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#10207 by KrK
Sat May 23, 2009 11:28 pm
Hello, I'm a brand new ooma customer.

So far, I'm very impressed.

OK, a little background:

I'm running my ooma on an at&t provisioned DSL only line.

At&t DSL is done via PPPOE, and the login and authentication was handled by my router prior to my ooma install.

When I installed ooma, I placed the router behind the ooma hub as was recommended, and once I learned about ooma.setup.com and to configure the Hub, I set it to PPPOE and I put in my login information, and bingo, the ooma hub connected.

I then changed my router settings, turning OFF PPPOE and turning on DHCP so that it would pull an IP from the ooma Hub and we'd go from there.

The rest of the discussion is speculative, but I'm posting it because I want people to discuss it and/or explain it to me (right or wrong!)

After ooma was up and running, I noticed my Internet connection seemed "slower". Not as snappy... pages appearing with the images not loaded, and then the images would load.... ie just more sluggish and less responsive.

I got to wondering why, to me it kind of reminded me of the old days before I "tweaked" my MTU and TCP windows under XP, etc etc

So I logged into my Dlink router. Sure enough, when I changed away from PPPOE and to DHCP the router reset my MTU back to the default 1500..... However, since my Internet connection was still having to go through a PPPOE connection for access, I knew this wasn't ideal.

So... I changed the routers MTU to 1454, saved and rebooted router.

Guess what---- it seems to me that the "snappyness" improved immediately. Does this make sense? Or is this all "Perception = reality"?

To me, it seems better. I should mention that speedtests showed the same speeds either way.... but to me it definitely seems more responsive now and back to speed.

Am I crazy? Is setting the MTU to 1454 still a good idea, being my connection is still PPPOE based?

Thanks for any information, input or comments!

KrK
#10293 by Bobby B
Tue May 26, 2009 10:38 am
Hi Krk,

No, I don't think you're crazy :)

When you are using PPPoE with your Internet connection, you want to set your MTU be at 1492, because the PPPoE header adds 8 bytes over the standard Ethernet connection.

Many routers and computers automatically make this adjustment, via a process called Path MTU discovery. Do you have this enabled on Windows XP? Or, can you check what your MTU was set to on your router?

If your MTU is too large (say 1500), then websites may send data that is too large for the PPPoE link to handle. This can cause IP fragmentation which would probably slow down your receive rate.

Thanks,
Bobby
#10316 by ggilman
Tue May 26, 2009 7:18 pm
I ran some tests as prescribed elsewhere on the Internet to determine what my MTU should be. Even though I have cable modem and not DSL, and no PPoE as well, my tests showed 1492, with fragmentation with larger MTUs. I adjusted my router's MTU accordingly. However, I see no way to adjust the MTU of my ooma. Is this necessary?

The ooma is sitting behind my router (Internet->Cable Modem->Router->ooma) & looking under "Ports" on the ooma, it shows the MTU to be 1500.
#10318 by ggilman
Tue May 26, 2009 8:06 pm
I read in a couple of places (none discussing ooma, just multiple pcs on a router) that all PCs on the network should have the same MTU as the router.
#10319 by scottlindner
Tue May 26, 2009 8:20 pm
ggilman wrote:I read in a couple of places (none discussing ooma, just multiple pcs on a router) that all PCs on the network should have the same MTU as the router.


That's for efficiency, but it'll work if that isn't true.
#10331 by ggilman
Wed May 27, 2009 6:49 am
Logically it would seem far more efficient for them to be the same. Say the ooma packages up a frame of 1500 bytes, then sends it to the router for transmission over the Internet. The router, being set to an MTU of 1492 bytes must then separate the single ooma frame into separate frames of 1492 and 8 bytes before transmitting.

Since transmission overhead is added to both frames transmitted out of the router, this would seem pretty inefficient. According to what I've read, "intermediate devices do not perform reassembly" (http://www.tcpipguide.com/free/t_IPMess ... rocess.htm) So, unless I'm mistaken, that means that each single 1500 byte ooma frame would be transmitted by my router as two separate frames, each with their own overhead. The router isn't capable of buffering multiple frames & reassembling.

Granted, MTU mismatches may well exist further down the line before the receiver that there is nothing you can do about, but since you control the ooma & router, shouldn't you ensure they match?

Shouldn't it be relatively easy to add an option into ooma to specify MTU?
#10355 by Bobby B
Wed May 27, 2009 9:56 am
Hi ggilman,

The Hub auto-adjusts the MTU to 1492 if you're using PPPoE on the Hub. If you're using PPPoE and the data is larger than 1500 bytes, then the Hub will correctly fragment the data.

Normally, path MTU discovery on the end-clients should take care of any MTU mismatches you have upstream of the computers (if Path MTU discovery is working, it'll just take the lowest MTU on the path).

-Bobby

ggilman wrote:Granted, MTU mismatches may well exist further down the line before the receiver that there is nothing you can do about, but since you control the ooma & router, shouldn't you ensure they match?

Shouldn't it be relatively easy to add an option into ooma to specify MTU?
#10362 by ggilman
Wed May 27, 2009 12:04 pm
Bobby B wrote:Hi ggilman,

The Hub auto-adjusts the MTU to 1492 if you're using PPPoE on the Hub. If you're using PPPoE and the data is larger than 1500 bytes, then the Hub will correctly fragment the data.

Normally, path MTU discovery on the end-clients should take care of any MTU mismatches you have upstream of the computers (if Path MTU discovery is working, it'll just take the lowest MTU on the path).

-Bobby


Yes, but as I stated initially, I'm not running PPPoE and since the ooma is behind my router, I can't turn it on this setting anyway. I tried... didn't work so well. Ooma will never connect. I assume it was due to empty username/password fields when I had PPPoE turned on. I set it to DHCP rather than PPPoE and all works fine, just showing a larger MTU of 1500.

As to why my MTU needs to be 1492 without PPPoE, I have no idea. I'm running a regular cable modem from Comcast, not DSL. The only reason I know I need it at 1492 is because I ran some ping tests I found elsewhere that you are supposed to perform to determine your MTU. I ran the same tests against a number of servers and continuously got the same end result, 1492.

In general, I understand how MTU discovery works. Does the ooma use MTU discovery such that it would know to switch to 1492 even though MTU is at 1500 on the device? If so, since I set my router to 1492, ooma in turn will always be adjusted to the same and thus I should have no worries. I assume it still shows 1500 since the display doesn't show the effect of discovery?
#10483 by zeitwatch1
Sat May 30, 2009 1:02 am
This is kind of along the same topic. I ran a Shields Up test this evening on my system, and it told me that I failed the ping test. When I disconnected the ooma device, I passed the ping test. Is there some security setting I need to tweak to secure my system? I am running a cable modem into the ooma hub, and the ooma hub into a Linksys router. Then from the router I have an ethernet cable running to my PC.

Thanks in advance!

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