An IP protocol frame, in the context of PPP (Point-to-Point Protocol), is a PPP frame that encapsulates an IP datagram for transmission over a PPP link. Essentially, it's how IP data travels across a PPP connection.
Here's a breakdown:
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PPP Frames: Think of PPP frames as envelopes for data transmission over a serial link (like a modem connection). These frames provide a structured way to send and receive data.
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IP Datagrams: IP datagrams are packets of data formatted according to the Internet Protocol (IP). These contain the source and destination IP addresses, payload data, and other control information necessary for routing the data across networks.
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Encapsulation: The IP datagram is placed inside the "information field" (also known as the "data field") of the PPP frame. This process is called encapsulation. The PPP frame adds its own header and trailer for link management and error detection.
In simpler terms: Imagine sending a letter (IP datagram) inside an envelope (PPP frame). The envelope provides address and postage information specific to the postal service (PPP link), while the letter contains the actual message and intended recipient (IP datagram).
Why use IP protocol frames within PPP?
PPP is a common protocol for establishing a direct connection between two nodes. Since IP is a dominant protocol for network communication, PPP provides a mechanism to transport IP traffic over these direct links.
Key characteristics of an IP Protocol Frame (within PPP):
- Contains a single IP datagram: Each PPP frame typically carries only one IP datagram in its information field.
- Standard PPP header and trailer: The PPP frame includes a header and trailer for tasks like link establishment, authentication, and error checking.
- Protocol field: The PPP header includes a "Protocol field" which indicates the type of protocol being carried in the information field. For an IP protocol frame, this field will be set to indicate that the payload is an IP datagram.
Example:
Let's say computer A (IP address 192.168.1.10) wants to send data to computer B (IP address 192.168.1.20) over a PPP link.
- Computer A creates an IP datagram with the data and the source and destination IP addresses.
- The PPP implementation on computer A takes this IP datagram and encapsulates it within a PPP frame. The PPP header will contain control information for managing the PPP link and a protocol field indicating that the payload is an IP datagram.
- The PPP frame is then transmitted over the PPP link.
- Computer B receives the PPP frame and extracts the IP datagram from the information field.
- Computer B then processes the IP datagram.
