1. RFC 5734
Network Working Group                                      S. Hollenbeck
Request for Comments: 5734                                VeriSign, Inc.
STD: 69                                                      August 2009
Obsoletes: 4934
Category: Standards Track

       Extensible Provisioning Protocol (EPP) Transport over TCP


   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 4934.

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 (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

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Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................2
   2. Session Management ..............................................2
   3. Message Exchange ................................................3
   4. Data Unit Format ................................................6
   5. Transport Considerations ........................................6
   6. Internationalization Considerations .............................7
   7. IANA Considerations .............................................7
   8. Security Considerations .........................................7
   9. TLS Usage Profile ...............................................8
   10. Acknowledgements ..............................................11
   11. References ....................................................11
      11.1. Normative References .....................................11
      11.2. Informative References ...................................12
   Appendix A.  Changes from RFC 4934 ................................13

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
   [RFC5730].  TCP is described in [RFC0793].  This document obsoletes
   RFC 4934 [RFC4934].

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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.

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   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

   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

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

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   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 [RFC5730].  General client-server message exchange using TCP
   transport is illustrated in Figure 1.

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                       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

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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

   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 [RFC5730]
   describes considerations to be addressed by protocol transport
   mappings.  This document 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 document describes how EPP sessions are mapped
      to TCP connections.

   -  Sections 2 and 3 of this document describe how the stateful nature
      of EPP is preserved through managed sessions and controlled
      message exchanges.

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   -  Section 3 of this document 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 document describes features to frame data units
      by explicitly specifying the number of octets used to represent a
      data unit.

6.  Internationalization Considerations

   This document 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 1.2 [RFC5246]),
   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.

   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

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   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.  TLS Usage Profile

   The client should initiate a connection to the server and then send
   the TLS Client Hello to begin the TLS handshake.  When the TLS
   handshake has finished, the client can then send the first EPP

   TLS implementations are REQUIRED to support the mandatory cipher
   suite specified in the implemented version:



   o  TLS 1.2 [RFC5246]: TLS_RSA_WITH_AES_128_CBC_SHA

   This document is assumed to apply to future versions of TLS, in which
   case the mandatory cipher suite for the implemented version MUST be

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   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 5280 [RFC5280].
   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.

   If the client has external information as to the expected identity of
   the server, the server name check MAY be omitted.  For instance, a
   client may be connecting to a machine whose address and server name
   are dynamic, but the client knows the certificate that the server
   will present.  In such cases, it is important to narrow the scope of
   acceptable certificates as much as possible in order to prevent man-
   in-the-middle attacks.  In special cases, it might be appropriate for
   the client to simply ignore the server's identity, but it needs to be
   understood that this leaves the connection open to active attack.

   During the TLS negotiation, the EPP client MUST check its
   understanding of the server name / IP address against the server's
   identity as presented in the server Certificate message in order to
   prevent man-in-the-middle attacks.  In this section, the client's
   understanding of the server's identity is called the "reference
   identity".  Checking is performed according to the following rules in
   the specified order:

   o  If the reference identity is a server name:

      *  If a subjectAltName extension of the dNSName [CCITT.X509.1988]
         type is present in the server's certificate, then it SHOULD be
         used as the source of the server's identity.  Matching is
         performed as described in Section 7.2 of [RFC5280], with the
         exception that wildcard matching (see below) is allowed for
         dNSName type.  If the certificate contains multiple names
         (e.g., more than one dNSName field), then a match with any one
         of the fields is considered acceptable.

      *  The '*' (ASCII 42) wildcard character is allowed in
         subjectAltName values of type dNSName, and then only as the
         left-most (least significant) DNS label in that value.  This
         wildcard matches any left-most DNS label in the server name.
         That is, the subject *.example.com matches the server names
         a.example.com and b.example.com, but does not match example.com
         or a.b.example.com.

