The CONNECT-IP HTTP Method

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Table of Contents

1. Introduction

This document describes the CONNECT-IP HTTP method. CONNECT-IP is similar to CONNECT-UDP, but allows transmitting IP packets, without being limited to just TCP like CONNECT or UDP like CONNECT-UDP.¶

CONNECT-IP allows endpoints to set up an IP tunnel between one another. This can be used to implement a consumer VPN, point-to-point, point-to-network, and network-to-network capabilities as described in [REQS].¶

2. The CONNECT-IP Method

The CONNECT-IP method establishes a stream to an endpoint server that then permits the exchange of control data, such as IP address information, reachable IP ranges, and other relevant information for successfully transmitting IP datagrams between hosts.¶

The request-target of a CONNECT-IP request is a URI [URI] which uses the "https" scheme and a client-specified path. When using HTTP/2 [H2] or later, CONNECT-IP requests use HTTP pseudo-headers with the following requirements:¶

• The ":method" pseudo-header field is set to "CONNECT-IP".¶
• The ":scheme" pseudo-header field is set to "https".¶
• The ":path" pseudo-header field is set to the value provided by the client. That value MUST NOT be empty.¶
• The ":authority" pseudo-header field contains the host and port of the proxy. The target of a CONNECT-IP request is the server providing the CONNECT-IP featureset, not an individual endpoint with which a connection is desired.¶

A CONNECT-IP request that does not conform to these restrictions is malformed (see [H2], Section 8.1.2.6).¶

Any 2xx (Successful) response indicates that the proxy is willing to open an IP tunnel between it and the client. Any response other than a successful response indicates that the tunnel has not yet been formed.¶

A proxy MUST NOT send any Transfer-Encoding or Content-Length header fields in a 2xx (Successful) response to CONNECT-IP. A client MUST treat a successful response to CONNECT-IP containing any Content-Length or Transfer-Encoding header fields as malformed.¶

A payload within a CONNECT-IP request message has no defined semantics; a CONNECT-IP request with a non-empty payload is malformed. Note that the CONNECT-IP stream is used to convey control messages, but they are not semantically part of the request or response themselves.¶

Responses to the CONNECT-IP method are not cacheable.¶

The lifetime of the tunnel is tied to the CONNECT-IP stream. Closing the stream (via the FIN bit on a QUIC STREAM frame, or a QUIC RESET_STREAM frame) closes the associated tunnel.¶

3. Transmitting IP Packets using HTTP Datagrams

IP packets are sent using HTTP Datagrams [HTTP-DGRAM]. The HTTP Datagram Payload contains a full IP packet, from the IP Version field until the last byte of the IP Payload. In order to use HTTP Datagrams, the CONNECT-IP client will first decide whether or not to use HTTP Datagram Contexts and then register its context ID (or lack thereof) using the corresponding registration capsule, see [HTTP-DGRAM].¶

Since HTTP Datagrams require prior negotiation (for example, in HTTP/3 it is necessary to both send and receive the H3_DATAGRAM SETTINGS Parameter), clients MUST NOT send any HTTP Datagrams until they have established support on a given connection. If negotiation of HTTP Datagrams fails (for example if an HTTP/3 SETTINGS frame was received without the H3_DATAGRAM SETTINGS Parameter), the client MUST consider its CONNECT-IP request as failed.¶

4. Forwarding of IP Packets

Since CONNECT-IP allows the transmission of IP packets over HTTP, CONNECT-IP endpoints will most often forward these packets to and from traditional IP interfaces. As such, CONNECT-IP endpoints act as IP routers. When a CONNECT-IP endpoint receives an HTTP Datagram containing an IP packet, it will parse the packet's IP header, perform any local policy checks (e.g., source address validation), check their routing table to pick an outbound interface, and then use an implementation-specific mechanism (such as raw sockets) to send the IP packet on that interface.¶

Conversely, when a CONNECT-IP endpoint receives an IP packet whose destination address does not match any local addresses, it consults its routing table to pick a forwarding destination, and if the table points to a CONNECT-IP tunnel, the endpoint performs the same forwarding checks before transmitting the packet inside the tunnel.¶

Note that CONNECT-IP endpoints will decrement the IP Hop Count (or TTL) upon encapsulation but not decapsulation. In other words, the Hop Count is decremented right before an IP packet is transmitted in an HTTP Datagram. This prevents infinite loops in the presence of routing loops, and matches the choices in IPsec [IPSEC].¶

Endpoints MAY implement additional filtering policies on the IP packets they forward.¶

