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
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
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.
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 [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
- Sections 2 and 3 of this mapping describe how the stateful nature
of EPP is preserved through managed sessions and controlled
- 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
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
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.
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
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.
10.1. 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,
[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.
2. 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.
3. Modified description of TCP operation as a result of IESG
4. Moved RFCs 2581 and 2914 from the normative reference section to
the informative reference section.
5. Added informative references to RFCs 3280 and 4346 and
descriptive text for each as a result of IESG evaluation.
6. Revised security considerations as a result of IESG evaluation.
7. Updated EPP references.
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