This layer says to the upper layer, “Just give me your data stream, with any instructions, and I’ll begin the process of getting your information ready to send.”
The following sections describe the two protocols at this layer:
- Transmission Control Protocol (TCP)
- User Datagram Protocol (UDP)
TCP
The figure shows the different fields within the TCP header.
UDP
This figure clearly illustrates UDP’s markedly low overhead as compared to TCP’s hungry usage.
Key concepts of Host to Host Protocols
TCP:
- Sequenced
- Reliable
- Connection-oriented
- Virtual circuit
- Acknowledgments
- Windowing flow control
UDP:
- Unsequenced
- Unreliable
- Connectionless
- Low overhead
- No acknowledgment
- No windowing or flow control
Port Numbers
Port number examples for TCP and UDP
Key Protocols and Port Numbers
TCP:
Telnet = 23
SMTP = 25
HTTP = 80
FTP = 21
DNS = 53
HTTPS = 443
UDP:
SNMP = 161
TFTP = 69
DNS = 53
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