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.