1. Chapter 17. Transport Protocols
Transport Services
Protocol Mechanisms
TCP
UDP

2. Transport Protocols MIME BGP FTP HTTP SMTP TELNET SNMP TCP UDP ICMP
OSPF IP

3. Transport Services Type of service Quality of service Data transfer
connection-oriented, connectionless
Quality of service
Acceptable error and loss levels
Desired average and maximum delay
Desired average and minimum throughput
Priority levels
Data transfer
Full-duplex service

4. Transport Services (cont)
User Interface
Connection management
Expedited delivery
Used to transfer occasional urgent data, such as break character or an alarm condition
Status reporting
Performance characteristics of a connection
Addresses (network, transport), etc.
Security

5. Protocol Mechanisms Reliable sequencing network service
Highly reliable packet-switching network with an X.25 interface
Frame relay using LAPF control protocol
IEEE LAN using the connection-oriented LLC service
Allows using a quite simple transport protocol
Issues need to be addressed
Addressing, Multiplexing, Flow control,
Connection establishment/termination

6. Reliable Sequencing Net. Service
Addressing
User identification, Transport entity identification
Station address, Network number
Multiplexing
by either port numbers or service access points
Flow control
The receiving transport entity can:
1. Do nothing

7. Reliable Seq. Net. Service (cont)
2. Refuse to accept further segments from the network service
Backpressure mechanism that relies on the network service to do the work
3. Use a fixed sliding-window protocol
As in link layer protocols, but no need for retx
The use of acknowledgements to advance the window
4. Use a credit scheme
A segment may be acknowledged without granting new credit

8. Credit Allocation Mechanism

9. Sending & Receiving Flow Control

10. Reliable Seq. Net. Service (cont)
Connection
setup
2-way
handshake

11. Reliable Seq. Net. Service (cont)

12. Unreliable Network Service
Examples
An internetwork using IP
Frame relay using only the LAPF core protocol
IEEE LAN using the unacknowledged connectionless LLC service
Issues to be addressed
Ordered delivery, Retransmission strategy
Duplicate detection, Flow control
Connection establishment/termination
Crash recovery

13. Transport Protocol Timers

14. Unreliable Network Service (cont)
E.g. incorrect duplicate detection
seq. space = 1600, win size = 600
Sender must not get confused if it receives multiple Ack to the same segment
Sequence number space must be long enough 1 2

15. Unreliable Network Service (cont)
Begin seq. number = 1
Solution: start each new connection with a different seq. # that is far removed from the last seq. #.
Using SYN i ?

16. Unreliable Network Service (cont)

17. E.g. Three-way Handshake

18. E.g. Three-way Handshake (cont)

19. TCP Entity State Diagram

20. TCP Entity State Diagram (cont)

21. Transmission Control Protocol
Two using facilities for labeling data
Data stream push
Ordinarily, TCP decides when sufficient data have accumulated to form a segment for transmission
TCP user can require TCP to transmit all outstanding data up to and including those labeled with a push flag
Urgent data signaling
Provides a means of informing the destination TCP user that significant or “urgent” data is in the upcoming data stream

22. TCP Service Request Primitives

23. TCP Service Response Primitives

24. TCP Header Format

25. Scope of Checksum for TCP/UDP

26. TCP Implementation Policy Options
Send policy
Deliver policy
Accept policy
In-order, In-window
Retransmit policy
First-only, Batch, Individual
Acknowledge policy
Immediate, Cumulative

27. TCP Congestion Control
Retransmission Timer Management
Simple Average
Exponential Average
RTT Variance Estimation (Jacobson’s Algorithm)
Exponential RTO Backoff
Karn’s Algorithm
Window Management
Slow start
Dynamic Window Sizing on Congestion

28. TCP Congestion Control (cont)
Simple Average
RTT(i): round-trip time observed for the ith transmitted segment
ARTT(K): average round-trip time for the first K segments or

29. TCP Congestion Control (cont)
Exponential Average
SRTT: smoothed round-trip time estimate
RTO: retransmission timer
RFC793:
Example values: a: 0.8 ~ 0.9, b: 1.3 ~ 2.0

30. Exponential Average

31. Exponential Average (cont)

32. RTT Variance Estimation
AERR(K): sample mean deviation measured at time K

33. RTT Variance Estimation (cont)
Jacobson’s Algorithm
g = 1/8 = 0.125, h = ¼ = 0.25, f = 2

34. Jacobson’s RTO

35. Jacobson’s RTO (cont)

36. Exponential RTO Backoff
TCP source increases its RTO each time the same segment is retransmitted; this is referred to as a backoff process
Multiply the RTO for a segment by a constant value for each retransmission
RTO = q x RTO
Most commonly used value of q is 2
Binary exponential backoff (the same as in CSMA/CD)

37. Karn’s Algorithm
Do not use the measured RTT for a retransmitted segment to update SRTT and SDEV
Using the backoff RTO when a retransmission occurs
Use the backoff RTO value for succeeding segments until an acknowledgement arrives for a segment that has not been retransmitted

38. Slow Start awnd = MIN [credit, cwnd] When a new connection is opened
awnd: allowed window, in segments
cwnd: congestion window, in segments
credit: the amount of unused credit granted in the most recent acknowledgement, in segments
When a new connection is opened
TCP entity initialized cwnd = 1
Each time an acknowledgement is received, the value of cwnd is increased by 1, up to some maximum value

39. Dynamic Window Sizing
Slow-start works effectively for initializing a connection
It enables the TCP sender to determine quickly a reasonable window size for the connection
When there is a surge in congestion?
That is, when a segment is lost (timeout)
Prudent procedure: reset cwnd = 1 and begin the slow-start process all over
Jacobson points out that “it is easy to drive a network into saturation but hard for the net to recover”
Thus, exponential growth of cwnd under slow start may be too aggressive  the use of slow-start to begin with, followed by a linear growth in cwnd

40. Dynamic Window Sizing (cont)
ssthresh = cwnd/2

41. User Datagram Protocol
optional