1. Transmission Control Protocol (TCP)

2. Puzzle
How can a toss be called over the phone (without requiring trust)?

3. 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

4. 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

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

6. 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

7. 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

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

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

10. Congestion Control Alternative: Fall to W/2 and start
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

11. 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

12. 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)

13. Sequencing Starting sequence number randomly chosen during connection3 1 2 4
Byte sequence numbers
TCP receiver buffers out of order segments and reassembles them later
Starting sequence number
randomly chosen
during connection
establishment
Why?
1 given to app
2 given to app
Loss
4 buffered (not given to app)
3 & 4 given to app
4 discarded

14. 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

15. 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 + intelligent fast recovery
TCP-Vegas, TCP-SACK

16. 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

17. 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

18. 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

19. TCP SACK
TCP (Tahoe, Reno, and New-Reno) uses cumulative acknowledgements
When there are multiple losses, TCP Reno and New-Reno can retransmit only one lost packet per round-trip time
What about TCP-Tahoe?
SACK enables receiver to give more information to sender about received packets allowing sender to recover from multiple-packet losses faster

20. TCP SACK (Example) Assume packets 5-25 are transmitted
Let packets 5, 12, and 18 be lost
Receiver sends back a CACK=5, and SACK=(6-11,13-17,19-25)
Sender knows that packets 5, 12, and 18 are lost and retransmits them immediately

21. Other TCP flavors TCP Vegas TCP FACK
Uses round-trip time as an early-congestion-feedback mechanism
Reduces losses
TCP FACK
Intelligently uses TCP SACK information to optimize the fast recovery mechanism further

22. 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

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

24. Recap TCP Connection management Reliability Flow control
Congestion control
TCP flavors
UDP

25. Puzzle You have a chain consisting of 63 inter-linked gold links
You have to stay at a motel where the charge per day is 1 gold link
You do not trust the motel manager and neither does he trust you
What is the minimum number of links you need to break in order to stay for 63 days?
Clue: The manager will not sell the links till you check out