Internet Engineering Task Force (IETF) A. Décimo
Request for Comments: 8968 IRIF, University of Paris-Diderot
Category: Standards Track D. Schinazi
ISSN: 2070-1721 Google LLC
J. Chroboczek
IRIF, University of Paris-Diderot
November 2020
January 2021
Babel Routing Protocol over Datagram Transport Layer Security
Abstract
The Babel Routing Protocol does not contain any means to authenticate
neighbours or provide integrity or confidentiality for messages sent
between them. This document specifies a mechanism to ensure these
properties using Datagram Transport Layer Security (DTLS).
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/rfc8968.
Copyright Notice
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Table of Contents
1. Introduction
1.1. Specification of Requirements
1.2. Applicability
2. Operation of the Protocol
2.1. DTLS Connection Initiation
2.2. Protocol Encoding
2.3. Transmission
2.4. Reception
2.5. Neighbour Table Entry
2.6. Simultaneous Operation of Babel over DTLS and Unprotected
Babel on a Node
2.7. Simultaneous Operation of Babel over DTLS and Unprotected
Babel on a Network
3. Interface Maximum Transmission Unit Issues
4. IANA Considerations
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Appendix A. Performance Considerations
Acknowledgments
Authors' Addresses
1. Introduction
The Babel routing protocol [RFC8966] does not contain any means to
authenticate neighbours or protect messages sent between them.
Because of this, an attacker is able to send maliciously crafted
Babel messages that could lead a network to route traffic to an
attacker or to an under-resourced target, causing denial of service.
This document specifies a mechanism to prevent such attacks using
Datagram Transport Layer Security (DTLS) [RFC6347].
1.1. Specification of Requirements
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.
1.2. Applicability
The protocol described in this document protects Babel packets with
DTLS. As such, it inherits the features offered by DTLS, notably
authentication, integrity, optional replay protection,
confidentiality, and asymmetric keying. It is therefore expected to
be applicable in a wide range of environments.
There exists another mechanism for securing Babel, namely Message
Authentication Code (MAC) authentication for Babel (Babel-MAC)
[RFC8967]. Babel-MAC only offers basic features, namely
authentication, integrity, and replay protection with a small number
of symmetric keys. A comparison of Babel security mechanisms and
their applicability can be found in [RFC8966].
Note that Babel over DTLS provides a single authentication domain,
meaning that all nodes that have the right credentials can convey any
and all routing information.
DTLS supports several mechanisms by which nodes can identify
themselves and prove possession of secrets tied to these identities.
This document does not prescribe which of these mechanisms to use;
details of identity management are left to deployment profiles of
Babel over DTLS.
2. Operation of the Protocol
Babel over DTLS requires some changes to how Babel operates. First,
DTLS is a client-server protocol, while Babel is a peer-to-peer
protocol. Second, DTLS can only protect unicast communication, while
Babel packets can be sent to both unicast and multicast destinations.
2.1. DTLS Connection Initiation
Babel over DTLS operates on a different port than unencrypted Babel.
All Babel over DTLS nodes MUST act as DTLS servers on a given UDP
port and MUST listen for unencrypted Babel traffic on another UDP
port, which MUST be distinct from the first one. The default port
for Babel over DTLS is registered with IANA as the "babel-dtls" port
(UDP port 6699, see Section 4), and the port exchanging unencrypted
Babel traffic is registered as the "babel" port (UDP port 6696, see
Section 5 of [RFC8966]).
When a Babel node discovers a new neighbour (generally by receiving
an unencrypted multicast Babel packet), it compares the neighbour's
IP address with its own, using network byte ordering. If a node's
address is lower than the recently discovered neighbour's address, it
acts as a client and connects to the neighbour. In other words, the
node with the lowest address is the DTLS client for this pairwise
relationship. As an example, fe80::1:2 is considered lower than
fe80::2:1.
The node acting as DTLS client initiates its DTLS connection from an
ephemeral UDP port. Nodes SHOULD ensure that new client DTLS
connections use different ephemeral ports from recently used
connections to allow servers to differentiate between the new and old
DTLS connections. Alternatively, nodes could use DTLS connection
identifiers [DTLS-CID] as a higher-entropy mechanism to distinguish
between connections.
When a node receives a new DTLS connection, it MUST verify that the
source IP address is either an IPv6 link-local address or an IPv4
address belonging to the local network; if it is neither, it MUST
reject the connection. Nodes use mutual authentication
(authenticating both client and server); clients MUST authenticate
servers and servers MUST authenticate clients. Implementations MUST
support authenticating peers against a local store of credentials.
If either node fails to authenticate its peer against its local
policy, it MUST abort the DTLS handshake. The guidance given in
[BCP195] MUST be followed to avoid attacks on DTLS. Additionally,
nodes MUST only negotiate DTLS version 1.2 or higher. Nodes MUST use
DTLS replay protection to prevent attackers from replaying stale
information. Nodes SHOULD drop packets that have been reordered by
more than two IHU (I Heard You) intervals, to avoid letting attackers
make stale information last longer. If a node receives a new DTLS
connection from a neighbour to whom it already has a connection, the
node MUST NOT discard the older connection until it has completed the
handshake of the new one and validated the identity of the peer.
