Re: Strange Network Problem (TTL?)

[Russell Coker <russell@coker.com.au>]

On Fri, 10 Mar 2000, Chris Wagner wrote:
>At 11:41 PM 3/9/00 +0100, Paul van Empelen wrote:
>>Time to live has nothing to do with time, although the name suggests it.
>>It is a hop count. You start at e.g. 255. On the next hop the IP packet
>>will have a ttl of 254 and so on.
>
>So it's really a max hops limit.  How did it get a name like TTL??  What
>function does it serve?  Besides providing a mechanism to expire lost
>packets...  What role does each host's TTL setting play in a ping or trace?

If you have a packet in your network buffers for 1 second you should
decrement the TTL.  That way a packet can survive on the net for a maximum of
255 seconds.  This is why there are ~4 minute timeouts on some parts of TCP.


Here is the relevant section from rfc791.  If you have installed the doc-rfc
package you'll have this on your system.

   Time to Live:  8 bits
 
    This field indicates the maximum time the datagram is allowed to
    remain in the internet system.  If this field contains the value
    zero, then the datagram must be destroyed.  This field is modified
    in internet header processing.  The time is measured in units of
    seconds, but since every module that processes a datagram must
    decrease the TTL by at least one even if it process the datagram in
    less than a second, the TTL must be thought of only as an upper
    bound on the time a datagram may exist.  The intention is to cause
    undeliverable datagrams to be discarded, and to bound the maximum
    datagram lifetime.


As for the so-called "open DNS".  What is the point of blocking TCP port 53? 
If information is private then don't publish it in the DNS!!!

-- 
My current location - X marks the spot.
X
X
X