Frag on xdsl router with PPPoA for a 1500 Bytes Paquet

osgpcqosg · ‎02-08-2011

Hi,

In a lnux box: ping -s 1472 -M do 8x.xx.xx.xx
PING 8x.xx.xx.xx (8x.xx.xx.xx) 1472(1500) bytes of data.
1480 bytes from 8x.xx.xx.xx: icmp_seq=1 ttl=61 time=73.9 ms
1480 bytes from 8x.xx.xx.xx: icmp_seq=2 ttl=61 time=72.5 ms

A tcpdump el linux: We could see two paquet for the answer !

16:10:03.172467 00:22:64:70:24:b0 > 08:1f:f3:e7:0e:65, ethertype IPv4 (0x0800), length 1514: 192.168.200.5 > 8x.xx.xx.xx: ICMP echo request, id 56898, seq 1, length 1480

16:10:03.215593 08:1f:f3:e7:0e:65 > 00:22:64:70:24:b0, ethertype IPv4 (0x0800), length 74: 8x.xx.xx.xx > 192.168.200.5: ICMP echo reply, id 56898, seq 1, length 40

16:10:03.245532 08:1f:f3:e7:0e:65 > 00:22:64:70:24:b0, ethertype IPv4 (0x0800), length 1474: 8x.xx.xx.xx > 192.168.200.5: icmp

16:10:04.174013 00:22:64:70:24:b0 > 08:1f:f3:e7:0e:65, ethertype IPv4 (0x0800), length 1514: 192.168.200.5 > 8x.xx.xx.xx: ICMP echo request, id 56898, seq 2, length 1480

16:10:04.216976 08:1f:f3:e7:0e:65 > 00:22:64:70:24:b0, ethertype IPv4 (0x0800), length 74: 8x.xx.xx.xx > 192.168.200.5: ICMP echo reply, id 56898, seq 2, length 40

16:10:04.246718 08:1f:f3:e7:0e:65 > 00:22:64:70:24:b0, ethertype IPv4 (0x0800), length 1474: 8x.xx.xx.xx > 192.168.200.5: icmp

At the cisco:

001321: *Jan 19 08:34:34.707 UTC: NAT-FRAG: tcpmss value
001322: *Jan 19 08:34:40.515 UTC: NAT*: s=xx.xx.1.202, d=xx.xx.1.252->192.168.200.5 [44501]
001323: *Jan 19 08:34:40.523 UTC: NAT*: s=192.168.200.5->xx.xx.1.252, d=xx.xx.1.202 [30600]
001324: *Jan 19 08:34:40.563 UTC: NAT*: s=xx.xx.1.202, d=xx.xx.1.252->192.168.200.5 [923]
001325: *Jan 19 08:34:41.031 UTC: NAT*: s=xx.xx.1.202, d=xx.xx.1.252->192.168.200.5 [37024]
001326: *Jan 19 08:34:41.039 UTC: NAT*: s=192.168.200.5->xx.xx.1.252, d=xx.xx.1.202 [30601]
001327: *Jan 19 08:34:41.043 UTC: NAT*: s=192.168.200.5->xx.xx.1.252, d=xx.xx.0.90 [0]
001328: *Jan 19 08:34:41.043 UTC: NAT*: s=192.168.200.5->xx.xx.1.252, d=xx.xx.1.202 [30602]
001329: *Jan 19 08:34:41.079 UTC: NAT*: s=xx.xx.1.202, d=xx.xx.1.252->192.168.200.5 [13791]
001330: *Jan 19 08:34:41.083 UTC: NAT*: s=xx.xx.0.90, d=xx.xx.1.252->192.168.200.5 [33593]
001331: *Jan 19 08:34:41.111 UTC: NAT*: fo 5, looking for fragment xx.xx.0.90 xx.xx.1.252 33593 1
001332: *Jan 19 08:34:41.111 UTC: NAT*: s=xx.xx.0.90, d=xx.xx.1.252->192.168.200.5 [33593]
001333: *Jan 19 08:34:41.115 UTC: NAT*: s=xx.xx.1.202, d=xx.xx.1.252->192.168.200.5 [17055]
001334: *Jan 19 08:34:41.115 UTC: NAT*: s=192.168.200.5->xx.xx.1.252, d=xx.xx.1.202 [30603]
001335: *Jan 19 08:34:41.155 UTC: NAT*: s=xx.xx.1.202, d=xx.xx.1.252->192.168.200.5 [13292]
001336: *Jan 19 08:34:42.043 UTC: NAT*: s=192.168.200.5->xx.xx.1.252, d=xx.xx.0.90 [0]
001337: *Jan 19 08:34:42.083 UTC: NAT*: s=xx.xx.0.90, d=xx.xx.1.252->192.168.200.5 [33594]
001338: *Jan 19 08:34:42.115 UTC: NAT*: fo 5, looking for fragment xx.xx.0.90 xx.xx.1.252 33594 1
001339: *Jan 19 08:34:42.115 UTC: NAT*: s=xx.xx.0.90, d=xx.xx.1.252->192.168.200.5 [33594]
001340: *Jan 19 08:34:42.119 UTC: NAT*: s=192.168.200.5->xx.xx.1.252, d=xx.xx.1.202 [30604]
001341: *Jan 19 08:34:42.163 UTC: NAT*: s=xx.xx.1.202, d=xx.xx.1.252->192.168.200.5 [47067]
001342: *Jan 19 08:34:43.644 UTC: NAT: expiring xx.xx.1.252 (192.168.200.5) icmp 61764 (61764)

