Network Working Group
Request for Comments: 4934
Obsoletes: 3734
Category: Standards Track
S. Hollenbeck
VeriSign, Inc.
May 2007

Extensible Provisioning Protocol (EPP) Transport over TCP

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The IETF Trust (2007).


This document describes how an Extensible Provisioning Protocol (EPP) session is mapped onto a single Transmission Control Protocol (TCP) connection. This mapping requires use of the Transport Layer Security (TLS) protocol to protect information exchanged between an EPP client and an EPP server. This document obsoletes RFC 3734.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
     1.1.  Conventions Used in This Document . . . . . . . . . . . . . 2
   2.  Session Management  . . . . . . . . . . . . . . . . . . . . . . 2
   3.  Message Exchange  . . . . . . . . . . . . . . . . . . . . . . . 2
   4.  Data Unit Format  . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Transport Considerations  . . . . . . . . . . . . . . . . . . . 5
   6.  Internationalization Considerations . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     10.1. Normative References  . . . . . . . . . . . . . . . . . . . 8
     10.2. Informative References  . . . . . . . . . . . . . . . . . . 8
   Appendix A.  Changes from RFC 3734  . . . . . . . . . . . . . . . . 9

1. Introduction

This document describes how the Extensible Provisioning Protocol (EPP) is mapped onto a single client-server TCP connection. Security services beyond those defined in EPP are provided by the Transport Layer Security (TLS) Protocol [RFC2246]. EPP is described in [RFC4930]. TCP is described in [RFC0793]. This document obsoletes RFC 3734 [RFC3734].

1.1. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. Session Management

Mapping EPP session management facilities onto the TCP service is straightforward. An EPP session first requires creation of a TCP connection between two peers, one that initiates the connection request and one that responds to the connection request. The initiating peer is called the "client", and the responding peer is called the "server". An EPP server MUST listen for TCP connection requests on a standard TCP port assigned by IANA.

The client MUST issue an active OPEN call, specifying the TCP port number on which the server is listening for EPP connection attempts. The EPP server MUST return an EPP <greeting> to the client after the TCP session has been established.

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 and close the TCP connection with a CLOSE call. A client MAY end an EPP session by issuing a CLOSE call.

A server MAY limit the life span of an established TCP connection. EPP sessions that are inactive for more than a server-defined period MAY be ended by a server issuing a CLOSE call. A server MAY also close TCP connections that have been open and active for longer than a server-defined period.

3. Message Exchange

With the exception of the EPP server greeting, EPP messages are initiated by the EPP client in the form of EPP commands. An EPP server MUST return an EPP response to an EPP command on the same TCP connection that carried the command. If the TCP connection is closed after a server receives and successfully processes a command but before the response can be returned to the client, the server MAY attempt to undo the effects of the command to ensure a consistent state between the client and the server. EPP commands are idempotent, so processing a command more than once produces the same net effect on the repository as successfully processing the command once.

An EPP client streams EPP commands to an EPP server on an established TCP connection. A client MUST NOT distribute commands from a single EPP session over multiple TCP connections. A client MAY establish multiple TCP connections to support multiple EPP sessions with each session mapped to a single connection. A server SHOULD limit a client to a maximum number of TCP connections based on server capabilities and operational load.

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. A client might be able to realize a slight performance gain by pipelining (sending more than one command before a response for the first command is received) commands with TCP transport, but this feature does not change the basic single command, single response operating mode of the core protocol.

Each EPP data unit MUST contain a single EPP message. Commands MUST be processed independently and in the same order as sent from the client.

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 and close an open TCP connection if a well-formed command is not received within the time limit.

A general state machine for an EPP server is described in Section 2 of [RFC4930]. General client-server message exchange using TCP transport is illustrated in Figure 1.

                       Client                  Server
                  |                                     |
                  |                Connect              |
                  | >>------------------------------->> |
                  |                                     |
                  |             Send Greeting           |
                  | <<-------------------------------<< |
                  |                                     |
                  |             Send <login>            |
                  | >>------------------------------->> |
                  |                                     |
                  |             Send Response           |
                  | <<-------------------------------<< |
                  |                                     |
                  |             Send Command            |
                  | >>------------------------------->> |
                  |                                     |
                  |             Send Response           |
                  | <<-------------------------------<< |
                  |                                     |
                  |            Send Command X           |
                  | >>------------------------------->> |
                  |                                     |
                  |    Send Command Y                   |
                  | >>---------------+                  |
                  |                  |                  |
                  |                  |                  |
                  |            Send Response X          |
                  | <<---------------(---------------<< |
                  |                  |                  |
                  |                  |                  |
                  |                  +--------------->> |
                  |                                     |
                  |            Send Response Y          |
                  | <<-------------------------------<< |
                  |                                     |
                  |             Send <logout>           |
                  | >>------------------------------->> |
                  |                                     |
                  |     Send Response & Disconnect      |
                  | <<-------------------------------<< |
                  |                                     |

Figure 1: TCP Client-Server Message Exchange

4. Data Unit Format

The EPP data unit contains two fields: a 32-bit header that describes the total length of the data unit, and the EPP XML instance. The length of the EPP XML instance is determined by subtracting four octets from the total length of the data unit. A receiver must successfully read that many octets to retrieve the complete EPP XML instance before processing the EPP message.

EPP Data Unit Format (one tick mark represents one bit position):

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |                           Total Length                        |
      |                         EPP XML Instance                      |

Total Length (32 bits): The total length of the EPP data unit measured in octets in network (big endian) byte order. The octets contained in this field MUST be included in the total length calculation.

