Is Comcast a "criminally braindead ISP"?

The following is copied from the iptables manpage. After getting a new cable modem from Comcast and finally getting the internet working by upgrading to dhcp3-client, the machines on the local network behind my Debian router could ping servers on the internet but not otherwise connect. The workaround below, which I added to my /etc/init.d/localnetwork script, fixed the problem. I think I had this problem in the past when I switched to DSL for a few months.

TCPMSS

This target allows to alter the MSS value of TCP SYN packets, to control
the maximum size for that connection (usually limiting it to your
outgoing interface's MTU minus 40). Of course, it can only be used
in conjunction with
-p tcp.

It is only valid in the
mangle

table.

This target is used to overcome criminally braindead ISPs or servers
which block ICMP Fragmentation Needed packets. The symptoms of this
problem are that everything works fine from your Linux
firewall/router, but machines behind it can never exchange large
packets:

1)

Web browsers connect, then hang with no data received.

2)

Small mail works fine, but large emails hang.

3)

ssh works fine, but scp hangs after initial handshaking.

Workaround: activate this option and add a rule to your firewall
configuration like:

iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
-j TCPMSS --clamp-mss-to-pmtu

--set-mss value

Explicitly set MSS option to specified value.

--clamp-mss-to-pmtu

Automatically clamp MSS value to (path_MTU - 40).

These options are mutually exclusive.