1. William Stallings Data and Computer Communications
Chapter 17
Transport Protocols

2. Transport Protocol - Summary
Provides end-to-end data transfer
Shields upper layer application protocols from the details of networks
TCP: complicated flow and error control since the underlying network service (IP) is unreliable
TCP is connection oriented
UDP is connectionless

3. Connection Oriented Transport Protocol Mechanisms
Logical connections between end users
Establishment
Maintenance
Termination
Reliable service
e.g. TCP

4. Connection Oriented Transport Protocol Mechanisms
TCP is complex because of IP, which is unreliable
Let’s assume the underlying network service is reliable (for simplicity)
frame relay (LAPF control protocol)
802.3 LAN with connection oriented LLC

5. Flow Control
Transport level flow control is more difficult than link-level one
transmission delay is variable due to network. That makes difficult to use timeouts
Recall from link level flow control
Backpressure
clumsy and coarse grained
Sliding window protocols
Works well on reliable network
Failure to receive ACK is taken as flow control indication
Does not work well on unreliable network
Can not distinguish between lost segment and flow control
Credit scheme

6. Credit Scheme Decouples flow control from ACK
May ACK without granting credit and vice versa
Each octet has a sequence number
Each transport segment has a header that contains
sequence number
ack number
window size

7. Use of Header Fields When sending ACK includes AN=i, W=j
seq. number (SN) of first octet in segment is included
ACK includes AN=i, W=j
All octets through SN=i-1 are acknowledged
Next expected octet is i
Permission to send additional window of W=j octets
i.e. octets through i+j-1

8. Credit Allocation (Example)

9. Connection Establishment and Termination
Necessary even with a reliable network services
Purposes
Allow each end to know the other exists
Negotiation of optional parameters
Triggers allocation of transport entity resources
Connection establishment is by mutual agreement
control messages are exchanged

10. Connection State Diagram

11. Connection Establishment

12. Not Listening
What happens if SYN comes when the receiver is in CLOSED state
3 options
Reject with RST (Reset) command
Request is queued until matching OPEN is issued
Notify upper layer about the pending request

13. Termination Either side may initiate termination By mutual agreement
Graceful termination
FIN WAIT state must accept incoming data until FIN received

14. Side Initiating Termination
TS user issues Close request
Transport entity sends FIN, requesting termination
Connection placed in FIN WAIT state
Continue to accept data and deliver data to user
Do not send any more data
When FIN received, inform user and close connection

15. Side Not Initiating Termination
FIN received
Inform TS user and place connection in CLOSE WAIT state
Continue to transmit data
TS user issues CLOSE primitive
Transport entity sends FIN
Connection closed
This procedure ensures that
both sides received all outstanding data
both sides agree to terminate

16. Unreliable Network Service
E.g.
internet using IP,
frame relay using LAPF core protocol
IEEE using unacknowledged connectionless LLC
Segments may get lost
Segments may arrive out of order
variable transit delays
Solutions will create other problems

17. Problems Ordered Delivery Retransmission strategy
Duplication detection
Flow control
Connection establishment
Connection termination

18. Ordered Delivery Segments may arrive out of order
General solution: number data units sequentially
TCP numbers each octet sequentially
implicit numbering
Segments are numbered by the first octet number in the segment

19. Retransmission Strategy
Segment may be damaged while in transit
Segment may fail to arrive
Positive acknowledgment: receiver must acknowledge successful receipt
Cumulative acknowledgment can be used
several segments can be acknowledged in one ack message
Waiting ACK for a long period of time (timeout period) triggers re-transmission

20. Timer Value Fixed timer Adaptive scheme: average round-trip delay
Based on typical network behavior
Can not adapt to changing network conditions
“Too small” leads to unnecessary re-transmissions
“Too large” causes slow response to lost segments
Should be a bit longer than round trip time (but this is not fixed)
Adaptive scheme: average round-trip delay
May not ACK immediately (cumulative ack)
Conditions may change suddenly
No complete solution