The mtu is 1500, if I change the cisco per un zyxel all paquet are 1514 lenght without problem (In PPPoA of course).

Can't understand why I have the frag.

Any Ideas ?

Vince.

osgpcqosg · ‎03-28-2011

I find a problem similar with tunel VPN: https://supportforums.cisco.com/message/3324823

So I look more deeper the case, here more information about this problem:

ping -c 1 -s 1464 80.94.0.90

client

12:11:44.811496 IP 192.168.3.2 > xx.xx.xx.90: ICMP echo request, id 12826, seq 1, length 1472

12:11:44.888315 IP xx.xx.xx.90 > 192.168.3.2: ICMP echo reply, id 12826, seq 1, length 1472

servidor
- 12:11:44.642439 IP xx.xx.xx.136 > xx.xx.xx.90: ICMP echo request, id 12826, seq 1, length 1472
- ```
12:11:44.642506 IP xx.xx.xx.90 > xx.xx.xx.136: ICMP echo reply, id 12826, seq 1, length 1472
```

ping -c 1 -s 1465 80.94.0.90

client: Aquí no es normal....

12:11:47.163509 IP 192.168.3.2 > xx.xx.xx.90: ICMP echo request, id 13082, seq 1, length 1473

12:11:47.236410 IP xx.xx.xx.90 > 192.168.3.2: ICMP echo reply, id 13082, seq 1, length 40

12:11:47.238760 IP xx.xx.xx.90 > 192.168.3.2: icmp

Servidor

12:11:46.993358 IP xx.xx.xx.136 > xx.xx.xx.90: ICMP echo request, id 13082, seq 1, length 1473

12:11:46.993398 IP xx.xx.xx.90 > xx.xx.xx.136: ICMP echo reply, id 13082, seq 1, length 1473

IOS test for now with bug

   -857 Cisco IOS Software, C850 Software (C850-ADVSECURITYK9-M), Version 12.4(15)T14, RELEASE SOFTWARE (fc2)
   -857 Cisco IOS Software, C850 Software (C850-ADVSECURITYK9-M), Version 12.4(15)T15, RELEASE SOFTWARE (fc3)
   -877 Cisco IOS Software, C870 Software (C870-ADVIPSERVICESK9-M), Version 12.4(15)T13, RELEASE SOFTWARE (fc3)
   -878 Cisco ISO Software, C870 Software (C870-ADVSECURITYK9-M), Version 12.4(24)T4, RELEASE SO)
   -878 Cisco ISO Software, C870 Software (C870-ADVSECURITYK9-M), Version 12.4(24)T5, RELEASE SO)

osgpcqosg · ‎03-28-2011

And here the full tcpdump in the client part, the server one is normal: one packet in, one paquet out.

