1. Computer Networks: Transmission Control Protocol (TCP)
Ivan Marsic
Rutgers University
Chapter 2 - TCP

2. Transmission Control Protocol (TCP)
Chapter 2

3. UDP Header

4. Topic: TCP Protocol  TCP Packet (“Segment”) Format
 Setting Retransmission Timers
 Flow Control

5. TCP in the Protocol Stack
End-to-end protocol
Most popular protocol in the Internet (together with IP, “TCP/IP protocol suite”

6. Transmission Control Protocol (TCP)
Notion of a “session” or “connection”, so TCP is said to be a connection-oriented protocol
TCP “Quality of Service”: Assured delivery
Congestion avoidance helps improve throughput

7. TCP Session
Continuous stream of bytes becomes available for application to use when a connection is established and disappears when the connection is closed

8. Encapsulation of TCP Data in IP Datagrams

9. TCP Header

10. Recall the IPv4 Packet Header10

11. Urgent Data Pointer
Last byte of urgent data (LBUD) = sequenceNumber+ urgentPointer
First byte of urgent data never explicitly defined
Any data in Receive buffer up to LBUD may be considered urgent

12. Retransmission Timer Calculation
Search the Web for RTT distribution measurement

13. Retransmission Timer Calculation Exponential Weighted Moving Average (EWMA)
Timeout Interval, set as +4 : TimeoutInterval(t) = EstimatedRTT(t) + 4  DevRTT(t)
Estimated mean value of RTT: EstimatedRTT(t) = (1)  EstimatedRTT(t  1) +   SampleRTT(t)
Estimated current standard deviation of RTT: DevRTT(t) = (1)  DevRTT(t  1) +   SampleRTT(t)  EstimatedRTT(t  1)
The initial value is set as: DevRTT(0) = 1/2 SampleRTT(0) for the first RTT measurement
Recommended values of the control parameters are  = and  = 0.25

14. Retransmission Timer Management (from Listing 2-1)
In method TCPSender.send(), Listing 2-1 // called by the application layer from above
in Line 24:
if (RTO timer not already running) {
set the RTO timer to the current value
as calculated in methods handle() and RTOtimeout();
start the timer; }
In method TCPSender.handle(), Listing 2-1 // called by IP layer when ACK arrives
in Lines :
if ((lastByteAcked < lastByteSent) {
calculate the new value of the RTO timer using Eq. (2.2);
re-start the RTO timer;
In method TCPSender.RTOtimeout(), Listing 2-1 // called when RTO timeout
in Line 43:
double the TimeoutInterval; // exponential backoff , Eq. (2.1)

15. TCP Connection Establishment
Visit to learn about network sockets

16. Example TCP Session

17. Flow vs. Congestion Control

18. Topic: TCP Congestion Control
 TCP Tahoe
 TCP Reno
 TCP NewReno 18

19. The Problem TCP is a window-based protocol
Window size represents the amount of resources available for a communication session
It limits the number of outstanding (unacknowledged) packets
However, unlike a simple ARQ (Chapter 1), “resources” include both receiver resources and network resources
How to know the available network resources?

20. Network Conceptual Model
Many sources and many receivers …
(a)
We don’t know when sources will start/end their sessions; also their datarates are variable

21. Simplified Network Model
The entire network is abstracted as a single router – “black box”
Receiver resources
Represented by
“Receive Window Size”
(b)
Network resources
Represented by “Congestion Window Size” 21

22. TCP Buffer Parameters

23. TCP Congestion Control Summary
Sender starts with a small window size
Send a “burst” of packets (size of the current sender window) into the network
Wait for a feedback about success rate (acknowledgements from the receiver end)
When the (positive/negative) feedback arrives:
+ If the success rate is greater than zero, increase the sender window size and go to Step 2
– If loss is detected, decrease the sender window size and go to Step 2

24. TCP Congestion Control Param’s
Variable
Definition
MSS
The maximum segment size that the sender can transmit. MSS does not include the TCP/IP headers and options—only the data payload. This value can be based on the maximum transmission unit (MTU) of the first link, the path MTU discovery algorithm, or other factors. By setting MSS to the path MTU, the sender may avoid packet fragmentation (Section ‎1.4.1), although this is difficult to achieve because routers change routes dynamically. [Note that RFC-2581 distinguishes the sender maximum segment size (SMSS) and the receiver maximum segment size (RMSS).]
RcvWindow
The size of the most recently advertised receiver window.
CongWindow
Sender’s current estimate of the available buffer space in the bottleneck router.
LastByteAcked
The highest sequence number currently acknowledged.
LastByteSent
The sequence number of the last byte the sender sent.
FlightSize
The amount of data that the sender has sent, but not yet had acknowledged.
EffectiveWindow
The maximum amount of data that the sender is currently allowed to send. At any given time, the sender must not send data with a sequence number higher than the sum of the highest acknowledged sequence number and the minimum of CongWindow and RcvWindow.
SSThresh
The slow start threshold used by the sender to decide whether to employ the slow-start or congestion-avoidance algorithm to control data transmission. The slow start algorithm is used when CongWindow < SSThresh, while the congestion avoidance algorithm is used when CongWindow > SSThresh. When CongWindow and SSThresh are equal the sender may use either slow start or congestion avoidance.

