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					TMA02                                                                       T305

Question 1

   (i)     What will be the value in the sequence number field of the next
           segment sent by Computer A?

The sequence number field of this segment sent by Computer A = 1465+1200=2665


   (ii)    After receiving the segment in Table 1 from Computer A,
           Computer B wishes to send further data to Computer A. At this
           point 1200 octets previously sent by Computer B to Computer A
           have not been acknowledged. How many octets can Computer B
           still send to Computer A without any acknowledgement being
           received?

The number of octets = 4000-1200=2800 octets

   (iii)   What was the sequence number of the highest value octet to be
           acknowledged by the TCP segment of Table 1?

The sequence number of the highest value octet to be acknowledged =
the acknowledgement number -1 =185382-1=185381


   (iv)     How many octets are there in the header of this segment?

The header length field contains 6 (counted in 32-bit words =4 octets).
So there are 6x4=24 0ctets in the header of this segment.


   (v)     (v) What will be the values of the source and destination port
           fields in segments sent by Computer B?

The source port number is 80 and the destination port number will be 120.




   b) For this part of the question you will need to refer to Figure 1.
   After successfully opening a connection and with normal data
   transfer having taken place between the two computers A and B,


                                                                            G.S
       TMA02                                                                  T305

          the connection is then closed down starting with a ‘close’ request
          from the application at Computer A. Using what you have learned
          from Block 1, Part 2, Communication Network




                  Computer A                                   Computer B
Application Layer       Transport Layer             Transport Layer       Application Layer

                            ESTABLISHED                   ESTABLISHED


                CLOSE Request
                                               FIN

                                   FIN WAIT
                                              ACK
                                                            CLOSED WAIT
                                   FIN WAIT
                                                                CLOSE Request
                                              FIN
                                                           LAST ACK

                                              ACK
                             TIMED WAIT
                                                                CLOSING Indication
              CLOSING Indication
                                                            CLOSED

                                   CLOSED
                                                                  TERMINATE
                                                                    Confirm
              TERMINATE Confirm




                                                                           G.S
TMA02                                                                         T305

   c) Describe, using a suitable diagram that includes a numerical
   example, how TCP regulates the flow of octets between two
   computers.


Flow control is used by the receiver to regulate the TCP octet stream by telling the
sender at appropriate times how many octets it can receive without overflow.




Numerical example:

      Octets 1 to 3 have already been sent and acknowledged by the receiver.
      The window offered by the receiver spans octets 4 to 9 and is six octets long.
      The first three octets in the window have already been sent but not yet been
       acknowledged by the receiver.
      The next three octets, 7 to 9, can be sent as soon as possible, depending on how
       busy the sender computer is with other things. The octets above 9 must not be
       sent until the window slides.




Question 2
                                                                           G.S
TMA02                                                                               T305


  a) Instances of the processes Transmitter and Receiver shown in
  Figures 3 and 4 are created and then two consecutive message
  instances M1 and M2 are sent from the environment to the process
  Transmitter in a time of less than 1 second. Describe concisely the
  detailed operation of the process Transmitter in response to the
  message instances, M1 and M2. There is no need to describe the
  detailed response of the process Receiver, but you should also
  briefly indicate how the process Transmitter handles variations that
  could arise in subsequent message exchanges. Please use the term
  local tag to distinguish the value of tag held by the process from
  the tag field in incoming messages.


Two message M1 and M2 are sent from the environment to the process Transmitter in a
time of less than 1 second.
The input signal send (M1) then the process Transmitter will respond to this the input
signal by giving an output signal trn (M1, o) to Receiver
The state of the process Transmitter will be changed to the state (Waiting) and the timer
value is set to 1+2=3. Three choices are available:
first one , the process Transmitter receive an input signal (t1) from the
environment(means that it receives another message like M2) then it will return to give
an output signal {trn (M2, o)} to the process Receiver and the state of the process
Transmitter will remain in the state (Waiting) .
Second one , give a save symbol that contain the name of a signal (send) indicating that
a signal instance is not in a state , but is retained in the input queue for later
consumption in another state.
Third one , the process Transmitter receive an input signal (ack(t)) from the process
Receiver then there is a decision point , if t is equal to tag value then the process
Transmitter will retuned back to state (Data) and the local tag value is set to( 1-tag )and
will reset the timer. If t is not equal to tag value then the process Transmitter will remain
in the state (Waiting).




