Unit 6
19.1
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DNS
To identify an entity, TCP/IP protocols use the IP
address, which uniquely identifies the connection of a
host to the Internet.
 However, people prefer to use names instead of numeric
addresses.
 Therefore, we need a system that can map a name to an
address or an address to a name.
 A user of an e-mail program may know the recipient’s email address; however, the IP protocol needs the IP
address.
 The DNS client program requests a DNS server to map
the e-mail address to the corresponding IP address.

19.2
Purpose of DNS
19.3
Namespace
To be unambiguous, the names assigned to
machines must be carefully selected from a
namespace with complete control over the
binding between the names and IP addresses.
 In other words, the names must be unique
because the addresses are unique.
 A name space that maps each address to a
unique name can be organized in two ways:


19.4
Flat or Hierarchical.
Flat Namespace
In a flat namespace, a name is assigned to an
address.
 A name in this space is a sequence of characters
without structure.
 The names may or may not have a standard
section; if they do, it has no meaning.
 The main disadvantage of a flat namespace is
that it cannot be used in an extensive system
such as the Internet because it must be centrally
controlled to avoid ambiguity and duplication.

19.5
Hierarchical Namespace
In a hierarchical namespace, each name is made of
several parts.
 The first part can define the nature of the
organization, the second part can define the name of
an organization,
 The third part can define departments in the
organization, and so on.
 In this case, the authority to assign and control the
namespaces can be decentralized

19.6
Hierarchical Namespace
A central authority can assign the part of the name
that defines the nature of the organization and the
name of the organization.
 The organization can add suffixes (or prefixes) to the
name to define its host or resources.
 The management of the organization need not worry
that the prefix chosen for a host is taken by another
organization because, even if part of an address is the
same, the whole address is different.

19.7
Hierarchical Namespace
19.8

Assume two colleges and a company call one of their
computers challenger.

The first college is given a name by the central authority such
as fhda.edu, the second college is given the name berkeley.edu,
and the company is given the name smart.com.

When each of these organizations adds the name challenger to
the name they have already been given, the end result is three
distinguishable names: challenger.fhda.edu,
challenger.berkeley.edu, and challenger.smart.com.
Domain Namespace
To have a hierarchical name space, a domain name space was
designed.
 In this design the names are defined in an inverted-tree
structure with the root at the top.
 The tree can have only 128 levels: level 0 (root) to level 127

19.9
Label
Each node in the tree has a label, a string with a
maximum of 63 characters.
 The root label is a null string (empty string).
 DNS requires that children of a node (nodes that
branch from the same node) have different labels,
guaranteeing the domain names' uniqueness.

19.10
Domain Name
Each node in the tree has a domain name. A full
domain name is a sequence of labels separated by
dots (.).
 The domain names are always read from the node up
to the root.
 The last label is the label of the root (null). This
means that a full domain name always ends in a null
label, which means the last character is a dot because
the null string is nothing

19.11
Domain Names and Labels
19.12
Fully Qualified Domain Name
(FQDN)
If a label is terminated by a null string, it is called a fully
qualified domain name (FQDN).
 An FQDN is a domain name that contains the full name of a
host.
 It contains all labels, from the most specific to the most
general, that uniquely define the host’s name.
 A DNS server can only match an FQDN to an address.
 That the name must end with a null label, but because null
means nothing, the label ends with a dot (.).

challenger.atc.fhda.edu
19.13
Partially Qualified Domain Name
(PQDN)
If a label is not terminated by a null string, it is called a
partially qualified domain name (PQDN).
 A PQDN starts from a node, but it does not reach the root. It is
used when the name to be resolved belongs to the same site as
the client.

19.14
Domain
A domain is a subtree of the domain name space. The name of
the domain is the name of the node at the top of the subtree.
 A domain may itself be divided into domains (or subdomains
as they are sometimes called).

19.15
Distribution of Name Space
The information contained in the domain name space
must be stored.
 However, it is very inefficient and also unreliable to have
just one computer store such a huge amount of
information.

