Proxy & travel distribution glossary

Glossary

The vocabulary of proxies, networking and travel distribution, one term per page, each defined in its first sentence.

A

Ancillary revenueTravel distribution: Ancillary revenue is the income an airline or other travel supplier earns beyond the base fare or room rate — checked bags, seat selection, priority boarding, meals, change fees, and commission on hotels, car hire or insurance sold alongside the trip.
Anonymous proxyProxies: An anonymous proxy is a proxy that forwards your requests without adding headers that reveal your own IP address, so the destination sees only the proxy's address; the familiar levels of transparent, anonymous and elite are an informal industry classification, not a standard.
ASN (autonomous system number)Networking: An ASN (autonomous system number) is the identifier of an autonomous system — a network such as an ISP, a mobile carrier or a cloud provider that announces its own IP prefixes to the internet under one routing policy — so looking up the ASN of an IP address tells you who operates the network it comes from.
ATPCO (Airline Tariff Publishing Company)Travel distribution: ATPCO (Airline Tariff Publishing Company) is an airline-owned company that collects the fares and fare rules airlines file and distributes them to global distribution systems, airline reservation systems and online travel agencies, making it the central clearing point for published airfare data.

B

Backconnect proxyProxies: A backconnect proxy is a single gateway address that a client connects to while the gateway forwards each connection out through one of many exit IPs behind it, so the client configures one host, port and set of credentials instead of a list of proxies.
Best available rate (BAR)Travel distribution: The best available rate (BAR) is a hotel's lowest publicly bookable rate for a given room type and night that any guest can book without a membership, corporate contract or other qualification; the exact definition varies by hotel group, and some count only flexible, unrestricted rates.

C

CAPTCHAData collection: A CAPTCHA — Completely Automated Public Turing test to tell Computers and Humans Apart — is a challenge a website presents to decide whether a request comes from a person or from software, ranging from distorted text and image selection to audio prompts and invisible checks on behaviour.
Carrier-grade NAT (CGNAT)Networking: Carrier-grade NAT (CGNAT) is network address translation run by an internet provider rather than in the customer's router: many subscribers share one public IPv4 address and are told apart by port, and RFC 6598 reserves 100.64.0.0/10 as the shared address space between the provider's NAT and its customers.
Channel managerTravel distribution: A channel manager is software that connects a hotel's property management or reservation system to its distribution channels — OTAs, GDS connections, metasearch and its own booking engine — pushing availability, rates and restrictions to all of them from one place and passing reservations back.

D

Datacenter proxyProxies: A datacenter proxy is a proxy that exits from a server in a hosting or cloud facility, on an IP address registered to that hosting provider rather than to a consumer ISP; it is the fastest and cheapest kind of proxy to run and the easiest for a website to recognise as infrastructure.
Dedicated proxyProxies: A dedicated proxy is a proxy IP address assigned to a single customer for the length of the contract, so no other user's traffic leaves through it and the address's history from then on is shaped only by that customer's use.
DNS leakNetworking: A DNS leak is when a client sends its traffic through a proxy or VPN but resolves hostnames through its own local resolver, so the lookups leave from the original network — exposing which sites you visit to that resolver and, for location-dependent services, possibly connecting you to a server picked for the wrong place.
Drip pricingTravel distribution: Drip pricing is showing a low headline price and adding further charges as the customer moves through the purchase, so the total only becomes clear late in the process; the UK Competition and Markets Authority describes it as "when prices are added as consumers proceed with a transaction".
Dynamic pricingTravel distribution: Dynamic pricing is setting prices that change over time in response to demand, remaining capacity, competitors' prices or time before the date of use, rather than holding one fixed price; airlines, hotels, car hire and event ticketing use it to sell the same product at different prices from one day or hour to the next.

F

Fare basis codeTravel distribution: A fare basis code is the alphanumeric code, typically three to eight characters, that names the specific published fare a ticket was priced with — and through it the rules attached to that fare, such as stay limits, seasons and change conditions.
Fare class (booking class, RBD)Travel distribution: A fare class, also called a booking class or reservation booking designator (RBD), is the single-letter code an airline uses to sell seats within a cabin at a particular price level and set of rules; it controls inventory, while the fare basis code — which almost always starts with the same letter — describes the fare's conditions.
Forward proxyProxies: A forward proxy is an intermediary that a client deliberately configures to send its requests onward to other servers on its behalf; RFC 9110 calls this simply a proxy, "a message-forwarding agent that is chosen by the client", and the destination sees the proxy's address instead of the client's.

