Packet Loss??

I have a coffeeshop I frequent that has free wifi, and since I got my new laptop that runs SL, I'd like to be able to go in-world while getting my caffeine transfusion.. The problem is this:
I get an excellent signal (54mb full-speed 802.11g), everything besides SL works great, but when I attempt to go into SL, I login ok, but get 4,000+ms pings both directions, packet loss of as high as 75%, and the b/w is down to 20kb/s or less.. Needless to say this is not acceptable... I regularly listen to 96kb streaming audio at this coffeeshop w/ very few dropouts.. In other words, their net connection -seems- to be ok.. I've gone into other wifi-equiped places with this system, along with my wireless connection at home, and SL works good everywhere else... I thought I might try pinging the sim server(s) that SL is talking to, but I'm unclear on exactly what the sim server url would be, besides something.agni.lindenlab.com .. I'd really like to figure this out, as at the present time, I spend about 2hrs/day here, both before and after work.. It's better to sit quietly with a nice cup of coffee, than fighting the congestion on the freeways.. :-<

TazPerth

Hi Tasman,

Have you seen this link? I'm willing to bet that your packet loss is due to out-of-order packets that have to be re-requested and resent. When packets are delivered to the SL client out of order, the client rejects them.

Have you tried throttling down your bandwidth under Preferences > Network? That may or may not help, as it seems wi-fi and SL sometimes just don't get along.

You can find the DNS name of the current sim under Help->About - it will be of the form simXXX.agni.lindenlab.com where XXX is a sim number. Pinging that might give you the information you seek.

It has been claimed by one of the Lindens that WiFi is not suitable for SL. I'm not familiar enough with the 802.11 specification to know why that should be, but based on my investigations, I can put forth an educated guess: the WiFi spec allows data-link layer packets to be received out of order (under one WiFi mode). SL has implemented a pretty weak transport layer (reliable stream akin to TCP) on top of the UDP substrate that SL does most of its comms over. Whereas TCP handles reassembly of SDUs robustly in the face of out-of-sequence IP packets, the SL transport layer falls apart under these conditions. In other words, SL reinvented the TCP wheel - for legitimate performance reasons - unfortunately, their "wheel" is squarish.

I've run SL across WiFi B (11 Mbps) and G (54 Mbps) and have never had a problem with it. I haven't mucked about to see if my gear is using the allowed out-of-order packet sequencing though. Unfortunately, if you approach LL support, they will likely tell you WiFi is "not supported" so you shouldn't expect much from that quarter. Finally, the WiFi mode that allows out-of-order delivery has not been visible at the Windows driver level or any home WiFi access point that I've seen, so you may not be able to tell if you are in a "good spot" or "bad spot" without really digging hard.

Well, that likely wasn't of much practical use but I hope it was vaguely understandable.

Malachi is dead on target!

Some wi-fi routers have a 'turbo' mode that allows for out-of-order packets, and that's probably why Tasman is seeing crappy performance at the coffee shop and not other places. Chances are they have this 'turbo' mode enabled (often it's on by default).

With the prevalence of wi-fi devices, it would seem LL would tweak their network code to handle these things.

Now that you mention it, I tried to figure out what was causing the loss bursts on my regular wired ethernet box. Grovelling through the SecondLife.log file was what showed me that the client complains bitterly for 10 to 20 packets when packet 16 of 36 gets lost before the transport layer gives up on that asset transfer.

So it would seem that the Turbo WiFi is simply an acute case of the SL transport layer falling over upon receipt of an out-of-sequence packet and that this defect could be causing major lossage on wired clients as well.

Alas, when I brought this analysis to LL, they more-or-less told me to take a flying fark at a rolling doughnut with my so called data so I don't think they'll be much impressed with this nor particularly keen on fixing it. I guess the question then becomes: is it their "loss" or the customers'?

Thanks all!!! I'd never thought of the out-of-sequence packet issue... Since SL/wifi works just fine everywhere else I've used it.. tmobile/starbucks, mickeyd's, panera, and many other freespots, I was puzzled at why this -one- really convienient freespot hozed SL sooo badly.. Further checking found that the internet connection I -thought- they were using, ie: a CoxHSI commercial cablemodem, which is what is used at other branches of this coffeehouse chain, was not the case at this branch.. In fact they are using one of those wireless-based WAN ISP's, in this case, Keyon Communications (www.keyon.com).. I wonder if this would contribute to the problem.. I'll try the adjustments mentioned above..

Again thanks all...

TazPerth