Download Unit-5 - IPEM Group of Institutions

COMPUTER NETWORKS
UNIT - 5
TCP – Transmission Control Protocol
• Communication takes place between processes or
application programs by using port addresses
• TCP is a connection-oriented protocol
• Does not mean it has a physical connection between sender and receiver
• TCP provides the function to allow a connection virtually exists – also called
virtual circuit
• TCP provides the functions:
• Dividing a chunk of data into segments
• Reassembly segments into the original chunk
• Provide further the functions such as reordering and data resend
• Offering a reliable byte-stream delivery service
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Sender
1
2
Timeout
retransmit
A1
3
A3
A2
1
3
2
Recipient
3
• A Typical Procedure
• Sender
•
•
•
•
TCP divides a message into segments
Add sequence no.
Send the segments in sequence and wait for acknowledgement
If an acknowledgement for a segment is not received for a certain
period of time, resend it until an acknowledgement is received
• Recipient
• When receiving segments, send the acknowledgement with correct
number
• Reassembly the segments back to the message
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Dividing and Reassembly
Source Port
TCP
Destination
Port
Sequence Number
Acknowledgement
Number
Message
Checksum
Message Data
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TCP Datagram
The TCP datagram format
TCP Datagram
A brief explanation of the functions of different fields are given below:
Source port (16 bits): It defines the port number of the application program in the host
of the sender
Destination port (16 bits): It defines the port number of the application program in the
host of the receiver
Sequence number (32 bits): It conveys the receiving host which octet in this sequence
comprises the first byte in the segment
Acknowledgement number (32 bits): This specifies the sequence number of the next
octet that receiver expects to receive
HLEN (4 bits): This field specifies the number of 32-bit words present in the TCP header
Control flag bits (6 bits):
URG: Urgent pointer
ACK: Indicates whether acknowledge field is valid
PSH: Push the data without buffering
RST: Resent the connection
SYN: Synchronize sequence numbers during connection establishment
FIN: Terminate the connection
Window (16 bits): Specifies the size of window
Checksum (16 bits): Checksum used for error detection.
User pointer (16 bits): Used only when URG flag is valid
Options: Optional 40 bytes of information
Port Numbers
Transport layer address is specified with the help a 16-bit Port number in the
range of 0 and 65535. Internet Assigned Number Authority (IANA) has
divided the addresses in three ranges:
Well-known ports: The ports in the range from 0 to 1023 are assigned and
controlled by IANA. These port numbers are commonly used as universal
port numbers in the servers for the convenience of many clients the servers
serve.
Registered ports: Registered ports in the range from 1024 to 49151 are not
assigned or controlled by IANA. However, they can only be registered with
IANA to avoid duplication.
Dynamic ports: Dynamic ports (49152 to 65535) are neither controlled by
IANA nor need to be registered. They can be defined at the client site and
chosen randomly by the transport layer software.
Port Multiplexing
• A computer may perform a number of network
applications at the same time
• FTP + SMTP + HTTP, etc.
• Each computer has only one network address, how
can it serve so many applications at the same time?
 by port multiplexing
FTP
Port 21
Network add:
158.132.161.99
SMTP
Port 25
HTTP
Port 80
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Well-known Port Numbers
• Some port numbers are reserved for some purposes
•
•
•
•
Port 21: FTP – file transfer
Port 25: SMTP – mail transfer
Port 23: TELNET – remote login
Port 80: HTTP – Web access
• These port numbers are well known to all
computers in the network
• E.g. whenever a client access port 25 of the server, it
means the client needs SMTP service
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The three types of addresses used in TCP/IP are shown in
Fig. TCP establishes a virtual path between the source and
destination processes before any data communication by
using two procedures, connection establishment to start
reliably and connection termination to terminate gracefully,
Three types of addresses used in TCP/IP
Connection-oriented service
TCP performs data communication in full-duplex mode, that is
both the sender and receiver processes can send segments
simultaneously. For connection establishment in full-duplex
mode, a four-way protocol can be used. However, the second
and third steps can be combined to form a three-way
handshaking protocol with the following three steps .
Step 1: The client sends SYN segment, which includes, source
and destination port numbers, and an initialization sequence
number (ISN), which is essentially the byte number to be sent
from the client to the server.
Step 2: The server sends a segment, which is a two-in-one
segment. It acknowledges the receipt of the previous segment
and it also acts as initialization segment for the server.
