Proxying Listener UDP in HTTP

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Table of Contents

1. Introduction

The mechanism to proxy UDP in HTTP [CONNECT-UDP] allows creating tunnels for communicating 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 ICE relies on the ability to send and receive UDP packets to multiple hosts. While in theory it might be possible to accomplish this using multiple UDP Proxying HTTP requests, HTTP semantics [HTTP] do not guarantee that distinct requests will be handled by the same server. This can lead to the UDP packets being sent from distinct IP addresses, thereby preventing ICE from operating correctly. Consequently, 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.¶

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 3.¶

When performing URI Template Expansion 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 UDP Proxying request, with its value set as the allocated context ID, see Section 4.¶

3. HTTP Datagram Payload Format

When HTTP Datagrams [HTTP-DGRAM] associated with this Listener UDP Proxying request contain the context ID in the connect-udp-listen header field, the format of their UDP Proxying Payload field (see Section 5 of [CONNECT-UDP]) is defined by Figure 1:¶

Listener UDP Proxying Payload {
  IP Version (8),
  IP Address (32..128),
  UDP Port (16),
  UDP Payload (..),
}

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

UDP Port:

The UDP Port of this proxied UDP packet in network byte order. When sent from client to proxy, this is the target port to which the proxy will send this UDP payload. 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]).¶

4. The connect-udp-listen Header Field

The "connect-udp-listen" header field’s value is an Integer. It is set as the Context ID allocated for Listener UDP Proxying; see Section 2. Any other value type MUST be handled as if the field were not present by the recipients (for example, if this field is defined multiple times, its type becomes a List and therefore is to 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.¶

5. Proxy behavior

After accepting the Connect-UDP Listener proxying request, the proxy uses a UDP port to transmit UDP payloads received from the client to the target IP Address and UDP Port specified in each Listener Datagram Payload received from the client. The proxy uses the same port to listen for UDP packets from any authorized target and encapsulates the packets in the Listener Datagram Payload format, specifying the IP and port of the target and forwards it to the client.¶

6. Security Considerations

The security considerations described in Section 7 of [CONNECT-UDP] also apply here. Since TURN can be run over this mechanism, implementors should review the security considerations in Section 21 of [TURN].¶

Since unextended UDP Proxying requests carry the target as part of the request, the proxy can protect unauthorized targets by rejecting requests before creating the tunnel, and communicate the rejection reason in response header fields. Listener UDP Proxying requests do not have this ability. Therefore, proxies MUST validate the target on every datagram and MUST NOT forward individual datagrams with unauthorized targets. Proxies can either silently discard such datagrams or abort the corresponding request stream.¶

7. IANA Considerations

This document will request IANA to register the following entry in the "HTTP Field Name" registry maintained at <https://www.iana.org/assignments/http-fields>:¶

Field Name:

connect-udp-listen¶

Template:

None¶

Status:

provisional (permanent if this document is approved)¶

Reference:

This document¶

Comments:

None¶

Appendix A. Example

In the example below, the client is configured with URI Template "https://example.org/.well-known/masque/udp/{target_host}/{target_port}/" and wishes to use WebRTC with another browser over a listener UDP Proxying tunnel. It contacts a STUN server at 192.0.2.42. The STUN server, in response, sends the proxy's IP address to the other browser at 203.0.113.33. Using this information, the other browser sends a UDP packet to the proxy, which is 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 = proxy.example.org
   connect-udp-listen = 2
   capsule-protocol = ?1

 DATAGRAM                       -------->
   Quarter Stream ID = 11
   Context ID = 2
   IP Version = 4
   IP Address = 192.0.2.42
   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 = 192.0.2.42
                         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 = 203.0.113.33
                         UDP Port = 4321
                         UDP Payload = Encapsulated UDP Payload

Appendix B. Comparison with CONNECT-IP

While the use-cases described in Section 1 could be supported using IP Proxying in HTTP [CONNECT-IP], it would require that every HTTP Datagram carries a complete IP header. This would lead to both inefficiencies in the wire encoding and reduction in available Maximum Transmission Unit (MTU). Furthermore, Web browsers would need to support IPv4 and IPv6 header generation, parsing, validation and error handling.¶

Acknowledgments

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

Authors' Addresses