Multi-homing APNs in GPRS/UMTS
Paul Vixie
vixie at sa.vix.com
Wed Jun 9 20:42:25 UTC 2004
phn at icke-reklam.ipsec.nu writes:
> > If APNs are mulithomed then some sort of redundancy or load balancing
> > can be implemented. Should the TTL for the APN be set to 0?
>
> DNS TTL should never be zero. ( neither should TTL in a IP-header)
In [RFC1034 3.6] we see the following text:
The meaning of the TTL field is a time limit on how long an RR can
be kept in a cache. This limit does not apply to authoritative
data in zones; it is also timed out, but by the refreshing policies
for the zone. The TTL is assigned by the administrator for the
zone where the data originates. While short TTLs can be used to
minimize caching, and a zero TTL prohibits caching, the realities
of Internet performance suggest that these times should be on the
order of days for the typical host. If a change can be
anticipated, the TTL can be reduced prior to the change to minimize
inconsistency during the change, and then increased back to its
former value following the change.
In [ibid 5.3.3]:
The ideal answer is one from a server authoritative for the query
which either gives the required data or a name error. The data is
passed back to the user and entered in the cache for future use if
its TTL is greater than zero.
In [RFC1035 3.2.1] we see:
TTL a 32 bit signed integer that specifies the time interval
that the resource record may be cached before the source
of the information should again be consulted. Zero
values are interpreted to mean that the RR can only be
used for the transaction in progress, and should not be
cached. For example, SOA records are always distributed
with a zero TTL to prohibit caching. Zero values can
also be used for extremely volatile data.
In [ibid 7.1]:
Note that using the timestamp is superior to using a current
time, since it allows RRs with TTLs of zero to be entered in
the cache in the usual manner, but still used by the current
request, even after intervals of many seconds due to system
load, query retransmission timeouts, etc.
I believe that a DNS TTL of zero is well specified, though ill-advised.
--
Paul Vixie
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