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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