The Source : Source 의 출처 제목 : Link Layer ( 수 ) 양 우 철

Contents  2.2 Ethernet and IEEE 802 Encapsulation  2.3 Trailer Encapsulation  2.4 SLIP : Serial Line IP  2.5 Compressed SLIP  2.6 PPP : Point-to-Point Protocol  2.7 Loopback Interface  2.8 MTU  2.9 Path MTU  2.10 Serial Line Throughput Calculations

2.2 Ethernet and IEEE 802 Encapsulation  Ethernet  A standard published in 1982 by Digital Equipment Corp., Intel Corp., Xerox Corp.  Predominant form of local area network technology used with TCP/IP today.  It uses an access method called CSMA/CD.  CSMA/CD : 10Mbits/sec, 48-bit addresses  : CSMA/CD networks  : Token bus networks  : Token ring networks  : Logical link control (LLC)

2.2 Ethernet and IEEE 802 Encapsulation  RFC 894 : The encapsulation of IP datagrams for Ethernets. (commonly used)  RFC 1042 : For IEEE 802 networks.  Requirement : Every Internet host connected to a 10Mbits/sec Ethernet cable.  1. Must be able to send and receive packets  2. Should be able to receive RFC1042 packets intermixed with RFC 894  3.May be able to send packets using RFC 1042 encapsulation.  If the host can send both types of packets, the type of packet sent must be configurable and the configuration option must default to RFC 894 packets.

2.2 Ethernet and IEEE 802 Encapsulation

2.3 Trailer Encapsulation  Defined in RFC 893  Another form of encapsulation used on Ethernets  Nowadays trailer encapsulation is deprecated.  The variable-length fields at the beginning of the data portion of the Ethernet frame were moved to the end. (right before CRC)  This allows the data portion of the frame to be mapped to a hardware page, saving a memory- to-memory copy when data is copied in the kernel.

2.4 SLIP : Serial Line IP  RFC 1055  Simple form of encapsulation for IP datagrams on serial line  Deficiencies  1.Each end must know the other’s IP address. There is no method for one end to inform the other of its IP address.  2.There is no type field. If a serial line is used for SLIP, it can’t be used for some other protocol at the same time.  3.There is no checksum added by SLIP. Therefore upper layers provide some form of CRC.

2.4 SLIP : Serial Line IP  1.The IP datagram is terminated by the special character called END (0xc0)  2.If a byte of the IP datagram equals the END character, the 2-byte sequence 0xdb, 0xdc is transmitted instead.  3.If a byte of the IP datagram equals the SLIP ESC character, the 2-byte sequence 0xdb, 0xdd is transmitted instead. SLIP ESC character

2.5 Compressed SLIP  A newer version of SLIP  Reduces 40-byte header to 3 or 5 bytes  Maintains the state of up to 16 TCP connections on each end of the CSLIP link.  Some of the fields in the two headers for a given connection normally don’t change. Of the fields that do change, most change by a small positive amount. These smaller headers greatly improve the interactive response time.

2.6 PPP : Point-to-Point Protocol  Corrects all the deficiencies in SLIP  PPP consists of three components.  1. A way to encapsulate IP datagrams on a serial link An Asynchronous link with 8 bits of data and no parity or bit- oriented synchronous links.  2.A link control protocol to establish, configure, and test the data-link connection.  3.A family of network control protocols (NCPs) specific to different network layer protocols.

2.6 PPP : Point-to-Point Protocol  Flag : 0x7E 각 프레임의 시작과 끝에 표시  Address : 0xff  Control : 0x30  Protocol ID : 2byte, 데이터 영역에 실린 상위계층표시  CRC : Frame 의 오류를 detecting

2.6 PPP : Point-to-Point Protocol  Since the byte 0x7e is the flag character, PPP needs to escape this byte  synchronous link : bit stuffing 기술 사용  Asynchronous link : Escape Character 로 0x7d 사용 Frame 의 Next character 의 sixth bit 을 완성 해야함 1.0x7e transmitted as the 2-byte sequence 0x7d, 0x5e ->escape of the flag byte 2.0x7d transmitted as the 2-byte sequence 0x7d, 0x5d ->escape of the escape byte 3.By default, a byte with a value less than 0x20 is also escaped.

2.6 PPP : Point-to-Point Protocol  Using the link control protocol, most implementations negotiate to omit the constant address and control fields and to reduce the size of the protocol field from 2bytes to 1byte.  SLIP 에 비해 PPP 가 제공하는 장점  1. 동적인 IP 할당  2. 데이터의 압축  3. 데이터의 암호화  4. 다양한 네트워크 프로토콜 지원  5. 모든 frame 에 대해 CRC  -> 단지 3bytes 를 추가.

2.7 Loopback Interface  동일 호스트상의 server 와 client 가 서로 다른 TCP/IP 를 사용하는 것을 포함, 통신할 수 있도록 허용.  Class A network ID 127 is reserved.  By convention, most systems assign the IP address of to this interface and assign it the name localhost.

2.7 Loopback Interface

2.8 MTU  해당 네트워크의 프레임에 실을 수 있는 최대 데이터의 크기  만약 IP 가 보낼 데이터를 갖고 있는데 link layer 의 MTU 보다 크면 IP 는 fragmentation 을 수행한다.

2.9 Path MTU  It is important that the smallest MTU of any data link that packets traverse between the two host.  The path MTU need not be the same in the two directions.

2.10 Serial Line Throughput Calculations  line speed : 960bytes/sec,Transfer 1024-byte packet -> 1066ms  Interactive App. 를 위해 SLIP 를 사용하면 wait time 은 533ms. Interactive App. 에서는 wait time 이 100ms~200ms 보다 크면 좋지 않다.  SLIP 에서는 MTU 크기를 256 으로 ->line 의 사용효율이 좋아진다.