It is inefficient because responding to requests from all
over the world places a heavy load on the system.
 It is not reliable because any failure makes the data
inaccessible

19.16
Hierarchy of Name Servers






19.17
The solution to these problems is to distribute the information
among many computers called DNS servers.
One way to do this is to divide the whole space into many domains
based on the first level.
In other words, we let the root stand alone and create as many
domains (subtrees) as there are first-level nodes.
Because a domain created in this way could be very large, DNS
allows domains to be divided further into smaller domains
(subdomains).
Each server can be responsible (authoritative) for either a large or a
small domain.
In other words, we have a hierarchy of servers in the same way
that we have a hierarchy of names
Hierarchy of Name Servers

The hierarchy is organized in a tree-like structure, and it includes
various types of name servers.
Root Servers:
1.
1.
2.
3.
Top Level Domain( TLD) Servers:
2.
1.
2.
3.
19.18
At the top of the DNS hierarchy are the 13 root name servers (labeled A through
M).
These servers maintain information about top-level domains (TLDs) like .com,
.org, .net, .gov, and country-code TLDs like .us, .uk, etc.
They don't contain information about specific domain names but rather direct
queries to the appropriate TLD servers.
These servers are responsible for specific top-level domains (TLDs).
For example, there are separate TLD servers for .com, .org, .net, .uk, and many
others.
They maintain information about second-level domains (SLDs) within their TLD.
Hierarchy of Name Servers

The hierarchy is organized in a tree-like structure, and it includes
various types of name servers.
3.
19.19
Authoritative Name Servers:
1.
These are the servers maintained by domain registrars or
organizations that have control over specific domain names.
2.
They store the actual DNS records (such as A records for IP
addresses or MX records for mail servers) for individual domain
names.
3.
Authoritative name servers are responsible for providing DNS
information for specific
Hierarchy of Name Servers

The hierarchy is organized in a tree-like structure, and it includes
various types of name servers.
4.
19.20
Recursive Solvers:
1.
These are typically operated by Internet Service Providers (ISPs)
or DNS resolver services like Google's public DNS or OpenDNS.
2.
Recursive resolvers are responsible for handling DNS queries from
end-user devices.
3.
When a user's device makes a DNS query, the recursive resolver
contacts the root servers, TLD servers, and authoritative name
servers to obtain the requested information
Resolution
Mapping a name to an address or an address to a name
is called name-address resolution.
 Resolver





19.21
DNS is designed as a client/server application.
A host that needs to map an address to a name or a name to an
address calls a DNS client called a resolver.
The resolver accesses the closest DNS server with a mapping
request.
If the server has the information, it satisfies the resolver;
otherwise, it either refers the resolver to other servers or asks
other servers to provide the information.
Resolution-Mapping Names to
Addresses





19.22
After the resolver receives the mapping, it interprets the response to
see if it is a real resolution or an error, and finally delivers the result to
the process that requested it.
In this case, the server checks the generic domains or the country
domains to find the mapping.
If the domain name is from the generic domains section, the resolver
receives a domain name such as "chal.atc.jhda.edu.".
The query is sent by the resolver to the local DNS server for resolution.
If the local server cannot resolve the query, it either refers the resolver
to other servers or asks other servers directly
Recursive Resolution
The client (resolver) can ask for a recursive answer from
a name server.
 This means that the resolver expects the server to
supply the final answer.
 If the server is the authority for the domain name, it
checks its database and responds.
 If the server is not the authority, it sends the request to
another server (the parent usually) and waits for the
response.

19.23
Recursive Resolution
If the parent is the authority, it responds; otherwise, it
sends the query to yet another server.
 When the query is finally resolved, the response travels
back until it finally reaches the requesting client.
 This is called recursive resolution

19.24
Iterative Resolution
If the client does not ask for a recursive answer, the
mapping can be done iteratively.
 If the server is an authority for the name, it sends the
answer.
 If it is not, it returns (to the client) the IP address of the
server that it thinks can resolve the query.
 The client is responsible for repeating the query to this
second server. If the newly addressed server can resolve
the problem, it answers the query with the IP address;
otherwise, it returns the IP address of a new server to
the client. Now the client must repeat the query to the
third server. This process is called iterative resolution
because the client repeats the same query to multiple
servers.

19.25
Iterative Resolution
If the newly addressed server can resolve the problem, it
answers the query with the IP address; otherwise, it
returns the IP address of a new server to the client.

Now the client must repeat the query to the third
server.
 This process is called iterative resolution because the

client repeats the same query to multiple servers.
19.26
Caching





19.27
Each time a server receives a query for a name that is not in its
domain, it needs to search its database for a server IP address.
Reduction of this search time would increase efficiency.
DNS handles this with a mechanism called caching.
When a server asks for a mapping from another server and receives
the response, it stores this information in its cache memory before
sending it to the client.
If the same or another client asks for the same mapping, it can
check its cache memory and solve the problem
Example of DNS Resolution
19.28
SMTP
The actual mail transfer is done through
message transfer agents.
 To send mail, a system must have the
client MTA( Mail transfer Agent), and to
receive mail, a system must have a server
MTA.
 The formal protocol that defines the MTA
client and server in the Internet is called
the Simple Mail Transfer Protocol (SMTP).

