Basic ip packet components, Basic protocol types, Basic ip packet components -21 – Netopia 4541 User Manual
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Security 11-21
Basic IP packet components
All IP packets contain the same basic header information, as follows:
This header information is what the packet filter uses to make filtering decisions. It is impor tant to note that a
packet filter does not look into the IP data stream (the User Data from above) to make filtering decisions.
Basic protocol types
TCP: Transmission Control Protocol. TCP provides reliable packet deliver y and has a retransmission
mechanism (so packets are not lost). RFC 793 is the specification for TCP.
UDP: User Datagram Protocol. Unlike TCP, UDP does not guarantee reliable, sequenced packet deliver y. If data
does not reach its destination, UDP does not retransmit the data. RFC 768 is the specification for UDP.
There are many more por ts defined in the Assigned Addresses RFC. The table that follows shows some of these
por t assignments.
Example TCP/UDP Ports
Source IP Address
163.176.132.18
Destination IP Address
163.176.4.27
Source Por t
2541
Destination Por t
80
Protocol
TCP
ACK Bit
Yes
DATA
User Data
TCP Port
Service
20/21
FTP
23
Telnet
25
SMTP
80
WWW
144
News
UDP Port
Service
161
SNMP