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Obsoletes:
Keywords: IKE, IKEv2, IPsec, TCP

Internet Engineering Task Force (IETF) T. Pauly
Request for Comments: 9329 Apple Inc.
Obsoletes: 8229 V. Smyslov
Category: Standards Track ELVIS-PLUS
ISSN: 2070-1721 November 2022
TCP Encapsulation of Internet Key Exchange Protocol (IKE) and IPsec
Packets
Abstract
This document describes a method to transport Internet Key Exchange
Protocol (IKE) and IPsec packets over a TCP connection for traversing
network middleboxes that may block IKE negotiation over UDP. This
method, referred to as "TCP encapsulation", involves sending both IKE
packets for Security Association (SA) establishment and Encapsulating
Security Payload (ESP) packets over a TCP connection. This method is
intended to be used as a fallback option when IKE cannot be
negotiated over UDP.
TCP encapsulation for IKE and IPsec was defined in RFC 8229. This
document clarifies the specification for TCP encapsulation by
including additional clarifications obtained during implementation
and deployment of this method. This documents obsoletes RFC 8229.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9329.
Copyright Notice
Copyright (c) 2022 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Prior Work and Motivation
1.2. Terminology and Notation
2. Configuration
3. TCP-Encapsulated Data Formats
3.1. TCP-Encapsulated IKE Message Format
3.2. TCP-Encapsulated ESP Packet Format
4. TCP-Encapsulated Stream Prefix
5. Applicability
5.1. Recommended Fallback from UDP
6. Using TCP Encapsulation
6.1. Connection Establishment and Teardown
6.2. Retransmissions
6.3. Cookies and Puzzles
6.3.1. Statelessness versus Delay of SA Establishment
6.4. Error Handling in IKE_SA_INIT
6.5. NAT-Detection Payloads
6.6. NAT-Keepalive Packets
6.7. Dead Peer Detection and Transport Keepalives
6.8. Implications of TCP Encapsulation on IPsec SA Processing
7. Interaction with IKEv2 Extensions
7.1. MOBIKE Protocol
7.2. IKE Redirect
7.3. IKEv2 Session Resumption
7.4. IKEv2 Protocol Support for High Availability
7.5. IKEv2 Fragmentation
8. Middlebox Considerations
9. Performance Considerations
9.1. TCP-in-TCP
9.2. Added Reliability for Unreliable Protocols
9.3. Quality-of-Service Markings
9.4. Maximum Segment Size
9.5. Tunneling ECN in TCP
10. Security Considerations
11. IANA Considerations
12. References
12.1. Normative References
12.2. Informative References
Appendix A. Using TCP Encapsulation with TLS
Appendix B. Example Exchanges of TCP Encapsulation with TLS 1.3
B.1. Establishing an IKE Session
B.2. Deleting an IKE Session
B.3. Re-establishing an IKE Session
B.4. Using MOBIKE between UDP and TCP Encapsulation
Acknowledgments
Authors' Addresses
1. Introduction
The Internet Key Exchange Protocol version 2 (IKEv2) [RFC7296] is a
protocol for establishing IPsec Security Associations (SAs) using IKE
messages over UDP for control traffic and using Encapsulating
Security Payload (ESP) messages [RFC4303] for encrypted data traffic.
Many network middleboxes that filter traffic on public hotspots block
all UDP traffic, including IKE and IPsec, but allow TCP connections
through because they appear to be web traffic. Devices on these
networks that need to use IPsec (to access private enterprise
networks, to route Voice over IP calls to carrier networks because of
security policies, etc.) are unable to establish IPsec SAs. This
document defines a method for encapsulating IKE control messages as
well as ESP data messages within a TCP connection. Note that
Authentication Header (AH) is not supported by this specification.
Using TCP as a transport for IPsec packets adds the third option
(below) to the list of traditional IPsec transports:
1. Direct. Usually, IKE negotiations begin over UDP port 500. If
no Network Address Translation (NAT) device is detected between
the Initiator and the Responder, then subsequent IKE packets are
sent over UDP port 500 and IPsec data packets are sent using ESP.
2. UDP Encapsulation. Described in [RFC3948]. If a NAT is detected
between the Initiator and the Responder, then subsequent IKE
packets are sent over UDP port 4500 with 4 bytes of zero at the
start of the UDP payload, and ESP packets are sent out over UDP
port 4500. Some implementations default to using UDP
encapsulation even when no NAT is detected on the path, as some
middleboxes do not support IP protocols other than TCP and UDP.
3. TCP Encapsulation. Described in this document. If the other two
methods are not available or appropriate, IKE negotiation packets
as well as ESP packets can be sent over a single TCP connection
to the peer.
Direct use of ESP or UDP encapsulation should be preferred by IKE
implementations due to performance concerns when using TCP
encapsulation (Section 9). Most implementations should use TCP
encapsulation only on networks where negotiation over UDP has been
attempted without receiving responses from the peer or if a network
is known to not support UDP.
1.1. Prior Work and Motivation
Encapsulating IKE connections within TCP streams is a common approach
to solve the problem of UDP packets being blocked by network
middleboxes. The specific goals of this document are as follows:
* To promote interoperability by defining a standard method of
framing IKE and ESP messages within TCP streams.
* To be compatible with the current IKEv2 standard without requiring
modifications or extensions.
* To use IKE over UDP by default to avoid the overhead of other
alternatives that always rely on TCP or Transport Layer Security
(TLS) [RFC5246] [RFC8446].
Some previous alternatives include:
Cellular Network Access:
Interworking Wireless LAN (IWLAN) uses IKEv2 to create secure
connections to cellular carrier networks for making voice calls
and accessing other network services over Wi-Fi networks. 3GPP has
recommended that IKEv2 and ESP packets be sent within a TLS
connection to be able to establish connections on restrictive
networks.
ISAKMP over TCP:
Various non-standard extensions to the Internet Security
Association and Key Management Protocol (ISAKMP) have been
deployed that send IPsec traffic over TCP or TCP-like packets.
Secure Sockets Layer (SSL) VPNs:
Many proprietary VPN solutions use a combination of TLS and IPsec
in order to provide reliability. These often run on TCP port 443.
IKEv2 over TCP:
IKEv2 over TCP as described in [IPSECME-IKE-TCP] is used to avoid
UDP fragmentation.
TCP encapsulation for IKE and IPsec was defined in [RFC8229]. This
document updates the specification for TCP encapsulation by including
additional clarifications obtained during implementation and
deployment of this method.
In particular:
* The interpretation of the Length field preceding every message is
clarified (Section 3).
* The use of the NAT_DETECTION_*_IP notifications is clarified
(Sections 5.1, 6.5, and 7.1).
* Retransmission behavior is clarified (Section 6.2).
* The use of cookies and puzzles is described in more detail
(Section 6.3).
* Error handling is clarified (Section 6.4).
* Implications of TCP encapsulation on IPsec SA processing are
expanded (Section 6.8).
* Section 7 describing interactions with other IKEv2 extensions is
added.
* The interaction of TCP encapsulation with IKEv2 Mobility and
Multihoming (MOBIKE) is clarified (Section 7.1).
