Extensible Provisioning Protocol (EPP) Transport over HTTP
Internet-Draft draft-loffredo-regext-epp-over-http-03
Mario Loffredo
IIT-CNR/Registro.it
Via Moruzzi,1
Pisa
IT
56124
mario.loffredo@iit.cnr.it
https://www.iit.cnr.it
Lorenzo Luconi Trombacchi
IIT-CNR/Registro.it
Via Moruzzi,1
Pisa
IT
56124
lorenzo.luconi@iit.cnr.it
https://www.iit.cnr.it
Maurizio Martinelli
IIT-CNR/Registro.it
Via Moruzzi,1
Pisa
IT
56124
maurizio.martinelli@iit.cnr.it
https://www.iit.cnr.it
Daniel Keathley
VeriSign, Inc.
12061 Bluemont Way
Reston
VA
20190
US
dkeathley@verisign.com
http://www.verisigninc.com
James Gould
VeriSign, Inc.
12061 Bluemont Way
Reston
VA
20190
US
jgould@verisign.com
http://www.verisigninc.com
ART
Internet Engineering Task Force
epp https
This document describes how an Extensible Provisioning Protocol (EPP) session is mapped onto a Hypertext Transfer Protocol (HTTP) connection. EPP over HTTP (EoH) requires the use of Transport Layer Security (TLS) to secure EPP information (i.e. HTTPS).

Introduction
This document describes how EPP [RFC5730] is mapped onto HTTP [RFC9110]. Note that there are several versions of HTTP currently in use, including: HTTP/1.1 [RFC9112], HTTP/2 [RFC9113], and HTTP/3 [RFC9114]. As the differences among such versions do not affect the EPP mapping described in this document, hereinafter the version number is omitted except for presenting the special features in the underlying layers of the HTTP stack.
HTTP represents a higher-level abstraction of a network connection, removing the need to directly deal with all of the lower-level details of transport protocols. This makes HTTP much more compatible with cloud-native infrastructures, and facilitates faster development times and reduced maintenance costs in such environments.
Security services beyond those defined in EPP are provided by TLS via HTTPS Section 4.2.2 of [RFC9110].
Conventions Used in This Document
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 [BCP14] when, and only when, they appear in all capitals, as shown here.

Session Management
Mapping EPP session management facilities onto HTTP is accomplished using existing HTTP methods, namely GET and POST. An EPP session exists between two peers, one that initiates the session request and one that responds to the session request. The initiating peer is called the "client", and the responding peer is called the "server". An EPP server implementing this specification MUST listen for HTTP session requests on a standard HTTP port assigned by IANA.
Even though HTTP itself is stateless, a stateful EPP session can be achieved using the mechanism described in [RFC6265]. This mechanism uses "Set-Cookie" and "Cookie" HTTP headers to facilitate a stateful HTTP session. Such a session is initiated by the client via sending a GET request to the sever. The GET request MUST include "application/epp+xml" (Appendix B of [RFC5730]) in the "Accept" HTTP header. The server MUST include the EPP Greeting in the response, and it also MUST use the "Set-Cookie" header to include a token that represents the identifier of the HTTP session. All subsequent HTTP requests that include the HTTP session identifier in the "Cookie" header will be treated as part of the session. The HTTP session represents an EPP connection, which is initiated by the initial GET request.
The EPP session begins with a successful EPP <login> command, and can be referred to as an EPP over HTTP (EoH) session.
An EPP session is normally ended by the client issuing an EPP <logout> command. A server receiving an EPP <logout> command MUST end the EPP session. A server MAY also end an EPP session that has been either active or inactve for longer than a server-defined period. A server MAY end the HTTP session after ending the EPP session.

Message Exchange
EPP describes client-server interaction as a command-response exchange where the client sends one command to the server and the server returns one response to the client. With the exception of the EPP Greeting, EPP messages are initiated by the EPP client in the form of EPP commands. An EPP client MUST send all commands as HTTP POST requests (Section 6.4 of [RFC9110]). Each POST request MUST include the HTTP session identifier in the "Cookie" header and "application/epp+xml" in the "Accept" header. An EPP server MUST return an EPP response to an EPP command in the HTTP response to the respective HTTP request.
HTTP does not guarantee that POST requests are idempotent. However, the semantics of EPP do require EPP commands to be idempotent, so processing a command more than once will produce the same net effect on the repository as successfully processing the command once.
The EPP command XML is framed by the content of the HTTP POST request, and the EPP response XML is framed by the content of the HTTP response. Each HTTP request MUST contain a single EPP message, and each HTTP response MUST contain a single EPP response. Commands MUST be processed independently and in the same order as received from the client.
Servers MUST NOT use HTTP return codes to signal clients about the failure of the EPP commands. The HTTP code 200 MUST be used for both successful and unsuccessful EPP requests. Servers MUST use HTTP codes to signal clients about the failure of the HTTP requests.
Servers MUST return an EPP 2002 response (i.e. Command use error) if the client issues an EPP command with either an empty or an invalid session ID.
A server SHOULD impose a limit on the amount of time required for a client to issue a well-formed EPP command. A server SHOULD end an EPP session if a well-formed command is not received within the time limit.
HTTP/2 and HTTP/3 support a multiplexing feature that was introduced to address head-of-line blocking issues in previous HTTP versions. In the context of multiple requests being sent on a single HTTP connection, multiplexing allows the delivery of responses in a different order from how the requests were made. Due to this behavior, pipelining MUST NOT be used by EoH clients. EoH clients MUST wait for a server response to a command before sending a subsequent command.
A general state machine for an EPP server is described in Section 2 of [RFC5730]. A general client-server message exchange using HTTP is illustrated in Figure 1.

