Network Crash Course

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Network Crash Course
OSI Model
Routing
Connection-oriented/Connectionless
Network Services
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OSI
Open Systems Interconnect Reference Model
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OSI Layers
IDS
firewall
router
repeater
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Physical Layer - 1
? Source receive frame from data link
transmit as raw bits
? Wires connections
? Signal levels
? Repeaters and hubs (amplifiers/splitters)
? Frame stupid
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Data Link Layer - 2
? 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)
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Ethernet
MAC address - 6 bytes
broadcast
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Network Layer - 3
? 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

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Internet Protocol (IP)
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
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Transport Layer - 4
? 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

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Session Layer - 5
? 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)
Presentation Layer - 6
? performs encryption/decryption if needed.
? note headers are added later so not encrypted
Application Layer - 7
? typical services SNMP, FTP, telnet, SMTP
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Routers
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
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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
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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?
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Connection-oriented or Connectionless
An issue of transport layer etiquette
Parties must shake hands before communicating.
TCP handshake
source
dest.
syn flood attack
Q How could a firewall block incoming traffic
still allow acks?
A
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Network Services (revisited)
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