21. Duplication Detection
If ACK lost, segments are re-transmitted
Receiver must recognize duplicates
Duplicate may arrive before the original one
Duplicate received prior to closing connection
Receiver assumes that ACK is lost and send ACK for the duplicate
Sender must not get confused with multiple ACKs for the same segment
Sequence number space large enough to not cycle within maximum life of segment
Duplicate received after closing connection

22. Incorrect Duplicate Detection
Sequence space is 1600
Sliding window with window size of 600

23. Flow Control Credit allocation scheme is quite robust and flexible
it is possible to increase credit without ack
after (AN = i, W = j), (AN = i, W = k), k >j
it is possible to ack without extra credit
after (AN = i, W = j), (AN =i + m, W = j - m), m: acked segment length
Lost ACK/CREDIT is generally no problem
future acks resynchronize the protocol
lost ack causes timeout and retransmission
retransmission triggers ack
but deadlock is still possible

24. Flow Control Possible deadlock case
receiver temporarily closes window with AN=i, W=0
later reopens with AN=i, W=j, but this is lost
Sender thinks window is still closed, receiver thinks it is open
SOLUTION: use window timer
timer for ACK/CREDIT segments
timer expires if no ACK/CREDIT segments are received within the timeout period
if timer expires, retransmit the previous ACK/CREDIT segment

25. Connection Establishment
Two way handshake
A send SYN, B replies with SYN
Lost SYN handled by re-transmission (special timer)
Can lead to duplicate SYNs
Ignore duplicate SYNs once connected
Lost or delayed data segments can cause connection problems
Segment from old connections (see next figure)

26. Two Way Handshake: Obsolete Data Segment

27. Solution to Obsolete Data Segment Problem
First segment number of the new connection must be distant from the last segment number of the previous connection
Need to specify the expected sequence numbers of the connections
Use SYN i
i is the sequence number of the first segment on that connection
acknowledged by SYN j
New Problem: Obsolete SYN (see next figure)

28. Two Way Handshake: Obsolete SYN Segment

29. Solution to Obsolete SYN Problem
Acknowledgments should also include the request’s SYN number (AN=i)
Three Way Handshake
Second ack is actually a data segment

30. Three Way Handshake: State Diagram

31. Connection Termination
Entity in CLOSE WAIT state sends last data segment, followed by FIN
FIN arrives before last data segment
Receiver accepts FIN
Closes connection
Loses last data segment
Add sequence number (number just after the last transmitted octet) to FIN
Receiver waits for all segments before FIN sequence number

32. Connection Termination
Graceful close
Send FIN i and receive AN i
Receive FIN j and send AN j
Wait twice maximum expected segment lifetime
FIN i
FIN i
AN i
AN i, FIN j
FIN j
AN j
AN j

33. TCP & UDP Transmission Control Protocol User Datagram Protocol (UDP)
Connection oriented
RFC 793
User Datagram Protocol (UDP)
Connectionless
RFC 768

34. TCP Reliable communication between pairs of processes
Variety of primitives
Passive/Active open
Send / Deliver data
Close primitives
Connections are identified by a pair of sockets
a socket is (IP address, port number)
ports less than 1024 are fixed
e.g. 23 telnet, 80 www
others are at operating systems disposal for individual needs

35. TCP Header

36. TCP Header Checksum includes IP addresses as well in a pseudo-header
check misdelivery
spoils protocol hierarchy
Question: What about segment length? It is not in the TCP header. How is the receiver TCP user be informed about the payload length?

