1. TCP

2. Transmission Control Protocol (TCP)
End-to-end transport protocol
Responsible for reliability, congestion control, flow control, and sequenced delivery
Applications that use TCP: http (web), telnet, ftp (file transfer), smtp ( ), chat
Applications that don’t: multimedia (typically) – use UDP instead

3. Ports, End-points, & Connections
IP Layer
TCP
UDP
http ftp smtptelnet A1 A2 A3
Transport
Port
IP address
Protocol ID
Thus, an end-point is represented by (IP address,Port)
Ports can be re-used between transport protocols
A connection is (SRC IP address, SRC port, DST IP address, DST port)
Same end-point can be used in multiple connections

4. TCP Connection Establishment Connection Maintenance
Reliability
Congestion control
Flow control
Sequencing
Connection Termination

5. Active and Passive Open
How do applications initiate a connection?
One end (server) registers with the TCP layer instructing it to “accept” connections at a certain port
The other end (client) initiates a “connect” request which is “accept”-ed by the server

6. Connection Establishment & Termination
3-way handshake used for connection establishment
Randomly chosen sequence number is conveyed to the other end
Similar FIN, FIN+ACK exchange used for connection termination
Server does passive open
Accept connection request
Send acceptance
Start connection
Active open
Send connection
request
SYN
SYN+ACK
ACK
DATA

7. Fundamental Mechanism
ack
data
data
retx
Simple stop and go protocol
Timeout based reliability (loss recovery)
Multiple unacknowledged packets (W)
ack
data
Sliding Window Protocol: ….

8. Congestion Control Slow Start Start with W=1 For every ACK, W=W+1
Congestion Avoidance (linear increase)
For every ACK,
W = W+1/W
Congestion Control (multiplicative decrease)
ssthresh = W/2
W = 1
Alternative: Fall to W/2 and start
congestion avoidance directly

9. Why LIMD? (fairness) W=1 W=W/2 100 10 diff = 90 1 1 diff = 0
Problem? – inefficient
W=W/2
diff = 45
51 6 diff = 45
52 7 diff = 45 ..
diff = 45
diff = 23.5
diff = 23.5
diff = 11.2

10. Reliability (Loss Recovery)
ack
data
Sequence Numbers
TCP uses cumulative Acknowledgments (ACKs)
Next expected in-sequence packet sequence number
Pros and cons?
Piggybacking
Timeout calculation
Rttavg = k*Rttavg + (1-k)*Rttsample
RTO = Rttavg + 4*Rttdeviation 5 1 2 3 4 3 1 2 4

11. Flow Control Prevent sender from overwhelming the receiver
Receiver in every ACK advertises the available buffer space at its end
Window calculation
MIN(congestion control window, flow control window)

12. Sequencing Byte sequence numbers
TCP receiver buffers out of order segments and reassembles them later
Starting sequence number randomly chosen during connection establishment
Why? 3 1 2 4
1 given to app
2 given to app
Loss
4 buffered (not given to app)
3 & 4 given to app
4 discarded

13. TCP Segment Format 16 bit SRC Port 16 bit DST Port
32 bit sequence number
32 bit ACK number HL
resvd
flags
16 bit window size
Flags: URG, ACK,
PSH, RST, SYN,
FIN
16 bit TCP checksum
16 bit urgent pointer
Options (if any)
Data

14. TCP Flavors TCP-Tahoe TCP-Reno TCP-newReno TCP-Vegas, TCP-SACK
W=1 adaptation on congestion
TCP-Reno
W=W/2 adaptation on fast retransmit, W=1 on timeout
TCP-newReno
TCP-Reno + fast recovery
TCP-Vegas, TCP-SACK

15. TCP Tahoe Slow-start Congestion control upon time-out or DUP-ACKs
When the sender receives 3 duplicate ACKs for the same sequence number, sender infers a loss
Congestion window reduced to 1 and slow-start performed again
Simple
Congestion control too aggressive

16. TCP Reno Tahoe + Fast re-transmit
Packet loss detected both through timeouts, and through DUP-ACKs
Sender reduces window by half, the ssthresh is set to half of current window, and congestion avoidance is performed (window increases only by 1 every round-trip time)
Fast recovery ensures that pipe does not become empty
Window cut-down to 1 (and subsequent slow-start) performed only on time-out

17. TCP New-Reno TCP-Reno with more intelligence during fast recovery
In TCP-Reno, the first partial ACK will bring the sender out of the fast recovery phase
Results in timeouts when there are multiple losses
In TCP New-Reno, partial ACK is taken as an indication of another lost packet (which is immediately retransmitted).
Sender comes out of fast recovery only after all outstanding packets (at the time of first loss) are ACKed

18. User Datagram Protocol (UDP)
Simpler cousin of TCP
No reliability, sequencing, congestion control, flow control, or connection management!
Serves solely as a labeling mechanism for demultiplexing at the receiver end
Use predominantly by protocols that do no require the strict service guarantees offered by TCP (e.g. real-time multimedia protocols)
Additional intelligence built at the application layer if needed

19. UDP Header Src Port Dst Port Length: length of header + data (min = 8)
Checksum

20. Puzzle Two great mathematicians S & P
S knows the sum of two positive integers (> 1) x and y
P knows the product of x and y
S calls P and says “You cannot find the two numbers”
P replies “I know the two numbers”
S responds “I know the two numbers too”
What are the two numbers?!!