On Tue, Oct 22, 2002 at 01:18:50PM -0700, Ted Deppner wrote: > On Tue, Oct 22, 2002 at 12:22:56PM -0700, Richard Crawford wrote: > > >From dist2-vlan10.scrm01.pbi.net (64.171.152.3): Time to live exceeded > > >From dist2-vlan10.scrm01.pbi.net (64.171.152.3): Time to live exceeded > > >From dist2-vlan10.scrm01.pbi.net (64.171.152.3): Time to live exceeded > > > > So what does "Time to live exceeded" mean? > > packets have a TTL value assigned to them by the originating computer (ie > where you did the ping from). Different machines have different ttl > values, most commonly being 64 but can be as high as 127 or 255 I believe. > Each router that forwards the packet decrements the TTL by 1, and when it > reaches zero, that router returns an ICMP packet with a TTL exceeded > error.
(Since everyone else has provided the answers to this, I'll just provide some links and a comment...) Yep. TTL is part of IP/Layer3 and is helpful with routing of packets. Generally, the host that drops the packet after 0 is reaches sends the ICMP response on TTL excession. Since it is an 8 bit field, the max value represented (unsigned) is 255. More info on IP fields and brief descriptions of what they mean/do: http://mike.passwall.com/networking/ippacket.html#TTL Some brief coverage of TCP, UDP and ICMP too if you like this kinda stuff: http://mike.passwall.com/networking/tcppacket.html http://mike.passwall.com/networking/udppacket.html http://mike.passwall.com/networking/icmppacket.html > This specific error usually means a link or router is down, and your > packet is being ping-ponged back and forth between the two routers just > before that downed link or downed router. This can most easily be seen by > a traceroute, where the packet hits rtr A, then rtr B, then A, then B, > etc. > > This problem of ping-pong is *always* a misconfiguration on the farthest > out reachable router, regardless of why the link or other router is down. > It indicates sloppiness on the part of the person configuring that > farthest out reachable router... in other words, he/she isn't a good > network admin guy. > > That said, the correct behavior is an ICMP "destination unreachable", > which is far more informative. > > To illustrate: > > You -> routerA -> routerB -> routerC -> Destination > this link goes down ^ > > routerB looses the knowledge that the Destination is reachable through > routerC, but doesn't tell routerA that it can no longer reach Destination. > routerB's default behavior takes over, which is to send traffic to > routerA. > > routerA, still believing routerB can reach Destination sends the packet to > routerB. routerB, not knowing better, sends the traffic to routerA. > rinse and repeat. > > -- > Ted Deppner > http://www.psyber.com/~ted/ > _______________________________________________ > vox-tech mailing list > [EMAIL PROTECTED] > http://lists.lugod.org/mailman/listinfo/vox-tech -- -----BEGIN GEEK CODE BLOCK----- Version: 3.12 GCS/CM$/IT$/LS$/S/O$ !d--(++) !s !a+++(-----) C++$(++++) U++++$(+$) P+$>+++ L+++$(++) E W+++$(+) N+ o K w+$>++>+++ O-@ M+$ V-$>- !PS !PE Y+ PGP++ t@-(++) 5+@ X@ R- tv- b++ DI+++ D+ G--@ e+>++>++++ h(++)>+ r*>? z? ------END GEEK CODE BLOCK------ decode: http://www.ebb.org/ungeek/ about: http://www.geekcode.com/geek.html Campus IT(/OS Security): Operating Systems Support Specialist Assistant _______________________________________________ vox-tech mailing list [EMAIL PROTECTED] http://lists.lugod.org/mailman/listinfo/vox-tech