Step3: The client sends an ACK segment, which acknowledges
Protocol for connection establishment
Similarly for connection termination, a four-way
handshaking protocol is necessary for termination of
connection in both directions as shown in Fig. The four
steps are as follows:
Step 1: The client sends a FIN segment to the server.
Step 2: The server sends an ACK segment indicating the
receipt of the FIN segment and the segment also acts as
initialization segment for the server.
Step3: The server can still continue to send data and when
the data transfer is complete it sends a FIN segment to the
client.
Step4: The client sends an ACK segment, which
acknowledges the receipt of the FIN segment sent by the
server.
Protocol for connection termination
Reliable Communication in TCP
Flow control: Flow control refers to a set of procedures used
to restrict the amount of data that the sender can send
before waiting for acknowledgement.
As incoming data must be checked and processed before they
can be used , The rate of such processing is often slower than
than the rate of transmission. Thus each receiving device has
a block of memory, called a buffer, reserved for storing
incoming data until they are processed.
an send acknowledgement any time.
TCP uses byte-oriented sliding window protocol, which allows
efficient transmission of data and at the same time the destination
host is not overwhelmed with data. The flow control operation is in
Fig. As shown in the figure, the receiver has a buffer size of 8
Kbytes. After receiving 4 K bytes, the window size is reduced to 4
Kbytes. After receiving another 3 K bytes, the window size reduces
to 1 K bytes. After the buffer gets empty by 4 K bytes, the widow
size increases to 7 K bytes. So it may be noted that the window size
is totally controlled by the receiver window size, which can be
increased or decreased dynamically by the destination. The
destination host can send acknowledgement any time.
Figure : Flow control in TCP
Error Control: Error control is both error detection
and error correction
Error control in TCP includes mechanism for
detecting corrupted segments with the help of
checksum field. Acknowledgement method is used
to confirm the receipt of uncorrupted data. If the
acknowledgement is not received before the timeout, it is assumed that the data or the
acknowledgement has been corrupted or lost. It
may
be noted that there is no negative
acknowledgement in TCP.
Error detection and correction in TCP is achieved
through the use of three simple three tools:
--checksum
-acknowledgement
-time-out.
User Datagram protocol (UDP)
UDP is responsible for differentiating among multiple
source and destination processes within one host.
Multiplexing and demultiplexing operations are
performed using the port mechanism.
Multiplexing and demultiplexing mechanism of
UDP
UDP Datagram
The UDP datagram format is shown in Fig. A
brief description of different fields of the
datagram are given below:
Source port (16 bits): It defines the port
number of the application program in the host
of the sender
Destination port (16 bits): It defines the port
number of the application program in the host
of the receiver
Length: It provides a count of octets in the UDP
datagram, minimum length = 8
Checksum: It is optional, 0 in case it is not in
use
Characteristics of the UDP
Key characteristics of UDP are given below:
•UDP provides an unreliable connectionless delivery
service using IP to transport messages between two
processes
•UDP messages can be lost, duplicated, delayed and
can be delivered out of order
•UDP is a thin protocol, which does not add
significantly to the functionality of IP
•It cannot provide reliable stream transport service
The above limitations can be overcome by using
connection-oriented transport layer protocol known
as Transmission Control Protocol (TCP).
• Application layer protocols define the rules when
implementing specific network applications
• Rely on the underlying layers to provide accurate
and efficient data delivery
• Typical protocols:
• FTP – File Transfer Protocol
• For file transfer
• Telnet – Remote terminal protocol
• For remote login on any other computer on the network
• SMTP – Simple Mail Transfer Protocol
• For mail transfer
• HTTP – Hypertext Transfer Protocol
• For Web browsing
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Example: SMTP
SMTP Server
Client
Virtual
SMTP
SMTP
TCP
TCP
IP, ARP, ICMP
Network Interface
Actual
IP, ARP, ICMP
Network Interface
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• The underlying layers have guaranteed accurate
data delivery
• We need to make a lot agreements with the server
in application layer before sending mail
1.
2.
3.
Agree on how data is represented
•
Binary or ASCII
Ensure the right recipient
• There may be 1000 users served by the server
Ensure the client has the right to send mail
•
Some clients are not welcome
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SMTP MODEL - Procedure
• The SMTP design is based on the following model of
communication: as the result of a user mail request, the
sender-SMTP establishes a two-way transmission channel to
a receiver-SMTP.
