Run make to build icmpdsl.  (There is no install target.)  It should be
relatively painless to cross-compile for your own router using OpenWRT's
toolchain.

On a DSL line with e.g. 1492-byte MTU, typically web servers reply to requests
but receive ICMP messages from your ISP's router to retransmit smaller segments.
This program pre-emptively attempts to inform hosts of your WAN MTU when
connecting over TCP.  With luck, the server sees these ICMP packets before its
initial response.  In my tests, this appeared to reduce round-trip latency
for some websites, but not all.

On Linux, it is possible to configure netfilter with iptables to pass the TCP
SYN-ACK to nfqdsl.  Then it doesn't need a BPF filter and it can delay relaying
the SYN-ACK until after the ICMP message gets transmitted.  I don't remember if
this program worked (I switched from DSL).