Cryptography Reference
In-Depth Information
the
traceroute
facility that comes with every Internet-capable computer to print
a list of the hops between you and any server on the Internet.
An example of a traceroute output is shown below:
[jdavies@localhost]:~$ traceroute www.travelocity.com
traceroute to www.travelocity.com (151.193.224.81), 30 hops max, 40 byte packets
1 192.168.0.1 (192.168.0.1) 0.174 ms 0.159 ms 0.123 ms
2 * * *
3 172.216.125.53 (172.216.125.53) 8.052 ms 7.978 ms 9.699 ms
4 10.208.164.65 (10.208.164.65) 10.731 ms 9.895 ms 9.489 ms
5 gig8-2.dllatxarl-t-rtr1.tx.rr.com (70.125.217.92) 12.593 ms 10.952 ms
13.003 ms
6 gig0-1-0.dllatxl3-rtr1.texas.rr.com (72.179.205.72) 69.604 ms 37.540 ms
14.015 ms
7 ae-4-0.cr0.dfw10.tbone.rr.com (66.109.6.88) 13.434 ms 13.696 ms 15.259 ms
8 ae-1-0.pr0.dfw10.tbone.rr.com (66.109.6.179) 15.498 ms 15.948 ms 15.555 ms
9 xe-7-0-0.edge4.Dallas3.Level3.net (4.59.32.17) 18.653 ms 22.451 ms 16.034
ms
10 ae-11-60.car1.Dallas1.Level3.net (4.69.145.3) 19.759 ms
ae-21-70.car1.Dallas1.Level3.net (4.69.145.67) 17.455 ms
ae-41-90.car1.Dallas1.Level3.net (4.69.145.195) 16.469 ms
11 EDS.car1.Dallas1.Level3.net (4.59.113.86) 28.853 ms 25.672 ms 26.337 ms
12 151.193.129.61 (151.193.129.61) 24.763 ms 26.032 ms 25.481 ms
13 151.193.129.99 (151.193.129.99) 28.727 ms 25.441 ms 26.507 ms
14 151.193.129.173 (151.193.129.173) 26.642 ms 23.995 ms 28.462 ms
15 * * *
Here, I've submitted a traceroute to
www.travelocity.com
. Each router along
the way is supposed to respond with a special packet called an ICMP timeout
packet, as described in RFC 793, with its own address. The routers that cannot
or will not do so are represented with
* * *
in the preceding code. Typically
the routers don't respond because they're behind a fi rewall that's confi gured
not to forward ICMP diagnostic packets. As you can see, there are quite a few
hops between my home router and Travelocity's main web server.
In network programming parlance, the tenuous connection between a sender
and a receiver is referred to as a
socket
. When one host — the
client
— is ready
to establish a connection with another — the
server
— it sends a
synchronize
(SYN) packet to the server. If the server is willing to accept the connection,
it responds with a SYN and acknowledge packet. Finally, the client acknowl-
edges the acknowledgment and both sides have agreed on a connection. This
three-packet exchange is referred to as the
TCP handshake
and is illustrated in
Figure 1-1. The connection is associated with a pair of numbers: the
source port
and the
destination port
, which are attached to each subsequent packet in the
communication. Because the server is sitting around, always listening for con-
nections, it must advertise its destination port ahead of time. How this is done
is protocol-specifi c; some protocols are lucky enough to have “magic numbers”
associated with them that are well-known (in other words, you, the programmer
are supposed to know them). This is the
Transport Control Protocol
(TCP); RFC
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