Subject:Advanced Computer Networks

Instructor :Bhaskaran Raman

Scribe: Srinivasa Rao Myla

Lecture  No:8

In a network environment there are many types of senders with different sending speed.
congestion control can be done  by the sender(eg: tcp) or  by the router's(eg:fair queuing).
Fair Queuing:-

 It deals with  
1. bandwidth among  packets send by the  different users which packet hash to choosen  so that each                         sender will get a fair  fraction of bandwidth.
2. how to allocate buffer space fairly among different  sender's.
 Buffer Space Allocation:
The objective of  of any router is   to give  for each sender an equal amount of buffer size.
router performs fair queuing by giving equal amount of  buffer space to each sender.
The sender can be identified as
1.source machine .ie buffur space can be allocated per souce machine.
2.destination machine  ie buffer space is allocated per one desination machine.
3.souce machine and destination machine buffer space is allocated per one source and destination pair .
4. individual process  based identification.                                               
Advantages:  sender  can not  can not be domonated by the other.
2.  in this approach  the most agressive sender  may lost it's packets.
1. Sender's may send their packets with the name of other senders.
2.If one sender is idle and other is sending very repidly then it may  loss packets even if  there is  an empty space  int the buffur.
 One possible correction will be using the empty  buffur space of  sender's and if  packet's of original sender's come then  the packet's of fast transmitting sender's will be deleted.
Bandwidth allocation: 
Bandwidth can be fairly allocated by sending packets of each sender.In roundrobin fashion.
This may not  work if  packets are  of variable length.
The ideal solution is  using bitwise roundrobin.Since it is very tedious it can not be implemnted because  sending one bit per packet wastes network resources.
So this algorithm  is simulated by keeping track of will be given to the sender's  which   used  less Bandwidth.
According to credit of each sender the router will decide which packet  to be send.

 When  router  router received a packet from highly credited sender while is sending  packet of another sender.It may preempt process or does not preempt the process .
Nonpreemption makes the reouter unfair.
Preemption  adds the extra overhead of  resending the preempted packet.
The disadvantage of this approach the router must keep track  of the information of  size of the bytes it  has  sent  and each time a packet has to be sent it must find which sender is to be choosen by  comparing   the  bytes sofar sent which is called perpacket over head.  
To address this problem Deficit round robin is implemented using Heap datastructure which can give minimum element in O(log(n)) time.    
Congestion Control In TCP:
TCP is a  connection oriented protocal which  promises ordered and reliable delivery of  messages to apllication system.
 It  keep track of  two windows one is receiver  window whose size is the  no of bytes handled by the receiver .It specifies this number when connection is established.
Another  window is congestion  window which is initially equal to the maximum size of segment.It contains another variable called  ssthreshold which commly 64k initially.
Initally it sends one segment on if it is acknowldged in time It will  double this .And the  process is continued until either receiver window  size is reached or  threshold value is reahed.
When it reaches threshold value it increses leniarly it's congestion window. this is called slow start.
When a timeout occurs it  reset's  it's congestion window to one.and sets ssthreshold to half of  the previous congestion window size.
When congestion occurs all the sender's set their window size to one. At a sudden   underusing the network resources.
The remedy for this is Fast retransmit.


Fast Retransmit:
Since TCP does not know whether a duplicate ACK is caused by a lost segment or just a reordering of segments, it waits for a small number of duplicate ACKs to be received.It is assumed that if there is just a reordering of the segments, there will be only one or two duplicate ACKs before the reordered segment is processed, which will then generate a new ACK.If three or more duplicate ACKs are received in a row, it is a strong indication that a segment has been lost.TCP then performs a retransmission of what appears to be the missing segment, without waiting for a retransmission timer to expire.
ast Recovery:
After fast retransmit sends what appears to be the missing segment,congestion avoidance, but not slow start is performed. This is the fast recovery algorithm.
It is an improvement that allows high throughput under moderate congestion,especially for large windows.
The reason for not performing slow start in this case is that the receipt of the duplicate ACKs tells TCP more than just a packet has been lost.Since the receiver can only generate the duplicate ACK when another segment is received, that segment has left the network and is in the receiver's buffer.
That is, there is still data flowing between the two ends, and TCP does not want to reduce the flow abruptly by going into slow start.
The fast retransmit and fast recovery algorithms are usually implemented together as follows.
1. When the third duplicate ACK in a row is received, set ssthresh to one-half the current congestion window,cwnd, but no less than two segments. Retransmit the missing segment. Set cwnd to ssthresh plus 3 times the segment size. This inflates the congestion window by the number of segments that have left the network and which the other end has cached (3).

2. Each time another duplicate ACK arrives, increment cwnd by the segment size. This inflates the congestion window for the additional segment that has left the network. Transmit a packet, if allowed by the new value of cwnd.

3. When the next ACK arrives that acknowledges new data, set cwndto ssthresh (the value set in step 1). This ACK should be the acknowledgment of the retransmission from step 1, one round-trip time after the retransmission. Additionally, this ACK should acknowledge all the intermediate segments sent between the lostpacket and the receipt of the first duplicate ACK. This step iscongestion avoidance, since TCP is down to one-half the rate it was at when the packet was lost.

 Bhaskaran Raman "Lecture 07 slides"
 Bhaskaran Raman "Lecture 08 Siledes"
[Nag87] John B. Nagle, "On Packet Switches with Infinite Storage", IEEE Transactions on Communication, 35 (4), Apr1987,pp.435-438.
 [DKS89] Alan Demers, Srinivasan Keshav, and Scott Shenker, "Analysis and Simulation of a Fair Queueing Algorithm", ACM SIGCOMM, Sep 1989.
[Jac88] Van Jacobson, "Congestion Avoidance and Control", ACM SIGCOMM, 1988.