.request from linux box

11:56:44.115141 IP 192.168.3.2 > xx.xx.xx.90: ICMP echo request, id 41495, seq 1, length 1473
    0x0000: 4500 05d5 0000 4000 4001 20c6 c0a8 0302 E.....@.@.......
    0x0010: 505e 005a 0800 7c1c a217 0001 5c5b 904d P^.Z..|.....\[.M
    0x0020: a9c1 0100 0809 0a0b 0c0d 0e0f 1011 1213 ................
    0x0030: 1415 1617 1819 1a1b 1c1d 1e1f 2021 2223 .............!"#
    0x0040: 2425 2627 2829 2a2b 2c2d 2e2f 3031 3233 $%&'()*+,-./0123
    0x0050: 3435 3637 3839 3a3b 3c3d 3e3f 4041 4243 456789:;<=>?@ABC
    0x0060: 4445 4647 4849 4a4b 4c4d 4e4f 5051 5253 DEFGHIJKLMNOPQRS
    0x0070: 5455 5657 5859 5a5b 5c5d 5e5f 6061 6263 TUVWXYZ[\]^_`abc
    0x0080: 6465 6667 6869 6a6b 6c6d 6e6f 7071 7273 defghijklmnopqrs
    0x0090: 7475 7677 7879 7a7b 7c7d 7e7f 8081 8283 tuvwxyz{|}~.....
    0x00a0: 8485 8687 8889 8a8b 8c8d 8e8f 9091 9293 ................
    0x00b0: 9495 9697 9899 9a9b 9c9d 9e9f a0a1 a2a3 ................
    0x00c0: a4a5 a6a7 a8a9 aaab acad aeaf b0b1 b2b3 ................
    0x00d0: b4b5 b6b7 b8b9 babb bcbd bebf c0c1 c2c3 ................
    0x00e0: c4c5 c6c7 c8c9 cacb cccd cecf d0d1 d2d3 ................
    0x00f0: d4d5 d6d7 d8d9 dadb dcdd dedf e0e1 e2e3 ................
    0x0100: e4e5 e6e7 e8e9 eaeb eced eeef f0f1 f2f3 ................
    0x0110: f4f5 f6f7 f8f9 fafb fcfd feff 0001 0203 ................
    0x0120: 0405 0607 0809 0a0b 0c0d 0e0f 1011 1213 ................
    0x0130: 1415 1617 1819 1a1b 1c1d 1e1f 2021 2223 .............!"#
    0x0140: 2425 2627 2829 2a2b 2c2d 2e2f 3031 3233 $%&'()*+,-./0123
    0x0150: 3435 3637 3839 3a3b 3c3d 3e3f 4041 4243 456789:;<=>?@ABC
    0x0160: 4445 4647 4849 4a4b 4c4d 4e4f 5051 5253 DEFGHIJKLMNOPQRS
    0x0170: 5455 5657 5859 5a5b 5c5d 5e5f 6061 6263 TUVWXYZ[\]^_`abc
    0x0180: 6465 6667 6869 6a6b 6c6d 6e6f 7071 7273 defghijklmnopqrs
    0x0190: 7475 7677 7879 7a7b 7c7d 7e7f 8081 8283 tuvwxyz{|}~.....
    0x01a0: 8485 8687 8889 8a8b 8c8d 8e8f 9091 9293 ................
    0x01b0: 9495 9697 9899 9a9b 9c9d 9e9f a0a1 a2a3 ................
    0x01c0: a4a5 a6a7 a8a9 aaab acad aeaf b0b1 b2b3 ................
    0x01d0: b4b5 b6b7 b8b9 babb bcbd bebf c0c1 c2c3 ................
