Date: Wed, 16 Jan 2002 23:00:53 -0700 From: Chris Fedde <chris@fedde.littleton.co.us> To: Evren Yurtesen <eyurtese@turkuamk.fi> Cc: freebsd-questions@FreeBSD.ORG Subject: Re: why TTL set to 64? Message-ID: <200201170600.g0H60r888277@fedde.littleton.co.us> In-Reply-To: <Pine.A41.4.10.10201170737460.144970-100000@bessel.tekniikka.turkuamk.fi>
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On Thu, 17 Jan 2002 07:39:40 +0200 (WET) Evren Yurtesen wrote:
+------------------
| I wonder why freebsd started to set the TTL to 64.
| Is there any reasons? Cisco uses 255 for example
| Doesnt this mean if a place is on a little slow network and many hops far
| away then the packets will be discarded by the routers if TTL reaches 0
| before the packet reaches destination?
+------------------
As you probably know ttl is just a hop count that gets decremented by each
router in the path. One of the more common kinds of routing failure
manifests itself as a route loop. That is to say that packets get bounced back
and forth between two or more routers and never make it to their
destination before the ttl expires.
If all the packets on the internet had ttl of 255 then when they
got stuck in these loops they would increase the load on those
routers and consuming forwarding resources and the bandwidth on
links between them. Thus making it harder for the backbone admins
to solve the problem.
On the other hand if the number is too small then the pakets die before
they reach their destination.
At one time 32 was a big enough ttl. But for a while right after
NSFnet stopped carrying most of the internet traffic many routes
accros the internet exceeded that. Now I rarely see routes longer than 24
hops but there are occasional routes that are much longer.
A ttl of 64 seems like a good compromise between too big and too small.
--
Chris Fedde
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