Return Routability Check for DTLS 1.2 and DTLS 1.3
draft-ietf-tls-dtls-rrc-05
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| Authors | Hannes Tschofenig , Thomas Fossati | ||
| Last updated | 2022-03-07 | ||
| Replaces | draft-tschofenig-tls-dtls-rrc | ||
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draft-ietf-tls-dtls-rrc-05
TLS H. Tschofenig, Ed.
Internet-Draft T. Fossati
Updates: 6347 (if approved) Arm Limited
Intended status: Standards Track 7 March 2022
Expires: 8 September 2022
Return Routability Check for DTLS 1.2 and DTLS 1.3
draft-ietf-tls-dtls-rrc-05
Abstract
This document specifies a return routability check for use in context
of the Connection ID (CID) construct for the Datagram Transport Layer
Security (DTLS) protocol versions 1.2 and 1.3.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Transport Layer
Security Working Group mailing list (tls@ietf.org), which is archived
at https://mailarchive.ietf.org/arch/browse/tls/.
Source for this draft and an issue tracker can be found at
https://github.com/tlswg/dtls-rrc.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 September 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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This document may contain material from IETF Documents or IETF
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Without obtaining an adequate license from the person(s) controlling
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it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 4
3. RRC Extension . . . . . . . . . . . . . . . . . . . . . . . . 4
4. The Return Routability Check Message . . . . . . . . . . . . 4
5. Off-Path Packet Forwarding . . . . . . . . . . . . . . . . . 5
6. Path Validation Procedure . . . . . . . . . . . . . . . . . . 9
7. Enhanced Path Validation Procedure . . . . . . . . . . . . . 10
7.1. Path Challenge Requirements . . . . . . . . . . . . . . 11
7.2. Path Response/Delete Requirements . . . . . . . . . . . . 12
7.3. Timer Choice . . . . . . . . . . . . . . . . . . . . . . 12
8. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9. Security and Privacy Considerations . . . . . . . . . . . . . 15
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 15
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
13.1. Normative References . . . . . . . . . . . . . . . . . . 15
13.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
In "classical" DTLS, selecting a security context of an incoming DTLS
record is accomplished with the help of the 5-tuple, i.e. source IP
address, source port, transport protocol, destination IP address, and
destination port. Changes to this 5 tuple can happen for a variety
reasons over the lifetime of the DTLS session. In the IoT context,
NAT rebinding is common with sleepy devices. Other examples include
end host mobility and multi-homing. Without CID, if the source IP
address and/or source port changes during the lifetime of an ongoing
DTLS session then the receiver will be unable to locate the correct
security context. As a result, the DTLS handshake has to be re-run.
Of course, it is not necessary to re-run the full handshake if
session resumption is supported and negotiated.
A CID is an identifier carried in the record layer header of a DTLS
datagram that gives the receiver additional information for selecting
the appropriate security context. The CID mechanism has been
specified in [I-D.ietf-tls-dtls-connection-id] for DTLS 1.2 and in
[I-D.ietf-tls-dtls13] for DTLS 1.3.
Section 6 of [I-D.ietf-tls-dtls-connection-id] describes how the use
of CID increases the attack surface by providing both on-path and
off-path attackers an opportunity for (D)DoS. It then goes on
describing the steps a DTLS principal must take when a record with a
CID is received that has a source address (and/or port) different
from the one currently associated with the DTLS connection. However,
the actual mechanism for ensuring that the new peer address is
willing to receive and process DTLS records is left open. This
document standardizes a return routability check (RRC) as part of the
DTLS protocol itself.
The return routability check is performed by the receiving peer
before the CID-to-IP address/port binding is updated in that peer's
session state database. This is done in order to provide more
confidence to the receiving peer that the sending peer is reachable
at the indicated address and port.
Note however that, irrespective of CID, if RRC has been successfully
negotiated by the peers, path validation can be used at any time by
either endpoint. For instance, an endpoint might use RRC to check
that a peer is still in possession of its address after a period of
quiescence.
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2. Conventions and Terminology
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.
This document assumes familiarity with the CID format and protocol
defined for DTLS 1.2 [I-D.ietf-tls-dtls-connection-id] and for DTLS
1.3 [I-D.ietf-tls-dtls13]. The presentation language used in this
document is described in Section 4 of [RFC8446].
