Foundation Summary 317
Table 5-38 contrasts typical transport layer functions as performed (or not performed) by UDP
or TCP.
Table 5-39 summarizes the ICMP unreachable codes.
Table 5-38
TCP and UDP Functional Comparison
Function
Description (TCP)
Description (UDP)
Data transfer
Continuous stream of ordered data
Message (datagram) delivery
Multiplexing
Process that allows receiving hosts to
decide the correct application for
which the data is destined, based on
port number
Process that allows receiving hosts to
decide the correct application for
which the data is destined, based on
port number
Reliable transfer
Acknowledgment of data using the
sequence and acknowledgment fields
in the TCP header
Not a feature of UDP
Flow control
Process used to protect buffer space
and routing devices
Not a feature of UDP
Connections
Process used to initialize port numbers
and other TCP header fields
UDP is connectionless
Table 5-39
ICMP Unreachable Codes
Unreachable
Code
When Used
Typically Sent
By
Network
Unreachable
No match exists in a routing table for the destination of the
packet.
Router
Host Unreachable
The packet can be routed to a router connected to the
destination subnet, but the host is not responding.
Router
Can't Fragment
The packet has the Don't Fragment bit set, and a router must
fragment to forward the packet.
Router
Protocol
Unreachable
The packet is delivered to the destination host, but the
transport layer protocol is not available on that host.
Endpoint host
Port Unreachable
The packet is delivered to the destination host, but the
destination port has not been opened by an application.
Endpoint host
ch05.fm Page 317 Monday, March 20, 2000 5:06 PM