a tool by Van Jakobson for CHARacterizing network PATHs on IP. Think traceroute, but vastly smarter (and heavier on the network).
pathchar is available for many platforms, notably including Windows and various flavours of UNIX.
It works by sending a number of ICMP probe packets of different sizes (by default from 48 byte to the MTU - usually 1500 byte).
pathchar analyzes in real time the delay and the packet loss, and provides a characterization of the speeds of each link. It sounds strange, but the software can measure the bandwidth of an Internet link away from you.

Pathchar has very minimal documentation, and it is in a sort of permanent alpha state.
Nonetheless I find it a useful and interesting tool.
At the very least you can use it to discover whether your IPS is short-selling you: or if he is trying to stuff 1000 DSL connection into a 10 Mb/s link.

What follows is a typical pathchar session. In italics, my commentary.

# nice -10 ./pathchar -f 4 -m 1500 foo.redescolar.ilce.edu.mx
I raise the priority with the nice command. pathchar
 is time critical. It also needs to run on a fast machine
 to meter fast links.
pathchar to foo.baz.edu (148.233.194.199)
 doing 32 probes at each of 45 sizes (64 to 1500 by 32)
 0 localhost
 |   9.8 Mb/s,   820 us (2.97 ms)
Local LAN. 10 Mb/s Ethernet. 820 microseconds of
propagation delay on this network - we are going
through a hub and a switch, otherwise it would
be less.
 1 gw (201.13.157.254)
 |   2.0 Mb/s,   418 us (9.96 ms)
A microwave link, part of our LAN. 2 megabits as
expected (and as paid for :-)
 2 221.37.183.254 (200.38.181.254)
 |   1.6 Mb/s,   499 us (18.6 ms)
A T1 link. Exiting the lan, to UNAM's routers.
 3 192.100.199.89 (192.100.199.89)
                        -> 148.223.192.6 (1505)            
                        -> 148.223.192.10 (1539)       
                        -> 148.223.192.18 (1482)  
 |   2.0 Mb/s,   10.3 ms (45.2 ms)
Another T1, from UNAM to UNINET (which is in
fact TELMEX). The extra addresses mean that those
machines have returned ICMP packets: it probably means
that TELMEX is doing fancy-smancy load balancing 
on its boundary routers.
 4?customer-148-235-192-14.uninet.net.mx (148.223.192.14)
 |    55 Mb/s,   65 us (45.6 ms),  +q 2.73 ms (18.7 KB) *2
Fast link ! Probably optical. Notice the 2.73 millisecond
queue in the router, and the 2, that indicates statistical
irregularities in the return times. Not big ones, though.
We enter here a big ISP's (TELMEX) fast internal network. 
 5 customer-148-235-173-118.uninet.net.mx (148.235.173.118)
 |   ?? b/s,   50 us (45.7 ms)
We were not able to measure the speed of this link.
Probably with more probes we could.
 6 customer-148-223-184-14.uninet.net.mx (148.223.184.14)
 |   143 Mb/s,   0 us (45.7 ms),  +q 2.97 ms (53.1 KB) *2
 7 tntroma2-2-49.telmex.net.mx (200.38.196.49)
This is the probably from the colonia Roma in
Mexico City to Jalisco
 |   190 Mb/s,   1.61 ms (49.0 ms),  +q 3.15 ms (74.8 KB) *2
 8 bb-jal-ctg-ps4-0.gdl.uninet.net.mx (200.38.192.137)
 |    50 Mb/s,   133 us (49.5 ms),  +q 4.04 ms (25.0 KB)
 9 inet-jal-ctg-4-gt5-0-0.gdl.uninet.net.mx (200.38.194.41)
 |   150 Mb/s,   12.3 ms (74.2 ms),  +q 2.55 ms (47.8 KB) *2
10 customer-148-223-65-114.uninet.net.mx (148.223.65.114)
 |   ?? b/s,   184 us (74.5 ms)
11 customer-38-200-38.uninet.net.mx (200.38.200.38)
 |    11 Mb/s,   2.68 ms (80.9 ms),  +q 5.37 ms (7.49 KB) *2
The bandwidth is falling off. We are approaching the end
system.
12 du-148-233-239-114.prodigy.net.mx (148.233.239.114)
13:  23   800   0     494
This is what you see during the probing. Still 23
 probes to go. Currently probing 800 bytes sized packet.
0 packet dropped. 494 ms of round trip time.

At this point the probe got really slow, which meant that we had hit the final modem link. I interrrupted it.
Notice that patchar consumes network resources. It can easily saturate a modem link. Be nice.

pathchar can be downloaded from ftp.ee.lbl.gov.

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