Networking glossary: performance, caching and measurement

Updated November 2026

Short, plain definitions of the terms used across this site’s guides to performance testing, caching and Internet measurement. Where a guide covers the topic in depth, the entry links to it.

A to C

Active measurement
Measuring a network by sending test traffic and timing the result, as ping, traceroute and iperf3 do. See active measurement.
AS_PATH
The list of autonomous system numbers a BGP route has passed through, newest first and origin last. Routers use it to detect loops and, among equally preferred routes, to prefer the shorter path.
Autonomous system (AS)
A network, or group of networks, run under one routing policy, such as an ISP, a cloud provider or a university. See autonomous systems and BGP.
AS number (ASN)
The number that identifies an autonomous system in BGP. Originally 16 bits (0–65535), now 32 bits (up to 4,294,967,295). Assigned by the Regional Internet Registries.
Bandwidth
The capacity of a link, in bits per second. A “1 Gbit/s” link can carry at most one billion bits each second, including protocol overhead.
Bandwidth-delay product (BDP)
Link speed multiplied by round-trip time: the amount of data that must be “in flight” to keep the link full. At 1 Gbit/s and 50 ms RTT, BDP = 1,000,000,000 × 0.05 = 50,000,000 bits, or 6.25 MB. A TCP window smaller than that cannot fill the link. See TCP window size.
BGP (Border Gateway Protocol)
The routing protocol autonomous systems use to tell each other which address ranges they can reach. The current version, BGP-4, runs over TCP port 179. Sessions between different autonomous systems are external BGP (eBGP); sessions inside one AS are internal BGP (iBGP).
Cache hit and cache miss
A hit is a request answered from a stored copy; a miss had to be fetched from the origin server. Squid logs them as, for example, TCP_HIT and TCP_MISS.
Cache-Control
The HTTP header that tells caches whether and how long they may store a response, using directives such as max-age, no-cache, no-store and private. See how web caching works.
Carrier-grade NAT (CGNAT)
Address sharing inside an ISP’s network, so many customers use a small pool of public IPv4 addresses. The range 100.64.0.0/10 is reserved for it. See IPv4 exhaustion.
CDN (content delivery network)
A network of shared caches in many locations that serves a website’s content from close to its users, on the site owner’s behalf.
CIDR notation
Writing an address block as address/prefix length. 192.0.2.0/24 means the first 24 bits are fixed, leaving 8 bits: 28 = 256 addresses.
Congestion window (cwnd)
The sender-side limit on how much unacknowledged data TCP may have in flight, which grows while the path seems clear and shrinks after loss. ss -ti shows it for each connection.
CONNECT
The HTTP method a browser uses to ask a proxy for a tunnel, usually to port 443 for HTTPS. The proxy relays encrypted bytes without reading them.

E to M

ETag
A version label a server attaches to a response. A cache sends it back in If-None-Match; if it still matches, the server replies 304 Not Modified with no body.
Flow record
A summary of one conversation seen by a router or switch: addresses, ports, protocol, byte and packet counts, and start and end times. Exported in formats such as NetFlow, IPFIX and sFlow.
Forward proxy
A proxy that acts for a group of users, fetching web content on their behalf, such as a school or office Squid server. Compare reverse proxy.
Freshness lifetime
How long a cache may reuse a stored response without checking with the server, taken from s-maxage, max-age or Expires, or estimated heuristically.
ICMP
The Internet Control Message Protocol, used for error and diagnostic messages such as echo request and reply (ping), “time exceeded” (traceroute) and “packet too big” (path MTU discovery).
ICP (Internet Cache Protocol)
A small UDP protocol that lets web caches ask their neighbors whether they hold a URL. Version 2 is documented in RFC 2186 and RFC 2187 (1997).
Interception proxy
A proxy that receives traffic redirected by a router or firewall, without any browser configuration. Also called a transparent proxy. See interception proxying.
IPFIX
IP Flow Information Export, the IETF standard for exporting flow records (RFC 7011), based on Cisco’s NetFlow version 9.
IPv4
Internet Protocol version 4, with 32-bit addresses: about 4.3 billion in total, now fully allocated.
IPv6
Internet Protocol version 6, with 128-bit addresses, specified in RFC 8200. Routers do not fragment IPv6 packets, and every link must carry at least 1,280 bytes.
Jitter (delay variation)
How much delay changes from packet to packet. Important for voice and video; iperf3 reports it in UDP tests.
Latency
The time it takes data to cross the network, measured one way or as a round trip. It includes propagation (distance), transmission, processing and queueing delay.
Looking glass
A public web page that runs routing commands, such as “show route” or traceroute, on a network’s routers so outsiders can see its view of the Internet.
MTU (maximum transmission unit)
The largest packet a link can carry without fragmentation; 1,500 bytes on standard Ethernet. Tunnels and PPPoE reduce it. See packet sizes.

