Chapter 23
Process-to-Process Delivery:
UDP, TCP, and SCTP
23.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
23-1 PROCESS-TO-PROCESS DELIVERY
The transport layer is responsible for process-to-
process delivery—the delivery of a packet, part of a
message, from one process to another. Two processes
communicate in a client/server relationship, as we will
see later.
Topics discussed in this section:
Client/Server Paradigm
Multiplexing and Demultiplexing
Connectionless Versus Connection-Oriented Service
Reliable Versus Unreliable
Three Protocols
23.2
Note
The transport layer is responsible for
process-to-process delivery.
23.3
Figure 23.1 Types of data deliveries
23.4
Figure 23.2 Port numbers
23.5
Figure 23.3 IP addresses versus port numbers
23.6
Figure 23.4 IANA ranges
23.7
Figure 23.5 Socket address
23.8
Figure 23.6 Multiplexing and demultiplexing
23.9
Figure 23.7 Error control
23.10
Figure 23.8 Position of UDP, TCP, and SCTP in TCP/IP suite
23.11
23-2 USER DATAGRAM PROTOCOL (UDP)
The User Datagram Protocol (UDP) is called a
connectionless, unreliable transport protocol. It does
not add anything to the services of IP except to provide
process-to-process communication instead of host-to-
host communication.
Topics discussed in this section:
Well-Known Ports for UDP
User Datagram
Checksum
UDP Operation
Use of UDP
23.12
Table 23.1 Well-known ports used with UDP
23.13
Example 23.1
In UNIX, the well-known ports are stored in a file
called /etc/services. Each line in this file gives the name
of the server and the well-known port number. We can use
the
grep utility to extract the line corresponding to the desired
application. The following shows the port for FTP. Note
that FTP can use port 21 with either UDP or TCP.
23.14
Example 23.1 (continued)
SNMP uses two port numbers (161 and 162), each for a
different purpose, as we will see in Chapter 28.
23.15
Figure 23.9 User datagram format
23.16
Note
UDP length
= IP length – IP header’s length
23.17
Figure 23.10 Pseudoheader for checksum calculation
23.18
Example 23.2
Figure 23.11 shows the checksum calculation for a very
small user datagram with only 7 bytes of data. Because
the number of bytes of data is odd, padding is added for
checksum calculation. The pseudoheader as well as the
padding will be dropped when the user datagram is
delivered to IP.
23.19
Figure 23.11 Checksum calculation of a simple UDP user datagram
23.20
Figure 23.12 Queues in UDP
23.21
23-3 TCP
TCP is a connection-oriented protocol; it creates a
virtual connection between two TCPs to send data. In
addition, TCP uses flow and error control mechanisms
at the transport level.
Topics discussed in this section:
TCP Services
TCP Features
Segment
A TCP Connection
Flow Control
Error Control
23.22
Table 23.2 Well-known ports used by TCP
23.23
Figure 23.13 Stream delivery
23.24
Figure 23.14 Sending and receiving buffers
23.25
Figure 23.15 TCP segments
23.26
Note
The bytes of data being transferred in
each connection are numbered by TCP.
The numbering starts with a randomly
generated number.
23.27
Example 23.3
The following shows the sequence number for each
segment:
23.28
Note
The value in the sequence number field
of a segment defines the
number of the first data byte
contained in that segment.
23.29
Note
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.
23.30
Figure 23.16 TCP segment format
23.31
Figure 23.17 Control field
23.32
Table 23.3 Description of flags in the control field
23.33
Figure 23.18 Connection establishment using three-way handshaking
23.34
Note
A SYN segment cannot carry data, but it
consumes one sequence number.
23.35
Note
A SYN + ACK segment cannot
carry data, but does consume one
sequence number.
23.36
Note
An ACK segment, if carrying no data,
consumes no sequence number.
23.37
Figure 23.19 Data transfer
23.38
Figure 23.20 Connection termination using three-way handshaking
23.39
Note
The FIN segment consumes one
sequence number if it does
not carry data.
23.40
Note
The FIN + ACK segment consumes
one sequence number if it
does not carry data.
23.41
Figure 23.21 Half-close
23.42
Figure 23.22 Sliding window
23.43
Note
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.
23.44
Example 23.4
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?
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.
23.45
Example 23.5
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?