* The recommendation for TLS encapsulation (Appendix A) now includes
TLS 1.3.
* Examples of TLS encapsulation are provided using TLS 1.3
(Appendix B).
* More security considerations are added.
1.2. Terminology and Notation
This document distinguishes between the IKE peer that initiates TCP
connections to be used for TCP encapsulation and the roles of
Initiator and Responder for particular IKE messages. During the
course of IKE exchanges, the role of IKE Initiator and Responder may
swap for a given SA (as with IKE SA rekeys), while the Initiator of
the TCP connection is still responsible for tearing down the TCP
connection and re-establishing it if necessary. For this reason,
this document will use the term "TCP Originator" to indicate the IKE
peer that initiates TCP connections. The peer that receives TCP
connections will be referred to as the "TCP Responder". If an IKE SA
is rekeyed one or more times, the TCP Originator MUST remain the peer
that originally initiated the first IKE SA.
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.
2. Configuration
One of the main reasons to use TCP encapsulation is that UDP traffic
may be entirely blocked on a network. Because of this, support for
TCP encapsulation is not specifically negotiated in the IKE exchange.
Instead, support for TCP encapsulation must be preconfigured on both
the TCP Originator and the TCP Responder.
Compliant implementations MUST support TCP encapsulation on TCP port
4500, which is reserved for IPsec NAT traversal.
Beyond a flag indicating support for TCP encapsulation, the
configuration for each peer can include the following optional
parameters:
* Alternate TCP ports on which the specific TCP Responder listens
for incoming connections. Note that the TCP Originator may
initiate TCP connections to the TCP Responder from any local port.
* An extra framing protocol to use on top of TCP to further
encapsulate the stream of IKE and IPsec packets. See Appendix A
for a detailed discussion.
Since TCP encapsulation of IKE and IPsec packets adds overhead and
has potential performance trade-offs compared to direct or UDP-
encapsulated SAs (as described in Section 9), implementations SHOULD
prefer ESP direct or UDP-encapsulated SAs over TCP-encapsulated SAs
when possible.
3. TCP-Encapsulated Data Formats
Like UDP encapsulation, TCP encapsulation uses the first 4 bytes of a
message to differentiate IKE and ESP messages. TCP encapsulation
also adds a 16-bit Length field that precedes every message to define
the boundaries of messages within a stream. The value in this field
is equal to the length of the original message plus the length of the
field itself, in octets. If the first 32 bits of the message are
zeros (a non-ESP marker), then the contents comprise an IKE message.
Otherwise, the contents comprise an ESP message. AH messages are not
supported for TCP encapsulation.
Although a TCP stream may be able to send very long messages,
implementations SHOULD limit message lengths to match the lengths
used for UDP encapsulation of ESP messages. The maximum message
length is used as the effective MTU for connections that are being
encrypted using ESP, so the maximum message length will influence
characteristics of these connections, such as the TCP Maximum Segment
Size (MSS).
Due to the fact that the Length field is 16 bits and includes both
the message length and the length of the field itself, it is
impossible to encapsulate messages greater than 65533 octets in
length. In most cases, this is not a problem. Note that a similar
limitation exists for encapsulation ESP in UDP [RFC3948].
The minimum size of an encapsulated message is 1 octet (for NAT-
keepalive packets, see Section 6.6). Empty messages (where the
Length field equals 2) MUST be silently ignored by receiver.
Note that this method of encapsulation will also work for placing IKE
and ESP messages within any protocol that presents a stream
abstraction, beyond TCP.
3.1. TCP-Encapsulated IKE Message Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Non-ESP Marker |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ IKE Message (RFC 7296) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IKE Message Format for TCP Encapsulation
The IKE message is preceded by a 16-bit Length field in network byte
order that specifies the length of the IKE message (including the
non-ESP marker) within the TCP stream. As with IKE over UDP port
4500, a zeroed 32-bit non-ESP marker is inserted before the start of
the IKE header in order to differentiate the traffic from ESP traffic
between the same addresses and ports.
Length (2 octets, unsigned integer): Length of the IKE message,
including the Length field and non-ESP marker. The value in the
Length field MUST NOT be 0 or 1. The receiver MUST treat these
values as fatal errors and MUST close the TCP connection.
Non-ESP Marker (4 octets): Four zero-valued bytes.
3.2. TCP-Encapsulated ESP Packet Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ESP Packet (RFC 4303) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: ESP Packet Format for TCP Encapsulation
The ESP packet is preceded by a 16-bit Length field in network byte
order that specifies the length of the ESP packet within the TCP
stream.
The Security Parameter Index (SPI) field [RFC7296] in the ESP header
MUST NOT be a zero value.
Length (2 octets, unsigned integer): Length of the ESP packet,
including the Length field. The value in the Length field MUST
NOT be 0 or 1. The receiver MUST treat these values as fatal
errors and MUST close TCP connection.
4. TCP-Encapsulated Stream Prefix
Each stream of bytes used for IKE and IPsec encapsulation MUST begin
with a fixed sequence of 6 bytes as a magic value, containing the
characters "IKETCP" as ASCII values.
0 1 2 3 4 5
+------+------+------+------+------+------+
| 0x49 | 0x4b | 0x45 | 0x54 | 0x43 | 0x50 |
+------+------+------+------+------+------+
Figure 3: TCP-Encapsulated Stream Prefix
This value is intended to identify and validate that the TCP
connection is being used for TCP encapsulation as defined in this
document, to avoid conflicts with the prevalence of previous non-
standard protocols that used TCP port 4500. This value is only sent
once, by the TCP Originator only, at the beginning of the TCP stream
of IKE and ESP messages.
Initiator Responder
---------------------------------------------------------------------
<new TCP connection is established by Initiator>
Stream Prefix|Length|non-ESP marker|IKE message -->
<-- Length|non-ESP marker|IKE message
Length|non-ESP marker|IKE message -->
<-- Length|non-ESP marker|IKE message
[...]
Length|ESP packet ->
<- Length|ESP packet
If other framing protocols are used within TCP to further encapsulate
or encrypt the stream of IKE and ESP messages, the stream prefix must
be at the start of the TCP Originator's IKE and ESP message stream
within the added protocol layer (Appendix A). Although some framing
protocols do support negotiating inner protocols, the stream prefix
should always be used in order for implementations to be as generic
as possible and not rely on other framing protocols on top of TCP.
5. Applicability
TCP encapsulation is applicable only when it has been configured to
be used with specific IKE peers. If a Responder is configured to
accept and is allowed to use TCP encapsulation, it MUST listen on the
configured port(s) in case any peers will initiate new IKE sessions.
Initiators MAY use TCP encapsulation for any IKE session to a peer
that is configured to support TCP encapsulation, although it is
recommended that Initiators only use TCP encapsulation when traffic
over UDP is blocked.
Since the support of TCP encapsulation is a configured property, not
a negotiated one, it is recommended that if there are multiple IKE
endpoints representing a single peer (such as multiple machines with
different IP addresses when connecting by Fully Qualified Domain Name
(FQDN), or endpoints used with IKE redirection), all of the endpoints
equally support TCP encapsulation.
