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Transport MASQUE quic http datagram udp proxy tunnels quic in quic turtles all the way down masque http-ng This document describes how to proxy UDP in HTTP, similar to how the HTTP CONNECT method allows proxying TCP in HTTP. More specifically, this document defines a protocol that allows HTTP clients to create a tunnel for UDP communications through an HTTP server that acts as a proxy. 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 MASQUE Working Group mailing list (), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction While HTTP provides the CONNECT method (see ) for creating a TCP tunnel to a proxy, it lacks a method for doing so for UDP traffic. This document describes a protocol for tunnelling UDP to a server acting as a UDP-specific proxy over HTTP. UDP tunnels are commonly used to create an end-to-end virtual connection, which can then be secured using QUIC or another protocol running over UDP. Unlike CONNECT, the UDP proxy itself is identified with an absolute URL containing the traffic's destination. Clients generate those URLs using a URI Template , as described in . This protocol supports all versions of HTTP by using HTTP Datagrams . When using HTTP/2 or HTTP/3 , it uses HTTP Extended CONNECT as described in and . When using HTTP/1.x , it uses HTTP Upgrade as defined in .

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. In this document, we use the term "UDP proxy" to refer to the HTTP server that acts upon the client's UDP tunnelling request to open a UDP socket to a target server, and generates the response to this request. If there are HTTP intermediaries (as defined in ) between the client and the UDP proxy, those are referred to as "intermediaries" in this document. Note that, when the HTTP version in use does not support multiplexing streams (such as HTTP/1.1), any reference to "stream" in this document represents the entire connection.

Client Configuration HTTP clients are configured to use a UDP proxy with a URI Template that has the variables "target_host" and "target_port". Examples are shown below:

URI Template Examples

The following requirements apply to the URI Template:

• The URI Template MUST be a level 3 template or lower.
• The URI Template MUST be in absolute form, and MUST include non-empty scheme, authority and path components.
• The path component of the URI Template MUST start with a slash "/".
• All template variables MUST be within the path or query components of the URI.
• The URI template MUST contain the two variables "target_host" and "target_port" and MAY contain other variables.
• The URI Template MUST NOT contain any non-ASCII unicode characters and MUST only contain ASCII characters in the range 0x21-0x7E inclusive (note that percent-encoding is allowed).
• The URI Template MUST NOT use Reserved Expansion ("+" operator), Fragment Expansion ("#" operator), Label Expansion with Dot-Prefix, Path Segment Expansion with Slash-Prefix, nor Path-Style Parameter Expansion with Semicolon-Prefix.

If the client detects that any of the requirements above are not met by a URI Template, the client MUST reject its configuration and fail the request without sending it to the UDP proxy. While clients SHOULD validate the requirements above, some clients MAY use a general-purpose URI Template implementation that lacks this specific validation. Since the original HTTP CONNECT method allowed conveying the target host and port but not the scheme, proxy authority, path, nor query, there exist proxy configuration interfaces that only allow the user to configure the proxy host and the proxy port. Client implementations of this specification that are constrained by such limitations MAY attempt to access UDP proxying capabilities using the default template, which is defined as: "https://$PROXY_HOST:$PROXY_PORT/.well-known/masque/udp/{target_host}/{target_port}/" where $PROXY_HOST and $PROXY_PORT are the configured host and port of the UDP proxy respectively. UDP proxy deployments SHOULD offer service at this location if they need to interoperate with such clients.

