Internet DRAFT - draft-smyslov-ipsecme-tcp-guidelines
draft-smyslov-ipsecme-tcp-guidelines
Network Working Group V. Smyslov
Internet-Draft ELVIS-PLUS
Intended status: Informational December 17, 2019
Expires: June 19, 2020
Clarifications and Implementation Guidelines for using TCP Encapsulation
in IKEv2
draft-smyslov-ipsecme-tcp-guidelines-03
Abstract
The Internet Key Exchange Protocol version 2 (IKEv2) defined in
[RFC7296] uses UDP transport for its messages. [RFC8229] specifies a
way to encapsulate IKEv2 and ESP (Encapsulating Security Payload)
messages in TCP, thus making possible to use them in network
environments that block UDP traffic. However, some nuances of using
TCP in IKEv2 are not covered by that specification. This document
provides clarifications and implementation guidelines for [RFC8229].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 19, 2020.
Copyright Notice
Copyright (c) 2019 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
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to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Notation . . . . . . . . . . . . . . . . . . 3
3. TCP Encapsulation Format . . . . . . . . . . . . . . . . . . 3
4. Falling back from UDP to TCP . . . . . . . . . . . . . . . . 4
5. Retransmissions . . . . . . . . . . . . . . . . . . . . . . . 4
6. Using Cookies and Puzzles . . . . . . . . . . . . . . . . . . 4
7. Error Handling in the IKE_SA_INIT . . . . . . . . . . . . . . 5
8. Interaction with IKEv2 Extensions . . . . . . . . . . . . . . 6
8.1. MOBIKE Protocol . . . . . . . . . . . . . . . . . . . . . 6
8.2. IKEv2 Redirect . . . . . . . . . . . . . . . . . . . . . 7
8.3. IKEv2 Session Resumption . . . . . . . . . . . . . . . . 7
8.4. IKEv2 Protocol Support for High Availability . . . . . . 7
9. TCP Encapsulation Influence on IPsec SAs . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The Internet Key Exchange version 2 (IKEv2) as it is defined in
[RFC7296] uses UDP as a transport protocol. As time passed the
network environment has been evolved and sometimes this evolution has
resulted in situations when UDP messages are dropped by network
infrastructure. This may happen either by incapability of network
devices to properly handle them (e.g. non-initial fragments of UDP
messages) of by deliberate configuration of network devices that
blocks UDP traffic.
Several standard solutions have been developed to deal with such
situations. In particular, [RFC7383] defines a way to avoid IP
fragmentation of large IKE messages and [RFC8229] specifies a way to
transfer IKEv2 and ESP (Encapsulated Security Payload) messages over
a stream protocol like TCP. This document focuses on the latter
specification and its goal is to give implementers guidelines how to
properly use reliable connection-oriented stream transport in IKEv2.
Since originally IKEv2 relied on unreliable transport, it was
designed to deal with this unreliability. IKEv2 has its own
retransmission timers, replay detection logic etc. Using reliable
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transport makes many of such things unnecessary. On the other hand,
connection-oriented transport require IKEv2 to keep the connection
alive and to restore it in case it is broken, the tasks that were not
needed before. [RFC8229] gives recommendations how peers must behave
in different situations to keep the connection. However,
implementation experience has revealed that not all situations are
covered in [RFC8229], that may lead to interoperability problems or
to suboptimal performance. This memo gives implementers more
guidelines how to use reliable stream transport in IKEv2 in
situations, which are not covered in [RFC8229].
2. Terminology and Notation
This document shares the terminology with [RFC8229]. In particular,
it uses terms "TCP Originator" and "TCP Responder" to refer to the
parties that initiate or responded to the TCP connection created for
the initial IKE SA (in a possible series of successive rekeys). More
details are given in Section 1.2 of [RFC8229].
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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. TCP Encapsulation Format
Section 3 of [RFC8229] describes how IKE and ESP packets are
encapsulated in TCP stream. For this purpose every IKE or ESP packet
is prepended with 16-bit Length field. However, the text in the
first para of the section is not very explicit on what the Length
field means - whether it indicates only the length of the following
IKE or ESP message or the length of field itself is also counted.
The following text in the same section clarifies it - the value of
the Length field includes the length of the field itself (2 octets).
It means that values 0 and 1 must never appear there. The receiver
MUST treat these values in the Length field as fatal error and MUST
close TCP session in this case.
