Glossary
RFC Basics

RFC Basics

What is an RFC?

An RFC (Request for Comments) is a formal document published by the Internet Engineering Task Force (IETF) that defines the standards and protocols that power the internet. Despite the informal-sounding name, RFCs are the authoritative specifications that all implementations must follow for interoperability.

HTTP/1.1 RFCs

HTTP/1.1 is defined by two key RFCs:

RFCTitleScope
RFC 9110HTTP SemanticsThe meaning of HTTP — methods, status codes, headers, content negotiation
RFC 9112HTTP/1.1 Message Syntax and RoutingThe wire format — how requests and responses are framed as bytes on a TCP connection

These replaced the older RFC 7230–7235 series in June 2022. Http11Probe tests against the current (9110/9112) requirements.

Requirement Levels

RFCs use specific keywords defined in RFC 2119 and RFC 8174:

KeywordMeaningIn Http11Probe
MUSTAbsolute requirement. Violating this means non-compliance.Test expects exactly the mandated response (e.g., only 400)
MUST NOTAbsolute prohibition.Test verifies the server does not exhibit prohibited behavior
SHOULDRecommended, but valid reasons to deviate may exist.Test expects the recommended response but accepts close
MAYOptional behavior.Test rewards stricter behavior but does not penalize lenience
“ought to”Weaker than SHOULD — a recommendation with less force.Test accepts multiple valid responses

How Http11Probe Maps Requirement Levels

  • MUST respond with 400 → Only 400 passes. Close or timeout is a fail.
  • MUST reject (no specific code) → 400 or connection close passes.
  • SHOULD respond with 400 → 400 or connection close passes.
  • MAY accept → Rejection (400/close) passes. Acceptance is RFC-compliant but noted.
  • “ought to” handle as error → 400 or connection close passes.

Reading Test IDs

Every test has an ID that encodes its source:

PrefixMeaningExample
RFC9112-X.Y-RFC 9112, section X.YRFC9112-2.2-BARE-LF-HEADER
RFC9110-X.Y-RFC 9110, section X.YRFC9110-5.4-DUPLICATE-HOST
COMP-General complianceCOMP-BASELINE
SMUG-Smuggling vectorSMUG-CL-TE-BOTH
MAL-Malformed inputMAL-BINARY-GARBAGE