Tunnelling UDP over HTTP To allow negotiation of a tunnel for UDP over HTTP, this document defines the "connect-udp" HTTP Upgrade Token. The resulting UDP tunnels use the Capsule Protocol (see ) with HTTP Datagram in the format defined in . To initiate a UDP tunnel associated with a single HTTP stream, clients issue a request containing the "connect-udp" upgrade token. The target of the tunnel is indicated by the client to the UDP proxy via the "target_host" and "target_port" variables of the URI Template, see . If the request is successful, the UDP proxy commits to converting received HTTP Datagrams into UDP packets and vice versa until the tunnel is closed. When sending its UDP proxying request, the client SHALL perform URI Template expansion to determine the path and query of its request. target_host supports using DNS names, IPv6 literals and IPv4 literals. Note that this URI Template expansion requires using pct-encoding, so for example if the target_host is "2001:db8::42", it will be encoded in the URI as "2001%3Adb8%3A%3A42". By virtue of the definition of the Capsule Protocol (see ), UDP proxying requests do not carry any message content. Similarly, successful UDP proxying responses also do not carry any message content.

UDP Proxy Handling Upon receiving a UDP proxying request, the recipient UDP proxy extracts the "target_host" and "target_port" variables from the URI it has reconstructed from the request headers, and establishes a tunnel by directly opening a UDP socket to the requested target. Unlike TCP, UDP is connection-less. The UDP proxy that opens the UDP socket has no way of knowing whether the destination is reachable. Therefore it needs to respond to the request without waiting for a packet from the target. However, if the target_host is a DNS name, the UDP proxy MUST perform DNS resolution before replying to the HTTP request. If errors occur during this process (for example, a DNS resolution failure), the UDP proxy MUST fail the request and SHOULD send details using an appropriate "Proxy-Status" header field . UDP proxies can use connected UDP sockets if their operating system supports them, as that allows the UDP proxy to rely on the kernel to only send it UDP packets that match the correct 5-tuple. If the UDP proxy uses a non-connected socket, it MUST validate the IP source address and UDP source port on received packets to ensure they match the client's request. Packets that do not match MUST be discarded by the UDP proxy. The lifetime of the socket is tied to the request stream. The UDP proxy MUST keep the socket open while the request stream is open. If a UDP proxy is notified by its operating system that its socket is no longer usable (for example, this can happen when an ICMP "Destination Unreachable" message is received, see ), it MUST close the request stream. UDP proxies MAY choose to close sockets due to a period of inactivity, but they MUST close the request stream when closing the socket. UDP proxies that close sockets after a period of inactivity SHOULD NOT use a period lower than two minutes, see . A successful response (as defined in and ) indicates that the UDP proxy has opened a socket to the requested target and is willing to proxy UDP payloads. Any response other than a successful response indicates that the request has failed, and the client MUST therefore abort the request. UDP proxies MUST NOT introduce fragmentation at the IP layer when forwarding HTTP Datagrams onto a UDP socket. In IPv4, the Don't Fragment (DF) bit MUST be set if possible, to prevent fragmentation on the path. Future extensions MAY remove these requirements.

HTTP/1.1 Request When using HTTP/1.1 , a UDP proxying request will meet the following requirements:

• the method SHALL be "GET".
• the request SHALL include a single "Host" header field containing the origin of the UDP proxy.
• the request SHALL include a "Connection" header field with value "Upgrade" (note that this requirement is case-insensitive as per ).
• the request SHALL include an "Upgrade" header field with value "connect-udp".

For example, if the client is configured with URI Template "https://proxy.example.org/.well-known/masque/udp/{target_host}/{target_port}/" and wishes to open a UDP proxying tunnel to target 192.0.2.42:443, it could send the following request:

Example HTTP/1.1 Request

In HTTP/1.1, this protocol uses the GET method to mimic the design of the WebSocket Protocol .

HTTP/1.1 Response The UDP proxy SHALL indicate a successful response by replying with the following requirements:

• the HTTP status code on the response SHALL be 101 (Switching Protocols).
• the reponse SHALL include a single "Connection" header field with value "Upgrade" (note that this requirement is case-insensitive as per ).
• the response SHALL include a single "Upgrade" header field with value "connect-udp".
• the response SHALL NOT include any "Transfer-Encoding" or "Content-Length" header fields.