    0x01e0: c4c5 c6c7 c8c9 cacb cccd cecf d0d1 d2d3 ................
    0x01f0: d4d5 d6d7 d8d9 dadb dcdd dedf e0e1 e2e3 ................
    0x0200: e4e5 e6e7 e8e9 eaeb eced eeef f0f1 f2f3 ................
    0x0210: f4f5 f6f7 f8f9 fafb fcfd feff 0001 0203 ................
    0x0220: 0405 0607 0809 0a0b 0c0d 0e0f 1011 1213 ................
    0x0230: 1415 1617 1819 1a1b 1c1d 1e1f 2021 2223 .............!"#
    0x0240: 2425 2627 2829 2a2b 2c2d 2e2f 3031 3233 $%&'()*+,-./0123
    0x0250: 3435 3637 3839 3a3b 3c3d 3e3f 4041 4243 456789:;<=>?@ABC
    0x0260: 4445 4647 4849 4a4b 4c4d 4e4f 5051 5253 DEFGHIJKLMNOPQRS
    0x0270: 5455 5657 5859 5a5b 5c5d 5e5f 6061 6263 TUVWXYZ[\]^_`abc
    0x0280: 6465 6667 6869 6a6b 6c6d 6e6f 7071 7273 defghijklmnopqrs
    0x0290: 7475 7677 7879 7a7b 7c7d 7e7f 8081 8283 tuvwxyz{|}~.....
    0x02a0: 8485 8687 8889 8a8b 8c8d 8e8f 9091 9293 ................
    0x02b0: 9495 9697 9899 9a9b 9c9d 9e9f a0a1 a2a3 ................
    0x02c0: a4a5 a6a7 a8a9 aaab acad aeaf b0b1 b2b3 ................
    0x02d0: b4b5 b6b7 b8b9 babb bcbd bebf c0c1 c2c3 ................
    0x02e0: c4c5 c6c7 c8c9 cacb cccd cecf d0d1 d2d3 ................
    0x02f0: d4d5 d6d7 d8d9 dadb dcdd dedf e0e1 e2e3 ................
    0x0300: e4e5 e6e7 e8e9 eaeb eced eeef f0f1 f2f3 ................
    0x0310: f4f5 f6f7 f8f9 fafb fcfd feff 0001 0203 ................
    0x0320: 0405 0607 0809 0a0b 0c0d 0e0f 1011 1213 ................
    0x0330: 1415 1617 1819 1a1b 1c1d 1e1f 2021 2223 .............!"#
    0x0340: 2425 2627 2829 2a2b 2c2d 2e2f 3031 3233 $%&'()*+,-./0123
    0x0350: 3435 3637 3839 3a3b 3c3d 3e3f 4041 4243 456789:;<=>?@ABC
    0x0360: 4445 4647 4849 4a4b 4c4d 4e4f 5051 5253 DEFGHIJKLMNOPQRS
    0x0370: 5455 5657 5859 5a5b 5c5d 5e5f 6061 6263 TUVWXYZ[\]^_`abc
    0x0380: 6465 6667 6869 6a6b 6c6d 6e6f 7071 7273 defghijklmnopqrs
    0x0390: 7475 7677 7879 7a7b 7c7d 7e7f 8081 8283 tuvwxyz{|}~.....
    0x03a0: 8485 8687 8889 8a8b 8c8d 8e8f 9091 9293 ................
    0x03b0: 9495 9697 9899 9a9b 9c9d 9e9f a0a1 a2a3 ................
    0x03c0: a4a5 a6a7 a8a9 aaab acad aeaf b0b1 b2b3 ................
    0x03d0: b4b5 b6b7 b8b9 babb bcbd bebf c0c1 c2c3 ................