25. TCP Sender State Diagram
“Send data” 
1. Set CongWin & SSThresh
2. Send EfctWin of data, if any
3. Re-start RTO timer
“Retransmit” 
1. Set CongWin & SSThresh
2. Re-send oldest outstanding segment
3. Double the RTO timer & re-start it

26. TCP Receiver State Diagram

27. How Much Sender Can Send
The sender must always ensure that: LastByteSent  LastByteAcked  min{CongWindow, RcvWindow}
 the amount of unacknowledged data should never exceed: FlightSize = LastByteSent  LastByteAcked  min{CongWindow, RcvWindow}
TCP session invariant (“allowed to send”): EffectiveWindow = min{CongWindow, RcvWindow}  FlightSize

28. How TCP Sender Detects Segment Loss
Retransmission timer (RTO) expiration
Reception of three duplicate ACKs (four identical ACKs without the arrival of any other intervening packets) Why three dupACKs:
If there is just a reordering of the segments, there will be only one or two dupACKs before the reordered segment is processed, which will then generate a regular ACK.
If three or more dupACKs are received in a row, it is a strong indication that a segment has been lost.

29. How TCP Sender Detects Segment Loss
Event
TCP Version
TCP Sender’s Action
Timeout
Tahoe
Set CongWindow = 1MSS
Reno 
3dup ACKs
Set CongWindow = max {½ FlightSize, 2MSS} + 3MSS

30. Slow Start Threshold
Determines when the “slow start” phase ends and the “congestion avoidance” phase begins
SSThresh = max {½ FlightSize, 2MSS}

31. TCP Tahoe Sender (Effective Window computed as before …)
When packet loss detected via 3  dupACKs:
SSThresh(t) = max {½ CongWindow(t1), 2MSS} (2.4)
Congestion Window computation
under Congestion Avoidance:
Congestion Window computation
under Congestion Avoidance:
[bytes] (2.5)

32. TCP-Tahoe Sender State Diagram
“Send data” 
1. Set CongWin & SSThresh
2. Send EfctWin of data, if any
3. Re-start RTO timer
“Retransmit” 
1. Set CongWin & SSThresh
2. Re-send oldest outstanding segment
3. Double the RTO timer & re-start it

33. TCP-Reno Sender State Diagram

34. Example 2.1: TCP Bottleneck Link

35. Detail of Transmission Round #5

37. RTO Timer Timeout Pattern
Move 8 UP

38. TCP Tahoe (2)
MSS = 1024 bytes, RcvWindow = 64 KB

39. TCP Tahoe (3)

40. TCP Sender States / Phases
Slow Start:
Defined by: Congestion Window (CW)  SSThresh
Congestion Window calculation:
CW(t) = CW(t – 1) + #ACKMSS
Fast Retransmit:
Defined as: When 3  dupACKs received,
retransmit the oldest outstanding packet without waiting for RTO timeout
Congestion Avoidance:
Defined by: Congestion Window (CW) > SSThresh
Congestion Window calculation:
CW(t) = CW(t – 1) + MSS
#ACKMSS
CW(t – 1)

41. Reno-Specific State / Phase
Same as for Tahoe:
Slow Start, Fast Retransmit, Congestion Avoidance
Fast Recovery:
Defined as: When 3  dupACKs received,
Congestion Window calculation:
CW(t) = max {½ FlightSize(t), 2MSS} + 3MSS
( Unlike TCP Tahoe sender,
which sets CW(t) = 1MSS
when 3  dupACKs received )

42. TCP Reno Sender (Effective Window computed as before …)
When packet loss detected via 3  dupACKs:
SSThresh(t) = max {½ FlightSize(t1), 2MSS} (2.6)
CongWindow(t) = SSThresh(t) + 3MSS (2.7)
Enter Fast Recovery: count each “old ACK” as +MSS to CongWin.
When “new ACK” received, exit Fast Recovery and enter Congestion Avoidance.
CongWindow = SSThresh (2.8)

43. TCP Reno (1)

44. TCP Reno (2)

45. Topic: TCP NewReno Protocol
 Partial ACKs for Pkt Loss Detection
 Example 45

46. How TCP Sender Discovers Packet Loss
Retransmission timer (RTO) expired
 3  dupACKs received
Partial ACK received
TCP Tahoe
&
TCP Reno
TCP NewReno

47. TCP NewReno – Partial ACKs
Defined at the time tloss when the sender discovers a packet loss (by 3  dupACKs):
Full acknowledgment: If the sender will receive an ACK for all packets that were outstanding at the time tloss
Partial acknowledgment: If the sender will receive an ACK for some of the packets that were outstanding at the time tloss

48. TCP NewReno tloss At this time define:
Full acknowledgment = ACK for all 6 outstanding pkts

49. TCP NewReno Fast Recovery
NewReno sender enters Fast Recovery when a loss is detected (by 3  dupACKs)
NewReno sender exits Fast Recovery when it receives the full acknowledgement (defined at time tloss)
After Fast Recovery, NewReno sender enters Congestion Avoidance

50. Congestion Window Calculation during Fast Recovery
NewlyAcked = LastByteAcked(t)  LastByteAcked(t  1)
If (NewlyAcked < MSS), then CongWindow(t) = CongWindow(t  1)  NewlyAcked
Else if (NewlyAcked  MSS), then CongWindow(t) = CongWindow(t  1)  NewlyAcked + MSS

51. TCP NewReno: Example (1)
txmit (Link 1) << txmit (Link 2)
tprop (Link 1) << tprop (Link 2)
tprop (Link 2) = 6  txmit (Link 2)

52. TCP NewReno: Example (2)52

53. Slow Start Fast Retransmit Fast Recovery
Neither in router nor acknowledged; black boxes symbolize packets that are sent out of order, for which the preceding packet was dropped at the router (due to the lack of router memory space)

54. Fast Recovery (cont’d)
Congestion Avoidance

55. Topic: TCP Over Wireless
 Reducing Link-Layer Overheads

56. TCP Over Wireless
Slow start until reaches the SSThresh size; set timer for every burst
If no ACK  congestion; halve the window size; slow start again

57. Wireless Link-layer Overhead