788.<br />

Code Contains the error code for the datagram reported by this<br />

ICMP message. The interpretation is dependent on the<br />

message type.<br />

Checksum Contains the checksum for the ICMP message starting<br />

with the ICMP Type field. If the checksum does not<br />

match the contents, the datagram is discarded.<br />

110 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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