APPLICATION LAYER
Application software issues
• Transport layer software supports the reliable
transfer of data between applications
• Application layer software supports:
1.
2.
3.
4.
5.
Initiating connections
An application programming interface (API)
Encoding data
User friendly naming
Definition of specific applications
The client server paradigm
• A widely used form of communication
• Server application waits passively for contact
from clients
• A server provides a specific service
• Client application actively initiates contact
with the server
• Information can flow in both directions
• Typical situation is many clients interacting
with each server
Characteristics of clients
• General application that becomes a client when
remote access is needed
• Invoked by the user
• Runs on users local computer
• Actively initiates contact with the server
• Can access multiple services as needed
Characteristics of servers
• Special purpose program to provide one
service
• can handle multiple clients at one time
• invoked when the system boots
• runs on a relatively powerful shared computer
• waits passively for contact from various kinds
of client
Transport protocols and client-server
interaction
• Clients and servers communicate using a
transport protocol - unaware of underlying
layers
Server class computers
• Often a single server class computer runs
several servers
Identifying a particular service
• Transport protocols assign a unique identifier
to each service
– server registers its ID with protocol software when
it boots
– client specifies the desired ID when it makes a
connection
• TCP uses protocol port numbers
Concurrency and servers
• Concurrent server offers supports multiple clients at
the same time
• Uses multiple threads of control
• Core part of server accepts new requests and
dynamically creates new servers as separate
threads to handle them
• Each thread deals with its client’s requests
• N concurrent clients => N + 1 threads
• TCP uses a combination of destination port and
source port to identify the right thread
Complex client-server interactions
• A client is not restricted to accessing a
single service
• A server can itself become a client of
another server
• Watch out for circular dependencies
Distributed programming
• Extend client-server paradigm to more general
distributed programming
• Provide greater transparency for programmers
– remote procedure calls (RPC)
– distributed objects and components
• Provide standard services for locating and
manufacturing other services
– traders
– factories
Traders
trader
2. Request
service
1. export
service offer
client
service
3. use
service
Factories
trader
2. request
create
service
factory
1. Request
service
3. create
service
client
service
4. use
service
Other forms of communication
peer
peer
Peer to peer
communication
peer
peer
Dynamic Host Configuration
Protocol (DHCP)
Dynamic Host Configuration Protocol
(DHCP)
Provides four pieces of information :
1. The IP address of the computer
2. The subnet mask of the computer
3. The IP address of a router
4. The IP address of a name server
DHCP operation
• The DHCP client and server can
1. Same network
2. Different networks
o Same network

the administrator may put the client and the server on the same network as shown
in Figure
Same Network
1.
2.
3.
The DHCP server issues a passive open command on UDP port number
67 and waits for a client.
A booted client issues an active open command on port number 68 . The
message is encapsulated in a UDP user datagram, using the destination
port number 67 and the source port number 68. The UDP user
datagram, in turn, is encapsulated in an IP datagram. The reader may ask
how a client can send an IP datagram when it knows neither its own IP
address (the source address) nor the server’s IP address (the destination
address). The client uses all 0s as the source address and all 1s as the
destination address.
The server responds with either a broadcast or a unicast message using
UDP source port number 67 and destination port number 68. The
response can be unicast because the server knows the IP address of the
client. It also knows the physical address of the client, which means it
does not need the services of ARP for logical to physical address
mapping. However, some systems do not allow the by passing of ARP,
resulting in the use of the broadcast address.
Different Networks
A client can be in one network and the server in another, separated by several other networks.
Figure shows the situation. However, there is one problem that must be solved. The DHCP
request is broadcast because the client does not know the IP address of the server. A broadcast
IP datagram cannot pass through any router. A router receiving such a packet discards it. Recall
that an IP address of all 1s is a limited broadcast address. To solve the problem, there is a need
for an intermediary. One of the hosts (or a router that can be configured to operate at the
application layer) can be used as a relay. The host in this case is called a relay agent. The relay
agent knows the unicast address of a DHCP server and listens for broadcast messages on port
67. When it receives this type of packet, it encapsulates the message in a unicast datagram and
sends the request to the DHCP server. The packet, carrying a unicast destination address, is
routed by any router and reaches the DHCP server. The DHCP server knows the message comes
from a relay agent because one of the fields in the request message defines the IP address of
the relay agent. The relay agent, after receiving the reply, sends it to the DHCP client.
DHCP packet format
1.
Operation code. This 8-bit field defines the type of DHCP packet: request (1) or reply (2).
2.
Hardware type. This is an 8-bit field defining the type of physical network. Each type of
network has been assigned an integer. For example, for Ethernet the value is 1.
3.
Hardware length. This is an 8-bit field defining the length of the physical address in bytes.
For example, for Ethernet the value is 6.
4.
Hop count. This is an 8-bit field defining the maximum number of hops the packet can
travel.
5.
Transaction ID. This is a 4-byte field carrying an integer. The transaction identification is set
by the client and is used to match a reply with the request. The server returns the same
value in its reply.
6.
Number of seconds. This is a 16-bit field that indicates the number of seconds elapsed
since the time the client started to boot.
7.
Flag. This is a 16-bit field in which only the leftmost bit is used and the rest of the bits
should be set to 00s, e.i:(0000000000000001). A leftmost bit specifies a forced broadcast
reply (instead of unicast) from the server.
8. Client IP address. This is a 4-byte field that contains the client IP address. If
the client does not have this information, this field has a value of 0.
9. Your IP address. This is a 4-byte field that contains the client IP address. It
is filled by the server (in the reply message) at the request of the client.
10. Server IP address. This is a 4-byte field containing the server IP address. It
is filled by the server in a reply message.
11. Gateway IP address. This is a 4-byte field containing the IP address of a
router. It is filled by the server in a reply message.
12. Client hardware address. This is the physical address of the client.
Although the server can retrieve this address from the frame sent by the
client, it is more efficient if the address is supplied explicitly by the client in
the request message.
13.Server name. This is a 64-byte field that is optionally
filled by the server in a reply packet. It contains a
null-terminated string consisting of the domain
name of the server. If the server does not want to fill
this field with data, the server must fill it with all 0s.
14.Boot filename. This is a 128-byte field that can be
optionally filled by the server in a reply packet. It
contains a null-terminated string consisting of the
full pathname of the boot file. The client can use this
path to retrieve other booting information. If the
server does not want to fill this field with data, the
server must fill it with all 0s.
15.Options. This is a 64-byte field. It can carry either
additional information such as the network mask or
default router address
File Transfer Protocol (FTP)
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.
FTP
File Transfer Protocol (FTP) is a TCP/IP clientserver application for copying files from one
host to another. FTP requires two connections
for data transfer: a control connection and a
data connection.
1- Control Connection
1. The server issues a passive open on the well-known port 21 and
waits for a client.
2. The client uses an ephemeral port and issues an active open.
The connection remains open during the entire process. The service
type, used by the IP protocol, is minimize delay because this is an
interactive connection between a user (human) and a server. The user
types commands and expects to receive responses without significant
delay.
2- Data Connection
Data Connection
The data connection uses the well-known port 20 at the
server site. However, the creation of a data connection is
different from what we have seen so far. The following shows
how FTP creates a data connection:
1. The client, not the server, issues a passive open using an
ephemeral port. This must be done by the client because
it is the client that issues the commands for transferring
files.
2. The client sends this port number to the server using the
PORT command.
3. The server receives the port number and issues an active
open using the well-known port 20 and the received
ephemeral port number