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Practical TCP/IP and Ethernet Networking- P34: 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 | Application layer protocols 147 Figure 8.6 DNS name resolution DNS frame format The message format for DNS messages is as follows. 0 1S 16 17 20 21 27 28 31 Bits ID QR OPCODE Flags RCODE QDCOUNT ANCOUNT NSCOUNT ARCOUNT Question Section Answa Section Aithority Section Additional Information Section Figure 8.7 DNS message format ID IDENTIFICATION a tracking number 16 bits used to correlate queries and responses QR a one-bit flag that identifies the message as a query QR 0 or a response QR 1 OPCODE. This 4-bit field further defines a query as follows 0 Standard query 1 Inverse query 2 Server status request The other opcodes 3-15 are not used Flags used to describe the message further. They are from right to left Authoritative answer AA Truncation TC Recursion desired RD Recursion available RA 148 Practical TCP IP and Ethernet Networking RCODE the last field in the first long-word is used for response codes with the following meanings 0 No error 1 Format error 2 Server error 3 Name error 4 Not used 5 Refused Four COUNT fields indicate the length of the fields to follow QDCOUNT gives the number of question entries ANCOUNT gives the number of resource records in the answer section NSCOUNT refers to the number of name server resource records in the Authority section ARCOUNT refers to the number of resource records in the additional records section Question section Contains queries in the format shown below. A query consists of a query domain name field containing the FQDN about which information is required a query type field specifying the type of information required and a query class field identifying the protocol suite with which the name is associated Answer section Contains information returned in response to a query in the format shown below. The resource domain name type and class fields are from the original query. The time to live field specifies how long this information can be used if it is cached at the local host. The format of the resource data field .
