More on PPP

1
More on PPP

• Point-to-Point Protocol

2
Data Link Layer Process

• Internet layer process passes EACH IP packet to
  the data link layer process for delivery over the
  data link

Internet LayerProcess
IP Packet
Data Link LayerProcess
3
PPP

• Point-to-Point Protocol
• Popular data link layer protocol for dial-in
  connections
• Supported by Microsoft Windows
• Between data link layer processes on user PC and
  first router
• Not between user PC and the destination host

PPP
DLL Process
DLL Process
User PC
First Router
4
PPP

• Negotiation Phase
• Before exchanging data frames, the two sides
• Negotiate conditions of PPP operation
• Also negotiate how specific protocols will be
  handled, such as IP
• Not limited to IP

PPP
DLL Process
DLL Process
User PC
First Router
5
PPP

• Data Frames
• Header
• Information field (IP packet or other
  information)
• Trailer with Frame Check Sequence field
• Error detection but not correction
• If an error is found, PPP frame is discarded

Trailer
Information Field
Header
IP Packet
6
PPP

• Flag Fields (1 Octet Each)
• Always contain 10000001
• Designates start of frame and end of frame
• No length field necessary

Flag
Addr
Ctrl
Prot
Info
CRC
Flag
7
PPP

• Address Field (1 Octet)
• PPP was designed to allow several devices at each
  end
• For modem-modem communication, only one device at
  each end
• Has fixed value 11111111

Flag
Addr
Ctrl
Prot
Info
CRC
Flag
8
PPP

• Control Field (1 Octet)
• PPP was designed to allow control information
• Not used in PPP included because PPP is based on
  an older protocol that used this field
• In PPP, the control field contains the fixed
  value 00000011

Flag
Addr
Ctrl
Prot
Info
CRC
Flag
9
PPP

• Protocol Field (2 Octets)
• Identifies the contents of the information field
• There are values for IP, IPX, other internet
  layer protocols that may be carried in the
  information field
• There are values for supervisory messages

Flag
Addr
Ctrl
Prot
Info
CRC
Flag
10
PPP

• Cyclical Redundancy Check (2 Octets)
• For error-checking information
• Allows receiver to detect a PPP frame with a
  transmission error
• Receiver discards such frames error detection
  but not reliability (no error correction)

Flag
Addr
Ctrl
Prot
Info
CRC
Flag
11
Physical Layer Process

• Data link layer process passes EACH data link
  layer frame to the physical layer process for
  delivery to the next computer (router or host)

Data Link LayerProcess
DL Frame
Physical LayerProcess
12
Physical Layer Process

• Physical layer process does not create a protocol
  data unit
• Sends one bit at a time over the data link
  connecting the sending computer to the next
  computer
• It is the data link layer process that organizes
  these bits into frames over the data link

Physical LayerProcess
Physical LayerProcess
10110
13
Reliability

• Only TCP is Reliable or Needs to Be
• Corrects errors, gives application programs clean
  data
• Corrects errors that occur a the transport layer
  or lower layers because only correctly received
  TCP segments are acknowledged