                                                                                G.S
   TMA02                                                                T305


     b) If the delay between transmitting a message and receiving its
     acknowledgement is 3 seconds (longer than the transmitter process
     time out) draw a signal sequence diagram (by extending Figure 5) to
     represent what happens in response to the message instances M1
     and M2. Explain how you arrived at your answer.




                     Transmitter                      Receiver



                  tag=0                                  tag=0

                       Data
Send (M1)                                            Active

                                    trn (M1,0)

                      Waiting
                                    ack (0)                      rec (M1)
                   tag=1
                                     tag=0               tag=1
                                                 z
                       Data
                                                     Active
Send (M2)
                                    trn (M2,1)

                  Waiting

                                    ack (1)                      rec (M2)
                                     tag=0
                                                 z
                   tag=0                                 tag=0
                    Data                               Active



   Question 3

                                                                     G.S
TMA02                                                                            T305

In the internet layer is the Internet protocol (IP).
         The IP protocol is used to get packets across the network in a robust (resistant
to malfunction) and flexible (network independent) manner, giving a best-effort and
connectionless service.
         TCP, UDP and SCTP are the protocols available in the transport layer. User
datagram protocol (UDP) is a connectionless protocol; for use with applications that do
not need sequencing and flow control.
SCTP (stream control transmission protocol) offers a point-to-point, connection-
oriented, reliable delivery transport service for applications communicating over an IP
net work. It inherits many of the functions developed for TCP over the past two decades,
including powerful congestion control and packet loss recovery functions. Indeed, any
application running over TCP can be ported to run over SCTP without loss of function,
but the many similarities between the two soon give way to several differences. The
most interesting of these differences revolve around SCTP s support for multihoming
and partial ordering. Multihoming enables an SCTP host to establish a “session” with
another SCTP host over multiple interfaces identified by separate IP addresses.

TCP requires a strict order-of-transmission delivery service for all data passed between
two hosts. This is too confining for applications that can accept per-stream sequential
delivery (partial ordering) or no sequential delivery (order-of- arrival delivery).
Partial ordering lets SCTP provide in-order delivery of one or more related sequences of
messages flowing between two hosts. Thus, SCTP can benefit applications that require
reliable delivery and fast processing of multiple, unrelated data streams.
      SCTP supports some features inherited from TCP and others that provide
         additional functionality •:Multiple delivery modes. SCTP supports several modes
         of delivery including strict order-of- transmission (like TCP), partially ordered
         (per- stream), and unordered delivery (like UDP).
      Multihoming support. SCTP sends packets to one destination IP address, but
         can reroute messages to an alternate if the current IP address becomes
         unreachable.

TCP -friendly congestion control. SCTP employs the standard techniques pioneered in
TCP for congestion control, including slow-start, congestion avoidance, and fast
retransmit. SCTP applications can thus receive their share of network resources when
coexisting with TCP applications.
Selective acknowledgments. SCTP employs a selective acknowledgment scheme,
derived from TCP, for packet loss recovery. The SCTP receiver provides feedback to the
sender about which messages to retransmit when any are lost.

IP networks today are typically resilient to network failure but are often subject to a re
convergence time during which the routing network “heals” itself. During this period,
traffic can be “black holed” or dropped within the network.

Multihomed SCTP end points might be less affected by network re convergence because
lost packets are retransmitted to an alternate address. The SCTP association should
thus recover faster and provide better throughput as long as the path to the alternate
destination is not affected by the re convergence. SCTP multihoming could be employed
to reduce stress on the Internet routing system.



                                                                              G.S
TMA02                                                                            T305
Multihoming offer network resilience to failed interfaces on the host and faster recovery
during network failures. However, the feature s effectiveness is reduced when an end-to-
end association path intersects with a single point of failure in the network — a single
link or router that all association traffic must pass through, for example, or a host that
communicates with only a single interface.




                                                                              G.S

				
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