Internet-Draft CONNECT-UDP Listen July 2022
Schinazi Expires 8 January 2023 [Page]
Intended Status:
Standards Track
D. Schinazi
Google LLC

Proxying Listener UDP in HTTP


The mechanism to proxy UDP in HTTP only allows each proxying request to transmit to a specific host and port. This is well suited for UDP client-server protocols such as HTTP/3, but is not sufficient for some UDP peer-to-peer protocols like WebRTC. This document proposes an extension to UDP Proxying in HTTP that enables those use-cases.

About This Document

This note is to be removed before publishing as an RFC.

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

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 8 January 2023.

Table of Contents

1. Introduction

The mechanism to proxy UDP in HTTP [CONNECT-UDP] allows proxying UDP payloads [UDP] to a fixed host and port. Combined with the HTTP CONNECT method (see Section 9.3.6 of [HTTP]), it allows proxying the majority of a Web Browser's HTTP traffic. However WebRTC [WebRTC] relies on ICE [ICE] to provide connectivity between two Web browsers, and that in turn relies on the ability to send and receive UDP packets to multiple hosts. While it would be possible in theory to accomplish this by using multiple UDP proxying HTTP requests, HTTP semantics [HTTP] do not guarantee that those distinct requests will be handled by the same server, which can lead to the UDP packets being sent from distinct IP addresses, which in turn prevents ICE from operating correctly. Because of this, UDP Proxying requests cannot enable WebRTC connectivity between peers.

This document describes an extension to UDP Proxying in HTTP that allows sending and receiving UDP payloads to multiple hosts within the scope of a single UDP proxying HTTP request.

1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

This document uses terminology from [CONNECT-UDP] and notational conventions from [QUIC].

2. Proxied UDP Listener Mechanism

In unextended UDP Proxying requests, the target host is encoded in the HTTP request path or query. For listener UDP proxying, it is instead conveyed in each HTTP Datagram, see Section 2.1.

When performing URI Template Exansion of the UDP proxying template (see Section 3 of [CONNECT-UDP]), the client sets both the target_host and the target_port variables to the '*' character (ASCII character 0x2A).

Before sending its UDP Proxying request to the proxy, the client allocates an even-numbered context ID, see Section 4 of [CONNECT-UDP]. The client then adds the "connect-udp-listen" header field to its proxying request, with the value equal to the context ID it has allocated, see Section 2.2.

2.1. HTTP Datagram Payload Format

When HTTP Datagrams [HTTP-DGRAM] associated with this listener UDP proxying request use the context ID sent with the connect-udp-listen header field, their Payload field (as defined in Section 5 of [CONNECT-UDP]) has the format defined in Figure 1:

Listener UDP Proxying HTTP Datagram Payload {
  IP Version (8),
  IP Address (32..128),
  UDP Port (16),
  UDP Payload (..),
Figure 1: Listener UDP Proxying HTTP Datagram Format
IP Version:

The IP Version of the following IP Address field. MUST be 4 or 6.

IP Address:

The IP Address of this proxied UDP packet. When sent from client to proxy, this is target host that the proxy will send this UDP payload to. When sent from proxy to client, this represents the source IP address of the UDP packet received by the proxy. This field has length 32 bits when the previous IP Version field value is 4, and 128 when the IP Version is 6.

UDP Port:

The UDP Port of this proxied UDP packet. When sent from client to proxy, this is target port that the proxy will send this UDP payload to. When sent from proxy to client, this represents the source UDP port of the UDP packet received by the proxy.

UDP Payload:

The unmodified UDP Payload of this proxied UDP packet (referred to as "data octets" in [UDP]).

2.2. The connect-udp-listen Header Field

The "connect-udp-listen" header field is an Item Structured Field, see Section 3.3 of [STRUCT-FIELD]; its value MUST be an Integer; any other value type MUST be handled as if the field were not present by recipients (for example, if this field is included multiple times, its type will become a List and the field will therefore be ignored). This document does not define any parameters for the connect-udp-listen header field value, but future documents might define parameters. Receivers MUST ignore unknown parameters.

3. Security Considerations

The security considerations described in Section 7 of [CONNECT-UDP] also apply here.

4. IANA Considerations

This document will request IANA to register the following entry in the "HTTP Field Name" registry maintained at <>:

Field Name:





provisional (permanent if this document is approved)


This document



5. References

5.1. Normative References

Schinazi, D., "Proxying UDP in HTTP", Work in Progress, Internet-Draft, draft-ietf-masque-connect-udp-15, , <>.
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, , <>.
Schinazi, D. and L. Pardue, "HTTP Datagrams and the Capsule Protocol", Work in Progress, Internet-Draft, draft-ietf-masque-h3-datagram-11, , <>.
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Nottingham, M. and P-H. Kamp, "Structured Field Values for HTTP", RFC 8941, DOI 10.17487/RFC8941, , <>.
Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, , <>.

5.2. Informative References

Pauly, T., Schinazi, D., Chernyakhovsky, A., Kuehlewind, M., and M. Westerlund, "IP Proxying Support for HTTP", Work in Progress, Internet-Draft, draft-ietf-masque-connect-ip-01, , <>.
Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal", RFC 8445, DOI 10.17487/RFC8445, , <>.
"WebRTC", W3C Recommendation, , <>.

Appendix A. Example

In the example below, the client is configured with URI Template "{target_host}/{target_port}/" and wishes to use WebRTC to another browser over a listener UDP proxying tunnel. It then contacts a STUN server at The STUN server then sends the proxy's IP address to the other browser at leading that other browser to send a UDP packet to the proxy, and that packets gets proxied over HTTP back to the client.

 Client                                             Server

 STREAM(44): HEADERS            -------->
   :method = CONNECT
   :protocol = connect-udp
   :scheme = https
   :path = /.well-known/masque/udp/*/*/
   :authority =
   connect-udp-listen = 2
   capsule-protocol = ?1

 DATAGRAM                       -------->
   Quarter Stream ID = 11
   Context ID = 2
   IP Version = 4
   IP Address =
   UDP Port = 1234
   UDP Payload = Encapsulated UDP Payload

            <--------  STREAM(44): HEADERS
                         :status = 200
                         capsule-protocol = ?1

/* Wait for STUN server to respond to UDP packet. */

            <--------  DATAGRAM
                         Quarter Stream ID = 11
                         Context ID = 2
                         IP Version = 4
                         IP Address =
                         UDP Port = 1234
                         UDP Payload = Encapsulated UDP Payload

/* Wait for the STUN server to send the proxy's IP and */
/* port to the other browser and for the other browser */
/* to send a UDP packet to the proxy. */

            <--------  DATAGRAM
                         Quarter Stream ID = 11
                         Context ID = 2
                         IP Version = 4
                         IP Address =
                         UDP Port = 4321
                         UDP Payload = Encapsulated UDP Payload

Appendix B. Comparison with CONNECT-IP

While the use-cases described in Section 1 could be solved using IP Proxying in HTTP [CONNECT-IP], that would require that every HTTP Datagram carry a complete IP header. This would not only cause inefficiencies in the wire encoding, it would additionally reduce the available Maximum Transmission Unit (MTU). Furthermore, it would require that Web browsers implement IPv4 and IPv6 header generation and parsing, alongside with validation and error handling.


This proposal is the result of many conversations with MASQUE working group participants.

Author's Address

David Schinazi
Google LLC
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America