If TCP encapsulation is being used for a specific IKE SA, all IKE
messages for that IKE SA and ESP packets for its Child SAs MUST be
sent over a TCP connection until the SA is deleted or IKEv2 Mobility
and Multihoming (MOBIKE) is used to change the SA endpoints and/or
the encapsulation protocol. See Section 7.1 for more details on
using MOBIKE to transition between encapsulation modes.
5.1. Recommended Fallback from UDP
Since UDP is the preferred method of transport for IKE messages,
implementations that use TCP encapsulation should have an algorithm
for deciding when to use TCP after determining that UDP is unusable.
If an Initiator implementation has no prior knowledge about the
network it is on and the status of UDP on that network, it SHOULD
always attempt to negotiate IKE over UDP first. IKEv2 defines how to
use retransmission timers with IKE messages and, specifically,
IKE_SA_INIT messages [RFC7296]. Generally, this means that the
implementation will define a frequency of retransmission and the
maximum number of retransmissions allowed before marking the IKE SA
as failed. An implementation can attempt negotiation over TCP once
it has hit the maximum retransmissions over UDP, or slightly before
to reduce connection setup delays. It is recommended that the
initial message over UDP be retransmitted at least once before
falling back to TCP, unless the Initiator knows beforehand that the
network is likely to block UDP.
When switching from UDP to TCP, a new IKE_SA_INIT exchange MUST be
initiated with the Initiator's new SPI and with recalculated content
of NAT_DETECTION_*_IP notifications.
6. Using TCP Encapsulation
6.1. Connection Establishment and Teardown
When the IKE Initiator uses TCP encapsulation, it will initiate a TCP
connection to the Responder using the Responder's preconfigured TCP
port. The first bytes sent on the TCP stream MUST be the stream
prefix value (Section 4). After this prefix, encapsulated IKE
messages will negotiate the IKE SA and initial Child SA [RFC7296].
After this point, both encapsulated IKE (Figure 1) and ESP (Figure 2)
messages will be sent over the TCP connection. The TCP Responder
MUST wait for the entire stream prefix to be received on the stream
before trying to parse out any IKE or ESP messages. The stream
prefix is sent only once, and only by the TCP Originator.
In order to close an IKE session, either the Initiator or Responder
SHOULD gracefully tear down IKE SAs with DELETE payloads. Once the
SA has been deleted, the TCP Originator SHOULD close the TCP
connection if it does not intend to use the connection for another
IKE session to the TCP Responder. If the TCP connection is no longer
associated with any active IKE SA, the TCP Responder MAY close the
connection to clean up IKE resources if the TCP Originator didn't
close it within some reasonable period of time (e.g., a few seconds).
An unexpected FIN or a TCP Reset on the TCP connection may indicate a
loss of connectivity, an attack, or some other error. If a DELETE
payload has not been sent, both sides SHOULD maintain the state for
their SAs for the standard lifetime or timeout period. The TCP
Originator is responsible for re-establishing the TCP connection if
it is torn down for any unexpected reason. Since new TCP connections
may use different IP addresses and/or ports due to NAT mappings or
local address or port allocations changing, the TCP Responder MUST
allow packets for existing SAs to be received from new source IP
addresses and ports. Note that the IPv6 Flow-ID header MUST remain
constant when a new TCP connection is created to avoid ECMP load
balancing.
A peer MUST discard a partially received message due to a broken
connection.
Whenever the TCP Originator opens a new TCP connection to be used for
an existing IKE SA, it MUST send the stream prefix first, before any
IKE or ESP messages. This follows the same behavior as the initial
TCP connection.
Multiple IKE SAs MUST NOT share a single TCP connection, unless one
is a rekey of an existing IKE SA, in which case there will
temporarily be two IKE SAs on the same TCP connection.
If a TCP connection is being used to continue an existing IKE/ESP
session, the TCP Responder can recognize the session using either the
IKE SPI from an encapsulated IKE message or the ESP SPI from an
encapsulated ESP packet. If the session had been fully established
previously, it is suggested that the TCP Originator send an
UPDATE_SA_ADDRESSES message if MOBIKE is supported and an empty
informational message if it is not.
The TCP Responder MUST NOT accept any messages for the existing IKE
session on a new incoming connection, unless that connection begins
with the stream prefix. If either the TCP Originator or TCP
Responder detects corruption on a connection that was started with a
valid stream prefix, it SHOULD close the TCP connection. The
connection can be corrupted if there are too many subsequent messages
that cannot be parsed as valid IKE messages or ESP messages with
known SPIs, or if the authentication check for an IKE message or ESP
message with a known SPI fails. Implementations SHOULD NOT tear down
a connection if only a few consecutive ESP packets have unknown SPIs
since the SPI databases may be momentarily out of sync. If there is
instead a syntax issue within an IKE message, an implementation MUST
send the INVALID_SYNTAX notify payload and tear down the IKE SA as
usual, rather than tearing down the TCP connection directly.
A TCP Originator SHOULD only open one TCP connection per IKE SA, over
which it sends all of the corresponding IKE and ESP messages. This
helps ensure that any firewall or NAT mappings allocated for the TCP
connection apply to all of the traffic associated with the IKE SA
equally.
As with TCP Originators, a TCP Responder SHOULD send packets for an
IKE SA and its Child SAs over only one TCP connection at any given
time. It SHOULD choose the TCP connection on which it last received
a valid and decryptable IKE or ESP message. In order to be
considered valid for choosing a TCP connection, an IKE message must
be successfully decrypted and authenticated, not be a retransmission
of a previously received message, and be within the expected window
for IKE message IDs. Similarly, an ESP message must be successfully
decrypted and authenticated, and must not be a replay of a previous
message.
Since a connection may be broken and a new connection re-established
by the TCP Originator without the TCP Responder being aware, a TCP
Responder SHOULD accept receiving IKE and ESP messages on both old
and new connections until the old connection is closed by the TCP
Originator. A TCP Responder MAY close a TCP connection that it
perceives as idle and extraneous (one previously used for IKE and ESP
messages that has been replaced by a new connection).
6.2. Retransmissions
Section 2.1 of [RFC7296] describes how IKEv2 deals with the
unreliability of the UDP protocol. In brief, the exchange Initiator
is responsible for retransmissions and must retransmit request
messages until a response message is received. If no reply is
received after several retransmissions, the SA is deleted. The
Responder never initiates retransmission, but it must send a response
message again in case it receives a retransmitted request.
When IKEv2 uses a reliable transport protocol, like TCP, the
retransmission rules are as follows:
* The exchange Initiator SHOULD NOT retransmit request message (*);
if no response is received within some reasonable period of time,
the IKE SA is deleted.
* If a new TCP connection for the IKE SA is established while the
exchange Initiator is waiting for a response, the Initiator MUST
retransmit its request over this connection and continue to wait
for a response.
* The exchange Responder does not change its behavior, but acts as
described in Section 2.1 of [RFC7296].
(*) This is an optimization; implementations may continue to use the
retransmission logic from Section 2.1 of [RFC7296] for simplicity.
