NETWORK SOFTWARE
Software structuring technique
Network Software
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Protocol Hierarchies
Design Issues for the Layers
Connection-Oriented and Connectionless Services
Service Primitives
The Relationship of Services to Protocols
Protocol Hierarchies
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Most networks are organized as a stack of layers
or levels, each one built upon the one below it
The name of each layer, its content and functions,
the number of layers, differ from network to
network
The purpose of each layer is to offer certain
services to the higher layers, shielding those layers
from the details of how the offered services are
actually implemented
Protocol Hierarchies - layers, protocols, and interfaces
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Protocol Hierarchies: Structure
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Layered structure:
 Protocol
- rules and convention of data exchange
between layer n of host1 and layer n of host2
 Peers - entities that locally implement the
functionality of a given layer (processes,
hardware devices, even humans); peers
communicate by using the protocol
 Interface - the set of primitive operations and
services that lower layer provides to the upper
 Physical media - through it the actual
communication occurs - there is the signal carrier
Network Software
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Considerations:
 Virtual
exchange between the equivalent
level layers of two hosts according the
protocols
 Physical exchange between the neighbor
layers of one host according to the
interface
 Portability of the layers: based on clear
simple interfaces and well defined set of
functions of each layer
Network Architecture
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Network architecture - the set of layers and
protocols; ignores the interfaces as the interfaces
of the hosts in a network may differ
Protocol stack - the list of protocol hierarchy in
the network; matches the layered structure
Analogy to the network protocol stack – two
philosophers (peer processes) speaking different
languages (English and French) want to
communicate
The philosopher-translator-secretary
architecture
Protocol Hierarchies
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How to provide communication to the top layer of a
five-layer network?
A message, M, is produced by an application process
running in layer 5 and given to layer 4 for transmission
Layer 4 puts a header in front of the message to
identify the message and passes the result to layer 3
The header includes control information, such as
sequence numbers, to allow layer 4 on the destination
machine to deliver messages in the right order if the
lower layers do not maintain sequence
Information flow supporting virtual
communication in layer 5.
Protocol Hierarchies
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In most networks, there is nearly always a limit imposed
by the layer 3 protocol, so layer 3 must break up the
incoming messages into smaller units, packets, adding a
layer 3 header to each packet - here M is split into two
parts, M1 and M2
Layer 2 adds not only a header to each piece, but also
a trailer, and gives the resulting unit to layer 1 for
physical transmission
At the receiving machine the message moves upward,
from layer to layer, with headers being stripped off as
it progresses. None of the headers for layers below n
are passed up to layer n
Network Architecture
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Receiving of the message M at the destination machine
consists in:
 moving
of its packets upward the layers,
 stripping of control header and trailers,
 merging the packets in a message and
 interpreting the message by the application
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Lower layers have hardware implementation
Medium layer[s] have firmware implementation
High layers have software implementation
Design Issues for the Layers
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Addressing
Error Control
Flow Control
Multiplexing
Routing
Addressing
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A network normally has many computers, some of
which have multiple processes
A mechanism is needed for a process on one
machine to specify with whom it wants to talk
As a consequence of having multiple destinations,
some form of addressing is needed in order to
specify a specific destination
Error control
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Physical communication circuits are not perfect –
errors occur
Many error-detecting and error-correcting codes
are known, but both ends of the connection must
agree on which one is being used
The receiver must have some way of telling the
sender which messages have been correctly
received and which have not
Flow control
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Not all communication channels preserve the order
of messages sent on them
The protocol must make explicit provision for the
receiver to allow the pieces to be reassembled
properly
How to keep a fast sender from swamping a slow
receiver with data – either have some kind of
feedback from the receiver to the sender, or limit
the sender to an agreed-on transmission rate
Multiplexing
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Sometimes it is expensive to set up a separate
connection for each pair of communicating
processes, so the underlying layer may decide to
use the same connection for multiple, unrelated
conversations – multiplexing
Must be done transparently and can be used by
any layer
Needed in the physical layer, for example, where
all the traffic for all connections has to be sent over
at most a few physical circuits
Routing
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When there are multiple paths between source and
destination, a route must be chosen
Sometimes this decision must be split over two or
more layers
To send data from London to Rome, a high-level
decision might have to be made to transit France or
Germany based on their respective privacy laws
A low-level decision might have to made to select
one of the available circuits based on the current
traffic load
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Connection-Oriented and Connectionless
Services
Connection-oriented service: establishes the connection
from point to point; caries the exchange, preserving the
order of the bitstream and releases the connection,
analogy - telephone system (dial, talk, hang up)
Connectionless service: each message is provided with full
destination address and it is routed through the system
independently to rest of message stream, analogy –
postal system
QoS (quality of service) - reliability to losing data
Quality of service
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For a file transfer, the owner of the file wants to be sure
that all the bits arrive correctly and in the same order
they were sent (i.e. need acknowledgements)
Acknowledge receipts produce
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communication overhead and
delays
For some applications, the transit delays introduced by
acknowledgements are unacceptable – for example digitized voice traffic – better users to hear a bit of
noise on the line from time to time than to experience a
delay waiting for acknowledgements, or during a video
conference, having a few pixels wrong is no problem
Quality of service
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Unreliable (meaning not acknowledged) connectionless
service is often called datagram service, in analogy
with telegram service, which also does not return an
acknowledgement to the sender
At some occasions not having to establish a connection to
send one short message is desired, but reliability is
essential – then an acknowledged datagram service
can be provided
For the request-reply service. In this service the sender
transmits a single datagram containing a request; the
reply contains the answer - client-server model: the client
issues a request and the server responds to it
Connection-Oriented and Connectionless Services
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Six different types of service.
