1. CSC 600 Internetworking with TCP/IP
Unit 3: Transport Layer
(Ch. 13, 12)
Dr. Cheer-Sun Yang
Spring 2001

2. Introduction
Transmission Control Protocol provides connection-oriented reliable transport services.
User Datagram Protocol (UDP) provides connectionless unreliable transport services.

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

5. Reliable vs. Unreliable
Reliable transport service handles error recovery at the transport level.
Unreliable transport service does not provide error recovery at at the transport level.

6. Connection-oriented vs. Connection-less
Connection-oriented service must establish connection between the source and the destination first.
Connection-less service does not establish connection first. It simply does store-and-forward.

7. Properties of the Reliable Delivery Service
Stream orientation - ordered delivery
Virtual circuit connection – connection establishment is must prior to segment delivery
Buffered transfer – data buffering is needed
Unstructured stream – TCP segments may not be as big as a record in a payroll application.
Full duplex connection – Connections provided by the TCP/IP stream service allow concurrent transfer in both direction.

8. Properties of the Reliable Delivery Service
TCP provides reliable transport service using sliding window protocol as defined in the Data Link Layer Protocol.

9. Transmission Control Protocol
TCP is a communication protocol, not a piece of software.

10. TCP vs. the Implementation
TCP is the communication protocol.
TCP is implemented by many venders in software as part of the Operating System.
The difference between a protocol and the software that implements it is analogous to the difference between the definition of a programming language and a compiler.

11. What does TCP Specify? Data segment format Timing
Meanings of header fields
Functions of TCP – also referred to as services provided by TCP

12. What does TCP not specify?
The user interface is not specified.
The underlying communication system can be a dialup telephone line, a local area network, a high speed fiber optical network, or a lower speed long haul network.

13. TCP Services Reliable communication between pairs of processes
Across variety of reliable and unreliable networks and internets
Two labeling facilities
Data stream push
TCP user can require transmission of all data up to push flag
Receiver will deliver in same manner
Avoids waiting for full buffers
Urgent data signal
Indicates urgent data is upcoming in stream
User decides how to handle it

15. TCP Header

17. Items Passed to IP TCP passes some parameters down to IP Precedence
Normal delay/low delay
Normal throughput/high throughput
Normal reliability/high reliability
Security

18. TCP Header Field Port Number
source and destination port numbers (why source port number?)
why not IP addresses?
Identifies an application
Together with IP address to form an end point

19. TCP Header Field Sequence Number 32 bits long
the range of sequence number is 0 <= seq <=
Each sequence number identifies the byte in the stream of data from the sending TCP to the receiving TCP where the first byte of data is located in the segment
Initial Sequence Number (ISN) of a connection is set during connection management
segment segment segment 3
(seq = 1) (seq = 201) (seq = 401)

20. TCP Header Field Acknowledgement Nubmer
Acknowledgements are piggybacked if there is a segment ready to be sent from the receiver to the sender
The acknowledgement segment consists of the next sequence number expected

21. TCP Header Field
Header Length
Why is this needed ?

22. TCP Header Field

23. TCP Header Field
Flags
URG - if the URG =1, the following bytes contain an urgent message: seq <= urgent message <= seq + urgent pointer
ACK: acknowledgement number is valid
PSH:
notification from sender to receiver to force the TCP on the receiver side to pass all data received to the application layer
Normally sent by the sender when the sender’s buffer is empty so the sender does not wait for more data
RST: Reset the connection
SYN: synchronization request for the sequence number
FIN: Finish flag

24. TCP Header Field Options: End of options: 1 byte NOP: 1 byte
Maximum segment size: 4 bytes
Window scale factor: 3 bytes
increases the TCP window size from 16 bits to 32 bits
1-byte shift count is between 0 and 14
used in the connection establishment for window size negotiation
Timestamp: 10 bytes
sender places a timestamp in a segment
receiver places an echo reply
this allows the sender to calculate the Round-Trip Time per window

25. TCP Header Field(Options)
End of options
NOP 1
MSS
Window scale
factor S
S: shift count
timestamp timestamp echo reply
Timestamp

