Computer Network - Transport Layer Services

Transport Layer Services

1. 23.1
Process-to-Process Delivery:
UDP, TCP, and SCTP
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2. 23-1 PROCESS-TO-PROCESS DELIVERY23-1 PROCESS-TO-PROCESS DELIVERY
The transport layer is responsible for process-to-The transport layer is responsible for process-to-
process delivery—the delivery of a packet, part of a process delivery—the delivery of a packet, part of a
message, from one process to another. Two processes message, from one process to another. Two processes
communicate in a client/server relationship, as we will communicate in a client/server relationship, as we will
see later. see later.
Process to Process Communications
Addressing : Port Numbers
Encapsulation and De-Encapsulation
Multiplexing and Demultiplexing
Flow Control
Error Control
Congestion Control

3. 23.3
The transport layer is responsible for
process-to-process delivery.
Note

4. Network Layer Versus Transport Layer

5. 23.5
Addressing : Port numbers

6. 23.6
IP addresses versus port numbers

7. 23.7
ICANN ranges

8. 23.8
Socket address

9. 23.9

10. 23.10
Figure 23.6 Multiplexing and demultiplexing

11. 23.11
Multiplexing and demultiplexing

12. Flow control

13. Error control

14. Figure 23.7 Error control

15. Connectionless Service

16. Connection Oriented Service

17. Connectionless and Connection Oriented Service represented as FSMs

18. Transport Layer ProtocolsTransport Layer Protocols
UDP UDP
UDP is an unreliable connectionless transport-layer UDP is an unreliable connectionless transport-layer
protocol used for its simplicity and efficiency in protocol used for its simplicity and efficiency in
applications where error control can be provided by the applications where error control can be provided by the
application-layer process. application-layer process.
TCP TCP
TCP is a reliable connection-oriented protocol that can TCP is a reliable connection-oriented protocol that can
be used in any application where reliability is importantbe used in any application where reliability is important
SCTPSCTP
SCTP is a new transport-layer protocol that combines SCTP is a new transport-layer protocol that combines
the features of UDP and TCP.the features of UDP and TCP.

19. Position of UDP, TCP, and SCTP in TCP/IP suite

20. Transport Layer ProtocolsTransport Layer Protocols
ServicesServices
Each protocol provides a different type of service and Each protocol provides a different type of service and
should be used appropriately.should be used appropriately.
Port Numbers Port Numbers
A transport-layer protocol usually has several A transport-layer protocol usually has several
responsibilities. One is to create a process-to-process responsibilities. One is to create a process-to-process
communication; these protocols use port numbers to communication; these protocols use port numbers to
accomplish this. Port numbers provide end-to-end accomplish this. Port numbers provide end-to-end
addresses at the transport layer. Table gives some addresses at the transport layer. Table gives some
common port numbers for the three protocols.common port numbers for the three protocols.

21. Some Well Known ports used with three transport protocols

22. Some Well Known ports used with three transport protocols

23. USER DATAGRAM PROTOCOL (UDP)USER DATAGRAM PROTOCOL (UDP)
 The The User Datagram Protocol (UDP) User Datagram Protocol (UDP) is a is a
connectionless, unreliable transport protocol. connectionless, unreliable transport protocol.
 It does not add anything to the services of IP It does not add anything to the services of IP
except for providing process-to-process except for providing process-to-process
communication instead of host-to-host communication instead of host-to-host
communication. communication.
 If UDP is so powerlessIf UDP is so powerless, why would a process want , why would a process want
to use it? With the disadvantages come some to use it? With the disadvantages come some
advantages. advantages.