This document reuses the definition of "anti-amplification limit"
from [RFC9000] to mean three times the amount of data received from
an unvalidated address. This includes all DTLS records originating
from that source address, excluding discarded ones.
3. RRC Extension
The use of RRC is negotiated via the rrc DTLS-only extension. On
connecting, the client includes the rrc extension in its ClientHello
if it wishes to use RRC. If the server is capable of meeting this
requirement, it responds with a rrc extension in its ServerHello.
The extension_type value for this extension is TBD1 and the
extension_data field of this extension is empty. The client and
server MUST NOT use RRC unless both sides have successfully exchanged
rrc extensions.
Note that the RRC extension applies to both DTLS 1.2 and DTLS 1.3.
4. The Return Routability Check Message
When a record with CID is received that has the source address of the
enclosing UDP datagram different from the one previously associated
with that CID, the receiver MUST NOT update its view of the peer's IP
address and port number with the source specified in the UDP datagram
before cryptographically validating the enclosed record(s) but
instead perform a return routability check.
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enum {
invalid(0),
change_cipher_spec(20),
alert(21),
handshake(22),
application_data(23),
heartbeat(24), /* RFC 6520 */
return_routability_check(TBD2), /* NEW */
(255)
} ContentType;
uint64 Cookie;
enum {
path_challenge(0),
path_response(1),
path_delete(2),
reserved(2..255)
} rrc_msg_type;
struct {
rrc_msg_type msg_type;
select (return_routability_check.msg_type) {
case path_challenge: Cookie;
case path_response: Cookie;
case path_delete: Cookie;
};
} return_routability_check;
The cookie is a 8-byte field containing arbitrary data.
The return_routability_check message MUST be authenticated and
encrypted using the currently active security context.
5. Off-Path Packet Forwarding
An off-path attacker that can observe packets might forward copies of
genuine packets to endpoints. If the copied packet arrives before
the genuine packet, this will appear as a NAT rebinding. Any genuine
packet will be discarded as a duplicate. If the attacker is able to
continue forwarding packets, it might be able to cause migration to a
path via the attacker. This places the attacker on-path, giving it
the ability to observe or drop all subsequent packets.
This style of attack relies on the attacker using a path that has
approximately the same characteristics as the direct path between
endpoints. The attack is more reliable if relatively few packets are
sent or if packet loss coincides with the attempted attack.
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A data packet received on the original path that increases the
maximum received packet number will cause the endpoint to move back
to that path. Eliciting packets on this path increases the
likelihood that the attack is unsuccessful.
Figure 1 demonstrates the case where a receiver receives a packet
with a new source IP address and/or new port number. The receiver
needs to determine whether this path change is caused by an attacker
and will send a RRC message of type path_challenge (RRC-1) on the old
path.
new +--------+ old
path | | path
+----->|Receiver|<-----+
| | | |
| +--------+ |
| |
| |
| |
| |
| |
+----------+ |
| Attacker?| |
+----------+ |
| |
| |
| |
| +--------+ |
| | | |
+------| Sender |------+
| |
+--------+
Figure 1: Off-Path Packet Forwarding Scenario
Three cases need to be considered:
Case 1: The old path is dead, which leads to a timeout of RRC-1.
As shown in Figure 2, a RRC message of type path_challenge (RRC-2)
needs to be sent on the new path. In this situation the switch to
the new path is considered legitimate. The sender will reply with
RRC-3 containing a path_response on the new path.
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...................>+--------+
. ********| |********
. *+----->|Receiver|<-----+*
. *| new | | old |*
. RRC-2 *| path +--------+ path |* RRC-1
. with *| |* with
. path- *| |* path-
. challenge *| |* challenge
. *| |*
. *| |*
. +----------+ |*
. | Attacker | |*
. +----------+ |*
. *| |v
. *| |timeout
. *| |
.RRC-3 *| +--------+ |
.with *| | | |
.path- *+------| Sender |------+
.response *******>| |
....................+--------+
Figure 2: Old path is dead
Case 2: The old path is alive but not preferred.
