Data Link Layer

Encapsulated Ethernet IEEE802

channel Point-to-point broadcasts

The purpose of sending and receiving data for the IP module is to send an ARP request for the ARP module and receive an ARP reply for the Rarp module to send RARP requests and receive RARP responses

Encapsulation of Ethernet and IEEE802
The most commonly used encapsulation format is the format defined by the RFC894

Type (upper data type): ipv4:0x0800 arp:0x0806 arp:0x8035 pppoe:0x8864 802.1Q tag:0x8100 ipv6:0x86dd MPLS label:0x8847

ARP

Ar_hrd:1, which means the Ethernet
AR_HLN and AR_PLN: for ARP requests or responses to IP addresses on Ethernet, their values are 6 and 4, respectively
Op:1, which represents the ARP request 2, which represents the ARP answer 3, represents RARP request 4, which represents the Rarp answer

When the system receives an ARP request message from the destination, he fills in the hardware address, replaces the two sender address with two destination addresses, and sends the Operation field to 2.

Example

0000 FF FF FF FF FF  0c F1 d4 D9-
0010 (  0c 
F1-D4-D9), XX 0020 XX  C0 A8 01 02  

Ethernet destination The FF FF FF FF FF is a broadcast address; 0c F1 D4 D9 60 is the Ethernet source address; frame type 0806, which refers to the data behind the ARP, which is the end of the Ethernet frame header. The hardware type 00 01 is Ethernet, the protocol type 08 00 is the IP protocol, the hardware address length and the protocol address length are 6 and 4;op 00 01 is a request packet; Sender MAC address 0c F1 d4 d9 60; Sender IP address c0 A8 0f;00 00 00 00 0 0 00 is the requested content; C0 A8 01 02 is the destination IP

MPLS Label
20-bit label (a number) 3-bit experimental field (usually used to carry IP precedence value) 1-bit Bottom-of-stack indicat Or (indicates whether the last label before the IP header) 8-bit TTL (equal to the TTL in IP header), used to Preve NT indefinite looping of packets.

Maximum Transmission Unit MTU
If the IP layer is transmitting a datagram that is larger than the MTU of the link layer, the IP layer will fragment the datagram

Ethernet Frame Length
Minimum length is 64 bytes (6+6+2+46+4)
Maximum length is 1518 bytes (6+6+2+1500+4)