Note, that since TCP header is longer than UDP header, and TCP
encapsulation also requires prepending of 16-bit Length field, some
very long ESP and IKE messages that could be sent over UDP cannot be
encapsulated in TCP, because their total length after encapsulation
would exceed 65535 and thus could not be represented in Length field.
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4. Falling back from UDP to TCP
Section 5.1 of [RFC8229] describes how the fallback from UDP to TCP
must be handled. It is recommended, that in the absence of prior
knowledge, implementations first try to use UDP and then fall back
TCP if no reply is received within some period of time after several
retransmissions. In this case a new IKE_SA_INIT exchange MUST be
initiated with new initiator's SPI and with recalculated content of
NAT_DETECTION_SOURCE_IP notification.
5. Retransmissions
Section 2.1 of [RFC7296] describes how IKEv2 deals with unreliability
of UDP protocol. In brief, exchange initiator is responsible for
retransmissions and must retransmit requests message until response
message is received. If no reply is received after several
retransmissions, the SA is deleted. The responder never retransmits
but must resend the response message in case it receives
retransmitted request.
When IKEv2 uses reliable transport protocol, most of these rules
become unnecessary. Since [RFC8229] doesn't provide clear guidance
on using retransmissions in case of TCP encapsulation, this memo
gives the following rules.
o the exchange initiator SHOULD NOT retransmit request message; if
no response is received within some reasonable period of time, the
IKE SA is deleted
o if TCP connection is broken and then restored while the exchange
initiator is waiting for the response, the initiator MUST
retransmit the request and continue to wait for the response
o the exchange responder acts as described in Section 2.1 of
[RFC7296], i.e. using TCP encapsulation doesn't change the
responder's behavior
6. Using Cookies and Puzzles
IKEv2 provides a DoS attack protection mechanism called Cookie, which
is described in Section 2.6 of [RFC7296]. [RFC8019] extends this
mechanism for protection against DDoS attacks by means of Client
Puzzles. Both mechanisms allow the responder to keep no state until
the initiator proves its IP address is real (and solves puzzle in the
latter case).
[RFC8229] gives no guidance on how these mechanisms should be used in
case of TCP encapsulation. However, the connection-oriented nature
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of TCP brings additional considerations for using these mechanisms.
In general, Cookie provides less value in case of TCP encapsulation,
because when the responder receives the IKE_SA_INIT request the TCP
session has already been established, so the initiator's IP address
has been verified. Moreover, TCP Responder creates state as far as
the SYN packer is received (unless SYN Cookies described in [RFC4987]
are employed), that violates the stateless nature of IKEv2 Cookies.
So, it makes little sense to send Cookie request in this situation,
unless the responder in concerned with the possibility of TCP
Sequence Number attacks (see [RFC6528] for details). On the other
hand, Puzzles still remain useful and their use requires using
Cookies.
The following considerations are applicable for using Cookie and
Puzzle mechanisms in case of TCP encapsulation.
o the exchange responder SHOULD NOT request Cookie unless the
responder has good reason to do it (like a concern of the
possibility of TCP Sequence Number attacks or Puzzle request is
sent in the same message)
o if the responder chooses to send Cookie request (possibly along
with Puzzle request), then the TCP connection that the IKE_SA_INIT
request message was received over SHOULD be closed, so that the
responder remains stateless at least until the Cookie (or Puzzle
Solution) is returned
* note, that if this TCP connection is closed, then the responder
MUST NOT include the initiator's TCP port into the Cookie
calculation (*), since the Cookie will be returned over a new
TCP connection with a different port
o the exchange initiator acts as described in Section 2.6 of
[RFC7296] and Section 7 of [RFC8019], i.e. using TCP encapsulation
doesn't change the initiator's behavior
(*) Examples of Cookie calculation methods are given in Section 2.6
of [RFC7296] and in Section 7.1.1.3 of [RFC8019] and they don't
include transport protocol ports. However these examples are given
for illustrative purposes, since Cookie generation algorithm is a
local matter and some implementations might include port numbers,
that won't work with TCP encapsulation.
7. Error Handling in the IKE_SA_INIT
Section 2.21.1 of [RFC7296] describes how error notifications should
be handled in the IKE_SA_INIT exchange. In particular, it is advised
that the initiator should not act immediately after receiving error
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notification and should instead wait some time for valid response,
since the IKE_SA_INIT messages are completely unauthenticated. This
advice has little sense in case of TCP encapsulation. If the
initiator receives the response message over TCP, then either this
message is genuine and was sent by the peer, or the TCP session was
hijacked and the message is forged, but in this case no genuine
messages from the responder will be received.