• The receiver-SMTP may be either the ultimate destination
or an intermediate.
• SMTP commands are generated by the sender-SMTP and
sent to the receiver-SMTP. SMTP replies are sent from the
receiver-SMTP to the sender-SMTP in response to the
commands.
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Mail message format
RFC(Request for Comment) 822:
standard for text message format:
• header lines, e.g.,
– To:
– From:
– Subject:
different from SMTP commands!
• body
– the “message”, ASCII characters
only.
– OK for text messages
header
blank
line
body
• What about sending a jpeg image, which is not ASCII
– Encode it to 7-bit ASCII
– How does the receiver know that the message is a jpeg image?
•  Need additional headers called the Multipurpose Internet Mail
Extension (MIME) headers
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Multipurpose Internet Mail Extensions
Multipurpose Internet Mail Extensions
(MIME) is an Internet standard that extends
the format of email (RFC 822 Standard) to
support:
Text in character sets other than ASCII
Non-text attachments
Message bodies with multiple parts
Header information in non-ASCII character
sets
MIME types
Content-Type: type/subtype; parameters
Text
• example subtypes: plain,
html
Image
• example subtypes: jpeg,
gif
Audio
Video
• example subtypes: mpeg,
quicktime
Application
• other data that must be
processed by reader before
“viewable”
• example subtypes: msword,
octet-stream
• example subtypes: basic
,(32 kbps coding)
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File Transfer Protocol
•
FTP works on the client/server principle. A client program enables
the user to interact with a server in order to access information and
services on the server computer.
•
Files that can be transferred are stored on computers called FTP
servers. To access these files, an FTP client program is used. This is
an interface that allows the user to locate the file(s) to be transferred
and initiate the transfer process.
• Process: To transfer files with FTP, you use a program often called
•
•
the "client." The FTP client program initiates a connection to a remote
computer running FTP "server" software.
After the connection is established, the client can choose to send
and/or receive copies of files, singly or in groups.
To connect to an FTP server, a client requires a username and
password as set by the administrator of the server.
Transfer Files in a Heterogeneous Host Environment
• Due to multiple hardware types and operating systems file are
converted to four environmentally neutral data type for
transport and the converted to local types at the destination
–
–
–
–
ASCII
EBCDIC
IMAGE
LOCAL
A
E
I
L
NVT-ASCII
EBCDIC Text
Raw binary, series of octets
Raw binary using a variable byte size
• Client responsibility to tell server data type to use
• Default data type, unless otherwise specified is ASCII
File Structures
• Operating System store files in different structures
• FTP defined file structures for transporting files
– File
– Record
– Page
F
R
P
Unstructured, sequence of bytes
Series of records
Series of data blocks (pages)
• Default file structure is File (F)
• File Structure specified using STRU command
Transmission Modes
• Mode is used to specify additional coding or sequencing
performed on data
• independent of data type and file structure
– Stream
S
– Block
B
– Compressed
C
stream of bytes, if record structure
EOF sent as record indication; if file
eof indicated by closing stream
file sent as sequence of blocks
preceded by header info allows restart
of an interruped transfer
data compressed using run length
encoding
Domain Name Server (RFC 1034 &
1035 Standard)
• Every computer has a network address
• e.g. 158.132.161.99
• To access a computer, we need to specify its
network address
• Human beings are weak in memorizing numbers
• We prefer computer name or domain name
• e.g. hkpu10.polyu.edu.hk
• Need a machine on the Internet to convert name to
number
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The Domain Name System is basically a distributed database of host
information associating many types of information (e.g. IP address, mail
exchanger, etc.) with domain names.
DNS's distributed database is indexed by domain names. Each domain
name is essentially just a path in a large tree, called the domain name
space.
Domain Name : Each node in the tree has a text label (without dots)
that can be up to 63 characters long. A null (zero-length) label is
reserved for the root. The full domain name of any node in the tree is
the sequence of labels on the path from that node to the root. Domain
names are always read from the node toward the root ("up" the tree),
with dots separating the names in the path.
Each node in the Domain Name Space has a text label (the root node has
a special zero-length label, "") and is uniquely identified by its domain
name, i.e. the list of the labels on the path from the node to the root,
separated by dots
Figure Domain name space
Figure 19.3
TCP/IP Protocol Suite
Domain names and labels
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