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      *  The server's identity MAY also be verified by comparing the
         reference identity to the Common Name (CN) [RFC4519] value in
         the leaf Relative Distinguished Name (RDN) of the subjectName
         field of the server's certificate.  This comparison is
         performed using the rules for comparison of DNS names in bullet
         1 above (including wildcard matching).  Although the use of the
         Common Name value is existing practice, it is deprecated, and
         Certification Authorities are encouraged to provide
         subjectAltName values instead.  Note that the TLS
         implementation may represent DNs in certificates according to
         X.500 or other conventions.  For example, some X.500
         implementations order the RDNs in a DN using a left-to-right
         (most significant to least significant) convention instead of
         LDAP's right-to-left convention.

   o  If the reference identity is an IP address:

      *  The iPAddress subjectAltName SHOULD be used by the client for
         comparison.  In such a case, the reference identity MUST be
         converted to the "network byte order" octet string
         representation.  For IP Version 4 (as specified in RFC 791
         [RFC0791]), the octet string will contain exactly four octets.
         For IP Version 6 (as specified in RFC 2460 [RFC2460]), the
         octet string will contain exactly sixteen octets.  This octet
         string is then compared against subjectAltName values of type
         iPAddress.  A match occurs if the reference identity octet
         string and value octet strings are identical.

   If the server identity check fails, user-oriented clients SHOULD
   either notify the user (clients MAY give the user the opportunity to
   continue with the EPP session in this case) or close the transport
   connection and indicate that the server's identity is suspect.
   Automated clients SHOULD return or log an error indicating that the
   server's identity is suspect and/or SHOULD close the transport
   connection.  Automated clients MAY provide a configuration setting
   that disables this check, but MUST provide a setting which enables

   During the TLS negotiation, the EPP server MUST verify that the
   client certificate matches the reference identity previously
   negotiated out of band, as specified in Section 8.  The server should
   match the entire subject name or the subjectAltName as described in
   RFC 5280.  The server MAY enforce other restrictions on the
   subjectAltName, for example if it knows that a particular client is
   always connecting from a particular hostname / IP address.

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   All EPP messages MUST be sent as TLS "application data".  It is
   possible that multiple EPP messages are contained in one TLS record,
   or that an EPP message is transferred in multiple TLS records.

   When no data is received from a connection for a long time (where the
   application decides what "long" means), a server MAY close the
   connection.  The server MUST attempt to initiate an exchange of
   close_notify alerts with the client before closing the connection.
   Servers that are unprepared to receive any more data MAY close the
   connection after sending the close_notify alert, thus generating an
   incomplete close on the client side.

10.  Acknowledgements

   RFC 3734 is a product of the PROVREG working group, which suggested
   improvements and provided many invaluable comments.  The author
   wishes to acknowledge the efforts of WG chairs Edward Lewis and Jaap
   Akkerhuis for their process and editorial contributions.  RFC 4934
   and this document are individual submissions, based on the work done
   in RFC 3734.

   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.

11.  References

11.1.  Normative References

              International Telephone and Telegraph Consultative
              Committee, "Information Technology - Open Systems
              Interconnection - The Directory: Authentication
              Framework", CCITT Recommendation X.509, November 1988.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [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.

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   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4519]  Sciberras, A., "Lightweight Directory Access Protocol
              (LDAP): Schema for User Applications", RFC 4519,
              June 2006.

   [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
              STD 69, RFC 5730, August 2009.

11.2.  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.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4934]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Transport Over TCP", RFC 4934, May 2007.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

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Appendix A.  Changes from RFC 4934

   1.  Changed "This document obsoletes RFC 3734" to "This document
       obsoletes RFC 4934".

   2.  Replaced references to RFC 3280 with references to 5280.

   3.  Replaced references to RFC 3734 with references to 4934.

   4.  Updated references to RFC 4346 and TLS 1.1 with references to
       5246 and TLS 1.2.

   5.  Replaced references to RFC 4930 with references to 5730.

   6.  Added clarifying TLS Usage Profile section and included

   7.  Moved the paragraph that begins with "Mutual client and server
       authentication" from the Security Considerations section to the
       TLS Usage Profile section.

Author's Address

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

   EMail: shollenbeck@verisign.com

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