                       Client                  Server
                  |                                     |
                  |             GET Server URL          |
                  | >>------------------------------->> |
                  |                                     |
                  |             Send Greeting           |
                  | <<-------------------------------<< |
                  |                                     |
                  |             POST <login>            |
                  | >>------------------------------->> |
                  |                                     |
                  |             Send Response           |
                  | <<-------------------------------<< |
                  |                                     |
                  |            POST Command X           |
                  | >>------------------------------->> |
                  |                                     |
                  |            Send Response X          |
                  | <<-------------------------------<< |
                  |                                     |
                  |            POST Command Y           |
                  | >>------------------------------->> |
                  |                                     |
                  |            Send Response Y          |
                  | <<-------------------------------<< |
                  |                  .                  |
                                     .
                                     .
                  |            POST <logout>            |
                  | >>------------------------------->> |
                  |                                     |
                  |            Send Response            |
                  | <<-------------------------------<< |

               Figure 1: HTTP Client-Server Message Exchange

The EPP server MUST follow the "EPP Server State Machine" procedure described in [RFC5730].

Transport Considerations
Section 2.1 of [RFC5730] describes considerations to be addressed by protocol transport mappings. This document addresses each of those considerations using a combination of features of the HTTP protocol itself and features of this document.
- Command Order: Section 4 includes a requirement for ordered message delivery.
- Session Mapping: EPP session management is described in Section 3 of this document.
- Stateful Nature: Achieving the stateful nature of EPP is described in Section 3.
- Frame Data Units: Section 4 of this document describes how each EPP command is framed within the content of HTTP requests and responses.
- Congestion Avoidance: Section 3.9.3 of [RFC8095] confirms congestion avoidance as a feature of HTTP.
- Reliability: Section 3.9.3 of [RFC8095] confirms reliable message delivery as a feature of HTTP.
- Pipelining: Section 4 of this document stipulates that command pipelining must not be used in EoH.

Internationalization Considerations
Servers MUST use the "charset" attribute in the HTTP "Content-Type" response header field to specify the character encoding (e.g. Content-Type: application/epp+xml; charset=UTF-8).

IANA Considerations
This specification does not request any actions by IANA.

Security Considerations
Since client credentials are included in the EPP <login> command, HTTPS (Section 4.2.2 of [RFC9110]) MUST be used to protect them from disclosure while in transit. HTTPS indicates that TLS is being used to secure the HTTP connection between the client and server. Transferring over TLS also prevents sniffing the session ID and, consequently, impersonating a client to perform actions on registrars' objects. Servers are REQUIRED to support TLS 1.2 [RFC8446][RFC9155] or higher.
As a further measure to enforce client identification, servers SHOULD require clients to present a digital certificate. Clients who possess and present a valid X.509 digital certificate, issued by a recognized Certification Authority (CA), could be identified and authenticated by a server who trusts the corresponding CA. This certificate-based mechanism is supported by HTTPS and can be used with EPP over HTTP.
Servers are RECOMMENDED to implement additional measures to verify the client. These measures include IP allow-listing, identifying the client by its IP address and locking the session ID to the client's IP address.
Session IDs SHOULD be randomly generated to mitigate the risk of obtaining a valid one through a brute-force search. A session ID SHOULD be at least 128 bits or 16 bytes long. An example of a reliable session ID is the Universally Unique Identifier (UUID). Servers MAY limit the lifetime of active sessions to avoid them being exchanged for a long time.
The following measures MAY also be taken to control cookies usage:
- Restricting their scope through the "Domain" and "Path" attributes
- Limiting their lifetime through the "Max-Age" and "Expire" attributes
Other attributes that are normally used to secure the cookies and prevent them to be accessed by unintended parties or scripts, such as "HttpOnly" and "Secure", are meaningless in this context. Finally, servers are RECOMMENDED to perform additional checks to limit the rate of open EPP sessions and HTTP connections to mitigate the risk of congestion of requests. Here again, IP allow-listing could also be implemented to prevent DDoS attacks.
If the EPP server is configured as a load balancer routing the requests to a pool of backend servers, some of the aforementioned checks SHOULD be implemented on the load balancer side.

Acknowledgements
The authors wish to acknowledge the input from the .IT technical team.

Normative References

Informative References
[RFC5730]
[RFC9110]
[RFC9112]
[RFC9113]
[RFC9114]
[RFC6265]
[RFC8095]
[RFC8446]
[RFC9155]
Change History
Change from 00 to 01
1. Added Dan Keathley and James Gould as co-authors.