Why unreliable communication?
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Reliable communication (in our sense, that is,
acknowledged) may not be available – (Ethernet
network - packets can occasionally be damaged in
transit and it is up to higher protocol levels to deal
with this problem)
Delays inherent in providing a reliable service may
be unacceptable, especially in real-time
applications such as multimedia
For these reasons, both reliable and unreliable
communication coexist
Service Primitives
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A service is formally specified by a set of primitives
(operations) available to a user process to access the
service
Primitives:
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request some elementary service action;
inform the service process for some event in the peer
entity
When the protocol stack is located in the operating
system, the primitives are normally system calls
The primitives for connection-oriented service are
different from those of connectionless service
Service primitives for implementing a
simple connection-oriented service
Service primitives for implementing a
simple connection-oriented service
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1. LISTEN – the server is prepared to accept incoming
connections;
makes a blocking system call - after executing the
primitive, the server process is blocked until a request
for connection appears
2. client process executes CONNECT to establish a
connection with the server
CONNECT call specifies who to connect to - parameter
giving the server's address
The OS sends a packet to the peer asking it to connect
and the client process is suspended until there is a
response
Simple client-server interaction on a
connection-oriented network
Service primitives for implementing a
simple connection-oriented service
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3. packet arrives at the server, processed by the OS
– it realizes that the packet requests a connection
and checks if there is a listener (if LISTEN was
executed earlier), if there is no listener – the result
is undefined, or waits for a short time a LISTEN to
appear
4. System unblocks the listener
5. System sends an acknowledgement (ACK)
6. After the client receives ACK, client and server
processes are running – a connection is established
Service primitives for implementing a
simple connection-oriented service
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7. Server executes RECEIVE to prepare to accept the first
request – this is done immediately after being released
from LISTEN, before ACK gets back to the client
The RECEIVE call blocks the server
8. Client executes SEND to transmit its request (3 in the
figure)
9. Client executes RECEIVE to get the reply
10. Server receives request, process is unblocked and
request is processed (work is done)
11. Server executes SEND to pass the answer to the client
Service primitives for implementing a
simple connection-oriented service
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12. Client receives the answer, unblocks the process, it
looks at the received data
- if there are additional requests – repeat the
procedure from 8
- if there are no other requests – 13.
13. Client sends DISCONNECT to terminate the
connection (usually – DISCONNECT is a blocking call,
suspending the client
14. Server gets the packet, issues its DISCONNECT and
releases the connection
15. Client receives DISCONNECT and its process gets
released
Relationship Services -Protocols
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Services and protocols are distinct concepts
A service is a set of primitives (operations) that a layer
provides to the layer above it
The service defines what operations the layer is
prepared to perform on behalf of its users
A service says nothing at all about how these
operations are implemented
A service relates to an interface between two layers,
with the lower layer being the service provider and the
upper layer being the service user
Relationship Services -Protocols
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A protocol is a set of rules governing the format and
meaning of the packets/messages that are exchanged
by the peer entities within a layer
Entities use protocols to implement their service
definitions
Entities are free to change their protocols at will,
provided they do not change the service visible to their
users
the service and the protocol are completely decoupled
services relate to the interfaces between layers
protocols relate to the packets sent between peer
entities on different machines
Services to Protocols Relationship