Mohamed G Gouda							CS 326E
Spring 2013							Midterm 1
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Open Notes							1 hour
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1. Consider the case where a packet is generated in Computer A, then sent to
   Computer B, then sent to Computer C, then sent to Computer D, then sent to
   Computer E, then sent to Computer F and finally sent to Computer G.

   i.   Which of Computers A to G is decidedly a router?
   ii.  Which of Computers A to G is decidedly a host?
   iii. Which of the Computers A to G can be a router?
   iv.  How many hops did this packet make?

Sol: 
   i.   Computers B,C,D,E and F are decidedly routers.
   ii.  None of the computers is decidedly a host.
   iii. Computers A and G can be routers.
   iv.  This packet made 6 hops.
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2. The internet service providers ISPs are supposedly connected in a 3-level
   tree. But, this tree is imperfect. Describe 5 ways in which the ISP tree 
   is imperfect.

Sol:

   i.   There are a dozen Tier-1 ISPs (rather than one).
   ii.  There are multiple regional ISPs for the same region.
   iii. Multihoming- An ISP in level i can be connected to two or more ISPs 
        in level (i-1).
   iv.  Peering - Two ISPs in the same level can be connected on 
        settlement-free	basis.
    v.  Shortcut - An ISP in level 3 can be directly connected to a Tier-1 
        ISP.
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3. Assume that there are 17 established TCP connections between one client 
   host and one HTTP server host.

   i.  How many sockets are needed in the client host to support these 
       connections? Explain your answer. 
   ii. How manty sockets are needed in the HTTP server host to support these
       connections? Explain your answer.

Sol: 

   i.  17 sockets are needed in the client host to support the connection 
       with the HTTP server. One socket is needed for each TCP connection.
   ii. 18 sockets are needed in the server host to support the connections 
       with the client. The HTTP server host needs one more socket to 
       act as a welcome socket for future connections.
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4. How many TCP connections need to be established in order to transmit an 
   email message from <user-1>@<email-server-1> to <user-2>@<email-server-2>.
   Name the client and server hosts for each one of these TCP connections.

Sol:

   Three TCP connections need to be established.

   For the first connection, the client host is the one that user-1 uses to 
   write and send the email message. The server host is the one that hosts
   email-server-1.
   
   For the second connection, the client host is the one that hosts
   email-server-1 and the server host is the one that hosts email-server-2.

   For the third connection, the client host is the one that user-2 uses to
   receive  and read the email message. The server host is the one that hosts
   email-server-2.
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