This case is shown in Figure 3 whereby the sender replies with a
RRC-2 path_delete message on the old path. This triggers the
receiver to send RRC-3 with a path-challenge along the new path. The
sender will reply with RRC-4 containing a path_response along the new
path.
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...................>+--------+<....................
. ********| |******** .
. *+----->|Receiver|<-----+* .
. *| new | | old |* .
. RRC-3 *| path +--------+ path |* RRC-1 .
. with *| |* with .
. path- *| |* path- .
. challenge *| |* challenge .
. *| |* .
. *| |* .
. +----------+ |* .
. | Attacker | |* .
. +----------+ |* .
. *| |* .
. *| |* .
. *| |* .
.RRC-4 *| +--------+ |* RRC-2.
.with *| | | |* with.
.path- *+------| Sender |------+* path-.
.response *******>| |<******* delete.
....................+--------+.....................
Figure 3: Old path is not preferred
Case 3: The old path is alive and preferred.
This is most likely the result of an attacker. The sender replies
with RRC-2 containing a path_response along the old path. The
interaction is shown in Figure 4. This results in the connection
being migrated back to the old path.
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+--------+<....................
| |******** .
+----->|Receiver|<-----+* .
| new | | old |* .
| path +--------+ path |* RRC-1 .
| |* with .
| |* path- .
| |* challenge .
| |* .
| |* .
+----------+ |* .
| Attacker | |* .
+----------+ |* .
| |* .
| |* .
| |* .
| +--------+ |* RRC-2.
| | | |* with.
+------| Sender |------+* path-.
| |<******* response.
+--------+.....................
Figure 4: Old path is preferred
Note that this defense is imperfect, but this is not considered a
serious problem. If the path via the attack is reliably faster than
the old path despite multiple attempts to use that old path, it is
not possible to distinguish between an attack and an improvement in
routing.
An endpoint could also use heuristics to improve detection of this
style of attack. For instance, NAT rebinding is improbable if
packets were recently received on the old path; similarly, rebinding
is rare on IPv6 paths. Endpoints can also look for duplicated
packets. Conversely, a change in connection ID is more likely to
indicate an intentional migration rather than an attack. Note,
however, changes in connection IDs are only supported in DTLS 1.3 but
not in DTLS 1.2.
6. Path Validation Procedure
Note: This algorithm does not take the Section 5 scenario into
account.
The receiver that observes the peer's address or port update MUST
stop sending any buffered application data (or limit the data sent to
the unvalidated address to the anti-amplification limit) and initiate
the return routability check that proceeds as follows:
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1. The receiver creates a return_routability_check message of type
path_challenge and places the unpredictable cookie into the
message.
2. The message is sent to the observed new address and a timer T
(see Section 7.3) is started.
3. The peer endpoint, after successfully verifying the received
return_routability_check message responds by echoing the cookie
value in a return_routability_check message of type
path_response.
4. When the initiator receives and verifies the
return_routability_check message contains the sent cookie, it
updates the peer address binding.
5. If T expires, or the address confirmation fails, the peer address
binding is not updated.
After this point, any pending send operation is resumed to the bound
peer address.
Section 7.1 and Section 7.2 contain the requirements for the
initiator and responder roles, broken down per protocol phase.
7. Enhanced Path Validation Procedure
Note: This algorithm also takes the Section 5 scenario into account.
The receiver that observes the peer's address or port update MUST
stop sending any buffered application data (or limit the data sent to
the unvalidated address to the anti-amplification limit) and initiate
the return routability check that proceeds as follows:
1. The receiver creates a return_routability_check message of type
path_challenge and places the unpredictable cookie into the
message.
2. The message is sent to the previously valid address, which
corresponds to the old path. Additionally, a timer T, see
Section 7.3, is started.
3. The peer endpoint verifies the received return_routability_check
message. The action to be taken depends on the preference of the
path through which the message was received:
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* If the path through which the message was received is
preferred, a return_routability_check message of type
path_response MUST be returned.
* If the path through which the message was received is not
preferred, a return_routability_check message of type
path_delete MUST be returned. In either case, the peer
endpoint echoes the cookie value in the response.
4. The initiator receives and verifies that the
return_routability_check message contains the previously sent
cookie. The actions taken by the initiator differ based on the
received message:
* When a return_routability_check message of type path_response
was received, the initiator MUST continue using the previously
valid address, i.e. no switch to the new path takes place and
the peer address binding is not updated.
