242005 - MTU network problem

Status: RESOLVED INVALID

(Core :: Networking, defect)

Description

[Enrique]

User-Agent:       Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7b) Gecko/20040316
Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.7b) Gecko/20040316

When I use default network MTU many pages stall infinetly (for example
mail.yahoo.com -I'm connecting from Spain-). The bug is more noticed when
browsing international connections (more "step-routers" ). By decreasing the MTU
size (for example to a minimun of 576) it work nice. It looks that a problem
similar to that of Virtual Private Networks is appearing (see docs in
openvpn.sourceforge.net for MTU network problems on the internet). Not really a
Mozilla bug, but Mozilla somethinglike has to prevent it when possible (by using
more conservatives socket values by default trying not to send tcp packets
bigger than 576 for example). The problem is increases a lot when connecting
through NAT (IPMasquerading) since NAT is very sensitive to MTU problems.

Reproducible: Always
Steps to Reproduce:
1. I try to connect with default MTU (1500 for ADSL 256/128). The Page randomly
stall.
2. I try to connect again with slower MTU (576, typical of an analog modem). The
page load with no problems and ALWAYS much faster than with bigger MTUs.


Actual Results:  
See above

Expected Results:  
See above

Comment 1

[Jo Hermans]

This is definitely not a Mozilla bug. If you have MTU problems, you should set a
lower maximum value in your network stack. It's not up to the application to
mess with this. In fact, it's the TCP-stack which should calculate this
automatically, with a path-mtu-discovery algorithm, but this has problems on its
own. See RFC2923 for more info.

Comment 2

[Matthias Versen [:Matti]]

-> invalid

This is a low-lewel TCP-Stack problem and Mozilla can't do anything.
You get the problem only if something in the route is blocking ICMP (many server
do that to avoid a ICMP DOS attack). 

576 is to low, you should try to get a better value with ping.
For exapmle "ping www.mozilla.org -l 1500 -f" will cause a error and 
"ping www.mozilla.org -l 1450 -f" works on my DSL line.