2.2. Protocol Encoding
Babel over DTLS sends all unicast Babel packets protected by DTLS.
The entire Babel packet, from the Magic byte at the start of the
Babel header to the last byte of the Babel packet trailer, is sent
protected by DTLS.
2.3. Transmission
When sending packets, Babel over DTLS nodes MUST NOT send any TLVs
over the unprotected "babel" port, with the exception of Hello TLVs
without the Unicast flag set. Babel over DTLS nodes MUST NOT send
any unprotected unicast packets. This ensures the confidentiality of
the information sent in Babel packets (e.g., the network topology) by
only sending it encrypted by DTLS. Unless some out-of-band neighbour
discovery mechanism is available, nodes SHOULD periodically send
unprotected Multicast Hellos to ensure discovery of new neighbours.
In order to maintain bidirectional reachability, nodes can either
rely entirely on unprotected Multicast Hellos, or send protected
Unicast Hellos in addition to the Multicast Hellos.
Since Babel over DTLS only protects unicast packets, implementors may
implement Babel over DTLS by modifying an implementation of Babel
without DTLS support and replacing any TLV previously sent over
multicast with a separate TLV sent over unicast for each neighbour.
TLVs previously sent over multicast can be replaced with the same
contents over unicast, with the exception of Hellos as described
above. Some implementations could also change the contents of IHU
TLVs when converting to unicast in order to remove redundant
information.
2.4. Reception
Babel over DTLS nodes can receive Babel packets either protected over
a DTLS connection or unprotected directly over the "babel" port. To
ensure the security properties of this mechanism, unprotected packets
are treated differently. Nodes MUST silently ignore any unprotected
packet sent over unicast. When parsing an unprotected packet, a node
MUST silently ignore all TLVs that are not of type Hello. Nodes MUST
also silently ignore any unprotected Hello with the Unicast flag set.
Note that receiving an unprotected packet can still be used to
discover new neighbours, even when all TLVs in that packet are
silently ignored.
2.5. Neighbour Table Entry
It is RECOMMENDED for nodes to associate the state of their DTLS
connection with their neighbour table. When a neighbour entry is
flushed from the neighbour table (Appendix A of [RFC8966]), its
associated DTLS state SHOULD be discarded. The node SHOULD send a
DTLS close_notify alert to the neighbour if it believes the link is
still viable.
2.6. Simultaneous Operation of Babel over DTLS and Unprotected Babel on
a Node
Implementations MAY implement both Babel over DTLS and unprotected
Babel. Additionally, a node MAY simultaneously run both Babel over
DTLS and unprotected Babel. However, a node running both MUST ensure
that it runs them on separate interfaces, as the security properties
of Babel over DTLS rely on ignoring unprotected Babel packets (other
than Multicast Hellos). An implementation MAY offer configuration
options to allow unprotected Babel on some interfaces but not others,
which effectively gives nodes on that interface the same access as
authenticated nodes; however, this SHOULD NOT be done unless that
interface has a mechanism to authenticate nodes at a lower layer
(e.g., IPsec).
2.7. Simultaneous Operation of Babel over DTLS and Unprotected Babel on
a Network
If Babel over DTLS and unprotected Babel are both operated on the
same network, the Babel over DTLS implementation will receive
unprotected Multicast Hellos and attempt to initiate a DTLS
connection. These connection attempts can be sent to nodes that only
run unprotected Babel, who will not respond. Babel over DTLS
implementations SHOULD therefore rate-limit their DTLS connection
attempts to avoid causing undue load on the network.
3. Interface Maximum Transmission Unit Issues
Compared to unprotected Babel, DTLS adds header, authentication tag,
and possibly block-size padding overhead to every packet. This
reduces the size of the Babel payload that can be carried. This
document does not relax the packet size requirements in Section 4 of
[RFC8966] but recommends that DTLS overhead be taken into account
when computing maximum packet size.
More precisely, nodes SHOULD compute the overhead of DTLS depending
on the ciphersuites in use and SHOULD NOT send Babel packets larger
than the interface maximum transmission unit (MTU) minus the overhead
of IP, UDP, and DTLS. Nodes MUST NOT send Babel packets larger than
the attached interface's MTU adjusted for known lower-layer headers
(at least UDP and IP) or 512 octets, whichever is larger, but not
exceeding 2^(16) - 1 adjusted for lower-layer headers. Every Babel
speaker MUST be able to receive packets that are as large as any
attached interface's MTU adjusted for UDP and IP headers or 512
octets, whichever is larger. Note that this requirement on reception
does not take into account the overhead of DTLS because the peer may
not have the ability to compute the overhead of DTLS, and the packet
may be fragmented by lower layers.
Note that distinct DTLS connections can use different ciphers, which
can have different amounts of per-packet overhead. Therefore, the
MTU to one neighbour can be different from the MTU to another
neighbour on the same link.
