Week 12 (Congestion Control & Queuing Theory) & Week 13 (Application Layer & Security)

Congestion Collapse

A state where hosts send packets as fast as possible, causing routers to drop packets due to full buffers, which leads to timeouts, retransmissions, and even more congestion.

Open Loop

A bandwidth allocation model where a fixed, static bandwidth is allocated to each host.

Closed Loop

A dynamic bandwidth allocation model where the host adjusts its transmission rate based on network feedback. TCP congestion control is an example of a closed-loop model.

AIMD (Additive Increase Multiplicative Decrease)

An algorithm used to avoid router buffer congestion. It works by increasing the congestion window linearly when no congestion is detected (Additive Increase) and decreasing it exponentially when a packet drop or timeout occurs (Multiplicative Decrease).

CongestionWindow

A variable set by the TCP source that determines the transmission rate based on the perceived level of network congestion.

Queuing Theory

A branch of mathematics focused on understanding and optimizing the efficiency of queues, including queue length and waiting times.

Little’s Law

A queuing theory principle stating that under steady-state conditions, the average number of items in a queue (L) equals the average rate at which items arrive multiplied by the average time an item spends in the queue.

Silly Window Syndrome

A performance degradation problem in poorly implemented TCP where the sender repeatedly transmits just one byte of data, or the receiver repeatedly accepts just one byte.

Nagle’s Algorithm

A solution to Silly Window Syndrome that prevents inefficient bandwidth use by sending the first byte and buffering the remaining data until an acknowledgment is received or the buffer reaches a certain size.

Clark’s Solution

Another solution to Silly Window Syndrome that prevents the receiver from advertising a window size of 1 byte, forcing it to wait until it has enough buffer capacity (typically 1 maximum segment size of half-empty).

HTTP (Hypertext Transfer Protocol)

The Web’s primary application-layer protocol based on a client/server model. It is considered “stateless” because the server does not maintain information about past client requests.

Persistent Connection (Connection Reuse)

An HTTP/1.1 innovation that allows multiple requests and responses to be sent over a single established TCP connection, reducing overhead.

Pipelining

Sending multiple HTTP requests in rapid succession without waiting for the corresponding responses, reducing server idle time.

Head-of-Line Blocking (HOL)

A performance bottleneck where a packet stuck at the front of a queue holds up the rest of the packets behind it.

QUIC (Quick Internet Connection)

An encrypted transport protocol built on top of UDP (used in HTTP/3). It combines the speed of UDP with the security of TLS and performs multiplexing to prevent head-of-line blocking.

SMTP (Simple Mail Transfer Protocol)

A “client push” protocol used for exchanging and delivering outgoing e-mail messaged reliably over TCP.

IMAP (Internet Mail Access Protocol)

An e-mail access protocol used by clients to retrieve, delete, and organize messages stored on a receiving mail server.

SNMP (Simple Network Management Protocol)

A protocol running on UDP used to query (GET) or modify (SET) the state of remote network devices like switches and routers.

DNS (Domain Name System)

A system that translates human-usable domain names (like www.lsu.edu) into machine-readable IP addresses.

Cryptography

The practice of encoding messages into a secret code (ciphertext) to make them unintelligible to anyone except the intended receiver.

Symmetric Encryption

An encryption scheme that uses the exact same key for both the encryption and decryption processes (e.g., AES, DES).

Asymmetric Encryption (Public-Key Cryptography)

A scheme where each user has a pair of keys: a public key used by others to encrypt messages sent to them, and a private key kept secret by the user to decrypt those messages (e.g., RSA).

Diffie-Hellman Key Agreement

A protocol that allows two parties to securely establish a shared session key over a public channel without needing to pre-distribute any keys.

PGP (Pretty Good Privacy)

A widely used cryptographic approach for securing electronic mail, providing confidentiality, authentication, data integrity, and nonrepudiation.