So, in case of TCP encapsulation the initiator SHOULD NOT wait for
additional messages in case it receives error notification from the
responder in the IKE_SA_INIT exchange.
8. Interaction with IKEv2 Extensions
8.1. MOBIKE Protocol
MOBIKE protocol, that allows IKEv2 SA to migrate between IP
addresses, is defined in [RFC4555], and [RFC4621] further clarifies
the details of the protocol. Section 8 of [RFC8229] describes how
interaction between MOBIKE and TCP encapsulation. This memo provides
clarifications and additional recommendations for using MOBIKE in
case of TCP encapsulation.
[RFC8229] recommends, that in case of IP address change, the
initiator first initiates the INFORMATIONAL exchange containing
UPDATE_SA_ADDRESSES notification using UDP transport and if no
response is received then send this notification over TCP transport.
However, this recommendation lacks some details. In particular, it
is not clear whether falling back from UDP to TCP requires initiating
a new INFORMATIONAL exchange or not.
From MOBIKE point of view UDP and TCP transports can be seen as two
different network attachments, so falling back from the former to the
latter is very similar to changing peer's IP address. For that
reason, the initiator first initiates the INFORMATIONAL exchange over
UDP, and if no response is received within some period of time after
several retransmissions, then the initiator changes transport from
UDP to TCP in this very exchange. New INFORMATIONAL exchange MUST
NOT be started in this situation.
It means that after switching to TCP the content of the
NAT_DETECTION_SOURCE_IP notification will in most cases be incorrect
(since UDP and TCP source ports will most probably be different), and
the peer will falsely think that there is a NAT in between. This
should not cause problems because in this case all traffic will be
encapsulated in TCP anyway, and TCP encapsulation is the same with
regardless of NAT presence.
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MOBIKE protocol defined the NO_NATS_ALLOWED notification that can be
used to detect the presence of NAT between peer and to refuse to
communicate in this situation. In case of TCP the NO_NATS_ALLOWED
notification SHOULD be ignored because TCP generally has no problems
with NAT boxes.
Section 3.7 of [RFC4555] describes an additional optional step in the
process of changing IP addresses called Return Routability Check. It
is performed by the responder in order to be sure that the new
initiator's address is in fact routable. In case of TCP
encapsulation this check has little value, since TCP handshake proves
routability of the TCP Originator's address. So, in case of TCP
encapsulation the Return Routability Check SHOULD NOT be performed.
8.2. IKEv2 Redirect
Redirect mechanism for IKEv2 is defined in [RFC5685]. This mechanism
allows security gateways to redirect clients to another gateway
either during IKE SA establishment or after it is set up. If a
client is connecting to a security gateway using TCP transport and
then is being redirected to another security gateway, then the client
must disregard the current transport. In other words, the client
MUST again first try UDP and then fall back to TCP while establishing
a new IKE SA, regardless of the transport of the SA the redirect
notification was received over (unless the client's configuration
instructs it to instantly use TCP for the gateway it is redirected
to).
8.3. IKEv2 Session Resumption
Session resumption for IKEv2 is defined in [RFC5723]. Once IKE SA is
established the server creates a resumption ticket where information
about this SA is stored, and transfers this ticket to the client.
The ticket may be later used to resume the IKE SA if it is deleted.
In the event of resumption the client presents the ticket in a new
exchange, called IKE_SESSION_RESUME. For the new SA some parameters
are taken from the ticket and some are re-negotiated (more details
are given in Section 5 of [RFC5723]). If TCP encapsulation was used
in an old SA, then the client SHOULD resume this SA using TCP,
without first trying to connect over UDP.
8.4. IKEv2 Protocol Support for High Availability
[RFC6311] defines a support for High Availability in IKEv2. The core
idea is that in case of cluster failover a new active node
immediately initiates the special INFORMATION exchange containing the
IKEV2_MESSAGE_ID_SYNC notification, which instructs the client to
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skip some number of Message IDs that might not be synchronized yet
between nodes at the time of failover.
