lunes, 24 de diciembre de 2018

Programming PDF – Section 1.5: layers of protocols and service models


Network logic

Section 1.5: layers of protocols and service models.

Topic 1: Content

2

1.1 What is the Internet?
1.2 The frontier of the network: access networks and physical media.
1.3 Network core.

 Packet switching, circuit switching, network network.

1.4 Delay, loss and transfer rate in the networks of

packet switching.

1.5 Layers of protocols and service models.
1.6 Attacks on networks.
1.7 History of the Internet and computer networks.

Topic 1. Computer Networks and the Internet: network logic.

Introduction: protocol layers, service models

3

 Network logic (logic for network operation).

 Different ways of approaching the design of communication networks.
 Structuring of software and communications hardware  Logic of

organized communication in layers / levels.
 Grouping of similar operations to transfer data between different programs

computers.

 Stack / tower / set of protocols organized in "N" layers / levels.
 Each layer / level built on the lower layer / level.

 Objectives:

 Offer services to the upper layers, hiding details of how they are performed.
 Use services from the lower layers.
 No. of layers, name and function differ according to network and model.

Topic 1. Computer Networks and the Internet: network logic.

Logic networks, fundamentals

4

function

of

 The

communication

is
decomposed into a set of modules,
layers or levels.

 The design of the network is simplified:

 The "big problem" is fragmented into
and

problems minus
manageable ("divide and conquer").

complexes

 Layered design makes things easier.
 P.e, the OSI model defines seven layers.

Topic 1. Computer Networks and the Internet: network logic.

5

Logic of networks, fundamentals
 Elements of logic and structuring of communications SW.

 Layer or level.
 Service.
 Entities.

 Peer entities in different systems.
 Entities do not stop in the same system.

 Communication.

 Horizontal -Virtual.
 Vertical – Real.

 Interface.
 Protocol.

10

Topic 1. Computer Networks and the Internet: network logic.

Network Logic, Fundamentals

6

 Layer or level.

 Set of well-defined functions, aimed at solving a given problem

.

 Service.

 Set of operations (primitives) that a layer provides to the layer

immediately above.

 Result of one or more functions defined by the corresponding protocol.

 Entities.

 Active elements HW or SW (single layer processes).
 They develop certain functions.
 Pairs and not pairs.

Topic 1. Computer Networks and the Internet: network logic.

Logic of networks, fundamentals

7

 Communication

 Horizontal – VIRTUAL.

 Runs on the basis of a protocol between peer entities (homologous layers) of

different machines.
 The physical medium of transmission is responsible for physical communication.

 Vertical – REAL.

 It runs between adjacent non-pairs entities (non-homologous layers) on the same

machine.

 A layer (entity) passes data and control information to the next (entity) layer

lower / upper.

 Interface.

 Defines rules for the interaction between layers of the same system.
 Defines which operations and services offers a layer to the next higher,
allowing replacement of the implementation while maintaining the same set of
services.

Topic 1. Computer Networks and the Internet: network logic.

Network Logic, Fundamentals

8

 Protocol

 Set of rules and conventions for communication between homologous layers

(even entities) of different systems.

 Actions / functions developed by peer entities to provide the service of

communication corresponding to a layer between different systems.

 Normalization.

 It comprises three key elements:

 Service definition (functional description of the service).
 Specification of the protocol.
 Addressing through the SAP (Service Access Point).

Service

SAPs

Layer N

Protocol

Topic 1. Computer Networks and the Internet: network logic.

Network Logic, Fundamentals

9

 SAP: Service Access Point.

 Identifies each entity (process) within a layer.
 Addressing mechanism between entities of different layers of the

same system.

 Through an SAP of layer "N" an entity of layer "N + 1" can:

 Access a service of layer "N".

 The SAPs allow the "N" layer to multiplex several users of the "N + 1" layer.
 Possible in all layers, but it is not usually done in all layers.

 Be addressed from the "N" layer and receive information from an "N" layer entity.

Topic 1. Computer Networks and the Internet: network logic.