6.3. Cookies and Puzzles
IKEv2 provides a DoS attack protection mechanism through Cookies,
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 avoid keeping state
until the Initiator proves its IP address is legitimate (and after
solving a puzzle if required).
The connection-oriented nature of TCP transport brings additional
considerations for using these mechanisms. In general, Cookies
provide less value in the case of TCP encapsulation; by the time a
Responder receives the IKE_SA_INIT request, the TCP session has
already been established and the Initiator's IP address has been
verified. Moreover, a TCP/IP stack creates state once a TCP SYN
packet is received (unless SYN Cookies described in [RFC4987] are
employed), which contradicts the statelessness of IKEv2 Cookies. In
particular, with TCP, an attacker is able to mount a SYN flooding DoS
attack that an IKEv2 Responder cannot prevent using stateless IKEv2
Cookies. Thus, when using TCP encapsulation, it makes little sense
to send Cookie requests without Puzzles unless the Responder is
concerned with a possibility of TCP sequence number attacks (see
[RFC6528] and [RFC9293] for details). Puzzles, on the other hand,
still remain useful (and their use requires using Cookies).
The following considerations are applicable for using Cookie and
Puzzle mechanisms in the case of TCP encapsulation:
* The exchange Responder SHOULD NOT send an IKEv2 Cookie request
without an accompanied Puzzle; implementations might choose to
have exceptions to this for cases like mitigating TCP sequence
number attacks.
* If the Responder chooses to send a Cookie request (possibly along
with Puzzle request), then the TCP connection that the IKE_SA_INIT
request message was received over SHOULD be closed after the
Responder sends its reply and no repeated requests are received
within some short period of time to keep the Responder stateless
(see Section 6.3.1). Note that the Responder MUST NOT include the
Initiator's TCP port into the Cookie calculation (*) since the
Cookie can be returned over a new TCP connection with a different
port.
* 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 the Cookie generation algorithm is a
local matter and some implementations might include port numbers that
won't work with TCP encapsulation. Note also that these examples
include the Initiator's IP address in Cookie calculation. In
general, this address may change between two initial requests (with
and without Cookies). This may happen due to NATs, which have more
freedom to change source IP addresses for new TCP connections than
for UDP. In such cases, cookie verification might fail.
6.3.1. Statelessness versus Delay of SA Establishment
There is a trade-off in choosing the period of time after which the
TCP connection is closed. If it is too short, then the proper
Initiator that repeats its request would need to re-establish the TCP
connection, introducing additional delay. On the other hand, if it
is too long, then the Responder's resources would be wasted in case
the Initiator never comes back. This document doesn't mandate the
duration of time because it doesn't affect interoperability, but it
is believed that 5-10 seconds is a good compromise. Also, note that
if the Responder requests that the Initiator solve a puzzle, then the
Responder can estimate how long it would take the Initiator to find a
solution and adjust the time interval accordingly.
6.4. Error Handling in IKE_SA_INIT
Section 2.21.1 of [RFC7296] describes how error notifications are
handled in the IKE_SA_INIT exchange. In particular, it is advised
that the Initiator should not act immediately after receiving an
error notification; instead, it should wait some time for a valid
response since the IKE_SA_INIT messages are completely
unauthenticated. This advice does not apply equally in the case of
TCP encapsulation. If the Initiator receives a 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. In the
latter case, no genuine messages from the Responder will be received.
Thus, in the case of TCP encapsulation, an Initiator SHOULD NOT wait
for additional messages in case it receives an error notification
from the Responder in the IKE_SA_INIT exchange.
In the IKE_SA_INIT exchange, if the Responder returns an error
notification that implies a recovery action from the Initiator (such
as INVALID_KE_PAYLOAD or INVALID_MAJOR_VERSION, see Section 2.21.1 of
[RFC7296]), then the Responder SHOULD NOT close the TCP connection
immediately in anticipation of the fact that the Initiator will
repeat the request with corrected parameters. See also Section 6.3.
6.5. NAT-Detection Payloads
When negotiating over UDP, IKE_SA_INIT packets include
NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP payloads to
determine if UDP encapsulation of IPsec packets should be used.
These payloads contain SHA-1 digests of the SPIs, IP addresses, and
ports as defined in [RFC7296]. IKE_SA_INIT packets sent on a TCP
connection SHOULD include these payloads with the same content as
when sending over UDP and SHOULD use the applicable TCP ports when
creating and checking the SHA-1 digests.
If a NAT is detected due to the SHA-1 digests not matching the
expected values, no change should be made for encapsulation of
subsequent IKE or ESP packets since TCP encapsulation inherently
supports NAT traversal. However, for the transport mode IPsec SAs,
implementations need to handle TCP and UDP packet checksum fixup
during decapsulation, as defined for UDP encapsulation in [RFC3948].
Implementations MAY use the information that a NAT is present to
influence keepalive timer values.
6.6. NAT-Keepalive Packets
Encapsulating IKE and IPsec inside of a TCP connection can impact the
strategy that implementations use to maintain middlebox port
mappings.
In general, TCP port mappings are maintained by NATs longer than UDP
port mappings, so IPsec ESP NAT-keepalive packets [RFC3948] SHOULD
NOT be sent when using TCP encapsulation. Any implementation using
TCP encapsulation MUST silently drop incoming NAT-keepalive packets
and not treat them as errors. NAT-keepalive packets over a TCP-
encapsulated IPsec connection will be sent as a 1-octet-long payload
with the value 0xFF, preceded by the 2-octet Length specifying a
length of 3 (since it includes the length of the Length field).
6.7. Dead Peer Detection and Transport Keepalives
Peer liveness should be checked using IKE informational packets
[RFC7296].
Note that, depending on the configuration of TCP and TLS on the
connection, TCP keep-alives [RFC1122] and TLS keep-alives [RFC6520]
MAY be used. These MUST NOT be used as indications of IKE peer
liveness, for which purpose the standard IKEv2 mechanism of
exchanging (usually empty) INFORMATIONAL messages is used (see
Section 1.4 of [RFC7296]).
6.8. Implications of TCP Encapsulation on IPsec SA Processing
Using TCP encapsulation affects some aspects of IPsec SA processing.
1. Section 8.1 of [RFC4301] requires all tunnel mode IPsec SAs to be
able to copy the Don't Fragment (DF) bit from inner IPv4 header
to the outer (tunnel) one. With TCP encapsulation, this is
generally not possible because the TCP/IP stack manages the DF
bit in the outer IPv4 header, and usually the stack ensures that
the DF bit is set for TCP packets to avoid IP fragmentation.
Note, that this behavior is compliant with generic tunneling
considerations since the outer TCP header acts as a link-layer
protocol and its fragmentation and reassembly have no correlation
with the inner payload.
2. The other feature that is less applicable 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 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; thus, they
will receive the same level of service (see Section 9.3). If
this functionality is needed, implementations should create
several IKE SAs each over separate TCP connections and assign a
corresponding DSCP value to each of them.
TCP encapsulation of IPsec packets may have implications on
performance of the encapsulated traffic. Performance considerations
are discussed in Section 9.