24. USER DATAGRAM PROTOCOL (UDP)USER DATAGRAM PROTOCOL (UDP)
 UDP is a UDP is a very simple protocol very simple protocol using a minimum of using a minimum of
overhead. overhead.
 If a process If a process wants to send a small messagewants to send a small message and does and does
not care much about reliability, it can use UDP.not care much about reliability, it can use UDP.
 Sending a small message using UDP takes much Sending a small message using UDP takes much less less
interaction between the sender and receiver than interaction between the sender and receiver than using using
TCPTCP

25. User datagram packetUser datagram packet
 UDP packets, called UDP packets, called user datagramsuser datagrams, have a fixed-size , have a fixed-size
header of 8 bytes made up of four fields, each of 2 bytes (16 header of 8 bytes made up of four fields, each of 2 bytes (16
bits). Figure 9.18 shows the format of a user datagram. bits). Figure 9.18 shows the format of a user datagram.
 The first two fields define the source and destination port The first two fields define the source and destination port
numbers, respectively. numbers, respectively.
 The third field defines the total length of the user datagram, The third field defines the total length of the user datagram,
header plus data. The 16 bits can define a total length of 0 to header plus data. The 16 bits can define a total length of 0 to
65,535 bytes65,535 bytes

26. User datagram packet FormatUser datagram packet Format

27. UDP SERVICESUDP SERVICES
Process –to-Process CommunicationProcess –to-Process Communication
 Connectionless servicesConnectionless services
 Flow ControlFlow Control
 Error ControlError Control
 ChecksumChecksum
 Congestion ControlCongestion Control
 Encapsulation and DecapsulationEncapsulation and Decapsulation
 Multiplexing and De multiplexingMultiplexing and De multiplexing

28. UDP SERVICESUDP SERVICES
Process –to-Process Communication Process –to-Process Communication
UDP provides process-to-process communication using UDP provides process-to-process communication using
socket addresses, which are a combination of IP addresses and socket addresses, which are a combination of IP addresses and
port numbers.port numbers.
 Connectionless servicesConnectionless services
UDP provides a connectionless service. This means that each UDP provides a connectionless service. This means that each
user datagram sent by UDP is an independent datagram. There is no user datagram sent by UDP is an independent datagram. There is no
relationship between the different user datagrams even if they are relationship between the different user datagrams even if they are
coming from the same source process and going to the same destination coming from the same source process and going to the same destination
program. The user datagrams are not numbered. Also, unlike TCP, program. The user datagrams are not numbered. Also, unlike TCP,
there is no connection establishment and no connection termination. there is no connection establishment and no connection termination.
This means that each user datagram can travel on a different path.This means that each user datagram can travel on a different path.

29. UDP SERVICESUDP SERVICES
Flow ControlFlow Control
UDP is a very simple protocol. There is no flow control, UDP is a very simple protocol. There is no flow control,
and hence no window mechanismand hence no window mechanism
 Error ControlError Control
There is no error-control mechanism in UDP except for There is no error-control mechanism in UDP except for
the checksum. This means that the sender does not know if a the checksum. This means that the sender does not know if a
message has been lost or duplicated. When the receiver detects message has been lost or duplicated. When the receiver detects
an error through the checksum, the user datagram is silently an error through the checksum, the user datagram is silently
discardeddiscarded
 ChecksumChecksum
UDP checksum calculation includes three sections: a UDP checksum calculation includes three sections: a
pseudoheader, the UDP header, and the data coming from the pseudoheader, the UDP header, and the data coming from the
application layer.application layer.

30. UDP SERVICESUDP SERVICES
Congestion ControlCongestion Control
Because UDP is a connectionless protocol, it does not provide congestion
control. UDP assumes that the packets sent are small and sporadic and cannot
create congestion in the network
 Encapsulation and DecapsulationEncapsulation and Decapsulation
To send a message from one process to another, UDP encapsulates and
decapsulates messages.
 Multiplexing and De multiplexingMultiplexing and De multiplexing
In a host running a TCP/IP protocol suite, there is only one UDP but
possibly several processes that may want to use the services of UDP. To handle this
situation, UDP multiplexes and demultiplexes