5. Routes

Endpoints have the ability to advertise and reject routes using the ROUTE_ADVERTISEMENT (Section 6.3) and ROUTE_REJECTION (Section 6.3) capsule. Note that these capsules are purely informational: receipt of a ROUTE_ADVERTISEMENT capsule does not require the recipient to start routing traffic to its peer. Additionally, if an endpoint receives a ROUTE_REJECTION for a given prefix that it had previously received a ROUTE_ADVERTISEMENT capsule for, then the two cancel out and the endpoint MUST remove its state from the ROUTE_ADVERTISEMENT capsule instead of installing new state for the ROUTE_REJECTION capsule. Conversely, the same is true of a ROUTE_ADVERTISEMENT that matches a previous ROUTE_REJECTION. Routes are handled via longest-prefix-first preference, meaning that if a given IP prefix is covered by multiple route advertisement and route rejections, the one with the longest prefix is used.¶

When processing ROUTE_ADVERTISEMENT capsules, endpoints MUST check their local policy before deciding whether to forward packets to their peer. Since ignoring these capsules is allowed by the protocol, such policy decisions will not prevent interoperability.¶

6. Capsules

ADDRESS_ASSIGN Capsule {
  IP Version (8),
  IP Address (32..128),
  IP Prefix Length (8),
}

ADDRESS_REQUEST Capsule {
  IP Version (8),
  IP Address (32..128),
  IP Prefix Length (8),
}

ROUTE_ADVERTISEMENT Capsule {
  IP Version (8),
  IP Address (32..128),
  IP Prefix Length (8),
}

ROUTE_REJECTION Capsule {
  IP Version (8),
  IP Address (32..128),
  IP Prefix Length (8),
}

ROUTE_RESET Capsule {
}

SHUTDOWN Capsule {
  Reason Phrase (..),
}

ATOMIC_START Capsule {
}

ATOMIC_END Capsule {
}

7. Extensibility Considerations

CONNECT-IP can be extended via multiple mechanisms to increase functionality. There are three main ways to extend CONNECT-IP: HTTP headers, Capsule Types, and HTTP Datagram Registration Extension Data. For example, an authentication extension could define an HTTP header that allows endpoints to send authentication credentials to their peer during the creation of the tunnel. Alternatively, one could specify an extension that defines a new Capsule Type which allows exchanging DNS configuration between endpoints. Additionally, an extension to CONNECT-IP can use multiple HTTP Datagram Contexts [HTTP-DGRAM] simultaneously to compress some IP packets by associating the compression context with an HTTP Datagram Context ID.¶

8. Security Considerations

There are significant risks in allowing arbitrary clients to establish a tunnel to arbitrary servers, as that could allow bad actors to send traffic and have it attributed to the proxy. Proxies that support CONNECT-IP SHOULD restrict its use to authenticated users. The HTTP Authorization header [AUTH] MAY be used to authenticate clients. More complex authentication schemes are out of scope for this document but can be implemented using CONNECT-IP extensions.¶

Since CONNECT-IP endpoints can proxy IP packets send by their peer, they SHOULD follow the guidance in [BCP38] to help prevent denial of service attacks.¶

In theory, endpoints could use ROUTE_ADVERTISEMENT capsules to divert traffic from naive endpoints. To avoid this, receivers of ROUTE_ADVERTISEMENT capsules MUST check their local policy before acting on such capsules, see Section 5.¶

9. IANA Considerations

  +-------------+------+------------+---------------+
  | Method Name | Safe | Idempotent |   Reference   |
  +-------------+------+------------+---------------+
  | CONNECT-IP  |  no  |     no     | This document |
  +-------------+------+------------+---------------+

+----------+---------------------+---------------------+---------------+
|   Value  |        Type         |      Description    |   Reference   |
+----------+---------------------+---------------------+---------------+
| 0xfff100 |   ADDRESS_ASSIGN    | Address Assignment  | This document |
| 0xfff101 |   ADDRESS_REQUEST   | Address Request     | This document |
| 0xfff102 | ROUTE_ADVERTISEMENT | Route Advertisement | This document |
| 0xfff103 |   ROUTE_REJECTION   | Route Rejection     | This document |
| 0xfff104 |     ROUTE_RESET     | Route Reset         | This document |
| 0xfff105 |      SHUTDOWN       | Shutdown Reason     | This document |
| 0xfff106 |    ATOMIC_START     | Atomic Start        | This document |
| 0xfff107 |     ATOMIC_END      | Atomic End          | This document |
+----------+---------------------+---------------------+---------------+

Appendix A. Examples

    Client                                             Server

    ADDRESS_REQUEST          -------->
      IP Version = 4
      IP Address = 0.0.0.0
      IP Prefix Length = 0

                             <--------  ADDRESS_ASSIGN
                                          IP Version = 4
                                          IP Address = 192.0.2.42
                                          IP Prefix Length = 32

                             <--------  ROUTE_ADVERTISEMENT
                                          IP Version = 4
                                          IP Address = 0.0.0.0
                                          IP Prefix Length = 0

Acknowledgments

The design of CONNECT-IP was inspired by discussions in the MASQUE working group around [REQS]. The authors would like to thank participants in those discussions for their feedback.¶

Authors' Addresses