26. Transport Layer Issues
Addressing
Connection establishment
Connection termination
Flow Control
Timeout and retransmission
Congestion Control
Multiplexing
Duplication detection
Crash recovery

27. TCP Mechanisms Connection establishment Data transfer Send policy
Deliver policy
Accept policy: in-order, in-window
Retransmission policy: first-only, batch, individual
Acknowledgement Policy

28. Addressing Target user specified by: User identification
Usually host, port
Called a socket in TCP
Port represents a particular transport service (TS) user
Transport entity identification
Generally only one per host
If more than one, then usually one of each type
Specify transport protocol (TCP, UDP)
Host address
An attached network device
In an internet, a global internet address
Network number

29. Finding Addresses Four methods Know address ahead of time
e.g. collection of network device stats
Well known addresses
Name server
Sending process request to well known address

30. Ports, Connections, and Endpoints
TCP uses the connection, not the protocol port, as its fundamental abstraction; connections are identified by a pair of endpoints, i.e., ( , 1069) and ( , 25).
An endpoint is a pair of integers = (host, port).
Because TCP identifies a connection by a pair of endpoints, a given TCP port number can be shared by multiple connections on the same machine.

31. Connection Establishment
Three way handshake
Between pairs of ports
One port can connect to multiple destinations

33. Passive and Active Opens
A client requests for a connection – an active open request.
A server must be waiting for the request for connection – a passive open.

34. Connection Establishment
Two way handshake
A send SYN, B replies with SYN
Lost SYN handled by re-transmission
Can lead to duplicate SYNs
Ignore duplicate SYNs once connected
Lost or delayed data segments can cause connection problems
Segment from old connections
Start segment numbers fare removed from previous connection
Use SYN i
Need ACK to include i
Three Way Handshake

35. Two Way Handshake: Obsolete Data Segment

36. Two Way Handshake: Obsolete SYN Segment

37. Three Way Handshake: Examples

38. Connection Establishment

39. Three Way Handshake: State Diagram

41. Initial Sequence Number
When a new connection is being established, the SYN flag is turned on. The sequence number field contains the ISN chosen by the host for this connection.
The sequence number of the first byte of data sent by the host will be the ISN plus one because the SYN flag consumes a sequence number.

42. 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
Associate sequence number with FIN
Receiver waits for all segments before FIN sequence number
Loss of segments and obsolete segments
Must explicitly ACK FIN

43. Data Transfer Data transfer Logical stream of octets
Octets numbered modulo 223
Flow control by credit allocation of number of octets
Data buffered at transmitter and receiver

44. Send Policy
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

45. Deliver Policy In absence of push, deliver data at own convenience
May deliver as each in order segment received
May buffer data from more than one segment

46. Accept Policy Segments may arrive out of order In order In windows
Only accept segments in order
Discard out of order segments
In windows
Accept all segments within receive window

47. Not Listening Reject with RST (Reset)
Queue request until matching open issued
Signal TS user to notify of pending request
May replace passive open with accept

48. 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

49. Termination Either or both sides By mutual agreement
Abrupt termination
Or graceful termination
Close wait state must accept incoming data until FIN received

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

52. Side Not Initiating Termination
FIN received
Inform TS user Place connection in CLOSE WAIT state
Continue to accept data from TS user and transmit it
TS user issues CLOSE primitive
Transport entity sends FIN
Connection closed
All outstanding data is transmitted from both sides
Both sides agree to terminate

54. Usage of tcpdump
A program called tcpdump on taz.cs.wcupa.edu has been installed for monitoring TCP mechanisms.
It requires root privilege. So Dr. Kline set up a script called TCPDUMP for us to run tcpdump.
For details, see homework sheet.

55. Output of tcpdump
On taz.cs.wcupa.edu, each segment sent is printed out twice. It looks odd.
TCPDUMP prints out each segment in the following format: source > destination: flags, where the flags represents S(SYN), F(FIN), R(RST), P(PSH), and a dot(.).
The sequence numbers are followed by the number of data bytes. For example: : (0) is a segment without data.

