1. Introduction to Computer Networks
Transport Layer
University of ilam
Dr. Mozafar Bag-Mohammadi

2. Outline Connection Establishment/Termination Sliding Window Revisited
Flow Control
Adaptive Timeout

3. End-to-End Protocols Underlying best-effort network
drop messages
re-orders messages
delivers duplicate copies of a given message
limits messages to some finite size
delivers messages after an arbitrarily long delay
Common end-to-end services
guarantee message delivery
deliver messages in the same order they are sent
deliver at most one copy of each message
support arbitrarily large messages
support synchronization
allow the receiver to flow control the sender
support multiple application processes on each host

4. Transport Layer Function(s)
Multiplexing/demultiplexing between network application processes.
Others?
Error Detection within a segment
Reliability
Flow Control.
Congestion control.
Connection Management.
Difference between Error detection and reliability?
Difference between flow control and congestion control?

5. Simple Demultiplexor (UDP)
Unreliable and unordered datagram service
Adds multiplexing
No flow control
Endpoints identified by ports
servers have well-known ports
see /etc/services on Unix
Header format
Optional checksum
pseudo header + UDP header + data 16 31
SrcPort
DstPort
Checksum
Length
Data

6. Using UDP Non-standard protocols can be implemented on top of UDP.
» Non-standard = non-TCP in practice
» use the port addressing provided by UDP
» implement their own reliability, flow control, ordering, congestion control
? Examples:
» remote procedure calls
» multimedia
» distributed computing communication libraries

7. TCP Overview Full duplex
Flow control: keep sender from overrunning receiver
Congestion control: keep sender from overrunning network
Connection-oriented
Byte-stream
app writes bytes
TCP sends segments
app reads bytes
Application process
Write
bytes
TCP
Send buffer
Segment T
ransmit segments
Read
Receive buffer …

8. High level TCP Characteristics
Connection-oriented reliable byte-stream protocol.
» Used for file transfers, telnet, web access, ….
Two way connections.
» control information for one direction piggy-backed on data flow in other direction
» header fields fall in three classes: general, forward flow, opposite flow
Protocol has evolved over time and will continue to do so.
» Nearly impossible to change the header
» Uses options to add information to the header
» Change processing at endpoints
» Backward compatibility is what makes it TCP

9. Data Link Versus Transport
Potentially connects many different hosts (logical connection)
need explicit connection establishment and termination
Potentially different RTT
need adaptive timeout mechanism
Potentially reordering packets. (How far?), Maximum segment life time. Currently 120 sec.
Potentially long delay in network
need to be prepared for arrival of very old packets
Delay X bandwidth?
Buffer size

10. Data Link Versus Transport (cont)
Potentially different capacity at destination
need to accommodate different node capacity
Potentially different network capacity
need to be prepared for network congestion

11. Data delivery in TCP
TCP is byte oriented, however transmit data in segments (messages).
How TCP knows the time for delivery?
Maximum Segment Size (MSS)
Pushed by the application. Like telnet.
Timer

12. Segment Format

13. Segment Format (cont) Each connection identified with 4-tuple:
(SrcPort, SrcIPAddr, DsrPort, DstIPAddr)
Sliding window + flow control
acknowledgment, SequenceNum, AdvertisedWinow
Checksum
pseudo header + TCP header + data
Sender
Data
(SequenceNum)
Acknowledgment +
AdvertisedWindow
Receiver

14. Segment Format (cont) Flags SYN, FIN, RESET, PUSH, URG, ACK
SYN and FIN for establish and tear down connection.
ACK indicates the Ack field is valid.
URG shows some part of data is urgent.(Up to UrgPtr).
PUSH shows the sender had push operation.
RESET shows confusion in receiver.

15. Connection Establishment and Termination
Active participant
Passive participant
(client)
(server)
Client does active connection.
Server must do passive
connection first.
SYN, SequenceNum = x y , 1
SYN + ACK, SequenceNum = x +
Closing is symmetric, both
side must tear down the
connection.
Acknowledgment =
ACK, Acknowledgment = y + 1
Three way handshaking.
Connection parameter are exchanged first.
Acknowledgment shows the next expected sequence number.

