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Transport QUIC QUIC STUN Address Discovery Unless they have out-of-band knowledge, QUIC endpoints have no information about their network situation. They neither know their external IP address and port, nor do they know if they are directly connected to the internet or if they are behind a NAT. This QUIC extension allows nodes to determine their public IP address and port for any QUIC path. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the QUIC Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction STUN () allows nodes to discover their reflexive transport address by asking a remote server to report the observed source address. While the QUIC () packet header was designed to allow demultiplexing from STUN packets, moving address discovery into the QUIC layer has a number of advantages:

1. STUN traffic is unencrypted, and can be observed and modified by on-path observers. By moving address discovery into QUIC's encrypted envelope it becomes invisible to observers.
2. When located behind a load balancer, QUIC packets may be routed based on the QUIC connection ID. Depending on the architecture, not using STUN might simplify the routing logic.
3. If QUIC traffic doesn't need to be demultiplexed from STUN traffic, implementations can enable QUIC bit greasing ().

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Negotiating Extension Use Endpoints advertise their support of the extension by sending the address_discovery (0x9f81a173) transport parameter () with an empty value. Implementations that understand this transport parameter MUST treat the receipt of a non-empty value as a connection error of type TRANSPORT_PARAMETER_ERROR. When using 0-RTT, both endpoints MUST remember the value of this transport parameter. This allows sending the frames defined by this extension in 0-RTT packets. If 0-RTT data is accepted by the server, the server MUST NOT disable this extension on the resumed connection.

Frames This extension defines three frames. These frames MUST only appear in the application data packet number space. These frames are "probing frames" as defined in .

REQUEST_ADDRESS The REQUEST_ADDRESS frame contains the following fields:

Sequence Number:

The sequence number of the request.

REQUEST_ADDRESS frames are ack-eliciting. If lost, it's the sender's decision if it wants to retransmit the frame. A sender MAY send a new REQUEST_ADDRESS frame with an incremented sequence number instead.

REQUEST_DECLINED The REQUEST_DECLINED frame contains the following fields:

Sequence Number:

The sequence number of the request that is being declined.

REQUEST_DECLINED frames are ack-eliciting, and SHOULD be retransmitted if lost.

OBSERVED_ADDRESS The OBSERVED_ADDRESS frame contains the following fields:

Sequence Number:

The sequence number of the request for which this response is intended.

IPv4:

The IPv4 address. Only present if the least significant bit of the frame type is 0.

IPv6:

The IPv6 address. Only present if the least significant bit of the frame type is 1.

Port:

The port number, in network byte order.

OBSERVED_ADDRESS frames are ack-eliciting, and SHOULD be retransmitted if lost.

Address Discovery An endpoint that wishes to determine the remote address of a path sends a REQUEST_ADDRESS frame on that path. The sequence number starts at 0 and is incremented for any subsequent request. Since the REQUEST_ADDRESS frame is a probing frame, the endpoint MAY bundle it with other probing frames during path validation (). The receiver of the REQUEST_ADDRESS frame SHOULD report the observed address using an OBSERVED_ADDRESS frame. The OBSERVED_ADDRESS frame does not need to be sent on the same path, since the requester can associate the response with the corresponding request using the sequence number. The receiver of a REQUEST_ADDRESS frame MAY decline to report the observed address by sending a REQUEST_DECLINED frame. The REQUEST_DECLINED frame also contains a sequence number, and therefore may be sent on any path. The sender MAY send a REQUEST_ADDRESS frame for the same path after a some time has elapsed. This allows it to detect when a NAT rebinding has happened. To speed up the discovery, it MAY also send another REQUEST_ADDRESS frame when the peer changes the connection ID used on the path. When receiving an OBSERVED_ADDRESS or a REQUEST_DECLINED frame with a sequence number value that was not previously sent in a REQUEST_ADDRESS frame, the receiver MUST close the connection with a PROTOCOL_VIOLATION error code if it can detect this condition.

Security Considerations

On the Requester Side In general, nodes cannot be trusted to report the correct address in OBSERVED_ADDRESS frames. If possible, endpoints might decide to only use this extension when connecting to trusted peers, or if that is not possible, define some validation logic (e.g. by asking multiple untrusted peers and observing if the responses are consistent). This logic is out of scope for this document.

On the Responder Side Depending on the routing setup, a node might not be able to observe the peer's reflexive transport address, and attempts to do so might reveal details about the internal network. In these cases, the node SHOULD NOT enable support the extension, or decline reporting the address using REQUEST_DECLINED frames.

IANA Considerations TODO: fill out registration request for the transport parameter and frame types

Normative References Session Traversal Utilities for NAT (STUN) is a protocol that serves as a tool for other protocols in dealing with NAT traversal. It can be used by an endpoint to determine the IP address and port allocated to it by a NAT. It can also be used to check connectivity between two endpoints and as a keep-alive protocol to maintain NAT bindings. STUN works with many existing NATs and does not require any special behavior from them. STUN is not a NAT traversal solution by itself. Rather, it is a tool to be used in the context of a NAT traversal solution. This document obsoletes RFC 5389. This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, and an exemplary congestion control algorithm. This document describes a method for negotiating the ability to send an arbitrary value for the second-most significant bit in QUIC packets. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

Acknowledgments TODO acknowledge.