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IP-ICMP
Reference standard: RFC 792 - Internet Control Message
Protocol
ICMP messages typically report errors in the processing of
datagrams: for example, when a datagram cannot reach its
destination, when the gateway does not have the buffering
capacity to forward a datagram, and when the gateway can direct
the host to send traffic on a shorter route.
ICMP uses the basic support of IP as if it were a higher
level protocol, however, ICMP is actually an integral part of IP.
ICMP messages are sent using the basic IP header:
Bit 0 is the most significant bit.
| bits 0 _ _ _ _ _ _ 7 |
Datagram field |
| octets 1-n |
Header |
| octet n+1 |
Type
of message |
| octet n+2 |
Code |
| octets n+3, n+4 |
Checksum of the message |
| octets (n+5)-p |
Data |
| bits 0 _ _ _ _ _ _ 7 |
Header |
| octet 1, bits 0-3 |
Version = 4. |
| octet 1, bits 4-7 |
IHL
Internet Header Length in 32 bits words which points to
the Type of message. |
| octet 2 |
Type of service = 0. |
| octets 3-4 |
Total length
Length of the datagram, measured in octets, including
header and all following data. |
| octets 5-6 |
Identification. |
| octet 7, bits 0-2 |
Flags |
octet 7, bits 3-7
octet 8 |
Fragment
offset. |
| octet 9 |
Time to live in seconds.
As this field is decremented at each machine in which the
datagram is processed, the value in this field should be
at least as great as the number of gateways which this
datagram will traverse. |
| octet 10 |
Protocol = 1. |
| octets 11-12 |
Header Checksum
A checksum on the header only. The checksum field is the
16 bits one's complement of the one's complement sum of
all 16 bits words in the header. |
| octets 13-14-15-16 |
Source address : the address of the gateway or
host that composes the ICMP message. |
| octets 17-18-19-20 |
Destination address : the address of the
gateway or host to which the message should be sent. |
| k octets 21-22-(n-1) |
Options. |
| octet n |
Padding
The internet header padding is used to ensure that the
header ends on a 32 bit boundary. The padding is zero. |
| bits 0 _ _ _ _ _ _ 7 |
Dec |
Type of message |
| 0 0 0 0 0 0 0 0 |
0 |
Echo Reply |
| 0 0 0 0 0 0 1 1 |
3 |
Destination Unreachable |
| 0 0 0 0 0 1 0 0 |
4 |
Source Quench |
| 0 0 0 0 0 1 0 1 |
5 |
Redirect |
| 0 0 0 0 1 0 0 0 |
8 |
Echo |
| 0 0 0 0 1 0 1 1 |
11 |
Time Exceeded |
| 0 0 0 0 1 1 0 0 |
12 |
Parameter Problem |
| 0 0 0 0 1 1 0 1 |
13 |
Timestamp |
| 0 0 0 0 1 1 1 0 |
14 |
Timestamp Reply |
| 0 0 0 0 1 1 1 1 |
15 |
Information Request |
| 0 0 0 1 0 0 0 0 |
16 |
Information Reply |
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