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OSI Model
Routing
Connection-oriented/Connectionless
Network Services
Destination
Source
Application Layer
Application Layer
Presentation Layer
Presentation Layer
Session Layer
Session Layer
Transport Layer
Transport Layer
Network Layer
Network Layer
Data Link Layer
Data Link Layer
Physical Layer
Network
Physical Layer
IDS
7 - Application Layer
Supports transmission from services
Unit: message
Uses: ftp, NFS, telnet
6 - Presentation Layer
Provides data translation
Unit: message
Uses: encryption, compression
5 - Session Layer
Maintains connectivity until task completion
Unit: message
Uses: RPC, netBIOS
firewall
4 - Transport Layer
router
3- Network Layer
bridge
switch
repeater
Partitions/reconstructs message
Unit: segment
Uses/standards: TCP, UDP
Delivers from logical device to logical device
Unit: packet
Uses/standards: IP, IPX, Appletalk
2 - Data Link Layer
Delivers from physical device to device
Unit: frame
Uses/standards: Ethernet, FDDI, T1
1- Physical Layer
Transmits raw data through net equipment
Unit: bit
Uses/standards: RS-232, 802.11
 Source:
• receive frame from data link
• transmit as raw bits
 Wires & connections
 Signal levels
 Repeaters and hubs (amplifiers/splitters)
 Frame stupid
 Logical link control
 Source:
• wraps packet within a frame
• forward frame to physical layer
 Destination:
• checks frames for integrity
• reconstruct packet from frame(s)
 Media Access Control (MAC) addresses
 Bridges and switches (connect by MAC)
Frame
preamble
8 bytes to establish
start of communication
data (payload)
46 to 1500 bytes
header
14 bytes including source
MAC, destination MAC,
frame length, frame type
MAC address - 6 bytes
this machine: 00:0d:93:87:80:10
broadcast:
CRC
4 byte cyclic
redundancy check
 Packets find their way through the network
 Source:
• wraps segment into a packet
• packet must contain network (IP) address
 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
• 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
Bridges vs. Routers
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
10.2.2.1
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
10.2.2.2
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
10.2.2.1
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
10.2.3.1
 Handles message partitioning/reconstructing
 Source:
• accepts message from session layer
• partitions message into sequence of
segments (will fit into frame)
 Destination:
• gathers together segments identified
by their sequence numbers
 Examples:
TCP - Transport Control Protocol
UDP - User Datagram Protocol
SPX - for IPX
ATP - for AppleTalk
 Maintains the complete “conversation”
 Source:
• attaches proper port address
 Destination:
• listens & directs from port to service
• maintains service integrity (e.g. directing
to proper window of web browser)
 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”
Connection-oriented
Parties must “shake hands” before communicating.
TCP handshake
syn = , ack =
syn = , ack =
source
syn = , ack =
dest.
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
MSRPC
MS remote procedure call
1433 / tcp
MS-SQL MS 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
HTTP
FTP
Telnet
UDP
TCP
IP
radio
pointEthernet packet to-point
TCP/IP Protocols