cyclic ip forwarding latency / delay on 2.2.6 ac1 (and previous)

Gabriel Montenegro (gab@eng.sun.com)
Fri, 23 Apr 1999 09:48:55 -0700 (PDT)


that pretty much says it all.

I set up a 2.2.6 ac1 redhat 5.2 box as an ip forwarder (no masquerading
or firewalling going on, and turning those on has no effect at all).

The forwarder has two 10BaseT cards in pcmcia slots 0 and 1
and has ip_forwarding turned on.

my setup is:

solaris box <===> linux forwarder <====> linux box

both linux boxes are 2.2.6 ac1 redhat 5.2.

pinging every second shows that the round trip time as seen
by either the solaris or the linux box cycles from <2 ms to
>21ms .

ping shows that with every ping (every second) the round
trip time increases by 0.1 ms

thus the cycle looks like this:

1.8 ms
1.9 ms
...
21.6 ms
and then it starts again:
1.8 ms
...

there will be some slight variations in the count, but the fact is that
something in the linux forwarder is producing this weird cycle.

any ideas?

tnx,

-gabriel

ps - i include sample output from the ping command below

Gabriel Montenegro (Networking and Security Center)
Sun Microsystems Laboratories
Sun Microsystems Inc. Email: gab@sun.com
901 San Antonio Road, MS UMPK15-214 Tel: (650)786-6288
Palo Alto, California 94303 Fax: (650)786-6445

....
64 bytes from 129.146.122.121: icmp_seq=2498 ttl=254 time=19.8 ms
64 bytes from 129.146.122.121: icmp_seq=2499 ttl=254 time=20.0 ms
64 bytes from 129.146.122.121: icmp_seq=2500 ttl=254 time=20.1 ms
64 bytes from 129.146.122.121: icmp_seq=2501 ttl=254 time=20.2 ms
64 bytes from 129.146.122.121: icmp_seq=2502 ttl=254 time=20.3 ms
64 bytes from 129.146.122.121: icmp_seq=2503 ttl=254 time=20.4 ms
64 bytes from 129.146.122.121: icmp_seq=2504 ttl=254 time=20.5 ms
64 bytes from 129.146.122.121: icmp_seq=2505 ttl=254 time=20.6 ms
64 bytes from 129.146.122.121: icmp_seq=2506 ttl=254 time=20.8 ms
64 bytes from 129.146.122.121: icmp_seq=2507 ttl=254 time=20.8 ms
64 bytes from 129.146.122.121: icmp_seq=2508 ttl=254 time=20.9 ms
64 bytes from 129.146.122.121: icmp_seq=2509 ttl=254 time=21.0 ms
64 bytes from 129.146.122.121: icmp_seq=2510 ttl=254 time=21.4 ms
64 bytes from 129.146.122.121: icmp_seq=2511 ttl=254 time=21.3 ms
64 bytes from 129.146.122.121: icmp_seq=2512 ttl=254 time=21.4 ms
64 bytes from 129.146.122.121: icmp_seq=2513 ttl=254 time=21.5 ms
64 bytes from 129.146.122.121: icmp_seq=2514 ttl=254 time=21.6 ms
64 bytes from 129.146.122.121: icmp_seq=2515 ttl=254 time=21.7 ms
64 bytes from 129.146.122.121: icmp_seq=2516 ttl=254 time=1.8 ms
64 bytes from 129.146.122.121: icmp_seq=2517 ttl=254 time=1.9 ms
64 bytes from 129.146.122.121: icmp_seq=2518 ttl=254 time=2.0 ms
64 bytes from 129.146.122.121: icmp_seq=2519 ttl=254 time=2.1 ms
64 bytes from 129.146.122.121: icmp_seq=2520 ttl=254 time=2.2 ms
64 bytes from 129.146.122.121: icmp_seq=2521 ttl=254 time=2.3 ms
64 bytes from 129.146.122.121: icmp_seq=2522 ttl=254 time=2.4 ms
64 bytes from 129.146.122.121: icmp_seq=2523 ttl=254 time=2.5 ms
64 bytes from 129.146.122.121: icmp_seq=2524 ttl=254 time=2.6 ms
64 bytes from 129.146.122.121: icmp_seq=2525 ttl=254 time=2.7 ms
64 bytes from 129.146.122.121: icmp_seq=2526 ttl=254 time=2.8 ms
64 bytes from 129.146.122.121: icmp_seq=2527 ttl=254 time=3.0 ms
64 bytes from 129.146.122.121: icmp_seq=2528 ttl=254 time=3.1 ms
64 bytes from 129.146.122.121: icmp_seq=2529 ttl=254 time=3.2 ms
64 bytes from 129.146.122.121: icmp_seq=2530 ttl=254 time=3.2 ms
....

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