TCP/IP protocol stack Analysis (2)

This section describes link layer.
In the TCP/IP protocol family, the link layer has three main objectives: (1) sending and receiving IP datagram for the IP module;
(2) send ARP requests to the ARP module and receive ARP responses; (3) Send RARP requests to RARP and receive RARP responses.
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1. encapsulation of Ethernet and IEEE 802: the Encapsulation Format is as follows:


2. SLIP-Serial Line IP Address: it is a simple form of encapsulation of IP data packets on a serial line. SLIP is not tested, and the upper layer needs to verify its data.
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3. PPP protocol: the control and verification components are added to the SLIP protocol, which is encapsulation of the PPP protocol:

The Protocol control field of PPP can run multiple protocols on a single serial line; CRC Field: Each pillow has a cyclic redundancy check.

4. loopback interface: To enable communication between client programs and server programs on the same host through TCP/IP, most products support loopback interfaces, generally, the system assigns the address 127.0.0.1 to the loopback interface and name it localhost. A datagram sent to the loopback interface cannot appear on any network.
The following describes how the loopback interface processes IP data packets:
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