Transcript PPT

The Source : Source의 출처
제목 : Link Layer
2005. 3. 16(수)
양우철
[email protected]
Contents
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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
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Ethernet
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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)
2.2 Ethernet and IEEE 802 Encapsulation
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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.
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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
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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 memoryto-memory copy when data is copied in the
kernel.
2.4 SLIP : Serial Line IP
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RFC 1055
Simple form of encapsulation for IP datagrams
on serial line
Deficiencies
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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
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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.
SLIP ESC character
3.If a byte of the IP datagram equals the SLIP ESC
character, the 2-byte sequence 0xdb, 0xdd is
transmitted instead.
2.5 Compressed SLIP
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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
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Corrects all the deficiencies in SLIP
PPP consists of three components.
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1. A way to encapsulate IP datagrams on a serial link
An Asynchronous link with 8 bits of data and no parity or bitoriented synchronous links.
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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
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Flag : 0x7E 각 프레임의 시작과 끝에 표시
Address : 0xff
Control : 0x30
Protocol ID : 2byte, 데이터 영역에 실린 상위계층표시
CRC : Frame의 오류를 detecting
2.6 PPP : Point-to-Point Protocol
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Since the byte 0x7e is the flag character, PPP
needs to escape this byte
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synchronous link : bit stuffing 기술 사용
Asynchronous link : Escape Character로 0x7d사용
Frame의 Next character의 sixth bit을 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
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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.
SLIP에 비해 PPP가 제공하는 장점
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1.동적인 IP 할당
2.데이터의 압축
3.데이터의 암호화
4.다양한 네트워크 프로토콜 지원
5.모든 frame에 대해 CRC
->단지 3bytes를 추가.
2.7 Loopback Interface
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동일 호스트상의 server와 client가 서로 다른
TCP/IP를 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.
2.7 Loopback Interface
2.8 MTU
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해당 네트워크의 프레임에 실을 수 있는 최대
데이터의 크기
만약 IP가 보낼 데이터를 갖고 있는데 link layer의
MTU보다 크면 IP는 fragmentation을 수행한다.
2.9 Path MTU
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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
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line speed : 960bytes/sec,Transfer 1024-byte
packet -> 1066ms
Interactive App.를 위해 SLIP를 사용하면 wait
time은 533ms. Interactive App.에서는 wait
time이 100ms~200ms보다 크면 좋지 않다.
SLIP에서는 MTU크기를 256으로->line의
사용효율이 좋아진다.