56. Output of tcpdump Option fields are printed out.
MSS - maximum segment size
WSCALE: window scale
NOP: no operation (used for padding a field length to a multiple of four bytes).
<mss 512,nop,wscale 0,nop,nop,timestamp >

57. Flow Control
Longer transmission delay between transport entities compared with actual transmission time
Delay in communication of flow control info
Variable transmission delay
Difficult to use timeouts
Flow may be controlled because:
The receiving user can not keep up
The receiving transport entity can not keep up
Results in buffer filling up

58. The idea Behind Sliding Windows
A simple positive acknowledgement protocol wastes a substantial amount of network bandwidth because it must delay sending a new packet until it receives an acknowledgement for the previous packet.

62. Window Size and Flow Control
TCP allows the window size to be changed over time.
Each ACK, which specifies how many octets have been received, contains a window advertisement that specifies how many additional octets of data the receiver is prepared to receive.
It is the receiver’s current buffer size.

63. Coping with Flow Control Requirements (1)
Do nothing
Segments that overflow are discarded
Sending transport entity will fail to get ACK and will retransmit
Thus further adding to incoming data
Refuse further segments
Clumsy
Multiplexed connections are controlled on aggregate flow

64. Coping with Flow Control Requirements (2)
Use fixed sliding window protocol
See chapter 7 for operational details
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
Use credit scheme

65. Credit Scheme Greater control on reliable network
More effective on unreliable network
Decouples flow control from ACK
May ACK without granting credit and vice versa
Each octet has sequence number
Each transport segment has seq number, ack number and window size in header

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

67. Credit Allocation

68. Sending and Receiving Perspectives

69. Unreliable Network Service
E.g.
internet using IP,
frame relay using LAPF
IEEE using unacknowledged connectionless LLC
Segments may get lost
Segments may arrive out of order

70. Ordered Delivery Segments may arrive out of order
Number segments sequentially
TCP numbers each octet sequentially
Segments are numbered by the first octet number in the segment

71. Retransmission Strategy
Segment damaged in transit
Segment fails to arrive
Transmitter does not know of failure
Receiver must acknowledge successful receipt
Use cumulative acknowledgement
Time out waiting for ACK triggers re-transmission

72. Timer Value Fixed timer Adaptive scheme
Based on understanding of network behavior
Can not adapt to changing network conditions
Too small leads to unnecessary re-transmissions
Too large and response to lost segments is slow
Should be a bit longer than round trip time
Adaptive scheme
May not ACK immediately
Can not distinguish between ACK of original segment and re-transmitted segment
Conditions may change suddenly

73. TCP Timers
Retransmission Timer: started during a transmission. A timeout causes a retransmission.
Persist Timer: ensures that window size information is transmitted even if no data is transmitted.
Keepalive Timer: detects crashes on the other end of connection.
Other Timers: delay ACK timer, timeout of connection setup, abort timeout, 2MSL(Maximum Segment Lifetime) timeout(closing timeout).

74. Acknowledgement Policy
Immediate
Cumulative

75. Congestion Control RFC 1122, Requirements for Internet hosts
Retransmission timer management
Estimate round trip delay by observing pattern of delay
Set time to value somewhat greater than estimate
Simple average
Exponential average
RTT Variance Estimation (Jacobson’s algorithm)

76. Congestion Control Avoidance
TCP must remember the size of the receiver’s window. To control congestion, TCP maintains a second limit, called the congestion window limit, that is used to restrict data flow to less than the receiver’s buffer size.
Multiplicative Decrease Congestion Avoidance: To estimate congestion window size, TCP assumes that most datagram loss comes from congestion and

77. Congestion Control Avoidance
Upon loss of a segment, the sender reduces the congestion window by half. For those segments tha remain in the allowed window, backoff retransmission timer exponentially.
Slow Start: When congestion ends, increase the congestion window exponentially until it reaches the receiver’s window limit.
The term slow start is a misnomer since the congestion window grows exponentially.

