Computer Network - Transport layer protocol

23.1
Process-to-Process Delivery:
UDP, TCP, and SCTP
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

UDP Applications
UDP Applications
UDP is preferable for some applications.
UDP is preferable for some applications.
UDP is suitable for a process that requires
UDP is suitable for a process that requires simple request-
simple request-
response communication with little concern for flow and error
response communication with little concern for flow and error
control
control. It is not usually used for a process such as FTP that
. It is not usually used for a process such as FTP that
needs to send bulk data
needs to send bulk data
UDP is a
UDP is a suitable transport protocol for multicasting
suitable transport protocol for multicasting.
.
Multicasting capability is embedded in the UDP software but
Multicasting capability is embedded in the UDP software but
not in the TCP software.
not in the TCP software.
UDP is
UDP is used for management processes
used for management processes such as SNMP.
such as SNMP.
UDP is used for some
UDP is used for some route updating protocols
route updating protocols such as Routing
such as Routing
Information Protocol (RIP)
Information Protocol (RIP)
UDP is normally used for
UDP is normally used for interactive real-time applications
interactive real-time applications that
that
cannot tolerate uneven delay between sections of a received
cannot tolerate uneven delay between sections of a received
message
message.
.

23.3
TCP
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:

TCP
TCP
 Transmission Control Protocol (TCP) is a
Transmission Control Protocol (TCP) is a
connection-oriented, reliable protocol
connection-oriented, reliable protocol.
.
 TCP explicitly
TCP explicitly defines connection
defines connection
establishment, data transfer, and connection
establishment, data transfer, and connection
teardown phases to provide a connection-oriented
teardown phases to provide a connection-oriented
service.
service.
 To provide reliability,
To provide reliability, TCP uses checksum (for
TCP uses checksum (for
error detection), retransmission of lost or
error detection), retransmission of lost or
corrupted packets, cumulative and selective
corrupted packets, cumulative and selective
acknowledgments, and timers.
acknowledgments, and timers.

TCP Services
TCP Services
Process –to-Process Communication
Process –to-Process Communication
 Stream Delivery Service
Stream Delivery Service
Sending and Receiving Buffers
Sending and Receiving Buffers
 TCP Segments
TCP Segments
 Full Duplex Communication
Full Duplex Communication
 Multiplexing and De multiplexing
Multiplexing and De multiplexing
 Connection oriented Services
Connection oriented Services
 Reliable Service
Reliable Service

TCP allows the sending process to deliver data as
TCP allows the sending process to deliver data as
a stream of bytes and allows the receiving process to
a stream of bytes and allows the receiving process to
obtain data as a stream of bytes.
obtain data as a stream of bytes.
TCP creates an environment in which the two processes
TCP creates an environment in which the two processes
seem to be
seem to be connected by an imaginary “tube” that
connected by an imaginary “tube” that
carries their bytes across the Internet
carries their bytes across the Internet

23.7
Figure 23.13 Stream delivery

Because the sending and the receiving processes may not
Because the sending and the receiving processes may not
necessarily write or read data at the same rate, TCP
necessarily write or read data at the same rate, TCP
needs buffers for storage.
needs buffers for storage.
There are
There are two buffers, the sending buffer and the
two buffers, the sending buffer and the
receiving buffer
receiving buffer, one for each direction.
, one for each direction.
One way to implement a buffer is to use a circular array
One way to implement a buffer is to use a circular array
of 1-byte locations.
of 1-byte locations.

23.9
Figure 23.14 Sending and receiving buffers

TCP Segments
TCP Segments
The network layer, as a service provider for TCP, needs to
The network layer, as a service provider for TCP, needs to
send data in packets, not as a stream of bytes.
send data in packets, not as a stream of bytes.
At the transport layer, TCP groups a number of bytes
At the transport layer, TCP groups a number of bytes
together into a packet called a segment.
together into a packet called a segment.
TCP adds a header to each segment (for control purposes)
TCP adds a header to each segment (for control purposes)
and delivers the segment to the network layer for
and delivers the segment to the network layer for
transmission.
transmission.
The segments are encapsulated in an IP datagram and
The segments are encapsulated in an IP datagram and
transmitted.
transmitted.

