R18:
Yes an organization’s mail server and Web server can have the same alias for a host
name. The MX record is used to map the mail server’s host name to its IP address.
R24:
The overlay network in a P2P file sharing system consists of the nodes participating
in the file sharing system and the logical links between the nodes. There is a logical
link (an “edge” in graph theory terms) from node A to node B if there is a
semi-permanent TCP connection between A and B. An overlay network does not
include routers. With Gnutella, when a node wants to join the Gnutella network, it
first discovers (“out of band”) the IP address of one or more nodes already in the
network. It then sends join messages to these nodes. When the node receives
confirmations, it becomes a member of the of Gnutella network. Nodes maintain
their logical links with periodic refresh messages.
R28
With the UDP server, there is no welcoming socket, and all data from different clients
enters the server through this one socket. With the TCP server, there is a welcoming
socket, and each time a client initiates a connection to the server, a new socket is
created. Thus, to support n simultaneous connections, the server would need n+1
sockets.
P4.
(a) The status code of 200 and the phrase OK indicate that the server was able to
locate the document successfully. The reply was provided on Tuesday, 07 Mar
2006 12:39:45GMT.
(b) 3874 bytes.
(c) The first five bytes of the returned document are : “<!doc” . The server agreed
to a persistent connection, as indicated by the Connection: Keep-Alive field.
(d) Sat, 10 Dec2005 18:27:46GMT.
P7.
(a) ∆ = (900,000 bits)/(15,000,000 bits/sec) = 0.06 sec
The traffic intensity on the link is (15 requests/sec)(0.06 sec/request) = 0.9 .
Thus, the average access delay is (0.06 sec)/(1 – 0.9) = 0.6 sec. The total
average response time is therefore 0.6 sec + 2 sec = 2.6 sec.
(b) The traffic intensity on the access link is reduced by 40% since the 40% of the
requests are satisfied within the institutional network. Thus the average access
delay is (0.06 sec)/(1 – 0.9 * 0.6) = 0.13 sec. The response time is
approximately zero if the request is satisfied by the cache (which happens with
probability 0.4); the average response time is 0.13 sec + 2 sec = 2.13 sec for
cache misses (which happens 60% of the time). So the average response time
is (0 sec) * 0.4 + (2.13 sec) * 0.6 = 1.278 sec.
P8.
2RTT0 + ∑ni=1 RTTi
P10.
Parallel download would only share the 150 bps bandwidth among the 10
connections (each getting just 15 bps), thus there is no significant advantage
here. With persistent HTTP we avoid the SYN and SYNACK exchange but that
only requires 2.66 seconds (1.33 seconds to send the 200 bit SYN message over
the 150 bps link, and 1.33 second to receive the ACK). Given that each object
takes 668 seconds to send request and receive the object, the use of pipelining
gives gain less than 1 percent.
P16.
For calculating the minimum distribution time for client-server distribution, we use
the following formula:
Similarly, for calculating the minimum distribution time for P2P distribution, we use
the following formula:
Where, F = 10 Gbits = 10 * 1024 Mbits
us = 20 Mbps
dmin = di = 1 Mbps
Client Server
u
N
10
100
1000
200Kbps
10240
51200
512000
600Kbps
10240
51200
512000
1Mbps
10240
51200
512000
10
100
1000
Peer-to-Peer
u
N
200Kbps
10240
25904.3
47559.33
600Kbps
10240
13029.6
16899.64
1Mbps
10240
10240
10240
P22.
With the original line, UDPClient does not specify a port number when it creates the
socket. In this case, the code lets the underlying operating system choose a port
number. With the replacement line, when UDPClient is executed, a UDP socket is
created with port number 5432 . UDPServer needs to know the client port number
so that it can send packets back to the correct client socket. Glancing at UDPServer,
we see that the client port number is not “hard-wired” into the server code; instead,
UDPServer determines the client port number by unraveling the datagram it receives
from the client (using the .getPort() method). Thus UDP server will work with any
client port number, including 5432. UDPServer therefore does not need to be
modified.
Before:
Client socket = x (chosen by OS)
Server socket = 9876
After:
Client socket = 5432