Solution
The size of the window is the smaller of rwnd and cwnd,
which is 3000 bytes.
23.46
Example 23.6
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.
23.47
Figure 23.23 Example 23.6
23.48
Note
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.
23.49
Note
ACK segments do not consume
sequence numbers and are not
acknowledged.
23.50
Note
In modern implementations, a
retransmission occurs if the
retransmission timer expires or three
duplicate ACK segments have arrived.
23.51
Note
No retransmission timer is set for an
ACK segment.
23.52
Note
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.
23.53
Figure 23.24 Normal operation
23.54
Figure 23.25 Lost segment
23.55
Note
The receiver TCP delivers only ordered
data to the process.
23.56
Figure 23.26 Fast retransmission
23.57
23-4 SCTP
Stream Control Transmission Protocol (SCTP) is a
new reliable, message-oriented transport layer
protocol. SCTP, however, is mostly designed for
Internet applications that have recently been
introduced. These new applications need a more
sophisticated service than TCP can provide.
Topics discussed in this section:
SCTP Services and Features
Packet Format
An SCTP Association
Flow Control and Error Control
23.58
Note
SCTP is a message-oriented, reliable
protocol that combines the best features
of UDP and TCP.
23.59
Table 23.4 Some SCTP applications
23.60
Figure 23.27 Multiple-stream concept
23.61
Note
An association in SCTP can involve
multiple streams.
23.62
Figure 23.28 Multihoming concept
23.63
Note
SCTP association allows multiple IP
addresses for each end.
23.64
Note
In SCTP, a data chunk is numbered
using a TSN.
23.65
Note
To distinguish between different
streams, SCTP uses an SI.
23.66
Note
To distinguish between different data
chunks belonging to the same stream,
SCTP uses SSNs.
23.67
Note
TCP has segments; SCTP has packets.
23.68
Figure 23.29 Comparison between a TCP segment and an SCTP packet
23.69
Note
In SCTP, control information and data
information are carried in separate
chunks.
23.70
Figure 23.30 Packet, data chunks, and streams
23.71
Note
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.
23.72
Note
In SCTP, acknowledgment numbers are
used to acknowledge only data chunks;
control chunks are acknowledged by
other control chunks if necessary.
23.73
Figure 23.31 SCTP packet format
23.74
Note
In an SCTP packet, control chunks come
before data chunks.
23.75
Figure 23.32 General header
23.76
Table 23.5 Chunks
23.77
Note
A connection in SCTP is called an
association.
23.78
Note
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.
23.79
Figure 23.33 Four-way handshaking
23.80
Note
In SCTP, only DATA chunks
consume TSNs;
DATA chunks are the only chunks
that are acknowledged.
23.81
Figure 23.34 Simple data transfer
23.82
Note
The acknowledgment in SCTP defines
the cumulative TSN, the TSN of the last
data chunk received in order.
23.83
Figure 23.35 Association termination
23.84
Figure 23.36 Flow control, receiver site
23.85
Figure 23.37 Flow control, sender site
23.86
Figure 23.38 Flow control scenario
23.87
Figure 23.39 Error control, receiver site
23.88
Figure 23.40 Error control, sender site
23.89
Chapter 24
Congestion Control and
Quality of Service
24.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
24-1 DATA TRAFFIC
The main focus of congestion control and quality of
service is data traffic. In congestion control we try to
avoid traffic congestion. In quality of service, we try to
create an appropriate environment for the traffic. So,
before talking about congestion control and quality of
service, we discuss the data traffic itself.
Topics discussed in this section:
Traffic Descriptor
Traffic Profiles
24.2
Figure 24.1 Traffic descriptors
24.3
Figure 24.2 Three traffic profiles
24.4
24-2 CONGESTION
Congestion in a network may occur if the load on the
network—the number of packets sent to the network—
is greater than the capacity of the network—the
number of packets a network can handle. Congestion
control refers to the mechanisms and techniques to
control the congestion and keep the load below the
capacity.
Topics discussed in this section:
Network Performance
24.5
Figure 24.3 Queues in a router
24.6
Figure Packet delay and throughput as functions of load
24.7
24-3 CONGESTION CONTROL
Congestion control refers to techniques and
mechanisms that can either prevent congestion, before
it happens, or remove congestion, after it has
happened. In general, we can divide congestion
control mechanisms into two broad categories: open-
loop congestion control (prevention) and closed-loop
congestion control (removal).
