"TCP/IP Details" Learning notes-2nd Chapter link Layer

There are three purposes of the data link layer:

（1）为IP模块发送和 接收IP数据报。（2）为ARP模块发送ARP请求和接收ARP应答。（3）为RARP发送RARP请 求和接收RARP应答。

ARP is called address Resolution Protocol and is a protocol that uses IP addresses to turn to MAC addresses.
Rarp is called the inverse Address Resolution Protocol, is a MAC address to the IP address of a protocol.

The data link layer belongs to the bottom of the computer network, and the channels used are two types of point-to-point channel and broadcast channel .

TCP/IP supports a number of different link layer protocols, depending on the hardware used by the network, such as Ethernet, Token Ring, FDDI (Fiber Distributed Data Interface), and RS-232 serial line.

Protocol Data unit of the data Link layer-frame: The datagram of the IP layer (network layer) is added to the header and the trailer is encapsulated into frames.
There are many kinds of data link layer protocols, which will solve three basic problems: package framing, transparent transmission, error detection.

The Maximum Transmission unit (Maximum transmission unit,mtu) refers to the maximum packet size (in bytes) that can be passed on a layer of a communication protocol.
If the IP layer is transmitting a datagram that is larger than the MTU of the link layer, then the IP layer will shard the datagram. A datagram is divided into slices, each with a size less than or equal to the MTU value of the link layer . When hosts on the same network communicate with each other, the MTU of the network is important to both sides of the communication. But when the host to communicate through many networks, the most important for both sides of communication is the minimum MTU in the communication path, because the link layer MTU of different networks on the communication path is different. The smallest MTU in the communication path is called the Path MTU.

csma/cd(Carrier sense multiple access/collision Detect) is a carrier-monitored multi-access/collision detection mechanism. The purpose is to prevent collisions between sites connected to the bus and to coordinate the protocols that work on the computers on the bus.
(1) Multipoint access: This is the total Linetype network, many computers connected to the bus in a multi-point way.
(2) Carrier monitoring: refers to each station before sending data to detect whether there are other computers on the bus to send data, if there is, temporarily do not send data to avoid collisions
(3) Collision detection: Edge to send data edge detection channel signal voltage size. When the signal voltage oscillation value exceeds a certain threshold value, it indicates that a collision occurred. Immediately stop sending, wait for a period of time to send again.

For loopback interfaces, there are a few areas to note:

(1) Any data transmitted to the loopback address (typically 127.0.0.1) is entered as I p.
(2) A copy of the datagram transmitted to the broadcast address or multicast address is transmitted to the loopback interface and then sent to the Ethernet. This is because the definition of broadcast delivery and multicast delivery contains the host itself.
(3) Any data passed to the IP address of the host is sent to the loopback interface.

"TCP/IP Details" Learning notes-2nd Chapter link Layer