31. 23.31
Figure 23.10 Pseudoheader for checksum calculation

32. 23.32
Figure 23.11 Checksum calculation of a simple UDP user datagram

33. 23.33
Figure 23.12 Queues in UDP

34. 23.34
23-3 TCP23-3 TCP
TCP is a connection-oriented protocol; it creates a TCP is a connection-oriented protocol; it creates a
virtual connection between two TCPs to send data. In virtual connection between two TCPs to send data. In
addition, TCP uses flow and error control mechanisms addition, TCP uses flow and error control mechanisms
at the transport level. at the transport level.
TCP Services
TCP Features
Segment
A TCP Connection
Flow Control
Error Control
Topics discussed in this section:Topics discussed in this section:

35. 23.35
Table 23.2 Well-known ports used by TCP

36. 23.36
Figure 23.13 Stream delivery

37. 23.37
Figure 23.14 Sending and receiving buffers

38. 23.38
Figure 23.15 TCP segments

39. 23.39
The bytes of data being transferred in
each connection are numbered by TCP.
The numbering starts with a randomly
generated number.
Note

40. 23.40
The following shows the sequence number for each
segment:
Example 23.3

41. 23.41
The value in the sequence number field
of a segment defines the
number of the first data byte
contained in that segment.
Note

42. 23.42
The value of the acknowledgment field
in a segment defines
the number of the next byte a party
expects to receive.
The acknowledgment number is
cumulative.
Note

43. 23.43
Figure 23.16 TCP segment format

44. 23.44
Figure 23.17 Control field

45. 23.45
Table 23.3 Description of flags in the control field

46. 23.46
Figure 23.18 Connection establishment using three-way handshaking

47. 23.47
A SYN segment cannot carry data, but it
consumes one sequence number.
Note

48. 23.48
A SYN + ACK segment cannot
carry data, but does consume one
sequence number.
Note

49. 23.49
An ACK segment, if carrying no data,
consumes no sequence number.
Note

50. 23.50
Figure 23.19 Data transfer

51. 23.51
Figure 23.20 Connection termination using three-way handshaking

52. 23.52
The FIN segment consumes one
sequence number if it does
not carry data.
Note

53. 23.53
The FIN + ACK segment consumes
one sequence number if it
does not carry data.
Note

54. 23.54
Figure 23.21 Half-close

55. 23.55
Figure 23.22 Sliding window

56. 23.56
A sliding window is used to make
transmission more efficient as well as
to control the flow of data so that the
destination does not become
overwhelmed with data.
TCP sliding windows are byte-oriented.
Note

57. 23.57
What is the value of the receiver window (rwnd) for host
A if the receiver, host B, has a buffer size of 5000 bytes
and 1000 bytes of received and unprocessed data?
Example 23.4
Solution
The value of rwnd = 5000 − 1000 = 4000. Host B can
receive only 4000 bytes of data before overflowing its
buffer. Host B advertises this value in its next segment to
A.

58. 23.58
What is the size of the window for host A if the value of
rwnd is 3000 bytes and the value of cwnd is 3500 bytes?
Example 23.5
Solution
The size of the window is the smaller of rwnd and cwnd,
which is 3000 bytes.

59. 23.59
Figure 23.23 shows an unrealistic example of a sliding
window. The sender has sent bytes up to 202. We assume
that cwnd is 20 (in reality this value is thousands of
bytes). The receiver has sent an acknowledgment number
of 200 with an rwnd of 9 bytes (in reality this value is
thousands of bytes). The size of the sender window is the
minimum of rwnd and cwnd, or 9 bytes. Bytes 200 to 202
are sent, but not acknowledged. Bytes 203 to 208 can be
sent without worrying about acknowledgment. Bytes 209
and above cannot be sent.
Example 23.6

60. 23.60
Figure 23.23 Example 23.6

61. 23.61
Some points about TCP sliding windows:
❏ The size of the window is the lesser of rwnd and
cwnd.
❏ The source does not have to send a full window’s
worth of data.
❏ The window can be opened or closed by the
receiver, but should not be shrunk.
❏ The destination can send an acknowledgment at
any time as long as it does not result in a shrinking
window.
❏ The receiver can temporarily shut down the
window; the sender, however, can always send a
segment of 1 byte after the window is shut down.
Note