EPP XML Instance (variable length): The EPP XML instance carried in the data unit.

5. Transport Considerations

Section 2.1 of the EPP core protocol specification [RFC4930] describes considerations to be addressed by protocol transport mappings. This mapping addresses each of the considerations using a combination of features described in this document and features provided by TCP as follows:

  • TCP includes features to provide reliability, flow control, ordered delivery, and congestion control. Section 1.5 of RFC 793 [RFC0793] describes these features in detail; congestion control principles are described further in RFC 2581 [RFC2581] and RFC 2914 [RFC2914]. TCP is a connection-oriented protocol, and Section 2 of this mapping describes how EPP sessions are mapped to TCP connections.
  • Sections 2 and 3 of this mapping describe how the stateful nature of EPP is preserved through managed sessions and controlled message exchanges.
  • Section 3 of this mapping notes that command pipelining is possible with TCP, though batch-oriented processing (combining multiple EPP commands in a single data unit) is not permitted.
  • Section 4 of this mapping describes features to frame data units by explicitly specifying the number of octets used to represent a data unit.

6. Internationalization Considerations

This mapping does not introduce or present any internationalization or localization issues.

7. IANA Considerations

System port number 700 has been assigned by the IANA for mapping EPP onto TCP.

User port number 3121 (which was used for development and test purposes) has been reclaimed by the IANA.

8. Security Considerations

EPP as-is provides only simple client authentication services using identifiers and plain text passwords. A passive attack is sufficient to recover client identifiers and passwords, allowing trivial command forgery. Protection against most other common attacks MUST be provided by other layered protocols.

When layered over TCP, the Transport Layer Security (TLS) Protocol version 1.0 [RFC2246] or its successors (such as TLS 1.1 [RFC4346]), using the latest version supported by both parties, MUST be used to provide integrity, confidentiality, and mutual strong client-server authentication. Implementations of TLS often contain a weak cryptographic mode that SHOULD NOT be used to protect EPP. Clients and servers desiring high security SHOULD instead use TLS with cryptographic algorithms that are less susceptible to compromise.

Mutual client and server authentication using the TLS Handshake Protocol is REQUIRED. Signatures on the complete certification path for both client machine and server machine MUST be validated as part of the TLS handshake. Information included in the client and server certificates, such as validity periods and machine names, MUST also be validated. A complete description of the issues associated with certification path validation can be found in RFC 3280 [RFC3280]. EPP service MUST NOT be granted until successful completion of a TLS handshake and certificate validation, ensuring that both the client machine and the server machine have been authenticated and cryptographic protections are in place.

Authentication using the TLS Handshake Protocol confirms the identity of the client and server machines. EPP uses an additional client identifier and password to identify and authenticate the client's user identity to the server, supplementing the machine authentication provided by TLS. The identity described in the client certificate and the identity described in the EPP client identifier can differ, as a server can assign multiple user identities for use from any particular client machine. Acceptable certificate identities MUST be negotiated between client operators and server operators using an out-of-band mechanism. Presented certificate identities MUST match negotiated identities before EPP service is granted.

There is a risk of login credential compromise if a client does not properly identify a server before attempting to establish an EPP session. Before sending login credentials to the server, a client needs to confirm that the server certificate received in the TLS handshake is an expected certificate for the server. A client also needs to confirm that the greeting received from the server contains expected identification information. After establishing a TLS session and receiving an EPP greeting on a protected TCP connection, clients MUST compare the certificate subject and/or subjectAltName to expected server identification information and abort processing if a mismatch is detected. If certificate validation is successful, the client then needs to ensure that the information contained in the received certificate and greeting is consistent and appropriate. As described above, both checks typically require an out-of-band exchange of information between client and server to identify expected values before in-band connections are attempted.

EPP TCP servers are vulnerable to common TCP denial-of-service attacks including TCP SYN flooding. Servers SHOULD take steps to minimize the impact of a denial-of-service attack using combinations of easily implemented solutions, such as deployment of firewall technology and border router filters to restrict inbound server access to known, trusted clients.

9. Acknowledgements

This document was originally written as an individual submission Internet-Draft. The PROVREG working group later adopted it as a working group document and provided many invaluable comments and suggested improvements. The author wishes to acknowledge the efforts of WG chairs Edward Lewis and Jaap Akkerhuis for their process and editorial contributions.

Specific suggestions that have been incorporated into this document were provided by Chris Bason, Randy Bush, Patrik Faltstrom, Ned Freed, James Gould, Dan Manley, and John Immordino.

10. References

10.1. Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.
   [RFC4930]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
              RFC 4930, May 2007.

10.2. Informative References

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.
   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, September 2000.
   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.
   [RFC3734]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Transport Over TCP", RFC 3734, March 2004.
   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

Appendix A. Changes from RFC 3734

  1. Minor reformatting as a result of converting I-D source format from nroff to XML.
  1. Updated Security Considerations to include strong authentication among the list of needed security services. Removed paragraph describing replay attacks because it's not specific to TCP. New text has been added to RFC 4930 to describe this issue.
  1. Modified description of TCP operation as a result of IESG evaluation.
  1. Moved RFCs 2581 and 2914 from the normative reference section to the informative reference section.
  1. Added informative references to RFCs 3280 and 4346 and descriptive text for each as a result of IESG evaluation.
  1. Revised security considerations as a result of IESG evaluation.
  1. Updated EPP references.

Author's Address

   Scott Hollenbeck
   VeriSign, Inc.
   21345 Ridgetop Circle
   Dulles, VA  20166-6503


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