7. Interaction with IKEv2 Extensions
7.1. MOBIKE Protocol
The MOBIKE protocol, which allows SAs to migrate between IP
addresses, is defined in [RFC4555]; [RFC4621] further clarifies the
details of the protocol. When an IKE session that has negotiated
MOBIKE is transitioning between networks, the Initiator of the
transition may switch between using TCP encapsulation, UDP
encapsulation, or no encapsulation. Implementations that implement
both MOBIKE and TCP encapsulation within the same connection
configuration MUST support dynamically enabling and disabling TCP
encapsulation as interfaces change.
When a MOBIKE-enabled Initiator changes networks, the INFORMATIONAL
exchange with the UPDATE_SA_ADDRESSES notification SHOULD be
initiated first over UDP before attempting over TCP. If there is a
response to the request sent over UDP, then the ESP packets should be
sent directly over IP or over UDP port 4500 (depending on if a NAT
was detected), regardless of if a connection on a previous network
was using TCP encapsulation. If no response is received within a
certain period of time after several retransmissions, the Initiator
ought to change its transport for this exchange from UDP to TCP and
resend the request message. A new INFORMATIONAL exchange MUST NOT be
started in this situation. If the Responder only responds to the
request sent over TCP, then the ESP packets should be sent over the
TCP connection, regardless of if a connection on a previous network
did not use TCP encapsulation.
The value of the timeout and the specific number of retransmissions
before switching to TCP can vary depending on the Initiator's
configuration. Implementations ought to provide reasonable defaults
to ensure that UDP attempts have a chance to succeed, but can shorten
the timeout based on historical data or metrics.
If the TCP transport was used for the previous network connection,
the old TCP connection SHOULD be closed by the Initiator once MOBIKE
finishes migration to a new connection (either TCP or UDP).
Since switching from UDP to TCP can happen during a single
INFORMATIONAL message exchange, the content of the NAT_DETECTION_*_IP
notifications will in most cases be incorrect (since UDP and TCP
ports will most likely be different), and the peer may incorrectly
detect the presence of a NAT. Section 3.5 of [RFC4555] states that a
new INFORMATIONAL exchange with the UPDATE_SA_ADDRESSES notify is
initiated in case the address (or transport) is changed while waiting
for a response.
Section 3.5 of [RFC4555] also states that once an IKE SA is switched
to a new IP address, all outstanding requests in this SA are
immediately retransmitted using this address. See also Section 6.2.
The MOBIKE protocol defines 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 the 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 Responders in order to be sure that the new
Initiator's address is, in fact, routable. In the case of TCP
encapsulation, this check has little value since a TCP handshake
proves the routability of the TCP Originator's address; thus, the
Return Routability Check SHOULD NOT be performed.
7.2. IKE Redirect
A 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 session setup.
If a client is connecting to a security gateway using TCP and then is
redirected to another security gateway, the client needs to reset its
transport selection. In other words, with the next security gateway,
the client MUST 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).
7.3. IKEv2 Session Resumption
Session resumption for IKEv2 is defined in [RFC5723]. Once an 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 after it
is deleted. In the event of resumption, the client presents the
ticket in a new exchange, called IKE_SESSION_RESUME. Some parameters
in the new SA are retrieved from the ticket and others are
renegotiated (more details are given in Section 5 of [RFC5723]).
Since network conditions may change while the client is inactive, the
fact that TCP encapsulation was used in an old SA SHOULD NOT affect
which transport is used during session resumption. In other words,
the transport should be selected as if the IKE SA is being created
from scratch.
7.4. IKEv2 Protocol Support for High Availability
[RFC6311] defines a support for High Availability in IKEv2. In case
of cluster failover, a new active node must immediately initiate a
special INFORMATION exchange containing the IKEV2_MESSAGE_ID_SYNC
notification, which instructs the client to skip some number of
Message IDs that might not be synchronized yet between nodes at the
time of failover.
Synchronizing states when using TCP encapsulation is much harder than
when using UDP; doing so requires access to TCP/IP stack internals,
which is not always available from an IKE/IPsec implementation. 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 because only the TCP Originator can do it.
For the client, the cluster failover event may remain undetected for
long time if it has no IKE or ESP traffic to send. Once the client
sends an ESP or IKEv2 packet, the cluster node will reply with TCP
RST and the client (as TCP Originator) will reestablish the TCP
connection so that the node will be able to initiate the
INFORMATIONAL exchange informing the client about the cluster
failover.
This document makes the following recommendation: if support for High
Availability in IKEv2 is negotiated and TCP transport is used, a
client that is a TCP Originator SHOULD periodically send IKEv2
messages (e.g., by initiating liveness check exchange) whenever there
is no 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 restoration of broken TCP
connections.
7.5. IKEv2 Fragmentation
IKE message fragmentation [RFC7383] is not required when using TCP
encapsulation since a TCP stream already handles the fragmentation of
its contents across packets. Since fragmentation is redundant in
this case, implementations might choose to not negotiate IKE
fragmentation. Even if fragmentation is negotiated, an
implementation SHOULD NOT send fragments when going over a TCP
connection, although it MUST support receiving fragments.
If an implementation supports both MOBIKE and IKE fragmentation, it
SHOULD negotiate IKE fragmentation over a TCP-encapsulated session in
case the session switches to UDP encapsulation on another network.
8. Middlebox Considerations
Many security networking devices, such as firewalls or intrusion
prevention systems, network optimization/acceleration devices, and
NAT devices, keep the state of sessions that traverse through them.
These devices commonly track the transport-layer and/or application-
layer data to drop traffic that is anomalous or malicious in nature.
While many of these devices will be more likely to pass TCP-
encapsulated traffic as opposed to UDP-encapsulated traffic, some may
still block or interfere with TCP-encapsulated IKE and IPsec traffic.
A network device that monitors the transport layer will track the
state of TCP sessions, such as TCP sequence numbers. If the IKE
implementation has its own minimal implementation of TCP, it SHOULD
still use common TCP behaviors to avoid being dropped by middleboxes.
Operators that intentionally block IPsec because of security
implications might want to also block TCP port 4500 or use other
methods to reject TCP encapsulated IPsec traffic (e.g., filter out
TCP connections that begin with the "IKETCP" stream prefix).
9. Performance Considerations
Several aspects of TCP encapsulation for IKE and IPsec packets may
negatively impact the performance of connections within a tunnel-mode
IPsec SA. Implementations should be aware of these performance
impacts and take these into consideration when determining when to
use TCP encapsulation. Implementations MUST favor using direct ESP
or UDP encapsulation over TCP encapsulation whenever possible.
9.1. TCP-in-TCP
If the outer connection between IKE peers is over TCP, inner TCP
connections may suffer negative effects from using TCP within TCP.
Running TCP within TCP is discouraged since the TCP algorithms
generally assume that they are running over an unreliable datagram
layer.