16. State Transition Diagram
CLOSED
LISTEN
SYN_RCVD
SYN_SENT
ESTABLISHED
CLOSE_WAIT
LAST_ACK
CLOSING
TIME_WAIT
FIN_WAIT_2
FIN_WAIT_1
Passive open
Close
Send/
SYN
SYN/SYN + ACK
SYN + ACK/ACK
ACK
/FIN
FIN/ACK
ACK + FIN/ACK
Timeout after two
segment lifetimes
Active open
/SYN

17. State Transition (cont)
Sliding window is in ESTABLISHED.
All connections start with CLOSED.
Arcs are tagged with event/action.
Triggering a transition.
Arriving a segment from a peer.
Envoking by an application.

18. Sliding Window Revisited
Sending application
LastByteWritten
TCP
LastByteSent
LastByteAcked
Receiving application
LastByteRead
LastByteRcvd
NextByteExpected
Sending side
LastByteAcked < = LastByteSent
LastByteSent < = LastByteWritten
buffer bytes between LastByteAcked and LastByteWritten
Receiving side
LastByteRead < NextByteExpected
NextByteExpected < = LastByteRcvd +1
buffer bytes between LastByteRead and LastByteRcvd

19. Flow Control Send buffer size: MaxSendBuffer
Receive buffer size: MaxRcvBuffer
Receiving side
LastByteRcvd - LastByteRead < = MaxRcvBuffer
AdvertisedWindow = MaxRcvBuffer - (NextByteExpected LastByteRead)
Sending side
LastByteSent - LastByteAcked < = AdvertisedWindow
EffectiveWindow = AdvertisedWindow - (LastByteSent LastByteAcked)
LastByteWritten - LastByteAcked < = MaxSendBuffer
block sender if (LastByteWritten - LastByteAcked) + y > MaxSenderBuffer
Always send ACK in response to arriving data segment
Persist when AdvertisedWindow = 0

20. Protection Against Wrap Around
32-bit SequenceNum
Bandwidth Time Until Wrap Around
T1 (1.5 Mbps) 6.4 hours
Ethernet (10 Mbps) 57 minutes
T3 (45 Mbps) 13 minutes
FDDI (100 Mbps) 6 minutes
STS-3 (155 Mbps) 4 minutes
STS-12 (622 Mbps) 55 seconds
STS-24 (1.2 Gbps) 28 seconds

21. Keeping the Pipe Full 16-bit AdvertisedWindow
Bandwidth Delay x Bandwidth Product
T1 (1.5 Mbps) 18KB
Ethernet (10 Mbps) 122KB
T3 (45 Mbps) 549KB
FDDI (100 Mbps) 1.2MB
STS-3 (155 Mbps) 1.8MB
STS-12 (622 Mbps) 7.4MB
STS-24 (1.2 Gbps) 14.8MB

22. TCP Extensions Implemented as header options
Store timestamp in outgoing segments
Extend sequence space with 32-bit timestamp (PAWS)
Shift (scale) advertised window

23. Adaptive Retransmission (Original Algorithm)
Measure SampleRTT for each segment/ ACK pair
Compute weighted average of RTT
EstRTT = a x EstRTT + b x SampleRTT
where a + b = 1
a between 0.8 and 0.9
b between 0.1 and 0.2
Set timeout based on EstRTT
TimeOut = 2 x EstRTT

24. Karn/Partridge Algorithm
Sender
Receiver
Sender
Receiver
Original transmission
Original transmission TT TT
ACK
Retransmission
SampleR
SampleR
Retransmission
ACK
Do not sample RTT when retransmitting
Double timeout after each retransmission

25. Jacobson/ Karels Algorithm
New Calculations for average RTT
Diff = SampleRTT - EstRTT
EstRTT = EstRTT + (d x Diff)
Dev = Dev + d( |Diff| - Dev)
where d is a factor between 0 and 1
Consider variance when setting timeout value
TimeOut = m x EstRTT + f x Dev
where m = 1 and f = 4
Notes
algorithm only as good as granularity of clock (500ms on Unix)
accurate timeout mechanism important to congestion control (later)