TCP
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... with the Truncation Bit set; in most cases the requester will then
retry using TCP. Furthermore, server delivery of truncated responses
varies widely and resolver handling of these responses also varies,
...
... transaction like DNS: a TCP connection requires 5 packets for setup
and tear down, excluding data packets, thus requiring at least 3
...
... DNS servers answer thousands of queries per
second, requiring them to use TCP will cause significant overhead and
delays.
...
... will not fit in a single 512-octet DNS message, resulting in a large
number of TCP query connections to the root servers ...
... fragments renders the
answer useless and the query must be retried. A TCP connection
requires a larger number of round trips for establishment, data
transfer ...
... standpoint. The open issue is the effect of losses on fragmented
messages. If connection has high loss ratio only TCP will allow
reliable transfer of DNS data, most links ...
... UDP packet in one round trip is better than
establishing a TCP connection to transfer a few thousand octets.
...