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The Source : Source 의 출처 제목 : Link Layer 2005. 3. 16( 수 ) 양 우 철

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Presentation on theme: "The Source : Source 의 출처 제목 : Link Layer 2005. 3. 16( 수 ) 양 우 철"— Presentation transcript:

1 The Source : Source 의 출처 제목 : Link Layer 2005. 3. 16( 수 ) 양 우 철 gregory@hufs.ac.kr

2 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

3 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  802.3 : CSMA/CD networks  802.4 : Token bus networks  802.5 : Token ring networks  802.2 : Logical link control (LLC)

4 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.

5 2.2 Ethernet and IEEE 802 Encapsulation

6 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.

7 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.

8 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

9 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.

10 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.

11 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

12 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.

13 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.

14 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 127.0.0.1 to this interface and assign it the name localhost.

15 2.7 Loopback Interface

16 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.

17 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.

18 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.


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