Protocol Glossary
Key networking and protocol terms – concise, accurate, grounded in the standards.
A
ACK
Acknowledgment. A control bit in TCP indicating the Acknowledgment field is significant. Also used informally to mean any acknowledgment of receipt.
AS
Autonomous System. A collection of IP prefixes under common administrative control, identified by an ASN (Autonomous System Number). BGP routes between ASes.
B
Bandwidth
The maximum rate of data transfer across a network path, typically measured in bits per second (bps, Mbps, Gbps).
C
Checksum
A computed value used to detect errors in transmitted data. The sender computes a checksum; the receiver recomputes it and compares to detect corruption.
CIDR
Classless Inter-Domain Routing. A method of allocating IP addresses and routing using variable-length subnet masks (VLSM), written as address/prefix-length (e.g. 192.0.2.0/24).
Congestion
Network congestion occurs when a link or node carries more data than it can handle, causing packet loss and increased latency. TCP's congestion control algorithms (slow start, AIMD) manage this.
D
Datagram
A self-contained, independent entity of data carrying sufficient information to be routed from source to destination without relying on earlier exchanges. Used to describe UDP packets and IP packets.
E
Encapsulation
The process of wrapping data from higher-level protocols inside lower-level protocol headers. HTTP data is encapsulated in TCP, which is encapsulated in IP, which is encapsulated in Ethernet.
F
Frame
A data link layer (Layer 2) packet including a header, payload, and trailer. An Ethernet frame carries an IP packet as its payload.
H
Handshake
An exchange of messages to establish or configure a communication channel. TCP uses a 3-way handshake (SYN/SYN-ACK/ACK); TLS uses a 1-RTT handshake in version 1.3.
Head-of-Line Blocking
A performance issue where a packet awaiting processing prevents subsequent packets from being processed. TCP suffers from HOL blocking; QUIC eliminates it at the transport layer.
I
IP Address
A numerical label assigned to each device on a network. IPv4 addresses are 32 bits (4.2 billion); IPv6 addresses are 128 bits (340 undecillion).
J
Jitter
Variation in packet arrival times. Caused by network congestion or routing changes. Critical for real-time applications like VoIP and video conferencing.
L
Latency
The time it takes for a packet to travel from source to destination. Composed of propagation delay (speed of light), transmission delay, processing delay, and queuing delay.
M
MIB
Management Information Base. A hierarchical database of network management information used by SNMP. Organized as a tree of OIDs (Object Identifiers).
MTU
Maximum Transmission Unit. The largest packet or frame that can be sent in one network transaction. Ethernet MTU is 1500 bytes; QUIC and modern stacks use Path MTU Discovery to find the optimal size.
N
NAT
Network Address Translation. Remaps IP addresses by modifying headers. Allows multiple devices to share a single public IP, masking internal addresses. Complicates peer-to-peer protocols.
O
OSI Model
Open Systems Interconnection model. A 7-layer conceptual framework: Physical, Data Link, Network, Transport, Session, Presentation, Application. The internet uses a simplified 4-layer model (Link/Internet/Transport/Application).
P
Packet
A formatted unit of data carried by a packet-switched network. Includes a header (addressing, control) and payload (data). IP packets contain TCP or UDP segments.
Port
A 16-bit number (0–65,535) that identifies a specific process or service on a host. Ports 0–1023 are well-known (HTTP=80, HTTPS=443, SSH=22); ports 1024–49151 are registered.
Q
QoS
Quality of Service. Mechanisms for prioritizing certain network traffic types over others. DiffServ (DSCP) marks packets; routers honor those marks to guarantee bandwidth or reduce latency for priority traffic.
R
RFC
Request for Comments. The official document format of the IETF for internet standards, proposals, and informational documents. RFCs are numbered sequentially and never changed – only superseded.
RTT
Round-Trip Time. The time for a signal to travel from sender to receiver and back. A key metric for network performance; TLS 1.3 and QUIC reduce connection setup to 1-RTT.
S
Socket
A combination of an IP address and port number that uniquely identifies a network endpoint. A TCP connection is identified by the 4-tuple: (source IP, source port, destination IP, destination port).
Stratum
The hierarchical level of an NTP clock source. Stratum 0 = atomic clocks/GPS receivers. Stratum 1 = servers directly connected to stratum 0. Each hop adds +1 stratum.
T
TCP/IP
The protocol suite underlying the internet. TCP provides reliable transport; IP provides addressing and routing. Together they form the foundation for HTTP, DNS, SMTP, and nearly all internet protocols.
TTL
Time to Live. An 8-bit field in IP headers decremented at each router hop. When it reaches 0, the packet is dropped and an ICMP Time Exceeded message is sent. Prevents routing loops.
U
UDP
User Datagram Protocol. A connectionless transport protocol with no delivery guarantees. Used where speed matters more than reliability: DNS, streaming, gaming, NTP.
W
Window Size
The amount of data a TCP receiver is willing to accept before requiring an acknowledgment. Flow control mechanism; sender cannot exceed receiver's advertised window.