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
16
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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