[Day 10] 11. Overcoming the Drawbacks of HDLC

Protocols in the Data Link Layer: Addressing HDLC Limitations

The Data Link Layer employs various protocols to overcome the limitations of HDLC (High-Level Data Link Control), with PPP (Point-to-Point Protocol) being a prominent example. PPP includes two critical components:

• NCP (Network Control Protocol): A framework within PPP that enables multiple network layer protocols to function over a single point-to-point link. An example is IPCP (Internet Protocol Control Protocol), used for configuring and managing IP connections.

• LCP (Link Control Protocol): Responsible for establishing, configuring, and testing data link connections, LCP was particularly useful during the era of traditional telephone lines for data transmission.

Key Features of LCP

1. Multilink Support
   LCP allows multiple physical links to be combined into a single logical link. This feature was crucial in the era of PSTN (Public Switched Telephone Network), where typical telephone lines offered low speeds (e.g., 28.8 kbps).

2. Error Detection
   Mechanisms like the Quality metric and Magic Number ensure data integrity during transmission, minimizing errors and avoiding link misidentification.

3. Compression
   LCP improves data transfer rates through compression techniques, such as:

   • Stacker: Uses a dictionary-based method to reduce data size before transmission.
   • Predictor: Optimizes data transmission by predicting patterns in the data.

4. Authentication
   LCP supports user authentication to enhance connection security, with two main protocols:

   • PAP (Password Authentication Protocol):

     • Transmits username and password in plain text, making it vulnerable to interception.
     • Authentication occurs only once at the start of the session.

   • CHAP (Challenge Handshake Authentication Protocol):

     • Employs a secure three-way handshake for authentication, occurring periodically (e.g., every 10 seconds) during the session.

     • Uses MD5 hashing for added security:

       1. Challenge: Router A sends a random challenge to Router B.
       2. Response: Router B combines the challenge with its password and sends the hashed result.
       3. Verification: Router A compares the received hash with its own calculation.

       Since the hash value changes with every challenge, CHAP provides robust protection against session hijacking and replay attacks.