503 due to content-length error? microwave issue?

[Caunter, Stefan]

There's nothing theoretical. Varnish talks HTTP. HTTP knows nothing, and needs to know nothing, about TCP/IP or MTU; it is a client and server protocol with various implementations of the relevant RFCs.

You cannot troubleshoot network or transport layer issues at the application layer. 

A more accurate conclusion would be that adding Varnish exposed existing TCP/IP (network/transport layer) issues. They were there before adding Varnish, and would still have been there had Varnish been removed. Changing the MTU corrected the issue.

Stefan Caunter :: Senior Systems Administrator :: TOPS
e: scaunter at topscms.com  ::  m: (416) 561-4871
www.thestar.com www.topscms.com


-----Original Message-----
From: varnish-misc-bounces at varnish-cache.org [mailto:varnish-misc-bounces at varnish-cache.org] On Behalf Of Alan Dixon
Sent: November-10-10 10:08 AM
To: varnish-misc at varnish-cache.org
Subject: Re: 503 due to content-length error? microwave issue?

Theoretically perhaps, but:

1. Adding varnish triggered the problem.
2. Changing the MTU solved the problem.
3. The system administrators of the ISP had seen a similar
problem/solution in the past.

My limited understanding is that varnish's role in the problem was
just being more demanding ("standards sensitive"), whereas Apache was
able to handle the bad data better.

  - Alan

On Wed, Nov 10, 2010 at 10:00 AM, Caunter, Stefan <scaunter at topscms.com> wrote:
> Irrelevant. Varnish (or any front end proxy) knows nothing about layer
> 2, 3 or 4 in the network stack. It's an application, handling requests
> and responses at layer 7 using the HTTP protocol.
>
> Stefan Caunter :: Senior Systems Administrator :: TOPS
> e: scaunter at topscms.com  ::  m: (416) 561-4871
> www.thestar.com www.topscms.com
>
>
> -----Original Message-----
> From: varnish-misc-bounces at varnish-cache.org
> [mailto:varnish-misc-bounces at varnish-cache.org] On Behalf Of Alan Dixon
> Sent: November-10-10 9:56 AM
> To: varnish-misc at varnish-cache.org
> Subject: Re: 503 due to content-length error? microwave issue?
>
> Okay, here's the answer: the problem was solved by the user's ISP:
> they just lowered the MTU.
>
> Conclusion: adding varnish can create subtle network problems [not
> unique to varnish - also true of any front end proxy or ssl].
>
> Not varnish's fault or problem, but good to know, and would be even
> better if it were easier to debug.
>
>  - Alan
>



-- 
Alan Dixon, Web Developer

Drupal and CiviCRM for Canadian nonprofits.
http://blackflysolutions.ca/

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