Chapter 2-Link Layer

Chapter 2 _ three purposes of The Link Layer of network protocol: sending and receiving IP datagram for the IP module, sending ARP requests for the ARP module, and receiving ARP responses; sends RARP requests to RARP and receives RARP responses. Ethernet is the main LAN technology used by TCP/IP today. It uses CSMA/CD (Multi-Channel Access for carrier listeners with Punch detection). The speed is 10 Mb/s and the address is 48bit. ARP and RARP are used to map 32-bit IP addresses and 48-bit hardware IP addresses. IP data packets include Ethernet encapsulation and IEEE 802 sub-assembly format, which are commonly used in Ethernet encapsulation. SLIP and PPP are often used in low-speed serial links. SLIP: IP address of a serial link. It is a simple form of encapsulation of IP data packets on a serial link. SLIP is suitable for RS-232 serial port and high-speed modem access to the Internet. RS-232 interface diagram: SLIP encapsulation is relatively simple, there are some defects: 1. Each end must know the other side of the IP address. There is no type field in the data frame. If a serial link is used for SLIP, other protocols cannot be used at the same time. 3. the checksum is not added to the data. Because the speed of the serial line is low, and the communication is often interactive, CSLIP (compressed SLIP), CSLIP takes 20 bytes of IP header and 20 bytes of TCP Header, a total of 40 bytes, to 3 or 5 bytes. The PPP Point-to-Point Protocol modifies all defects in the SLIP protocol. PPP has the following advantages over SLIP: (1) PPP supports running multiple protocols on a single serial line, not just the IP protocol; (2) each frame has a cyclic redundancy test; (3) the communication parties can conduct dynamic negotiation of the I P address (using the IP Network Control Protocol); (4) compression of the TCP and IP packet headers similar to CSLIP; (5) the Link Control Protocol can be used to set multiple data link options. The price paid for these advantages is that three bytes are added at the beginning of each frame. When a link is established, several frames of negotiation data are sent, and more complex implementations are required. Loop interface: allows communication between client programs and server programs running on the same host through TCP/IP. The general loopback address is 127.0.0.1. Maximum Transmission Unit (MTU): Ethernet MTU is 1500 bytes, IEEE802 MTU is 1492, and SLIP is 296 bytes. If there is a datagram to be transmitted on the IP layer, but the data length is greater than the MTU on the link layer, the IP layer needs to split the datagram into several slices so that each piece is smaller than the MTU. Path MTU: The minimum MTU in the path of the two communication hosts. The path MTU between the two hosts is not necessarily a constant, depending on the route selected at that time. The path MTU is not necessarily the same in both directions.