Physical Layer

volleyballbeginnerNetworking and Communications

Oct 27, 2013 (3 years and 5 months ago)

120 views

OSI Model

Routing

Connection
-
oriented/Connectionless

Network Services

Source

Destination

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Network

7
-

Application Layer

6
-

Presentation Layer

5
-

Session Layer

4
-

Transport Layer

3
-

Network Layer

2
-

Data Link Layer

1
-

Physical Layer

Supports transmission from services

Uses:

ftp, NFS, telnet

Unit:

message

Provides data translation

Uses:

encryption, compression

Unit:

message

Maintains connectivity until task completion

Uses:

RPC, netBIOS

Unit:

message

Partitions/reconstructs message

Uses/standards:

TCP, UDP

Unit:

segment

Delivers from logical device to logical device

Uses/standards:

IP, IPX, Appletalk

Unit:

packet

Delivers from physical device to device

Uses/standards:

Ethernet, FDDI, T1

Unit:

frame

Transmits raw data through net equipment

Uses/standards:

RS
-
232, 802.11

Unit:

bit

IDS

firewall

router

bridge

switch

repeater



Source:




receive frame from data link




transmit as raw bits



Wires & connections



Signal levels



Repeaters and hubs (amplifiers/splitters)



Frame stupid



Logical link control



Destination:




checks frames for integrity




reconstruct packet from frame(s)



Media Access Control (MAC) addresses



Bridges and switches (connect by MAC)



Source:




wraps packet within a frame




forward frame to physical layer

Frame


preamble

8 bytes to establish

start of communication


header

14 bytes including source

MAC, destination MAC,

frame length, frame type


data (payload)

46 to 1500 bytes


CRC

4 byte cyclic

redundancy check

MAC address
-

6 bytes

this machine:

00:0d:93:87:80:10



broadcast
:



Packets find their way through the network



Destination:




packets received only if the logical



device address matches the packet


• strip away frame header & CRC



Routers select network path based on


logical address of destination



Source:




wraps segment into a packet




packet must contain network (IP) address

Bridges vs. Routers

• An IPv4 address consists of 4 octets.

• The range of an octet is 0 through 255.

Class

1st Octet

Subnet Mask

Available Addresses


A


1
-

127


B

128
-

191


C

192
-

223


D

224
-

255

reserved for broadcast

10.2.2.1

10.2.2.2

10.2.2.1

10.2.3.1



Handles message partitioning/reconstructing



Destination:




gathers together segments identified



by their sequence numbers



Examples:


TCP
-

Transport Control Protocol


UDP
-

User Datagram Protocol


SPX
-

for IPX


ATP
-

for AppleTalk



Source:




accepts message from session layer




partitions message into sequence of


segments (will fit into frame)



Maintains the complete “conversation”



Destination:




listens &
directs from port to service


• maintains service integrity (e.g. directing



to proper window of web browser)



Source:




attaches proper port address



performs encryption/decryption if needed.



note: headers are added later so not encrypted



typical services: SNMP, FTP, telnet, SMTP

A router connects logical networks.

Its purpose is to route packets between subnets.

Routing is performed according to routing tables.

Four types of routers



static



distance vector



link state



label switching

Static Routing

The routing table is manually configured.

• simple

• efficient routing

• good security (if properly configured)

• requires the most maintenance

Distance Vector Routing

The table is built from Routing Information Protocol.

• oldest, most popular, routing

• tables rely upon “advertised” hop information

• distance vector used to determine “best” routes

• vulnerable to spoofing

Link State Routing

The table is built from Link State Protocol.

• LSP sends
actual

hop data.

• LSP frames can be requested from other routers

• uses some authentication (password & MD)

Label Switching Routing

The table is built from Multiprotocol Label Switching

• MPLS faster by permitting by using MAC

• packets include label(s) of routing info

• route efficiency (not just hop count) is used

• standards?

An issue of transport layer “etiquette”

Parties must “shake hands” before communicating.

TCP handshake

Connection
-
oriented

source

dest.

syn = , ack =

syn = , ack =

syn = , ack =

syn flood attack

Q:

How could a firewall block incoming traffic & still allow acks?


A:


Port/protocol

Service

Purpose

20 / tcp

ftp data

transfers file content

21 / tcp

ftp

transfers ftp commands

22 / tcp

ssh

secure shell (remote access)

23 / tcp

telnet

remote computer login

25 / tcp

smtp

email delivery

43 / tcp

whois

Internet domain lookup

80 / tcp

http

web browser

110 / tcp

pop

pop email service

119 / tcp

nntp

network news

143 / tcp

imap

imap email service

161 / udp

snmp

remote system management

443 / tcp

ssl

secure socket layer (tunnel)

445 / tcp

smb

MS network file system

593 /tcp

MS
-
RPC

MS remote procedure call

1433 / tcp

MS
-
SQL

MS SQL server

1521

Oracle

Oracle SQL server

• Ports 0
-
1023 statically assigned

• Ports 1024
-
65535
-

upper ports


can be dynamically assigned

• Vulnerability:


dynamic port assignment

• A defense:

7
-

Application Layer

6
-

Presentation Layer

5
-

Session Layer

4
-

Transport Layer

3
-

Network Layer

2
-

Data Link Layer

1
-

Physical Layer

OSI Model

TCP/IP Protocols

HTTP

FTP

Telnet

TCP

UDP

Ethernet

radio

packet

point
-

to
-
point

IP