Extreme Slowness

Fri Oct 27 10:25:52 UTC 2006

On Fri, 27 Oct 2006, Michael.Dillon at btradianz.com wrote:

> For the record, TCP traceroute and similar TCP based
> tools rely on the fact that if you send a TCP SYN
> packet to a host it will respond with either a
> TCP RST (if the port is NOT listening) or a TCP
> SYN/ACK. The round trip time of this provides useful
> information which is unaffected by any ICMP chicanery
> on the part of routers or firewalls. A polite application
> such as TCP traceroute will reply to the SYN/ACK with
> an RST packet so it is reasonably safe to use this tool
> with live services.

Intermediate nodes are still discovered by "ICMP TTL Exceeded in transit" 
just like UDP based traceroute, ie the outgoing TCP SYN packet has a low 
TTL.

So yes, tcptraceroute is good for getting thru firewalls in the forward 
direction, but intermediate routers are discovered in the same way by you 
getting an ICMP back because the TTL ran out.

-- 
Mikael Abrahamsson    email: swmike at swm.pp.se