May 2006
Intermediate to advanced
480 pages
12h 20m
English
Content preview from Understanding TCP/IPBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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5.2. Internet Control Message Protocol
ICMP is a service protocol that is part of IP. It is used to signal abnormal events in networks built on the IP protocol. ICMP wraps its packets into an IP datagram, i.e., if we capture transported datagrams, we can later find a link header then an IP header followed by the header of the ICMP packet.
It is possible to signal various states with ICMP; however, the reality is that any specific implementation of TCP/IP can only support a certain number of these signals and, above all, many ICMP signals may be discarded by routers for security reasons.
An ICMP packet header is always 8-bytes long (see Figure 5.10). The first four bytes always have the same meaning, and the contents of the remaining four depend ...
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