Topics discussed in this section:
Open-Loop Congestion Control
Closed-Loop Congestion Control
24.8
Figure 24.5 Congestion control categories
24.9
Figure 24.6 Backpressure method for alleviating congestion
24.10
Figure 24.7 Choke packet
24.11
24-4 TWO EXAMPLES
To better understand the concept of congestion
control, let us give two examples: one in TCP and the
other in Frame Relay.
Topics discussed in this section:
Congestion Control in TCP
Congestion Control in Frame Relay
24.12
Figure 24.8 Slow start, exponential increase
24.13
Note
In the slow-start algorithm, the size of
the congestion window increases
exponentially until it reaches a
threshold.
24.14
Figure 24.9 Congestion avoidance, additive increase
24.15
Note
In the congestion avoidance algorithm,
the size of the congestion window
increases additively until
congestion is detected.
24.16
Note
An implementation reacts to congestion
detection in one of the following ways:
❏ If detection is by time-out, a new slow
start phase starts.
❏ If detection is by three ACKs, a new
congestion avoidance phase starts.
24.17
Figure 24.10 TCP congestion policy summary
24.18
Figure 24.11 Congestion example
24.19
Figure 24.12 BECN
24.20
Figure 24.13 FECN
24.21
Figure 24.14 Four cases of congestion
24.22
24-5 QUALITY OF SERVICE
Quality of service (QoS) is an internetworking issue
that has been discussed more than defined. We can
informally define quality of service as something a
flow seeks to attain.
Topics discussed in this section:
Flow Characteristics
Flow Classes
24.23
Figure 24.15 Flow characteristics
24.24
24-6 TECHNIQUES TO IMPROVE QoS
In Section 24.5 we tried to define QoS in terms of its
characteristics. In this section, we discuss some
techniques that can be used to improve the quality of
service. We briefly discuss four common methods:
scheduling, traffic shaping, admission control, and
resource reservation.
Topics discussed in this section:
Scheduling
Traffic Shaping
Resource Reservation
Admission Control
24.25
Figure 24.16 FIFO queue
24.26
Figure 24.17 Priority queuing
24.27
Figure 24.18 Weighted fair queuing
24.28
Figure 24.19 Leaky bucket
24.29
Figure 24.20 Leaky bucket implementation
24.30
Note
A leaky bucket algorithm shapes bursty
traffic into fixed-rate traffic by averaging
the data rate. It may drop the packets if
the bucket is full.
24.31
Note
The token bucket allows bursty traffic at
a regulated maximum rate.
24.32
Figure 24.21 Token bucket
24.33
24-7 INTEGRATED SERVICES
Two models have been designed to provide quality of
service in the Internet: Integrated Services and
Differentiated Services. We discuss the first model
here.
Topics discussed in this section:
Signaling
Flow Specification
Admission
Service Classes
RSVP
Problems with Integrated Services
24.34
Note
Integrated Services is a flow-based QoS
model designed for IP.
24.35
Figure 24.22 Path messages
24.36
Figure 24.23 Resv messages
24.37
Figure 24.24 Reservation merging
24.38
Figure 24.25 Reservation styles
24.39
24-8 DIFFERENTIATED SERVICES
Differentiated Services (DS or Diffserv) was
introduced by the IETF (Internet Engineering Task
Force) to handle the shortcomings of Integrated
Services.
Topics discussed in this section:
DS Field
24.40
Note
Differentiated Services is a class-based
QoS model designed for IP.
24.41
Figure 24.26 DS field
24.42
Figure 24.27 Traffic conditioner
24.43
24-9 QoS IN SWITCHED NETWORKS
Let us now discuss QoS as used in two switched
networks: Frame Relay and ATM. These two networks
are virtual-circuit networks that need a signaling
protocol such as RSVP.
Topics discussed in this section:
QoS in Frame Relay
QoS in ATM
24.44
Figure 24.28 Relationship between traffic control attributes
24.45
Figure 24.29 User rate in relation to Bc and Bc + Be
24.46
Figure 24.30 Service classes
24.47
Figure 24.31 Relationship of service classes to the total capacity of the network
24.48
