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55 % 9: < ) 7 F % %,' $' /!.F NI2L'.F NI21 ( Transport The DNS assumes that messages will be transmitted as datagrams R $! or in a byte stream carried by a virtual circuit R *$!. While virtual circuits can be used for any DNS activity, datagrams are preferred for queries due to their lower overhead and better performance. Zone refresh activities must use virtual circuits because of the need for reliable transfer.

56 UDP usage Messages sent using UDP user server port 53 (decimal). Messages carried by UDP are restricted to 512 bytes (not counting the IP or UDP headers). Longer messages are truncated and the TC bit is set in the header. UDP is not acceptable for zone transfers, but is the recommended method for standard queries in the Internet. Queries sent using UDP may be lost, and hence a retransmission strategy is required. Queries or their responses may be reordered by the network, or by processing in name servers, so resolvers should not depend on them being returned in order TCP usage Messages sent over TCP connections use server port 53 (decimal). The message is prefixed with a two byte length field which gives the message length, excluding the two byte length field. This length field allows the low-level processing to assemble a complete message before beginning to parse it. Several connection management policies are recommended: The server should not block other activities waiting for TCP data. The server should support multiple connections. The server should assume that the client will initiate connection closing, and should delay closing its end of the connection until all outstanding client requests have been satisfied....

57 8!!...The top level format of message is divided into 5 sections (some of which are empty in certain cases) shown below: Header Question the question for the name server Answer RRs answering the question Authority RRs pointing toward an authority Additional RRs holding additional information The header section is always present. The header includes fields that specify which of the remaining sections are present, and also specify whether the message is a query or a response, a standard query or some other opcode, etc.... 8!! The query would look like: The response from C.ISI.EDU would be: The header of the response looks like the header of the query, except that the RESPONSE bit is set... The question section of the response matches the question section of the query.

58 8!! Header section format The header contains the following fields: where:... OPCODE... QDCOUNT... A four bit field that specifies kind of query in this message. This value is set by the originator of a query and copied into the response. The values are: an unsigned 16 bit integer specifying the number of entries in the question section.

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