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Standard User DerbyshireChris
(newbie) Tue 06-Sep-16 12:44:55
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MTU and Amazon - Curious behaviour


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Hi all,

Weird techie question now...

I've been having issues browsing the amazon website from my Sky Fibre connected home. I'd seen issues like this before so I've shrunk the MTU value to 1400 and everything is immediately fine. I've done some checking and tested the MTU of the connection and can ping 8.8.8.8 with an MTU of 1500 without any problems. Unfortunately I can't ping Amazon at all.

I'm only having this issue with Amazon (currently!) and it's only started in the last couple of weeks. Has anyone else noticed anything like this or can anyone add any insight?

Cheers

Chris.
Standard User Oliver341
(eat-sleep-adslguide) Tue 06-Sep-16 13:18:00
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Re: MTU and Amazon - Curious behaviour


[re: DerbyshireChris] [link to this post]
 
In reply to a post by DerbyshireChris:
Unfortunately I can't ping Amazon at all.

www.amazon.co.uk like many other web servers is configured not to respond to pings.

Oliver.
Standard User DerbyshireChris
(newbie) Tue 06-Sep-16 13:22:24
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Re: MTU and Amazon - Curious behaviour


[re: Oliver341] [link to this post]
 
Hi Oliver,

Yes, I assumed as much, however if it did respond it would allow me to test the maximum mtu I could talk to it on. That's why it's unfortunate.

Chris.


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Standard User Oliver341
(eat-sleep-adslguide) Tue 06-Sep-16 14:07:31
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Re: MTU and Amazon - Curious behaviour


[re: DerbyshireChris] [link to this post]
 
Yep. In any case I haven't seen this issue. Pretty much everyone on Sky uses MTU 1500 so if it was breaking Amazon we'd see tons of complaints. It is probably a local issue to your setup therefore.

Oliver.
Standard User DerbyshireChris
(newbie) Tue 06-Sep-16 15:16:42
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Re: MTU and Amazon - Curious behaviour


[re: Oliver341] [link to this post]
 
I'll have it that it's local to me, but does anyone have any ideas on how to diagnose further to get closer to the answer of what is causing it?

Chris.
Standard User Ignitionnet
(knowledge is power) Tue 06-Sep-16 16:09:55
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Re: MTU and Amazon - Curious behaviour


[re: DerbyshireChris] [link to this post]
 
Hi Chris,

Amazon is responding to me offering an MSS of 1440, so MTU 1480.

I'm not sure why you are experiencing problems, assuming you are receiving the same your kit should just adapt to the smallest MSS of the two.
Standard User jchamier
(eat-sleep-adslguide) Tue 06-Sep-16 17:05:01
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Re: MTU and Amazon - Curious behaviour


[re: Ignitionnet] [link to this post]
 
In reply to a post by Ignitionnet:
I'm not sure why you are experiencing problems, assuming you are receiving the same your kit should just adapt to the smallest MSS of the two.

Maybe his router is blocking Path-MTU discovery somehow. Maybe blocking ICMP echo from Amazon's end.

plusnet unlimited fibre 80/20 since 2 Jun 14 - Sync as of 7th Aug 16: 55,355/10,291 kbps with G.INP
17 years of UK broadband since 1999 ntl:cable modem trial -Router: Asus RT-AC68U with HG612 - BQM
Standard User Ignitionnet
(knowledge is power) Tue 06-Sep-16 22:17:26
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Re: MTU and Amazon - Curious behaviour


[re: jchamier] [link to this post]
 
In reply to a post by jchamier:
In reply to a post by Ignitionnet:
I'm not sure why you are experiencing problems, assuming you are receiving the same your kit should just adapt to the smallest MSS of the two.

Maybe his router is blocking Path-MTU discovery somehow. Maybe blocking ICMP echo from Amazon's end.


You open up a session to a server it responds as part of the SYN-ACK with the MSS it will support. ICMP echo is always irrelevant, as is PMTU unless there's something in the path that both won't fragment and has a smaller MTU than either client or server which shouldn't be the case.

By default a device won't do PMTU discovery, it'll rely on the end server. The SYN-ACK must contain the MSS the server will support, this isn't negotiable.

Given most intermediate devices will have an MTU that doesn't bottleneck, or will at very least be happy to fragment this is a reasonable approach.

Edited by Ignitionnet (Tue 06-Sep-16 22:25:33)

Standard User jchamier
(eat-sleep-adslguide) Wed 07-Sep-16 10:16:54
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Re: MTU and Amazon - Curious behaviour


[re: Ignitionnet] [link to this post]
 
In reply to a post by Ignitionnet:
You open up a session to a server it responds as part of the SYN-ACK with the MSS it will support. ICMP echo is always irrelevant, as is PMTU unless there's something in the path that both won't fragment and has a smaller MTU than either client or server which shouldn't be the case.

Thanks that makes sense.

By default a device won't do PMTU discovery, it'll rely on the end server. The SYN-ACK must contain the MSS the server will support, this isn't negotiable.

Given most intermediate devices will have an MTU that doesn't bottleneck, or will at very least be happy to fragment this is a reasonable approach.


Yes that does make a lot of sense, and so its only something "broken" in between that causes things to fail.

plusnet unlimited fibre 80/20 since 2 Jun 14 - Sync as of 7th Aug 16: 55,355/10,291 kbps with G.INP
17 years of UK broadband since 1999 ntl:cable modem trial -Router: Asus RT-AC68U with HG612 - BQM
Standard User Ignitionnet
(knowledge is power) Wed 07-Sep-16 11:40:45
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Re: MTU and Amazon - Curious behaviour


[re: jchamier] [link to this post]
 
I should be clear: many devices do run PMTU all the time, setting the DF bit in the headers, but for TCP connections the MSS is defined by the TCP handshake.

If there's something in the path that can't handle that MSS / MTU it should inform whenever an overlarge packet is sent, or can just fragment them, though that does involve a performance hit.

Kinda two different controls in use, one at the TCP level, MSS, and one at the IP level, MTU.
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