    0x03e0: c4c5 c6c7 c8c9 cacb cccd cecf d0d1 d2d3 ................
    0x03f0: d4d5 d6d7 d8d9 dadb dcdd dedf e0e1 e2e3 ................
    0x0400: e4e5 e6e7 e8e9 eaeb eced eeef f0f1 f2f3 ................
    0x0410: f4f5 f6f7 f8f9 fafb fcfd feff 0001 0203 ................
    0x0420: 0405 0607 0809 0a0b 0c0d 0e0f 1011 1213 ................
    0x0430: 1415 1617 1819 1a1b 1c1d 1e1f 2021 2223 .............!"#
    0x0440: 2425 2627 2829 2a2b 2c2d 2e2f 3031 3233 $%&'()*+,-./0123
    0x0450: 3435 3637 3839 3a3b 3c3d 3e3f 4041 4243 456789:;<=>?@ABC
    0x0460: 4445 4647 4849 4a4b 4c4d 4e4f 5051 5253 DEFGHIJKLMNOPQRS
    0x0470: 5455 5657 5859 5a5b 5c5d 5e5f 6061 6263 TUVWXYZ[\]^_`abc
    0x0480: 6465 6667 6869 6a6b 6c6d 6e6f 7071 7273 defghijklmnopqrs
    0x0490: 7475 7677 7879 7a7b 7c7d 7e7f 8081 8283 tuvwxyz{|}~.....
    0x04a0: 8485 8687 8889 8a8b 8c8d 8e8f 9091 9293 ................
    0x04b0: 9495 9697 9899 9a9b 9c9d 9e9f a0a1 a2a3 ................
    0x04c0: a4a5 a6a7 a8a9 aaab acad aeaf b0b1 b2b3 ................
    0x04d0: b4b5 b6b7 b8b9 babb bcbd bebf c0c1 c2c3 ................
    0x04e0: c4c5 c6c7 c8c9 cacb cccd cecf d0d1 d2d3 ................
    0x04f0: d4d5 d6d7 d8d9 dadb dcdd dedf e0e1 e2e3 ................
    0x0500: e4e5 e6e7 e8e9 eaeb eced eeef f0f1 f2f3 ................
    0x0510: f4f5 f6f7 f8f9 fafb fcfd feff 0001 0203 ................
    0x0520: 0405 0607 0809 0a0b 0c0d 0e0f 1011 1213 ................
    0x0530: 1415 1617 1819 1a1b 1c1d 1e1f 2021 2223 .............!"#
    0x0540: 2425 2627 2829 2a2b 2c2d 2e2f 3031 3233 $%&'()*+,-./0123
    0x0550: 3435 3637 3839 3a3b 3c3d 3e3f 4041 4243 456789:;<=>?@ABC
    0x0560: 4445 4647 4849 4a4b 4c4d 4e4f 5051 5253 DEFGHIJKLMNOPQRS
    0x0570: 5455 5657 5859 5a5b 5c5d 5e5f 6061 6263 TUVWXYZ[\]^_`abc
    0x0580: 6465 6667 6869 6a6b 6c6d 6e6f 7071 7273 defghijklmnopqrs
    0x0590: 7475 7677 7879 7a7b 7c7d 7e7f 8081 8283 tuvwxyz{|}~.....
    0x05a0: 8485 8687 8889 8a8b 8c8d 8e8f 9091 9293 ................
    0x05b0: 9495 9697 9899 9a9b 9c9d 9e9f a0a1 a2a3 ................
    0x05c0: a4a5 a6a7 a8a9 aaab acad aeaf b0b1 b2b3 ................
    0x05d0: b4b5 b6b7 b8                             .....