If any of these requirements are not met, the client MUST treat this proxying attempt as failed and abort the connection. For example, the UDP proxy could respond with:

Example HTTP/1.1 Response

HTTP/2 and HTTP/3 Requests When using HTTP/2 or HTTP/3 , UDP proxying requests use Extended CONNECT. This requires that servers send an HTTP Setting as specified in and , and that requests use HTTP pseudo-header fields with the following requirements:

• The ":method" pseudo-header field SHALL be "CONNECT".
• The ":protocol" pseudo-header field SHALL be "connect-udp".
• The ":authority" pseudo-header field SHALL contain the authority of the UDP proxy.
• The ":path" and ":scheme" pseudo-header fields SHALL NOT be empty. Their values SHALL contain the scheme and path from the URI Template after the URI template expansion process has been completed.

A UDP proxying request that does not conform to these restrictions is malformed (see ). For example, if the client is configured with URI Template "https://proxy.example.org/{target_host}/{target_port}/" and wishes to open a UDP proxying tunnel to target 192.0.2.42:443, it could send the following request:

Example HTTP/2 Request

HTTP/2 and HTTP/3 Responses The UDP proxy SHALL indicate a successful response by replying with any 2xx (Successful) HTTP status code, without any "Transfer-Encoding" or "Content-Length" header fields. If any of these requirements are not met, the client MUST treat this proxying attempt as failed and abort the request. For example, the UDP proxy could respond with:

Example HTTP/2 Response

Note About Draft Versions [[RFC editor: please remove this section before publication.]] In order to allow implementations to support multiple draft versions of this specification during its development, we introduce the "connect-udp-version" header field. When sent by the client, it contains a list of draft numbers supported by the client (e.g., "connect-udp-version: 0, 2"). When sent by the UDP proxy, it contains a single draft number selected by the UDP proxy from the list provided by the client (e.g., "connect-udp-version: 2"). Sending this header field is RECOMMENDED but not required. The "connect-udp-version" header field is a List Structured Field, see . Each list member MUST be an Integer.

Context Identifiers This protocol allows future extensions to exchange HTTP Datagrams which carry different semantics from UDP payloads. Some of these extensions can augment UDP payloads with additional data, while others can exchange data that is completely separate from UDP payloads. In order to accomplish this, all HTTP Datagrams associated with UDP Proxying request streams start with a context ID, see . Context IDs are 62-bit integers (0 to 2⁶²-1). Context IDs are encoded as variable-length integers, see . The context ID value of 0 is reserved for UDP payloads, while non-zero values are dynamically allocated: non-zero even-numbered context IDs are client-allocated, and odd-numbered context IDs are proxy-allocated. The context ID namespace is tied to a given HTTP request: it is possible for a context ID with the same numeric value to be simultaneously allocated in distinct requests, potentially with different semantics. Context IDs MUST NOT be re-allocated within a given HTTP namespace but MAY be allocated in any order. The context ID allocation restrictions to the use of even-numbered and odd-numbered context IDs exist in order to avoid the need for synchronisation between endpoints. However, once a context ID has been allocated, those restrictions do not apply to the use of the context ID: it can be used by any client or UDP proxy, independent of which endpoint initially allocated it. Registration is the action by which an endpoint informs its peer of the semantics and format of a given context ID. This document does not define how registration occurs. Future extensions MAY use HTTP header fields or capsules to register contexts. Depending on the method being used, it is possible for datagrams to be received with Context IDs which have not yet been registered, for instance due to reordering of the packet containing the datagram and the packet containing the registration message during transmission.

HTTP Datagram Payload Format When HTTP Datagrams (see ) are associated with UDP proxying request streams, the HTTP Datagram Payload field has the format defined in . Note that when HTTP Datagrams are encoded using QUIC DATAGRAM frames, the Context ID field defined below directly follows the Quarter Stream ID field which is at the start of the QUIC DATAGRAM frame payload:

UDP Proxying HTTP Datagram Format

Context ID:

A variable-length integer (see ) that contains the value of the Context ID. If an HTTP/3 datagram which carries an unknown Context ID is received, the receiver SHALL either drop that datagram silently or buffer it temporarily (on the order of a round trip) while awaiting the registration of the corresponding Context ID.

