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Site to Site VPN + RDP

 
 
robertm@performancematerials.com
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      08-31-2006
Hello,

I have a Cisco router in the US and a Symantec Router in China. I have
established a Site to Site VPN. I can ping each network from both
sides. The problem is that when I use Remote Desktop from the US to
China, I am able to connect but I get a black screen, or the connection
is very very slow. Any Suggestions?

 
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Merv
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      08-31-2006


RDP is a VERY chatty prtocol so do not be surprised if it will not
work well/at all over a VPN especailly with many hops.


Couple of things you might try:

1. RDP sets the do-not-fragment bit in its TCP packet
so do a path MTU discovery manuall using ping.
Start with a ping packet lenght of 1500 and reduce until you have
successful ping.

Adjust you NIC to use the discovered maximum path MTU size

ping -l 1500 -f <IP address>


2. On Windows increase the TCP retransmissions retries count via
REGEDIT:

Add a value to the TcpMaxDataRetransmissions subkey under the
following registry key:


HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servic es\Tcpip\Parameters\TcpMaxDataRetransmissions
= 10

If the value does not exist, highlight PARAMETERS, go to Edit and
select add value. Choose REG_ DWORD format

 
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Bod43@hotmail.co.uk
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      09-01-2006

Merv wrote:
> RDP is a VERY chatty prtocol so do not be surprised if it will not
> work well/at all over a VPN especailly with many hops.

Hmmm.

I don't think that it is chatty! Maybe compared to telnet but you are
getting
more bang.

> Couple of things you might try:
>
> 1. RDP sets the do-not-fragment bit in its TCP packet
> so do a path MTU discovery manuall using ping.
> Start with a ping packet lenght of 1500 and reduce until you have
> successful ping.


This is true, I checked with ethereal. Beyond belief. !!!

>
> Adjust you NIC to use the discovered maximum path MTU size
>
> ping -l 1500 -f <IP address>
>
>
> 2. On Windows increase the TCP retransmissions retries count via
> REGEDIT:
>
> Add a value to the TcpMaxDataRetransmissions subkey under the
> following registry key:
>
>
> HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servic es\Tcpip\Parameters\TcpMaxDataRetransmissions
> = 10
>
> If the value does not exist, highlight PARAMETERS, go to Edit and
> select add value. Choose REG_ DWORD format


OK.

If you find that you are getting MTU issue than I would drop the MTU on
one end
(at least) to an appropriate value. Fixing one is sufficient.
e.g. 1300.

Alternatively if you have a cisco router in the path you could apply
the "ip tcp mss-adjust 1300" [from memory] (or whatever value
you need) command to an interface on the path, Note that
the length in ping commands is often the ICMP data length
and not the MTU or the TCP mss.

Don't worry if you are 50 or 100 bytes below the true min, the extra
overhead is negligable.
Get it working and go and relax.


I think that the absolute most important thing after MTU is to make
sure that you are
doing selective ack. This is the default in XP.
http://support.microsoft.com/kb/q224829/

Tune up the RDP parameters. Choose a smaller screen size if necessary.
Use as few coulours as you can.
Turn off Desktop background, Show contents while dragging, Menu
animation, Themes.
Turn on bitmap caching.

Also if you are having to wait on the initial splash screeen look
into:-

Remote Desktop RDP mstsc splash screen slow links

http://www.winguides.com/registry/display.php/103/
Change the Logon Screen Wallpaper (All Windows) Popular
When you setup a background wallpaper for your desktop,
the initial logon screen wallpaper is not changed and
stays as the default Windows appearance. This tweak allows
you to change the default wallpaper 'c:\windows\mylogo.bmp'

Registry Settings
System Key: [HKEY_USERS\.DEFAULT\Control Panel\Desktop]
Value Name: Wallpaper, TitleWallpaper
Data Type: REG_SZ (String Value)

Wallpaper "A filename" Without quotes
Pattern "(None)" Without quotes
TileWallpaper 0 = don't
Old PC Wallpaper was - C:\Windows\cpq800h.bmp

NO REBOOT NEEDED has immediate effect.

I do feel that Merv has on this occasion been more negative that I am
about RDP. We have people using it from Hong Kong to London over a VPN.

I think that, aside from the MTU thing, the VPN makes no difference.

One thing to do is to make sure that RDP is OK on the target machine.
Get someone local to try it out.

There is a nice cisco doc on MTU. I will have a look for it tomorrow.
I think if you search for MTU GRE IPSEC you should get it.

Finally my bet is on Merv being right about the MTU.

 
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Hansang Bae
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      09-01-2006
Merv wrote:
> RDP is a VERY chatty prtocol so do not be surprised if it will not
> work well/at all over a VPN especailly with many hops.


It has to be chatty since it's moving mouse/key movements around. I
don't know that you can change that since it's the nature of the
application.

>
>
> Couple of things you might try:
>
> 1. RDP sets the do-not-fragment bit in its TCP packet
> so do a path MTU discovery manuall using ping.
> Start with a ping packet lenght of 1500 and reduce until you have
> successful ping.

[snip]

You mean it does *not* set the DF bit? If you set the DF bit, that's
how you do PMTU-D so I think I'm missing something here.


>
> Adjust you NIC to use the discovered maximum path MTU size
> ping -l 1500 -f <IP address>


1472 should be the first starting point. 20 bytes for IP header, 8
bytes for ICMP. So you have 1472 left over.


--

hsb


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Merv
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      09-01-2006

> > 1. RDP sets the do-not-fragment bit in its TCP packet


RDP does not allow it's packets to be fragmented, so you most ensure
MTU sent does not exceed path MTU.

 
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RobMarsh
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      09-01-2006
My thanks to everyone who is providing me with help, I tried all of the
above mentioned, but nothing works. I have also tried browsing to the
servers share: \\server-over-vpn\share
but that times out and I can't access that either. Could the two
problems be related?


Merv wrote:
> > > 1. RDP sets the do-not-fragment bit in its TCP packet

>
> RDP does not allow it's packets to be fragmented, so you most ensure
> MTU sent does not exceed path MTU.


 
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Merv
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      09-01-2006

What happens for ping with large packet size ( ie. max MTU and lartge
number of packets ( 1000 or more.

What is the latency tiem like ?

 
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RobMarsh
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      09-05-2006
ping -l 1500 172.16.64.5

Pinging 172.16.64.5 with 1500 bytes of data:

Reply from 172.16.64.5: bytes=1500 time=295ms TTL=127
Reply from 172.16.64.5: bytes=1500 time=294ms TTL=127
Reply from 172.16.64.5: bytes=1500 time=290ms TTL=127

if I try this:

ping -f -l 1500 172.16.64.5

I can't get a good ping. so I tried reducing the mtu but when I did
that web browsing became so slow that it was a problem.


Merv wrote:
> What happens for ping with large packet size ( ie. max MTU and lartge
> number of packets ( 1000 or more.
>
> What is the latency tiem like ?


 
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Merv
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      09-06-2006

Reply from 172.16.64.5: bytes=1500 time=295ms TTL=127

ouch !!!

> ping -f -l 1500 172.16.64.5


with the -f flag set what is the largest MTU size for which you get a
ping response ?

does ping -f -l 1472 172.16.64.5 give you a ping response ?

 
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