*Answer in the client side once again:

** first

11:56:44.190032 IP xx.xx.xx.90 > 192.168.3.2: ICMP echo reply, id 41495, seq 1, length 40 0x0000: 4500 003c c7a3 2000 3d01 81bb 505e 005a E..<....=...P^.Z 0x0010: c0a8 0302 0000 841c a217 0001 5c5b 904d ............\[.M 0x0020: a9c1 0100 0809 0a0b 0c0d 0e0f 1011 1213 ................ 0x0030: 1415 1617 1819 1a1b 1c1d 1e1f ............

** Segon

11:56:44.192402 IP xx.xx.xx90 > 192.168.3.2: icmp 0x0000: 4500 05ad c7a3 0005 3d01 9c45 505e 005a E.......=..EP^.Z 0x0010: c0a8 0302 2021 2223 2425 2627 2829 2a2b .....!"#$%&'()*+ 0x0020: 2c2d 2e2f 3031 3233 3435 3637 3839 3a3b ,-./0123456789:; 0x0030: 3c3d 3e3f 4041 4243 4445 4647 4849 4a4b <=>?@ABCDEFGHIJK 0x0040: 4c4d 4e4f 5051 5253 5455 5657 5859 5a5b LMNOPQRSTUVWXYZ[ 0x0050: 5c5d 5e5f 6061 6263 6465 6667 6869 6a6b \]^_`abcdefghijk 0x0060: 6c6d 6e6f 7071 7273 7475 7677 7879 7a7b lmnopqrstuvwxyz{ 0x0070: 7c7d 7e7f 8081 8283 8485 8687 8889 8a8b |}~............. 0x0080: 8c8d 8e8f 9091 9293 9495 9697 9899 9a9b ................ 0x0090: 9c9d 9e9f a0a1 a2a3 a4a5 a6a7 a8a9 aaab ................ 0x00a0: acad aeaf b0b1 b2b3 b4b5 b6b7 b8b9 babb ................ 0x00b0: bcbd bebf c0c1 c2c3 c4c5 c6c7 c8c9 cacb ................ 0x00c0: cccd cecf d0d1 d2d3 d4d5 d6d7 d8d9 dadb ................ 0x00d0: dcdd dedf e0e1 e2e3 e4e5 e6e7 e8e9 eaeb ................ 0x00e0: eced eeef f0f1 f2f3 f4f5 f6f7 f8f9 fafb ................ 0x00f0: fcfd feff 0001 0203 0405 0607 0809 0a0b ................ 0x0100: 0c0d 0e0f 1011 1213 1415 1617 1819 1a1b ................ 0x0110: 1c1d 1e1f 2021 2223 2425 2627 2829 2a2b .....!"#$%&'()*+ 0x0120: 2c2d 2e2f 3031 3233 3435 3637 3839 3a3b ,-./0123456789:; 0x0130: 3c3d 3e3f 4041 4243 4445 4647 4849 4a4b <=>?@ABCDEFGHIJK 0x0140: 4c4d 4e4f 5051 5253 5455 5657 5859 5a5b LMNOPQRSTUVWXYZ[ 0x0150: 5c5d 5e5f 6061 6263 6465 6667 6869 6a6b \]^_`abcdefghijk 0x0160: 6c6d 6e6f 7071 7273 7475 7677 7879 7a7b lmnopqrstuvwxyz{ 0x0170: 7c7d 7e7f 8081 8283 8485 8687 8889 8a8b |}~............. 0x0180: 8c8d 8e8f 9091 9293 9495 9697 9899 9a9b ................ 0x0190: 9c9d 9e9f a0a1 a2a3 a4a5 a6a7 a8a9 aaab ................ 0x01a0: acad aeaf b0b1 b2b3 b4b5 b6b7 b8b9 babb ................ 0x01b0: bcbd bebf c0c1 c2c3 c4c5 c6c7 c8c9 cacb ................ 0x01c0: cccd cecf d0d1 d2d3 d4d5 d6d7 d8d9 dadb ................ 0x01d0: dcdd dedf e0e1 e2e3 e4e5 e6e7 e8e9 eaeb ................ 0x01e0: eced eeef f0f1 f2f3 f4f5 f6f7 f8f9 fafb ................ 0x01f0: fcfd feff 0001 0203 0405 0607 0809 0a0b ................ 0x0200: 0c0d 0e0f 1011 1213 1415 1617 1819 1a1b ................ 0x0210: 1c1d 1e1f 2021 2223 2425 2627 2829 2a2b .....!"#$%&'()*+ 0x0220: 2c2d 2e2f 3031 3233 3435 3637 3839 3a3b ,-./0123456789:; 0x0230: 3c3d 3e3f 4041 4243 4445 4647 4849 4a4b <=>?@ABCDEFGHIJK 0x0240: 4c4d 4e4f 5051 5253 5455 5657 5859 5a5b LMNOPQRSTUVWXYZ[ 0x0250: 5c5d 5e5f 6061 6263 6465 6667 6869 6a6b \]^_`abcdefghijk 0x0260: 6c6d 6e6f 7071 7273 7475 7677 7879 7a7b lmnopqrstuvwxyz{ 0x0270: 7c7d 7e7f 8081 8283 8485 8687 8889 8a8b |}~............. 0x0280: 8c8d 8e8f 9091 9293 9495 9697 9899 9a9b ................ 0x0290: 9c9d 9e9f a0a1 a2a3 a4a5 a6a7 a8a9 aaab ................ 0x02a0: acad aeaf b0b1 b2b3 b4b5 b6b7 b8b9 babb ................ 0x02b0: bcbd bebf c0c1 c2c3 c4c5 c6c7 c8c9 cacb ................ 0x02c0: cccd cecf d0d1 d2d3 d4d5 d6d7 d8d9 dadb ................ 0x02d0: dcdd dedf e0e1 e2e3 e4e5 e6e7 e8e9 eaeb ................ 0x02e0: eced eeef f0f1 f2f3 f4f5 f6f7 f8f9 fafb ................ 0x02f0: fcfd feff 0001 0203 0405 0607 0809 0a0b ................ 0x0300: 0c0d 0e0f 1011 1213 1415 1617 1819 1a1b ................ 0x0310: 1c1d 1e1f 2021 2223 2425 2627 2829 2a2b .....!"#$%&'()*+ 0x0320: 2c2d 2e2f 3031 3233 3435 3637 3839 3a3b ,-./0123456789:; 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osgpcqosg · ‎04-20-2011

The "sh ip traffic" show we haven't any frag here.