If the outer (tunnel) TCP connection experiences packet loss, this
loss will be hidden from any inner TCP connections since the outer
connection will retransmit to account for the losses. Since the
outer TCP connection will deliver the inner messages in order, any
messages after a lost packet may have to wait until the loss is
recovered. This means that loss on the outer connection will be
interpreted only as delay by inner connections. The burstiness of
inner traffic can increase since a large number of inner packets may
be delivered across the tunnel at once. The inner TCP connection may
interpret a long period of delay as a transmission problem,
triggering a retransmission timeout, which will cause spurious
retransmissions. The sending rate of the inner connection may be
unnecessarily reduced if the retransmissions are not detected as
spurious in time.
The inner TCP connection's round-trip-time estimation will be
affected by the burstiness of the outer TCP connection if there are
long delays when packets are retransmitted by the outer TCP
connection. This will make the congestion control loop of the inner
TCP traffic less reactive, potentially permanently leading to a lower
sending rate than the outer TCP would allow for.
TCP-in-TCP can also lead to "TCP meltdown", where stacked instances
of TCP can result in significant impacts to performance
[TCP-MELTDOWN]. This can occur when losses in the lower TCP (closer
to the link) increase delays seen by the higher TCP (closer to the
application) that create timeouts, which, in turn, cause
retransmissions that can then cause losses in the lower TCP by
overrunning its buffer. The very mechanism intended to avoid loss
(retransmission) interacts between the two layers to increase loss.
To limit this effect, the timeouts of the two TCP layers need to be
carefully managed, e.g., such that the higher layer has a much longer
timeout than the lower layer.
Note that any negative effects will be shared among all flows going
through the outer TCP connection. This is of particular concern for
any latency-sensitive or real-time applications using the tunnel. If
such traffic is using a TCP-encapsulated IPsec connection, it is
recommended that the number of inner connections sharing the tunnel
be limited as much as possible.
9.2. Added Reliability for Unreliable Protocols
Since ESP is an unreliable protocol, transmitting ESP packets over a
TCP connection will change the fundamental behavior of the packets.
Some application-level protocols that prefer packet loss to delay
(such as Voice over IP or other real-time protocols) may be
negatively impacted if their packets are retransmitted by the TCP
connection due to packet loss.
9.3. Quality-of-Service Markings
Quality-of-Service (QoS) markings, such as the Differentiated
Services Code Point (DSCP) and Traffic Class, should be used with
care on TCP connections used for encapsulation. Individual packets
SHOULD NOT use different markings than the rest of the connection
since packets with different priorities may be routed differently and
cause unnecessary delays in the connection.
9.4. Maximum Segment Size
A TCP connection used for IKE encapsulation SHOULD negotiate its MSS
in order to avoid unnecessary fragmentation of packets.
9.5. Tunneling ECN in TCP
Since there is not a one-to-one relationship between outer IP packets
and inner ESP/IP messages when using TCP encapsulation, the markings
for Explicit Congestion Notification (ECN) [RFC3168] cannot easily be
mapped. However, any ECN Congestion Experienced (CE) marking on
inner headers should be preserved through the tunnel.
Implementations SHOULD follow the ECN compatibility mode for tunnel
ingress as described in [RFC6040]. In compatibility mode, the outer
tunnel TCP connection marks its packet headers as not ECN-capable.
Upon egress, if the arriving outer header is marked with CE, the
implementation will drop the inner packet since there is not a
distinct inner packet header onto which to translate the ECN
markings.
10. Security Considerations
IKE Responders that support TCP encapsulation may become vulnerable
to new Denial-of-Service (DoS) attacks that are specific to TCP, such
as SYN-flooding attacks. TCP Responders should be aware of this
additional attack surface.
TCP connections are also susceptible to RST and other spoofing
attacks [RFC4953]. This specification makes IPsec tolerant of sudden
TCP connection drops, but if an attacker is able to tear down TCP
connections, IPsec connection's performance can suffer, effectively
making this a DoS attack.
TCP data injection attacks have no effect on application data since
IPsec provides data integrity. However, they can have some effect,
mostly by creating DoS attacks:
* If an attacker alters the content of the Length field that
separates packets, then the Receiver will incorrectly identify the
boundaries of the following packets and will drop all of them or
even tear down the TCP connection if the content of the Length
field happens to be 0 or 1 (see Section 3).
* If the content of an IKE message is altered, then it will be
dropped by the receiver; if the dropped message is the IKE request
message, then the Initiator will tear down the IKE SA after some
timeout since, in most cases, the request message will not be
retransmitted (as advised in Section 6.2); thus, the response will
never be received.
* If an attacker alters the non-ESP marker, then IKE packets will be
dispatched to ESP (and sometimes visa versa) and those packets
will be dropped.
* If an attacker modifies TCP-Encapsulated stream prefix or
unencrypted IKE messages before IKE SA is established, then in
most cases this will result in failure to establish IKE SA, often
with false "authentication failed" diagnostics.
[RFC5961] discusses how TCP injection attacks can be mitigated.
Note that data injection attacks are also possible on IP level (e.g.,
when IP fragmentation is used), resulting in DoS attacks even if TCP
encapsulation is not used. On the other hand, TCP injection attacks
are easier to mount than the IP fragmentation injection attacks
because TCP keeps a long receive window open that's a sitting target
for such attacks.
If an attacker successfully mounts an injection attack on a TCP
connection used for encapsulating IPsec traffic and modifies a Length
field, the receiver might not be able to correctly identify the
boundaries of the following packets in the stream since it will try
to parse arbitrary data as an ESP or IKE header. After such a
parsing failure, all following packets will be dropped.
Communication will eventually recover, but this might take several
minutes and can result in IKE SA deletion and re-creation.
To speed up the recovery from such attacks, implementations are
advised to follow recommendations in Section 6.1 and close the TCP
connection if incoming packets contain SPIs that don't match any
known SAs. Once the TCP connection is closed, it will be re-created
by the TCP Originator as described in Section 6.1.
To avoid performance degradation caused by closing and re-creating
TCP connections, implementations MAY alternatively try to resync
after they receive unknown SPIs by searching the TCP stream for a
64-bit binary vector consisting of a known SPI in the first 32 bits
and a valid Sequence Number for this SPI in the second 32 bits.
Then, they can validate the Integrity Check Value (ICV) of this
packet candidate by taking the preceding 16 bits as the Length field.
They can also search for 4 bytes of zero (non-ESP marker) followed by
128 bits of IKE SPIs of the IKE SA(s) associated with this TCP
connection and then validate the ICV of this IKE message candidate by
taking the 16 bits preceding the non-ESP marker as the Length field.
Implementations SHOULD limit the attempts to resync, because if the
injection attack is ongoing, then there is a high probability that
the resync process will not succeed or will quickly come under attack
again.
An attacker capable of blocking UDP traffic can force peers to use
TCP encapsulation, thus, degrading the performance and making the
connection more vulnerable to DoS attacks. Note that an attacker
that is able to modify packets on the wire or to block them can
prevent peers from communicating regardless of the transport being
used.
TCP Responders should be careful to ensure that the stream prefix
"IKETCP" uniquely identifies incoming streams as streams that use the
TCP encapsulation protocol.
Attackers may be able to disrupt the TCP connection by sending
spurious TCP Reset packets. Therefore, implementations SHOULD make
sure that IKE session state persists even if the underlying TCP
connection is torn down.
