1. Review: Switch Model: Reduce the Mess

2. Question
Why you fly, you can check a map to determine what cities are on the route.
However, there is no map of the internet, so how do you know what route a packet is taking?

3. An Analogy State College Philly NYC Boston Remote host: Jack
Local host: Alice

4. Traceroute
Widely used command-line tool to let hosts peek inside the network
On all OSes (tracert on Windows)
Provides delay measurement from source to router along end-end Internet path towards destination.

5. Traceroute Probe successive hops to find network path
sends 3 packets that will reach each router on path towards destination
Each router returns packets to sender
sender times interval between transmission and reply.
3 probes
3 probes
3 probes
Remote host: Jack
Local host: Alice

6. Traceroute
Technically, a hop is defined as the action that occurs when a packet “jumps” from one router to the next.
Remote host: Jack
Local host: Alice
1 hop
2 hop
3 hop
N-1 hop
N hop

7. Traceroute Question 1
How many computers (incl. local host, remote host, and intermediate routers) does this route have?
Answer: 4

8. Traceroute Question 2
How many hops does this route have?
Answer: 3

9. Traceroute Question 3
Remote host: Jack
Local host: Alice
1 hop
2 hop
3 hop
N-1 hop
N hop
Answer: N+1
For a route with N hops, how many computer does it have?

10. Traceroute Question 4
Remote host: Jack
Local host: Alice
1 hop
2 hop
3 hop
N-1 hop
N hop
Answer: 3N
Local host sends 3 packets to each computer on the route (incl. routers and remote host). If the route has N hops, how many packets does local host send in total?

11. Traceroute Question 5
Remote host: Jack
Local host: Alice
1 hop
2 hop
3 hop
N-1 hop
N hop
Answer: it forwards 3 packets to each of the (N-1) remaining computers, AND forwards 3 packets from each of them back to the local host. So the total is 3*(N-1) + 3*(N-1) = 6N-6
For a route with N hops, how many packets are forwarded by the router that is closest to the local host?

12. Traceroute Example 1 C:\> tracert allspice.lcs.mit.edu
1 20ms 25ms 15ms philadelphia4.t3.ans.net ( )
ms 120ms 140ms New-York2.t3.ans.net ( )
ms 280ms 260ms Cambridge1.bbnplanet.net ( )
ms 279ms 279ms Allspice.lcs.mit.edu ( )
3 delay measurements

13. Traceroute Question 6 C:\> tracert allspice.lcs.mit.edu
1 20ms 25ms 15ms philadelphia4.t3.ans.net ( )
ms 120ms 140ms New-York2.t3.ans.net ( )
ms 280ms 260ms Cambridge1.bbnplanet.net ( )
ms 279ms 279ms Allspice.lcs.mit.edu ( )
How many computers are there in this route? How many hops?
Answer : 5

14. Traceroute Question 7 C:\> tracert allspice.lcs.mit.edu
1 20ms 25ms 15ms philadelphia4.t3.ans.net ( )
ms 120ms 140ms New-York2.t3.ans.net ( )
ms 280ms 260ms Cambridge1.bbnplanet.net ( )
ms 279ms 279ms Allspice.lcs.mit.edu ( )
How many packets are sent by local host?
How many packets are forwarded by New York (New-York2.t3.ans.net )?
Answer 1: 3 * 4 = 12
Answer 2: = 12

15. Traceroute Question 8 C:\> tracert allspice.lcs.mit.edu
1 20ms 25ms 15ms philadelphia4.t3.ans.net ( )
ms 120ms 140ms New-York2.t3.ans.net ( )
ms 280ms 260ms Cambridge1.bbnplanet.net ( )
ms 279ms 279ms Allspice.lcs.mit.edu ( )
What’s the average round trip time form local host to Cambridge1.bbnplanet.net?
Answer: ( ) / 3 = 280ms

16. Traceroute Example 2 C:\> tracert www.oxford.ac.uk comcast
ms ms ms
ms ms ms c hsd1.pa.comcast.net [ ]
ms ms ms te sur02.statecollege.pa.pitt.comcast.net [ ]
ms ms ms be-3-ar03.lowerpaxton.pa.pitt.comcast.net [ ]
ms ms ms he ar01.pittsburgh.pa.pitt.comcast.net[ ]
ms ms ms be-7016-cr01.newyork.ny.ibone.comcast.net [ ]
ms ms ms ae12.edge1.NewYork2.level3.net [ ]
ms ms ms ae-4-4.car1.Manchesteruk1.Level3.net [ ]
ms ms ms ae-4-4.car1.Manchesteruk1.Level3.net [ ]
ms ms ms
ms ms ms ae29.erdiss-sbr1.ja.net [ ]
ms ms ms ae31.londpg-sbr1.ja.net [ ]
ms ms ms ae21.read-rbr3.ja.net [ ]
ms ms ms ae1.read-rbr2.ja.net [ ]
ms ms ms ae2.oxfo-rbr2.ja.net [ ]
ms ms ms Oxford-University-2.ja.net [ ]
ms ms ms csurb.backbone.ox.ac.uk [ ]
ms ms ms boucs.backbone.ox.ac.uk [ ]
ms ms ms boucs-lompi1.sdc.ox.ac.uk [ ]
ms ms ms aurochs-web-154.nsms.ox.ac.uk [ ]
Trace complete.
comcast
Level3: Trans-atlantic link
Janet: UK’s research network
oxford

17. Traceroute Exercise 1 Step 1: Start  Run  cmd Step 2: Trace route to
Somewhere in the U.S.
Somewhere outside U.S.
Discuss the results with classmates next to you
How may hops are there in the path?
What’s the average delay from you computer to the remote host?
How many major networks does the path traverse? What are they?
For destination outside US, from which router do the packets leave US?

18. Traceroute Exercise 2
For this exercise, you need to use the traceroute server at When you view this web page, execute a traceroute (trace) to:
your machine
Somewhere in US
Somewhere outside US

19. Traceroute Exercise 3 At the command prompt, type:
tracert
Describe what is strange about the observed output, and why traceroute gives you such an output