Cybersecurity Fundamentals: Transport and Network Layer Protocols

Vocabulary flashcards covering the core concepts, protocols, packet formats, and control mechanisms of the Internet Transport and Network layers.

Internet Protocol Stack (5-Layer Model)

The layered Internet network architecture consisting from top to bottom of the application layer, transport layer, network layer, link layer, and physical layer.

Elastic Applications

Network applications such as file transfer, e-mail, and Web document browsing that can adapt to utilize whatever network throughput is available rather than requiring a strict minimum bandwidth guarantee.

Transmission Control Protocol (TCP)

A connection-oriented transport protocol that provides reliable, in-order byte stream delivery between processes, flow control, and congestion control, without guaranteeing delivery timing, minimum throughput, or security.

User Datagram Protocol (UDP)

A simple, lightweight, connectionless Internet transport protocol providing unreliable 'best effort' data transfer without flow control, congestion control, connection setup delays, or bandwidth guarantees.

Logical Communication (Transport vs. Network Layer)

The distinction where the network layer provides logical communication between end hosts, whereas the transport layer provides logical communication between application processes running on those hosts.

Multiplexing

The transport layer function at a sending host of collecting data chunks from multiple application sockets, adding transport header fields to create segments, and passing them down to the network layer.

Demultiplexing

The transport layer process at a receiving host of inspecting segment header fields (such as port numbers and IP addresses) to direct incoming segments to the appropriate application socket.

Connectionless Demultiplexing

The demultiplexing method used by UDP where incoming segments are delivered to sockets based solely on the destination IP address and destination port number, directing datagrams with different source addresses to the same socket if their destination port matches.

TCP 4-Tuple

The four specific header values—source IP address, source port number, destination IP address, and destination port number—used together to uniquely identify a TCP socket in connection-oriented demultiplexing.

UDP Segment Header

An $8\text{-byte}$ binary header comprising four fields: source port number ($16$ bits), destination port number ($16$ bits), segment length in bytes (including header and data), and Internet checksum.

Maximum Segment Size (MSS)

The maximum amount of application-layer data, typically set to $1460\text{ bytes}$, that can be transmitted within a single TCP segment, excluding TCP and IP headers.

TCP Sequence Number

The byte-stream number assigned to the first data byte contained within a TCP segment, indexing bytes rather than individual segments.

TCP Cumulative Acknowledgement (ACK)

An acknowledgement mechanism where the ACK number sent by the receiver denotes the sequence number of the next byte expected, implicitly confirming receipt of all preceding bytes in the stream.

TCP Three-Way Handshake

The connection setup procedure in which the client transmits a SYN segment ($\text{SYNbit} = 1$), the server responds with a SYNACK segment ($\text{SYNbit} = 1, \text{ACKbit} = 1$), and the client concludes with an ACK segment ($\text{ACKbit} = 1$) before exchanging data.

TCP Connection Teardown

The connection termination sequence where a host sends a segment with $\text{FINbit} = 1$, the receiver acknowledges it with an ACK, and the process repeats in reverse, requiring the initiating client to wait in $\text{TIMED\_WAIT}$ for $2 \times \text{max segment lifetime}$.

Receive Window (rwnd)

A TCP header field used for flow control that specifies the number of bytes the receiving host's buffer is currently willing and able to accept from the sender.

Network Layer Data Plane

The local, per-router network layer function that determines how an arriving datagram on an input port is forwarded directly to the correct output port.

Network Layer Control Plane

The network-wide logic that determines the routing path taken by datagrams across routers from source host to destination host, implemented via traditional routing algorithms or Software-Defined Networking (SDN).

Best Effort Service Model

The Internet network-layer service model that makes no quality-of-service guarantees regarding packet delivery, packet ordering, transmission delay, or available end-to-end bandwidth.

IPv4 Address

A $32\text{-bit}$ numerical identifier structured as four $8\text{-bit}$ decimal numbers ($0\text{--}255$) separated by dots that uniquely labels a single network interface on an IP network.

Longest Prefix Matching

A forwarding table mechanism in routers where an incoming packet's destination IP address is compared against table entries and forwarded along the link interface corresponding to the entry sharing the highest number of matching leading bits.

Longest Prefix Match Forwarding Table

A table data structure within a router that directs packets to specific output link interfaces based on prefixes of destination bit strings.

Time to Live (TTL)

A header field in an IPv4 datagram specifying the maximum remaining number of router hops allowed, decremented by $1$ at each router to prevent packets from circulating endlessly.

TCP/IP Header Overhead

The combined $40\text{ bytes}$ of baseline protocol overhead added to application payload data, comprising $20\text{ bytes}$ of TCP header and $20\text{ bytes}$ of IP header.

Internet Control Message Protocol (ICMP)

A network-layer protocol utilized by hosts and routers to report transmission errors and carry operational network-level signaling messages.

Dynamic Host Configuration Protocol (DHCP)

A plug-and-play client-server protocol enabling a device joining a network to dynamically obtain an IP address, first-hop router address, DNS server address, and network mask.

DHCP Protocol Encapsulation Stack

The network encapsulation path where a DHCP message is encapsulated inside a UDP segment, placed into an IP datagram, enclosed within an Ethernet frame broadcast to $\text{FFFFFFFFFFFF}$, and transmitted physically.

DHCP Four-Step Exchange (DORA)

The sequential protocol exchange consisting of DHCP Discover (broadcast from client), DHCP Offer (broadcast from server), DHCP Request (broadcast from client requesting offered address), and DHCP ACK (broadcast from server confirming lease).