37. TCP Mechanisms (1) Connection establishment Three way handshake
Between pairs of ports
One port can connect to multiple destinations

38. TCP Mechanisms (2) Data transfer Logical stream of octets
Octets numbered modulo 232
Segments contain sequence number of the first octet
Flow control by credit allocation of number of octets
Data buffered at transmitter and receiver
TCP decides when to construct a segment
exception is PUSH
RST is sent when
obsolete and duplicate SYNs are received
TCP segment is misdelivered

39. TCP Mechanisms (3) Connection termination Graceful close
TCP users issues CLOSE primitive
Transport entity sets FIN flag on last segment sent
Abrupt termination by ABORT primitive
Entity abandons all attempts to send or receive data
RST segment transmitted

40. Implementation Policy Options
Send
Deliver
Accept
Retransmit
Acknowledge

41. Send If no push or close TCP entity transmits at its own convenience
Data buffered at transmit buffer
May construct segment per data batch
May wait for certain amount of data
Trade-off
infrequent and large segments
low processing overhead, slow response
frequent and small segments
quick response, but high processing overhead

42. Deliver In absence of push, TCP delivers data at its own convenience
May deliver as segments are received
May buffer several segments and then deliver
Performance trade-off
response time versus processing overhead/interrupts

43. Accept Segments may arrive out of order In order In windows
Only accept in-order segments
Discard out-of-order segments
easy implementation, simple
lots of retransmissions
In windows
Accept all segments within receive window
complicated
large buffers
less retransmissions

44. Retransmit TCP will retransmit if does not receive ACK in given time
First only
single timer for all segments waiting ack
if expires the oldest segment waiting ack is retransmitted
longer delays for the other segments
Batch
if expires all segments waiting ack are retransmitted
smaller delays but unnecessary retransmissions
Individual
one timer per segment
complex implementation

45. Acknowledgment Immediate Cumulative limits unnecessary retransmissions
increase the load by acks
Cumulative
typical method
problem in estimating timer values

46. Retransmission Timer Management
Estimate round trip time (RTT) by observing pattern of delay
Set time to value a bit greater than estimate
Simple average
average the RTTs over a number of segments
Exponential average
later segments have more weight
smaller  means greater weight to the last RTT

47. Use of Exponential Averaging

48. How to determine RTO (Retransmission Timeout)
Add fixed  to estimated RTT
Multiply estimated RTT with a fixed factor
Both not good if the observed RTT has variation
It is better if the RTO depends on the estimated RTT and standard deviation in RTT
Jacobson’s method

49. Jacobson’s RTO Calculation
Exp. Average is not good if variation is too much
RTT Variance Estimation (Jacobson’s algorithm)
Includes standard deviation in the calculations
f: the factor that effects RTO

50. Exponential RTO Backoff
What RTO will be used for a retransmitted segment?
Since timeout is probably due to congestion (dropped packet or long round trip), maintaining the same RTO is not good idea
RTO increased each time a segment is re-transmitted
RTO = 2*RTO
Binary exponential backoff

51. Karn’s Algorithm
If a segment is retransmitted, the ACK arriving may be:
For the first copy of the segment
RTT longer than expected
For second copy
No way to tell
Karn’s rules
Do not measure RTT for retransmitted segments
Calculate backoff when re-transmission occurs
Use backoff RTO until ACK arrives for segment that has not been re-transmitted

52. Window Management (1) Slow start
It is not a good idea to start with a large window since the network situation is not known
Start connection with a small window (to fit one segment only)
Enlarge window at each ACK, to some max
Actually not a slow procedure
window growth is exponential

53. Window Management (2) Dynamic windows sizing on congestion
When a timeout occurs
Run a slow start until a threshold
threshold = half of the current window at which timeout occurred.
Increasing window size by 1 segment for every ACK
After threshold, increase window by one segment for each RTT
linear increase in window size

54. UDP User datagram protocol RFC 768
Connectionless service for application level procedures
Unreliable
Delivery and duplication control not guaranteed
Reduced overhead

55. UDP Uses
Applications for which the loss of an occasional data does not cause too much problems
Inward data collection
periodic reports
real-time monitoring
Outward data dissemination
real-time clock
Real time application
e.g. video

56. UDP Header