[FUGSPBR] MTU Discovery

[Jean M. Duarte]

        Ola.

Olha ainda com problemas de MTU com a Braslink.
Ja olhei e li tudo.

Pelo que entendi o problema pode ate estar no meio do Caminho.
Pois agora eles estao dizendo que o pacote sai de de la com MTU 1500.

Agora dei um TCP dump com a opcao -v vejam o
resultado:

O Porque desse bad tcp cksum ?

O mss(MTU) eh 1460, Claro MTU(ethernet 1500) - 40 !

Vejam tambem que os pacotes deles vem com a opcao DF (Dont Fragment),
entao o problema nao deveria ser fragmentos, nao eh isso.

Vi nesse tutorial, http://www.daemonnews.org/200101/pppoe.html#pmtu
Que existe um Daemon, o tcpmssd , que iria reescrever os pacotes consertando
a MTU, mas acho que isso na verdade eh uma Gambiarra !

Aproveitando, agradeco muito ao Ricardo pela atencao.

Jean Duarte


13:36.086315 10.10.10.10.2615 > 20.20.20.20.80: S [bad tcp cksum cec3!]
354736754:354736754(0) win 32768 <mss 1460> (DF) (ttl 64, id 40764, len 44,
bad cksum 0!)
13:43:39.084585 10.10.10.10.2615 > 20.20.20.20.80: S [bad tcp cksum cec3!]
354736754:354736754(0) win 32768 <mss 1460> (DF) (ttl 64, id 41259, len 44,
bad cksum 0!)
13:43:40.006258 20.20.20.20.80 > 10.10.10.10.2615: S [tcp sum ok]
1264497124:1264497124(0) ack 354736755 win 5840 <mss 1460> (DF) (ttl 43, id
0, len 44)
13:43:40.006706 10.10.10.10.2615 > 20.20.20.20.80: . [bad tcp cksum 6bd9!]
ack 1 win 34000 (DF) (ttl 64, id 41441, len 40, bad cksum 0!)
13:43:40.007451 10.10.10.10.2615 > 20.20.20.20.80: P 1:639(638) ack 1 win
34000 (DF) (ttl 64, id 41442, len 678, bad cksum 0!)
13:43:40.007869 10.10.10.10.2615 > 20.20.20.20.80: P 639:758(119) ack 1 win
34000 (DF) (ttl 64, id 41443, len 159, bad cksum 0!)
13:43:40.569754 20.20.20.20.80 > 10.10.10.10.2615: . [tcp sum ok] ack 758
win 7018 (DF) (ttl 43, id 21891, len 40)
13:43:40.646103 20.20.20.20.80 > 10.10.10.10.2615: P 1:415(414) ack 758 win
7018 (DF) (ttl 43, id 21892, len 454)
13:43:40.646109 20.20.20.20.80 > 10.10.10.10.2615: F [tcp sum ok] 415:415(0)
ack 758 win 7018 (DF) (ttl 43, id 21893, len 40)
13:43:40.647650 10.10.10.10.2615 > 20.20.20.20.80: . [bad tcp cksum d7d4!]
ack 416 win 34000 (DF) (ttl 64, id 41523, len 40, bad cksum 0!)
13:43:40.648323 10.10.10.10.2615 > 20.20.20.20.80: F [bad tcp cksum d6d4!]
758:758(0) ack 416 win 34000 (DF) (ttl 64, id 41525, len 40, bad cksum 0!)
13:43:40.695148 10.10.10.10.2645 > 20.20.20.20.80: S [bad tcp cksum fba9!]
529328575:529328575(0) win 32768 <mss 1460> (DF) (ttl 64, id 41534, len 44,
bad cksum 0!)
13:43:41.115938 20.20.20.20.80 > 10.10.10.10.2615: . [tcp sum ok] ack 759
win 7018 (DF) (ttl 43, id 21894, len 40)
13:43:41.115944 20.20.20.20.80 > 10.10.10.10.2645: S [tcp sum ok]
1278597907:1278597907(0) ack 529328576 win 5840 <mss 1460> (DF) (ttl 43, id
0, len 44)
13:43:41.117137 10.10.10.10.2645 > 20.20.20.20.80: . [bad tcp cksum 9295!]
ack 1 win 34000 (DF) (ttl 64, id 41594, len 40, bad cksum 0!)
13:43:41.117815 10.10.10.10.2645 > 20.20.20.20.80: P 1:627(626) ack 1 win
34000 (DF) (ttl 64, id 41595, len 666, bad cksum 0!)
13:43:41.475926 20.20.20.20.80 > 10.10.10.10.2645: . [tcp sum ok] ack 627
win 6886 (DF) (ttl 43, id 16335, len 40)

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