TTLs


stan@h1.gcy.nytel.com
Tue 21 Jun 88 07:26:14


There has recently been considerable discussion on this list about the
increasing loss of network connectivity. I have not seen any discussion
about TTL problems, however many of the problems that I have had to shoot
with regard to loss of connectivity have been due to inappropriate TTL
values. The increase in the size of the regionals and of the Internet as
a whole has not been accounted for and TTL values are therefore set too low.
This results in loss of connectivity, the cause of which is not readily
apparent to the user.

The matter is often further complicated by different TTL values for ping,
ftp, tftp and telnet. Generally I have found that the highest TTL values
are associated with the ping servers, so that when the user pings the host
the network appears to be OK, but because ftp or telnet have a lower TTL,
it is assumed that there is a problem with those servers and that they are
not responding. The fact is that their TTL has gone to 0 somewhere in the
network. There appears to be no consistency across vendor software
implemenations, as I have seen TTL's for ftp range from 24 to 255.

In my opinion, TTL for all services should be set to 255 as the network is
not currently plagued with major looping problems and it seems to be more
important to improve connectivity than to prevent the looping of a packet
at 32 or 64 hops. Comments? Opinions? Similar experiences? Other
solutions?

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