If MOBIKE is being used, all of the security considerations outlined
for MOBIKE apply [RFC4555].
Similar to MOBIKE, TCP encapsulation requires a TCP Responder to
handle changes to source address and port due to network or
connection disruption. The successful delivery of valid new IKE or
ESP messages over a new TCP connection is used by the TCP Responder
to determine where to send subsequent responses. If an attacker is
able to send packets on a new TCP connection that pass the validation
checks of the TCP Responder, it can influence which path future
packets will take. For this reason, the validation of messages on
the TCP Responder must include decryption, authentication, and replay
checks.
11. IANA Considerations
TCP port 4500 is already allocated to IPsec for NAT traversal in the
"Service Name and Transport Protocol Port Number Registry". This
port SHOULD be used for TCP-encapsulated IKE and ESP as described in
this document.
This document updates the reference for TCP port 4500 from RFC 8229
to itself:
Service Name: ipsec-nat-t
Port Number / Transport Protocol: 4500/tcp
Description: IPsec NAT-Traversal
Reference: RFC 9329
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>.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, DOI 10.17487/RFC3948, January 2005,
<https://www.rfc-editor.org/info/rfc3948>.
[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>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <https://www.rfc-editor.org/info/rfc6040>.
[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>.
[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>.
12.2. Informative References
[IPSECME-IKE-TCP]
Nir, Y., "A TCP transport for the Internet Key Exchange",
Work in Progress, Internet-Draft, draft-ietf-ipsecme-ike-
tcp-01, 3 December 2012,
<https://datatracker.ietf.org/doc/html/draft-ietf-ipsecme-
ike-tcp-01>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC2817] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, DOI 10.17487/RFC2817, May 2000,
<https://www.rfc-editor.org/info/rfc2817>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006,
<https://www.rfc-editor.org/info/rfc4555>.
[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>.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007,
<https://www.rfc-editor.org/info/rfc4953>.
[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>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[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>.
[RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961,
DOI 10.17487/RFC5961, August 2010,
<https://www.rfc-editor.org/info/rfc5961>.
[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>.
[RFC6520] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) Heartbeat Extension", RFC 6520,
DOI 10.17487/RFC6520, February 2012,
<https://www.rfc-editor.org/info/rfc6520>.
[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>.
[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.
[RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 9325, DOI 10.17487/RFC9325, November 2022,
<https://www.rfc-editor.org/info/rfc9325>.
[TCP-MELTDOWN]
Honda, O., Ohsaki, H., Imase, M., Ishizuka, M., and J.
Murayama, "Understanding TCP over TCP: effects of TCP
tunneling on end-to-end throughput and latency", October
2005, <https://doi.org/10.1117/12.630496>.
Appendix A. Using TCP Encapsulation with TLS
This section provides recommendations on how to use TLS in addition
to TCP encapsulation.
When using TCP encapsulation, implementations may choose to use TLS
1.2 [RFC5246] or TLS 1.3 [RFC8446] on the TCP connection to be able
to traverse middleboxes, which may otherwise block the traffic.
If a web proxy is applied to the ports used for the TCP connection
and TLS is being used, the TCP Originator can send an HTTP CONNECT
message to establish an SA through the proxy [RFC2817].
The use of TLS should be configurable on the peers and may be used as
the default when using TCP encapsulation or may be used as a fallback
when basic TCP encapsulation fails. The TCP Responder may expect to
read encapsulated IKE and ESP packets directly from the TCP
connection, or it may expect to read them from a stream of TLS data
packets. The TCP Originator should be preconfigured regarding
whether or not to use TLS when communicating with a given port on the
TCP Responder.
When new TCP connections are re-established due to a broken
connection, TLS must be renegotiated. TLS session resumption is
recommended to improve efficiency in this case.
The security of the IKE session is entirely derived from the IKE
negotiation and key establishment and not from the TLS session
(which, in this context, is only used for encapsulation purposes);
therefore, when TLS is used on the TCP connection, both the TCP
Originator and the TCP Responder SHOULD allow the NULL cipher to be
selected for performance reasons. Note that TLS 1.3 only supports
AEAD algorithms and at the time of writing this document there was no
recommended cipher suite for TLS 1.3 with the NULL cipher. It is
RECOMMENDED to follow [RFC9325] when selecting parameters for TLS.
Implementations should be aware that the use of TLS introduces
another layer of overhead requiring more bytes to transmit a given
IKE and IPsec packet. For this reason, direct ESP, UDP
encapsulation, or TCP encapsulation without TLS should be preferred
in situations in which TLS is not required in order to traverse
middleboxes.
Appendix B. Example Exchanges of TCP Encapsulation with TLS 1.3
This appendix contains examples of data flows in cases where TCP
encapsulation of IKE and IPsec packets is used with TLS 1.3. The
examples below are provided for illustrative purpose only; readers
should refer to the main body of the document for details.
B.1. Establishing an IKE Session
Client Server
---------- ----------
1) -------------------- TCP Connection -------------------
(IP_I:Port_I -> IP_R:Port_R)
TcpSyn ------->
<------- TcpSyn,Ack
TcpAck ------->
2) --------------------- TLS Session ---------------------
ClientHello ------->
ServerHello
{EncryptedExtensions}
{Certificate*}
{CertificateVerify*}
<------- {Finished}
{Finished} ------->
3) ---------------------- Stream Prefix --------------------
"IKETCP" ------->
4) ----------------------- IKE Session ---------------------
Length + Non-ESP Marker ------->
IKE_SA_INIT
HDR, SAi1, KEi, Ni,
[N(NAT_DETECTION_SOURCE_IP)],
[N(NAT_DETECTION_DESTINATION_IP)]
<------- Length + Non-ESP Marker
IKE_SA_INIT
HDR, SAr1, KEr, Nr,
[N(NAT_DETECTION_SOURCE_IP)],
[N(NAT_DETECTION_DESTINATION_IP)]
Length + Non-ESP Marker ------->
first IKE_AUTH
HDR, SK {IDi, [CERTREQ]
CP(CFG_REQUEST), IDr,
SAi2, TSi, TSr, ...}
<------- Length + Non-ESP Marker
first IKE_AUTH
HDR, SK {IDr, [CERT], AUTH,
EAP, SAr2, TSi, TSr}
Length + Non-ESP Marker ------->
IKE_AUTH (repeat 1..N times)
HDR, SK {EAP}
<------- Length + Non-ESP Marker
IKE_AUTH (repeat 1..N times)
HDR SK {EAP}
Length + Non-ESP Marker ------->
final IKE_AUTH
HDR, SK {AUTH}
<------- Length + Non-ESP Marker
final IKE_AUTH
HDR, SK {AUTH, CP(CFG_REPLY),
SA, TSi, TSr, ...}
-------------- IKE and IPsec SAs Established ------------
Length + ESP Frame ------->
1. The client establishes a TCP connection with the server on port
4500 or on an alternate preconfigured port that the server is
listening on.
