Chapter 1 Overview

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Chapter 1


Networking Components
1

Chapter
1
Overview


Review

o
OSI reference model

o
TCP/IP (or Internet) reference model

o
Compare OSI and TCP/IP models

o
Format of Ethernet frames

o
Function of various network devices


Overview of demonstration network


Chapter 1


Networking Components
2

OSI Reference Model


Application:

supporting network
applications: FTP, SMTP, HTTP, etc.


Presentation:
handle different data
representations (e.g., encryption)


Session:
connections between apps


Transport:

host
-
host: TCP, UDP


Network:

routing of datagrams from
source to dest: IP, routing protocols


Link:

data transfer between adjacent
network elements: PPP, Ethernet


Physical:

bits “on the wire”

Application (
7
)

Presentation (
6
)

Session (
5
)

Transport (
4
)

Network (
3
)

Data link (2)

Physical (1)


Chapter 1


Networking Components
3

OSI Reference Model


Each layer provides service to the layer
above

o
Link layer relies on physical layer

o
Network layer relies on link layer, etc.


Protocol “stack” is conceptual


Layering is supposed to simplify networking

o
Believe it or not!


Chapter
1


Networking Components
4

Example Network


Hosts

live at edge of
network

o
Computers, servers, etc.


Routers are at the
core


This network has

o
WAN


Connected by satellite

o
2 LANs

o
2 segments on LAN 1


Connected by bridge


Chapter 1


Networking Components
5

Physical Layer


Bits on the “wire”


We don’t care
much about this


Repeaters and
hubs are physical
layer devices


Chapter 1


Networking Components
6

Data Link Layer


2
sublayers:


LLC sublayer

o
Logical Link Control


MAC sublayer

o
Medium Access
Control

o
CSMA/CD (Ethernet)


Chapter
1


Networking Components
7

Data Link Layer


Link layer data unit is a
frame


Recall that a
bridge

operates at the
link layer

o
Bridge looks at MAC address to decide
which interface(s) to forward frame

o
Bridges are self
-
learning

o
Bridges separate segments (separate
collision domains)


Chapter 1


Networking Components
8

Frame Structure


Preamble
---

allows NIC to detect frame


Start Frame Delimiter (SFD)
---

marks start of
frame


Dest. Address
---

hardware address of dest


Source Address
---

hardware address of source


Chapter 1


Networking Components
9

Frame Structure


Length
---

number of bytes in packet


Logical Link Control (LLC)
---

For reliable
communication on the same LAN


Packet
---

“data” from link layer perspective


PAD
---

pad frame if less than
64
bytes


Frame Check Sequence (FCS)
---

error checking


Chapter 1


Networking Components
10

Network Layer


Route packets over the
network

o
From source to dest


Data unit is a
packet

or
datagram


Routers

live at the
network layer


“Core” of the network


Chapter 1


Networking Components
11

Transport Layer


Logical host
-
to
-
host communication


TCP is reliable


UDP is unreliable


Note that SNMP uses UDP

o
Why?


Transport layer operates in hosts


Routers don’t care about transport layer


Chapter
1


Networking Components
12

Session Layer


Handles multiple logical processes on
hosts


For example, you can browse the
Web, send email, FTP and telnet
simultaneously

o
Must keep these processes separate


Is this really worth an entire “layer”?


Chapter 1


Networking Components
13

Presentation Layer


Compression, encryption, encoding, etc., are
the job of presentation layer


Also deals with

o
Abstract Syntax Notation One (ASN.1) which is
discussed in Appendix B

o
Basic Encoding Rules (BER) which is covered in
Appendix D


Is this really worth an entire “layer”?


Chapter 1


Networking Components
14

Application Layer


Like an API: transparently provides access
to lower layer services


No formal layer above, but application
layer provides service to applications


System Management Application Entity
(SMAE) provides support to network
management applications


Remote Operations Service Element
(ROSE) is contained in SMAE


Chapter
1


Networking Components
15

Application Layer


Common Management Information Protocol
(CMIP) makes use of ROSE


Note that we’ll be interested in SNMP

o
SNMP is
not

part of the OSI model

o
Instead, it’s part of the TCP/IP model


CMIP is the analog of SNMP in the OSI
model


Chapter 1


Networking Components
16

OSI Reference Model:

The Bottom Line


OSI model has everything and the kitchen
sink


As a result, it’s complex

o
Over
-
engineered?


OSI model took a long time to develop


So people started using a simpler model:
TCP/IP (or Internet) model


TCP/IP model applies to the Internet


Chapter 1


Networking Components
17

TCP/IP Protocol Stack

application


transport


network


link


physical


application:

includes OSI
application, presentation and
session layers


transport:

same as OSI


network:

same as OSI


link:

same as OSI

o
Author calls this “Network
Interface” layer with MAC
implementation


physical:

same as OSI


Chapter
1


Networking Components
18

Network Interface Layer


Network interface layer implements
Network Device Interface Specification
(NDIS)


NDIS is interfaces between higher layers
and NIC


Drivers for Ethernet, FDDI, etc.


NDIS allows higher layers to be
independent of media access method


Chapter 1


Networking Components
19

Network Interface Layer


Chapter 1


Networking Components
20

OSI vs TCP/IP Stacks


Link layer is slightly different

o
OSI designed with reliable link layer


OSI model “has it all”

o
But more complex


TCP/IP is “good enough”

o
Simpler


Chapter
1


Networking Components
21

Demo Network


LAN used in book


Chapter 1


Networking Components
22

Demo Network


Repeater is a physical layer device

o
Amplifies the signal

o
Can extend the length of the LAN


Hub is a repeater with multiple I/O ports

o
A physical layer device

o
Demo network has a hub in subnet 2


Bridge is a link layer device

o
Only forwards frame onto appropriate link(s)

o
“Transparent” since self
-
learning


Chapter 1


Networking Components
23

Demo Network


A switch is a fancy bridge

o
So a switch is a layer
2
device

o
In “switched Ethernet”, can have simultaneous
comm. between hosts on LAN without collisions


Layer
3
and layer
4
switches exist

o
These switches can use info in higher layers

o
Eg, layer
4
switch could give retransmission
priority to packets that require high QoS


Chapter 1


Networking Components
24

Demo Network


Routers

o
Layer 3 devices

o
Like bridges/switches, routers isolate collision
domains

o
Routers also isolate broadcast domains

o
Routing tables use IP address

o
For small network, static table is OK

o
For larger network, use RIP, OSPF, etc.

o
Routers route between networks, not hosts


Chapter 1


Networking Components
25

Demo Network


Probes

o
Gather information and pass it to “management
station”

o
Management station analyzes the network
traffic

o
Probe also called a “remote monitor”

o
In demo network, Probe 1 can monitor 2
segments simultaneously


Chapter
1


Networking Components
26

Chapter
1
Summary


Overview of networking


OSI reference model


TCP/IP protocol stack


Networking devices


Demo network


Book includes lots of pictures of hardware