ICMP CHECKSUM CODE
Code field- The code field is an 8-bit field that provides further information about the ICMP message.These type field values are displayed in Table 1-2. Checksum - The Checksum is a 16 bit field containing a ones complement of the ones complement of the headers starting with the ICMP type and down. Type field- The type field is an 8-bit field that identifies the message.
These ICMP messages contain three fields at the beginning of the packet: Routers usually generate ICMP messages as they receive and route the IP packet. The main use of ICMP is to provide a reporting function that identifies error conditions on network devices. IP htadtr - ' rat ti h ts er Echo Revest datagram Remember that this ICMP message is imbedded within the data portion of an IP packet, which is in turn encapsulated within another protocol, such as Ethernet. The formats of echo request, echo reply, and destination unreachable messages are shown in Figure 1-10. Because the most critical information of a higher-level protocol using IP is carried within these first 8 bytes, this helps with detecting what caused the error. All ICMP messages that report errors also carry the header and the first 8 bytes of the datagram that caused the error. However, the ICMP checksum only pertains to the ICMP portion of the IP message, not to the whole IP packet. The ICMP checksum is a 2-byte field that uses the same algorithm as the IP checksum field. Code is a single byte field whose purpose is to explain the type field further. These fields are type, code, and checksum.
RFCs 7 define ICMP.Įven though ICMP message formats vary based on which service is requested, all ICMP messages have the first three fields in common. However, ICMP is an integral part of the IP protocol suite that still is encapsulated within the data portion of an IP packet. Usually, only higher-level protocols are encapsulated within another protocol. Using a connectionless, unreliable transfer mechanism, ICMP is used to report errors within a network. ICMP messages are encapsulated within IP packets.
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