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Abstract:
This document, RFC 9369, specifies QUIC version 2, a Standards Track protocol published by the Internet Engineering Task Force (IETF). Designed primarily to counter protocol ossification risks and test the version negotiation framework, QUIC version 2 is intentionally specified to be nearly identical to QUIC version 1, differing only in minimal details. Key changes include a distinct wire version number (not '2'), altered long header packet types, and updated cryptographic parameters (initial salt, HKDF labels, Retry integrity tag). The document highlights the mandatory use of version negotiation for endpoints supporting multiple QUIC versions to prevent downgrade attacks, emphasizes the compatibility between versions 1 and 2, and provides technical details and examples for implementation verification.
Exploring QUIC Version 2 (RFC 9369): Fighting Ossification and Exercising Version Negotiation
Core Purpose: QUIC version 2 (RFC 9369) was developed by the IETF primarily to combat the risk of network middleboxes "ossifying" on QUIC version 1 specifics and to exercise the protocol's defined version negotiation framework. It also serves as a template for minimal changes in future QUIC versions.
Minimal Differences from Version 1: Apart from specific, deliberate changes detailed below, QUIC version 2 endpoints are required to implement the QUIC version 1 specification ([QUIC], [QUIC-TLS], [QUIC-RECOVERY]).
Distinct Wire Version Number: The version number used in the long header of version 2 packets is not '2' but a randomly generated value, specifically 0x6b3343cf, derived from a hash, to actively challenge ossification based on expected version values.
Changed Long Header Packet Types: The binary values used to identify Long Header packet types are different in version 2 compared to version 1 (e.g., Initial is 0b01, 0-RTT is 0b10, Handshake is 0b11, Retry is 0b00).
Updated Cryptographic Parameters: QUIC version 2 uses a different initial salt value, changes the labels used in HKDF (HMAC-based Key Derivation Function) from "quic *" to "quicv2 *", and employs a new secret, key, and nonce for the Retry Integrity Tag.
Mandatory Version Negotiation: Any endpoint supporting QUIC version 2 MUST implement and correctly process the version_information transport parameter defined in [QUIC-VN] to protect against potential version downgrade attacks.
Compatibility with Version 1: QUIC version 2 is defined as a "compatible" version with version 1 according to [QUIC-VN]. Endpoints supporting both versions SHOULD use compatible version negotiation to avoid adding an extra round trip (RTT) at the start of a connection.
Version-Specific TLS Tokens: TLS session tickets and NEW_TOKEN tokens issued by a server are specific to the QUIC version of the connection they originate from. Clients MUST NOT use a token from a version 1 connection to initiate a version 2 connection, and vice versa. Servers MUST validate the originating version of any token.
Same Application Capabilities: QUIC version 2 provides the same capabilities to applications as version 1. Application-Layer Protocol Negotiation (ALPN) codepoints (like "h3" and "doq") defined for version 1 are also applicable to version 2. Most QUIC extensions defined for version 1 are expected to work with version 2.
Security Properties Unchanged: QUIC version 2 does not introduce any changes to the security or privacy properties inherent in QUIC version 1.
Glossary:
ALPN (Application-Layer Protocol Negotiation): A TLS extension that allows the application layer to negotiate the protocol to use over a secure connection (e.g., negotiating HTTP/3 over QUIC).
HKDF (HMAC-based Key Derivation Function): A function used in cryptography to derive one or more cryptographically strong secret keys from a less strong source key material, often combined with context-specific information.
IETF (Internet Engineering Task Force): A standards organization that develops and promotes voluntary Internet standards, including the protocols that make up the TCP/IP suite.
Middlebox: An intermediary network device (like a firewall, NAT, or traffic shaper) that sits between endpoints and performs functions beyond simple packet forwarding. They can sometimes cause issues if they make fixed assumptions about protocol formats.
Ossification: In network protocols, the phenomenon where parts of the protocol specification that were intended to be flexible or extensible become rigid in practice, often because intermediary devices (middleboxes) or implementations make fixed assumptions about their values or structure. This hinders protocol evolution.
QUIC (Quick UDP Internet Connections): A transport layer network protocol that runs over UDP, designed for performance and security. It offers features like stream multiplexing, flow control, and integrated TLS encryption.
RFC (Request for Comments): A formal document series used by the IETF to publish specifications, memos, and other information related to the Internet. Some RFCs define official Internet standards.
Retry Packet: A specific type of QUIC packet sent by a server in response to an Initial packet, primarily used for address validation of the client.
RTT (Round-Trip Time): The time delay required for a signal or data packet to travel from a specific source to a specific destination and back again. It's a key measure of network latency.
TLS (Transport Layer Security): A cryptographic protocol used to provide secure communication over a computer network. QUIC integrates TLS 1.3.
Version Negotiation: The process by which the two endpoints of a network connection determine and agree upon which version of a communication protocol they will use for the session.