No-latency SSH sessions on a 5Ghz WiFi router with 250mw radio

Disclaimer: You may brick your fancy new Linksys router by following the advice in this blog entry. A large number of folks have installed this software successfully including me. But consider yourself warned in case you’re the unlucky one.

I use SSH a lot. My wife and nephew love streaming video like Hulu instead of regular cable. For the last few years there’s been a cold war simmering. I’m working late, they start streaming, and my SSH session to my server gets higher latency. So every time I hit a keystroke it takes 0.3 seconds to appear instead of 0.01. Try hitting 10,000 keystrokes in an evening and you’ll begin to understand why this sucks.

I’ve tried screwing with the QoS settings on my Linksys routers but it doesn’t help at all. I ran across a bunch of articles explaining how it’s useless to try to use QoS because it only modifies your outgoing bandwidth and can’t change the speed at which routers on the Internet send you traffic.

Well that’s all bullshit. Here’s how you fix it:

Upgrade the firmware on your router to DD-WRT. Here’s the list of supported devices. I have a WRT320N Linksys router. It’s a newer router that has both a 2.4 Ghz and 5Ghz radio. Many routers that look new and claim to support “N” actually just have 2.4Ghz radios in them.

The DD-WRT firmware for the WRT320N router is very very new, but it works perfectly. Here’s how you upgrade:

Read Eko’s (DD-WRT author) announcement about WRT320N support here. The standard DD-WRT installation instructions are here so you may want to reference them too. Here’s how I upgraded without bricking my router:

  1. Download the ‘mini’ DD-WRT here.
  2. Open all the links in this blog entry in other browser windows in case you need to refer to them for troubleshooting. You’re about to lose your Internet access.
  3. Visit your router’s web interface and take not of all settings – not just your wireless SSID and keys but your current MAC address on your Internet interface too. I had to clone this once DD-WRT started up because my ISP hard-codes MAC addresses on their side and filters out any unauthorized MAC’s. I’d suggest printing the settings direct from your web browser.
  4. Use the web interface (visit usually) and reset your router to factory default settings.
  5. You’ll need to log into your router again. For linksys the default login is a blank username and the password ‘admin’.
  6. Use Internet Explorer to upgrade the firmware using your router’s web interface. Apparently Firefox has a bug on some Linksys routers so don’t use that.
  7. Wait for the router to reboot.
  8. Hit with your web browser and change your router’s default username and password.
  9. Go to the Clone MAC address option and set it to your old Internet MAC address
  10. Set up your wireless with the old SSID and key
  11. Confirm you can connect to the router via WiFi and have Internet Access.

Now the fun part:

  1. Go to Wireless, Advanced settings, and scroll down to TX Power. You can boost your transmit signal all the way to 251mw. Boosting it by about 70mw should be safe according to the help. I’ve actually left mine as is to increase my radio’s life, but nice to know I have that.
  2. Go to the NAT/QoS menu and hit the QoS tab on the right. Enable QoS. Add your machine’s MAC address. Set the priority to Premium (not Exempt because that does nothing). Hit Apply Settings. Every other machine now has a default priority of Standard and your traffic will be expedited.
  3. For Linux Geeks: Click the services tab and enable SSHd. Then ssh to your router’s IP, usually Log in as root and whatever password you chose for your router. I actually changed my username to ‘admin’ but the username seems to stay root for ssh.

You can use a lot of standard linux commands in SSH – it’s busybox linux. Type:

cat /proc/net/ip_conntrack | grep <YourIPAddress>

Close to the end of each line you’ll see a mark= field. For your IP address it should have mark=10 for all your connections. Everyone else should be mark=0. The values mean:

  • Exempt: 100
  • Premium: 10
  • Express: 20
  • Standard: 30
  • Bulk: 40
  • (no QoS matched): 0

Remember if no QoS rule is matched the traffic is Standard priority if you have QoS enabled on the router. So you are Premium and everyone else is standard. Much more detail is available on the QoS DD-WRT Wiki here.

The Linux distro is quite amazing. There are over 1000 packages available for DD-WRT including Perl, PHP and MySQL in case you’d like to write a blogging platform for your Linksys router. To use this you’re going to have to upgrade your firmware to the ‘big’ version of the WRT320N binary. Don’t upgrade directly from Linksys firmware to the ‘big’ DD-WRT – Ecko recommends upgrading to mini first and then upgrading to ‘big’. Also note I haven’t tried running ‘big’ on the WRT320N because I’m quite happy with QoS and a more powerful radio.