* When a return_routability_check message of type path_delete
was received, the initiator MUST perform a return routability
check on the observed new address, as described in Section 6.
5. If T expires, or the address confirmation fails, the peer address
binding is not updated. In this case, the initiator MUST perform
a return routability check on the observed new address, as
described in Section 6.
After the path validation procedure is completed, any pending send
operation is resumed to the bound peer address.
Section 7.1 and Section 7.2 contain the requirements for the
initiator and responder roles, broken down per protocol phase.
7.1. Path Challenge Requirements
* The initiator MAY send multiple return_routability_check messages
of type path_challenge to cater for packet loss on the probed
path.
- Each path_challenge SHOULD go into different transport packets.
(Note that the DTLS implementation may not have control over
the packetization done by the transport layer.)
- The transmission of subsequent path_challenge messages SHOULD
be paced to decrease the chance of loss.
- Each path_challenge message MUST contain random data.
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* The initiator MAY use padding using the record padding mechanism
available in DTLS 1.3 (and in DTLS 1.2, when CID is enabled on the
sending direction) up to the anti-amplification limit to probe if
the path MTU (PMTU) for the new path is still acceptable.
7.2. Path Response/Delete Requirements
* The responder MUST NOT delay sending an elicited path_response or
path_delete messages.
* The responder MUST send exactly one path_response or path_delete
message for each received path_challenge.
* The responder MUST send the path_response or the path_delete on
the path where the corresponding path_challenge has been received,
so that validation succeeds only if the path is functional in both
directions. The initiator MUST NOT enforce this behaviour.
* The initiator MUST silently discard any invalid path_response it
receives.
Note that RRC does not cater for PMTU discovery on the reverse path.
If the responder wants to do PMTU discovery using RRC, it should
initiate a new path validation procedure.
7.3. Timer Choice
When setting T, implementations are cautioned that the new path could
have a longer round-trip time (RTT) than the original.
In settings where there is external information about the RTT of the
active path, implementations SHOULD use T = 3xRTT.
If an implementation has no way to obtain information regarding the
RTT of the active path, a value of 1s SHOULD be used.
Profiles for specific deployment environments -- for example,
constrained networks [I-D.ietf-uta-tls13-iot-profile] -- MAY specify
a different, more suitable value.
8. Example
The example TLS 1.3 handshake shown in Figure 5 shows a client and a
server negotiating the support for CID and for the RRC extension.
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Client Server
Key ^ ClientHello
Exch | + key_share
| + signature_algorithms
| + rrc
v + connection_id=empty
-------->
ServerHello ^ Key
+ key_share | Exch
+ connection_id=100 |
+ rrc v
{EncryptedExtensions} ^ Server
{CertificateRequest} v Params
{Certificate} ^
{CertificateVerify} | Auth
<-------- {Finished} v
^ {Certificate}
Auth | {CertificateVerify}
v {Finished} -------->
[Application Data] <-------> [Application Data]
+ Indicates noteworthy extensions sent in the
previously noted message.
* Indicates optional or situation-dependent
messages/extensions that are not always sent.
{} Indicates messages protected using keys
derived from a [sender]_handshake_traffic_secret.
[] Indicates messages protected using keys
derived from [sender]_application_traffic_secret_N.
Figure 5: Message Flow for Full TLS Handshake
Once a connection has been established the client and the server
exchange application payloads protected by DTLS with an unilaterally
used CIDs. In our case, the client is requested to use CID 100 for
records sent to the server.
At some point in the communication interaction the IP address used by
the client changes and, thanks to the CID usage, the security context
to interpret the record is successfully located by the server.
However, the server wants to test the reachability of the client at
his new IP address.
