Telecommunications Essentials

1
TelecommunicationsEssentials

• John R. Durrett
• July 5, 2005

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CISSP CIA Triad

• Confidentiality
• Integrity
• Availability

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Basic Definitions

• Protocol a standard set of rules for computer
  to computer communications
• Describes format of messages
• Describes way messages are transmitted
• Enables different computers to communicate
• Allows extensions of existing standards
• Typically uses a layered model

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Layered Architectures

• Utilizes logical groups called layers
• Helps clarify communications process
• Breaks down complexity
• Enables interoperability
• Allows changing of individual layers without
  changing others
• Makes for easier troubleshooting

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TCP/IP
Application
Transport Layer
Transport Layer
Network Layer
Network Layer
Network Layer
Network Layer
Alice
Router
Bob
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Transmission Control Protocol

• Full duplex
• Connection Oriented, packets are sequenced
• Costly in terms of network overhead
• Slower than UDP
• Syn, Ack-Syn, Ack

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TCP Fields
Source port Destination port Sequence
number Acknowledgment number Data offset
Reserved Window Checksum Urgent
pointer Options Padding data
U R P
A C K
P S H
R S T
S Y N
F I N
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User Datagram Protocol

• Best effort delivery only
• Connectionless
• No sequence, no error correction
• unreliable protocol

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UDP Fields
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Ports
Ports are used in the TCP RFC793 to name the
ends of logical connections which carry long term
conversations. For the purpose of providing
services to unknown callers, a service contact
port is defined. This list specifies the port
used by the server process as its contact port.
The contact port is sometimes called the
"well-known port".
PORT USE
17 Quote of the Day
20 File Transfer Data
21 File Transfer Control
22 SSH
23 Telnet
25 SMTP
53 DNS
gt 1024

• Source port
• Destination port
• Logical connection
• Privileged unprivileged ports
• Netstat na

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Internet Protocol

• Network Layer Protocol
• Logical ID called an IP address
• IP sender IP recipient
• Routing decisions based on destination IP address

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IP Address (IPv4)

• uniquely identifies a computer on a network
• 32 bits, 4 bytes of 8 bits each
• xxxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
• dotted quad notation system
• Example 198.113.201.23
• There are five classes of addresses A - E.

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Identifying a Class
Address Identifier
Network Address
Host Address
16,277,214 hosts per network ID
Class A
0-126
7 bits of network address
24 bits of host address
0
Last three bytes
First byte
65,354 hosts
Class B
128-191
14 bits of network address
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16 bits of host address
First two bytes
Last two bytes
254 hosts
Class C
192-223
8 bits of host address
21 bits of network address
110
First three bytes
Last byte
224-239
Class D
1110
Multicast address in the range of 224.0.0.0 -
239.255.255.255
Class E
240-255
11110
Class E - Reserved for future use
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Subnetting
Customer Site
130.1.1.0 130.1.2.0 130.1.3.0 . . . . . .
. 130.1.255.0
130.1.0.0
Internet

• Each address consists of two componentsNetwork
  address and Host address
• Determined by Netmask
• 10.21.41.3 00001010 00010101 00101001 00000011
• 255.255.0.0 11111111 11111111 00000000 00000000
• Network address is IP XORed with netmask