Payload:

The payload of the datagram, whose semantics depend on value of the previous field. Note that this field can be empty.

UDP packets are encoded using HTTP Datagrams with the Context ID set to zero. When the Context ID is set to zero, the Payload field contains the unmodified payload of a UDP packet (referred to as "data octets" in ). Clients MAY optimistically start sending UDP packets in HTTP Datagrams before receiving the response to its UDP proxying request. However, implementors should note that such proxied packets may not be processed by the UDP proxy if it responds to the request with a failure, or if the proxied packets are received by the UDP proxy before the request. By virtue of the definition of the UDP header , it is not possible to encode UDP payloads longer than 65527 bytes. Therefore, endpoints MUST NOT send HTTP Datagrams with a Payload field longer than 65527 using Context ID zero. An endpoint that receives a DATAGRAM capsule using Context ID zero whose Payload field is longer than 65527 MUST abort the stream. If a UDP proxy knows it can only send out UDP packets of a certain length due to its underlying link MTU, it SHOULD discard incoming DATAGRAM capsules using Context ID zero whose Payload field is longer than that limit without buffering the capsule contents.

Performance Considerations UDP proxies SHOULD strive to avoid increasing burstiness of UDP traffic: they SHOULD NOT queue packets in order to increase batching. When the protocol running over UDP that is being proxied uses congestion control (e.g., ), the proxied traffic will incur at least two nested congestion controllers. This can reduce performance but the underlying HTTP connection MUST NOT disable congestion control unless it has an out-of-band way of knowing with absolute certainty that the inner traffic is congestion-controlled. If a client or UDP proxy with a connection containing a UDP proxying request stream disables congestion control, it MUST NOT signal ECN support on that connection. That is, it MUST mark all IP headers with the Not-ECT codepoint. It MAY continue to report ECN feedback via ACK_ECN frames, as the peer may not have disabled congestion control. When the protocol running over UDP that is being proxied uses loss recovery (e.g., ), and the underlying HTTP connection runs over TCP, the proxied traffic will incur at least two nested loss recovery mechanisms. This can reduce performance as both can sometimes independently retransmit the same data. To avoid this, UDP proxying SHOULD be performed over HTTP/3 to allow leveraging the QUIC DATAGRAM frame.

MTU Considerations When using HTTP/3 with the QUIC Datagram extension , UDP payloads are transmitted in QUIC DATAGRAM frames. Since those cannot be fragmented, they can only carry payloads up to a given length determined by the QUIC connection configuration and the path MTU. If a UDP proxy is using QUIC DATAGRAM frames and it receives a UDP payload from the target that will not fit inside a QUIC DATAGRAM frame, the UDP proxy SHOULD NOT send the UDP payload in a DATAGRAM capsule, as that defeats the end-to-end unreliability characteristic that methods such as Datagram Packetization Layer Path MTU Discovery (DPLPMTUD) depend on . In this scenario, the UDP proxy SHOULD drop the UDP payload and send an ICMP "Packet Too Big" message to the target, see .

Tunneling of ECN Marks UDP proxying does not create an IP-in-IP tunnel, so the guidance in about transferring ECN marks between inner and outer IP headers does not apply. There is no inner IP header in UDP proxying tunnels. Note that UDP proxying clients do not have the ability in this specification to control the ECN codepoints on UDP packets the UDP proxy sends to the target, nor can UDP proxies communicate the markings of each UDP packet from target to UDP proxy. A UDP proxy MUST ignore ECN bits in the IP header of UDP packets received from the target, and MUST set the ECN bits to Not-ECT on UDP packets it sends to the target. These do not relate to the ECN markings of packets sent between client and UDP proxy in any way.

