4. What is Point-to-Point Protocol ?
What is the difference between point to point and peer to peer?
Answer:
- Point to point is a protocol for communication between two
computers using a serial interface (phone).
- Peep to peer (P2P) is a communication model in which each
party has the same capabilities and either party can initiate a
communication session (BitTorrent).
5. What is Point-to-Point Protocol ?
The Point-to-Point Protocol (PPP) provides a standard method for transporting
multi-protocol datagrams over point-to-point links. PPP is comprised of three
main components:
• A method for encapsulating multi-protocol datagrams.
• A Link Control Protocol (LCP) for establishing, configuring, and testing the
data-link connection.
• A family of Network Control Protocols (NCPs) for establishing and configuring
different network-layer protocols.
6. Design requirements?
• Packet framing: Packet (from network-layer) Frames (start & end of frame)
• Transparency.To network layer packet (headers or data).
• Multiple network layer protocols. Able to support multiple network layer protocols (e.g., IP, RIP,
ICMP) running over the same physical link at the same time.
• Multiple types of links. Operate over a wide variety of link types: serial or parallel, synchronous or
asynchronous, low speed or high speed, electrical or optical.
• Error detection. Able to detect bit errors in the received frame.
• Connection liveness: detect a failure at the link level (e.g., the inability to transfer data from the
sending side of the link to the receiving side of this link) and signal this error condition to the
network layer.
• Network Layer Address Negotiation. mechanism for the communicating network layers (e.g., IP)
to learn or configure each other's network layer address.
• Simplicity.The first and foremost among all requirements . RFC 1547 states "the watchword for a
point-to-point protocol should be simplicity."
7. Design Non-requirements?
• Error correction. Required to detect bit errors but
is NOT required to correct
• Flow control. receive frames at the full rate of the underlying
physical layer .NO need to control for upper layers
• Sequencing. NOT required to deliver frames in the same order
• Multipoint links. Only a single sender and a single receiver.
8. Data frame format
01111110 11111111 00000011 Protocol Info Padding Check 01111110
• Flag: delimiter (framing)
• Address: not used in PPP as no support addressing(used in HDLC broadcast address)
• Control: always 00000011, transmission not sequenced, connectionless link.
• Protocol: upper layer protocol to which frame delivered (eg,IP 0x0021, IPX 0x002B)
• Info: upper layer data being carried
• Padding: In some cases, additional dummy bytes may be added to pad out the size
of the PPP frame.
• Check: cyclic redundancy check for error detection (2 0r 4 bytes, default 2 bytes)
Flag FlagAddress Control
1 byte 1 byte 1 byte 1 or 2 byte
Variable
length 2 or 4 bytes 1 byte
Variable
length
9. Stuffing
• Question: if receive <01111110> => It is data or flag?
• Sender: adds (“stuffs”) extra < 01111110> byte after
each < 01111110> data byte
• Receiver:
- two 01111110 bytes in a row: discard first byte,
continue data reception
- single 01111110: flag byte
PPP PPP
A5
BB
01111110
B4
B4
01111110
BB
A5
A5 BB 01111110 01111110 B4
RIP, Routing Information ProtocolICMP, Internet Control Message Protocol
Link Dead: This phase occurs when the link fails, or one side has been told to disconnect (e.g. a user has finished his or her dialup connection.)Link Establishment Phase: This phase is where Link Control Protocol negotiation is attempted. If successful, control goes either to the authentication phase or the Network-Layer Protocol phase, depending on whether authentication is desired.Authentication Phase: This phase is optional. It allows the sides to authenticate each other before a connection is established. If successful, control goes to the network-layer protocol phase.Network-Layer Protocol Phase: This phase is where each desired protocols' Network Control Protocols are invoked. For example, IPCP is used in establishing IP service over the line. Data transport for all protocols which are successfully started with their network control protocols also occurs in this phase. Closing down of network protocols also occur in this phase.Link Termination Phase: This phase closes down this connection. This can happen if there is an authentication failure, if there are so many checksum errors that the two parties decide to tear down the link automatically, if the link suddenly fails, or if the user decides to hang up his connection.
Weak-point: passwork and user are sent in clear text.
