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rfc:rfc5734

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

Abstract

 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.

Hollenbeck Standards Track [Page 1] RFC 5734 EPP TCP Transport August 2009

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",
 "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.

Hollenbeck Standards Track [Page 2] RFC 5734 EPP TCP Transport August 2009

 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.

Hollenbeck Standards Track [Page 3] RFC 5734 EPP TCP Transport August 2009

 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.

Hollenbeck Standards Track [Page 4] RFC 5734 EPP TCP Transport August 2009

                     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

Hollenbeck Standards Track [Page 5] RFC 5734 EPP TCP Transport August 2009

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 [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:
  1. 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.
  1. Sections 2 and 3 of this document describe how the stateful nature

of EPP is preserved through managed sessions and controlled

    message exchanges.

Hollenbeck Standards Track [Page 6] RFC 5734 EPP TCP Transport August 2009

  1. 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.
  1. 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

Hollenbeck Standards Track [Page 7] RFC 5734 EPP TCP Transport August 2009

 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
 message.
 TLS implementations are REQUIRED to support the mandatory cipher
 suite specified in the implemented version:
 o  TLS 1.0 [RFC2246]: TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA
 o  TLS 1.1 [RFC4346]: TLS_RSA_WITH_3DES_EDE_CBC_SHA
 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
 supported.

Hollenbeck Standards Track [Page 8] RFC 5734 EPP TCP Transport August 2009

 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.

Hollenbeck Standards Track [Page 9] RFC 5734 EPP TCP Transport August 2009

  • 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
 it.
 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.

Hollenbeck Standards Track [Page 10] RFC 5734 EPP TCP Transport August 2009

 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

 [CCITT.X509.1988]
            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.

Hollenbeck Standards Track [Page 11] RFC 5734 EPP TCP Transport August 2009

 [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.

Hollenbeck Standards Track [Page 12] RFC 5734 EPP TCP Transport August 2009

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
     references.
 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
 US
 EMail: shollenbeck@verisign.com

Hollenbeck Standards Track [Page 13]

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