Security Considerations There are significant risks in allowing arbitrary clients to establish a tunnel to arbitrary targets, as that could allow bad actors to send traffic and have it attributed to the UDP proxy. HTTP servers that support UDP proxying ought to restrict its use to authenticated users. Because the CONNECT method creates a TCP connection to the target, the target has to indicate its willingness to accept TCP connections by responding with a TCP SYN-ACK before the CONNECT proxy can send it application data. UDP doesn't have this property, so a UDP proxy could send more data to an unwilling target than a CONNECT proxy. However, in practice denial of service attacks target open TCP ports so the TCP SYN-ACK does not offer much protection in real scenarios. While a UDP proxy could potentially limit the number of UDP packets it is willing to forward until it has observed a response from the target, that is unlikely to provide any protection against denial of service attacks because such attacks target open UDP ports where the protocol running over UDP would respond, and that would be interpreted as willingness to accept UDP by the UDP proxy. UDP sockets for UDP proxying have a different lifetime than TCP sockets for CONNECT, therefore implementors would be well served to follow the advice in if they base their UDP proxying implementation on a preexisting implementation of CONNECT. The security considerations described in also apply here.

IANA Considerations

HTTP Upgrade Token This document will request IANA to register "connect-udp" in the "HTTP Upgrade Tokens" registry maintained at <>.

Value:

connect-udp

Description:

Proxying of UDP Payloads

Expected Version Tokens:

None

Reference:

This document

Well-Known URI This document will request IANA to register "masque/udp" in the "Well-Known URIs" registry maintained at <>.

URI Suffix:

masque/udp

Change Controller:

IETF

Reference:

This document

Status:

permanent (if this document is approved)

Related Information:

Includes all resources identified with the path prefix "/.well-known/masque/udp/"

