TCP/IP Tutorial and Technical Overview - IBM Redbooks

12.1.13 Transport
Domain Name System messages are transmitted either as datagrams (UDP) or
through stream connection (TCP):
► UDP usage: Server port 53 (decimal).
Messages carried by UDP are restricted to 512 bytes. Longer messages are
truncated and the truncation (TC) bit is set in the header. Because UDP
frames can be lost, a retransmission strategy is required.
► TCP usage: Server port 53 (decimal).
In this case, the message is preceded by a 2-byte field indicating the total
message frame length.
► STD 3 – Host Requirements requires that:
– A Domain Name System resolver or server that is sending a
non-zone-transfer query must send a UDP query first. If the answer
section of the response is truncated and if the requester supports TCP, it
tries the query again using TCP. UDP is preferred over TCP for queries
because UDP queries have much lower overall processing cost, and the
use of UDP is essential for a heavily loaded server. Truncation of
messages is rarely a problem given the current contents of the Domain
Name System database, because typically 15 response records can be
accommodated in the datagram, but this might change as new record
types continue to be added to the Domain Name System.
– TCP must be used for zone transfer activities because the 512-byte limit
for a UDP datagram will always be inadequate for a zone transfer.
– Name servers must support both types of transport.
As IPv6 becomes more pervasive throughout the Internet community, some
problems are forecasted as a result of mixed IPv4/IPv6 network segments.
Primarily, if a resolver that can only use IPv4 is forwarded to across a network
segment that supports only IPv6, the resolver and the name server will be unable
to communicate. As a result, the hierarchical namespace becomes fragmented
into two sets of segments: those that support IPv4 and IPv6, and those that
support only IPv6. This impending issue has been named the Problem of Name
Space Fragmentation, and documented in RFC 3901. In order to preserve
namespace continuity, RFC 3901 recommends the following:
► Every recursive name server should be either IPv4 only, or dual IPv4 and
IPv6.
► Every DNS zone should be served by at least one IPv4-reachable
authoritative name server.
Additional suggestions about configuring IPv6 DNS servers are in RFC 4339.
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