1. - Reliable Stream Transport Service
TCP (Part 1)
- Reliable Stream Transport Service
D.E. Comer “Internetworking with TCP/IP: Principles, Protocols and Architectures”, Ch. 13, Prentice Hall, 2000
Presented by Ming Su

2. Content TCP Introduction Issue in TCP --- Flow Control Summary
Sliding window protocol
TCP acknowledgement scheme
TCP segment
TCP timeout
Summary

3. TCP Introduction

4. Introduction - TCP TCP Purpose Transmission Control Protocol
TCP is not a software
Purpose
Providing reliable stream delivery
Isolating application programs from the details of networking

5. TCP Conceptual Layering
Application
Reliable Stream (TCP)
User Datagram (UDP)
Internet (IP), ICMP
Network Interface

6. Properties of the reliable delivery service
Stream Orientation
data as a stream of bits, divided into 8-bit octets
Virtual Circuit Connection
Buffered Transfer
transfer more efficiently & minimize network traffic
Unstructured Stream
Full Duplex Connection
Two-way connection
Unstructured stream: application must understand stream content and agree on stream format before they initiate a connection.

7. Issues in TCP

8. Two issues in TCP Flow Control Virtual Circuit Connection
End-to-end flow control problem
Congestion control problem
Virtual Circuit Connection
Congestion: router overload. There is trouble to route.

9. Flow Control - Sliding window protocol

10. End-to-end flow control
Problem
Sender can send more traffic that receiver can handle. (Too fast)
Solution
variable sliding window protocol
each acknowledgement, which specifies how many octets have been received, contains a window advertisement that specifies how many additional octets receiver are prepared to accept.
Solution: sliding window protocol. TCP: variable sliding window protocol

11. Transmitter Window Size Free space in buffer to fill
Variable Window Size … …
Window Advertisement
Receiver
Transmitter
Transmitter Window Size
Value of
Window Advertisement
Free space in buffer to fill
increase
bigger
decrease
smaller
Stop transmissions
full

12. Sliding window protocol in TCP
TCP allows the window size to vary over time.
Window size changes at the time it slides forward.
Advantage: it provides flow control as well as reliable transfer.

13. Flow Control - TCP acknowledgement scheme

14. Acknowledgements and Retransmission
Cumulative acknowledgement scheme is used in TCP.
A TCP ACK specifies “the sequence number of the next octet that the receiver expects to receive”.
The scheme is called cumulative 渐增的;累加的 because it reports how much of the stream has accumulated. Go-Back-N protocols which retransmit all packets that follow a single erred packet. Selective Retransmission protocols which only retransmit erred packets using ACKs and NAKs (TCP).

15. Acknowledgements and Retransmission
Adv.
Easy to generate and unambiguous.
The lost ACKs don’t force retransmission.
Disadv.
No information about all successful transmissions for the sender, but only a single position in the stream.
ACK for p position loses, but the sender receive ACK for p+1 position, then sender doesn’t resend p.
Disadv.: sends 5 segments, only first one loses. ACK only for 1st . Sender doesn’t know about 2nd to 5th segments successful.

16. Flow Control - TCP segment

17. TCP segment format Source Port Destination Port Sequence Number4 16 24 31
Source Port
Destination Port
Sequence Number
Acknowledge Number
HLEN
Reserved
Code Bits
Window
Standard MSS = 536
IP datagram standard = 576
This allows 20 bytes for the IP header and the TCP header.
HLEN: length of the segment header. The Options field varies in length.
Sequence numbers go up to at least 2^{32}
Code bits to determine the purpose and contents of the segment.
Window: window advertisement.
Checksum
Urgent Pointer
Options (if any)
Padding
Data …

18. Flow Control - TCP timeout

19. Timeout and Retransmission
Question
How to determine timeout?
Is the timeout always a constant?
A plot of internet round trip times as measured for 100 successive IP datagrams.

20. Timeout and Retransmission
An adaptive retransmission algorithm is used in TCP.
TCP monitors the performance of each connection and adjust its timeout parameter accordingly
Timeout value may change.
Timeout is adjusted when a new round trip sample ( RTT ) is obtained.

21. Timeout and Retransmission
RTT =
(α * Old_RTT) + ((1 – α) * new_RTT_sample )
0 < α < 1
α close to 1 => no change in a short time
α close to 0 => RTT changes too quickly
Timeout = β * RTT
β >1
Recommended setting, β= 2
Immune:免疫. α , βare constant weighting factor. If β=1, any small delay cause an unnecessary retransmission.

22. Accurate Measurement of Round Trip Samples
TCP acknowledgements are ambiguous.
It is caused by the cumulative acknowledgement scheme.
It happens when retransmission.
It causes the question that the first received ACK does correspond the original datagram or the retransmitted datagram.
RTT couldn’t be measured accurately if the above question cannot be answered.
How to do?
Cumulative ack. Refers to data received, and not to the instance of a specific datagram that carried the data. When time out, after resend, the first received ack corresponds the original or retransmitted datagram?

23. Accurate Measurement of Round Trip Samples --- Karn’s Algorithm and Timer Backoff
Karn’s algorithm: when computing the round trip estimate, ignore samples that correspond to retransmitted segments, but use a back-off strategy, and retain the timeout value from a retransmitted packet for subsequent packets until a valid sample is obtained.
Timer back-off strategy:
New_timeout = γ * timeout ( typically, γ =2 )
Each time timer expires (retransmit happens), TCP increases timeout value.

24. Karn’s Algorithm Use RTT to compute Timeout
When retransmission happens, Timeout increases in γtimes continuously, until transfer successfully. [Backoff strategy]
Use the timeout in the final turn of the last step to send next segment. [Backoff strategy]
When an acknowledgement arrives corresponding to a segment that did not require retransmission, then TCP re-computes the RTT and reset the timeout accordingly

25. Responding to High Variance in Delay
Queuing theory: variation in round trip delay is proportional to 1/(1-L), where L is the current network load, 0<L<1
( Timeout = β * RTT ) & (β=2 ) => L < 30%
Not efficient
1989 TCP specification requires to use estimated variance in place of β
E ∞ 1/(1-L) E=10 => L=80%; E=4 => L=50%;

26. New RTT and Timeout Algorithm
DIFF = sample – old_RTT
Smoothed_RTT = old_RTT + d * DIFF
DEV = old_DEV + p (|DIFF| - old_DEV)
Timeout = Smoothed_RTT + g * DEV
DEV estimated mean deviation
d, a fraction between 0 and 1 to control how quickly the new sample affects the weighted average
p, a fraction between 0 and 1 to control how quickly the new sample affects mean deviation
g, a factor controls how much deviation affects round trip timeout
Research suggests: d=1/8, p=1/4 and g=4

27. Summary Purpose of TCP End-end Flow Control issue
Variable Sliding window protocol in TCP
TCP acknowledgement scheme
TCP segment
TCP timeout
RTT-Timeout Calculation and Karn’s Algorithm
New RTT and Timeout Algorithm

28. Thanks