G

GDS (global distribution system)Travel distribution: A GDS (global distribution system) is a computerised reservation network that aggregates schedules, availability and fares from many airlines — and often hotels and car hire — so travel agencies and online sellers can search, book and ticket across suppliers in one system; EU law calls it a computerised reservation system (CRS).
Geo-blockingData collection: Geo-blocking is restricting, redirecting or changing the terms of access to a website or offer according to where the visitor is — usually judged from the IP address, sometimes from the payment card or delivery address — so people in different countries cannot reach, or cannot buy, the same thing.
Geo-targetingData collection: Geo-targeting, for proxies, is choosing the country, region or city your request exits from, so a website serves you the prices, language, search results and availability it shows to people in that location.

H

Headless browserData collection: A headless browser is a real web browser engine running without a visible window and controlled by code instead of a person; it loads pages, executes JavaScript and builds the DOM like a normal browser, which makes it the tool for pages whose content only appears after scripts run.
HTTP CONNECTNetworking: HTTP CONNECT is the HTTP request method a client sends to a proxy to ask for a tunnel to a host and port; once the proxy answers with a 2xx status it blindly forwards bytes in both directions, which is how HTTPS passes through an HTTP proxy with TLS intact end to end (RFC 9110, section 9.3.6).

I

IP reputationNetworking: IP reputation is the assessment that websites, mail servers and security services attach to an IP address or range — based on its observed history of spam, attacks or automated traffic and on what kind of network it belongs to — and use to decide whether to accept, challenge, slow down or refuse its traffic.
IPv6 proxyNetworking: An IPv6 proxy is a proxy whose exit address is an IPv6 address — 128 bits long under RFC 8200, against 32 bits for IPv4 — so it can only reach sites that accept IPv6 connections and is seen by them as an IPv6 client.
ISP proxyProxies: An ISP proxy is a proxy whose IP address is registered to a consumer internet service provider but runs on a server in a datacentre, combining an address that does not read as a hosting range with the throughput and uptime of hosted hardware.

L

Look-to-book ratioTravel distribution: The look-to-book ratio is the number of availability or price searches a travel system answers for each booking it receives — written as, say, 1,000:1 — and it measures how much computing an airline, hotel or distribution system spends to sell one trip.
Low-cost carrier (LCC)Travel distribution: A low-cost carrier (LCC) is an airline with a lower cost structure than comparable carriers that uses it to sell lower fares — the working definition ICAO gives — typically by flying one aircraft type point to point, selling directly and charging separately for bags, seats and other services.

M

MetasearchTravel distribution: Travel metasearch is a search service that queries many sellers — airlines, hotels and online travel agencies — for the same trip, shows their prices side by side, and sends the traveller to the chosen seller to book instead of selling the trip itself.
Mobile proxyProxies: A mobile proxy is a proxy that sends your traffic out through a mobile network operator's 4G or 5G network, so the destination sees an address from the range that carrier assigns to phones on its network.

N

NDC (New Distribution Capability)Travel distribution: NDC (New Distribution Capability) is an IATA programme and XML-based data exchange standard that lets airlines build and send their own offers — fares, bundles, ancillaries and rich content — to travel sellers through any channel, instead of relying only on fares filed and priced in a GDS.

O

OTA (online travel agency)Travel distribution: An OTA (online travel agency) is a website or app that sells travel from many suppliers — hotel rooms, flights, car hire, packages — and completes the booking itself, either as an agent passing the reservation to the supplier or as a merchant reselling inventory at a price it sets.

P

Per-GB pricingProxies: Per-GB pricing is a proxy billing model in which you pay for the volume of traffic that passes through the proxy, measured in gigabytes, rather than per IP address, per request or per month; the number of threads, requests and exit IPs you use does not change the price.
Point of sale (POS)Travel distribution: Point of sale (POS), in travel, is the market in which a fare or rate is sold — usually the country of the seller or of the website edition the traveller uses — and airlines, hotels and other suppliers can set different prices, conditions and availability for each one.
Proxy authenticationProxies: Proxy authentication is how a proxy confirms that a client may use it: under RFC 9110 the proxy answers with status 407 Proxy Authentication Required and a Proxy-Authenticate challenge, and the client sends its credentials in a Proxy-Authorization header.
Proxy poolProxies: A proxy pool is the set of exit IP addresses a proxy service can route your traffic through; each request or session draws an address from it, and the pool's size, address type and geographic spread decide how widely load can be spread and which locations you can appear in.