78. Congestion Control Slow start Dynamic windows sizing on congestion
awnd = MIN[credit, cwnd]
Start connection with cwnd=1
Increment cwnd at each ACK, to some max
Dynamic windows sizing on congestion
When a timeout occurs
Set slow start threshold to half current congestion window
ssthresh=cwnd/2
Set cwnd = 1 and slow start until cwnd=ssthresh
Increasing cwnd by 1 for every ACK
For cwnd >=ssthresh, increase cwnd by 1 for each RTT

79. Response to Congestion
How can a router avoid global congestion?
Tail drop: if the input queue is fulled when a datagram arrives, discard the datagram.
Random Early Discard(RED)

80. RED
A router uses two threshold values to mark positions in the queue: Tmin and Tmax. The general operation of RED can be described by three rules that determine the disposition of each arriving datagram:
if the queue currently contains fewer than Tmin datagrams, add the new datagram to the queue.
If the queue contains more than Tmax datagrams, discard the new datagram.
If the queue contains between Tmin and Tmax datagrams, randomly discard the datagram according to a probability p.

81. Timeout and Retransmit
TCP maintains queue of segments transmitted but not acknowledged
TCP will retransmit if not ACKed in given time
Measurements of round trip times vary dramatically over time.

83. Exponential RTO Backoff
Since timeout is probably due to congestion (dropped packet or long round trip), maintaining a constant RTT is not a good idea
RTT increased each time a segment is re-transmitted

85. Timeout and Retransmit
Adaptive retransmission algorithm(RFC 793)
RTT =  * old RTT + (1 - ) * new Round Trip Sample
RTO =  * RTT (usually  = 2)

86. Use of Exponential Averaging

87. Responding to High Variance in Delay - Jacobson’s Algorithm
DIFF = Sample - old RTT
smoothed RTT = old RTT +  * DIFF
mean deviation = old mean deviation +  (|DIFF| - old mean deviation)
RTO = smoothed RTT +  * mean deviation
 : between 0 and 1
 : inverse of a power of 2
 : inverse of a power of 2

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

90. Silly Window Syndrome
A problem occurs when the sender and the receiver operate at different speeds.
When the receiving application reads an octet of data from a full buffer, one octet of space becomes available. The TCP on the receiver generates a segment the inform the sender that 1 octet is available.
The sender sends out a segment of one byte.
This results in a series of small data segment - silly window syndrome (SWS).

91. Silly Window Syndrome Avoidance-Nagle Algorithm
Receiver-side avoidance : delay acknowledgement
Sender-side avoidance: delay transmission adaptively.

92. Multiplexing Multiple users employ same transport protocol
User identified by port number or service access point (SAP)
May also multiplex with respect to network services used
e.g. multiplexing a single virtual X.25 circuit to a number of transport service user
X.25 charges per virtual circuit connection time

93. Duplication Detection
If a segment is lost and retransmitted, no confusion will result.
If, however, an ACK is lost, one or more segments will be retransmitted and, if they arrive successfully, the receiver must be able to recognizes duplicates.

94. Duplication Detection
Duplicate received prior to closing connection
Receiver assumes ACK lost and ACKs duplicate
Sender must not get confused with multiple ACKs
Sequence number space large enough to not cycle within maximum life of segment
Duplicate received after closing connection

95. Crash Recovery After restart all state info is lost
Connection is half open
Side that did not crash still thinks it is connected
Close connection using persistence timer
Wait for ACK for (time out) * (number of retries)
When expired, close connection and inform user
Send RST i in response to any i segment arriving
User must decide whether to reconnect
Problems with lost or duplicate data

96. UDP User datagram protocol RFC 768
Connectionless service for application level procedures
Unreliable
Delivery and duplication control not guaranteed
Reduced overhead
e.g. network management (Chapter 19)

97. UDP Uses Inward data collection Outward data dissemination
Request-Response
Real time application

98. UDP Header

99. Recommended Reading Comer: chapter 12, chapter 13
Stallings: chapter 17
RFCs