OSI Layer 3: Routed and Routing Protocols
49
Internet Control Message Protocol (ICMP)
The Internet Control Message Protocol (ICMP) is used throughout IP net-
works. ICMP was designed to provide routing failure information to the
source system. This protocol provides four different types of feedback that
are used to make the IP routing environment more efficient. These feedback
types are as follows:
Reachability This is determined by using ICMP echo and reply mes-
sages.
Redirects These messages tell hosts to redirect traffic or choose alterna-
tive routes.
Timeouts These messages indicate that a packet's designated TTL is
expired.
Router Discovery These messages discover directly connected routers'
IP addresses. Router discovery actually uses ICMP Router Discovery
Protocol to do this. This passive method gathers directly connected IP
addresses without having to understand specific routing protocols.
Here is a look at a couple of ICMP packets (echo request and reply):
ICMP - Internet Control Messages Protocol
ICMP Type: 8 Echo Request
Code: 0
Checksum: 0x495c
Identifier: 0x0200
Sequence Number: 512
ICMP Data Area:
abcdefghijklmnop 61 62 63 64 65 66 67 68 69 6a 6b 6c
6d 6e 6f 70
qrstuvwabcdefghi 71 72 73 74 75 76 77 61 62 63 64 65
66 67 68 69
Frame Check Sequence: 0x342e3235
ICMP - Internet Control Messages Protocol
ICMP Type: 0 Echo Reply
Code: 0
Checksum: 0x515c
Identifier: 0x0200
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