R

Rate limitingData collection: Rate limiting is a server's cap on how many requests a client may make in a given time; when the cap is exceeded the standard response is HTTP 429 Too Many Requests (RFC 6585), often with a Retry-After header saying how long to wait.
Rate parityTravel distribution: Rate parity is a hotel offering the same room, for the same dates and conditions, at the same price on every distribution channel — its own website, online travel agencies and metasearch listings — whether by its own choice or because a contract's parity clause requires it.
Rate shoppingTravel distribution: Rate shopping is a hotel regularly collecting the public room rates of a chosen set of competitors — for the same dates, room type and conditions — to inform its own pricing; the tools that automate it are called rate shoppers.
Residential proxyProxies: A residential proxy is a proxy server whose exit IP address belongs to an ordinary home internet connection supplied by a consumer ISP, so the site you request sees a household address in the country or city you chose rather than a hosting range.
Revenue management system (RMS)Travel distribution: A revenue management system (RMS) is software that forecasts demand and recommends or sets prices and inventory controls — which booking classes stay open on a flight, which rates and stay restrictions apply to a hotel night — to earn the most from fixed, perishable capacity.
Reverse proxyNetworking: A reverse proxy is a server that sits in front of one or more web servers and receives requests as if it were the site, then forwards them inward; RFC 9110 calls it a gateway, "an intermediary that acts as an origin server for the outbound connection", and it is chosen by the site operator, not by the visitor.
robots.txtData collection: robots.txt is a plain-text file at the root of a website, /robots.txt, in which the operator tells automated crawlers which paths they may and may not fetch, grouped by crawler name; the Robots Exclusion Protocol behind it dates from 1994 and was standardised by the IETF as RFC 9309 in September 2022.
Rotating proxyProxies: A rotating proxy is a proxy endpoint that automatically assigns a different exit IP address from a pool to new connections or requests, so one client's traffic is spread across many addresses without the client managing a list.

S

Shared proxyProxies: A shared proxy is a proxy exit IP that more than one customer uses, either at the same time or one after another, so whatever a site has learned about that address applies to everyone sending traffic through it.
SOCKS5Networking: SOCKS5 is a proxy protocol, standardised in RFC 1928 in 1996, that relays TCP connections and UDP datagrams between a client and a destination without interpreting the application traffic, with username/password authentication defined separately in RFC 1929.
Sticky sessionProxies: A sticky session is a setting on a rotating proxy that keeps the same exit IP address for a chosen period, usually by attaching a session identifier to the connection, so a multi-step task appears to come from one address from start to finish.
Subnet (CIDR block)Networking: A subnet is a contiguous block of IP addresses that share the same leading bits, written in CIDR notation as an address, a slash and the number of fixed bits — 198.51.100.0/24, for example, is 256 IPv4 addresses — as defined for IPv4 in RFC 4632.

T

Transparent proxyNetworking: A transparent proxy, in the terminology of RFC 9110 an interception proxy, is an intermediary that the client did not choose: it sits on the network path and filters or redirects outgoing web traffic without being configured in the client's software.

U

User-AgentData collection: User-Agent is the HTTP request header in which a client identifies its software — one or more product tokens such as curl/8.5.0, with optional versions and comments — and which RFC 9110 says servers use to diagnose compatibility problems, tailor responses and compile browser and operating-system statistics.

W

Web crawlerData collection: A web crawler, also called a spider or bot, is a program that automatically discovers and fetches web pages — usually by following links from page to page or reading sitemaps — to build a search index, an archive or a dataset.
Web scrapingData collection: Web scraping is the automated extraction of specific data — prices, listings, reviews, text — from web pages into a structured form such as rows or JSON, by fetching pages with an HTTP client or a browser and parsing the content.

X

X-Forwarded-ForNetworking: X-Forwarded-For is a widely used but non-standard HTTP request header in which proxies and load balancers record the IP address they received a request from, building a comma-separated chain; RFC 7239 (2014) standardised the same idea as the Forwarded header.