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Client Server
------ ------
Application Data ========>
<CID=100>
Src-IP=A
Dst-IP=Z
<======== Application Data
Src-IP=Z
Dst-IP=A
<<------------->>
<< Some >>
<< Time >>
<< Later >>
<<------------->>
Application Data ========>
<CID=100>
Src-IP=B
Dst-IP=Z
<<< Unverified IP
Address B >>
<-------- Return Routability Check
path_challenge(cookie)
Src-IP=Z
Dst-IP=B
Return Routability Check -------->
path_response(cookie)
Src-IP=B
Dst-IP=Z
<<< IP Address B
Verified >>
<======== Application Data
Src-IP=Z
Dst-IP=B
Figure 6: Return Routability Example
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9. Security and Privacy Considerations
Note that the return routability checks do not protect against
flooding of third-parties if the attacker is on-path, as the attacker
can redirect the return routability checks to the real peer (even if
those datagrams are cryptographically authenticated). On-path
adversaries can, in general, pose a harm to connectivity.
10. IANA Considerations
IANA is requested to allocate an entry to the TLS ContentType
registry, for the return_routability_check(TBD2) message defined in
this document. The return_routability_check content type is only
applicable to DTLS 1.2 and 1.3.
IANA is requested to allocate the extension code point (TBD1) for the
rrc extension to the TLS ExtensionType Values registry as described
in Table 1.
+=======+===========+=====+===========+=============+===========+
| Value | Extension | TLS | DTLS-Only | Recommended | Reference |
| | Name | 1.3 | | | |
+=======+===========+=====+===========+=============+===========+
| TBD1 | rrc | CH, | Y | N | RFC-THIS |
| | | SH | | | |
+-------+-----------+-----+-----------+-------------+-----------+
Table 1: rrc entry in the TLS ExtensionType Values registry
11. Open Issues
Issues against this document are tracked at https://github.com/tlswg/
dtls-rrc/issues
12. Acknowledgments
We would like to thank Achim Kraus, Hanno Becker, Hanno Boeck, Manuel
Pegourie-Gonnard, Mohit Sahni and Rich Salz for their input to this
document.
13. References
13.1. Normative References
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[I-D.ietf-tls-dtls-connection-id]
Rescorla, E., Tschofenig, H., Fossati, T., and A. Kraus,
"Connection Identifiers for DTLS 1.2", Work in Progress,
Internet-Draft, draft-ietf-tls-dtls-connection-id-13, 22
June 2021, <https://datatracker.ietf.org/doc/html/draft-
ietf-tls-dtls-connection-id-13>.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-43, 30 April 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
dtls13-43>.
[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/rfc/rfc2119>.
[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/rfc/rfc8174>.
[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/rfc/rfc8446>.
13.2. Informative References
[I-D.ietf-uta-tls13-iot-profile]
Tschofenig, H. and T. Fossati, "TLS/DTLS 1.3 Profiles for
the Internet of Things", Work in Progress, Internet-Draft,
draft-ietf-uta-tls13-iot-profile-04, 7 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-uta-
tls13-iot-profile-04>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
Appendix A. History
// RFC EDITOR: PLEASE REMOVE THIS SECTION
draft-ietf-tls-dtls-rrc-05
Tschofenig & Fossati Expires 8 September 2022 [Page 16]
Internet-Draft DTLS Return Routability Check March 2022
* Added text about off-path packet forwarding
draft-ietf-tls-dtls-rrc-04
* Re-submitted draft to fix references
draft-ietf-tls-dtls-rrc-03
* Added details for challenge-response exchange
draft-ietf-tls-dtls-rrc-02
* Undo the TLS flags extension for negotiating RRC, use a new
extension type
draft-ietf-tls-dtls-rrc-01
* Use the TLS flags extension for negotiating RRC
* Enhanced IANA consideration section
* Expanded example section
* Revamp message layout:
- Use 8-byte fixed size cookies
- Explicitly separate path challenge from response
draft-ietf-tls-dtls-rrc-00
* Draft name changed after WG adoption
draft-tschofenig-tls-dtls-rrc-01
* Removed text that overlapped with draft-ietf-tls-dtls-connection-
id
draft-tschofenig-tls-dtls-rrc-00
* Initial version
Authors' Addresses
Hannes Tschofenig (editor)
Arm Limited
Email: hannes.tschofenig@arm.com
Tschofenig & Fossati Expires 8 September 2022 [Page 17]
Internet-Draft DTLS Return Routability Check March 2022
Thomas Fossati
Arm Limited
Email: thomas.fossati@arm.com
Tschofenig & Fossati Expires 8 September 2022 [Page 18]