References Normative References Adobe Fastly greenbytes GmbH The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document specifies the HTTP/1.1 message syntax, message parsing, connection management, and related security concerns. This document obsoletes portions of RFC 7230. Mozilla Apple Inc. This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced latency by introducing field compression and allowing multiple concurrent exchanges on the same connection. This document obsoletes RFC 7540 and RFC 8740. Akamai The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. This document also identifies HTTP/2 features that are subsumed by QUIC, and describes how HTTP/2 extensions can be ported to HTTP/3. DO NOT DEPLOY THIS VERSION OF HTTP DO NOT DEPLOY THIS VERSION OF HTTP/3 UNTIL IT IS IN AN RFC. This version is still a work in progress. For trial deployments, please use earlier versions. Note to Readers Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org), which is archived at https://mailarchive.ietf.org/arch/search/?email_list=quic. Working Group information can be found at https://github.com/quicwg; source code and issues list for this draft can be found at https://github.com/quicwg/base-drafts/labels/-http. Adobe Fastly greenbytes GmbH The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFC 2818, RFC 7231, RFC 7232, RFC 7233, RFC 7235, RFC 7538, RFC 7615, RFC 7694, and portions of RFC 7230. 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. A URI Template is a compact sequence of characters for describing a range of Uniform Resource Identifiers through variable expansion. This specification defines the URI Template syntax and the process for expanding a URI Template into a URI reference, along with guidelines for the use of URI Templates on the Internet. [STANDARDS-TRACK] Google LLC Cloudflare This document describes HTTP Datagrams, a convention for conveying multiplexed, potentially unreliable datagrams inside an HTTP connection. In HTTP/3, HTTP Datagrams can be conveyed natively using the QUIC DATAGRAM extension. When the QUIC DATAGRAM frame is unavailable or undesirable, they can be sent using the Capsule Protocol, a more general convention for conveying data in HTTP connections. HTTP Datagrams and the Capsule Protocol are intended for use by HTTP extensions, not applications. This document defines a mechanism for running the WebSocket Protocol (RFC 6455) over a single stream of an HTTP/2 connection. Google The mechanism for running the WebSocket Protocol over a single stream of an HTTP/2 connection is equally applicable to HTTP/3, but the HTTP version-specific details need to be specified. This document describes how the mechanism is adapted for HTTP/3. 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. Fastly Google This document defines the Proxy-Status HTTP field to convey the details of intermediary response handling, including generated errors. This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values. This document defines an extension to the QUIC transport protocol to add support for sending and receiving unreliable datagrams over a QUIC connection. Informative References This document describes the format of a set of control messages used in ICMPv6 (Internet Control Message Protocol). ICMPv6 is the Internet Control Message Protocol for Internet Protocol version 6 (IPv6). [STANDARDS-TRACK] This document defines basic terminology for describing different types of Network Address Translation (NAT) behavior when handling Unicast UDP and also defines a set of requirements that would allow many applications, such as multimedia communications or online gaming, to work consistently. Developing NATs that meet this set of requirements will greatly increase the likelihood that these applications will function properly. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The WebSocket Protocol enables two-way communication between a client running untrusted code in a controlled environment to a remote host that has opted-in to communications from that code. The security model used for this is the origin-based security model commonly used by web browsers. The protocol consists of an opening handshake followed by basic message framing, layered over TCP. The goal of this technology is to provide a mechanism for browser-based applications that need two-way communication with servers that does not rely on opening multiple HTTP connections (e.g., using XMLHttpRequest or <iframe>s and long polling). [STANDARDS-TRACK] This document specifies Datagram Packetization Layer Path MTU Discovery (DPLPMTUD). This is a robust method for Path MTU Discovery (PMTUD) for datagram Packetization Layers (PLs). It allows a PL, or a datagram application that uses a PL, to discover whether a network path can support the current size of datagram. This can be used to detect and reduce the message size when a sender encounters a packet black hole. It can also probe a network path to discover whether the maximum packet size can be increased. This provides functionality for datagram transports that is equivalent to the PLPMTUD specification for TCP, specified in RFC 4821, which it updates. It also updates the UDP Usage Guidelines to refer to this method for use with UDP datagrams and updates SCTP. The document provides implementation notes for incorporating Datagram PMTUD into IETF datagram transports or applications that use datagram transports. This specification updates RFC 4960, RFC 4821, RFC 6951, RFC 8085, and RFC 8261. This document redefines how the explicit congestion notification (ECN) field of the IP header should be constructed on entry to and exit from any IP-in-IP tunnel. On encapsulation, it updates RFC 3168 to bring all IP-in-IP tunnels (v4 or v6) into line with RFC 4301 IPsec ECN processing. On decapsulation, it updates both RFC 3168 and RFC 4301 to add new behaviours for previously unused combinations of inner and outer headers. The new rules ensure the ECN field is correctly propagated across a tunnel whether it is used to signal one or two severity levels of congestion; whereas before, only one severity level was supported. Tunnel endpoints can be updated in any order without affecting pre-existing uses of the ECN field, thus ensuring backward compatibility. Nonetheless, operators wanting to support two severity levels (e.g., for pre-congestion notification -- PCN) can require compliance with this new specification. A thorough analysis of the reasoning for these changes and the implications is included. In the unlikely event that the new rules do not meet a specific need, RFC 4774 gives guidance on designing alternate ECN semantics, and this document extends that to include tunnelling issues. [STANDARDS-TRACK]

Acknowledgments This document is a product of the MASQUE Working Group, and the author thanks all MASQUE enthusiasts for their contibutions. This proposal was inspired directly or indirectly by prior work from many people. In particular, the author would like to thank Eric Rescorla for suggesting to use an HTTP method to proxy UDP. The author is indebted to Mark Nottingham and Lucas Pardue for the many improvements they contributed to this document. The extensibility design in this document came out of the HTTP Datagrams Design Team, whose members were Alan Frindell, Alex Chernyakhovsky, Ben Schwartz, Eric Rescorla, Lucas Pardue, Marcus Ihlar, Martin Thomson, Mike Bishop, Tommy Pauly, Victor Vasiliev, and the author of this document.