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2
Link Layer
2
Ethernet and IEEE 802 Encapsulation
MUST be able to send and receive Ethernet
SHOULD be able to receive IEEE 802
MAY be able to send Ethernet and IEEE 802
configurable
default Ethernet
2
SLIP - Serial Line IP
A way to encapsulate IP-datagrams on a serial link
Simple - END + ESC character
no address negotiation
no type field - 1 protocol
Error prone - no CRC
CSLIP
Old fashioned
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c0c0 dbdb
c0c0 dbdb dcdc
IP datagram
SLIP encapsulation
ENDEND
dbdb dddd c0c0ESC ESC
SLIP - Framing
2
PPP - Point to Point Protocol
A way to encapsulate IP-datagrams on a serial link
Asynchronous AND synchronous communication
Link Control Protocol (LCP)
negotiation for datalink options
Conf-req * Conf-ack * Conf-nack * Conf-rej
Network Control Protocol (NCP)
negotiation for network protocol options
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PPP Framing
flagflag addraddr contolcontol protocolprotocol informationinformation CRCCRC flagflag
protocolprotocol IP datagramIP datagram
protocolprotocol Link control dataLink control data
protocolprotocol Network control dataNetwork control data1
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3
Link setup
2
PPP - LCP Negotiation options
Maximum Receive Unit (MRU)
Asynchronous Control Character Map
Authentication Protocol
Address and Control field compression
Multi-link procedure
Call-back
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Password authentication protocol
Snooping gets you the password
Not compatible with bidirectional authorization
PAP Authentication
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Challenge authentication protocol Follows establishment of LCP Identifies user Random number challenge,
crypto response Snooping does not discover password
CHAP Authentication
2
CHAP
LCP
Link (Async, ISDN)
CHAP
LCP
Link (Async, ISDN)
Challenge
Response
CHAP Authentication
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LCPLCP
LinkLink
LCPLCP
LinkLink
LCPLCP
LinkLink
MLP BundleMLP Bundle
IPCPIPCP IPXCPIPXCP
Multilink PPP
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PPP - IPCP options
IP-Address
IP-Compression-Protocol
Unnumbered mode
Primary-DNS-Address
Secondary-DNS-Address
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PPP summary
Multiple protocols on one link
CRC for every frame
Dynamic IP address negotiation (LCP)
TCP and IP header compression
Negotiation of datalink options (NCP)
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Serial line troughput calculation
33.6 kbps - 8 bits / byte - 1 start - 1 stop = 3360 bytes /sec
1024-byte + 20(TCP) + 20(IP) + 8(PPP) --> 319 msec
RTT = 638 ms
> 100 - 200 ms is bad for interactive response
for multiple sessions use small frames or better use queuing
