8. TCP Message Length Field

DNS clients and servers SHOULD pass the two-octet length field, and the message described by that length field, to the TCP layer at the same time (e.g., in a single "write" system call) to make it more likely that all the data will be transmitted in a single TCP segment. This is for reasons of both efficiency and to avoid problems due to some DNS server implementations behaving undesirably when reading data from the TCP layer (due to a lack of clarity in previous documents). For example, some DNS server implementations might abort a TCP session if the first "read" from the TCP layer does not contain both the length field and the entire message.

To clarify, DNS servers MUST NOT close a connection simply because the first "read" from the TCP layer does not contain the entire DNS message, and servers SHOULD apply the connection timeouts as specified in Section 6.2.3.