Logic of networks, fundamentals

10

Generic example of the
communication logic
based on five layers

Basic elements of the logic of
communication related in
slide 5, except "entities" and
"services"

Topic 1. Computer and Internet networks: network logic.

Layer 1 procedure

Caption:
Physical communication, actual:
Logical, virtual communication ( protocol):

Logic of networks, fundamentals

11

 Battery / tower of protocols.

 Set of protocols in a system, one per layer in the context of a communication.
 Each protocol independent of the others. Together they allow communication to

through the network.

 Network logical architecture

 Set of layers (processes) and protocols.
 Does not include implementation details or interfaces specification, which may NOT

be the same in different systems (machines).

 Contains information for HW developers and SW communications.

 Distinction between services and protocols.

 Service: set of primitives (operations) that offers a layer that is by

above.

 Protocol: rules that govern the exchange of data structures between entities

pairs on different machines.

 The services and protocols are normalized.

Topic 1. Computer Networks and the Internet: network logic.

12

Network logic, basics
 General functions of the protocols and network logic.

 Encapsulated / de-encapsulated.
 Segmentation / assembly and locking / unlocking.
 Transfer services:
 Confirmed and not confirmed.
 Connective and non-connective.

 Flow, error and sequence control.
 Addressing (machine and process).
 Routing.
 Logic multiplexing / demultiplexing (non-physical).
 Other: priorities, quality of service, security, etc.
 There are functions that only develop in certain layers, or in a single layer, others

in several layers.

 Not all protocols provide all functions.

Topic 1. Computer Networks and the Internet: network logic.

Network Logic, Fundamentals

13

 Encapsulated (in the sending host).

 Each process per layer adds control information (usually header) that

is transmitted to the lower layer.

 De-encapsulated (in the receiving host).

 Each process per layer removes control information (usually header) that

is not transmitted to the upper layer.

1

M = M1 + M2

This example also shows
"operations"
the
data units that are
discussed below.

on

Origin

Destination
Topic 1. Computer Networks and the Internet: network logic.

Network Logic, Fundamentals

14

 PDU, protocol data unit.

 Information structure on which all protocol is based.
 Defined in each layer of the protocol stack.
 It is what is transmitted to the peer entity in another remote system.
 It can be DATA or CONTROL.
 Structure: PDU = PCI + SDU

 PCI, protocol control information (headers).

 It supports the logic of the corresponding communication protocol.

 SDU, service data unit.

 Contains the "data" from the perspective of the corresponding layer.
 It is the PDU of the next higher layer.

 Size limits

 The service does not necessarily impose limits on the size of the SDU.
 The protocol specification may place limits on the size of the PDU.

Topic 1. Computer Networks and the Internet: network logic.

Network Logic, Fundamentals
 Operations on Data Units

15

 Only when required:

 Segmentation  Fragmentation of PDUN to form several SDUN-1, due to :

 Limitations on the size of SDUN-1.
 Layer "N" delivers information continuously.
 Requires re-assembly, reverse operation at the other end of the communication.

 Blocking  Union / grouping of several PDUNs to form the SDUN-1.

 To take better advantage of available transmission capacity.
 Requires unlocking, reverse operation at the other end of the communication.

Segmentation operation

PDU n + 1

No operation

PDU n + 1

PCI n

SDU n

PCI n

SDU n

PCI n

SDU n

Block operation

PDU n + 1

PDU n + 1

PCI n

SDU n

Which operation to develop?
Depends on the context, there are cases that justify
one or the other, or none at all. Each has advantages
inconvenient disadvantages.

Topic 1. Computer Networks and the Internet: network logic.

Network Logic, Fundamentals

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 Segmentation is justified when:

 The network only accepts PDUs of limited size. Pe:

 In ATM networks, PDUmax = 53 octets
 In Ethernet networks, PDUmax = 1518 bytes

 You want efficiency in error correction, smaller PDU requires less

bits to relay.

 You want to better share the transmission capabilities by avoiding very large PDUs

that monopolize the transmission capabilities.

 We want to reduce the storage capacities in the buffers of

.



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