23.11
Figure 23.15 TCP segments

TCP Other Services
TCP Other Services
Full-Duplex Communication
TCP offers full-duplex service, where data can flow in both directions at the
same time. Each TCP endpoint then has its own sending and receiving buffer,
and segments move in both directions.
Multiplexing and Demultiplexing
Like UDP, TCP performs multiplexing at the sender and demultiplexing at the
receiver. However, because TCP is a connection-oriented protocol, a
connection needs to be established for each pair of processes.

TCP Other Services
TCP Other Services
Connection-Oriented Service
TCP, unlike UDP, is a connection-oriented protocol. When a process at site A wants to send to
and receive data from another process at site B, the following three phases occur:
1. The two TCP’s establish a logical connection between them.
2. Data are exchanged in both directions.
3. The connection is terminated.
Reliable Service
TCP is a reliable transport protocol. It uses an acknowledgment mechanism to check the safe
and sound arrival of data

23.14
Figure 23.16 TCP segment format

23.15
Figure 23.17 Control field

23.16
Table 23.3 Description of flags in the control field

23.17
Figure 23.18 Connection establishment using three-way handshaking

23.18
A SYN segment cannot carry data, but it
consumes one sequence number.
Note

23.19
A SYN + ACK segment cannot
carry data, but does consume one
sequence number.
Note

23.20
An ACK segment, if carrying no data,
consumes no sequence number.
Note

23.21
Figure 23.19 Data transfer

23.22
Figure 23.20 Connection termination using three-way handshaking

23.23
The FIN segment consumes one
sequence number if it does
not carry data.
Note

23.24
The FIN + ACK segment consumes
one sequence number if it
does not carry data.
Note

23.26
Figure 23.22 Sliding window

23.27
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

23.28
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.

23.29
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.

23.30
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

23.31
Figure 23.23 Example 23.6

23.32
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

23.33
ACK segments do not consume
sequence numbers and are not
acknowledged.
Note

23.34
In modern implementations, a
retransmission occurs if the
retransmission timer expires or three
duplicate ACK segments have arrived.
Note

23.35
No retransmission timer is set for an
ACK segment.
Note

23.36
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

23.37
Figure 23.24 Normal operation

23.38
Figure 23.25 Lost segment

23.39
The receiver TCP delivers only ordered
data to the process.
Note

23.40
Figure 23.26 Fast retransmission

23.41
23-4 SCTP
23-4 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

23.42
SCTP is a message-oriented, reliable
protocol that combines the best features
of UDP and TCP.
Note

23.43
Table 23.4 Some SCTP applications

23.44
Figure 23.27 Multiple-stream concept

23.45
An association in SCTP can involve
multiple streams.
Note

23.46
Figure 23.28 Multihoming concept

23.47
SCTP association allows multiple IP
addresses for each end.
Note

23.48
In SCTP, a data chunk is numbered
using a TSN.
Note

23.49
To distinguish between different
streams, SCTP uses an SI.
Note

23.50
To distinguish between different data
chunks belonging to the same stream,
SCTP uses SSNs.
Note

23.51
TCP has segments; SCTP has packets.
Note

23.52
Figure 23.29 Comparison between a TCP segment and an SCTP packet

23.53
In SCTP, control information and data
information are carried in separate
chunks.
Note

23.54
Figure 23.30 Packet, data chunks, and streams

23.55
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

23.56
In SCTP, acknowledgment numbers are
used to acknowledge only data chunks;
control chunks are acknowledged by
other control chunks if necessary.
Note

23.57
Figure 23.31 SCTP packet format

23.58
In an SCTP packet, control chunks come
before data chunks.
Note

23.59
Figure 23.32 General header

23.61
A connection in SCTP is called an
association.
Note

23.62
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

23.63
Figure 23.33 Four-way handshaking

23.64
In SCTP, only DATA chunks
consume TSNs;
DATA chunks are the only chunks
that are acknowledged.
Note

23.65
Figure 23.34 Simple data transfer

23.66
The acknowledgment in SCTP defines
the cumulative TSN, the TSN of the last
data chunk received in order.
Note

23.67
Figure 23.35 Association termination

23.68
Figure 23.36 Flow control, receiver site

23.69
Figure 23.37 Flow control, sender site

23.70
Figure 23.38 Flow control scenario

23.71
Figure 23.39 Error control, receiver site

23.72
Figure 23.40 Error control, sender site

Computer Network - Transport layer protocol

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