I'm seeing similar output to that on the traceroute manpage, but I've always been curious on why ping times go down between hops? I.e., I'd always expect values to increment from hop to hop like:
[yak 71]% traceroute nis.nsf.net.
traceroute to nis.nsf.net (35.1.1.48), 64 hops max, 38 byte packet
1 helios.ee.lbl.gov (128.3.112.1) 19 ms 19 ms 0 ms
2 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 39 ms 19 ms
3 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 39 ms 40 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (128.32.168.22) 39 ms 39 ms 39 ms
6 128.32.197.4 (128.32.197.4) 40 ms 59 ms 59 ms
7 131.119.2.5 (131.119.2.5) 59 ms 59 ms 59 ms
8 129.140.70.13 (129.140.70.13) 99 ms 99 ms 80 ms
9 129.140.71.6 (129.140.71.6) 139 ms 239 ms 319 ms
10 129.140.81.7 (129.140.81.7) 220 ms 199 ms 199 ms
11 nic.merit.edu (35.1.1.48) 239 ms 239 ms 239 ms
But what is the reasoning for:
[yak 72]% traceroute allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (18.26.0.115), 64 hops max
1 helios.ee.lbl.gov (128.3.112.1) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (128.32.216.1) 19 ms 19 ms 19 ms
3 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 19 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 19 ms 39 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (128.32.168.22) 20 ms 39 ms 39 ms
6 128.32.197.4 (128.32.197.4) 59 ms 119 ms 39 ms
7 131.119.2.5 (131.119.2.5) 59 ms 59 ms 39 ms
8 129.140.70.13 (129.140.70.13) 80 ms 79 ms 99 ms
9 129.140.71.6 (129.140.71.6) 139 ms 139 ms 159 ms
10 129.140.81.7 (129.140.81.7) 199 ms 180 ms 300 ms
11 129.140.72.17 (129.140.72.17) 300 ms 239 ms 239 ms
12 * * *
13 128.121.54.72 (128.121.54.72) 259 ms 499 ms 279 ms
14 * * *
15 * * *
16 * * *
17 * * *
18 ALLSPICE.LCS.MIT.EDU (18.26.0.115) 339 ms 279 ms 279 ms
Why is the time from hops 3 to 4 go from 39ms to 19ms?
They are not just different "hops" but indeed totally different events, at different times, and possibly under different conditions.
The way traceroute works is, it sends a packet with at TTL of 1 and sees how long it takes for an answer to arrive. The program repeats this three times to account for some variation that is possible and normal (note that even the three numbers in "the same hop" are sometimes very different, since they are totally separate things, too).
Then, traceroute sends a packet with a TTL of 2 (and then 3, and 4, etc.) and sees how long it takes for an answer to arrive. In the mean time, the load on the first router may have lowered, so your packets are actually forwarded faster. Or, the physical cable where your packets go out may not be busy because your roommate just finished downloading his porn (whereas before your network card would have to wait for the cable to get idle). This is something that happens all the time, and you have no way of knowing or controlling that.
There is no way of going back in time, so you can't change the fact that a second ago it took a bit longer for your packets to make it through. You're probing at two different times, so all you can get is a reasonable guestimate. Yes, generally, the time from hop to hop should gradually increase, but it needs not.
That doesn't mean that the round trip time is really going down between hops 3 and 4. It only means that it went down in the meantime while you you independently tried to get an estimate of how long 3 hops and 4 hops might take.