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RWIN MTU MSS etc. ...

Razer
Grafter
Posts: 1,398
Thanks: 6
Registered: 17-11-2012

RWIN MTU MSS etc. ...

... It's all gobbledegook to me. An important warning:
I DO NOT HAVE A MATHEMATICAL MIND.
Anyway, basically understanding that the settings need to be right, I was looking into having a ... fiddle. So I went to this site:
https://www.broadbandadvice.org.uk/Website/Computer%20Tweaks/RWIN%20for%20XP/Setting%20RWIN%20for%20... because I can't use the one members are most often directed to on this forum as it's a Java thing.
After clicking the top link to the analyser, I get this:
Quote
TCP options string = 020405b40103030001010402
MTU = 1500
MTU is fully optimized for broadband.
MSS = 1460
Maximum useful data in each packet = 1460, which equals MSS.
Default TCP Receive Window (RWIN) = 65535
RWIN Scaling (RFC1323) = 0 bits
Unscaled TCP Receive Window = 65535
Note: TCP 1323 Options need to be enabled for RWIN over 2^16 (65535).
For optimum performance, consider changing RWIN to a multiple of MSS.
Other RWIN values that might work well with your current MTU/MSS:
64240  (up to 2 Mbit lines, depending on latency. MSS * 44)
128480 (1-5 Mbit lines, depending on latency. MSS * 44 * 2)
256960 (2-14 Mbit lines, depending on latency. MSS * 44 * 2^2)
513920 (8-30 Mbit lines, depending on latency. MSS * 44 * 2^3)
1027840 (25-60 Mbit lines depending on latency. MSS * 44 * 2^4)
bandwidth * delay product (Note this is not a speed test):
Your TCP Window limits you to: 2621 kbps (328 KBytes/s) @ 200ms
Your TCP Window limits you to: 1049 kbps (131 KBytes/s) @ 500ms
MTU Discovery (RFC1191) = ON
Time to live left = 110 hops
TTL value is ok.
Timestamps (RFC1323) = OFF
Selective Acknowledgements (RFC201Cool = ON
IP type of service field (RFC1349) = 00000000 (0)

As I (barely) understand it, the RWIN should ideally be an even multiplication of MSS and something or other. If this is so, why is the default value 65535?
I would also like to check that I have it right in my head that from the recommendations I should set RWIN to 513920 because my line is ADSL2+ (a 24mb connection - though I don't get the full speed). And is there not also something to do with my router, that the router settings should match my computer settings in this regard?
6 REPLIES
Superuser
Superuser
Posts: 9,770
Thanks: 1,151
Fixes: 63
Registered: 06-04-2007

Re: RWIN MTU MSS etc. ...

Does your computer run Windows XP (not Vista, 7 or Cool?
Assuming it is XP, download DrTCP (choose the .exe file) and save it in a convenient location. Then double-click the file to run it (it doesn't install anything on your computer).
I suggest entering 256960 (double that is too extreme in my opinion) in the top TCP Receive Window box.
Make sure your network card (description) shows in the Adapter Setting box and check the MTU alongside. It might be 1500 or perhaps blank. Either of those is acceptable. If it's any other value change it to 1500.
Then click Save, and Exit.
Now restart your computer so that any changed settings take effect. (No need to restart your router.)
Assuming your router is a Plusnet supplied TG585 or TG582 its MTU will be 1500 so no changes needed.
(I wouldn't worry about 65535 not being an even multiple of MSS. The issue isn't as clean cut as that site implies.)
David
Razer
Grafter
Posts: 1,398
Thanks: 6
Registered: 17-11-2012

Re: RWIN MTU MSS etc. ...

Hi spraxyt,
Thanks for your reply. Yeah, I'm on XP, sorry I forgot to say and forgot to specify my router is the TG582n (my head was too wrapped up in the RWIN figures and not understanding them). OK, I'll give your suggestion a whirl. Thanks again.
EDIT:
Actually, I'm not sure which is the right network card to change it for. I think it should be the Realtek RTL8168/8111 PCI-E Gigabit Ethernet NIC one, but just having a quick check and I have two listed that both show as connected in Network Connections. The other is listed at 1394 Net Adapter.
Community Veteran
Posts: 5,172
Thanks: 480
Fixes: 20
Registered: 10-06-2010

Re: RWIN MTU MSS etc. ...

1394 is firewire, you won't be connecting to the router with that.
Razer
Grafter
Posts: 1,398
Thanks: 6
Registered: 17-11-2012

Re: RWIN MTU MSS etc. ...

Oh, that thing I've never used! ha. OK, thanks for that ejs. (I don't know why it says 'connected' when I'm connected to the internet, though.)
Superuser
Superuser
Posts: 9,770
Thanks: 1,151
Fixes: 63
Registered: 06-04-2007

Re: RWIN MTU MSS etc. ...

I don't think firewire "connected" means what you assume it means, it probably means the socket is live in that a camcorder (say) plugged into it would immediately be detected with a connection window opening.
David
Razer
Grafter
Posts: 1,398
Thanks: 6
Registered: 17-11-2012

Re: RWIN MTU MSS etc. ...

Ah, right. Thanks spraxyt. At least now I know what it is and why it says that. And I should also be able to finally remember when I'm fiddling about (like changing DNS settings, for example) to stop incorrectly clicking for the properties on the 1394 instance ... after all these years. lol