The problem is that TCP states are much harder to synchronize than
IKE states - it requires access to TCP/IP stack internals, which is
not always available for IKE/IPsec implementations. If a cluster
implementation doesn't synchronize TCP states between nodes, then
after failover event the new active node will not have any TCP
connection with the client, so the node cannot initiate the
INFORMATIONAL exchange as required by [RFC6311]. Since the cluster
usually acts as TCP Responder, the new active node cannot re-
establish TCP connection, since only the TCP Originator can do it.
And for the client the situation of cluster failover may remain
unknown for long time if it has no IKE or ESP traffic to send. Once
the client sends any ESP or IKEv2 packet, the cluster node will reply
with TCP RST and the client (as TCP Originator) will restore the TCP
connection so that the node will be able to initiate the
INFORMATIONAL exchange informing the client about the cluster
failover.
This memo makes the following recommendation: if support for High
Availability in IKEv2 is negotiated and TCP transport is used and a
client is TCP Originator, then the client SHOULD periodically send
IKEv2 messages (e.g. by initiating liveness check exchange) whenever
there is no any IKEv2 or ESP traffic. This differs from the
recommendations given in Section 2.4 of [RFC7296] in the following:
the liveness check should be periodically performed even if the
client has nothing to send over ESP. The frequency of sending such
messages should be high enough to allow quick detection and restoring
of broken TCP connection.
9. TCP Encapsulation Influence on IPsec SAs
Using TCP encapsulation makes impossible to use some features of
IPsec SA processing. In particular, there are two features that are
affected.
First, Section 8.1 of [RFC4301] requires all tunnel mode IPsec SAs to
be able to copy the Don't Fragment (DF) bit from inner IP header to
the outer (tunnel) one. With TCP encapsulation it's generally
impossible, because TCP/IP stack manages DF bit in the outer IP
header, and usually the stack ensures that the DF bit is set for TCP
packets to avoid IP fragmentation.
The other feature that is degraded with TCP encapsulation is an
ability to split traffic of different QoS classes into different
IPsec SAs, created by a single IKE SA. In this case the
Differentiated Services Code Point (DSCP) field is usually copied
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from the inner IP header to the outer (tunnel) one, ensuring that
IPsec traffic of each SA receives the corresponding level of service.
With TCP encapsulation all IPsec SAs created by a single IKE SA will
share a single TCP connection and thus will receive the same level of
service. If this functionality is needed, implementations should
create several IKE SAs over TCP and assign a corresponding DSCP value
to each of them.
10. Security Considerations
Security considerations concerning using TCP encapsulation in IKEv2
and ESP are given in [RFC8229]. This memo doesn't provide additional
security considerations.
11. Acknowledgements
Author would like to thank Tommy Pauly and Tero Kivinen for their
valuable comments.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006,
<https://www.rfc-editor.org/info/rfc4555>.
[RFC5685] Devarapalli, V. and K. Weniger, "Redirect Mechanism for
the Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5685, DOI 10.17487/RFC5685, November 2009,
<https://www.rfc-editor.org/info/rfc5685>.
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
DOI 10.17487/RFC5723, January 2010,
<https://www.rfc-editor.org/info/rfc5723>.
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[RFC6311] Singh, R., Ed., Kalyani, G., Nir, Y., Sheffer, Y., and D.
Zhang, "Protocol Support for High Availability of IKEv2/
IPsec", RFC 6311, DOI 10.17487/RFC6311, July 2011,
<https://www.rfc-editor.org/info/rfc6311>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC8019] Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
Protocol Version 2 (IKEv2) Implementations from
Distributed Denial-of-Service Attacks", RFC 8019,
DOI 10.17487/RFC8019, November 2016,
<https://www.rfc-editor.org/info/rfc8019>.
[RFC8229] Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
August 2017, <https://www.rfc-editor.org/info/rfc8229>.
12.2. Informative References
[RFC4621] Kivinen, T. and H. Tschofenig, "Design of the IKEv2
Mobility and Multihoming (MOBIKE) Protocol", RFC 4621,
DOI 10.17487/RFC4621, August 2006,
<https://www.rfc-editor.org/info/rfc4621>.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>.
[RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
2012, <https://www.rfc-editor.org/info/rfc6528>.
[RFC7383] Smyslov, V., "Internet Key Exchange Protocol Version 2
(IKEv2) Message Fragmentation", RFC 7383,
DOI 10.17487/RFC7383, November 2014,
<https://www.rfc-editor.org/info/rfc7383>.
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Author's Address
Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd) 124460
RU
Phone: +7 495 276 0211
Email: svan@elvis.ru
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