Practical TCP/IP and Ethernet Networking- P27: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | 112 Practical TCP IP and Ethernet Networking The values relating the code values in the above unreachable message are as follows. CODE DESCRIPTION 0 Network unreachable 1 Host unreachable 2 Protocol unreachable 3 Port unreachable 4 Fragmentation needed and DF set 5 Source route failed 6 Destination network unknown 7 Destination node unknown 8 Source host isolated 9 Communication with destination network prohibited 10 Communication with destination node prohibited 11 Network unreachable for type of service 12 Host unreachable for type of service Figure Typical code messages ICMP query messages In addition to the reports on errors and exceptional conditions there is a set of ICMP messages to request information and to reply to such request. Echo request and reply An echo request message is sent to the destination node. This message essentially enquires Are you alive A reply indicates that the pathway . the network s in between the gateways routers and the destination node are all operating correctly. The structure of the request and reply are indicated below. Figure ICMP echo request and reply The first three fields have already been discussed. The additional fields are Type 8 for an echo request and 0 for a reply Identifier A 16-bit random number used to match a reply message with its associated request message Sequence number Used to identify each individual request or reply in a sequence of associated requests or replies with the same source and destination Internet layer protocols 113 Data Generated by the sender and echoed back by the echoer. This field is variable in length its length and contents are set by the echo request sender. It usually consists of the ASCII characters a b c d etc Time-stamp request and replies This can be used to estimate to synchronize the clock of a host with that of a timeserver. 0 4 8 16 24 31 Type 13or14 Code 0 Checksum Identifier Sequence Number Originate Timestamp Receive Timestamp Transit Timestamp Figure