So don't know if are frag paquet or only a "little useless 40 bytes packet" !

osgpcqosg · ‎08-09-2011

Some news,

I used the so usefull way to capture the traffic on a router port, the IP Traffic export feature

http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gt_rawip.html

(Edit: We use this in the dialer0).

So the problems seems to be between us and our provider:

upload MTU 1500.

download MTU lower than 1500.

We are looking to make a configuration change, I will explain the result then it'll be done.

I'm really sorry for the noise...

osgpcqosg · ‎01-20-2012

I finally find the way to see the same problem with a another router: Dkink DSL-524T

(In fact inside it's a linux box).

I done two ping -M do -s 1472 from the linux box and in the same time, some ifconfig command in the modem:

before / after ping

TX 1651 -> 1653 (2 packets)

RX 2007 -> 2011 (4 packets)

We can observe in the linux netbook a full 1500 packet in and out.

The Dlink DSL-524T has done the reassembly of the packet before sending a full 1500 mtu packet to the linux.

It's for this reason I could observe different results with different modem branding !

This test once again show what the problem must be in the provider side.

I'm still trying to have some dump from the provider...

Konstantin Dunaev · ‎01-20-2012

Hi,

I'm quite sure that on provider site the MTU is smaller 1500, because in other case Provider would need to fragment too many packets which would cost too much performance. and it's quite normal that MTU on SDLS lines is smaller then 1500

paolo bevilacqua · ‎01-20-2012

IP MTU can and should be 1500 with PPPoA

Link MTU, 1514 as minimum.

These are different things.

osgpcqosg · ‎01-23-2012

In fact we already look about MTU with the provider, I'm waiting an answer from when, to have some dump in their red...

For information the chain used is:

dell-notebook OSGPCQ (MTU 1500) //cisco800 OSGPCQ (MTU 1500 with PPPoA) /...

.../Provider J-ES DSLAM 2G (MTU 1800) // Provider J-ES core 2G (MTU 9600) // Provider J-ES BRAS (MTU 4700)/.

.../ Switch OSGPCQ (MTU 1998) // (MTU 1998) Cisco OSGPCQ 2821 (MTU 1500)/...

.../gw OSGPCQ (MTU 1500) //server OSGPCQ (MTU 1500)

And seems the problem come from the red of this provider:

Upload : Packet of 1500 everywhere.

Download: Packet of 1500 at the LNS: Cisco OSGPCQ 2821.

2 Packets 1480 + 60 at the CPE: Cisco OSGPCQ 877. (Dialer)

And yes Konstantin Dunaev this look strange, we already have some performance problem in the CPE, so in the provider side ...

If anybody have a idea to diagnose this more deeper ?

Konstantin Dunaev · ‎01-23-2012

try to activate "Path discavery protocol" and you end station and see what value it find on the way along.

osgpcqosg · ‎01-24-2012

Can't find the way to do what ! Could you explain me how must do this ?

Does the cisco 2821 support it ?

Does the cisco 87x support it ?

Konstantin Dunaev · ‎01-24-2012

you should activate it on you end-PC, not on the router, with this option PC will find out the optimal MTU for each connection.

for PPPoA packets the "best" MTU is 1478 bytes (ATM cells overhead), if ISP's infrastructure doesn't use interlinks with bigger MTU (eg. bigger as 1500 on ethernet links) then you will always see a fragmentation. You can "tweak" a MTU value and avoid a fragmantation if you activaze on you "LAN" interface the "tcp window option", on ciso router it'S some like this:

"ip tcp adjust-mss 1438" (1438 + 20 TCP Hdr + 20 IPHdr = 1478 MAX MTU).

if you traffic mosty TCP (usually it is ) then you'll solve you fragmentation issue.

osgpcqosg · ‎02-09-2012

In my linux box I always have PMTU active. (cat /proc/sys/net/ipv4/ip_no_pmtu_disc).

And the "ip route show table cache" commant give me:

x.x.x.x via y.y.y.y dev eth0 src z.z.z.z

cache mtu 1500 advmss 1460 hoplimit 64

For now I'm not looking to Optimize the MTU with the ATM fragmentation, but only have a full path 1500Bytes connection.

(I will leave the mtu optimization for later, because for me the compres should change a lot the theorical calcul and I will must try this with a lot of time, check the atm packet number... Have you already test it and have some result to present me ?).