2. If configured to use TLS, the client initiates a TLS handshake.
During the TLS handshake, the server SHOULD NOT request the
client's certificate since authentication is handled as part of
IKE negotiation.
3. The client sends the stream prefix for TCP-encapsulated IKE
(Section 4) traffic to signal the beginning of IKE negotiation.
4. The client and server establish an IKE connection. This example
shows EAP-based authentication, although any authentication type
may be used.
B.2. Deleting an IKE Session
Client Server
---------- ----------
1) ----------------------- IKE Session ---------------------
Length + Non-ESP Marker ------->
INFORMATIONAL
HDR, SK {[N,] [D,]
[CP,] ...}
<------- Length + Non-ESP Marker
INFORMATIONAL
HDR, SK {[N,] [D,]
[CP], ...}
2) --------------------- TLS Session ---------------------
close_notify ------->
<------- close_notify
3) -------------------- TCP Connection -------------------
TcpFin ------->
<------- Ack
<------- TcpFin
Ack ------->
-------------------- IKE SA Deleted -------------------
1. The client and server exchange informational messages to notify
IKE SA deletion.
2. The client and server negotiate TLS session deletion using TLS
CLOSE_NOTIFY.
3. The TCP connection is torn down.
The deletion of the IKE SA should lead to the disposal of the
underlying TLS and TCP state.
B.3. Re-establishing an IKE Session
Client Server
---------- ----------
1) -------------------- TCP Connection -------------------
(IP_I:Port_I -> IP_R:Port_R)
TcpSyn ------->
<------- TcpSyn,Ack
TcpAck ------->
2) --------------------- TLS Session ---------------------
ClientHello ------->
ServerHello
{EncryptedExtensions}
<------- {Finished}
{Finished} ------->
3) ---------------------- Stream Prefix --------------------
"IKETCP" ------->
4) <---------------------> IKE/ESP Flow <------------------>
1. If a previous TCP connection was broken (for example, due to a
TCP Reset), the client is responsible for re-initiating the TCP
connection. The TCP Originator's address and port (IP_I and
Port_I) may be different from the previous connection's address
and port.
2. The client SHOULD attempt TLS session resumption if it has
previously established a session with the server.
3. After TCP and TLS are complete, the client sends the stream
prefix for TCP-encapsulated IKE traffic (Section 4).
4. The IKE and ESP packet flow can resume. If MOBIKE is being used,
the Initiator SHOULD send an UPDATE_SA_ADDRESSES message.
B.4. Using MOBIKE between UDP and TCP Encapsulation
Client Server
---------- ----------
1) --------------------- IKE_session ----------------------
(IP_I1:UDP500 -> IP_R:UDP500)
IKE_SA_INIT ------->
HDR, SAi1, KEi, Ni,
[N(NAT_DETECTION_SOURCE_IP)],
[N(NAT_DETECTION_DESTINATION_IP)]
<------- IKE_SA_INIT
HDR, SAr1, KEr, Nr,
[N(NAT_DETECTION_SOURCE_IP)],
[N(NAT_DETECTION_DESTINATION_IP)]
(IP_I1:UDP4500 -> IP_R:UDP4500)
Non-ESP Marker ------->
IKE_AUTH
HDR, SK { IDi, CERT, AUTH,
SAi2, TSi, TSr,
N(MOBIKE_SUPPORTED) }
<------- Non-ESP Marker
IKE_AUTH
HDR, SK { IDr, CERT, AUTH,
SAr2, TSi, TSr,
N(MOBIKE_SUPPORTED) }
<---------------------> IKE/ESP Flow <------------------>
2) ------------ MOBIKE Attempt on New Network --------------
(IP_I2:UDP4500 -> IP_R:UDP4500)
Non-ESP Marker ------->
INFORMATIONAL
HDR, SK { N(UPDATE_SA_ADDRESSES),
N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP) }
3) -------------------- TCP Connection -------------------
(IP_I2:Port_I -> IP_R:Port_R)
TcpSyn ------->
<------- TcpSyn,Ack
TcpAck ------->
4) --------------------- TLS Session ---------------------
ClientHello ------->
ServerHello
{EncryptedExtensions}
{Certificate*}
{CertificateVerify*}
<------- {Finished}
{Finished} ------->
5) ---------------------- Stream Prefix --------------------
"IKETCP" ------->
6) ------------ Retransmit Message from step 2 -------------
Length + Non-ESP Marker ------->
INFORMATIONAL
HDR, SK { N(UPDATE_SA_ADDRESSES),
N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP) }
<------- Length + Non-ESP Marker
INFORMATIONAL
HDR, SK { N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP) }
7) -- New Exchange with recalculated NAT_DETECTION_*_IP ---
Length + Non-ESP Marker ------->
INFORMATIONAL
HDR, SK { N(UPDATE_SA_ADDRESSES),
N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP) }
<------- Length + Non-ESP Marker
INFORMATIONAL
HDR, SK { N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP) }
8) <---------------------> IKE/ESP Flow <------------------>
1. During the IKE_AUTH exchange, the client and server exchange
MOBIKE_SUPPORTED notify payloads to indicate support for MOBIKE.
2. The client changes its point of attachment to the network and
receives a new IP address. The client attempts to re-establish
the IKE session using the UPDATE_SA_ADDRESSES notify payload, but
the server does not respond because the network blocks UDP
traffic.
3. The client brings up a TCP connection to the server in order to
use TCP encapsulation.
4. The client initiates a TLS handshake with the server.
5. The client sends the stream prefix for TCP-encapsulated IKE
traffic (Section 4).
6. The client sends the UPDATE_SA_ADDRESSES notify payload in the
INFORMATIONAL exchange on the TCP-encapsulated connection. Note
that this IKE message is the same as the one sent over UDP in
step 2; it should have the same message ID and contents.
7. Once the client receives a response on the TCP-encapsulated
connection, it immediately starts a new INFORMATIONAL exchange
with an UPDATE_SA_ADDRESSES notify payload and recalculated
NAT_DETECTION_*_IP notify payloads in order to get correct
information about the presence of NATs.
8. The IKE and ESP packet flow can resume.
Acknowledgments
Thanks to the authors of RFC 8229 (Tommy Pauly, Samy Touati, and Ravi
Mantha). Since this document clarifies and obsoletes RFC 8229, most
of its text was borrowed from the original document.
The following people provided valuable feedback and advice while
preparing RFC 8229: Stuart Cheshire, Delziel Fernandes, Yoav Nir,
Christoph Paasch, Yaron Sheffer, David Schinazi, Graham Bartlett,
Byju Pularikkal, March Wu, Kingwel Xie, Valery Smyslov, Jun Hu, and
Tero Kivinen. Special thanks to Eric Kinnear for his implementation
work.
The authors would like to thank Tero Kivinen, Paul Wouters, Joseph
Touch, and Christian Huitema for their valuable comments while
preparing this document.
Authors' Addresses
Tommy Pauly
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
United States of America
Email: tpauly@apple.com
Valery Smyslov
ELVIS-PLUS
PO Box 81
Moscow (Zelenograd)
124460
Russian Federation
Phone: +7 495 276 0211
Email: svan@elvis.ru