Commit 8ddd5c2f authored by Michał Kępień's avatar Michał Kępień

Update "edns-udp-size" documentation in the ARM

Update the description of the process for determining the advertised UDP
buffer size in outgoing queries so that it matches the code.
parent fb123df2
......@@ -3401,16 +3401,23 @@ Tuning
buffer size of 512, as this has the greatest chance of success on the
first try.
If the initial query is successful with EDNS advertising a buffer size of
512, then ``named`` will advertise progressively larger buffer sizes on
successive queries, until responses begin timing out or ``edns-udp-size`` is
reached.
The default buffer sizes used by ``named`` are 512, 1232, 1432, and
4096, but never exceeding ``edns-udp-size``. (The values 1232 and
1432 are chosen to allow for an IPv4/IPv6 encapsulated UDP message to
be sent without fragmentation at the minimum MTU sizes for Ethernet
and IPv6 networks.)
If the initial query is successful with EDNS advertising a buffer
size of 512, then ``named`` will switch to advertising a buffer size
of 4096 bytes (unless ``edns-udp-size`` is lower, in which case the
latter will be used).
Query timeouts observed for any given server will affect the buffer
size advertised in queries sent to that server. Depending on
observed packet dropping patterns, the advertised buffer size will be
lowered to 1432 bytes, 1232 bytes, 512 bytes, or the size of the
largest UDP response ever received from a given server, and then
clamped to the ``<512, edns-udp-size>`` range. Per-server EDNS
statistics are only retained in memory for the lifetime of a given
server's ADB entry.
(The values 1232 and 1432 are chosen to allow for an IPv4/IPv6
encapsulated UDP message to be sent without fragmentation at the
minimum MTU sizes for Ethernet and IPv6 networks.)
Any server-specific ``edns-udp-size`` setting has precedence over all
the above rules.
......
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