IP and ICMP protocols

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IP protocal resides in Network Layer, which provide service to the Transport Layer protocals TCP and UDP.

IP packet structure

bit offset	0–3				4–7				8–15								16–18			19–31
0	Version				Header length				Differentiated Services								Total Length
32	Identification																Flags			Fragment Offset
64	Time to Live								Protocol								Header Checksum
96	Source Address
128	Destination Address
160	Options
160 or 192+	Data

The header consists of 13 fields, of which 12 are required. The 13^th field is optional (red background in table) and aptly named: options.

IP is the primary protocol in the Network Layer (Layer 3) of the OSI Model and has the task of delivering packets from the source host to the destination host solely based on their addresses. IP is considered a best-effort protocol because IP does not provide acknowledgement of the data delivery. Its service can also be characterized as unreliable. In network architectural language it is a connection-less protocol, in contrast to so-called connection-oriented protocols such as TCP and Frame Relay.

ICMP packet structure

The ICMP header starts after bit 160 of the IP header (unless IP options are used). Although ICMP messages are contained within standard IP datagrams, ICMP messages are usually processed as a special case.
ICMP messages are constructed from a normal IP datagram that has generated an ICMP response. IP encapsulates the appropriate ICMP message with a new IP header to get the ICMP message back to the original sending host and transmits the resulting datagram in the usual manner.

Bits	160-167	168-175	176-183	184-191
160	Type	Code	Checksum
192	ID		Sequence

ICND1 and ICND2 break down