19.29
Figure 26.16 SMTP range
26.30
Figure 26.17 Commands and responses
26.31
Figure 26.18 Command format
26.32
SMTP
SMTP is used two times, between the sender and the
sender's mail server and between the two mail servers.
 Another protocol is needed between the mail server and
the receiver.

19.33
Mail Transfer Phases
The process of transferring a mail message occurs in three
phases: connection establishment, mail transfer, and
connection termination.
 Connection Establishment
 A client initiates a connection to the SMTP server of the
recipient's domain on port 25 (or an alternate port if
specified).
 The client and server engage in a handshake protocol to
establish a connection. This typically involves a 3-way
handshake.
 Once the connection is established, the server is ready
to receive commands from the client.

19.34
Mail Transfer Phases





19.35
Mail Transfer:
The client (sender) initiates the mail transfer phase by sending a series of
SMTP commands to the server.
Common SMTP commands during the mail transfer phase include:

HELO/EHLO: The client identifies itself to the server.

MAIL FROM: The client specifies the email address of the sender.

RCPT TO: The client specifies the recipient's email address.

DATA: The client begins sending the email content.
The email content includes the header, message body, and any
attachments. The client sends this data to the server using the DATA
command. The data is terminated with a period (.) on a line by itself.
The server processes the email and, if all is well, queues it for delivery to
the recipient's mailbox. If there are issues, the server may send back error
messages to the client.
Mail Transfer Phases

Connection Termination
Once the email transfer is complete, the client
can initiate the connection termination phase.
 This typically involves sending the QUIT
command to the server to gracefully close the
connection.
 The server acknowledges the QUIT command,
and the connection is terminated.

19.36
FILE TRANSFER
Transferring files from one computer to another is one of the most common tasks
expected from a networking or internetworking environment. As a matter of fact,
the greatest volume of data exchange in the Internet today is due to file transfer.
File Transfer Protocol (FTP)
Anonymous FTP
26.37
Note
FTP uses the services of TCP. It needs two TCP connections.
The well-known port 21 is used for the control connection
and the well-known port 20 for the data connection.
26.38
FTP
26.39
Using the control connection
26.40
Communication over Control Section
FTP uses the same approach as SMTP to communicate
across the control connection.

It uses the 7-bit ASCII character set Communication is
achieved through commands and responses.
 This simple method is adequate for the control
connection because we send one command (or
response) at a time.
 Each command or response is only one short line, so we
need not worry about file format or file structure.
 Each line is terminated with a two-character (carriage
return and line feed) end-of-line token.

19.41
Using the data connection
26.42





19.43
The purpose of the data connection is different from that of the control
connection.
We want to transfer files through the data connection. File transfer occurs
over the data connection under the control of the commands sent over the
control connection.
However, we should remember that file transfer in FTP means one of three
things: A file is to be copied from the server to the client.
This is called retrieving after. It is done under the supervision of the RETR
command,
A file is to be copied from the client to the server. This is called storing
after.


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
19.44
It is done under the supervision of the STOR command.
A list of directory or file names is to be sent from the server to the
client.
This is done under the supervision of the LIST command.
Note that FTP treats a list of directory or file names as a file.
It is sent over the data connection.
The client must define the type of file to be transferred, the
structure of the data, and the transmission mode.
Before sending the file through the data connection, we prepare for
transmission through the control connection.
The heterogeneity problem is resolved by defining three attributes
of communication: file type, data structure, and transmission mode
HTTP
The Hypertext Transfer Protocol (HTTP) is a protocol used
mainly to access data on the World Wide Web.
 HTTP functions as a combination of FTP and SMTP.
 Unlike SMTP, the HTTP messages are not destined to be
read by humans; they are read and interpreted by the HTTP
server and HTTP client (browser).
 SMTP messages are stored and forwarded, but HTTP
messages are delivered immediately.
 The contents of the requested file or other information are
embedded in a response message.
 HTTP uses the services of TCP on well-known port 80

19.45
HTTP
Although HTTP uses the services of TCP, HTTP itself is a
stateless protocol.
 The client initializes the transaction by sending a request
message.
 The server replies by sending a response.

19.46
HTTP Transaction
HTTP uses the services of TCP, HTTP itself
is a stateless protocol.
 The client initializes the transaction by
sending a request message.
 The server replies by sending a response.
 Messages

A request message consists of a request line,
a header, and sometimes a body.
 A response message consists of a status line,
a header, and sometimes a body

19.47
HTTP messages are of two types: request
and response.
 Both the message types follow the same
message format.
 Request Message: The request message is sent

by the client that consists of a request line, headers, and
sometimes a body.

Response Message: The response message is
sent by the server to the client that consists of a status
line, headers, and sometimes a body.
19.48
Request Message
19.49
Response Message