N to R

NAT (network address translation)
Rewriting addresses in packets so many devices with private addresses can share one public address. Your home router does it.
NetFlow
Cisco’s flow-export format. Version 9 (RFC 3954) is template-based and was the basis for IPFIX.
One-way delay
The time for a packet to travel in one direction only. Measuring it needs synchronized clocks at both ends.
OWAMP
The One-way Active Measurement Protocol (RFC 4656), a standard way to measure one-way delay and loss between two hosts.
PAC file
Proxy auto-config: a JavaScript function, FindProxyForURL(url, host), that tells the browser which proxy to use for each request. Browsers can locate it automatically through WPAD.
Packet loss
The share of packets that never arrive. TCP treats loss as a sign of congestion and slows down, so even small loss rates hurt throughput on long paths.
Passive measurement
Measuring by observing existing traffic, through packet capture or flow records, without sending test traffic. See passive measurement.
Path MTU discovery
The way a sender finds the largest packet size that fits the whole path: it sends packets marked “don’t fragment” and shrinks them when a router reports they are too big. Blocked ICMP breaks it.
pcap and pcapng
File formats for captured packets, written by tcpdump (-w) and read by Wireshark and most analysis tools.
perfSONAR
Open-source software for scheduled end-to-end tests of delay, loss, throughput and path between sites, developed by research and education networks including ESnet, GÉANT and Internet2.
Regional Internet Registry (RIR)
One of five organizations that hand out IP addresses and AS numbers in their region: AFRINIC, APNIC, ARIN, LACNIC and the RIPE NCC.
Reverse proxy
A proxy that sits in front of web servers and acts on their behalf, often caching responses and terminating TLS. CDNs are large distributed reverse proxies. Squid calls this accelerator mode.
RIPE Atlas
The RIPE NCC’s global network of more than 14,000 measurement probes and anchors that run ping, traceroute, DNS, TLS and NTP measurements.
Round-trip time (RTT)
The time from sending a packet to receiving the reply. What ping reports.
RPKI and ROA
The Resource Public Key Infrastructure lets address holders sign Route Origin Authorizations (ROAs) stating which AS may announce their prefixes. Routers then mark BGP routes Valid, Invalid or NotFound (RFC 6811).

S to V

sFlow
A traffic-monitoring method for switches and routers that samples 1 in N packets and exports interface counters over UDP (RFC 3176).
Shared cache and private cache
A shared cache (a proxy or CDN) stores responses for many users; a private cache (your browser’s) stores them for one. Cache-Control: private keeps a response out of shared caches.
SSL-Bump
Squid’s feature for handling TLS connections: it can peek at the requested server name, splice the connection through untouched, or decrypt it with a locally trusted certificate authority.
Throughput
The data rate actually achieved by a transfer, as opposed to the link’s bandwidth. Window size, loss and RTT usually limit it. iperf3 measures it.
TTL (time to live)
A counter in each IPv4 packet (called Hop Limit in IPv6) that every router decreases by one; at zero the packet is dropped and an ICMP “time exceeded” message is sent back. Traceroute relies on this.
TWAMP
The Two-Way Active Measurement Protocol (RFC 5357), which measures round-trip delay and loss with a reflector that timestamps each test packet.
Vary
An HTTP response header that lists the request headers a response depends on, such as Accept-Encoding, so a cache keeps separate copies for each variant.

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