4. IANA Considerations
IANA has registered a UDP port number, called "babel-dtls", for use
by Babel over DTLS:
Service Name: babel-dtls
Port Number: 6699
Transport Protocols: UDP only
Description: Babel Routing Protocol over DTLS
Assignee: IESG, iesg@ietf.org
Contact: IETF Chair, chair@ietf.org
Reference: RFC 8968
Service Code: None
5. Security Considerations
A malicious client might attempt to perform a high number of DTLS
handshakes with a server. As the clients are not uniquely identified
by the protocol until the handshake completes and can be obfuscated
with IPv6 temporary addresses, a server needs to mitigate the impact
of such an attack. Note that attackers might attempt to keep in-
progress handshakes open for as long as possible by using variants on
the attack commonly known as Slowloris [SLOWLORIS]. Mitigating these
attacks might involve limiting the rate of handshakes from a given
subnet or more advanced denial of service avoidance techniques beyond
the scope of this document.
Babel over DTLS allows sending Multicast Hellos unprotected;
attackers can therefore tamper with them. For example, an attacker
could send erroneous values for the Seqno and Interval fields,
causing bidirectional reachability detection to fail. While
implementations MAY use Multicast Hellos for link quality estimation,
they SHOULD also emit protected Unicast Hellos to prevent this class
of denial-of-service attack.
While DTLS provides protection against an attacker that replays valid
packets, DTLS is not able to detect when an active on-path attacker
intercepts valid packets and resends them at a later time. This
attack could be used to make a node believe it has bidirectional
reachability to a neighbour even though that neighbour has
disconnected from the network. To prevent this attack, nodes MUST
discard the DTLS state associated with a neighbour after a finite
time of not receiving valid DTLS packets. This can be implemented
by, for example, discarding a neighbour's DTLS state when its
associated IHU timer fires. Note that relying solely on the receipt
of Hellos is not sufficient as Multicast Hellos are sent unprotected.
Additionally, an attacker could save some packets and replay them
later in hopes of propagating stale routing information at a later
time. This can be mitigated by discarding received packets that have
been reordered by more than two IHU intervals.
6. References
6.1. Normative References
[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, May 2015,
<https://www.rfc-editor.org/info/bcp195>.
[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>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[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>.
[RFC8966] Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", RFC 8966, DOI 10.17487/RFC8966, November 2020, January 2021,
<https://www.rfc-editor.org/info/rfc8966>.
6.2. Informative References
[DTLS-CID] Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
Identifiers for DTLS 1.2", Work in Progress, Internet-
Draft, draft-ietf-tls-dtls-connection-id-08, 2 November
2020, <https://tools.ietf.org/html/draft-ietf-tls-dtls-
connection-id-08>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>.
[RFC7918] Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", RFC 7918,
DOI 10.17487/RFC7918, August 2016,
<https://www.rfc-editor.org/info/rfc7918>.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016,
<https://www.rfc-editor.org/info/rfc7924>.
[RFC8094] Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
Transport Layer Security (DTLS)", RFC 8094,
DOI 10.17487/RFC8094, February 2017,
<https://www.rfc-editor.org/info/rfc8094>.
[RFC8967] Do, Dô, C., Kolodziejak, W., and J. Chroboczek, "Message
Authentication Code (MAC) "MAC
Authentication for the Babel Routing Protocol", RFC 8967,
DOI 10.17487/RFC8967,
November 2020, January 2021,
<https://www.rfc-editor.org/info/rfc8967>.
[SLOWLORIS]
Hansen, R., "Slowloris HTTP DoS", June 2009,
<https://web.archive.org/web/20150315054838/
http://ha.ckers.org/slowloris/>.
Appendix A. Performance Considerations
To reduce the number of octets taken by the DTLS handshake,
especially the size of the certificate in the ServerHello (which can
be several kilobytes), Babel peers can use raw public keys [RFC7250]
or the Cached Information Extension [RFC7924]. The Cached
Information Extension avoids transmitting the server's certificate
and certificate chain if the client has cached that information from
a previous TLS handshake. TLS False Start [RFC7918] can reduce round
trips by allowing the TLS second flight of messages
(ChangeCipherSpec) to also contain the (encrypted) Babel packet.
Acknowledgments
The authors would like to thank Roman Danyliw, Donald Eastlake,
Thomas Fossati, Benjamin Kaduk, Gabriel Kerneis, Mirja Kühlewind,
Antoni Przygienda, Henning Rogge, Dan Romascanu, Barbara Stark,
Markus Stenberg, Dave Taht, Martin Thomson, Sean Turner, and Martin
Vigoureux for their input and contributions. The performance
considerations in this document were inspired from the ones for DNS
over DTLS [RFC8094].
Authors' Addresses
Antonin Décimo
IRIF, University of Paris-Diderot
Paris
France
Email: antonin.decimo@gmail.com
David Schinazi
Google LLC
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: dschinazi.ietf@gmail.com
Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris CEDEX 13
France
Email: jch@irif.fr