There are detailed instructions on how to get Optware up and running once you’re running ‘big’ on the Wiki. It includes info on how to install a throttling HTTP server, Samba2 for windows networking and a torrent client.

If you’d like to run your WRT320N at 5Ghz the DD-WRT forums suggest switching wireless network mode to ‘NA-only’ but that didn’t work for my Snow Leopard OS X machine. When I was running Linksys I had to use 802.11A to make 5Ghz work for my macbook. And likewise for this router I run A-only. You can confirm you’re at 5Ghz by holding down the ‘option’ key on your macbook and clicking the wifi icon on top right.

I prefer 5Ghz because the spectrum is quieter, but 5Ghz doesn’t have the distance through air that 2.4 Ghz does. So boosting your TX power will give you the same distance with a clear spectrum while all your neighbors fight over teh 2.4Ghz band.

SSL Network problem follow-up

It’s now exactly a week since I blogged about my SSL issues over our network. To summarize, when fetching documents on the web via HTTPS from my servers, the connection would just hang halfway through until it timed out. I had confirmed that it wasn’t the infamous PMTU ICMP issue that is common if you’re fetching documents via HTTPS from a misconfigured web server. It was being caused by inbound HTTPS data packets getting dropped and when the retransmit would occur the retransmitted packets would also get dropped. Exactly the same packet every time would get dropped.

Last night we solved it. We’ve been working with Cisco for the last week and have been through several of their engineers with no progress. I was seeing packets arriving on my provider’s switch (we have a great working relationship and share a lot of data like sniffer logs) – but the packet was not arriving on my switch We had isolated it to the layer 2 infrastructure.

Last night we decided to throw a hail-mary and my provider changed the switch module my two HSRP uplinks were connected to from one 24 port module to another. And holycrap it fixed the problem. We then reconfigured routes and everything else so that the only thing that had changed was the 24 port module. And it was still fixed.

This is the strangest thing I’ve seen and the Cisco guys we were working with echoed that. It’s extremely rare for Layer 2 infrastructure which is fairly brain-dead to cause errors with packets that have a higher level protocol like HTTPS in common. These devices examine the layer-2 header with the MAC address and either forward the entire packet or not. The one thing we did notice is that the packets that were getting dropped were the last data packet in a PDU (protocol data unit) and were therefore slightly shorter by about 100 bytes than the other packets in the PDU that were stuffed full of data.

But we’ve exorcised the network ghosts and data is flowing smoothly again.

Routers treat HTTPS and HTTP traffic differently

OSI Network Model

Well the title says it all. Internet routers live at Layer 3 [the Network Layer] of the OSI model which I’ve included to the left. HTTP and HTTPS live at Layer 7 (Application layer) of the OSI model, although some may argue HTTPS lives at Layer 6.

So how is it that Layer 3 devices like routers treat HTTPS traffic differently?

Because HTTPS servers set the DF or Do Not Fragment IP flag on packets and regular HTTP servers do not.

This matters because HTTP and HTTPS usually transfer a lot of data. That means that the packets are usually quite large and are often the maximum allowed size.

So if a server sends out a very big HTTP packet and it goes through a route on the network that does not allow packets that size, then the router in question simply breaks the packet up.

But if a server sends out a big HTTPS packet and it hits a route that doesn’t allow packets that size, the routers on that route can’t break the packet up. So they drop the packet and send back an ICMP message telling the machine that sent the big packet to adjust it’s MTU (maximum transfer unit) size and resend the packet. This is called Path MTU Discovery.

This can create some interesting problems that don’t exist with plain HTTP. For example, if your ops team has gotten a little overzealous with security and decided to filter out all ICMP traffic, your web server won’t receive any of those ICMP messages I’ve described above telling it to break up it’s packets and resend them. So large secure packets that usually are sent halfway through a secure HTTPS connection will just be dropped. So visitors to your website who are across network paths that need to have their packets broken up into smaller pieces will see half-loaded pages from the secure part of your site.

If you have the problem I’ve described above there are two solutions: If you’re a webmaster, make sure your web server can receive ICMP messages [You need to allow ICMP code 4 "Fragmentation needed and DF bit set"]. If you’re a web surfer (client) and are trying to access a secure site that has ICMP disabled, adjust your network card’s MTU to be smaller than the default (usually the default is 1500 for ethernet).

But the bottom line is that if everything else is working fine and you are having a problem sending or receiving HTTPS traffic, know that the big difference with HTTPS traffic over regular web traffic is that the packets can’t be broken up.