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Possible MSS Issue

asked 2018-07-16 14:07:05 +0000

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I have configured PE Router with MSS of 1410 and am testing. Some websites are loading fine and some are not. As a relevant test site to show th4e data with, I chose

The issue with the website is that the Writing appears immediately, as it should, but then I wait for 15 - 20 seconds for all images to appear at the same time. On other websites this can be even slower (even with basic connection). To make it clearer, the websites have also been tested when cached, with the same problem.

Here is the initial SYN - SYN/ACK agreement for the MSS:

   6    2.382020   TCP 66  50783 → 443 [SYN] Seq=0 Win=65535 Len=0 MSS=1460 WS=8 SACK_PERM=1
  54    2.420046    TCP 58  443 → 50783 [SYN, ACK] Seq=0 Ack=1 Win=0 Len=0 MSS=1410

This shows in the SYN/ACK the MSS of 1410, which is fantastic. Exactly as configured. From my understanding this is a one time negotiation and this MSS should be used throughout the rest of the connection communication.

So, why is it that the text appears immediately, but I am having to always wait for the images?

Has anyone seen this before?

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That is a question that would require having all packets for the whole page loading process, including the pictures. The handshake is nice, but to see where the delays are the rest of the packets need to be looked at. Can you share a PCAP?

Jasper gravatar imageJasper ( 2018-07-16 14:56:01 +0000 )edit

I can't load a PCAP as I apparently don't have enough points.

adgwytc gravatar imageadgwytc ( 2018-07-16 15:04:48 +0000 )edit

Okay, can you put it on a file hoster, or CloudShark, and post the link? Make sure that it's visible to the public (CloudShark makes uploaded PCAPs private by default IIRC)

Jasper gravatar imageJasper ( 2018-07-16 15:10:14 +0000 )edit

Try this:

ip.addr== as the filter for the website address

adgwytc gravatar imageadgwytc ( 2018-07-16 15:17:19 +0000 )edit

It requires a login, so I guess it's still set to private ;-)

Jasper gravatar imageJasper ( 2018-07-16 15:53:34 +0000 )edit

1 Answer

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answered 2018-07-16 19:44:02 +0000

mrEEde gravatar image

Your problem seems that none of your IPV6 SYN packets get through. Is your router configured to route IPV6 ?

Regards Matthias

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Yes, the routers that I have configured are for dual stack and we are advertsing and receiving IPv6 via BGP.

Why would that cause an issue with the IPv4 though and the loading of the images?

IPv6 is working well to IPv6 addressing but I am contacting via IPv4, or at least I hope I am....

I will try a different CPE tomorrow and see what the result is.

adgwytc gravatar imageadgwytc ( 2018-07-16 20:16:42 +0000 )edit

The reason you see SYN packets going out via IPV6 is because some of the 'bbc' host names are successfully resolved to AAAA records: 2a02:26f0:c800:28c::f33 I guess this is where your pictures are located ...

mrEEde gravatar imagemrEEde ( 2018-07-17 04:41:53 +0000 )edit

Thank you for the answer.

Strangely, I now have IPv6 working and can ping and traceroute through the CPE to the IPv6 address stated and now I get no images at all on the bbc website.


adgwytc gravatar imageadgwytc ( 2018-07-17 07:55:00 +0000 )edit

Okay. I have narrowed it down. I turned off IPv6 on the CPE and everything worked perfectly. So, it is definitely an IPv6 issue.

adgwytc gravatar imageadgwytc ( 2018-07-17 08:48:51 +0000 )edit

@adgwytc I have converted your answer to comment. As it is more a comment.

Christian_R gravatar imageChristian_R ( 2018-07-17 08:55:04 +0000 )edit

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Asked: 2018-07-16 14:07:05 +0000

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Last updated: Jul 17