CS610 Lec 37

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					CS610-Computer Networks                                                    Lecture No.37



Lecture Handout
Computer Networks
Lecture No. 37


CONGESTION CONTROL:
        The goal of congestion control is to avoid adding retransmissions to an already
congested network. Reducing the window size quickly in response to the lost messages
does it. It is assumed that loss is due to congestion.
        We have to resume carefully. Otherwise the network will swing wildly between
congestion and under utilization.


TCP SEGMENT FORMAT:
       It is shown in the figure below. TCP uses single format for all messages. TCP
uses the term segment to refer to a message. Each message sent from TCP on one
machine to TCP on another machine uses this format including data and
acknowledgement.




                                         Figure 37.1




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                           © Copyright Virtual University of Pakistan
CS610-Computer Networks                                                      Lecture No.37


CHAPTER 26

NETWORK ADDRESS TRANSLATION
(NAT)



NETWORK ADDRESS TRANSLATION:
         It is the extension of original addressing scheme and was motivated by exhaustion
of IP address space. It allows multiple computers to share a single address. It requires
device to perform packet translation.
Its implementations are available e.g.,
•    Stand-alone hardware device
•    IP router with NAT functionality embedded



NAT DETAILS:
       Site that consists of more than one computer, obtains a single valid IP address. It
assigns a private address to each computer and uses NAT box to connect to the Internet.
NAT translates address in IP datagrams.



ILLUSTRATION OF NAT:
        The figure illustrates the NAT functionality. When a computer in the site
communicates to the internet, the NAT device, as shown in the figure below, translates its
private address in the site to the global IP address and vice versa.




                                        Figure 37.2




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                          © Copyright Virtual University of Pakistan
CS610-Computer Networks                                                        Lecture No.37



NAT EXAMPLE:
        For example, a site uses private network 10.0.0.0/8 internally.
First computer is assigned 10.0.0.1, second computer is assigned 10.0.0.2 and so on…
        Site obtains a valid IP address (e.g. 128.210.24.6). Let’s assume that the
computer 10.0.0.1 sends to 128.211.134.4 (another global IP address). NAT translates the
IP source address of the outgoing datagram to the global IP address. NAT also translates
the destination address of incoming datagram to the private site address. It is shown in the
figure below.




                                         Figure 37.3




ILLUSTRATION OF NAT TRANSLATION:
       It is transparent to each end i.e. computer at site sends and receives datagrams
normally and computer at Internet receives datagrams from NAT box.

IMPLEMENTATION OF NAT:
       The figure below shows the implementation of NAT. We can see that the old and
new values of IP source field and destination field are shown with their directions.




                                         Figure 37.4


      NAT device stores state information in table. The value is entered in the table
when NAT box receives outgoing datagram from new




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