62. 23.62
ACK segments do not consume
sequence numbers and are not
acknowledged.
Note

63. 23.63
In modern implementations, a
retransmission occurs if the
retransmission timer expires or three
duplicate ACK segments have arrived.
Note

64. 23.64
No retransmission timer is set for an
ACK segment.
Note

65. 23.65
Data may arrive out of order and be
temporarily stored by the receiving TCP,
but TCP guarantees that no out-of-order
segment is delivered to the process.
Note

66. 23.66
Figure 23.24 Normal operation

67. 23.67
Figure 23.25 Lost segment

68. 23.68
The receiver TCP delivers only ordered
data to the process.
Note

69. 23.69
Figure 23.26 Fast retransmission

70. 23.70
23-4 SCTP23-4 SCTP
Stream Control Transmission Protocol (SCTP) is a Stream Control Transmission Protocol (SCTP) is a
new reliable, message-oriented transport layer new reliable, message-oriented transport layer
protocol. SCTP, however, is mostly designed for protocol. SCTP, however, is mostly designed for
Internet applications that have recently been Internet applications that have recently been
introduced. These new applications need a more introduced. These new applications need a more
sophisticated service than TCP can provide. sophisticated service than TCP can provide.
SCTP Services and Features
Packet Format
An SCTP Association
Flow Control and Error Control
Topics discussed in this section:Topics discussed in this section:

71. 23.71
SCTP is a message-oriented, reliable
protocol that combines the best features
of UDP and TCP.
Note

72. 23.72
Table 23.4 Some SCTP applications

73. 23.73
Figure 23.27 Multiple-stream concept

74. 23.74
An association in SCTP can involve
multiple streams.
Note

75. 23.75
Figure 23.28 Multihoming concept

76. 23.76
SCTP association allows multiple IP
addresses for each end.
Note

77. 23.77
In SCTP, a data chunk is numbered
using a TSN.
Note

78. 23.78
To distinguish between different
streams, SCTP uses an SI.
Note

79. 23.79
To distinguish between different data
chunks belonging to the same stream,
SCTP uses SSNs.
Note

80. 23.80
TCP has segments; SCTP has packets.
Note

81. 23.81
Figure 23.29 Comparison between a TCP segment and an SCTP packet

82. 23.82
In SCTP, control information and data
information are carried in separate
chunks.
Note

83. 23.83
Figure 23.30 Packet, data chunks, and streams

84. 23.84
Data chunks are identified by three
items: TSN, SI, and SSN.
TSN is a cumulative number identifying
the association; SI defines the stream;
SSN defines the chunk in a stream.
Note

85. 23.85
In SCTP, acknowledgment numbers are
used to acknowledge only data chunks;
control chunks are acknowledged by
other control chunks if necessary.
Note

86. 23.86
Figure 23.31 SCTP packet format

87. 23.87
In an SCTP packet, control chunks come
before data chunks.
Note

88. 23.88
Figure 23.32 General header

89. 23.89
Table 23.5 Chunks

90. 23.90
A connection in SCTP is called an
association.
Note

91. 23.91
No other chunk is allowed in a packet
carrying an INIT or INIT ACK chunk.
A COOKIE ECHO or a COOKIE ACK
chunk can carry data chunks.
Note

92. 23.92
Figure 23.33 Four-way handshaking

93. 23.93
In SCTP, only DATA chunks
consume TSNs;
DATA chunks are the only chunks
that are acknowledged.
Note

94. 23.94
Figure 23.34 Simple data transfer

95. 23.95
The acknowledgment in SCTP defines
the cumulative TSN, the TSN of the last
data chunk received in order.
Note

96. 23.96
Figure 23.35 Association termination

97. 23.97
Figure 23.36 Flow control, receiver site

98. 23.98
Figure 23.37 Flow control, sender site

99. 23.99
Figure 23.38 Flow control scenario

100. 23.100
Figure 23.39 Error control, receiver site

101. 23.101
Figure 23.40 Error control, sender site