1. TCP Overview

2. End to end issues

3. Segment format

4. Connection establishment

6. TCP sliding window

8. Stream control Transmission Protocol

9. Simple demultiplexor

10. TCP Congestion Control
Determines the network capacity
Adjust the number of packets that can have safely in transit
Acks to pace the transmission of packets
TCP is self clocking
Avoids congestion
Maxwindow=MIN(CongestionWindow,AdvertisedWindow)
EffectiveWindow=MaxWindow-(LastByteSent-LastByteAcked)

11. Caused By the shortage of buffer space. slow links. slow processors
Possible solutions
End-to-end versus link-by-link control
Rate-Based versus Credit-Based control
The rate-based traffic-flow technique constantly
Integrated congestion control

12. Principles of Congestion Control
informally: “too many sources sending too much data too fast for network to handle”
different from flow control!
manifestations:
lost packets (buffer overflow at routers)
long delays (queueing in router buffers)
a top-10 problem!

13. Scenario 1: Queuing Delays
unlimited shared output link buffers
Host A
lin : original data
Host B
lout
two senders, two receivers
one router, infinite buffers
no retransmission
large delays when congested
maximum achievable throughput

14. Scenario 2: Retransmits
one router, finite buffers
sender retransmission of lost packet
Host A
lout
lin : original data
l'in : original data, plus retransmitted data
Host B
finite shared output link buffers

15. Scenario 3: Congestion Near Receiver
four senders
multihop paths
timeout/retransmit l in
Q: what happens as and increase ? l in
Host A
lout
lin : original data
l'in : original data, plus retransmitted data
finite shared output link buffers
Host B

16. Approaches towards congestion control
Two broad approaches towards congestion control:
End-end congestion control:
no explicit feedback from network
congestion inferred from end-system observed loss, delay
approach taken by TCP
Network-assisted congestion control:
routers provide feedback to end systems
single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM)
explicit rate sender should send at

17. TCP Congestion Control
end-end control (no network assistance)
sender limits transmission:
LastByteSent-LastByteAcked
 CongWin
Roughly,
CongWin is dynamic, function of perceived network congestion
How does sender perceive congestion?
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms:
AIMD
slow start
conservative after timeout events
rate =
CongWin
RTT
Bytes/sec

18. TCP AIMD multiplicative decrease: cut CongWin in half after loss event
additive increase: increase CongWin by 1 MSS every RTT in the absence of loss events: probing
Long-lived TCP connection

19. TCP Slow Start
When connection begins, increase rate exponentially fast until first loss event
When connection begins, CongWin = 1 MSS
Example: MSS = 500 bytes & RTT = 200 msec
initial rate = 20 kbps
available bandwidth may be >> MSS/RTT
desirable to quickly ramp up to respectable rate

20. TCP Slow Start (more)
When connection begins, increase rate exponentially until first loss event:
double CongWin every RTT
done by incrementing CongWin for every ACK received
Summary: initial rate is slow but ramps up exponentially fast
Host A
Host B
one segment
RTT
two segments
four segments
time

21. Refinement (more) Implementation: Variable Threshold
Q: When should the exponential increase switch to linear?
A: When CongWin gets to 1/2 of its value before timeout.
Implementation:
Variable Threshold
At loss event, Threshold is set to 1/2 of CongWin just before loss event

22. TCP sender congestion control
Event
State
TCP Sender Action
Commentary
ACK receipt for previously unacked data
Slow Start (SS)
CongWin = CongWin + MSS,
If (CongWin > Threshold)
set state to “Congestion Avoidance”
Resulting in a doubling of CongWin every RTT
Congestion
Avoidance (CA)
CongWin = CongWin+MSS * (MSS/CongWin)
Additive increase, resulting in increase of CongWin by 1 MSS every RTT
Loss event detected by triple duplicate ACK
SS or CA
Threshold = CongWin/2,
CongWin = Threshold,
Set state to “Congestion Avoidance”
Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS.
Timeout
CongWin = 1 MSS,
Set state to “Slow Start”
Enter slow start
Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed

23. Congestion Avoidance Mechanisms
Helps to avoid congestion
Additional functionality into the router to assist in anticipation of congestion
to  control  congestion  once  it  happens
to  repeatedly  increase  load  in  an  effort  to  find  the  point  at  which  congestion  occurs,  and  then  back  off

24. Mechanisms router-centric: DECbit and RED Gateways
host-centric:  TCP  Vegas

25. DECbit

26. DECbit Add binary congestion bit to each packet header Router
monitors  average  queue  length  over  last  busy+idle  cycle
set  congestion  bit  if  average  queue  length  greater  than  1  when  packet  arrives
attempts  to  balance  throughput  against  delay

27. DECbit End Hosts destination echos bit back to source
source  records  how  many  packets  resulted  in  set  bit
if  less  than  50%  of  last  window's  worth  had  bit  set,  then  increase  CongestionWindow  by  1  packet
if  50%  or  more  of  last  window's  worth  had  bit  set,  then  decrease  CongestionWindow  by  0.875  times

28. UDP—User Datagram Protocol
An unreliable, connectionless transport layer protocol
UDP format. See picture
Two additional functions beyond IP:
Demultiplexing: deliver to different upper layer entities such as DNS, RTP, SNMP based on the destination port # in the header. i.e., UDP can support multiple applications in the same end systems.
(Optionally) check the integrity of entire UDP. (recall IP only checks the integrity of IP header.)
If source does not want to compute checksum, fill checksum with all 0s.
If compute checksum and the checksum happens to be 0s, then fill all 1s.
UDP checksum computation is similar to IP checksum, with two more:
Add extra 0s to entire datagram if not multiple of 16 bits.
Add pseudoheader to the beginning of datagram. UDP pseudoheader

29. Back to UDP—User Datagram Protocol
UDP datagram
Source Port Destination Port
UDP Length UDP Checksum
Data
Back to UDP—User Datagram Protocol
Figure 8.16

30. Back to UDP—User Datagram Protocol
UDP pseudoheader
Source IP Address
Destination IP Address
Protocol = UDP Length
1.Pseudoheader is to ensure that the datagram has indeed
reached the correct destination host and port.
2. The padding of 0s and pseudoheader is only for the
computation of checksum and not be transmitted.
Figure 8.17

31. Thank u