Additional data (trailer data) and functions of the link layer in TCP/IP

What is the additional data for the link layer in the 1.TCP/IP?

When the message is opened with Wireshark, the trailer data displayed by the link layer is the additional data,

2. How to Generate

1. For example, Ethernet automatically fills messages that are less than 64 bytes in size (not experimentally).

2. When writing data using link-layer sockets, the actual data size > (IP header message size + link layer header)

3. Life cycle

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4. Role

Suppose there are 2 devices A and B under Layer two switches

1. Added an additional channel for the operating system to communicate with the outside world

Imagine, how to get the system time of B device from device A when the protocol is not supported?

When the B device sends data to a device (any protocol), writes its own system time to the additional data, and a uses a link-layer socket (such as a socket used by tcpdump) to obtain the link-layer data to

Additional data to do the parsing.

2. Controlling the behavior of upper-level applications

The behavior of the application is affected by the received message, and the approximate flow of the application ticker is as follows:

Network card received message--Kernel processing->TCP/IP protocol stack processing--upper application

Visible kernel processing can tamper with the message (for example, Iptables's mangle) to "spoof" the application.

For example, upper-level applications only serve specific IPs, such as 1.1.1.1, if a request from 2.2.2.2 is rejected, the upper application is not changed

Behavior, the message can be modified at the kernel to change the 2.2.2.2 to 1.1.1.1 (and of course, the kernel also changes the 1.1.1.1 to 2.2.2.2 when the application contracts)

The role of additional data is to provide a strategy or basis for such message modification.

Additional data (trailer data) and functions of the link layer in TCP/IP