Network Working Group A. Decimo
Internet-Draft IRIF, University of Paris-Diderot
Updates: 6126bis (if approved) D. Schinazi
Intended status: Standards Track Google LLC
Expires: August 10, 2019 J. Chroboczek
IRIF, University of Paris-Diderot
February 6, 2019
Babel Routing Protocol over Datagram Transport Layer Security
draft-ietf-babel-dtls-04
Abstract
The Babel Routing Protocol does not contain any means to authenticate
neighbours or protect messages sent between them. This documents
specifies a mechanism to ensure these properties, using Datagram
Transport Layer Security (DTLS). This document updates RFC 6126bis.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Specification of Requirements . . . . . . . . . . . . . . 2
1.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 3
2. Operation of the Protocol . . . . . . . . . . . . . . . . . . 3
2.1. DTLS Connection Initiation . . . . . . . . . . . . . . . 3
2.2. Protocol Encoding . . . . . . . . . . . . . . . . . . . . 4
2.3. Transmission . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Reception . . . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Neighbour table entry . . . . . . . . . . . . . . . . . . 5
2.6. Simultaneous operation of both Babel over DTLS and
unprotected Babel . . . . . . . . . . . . . . . . . . . . 5
3. Interface Maximum Transmission Unit Issues . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Performance Considerations . . . . . . . . . . . . . 8
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
The Babel Routing Protocol [RFC6126bis] 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 which could lead a network to route traffic to an
attacker or to an under-resourced target causing denial of service.
This documents 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.
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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, 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 Babel HMAC
authentication [BABEL-HMAC]. HMAC only offers basic features, namely
authentication, integrity and replay protection with a small number
of symmetric keys.
Since HMAC authentication is simpler, requires fewer changes to the
Babel protocol, and avoids a dependency on DTLS, its use is
RECOMMENDED in deployments where both protocols are equally
applicable.
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 over 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 DTLS port,
and MUST listen for unencrypted Babel traffic on an unencrypted port,
which MUST be distinct from the DTLS port. The default port for
Babel over DTLS is registered with IANA as the "babel-dtls" port (UDP
port TBD, see Section 4), and the unencrypted port is registered as
the "babel" port (UDP port 6696). Nodes SHOULD use these default
ports.
When a Babel node discovers a new neighbor (generally by receiving an
unencrypted multicast Babel packet), it compares the neighbour's IPv6
link-local address with its own, using network byte ordering. If a
node's address is lower than the recently discovered neighbor's
address, it acts as a client and connects to the neighbor. 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
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connections use different ephemeral ports from recently used
connections to allow servers to differentiate between the new and old
DTLS connections. Alternatively, nodes MAY 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 an IPv6 link-local address; if it is not, it
MUST reject the connection. Nodes use mutual authentication
(authenticating both client and server); servers MUST send a
CertificateRequest message and subsequently authenticate the client.
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. 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 several IHU intervals, to avoid letting attackers make
stale information last longer.
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 neighbor
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
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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 neighbors, 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 [RFC6126bis]), 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.
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.
2.6. Simultaneous operation of both Babel over DTLS and unprotected
Babel
Implementations MAY implement both Babel over DTLS and unprotected
Babel. However, accepting unprotected Babel packets (other than
multicast Hellos) loses the security properties of Babel over DTLS.
A node MAY allow configuration options to allow unprotected Babel on
some interfaces but not others; this effectively gives nodes on that
interface the same access as authenticated nodes, and SHOULD NOT be
done unless that interface has a mechanism to authenticate nodes at a
lower layer (e.g. IPsec).
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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
[RFC6126bis], 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 ciphers 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.
4. IANA Considerations
If this document is approved, IANA is requested to register a UDP
port number, called "babel-dtls", for use by Babel over DTLS. The
IANA registry will include a reference to this document.
5. Security Considerations
Confidential interaction between two Babel peers requires Datagram
Transport Layer Security (DTLS) with a cipher suite offering
confidentiality protection. The guidance given in [RFC7525] MUST be
followed to avoid attacks on DTLS.
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 and can be obfuscated with IPv6 temporary addresses,
a server needs to mitigate the impact of such an attack. Such
mitigation might involve rate limiting handshakes from a given subnet
or more advanced denial of service avoidance techniques beyond the
scope of this document.
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6. References
6.1. 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>.
[RFC6126bis]
Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", Internet Draft draft-ietf-babel-rfc6126bis-07,
November 2018.
[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>.
6.2. Informative References
[BABEL-HMAC]
Do, C., Kolodziejak, W., and J. Chroboczek, "Babel
Cryptographic Authentication", Internet Draft draft-ietf-
babel-hmac-03, November 2018.
[DTLS-CID]
Rescorla, E., Tschofenig, H., Fossati, T., and T. Gondrom,
"Connection Identifiers for DTLS 1.2", Internet Draft
draft-ietf-tls-dtls-connection-id-02, October 2018.
[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>.
[RFC7525] 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, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
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[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>.
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.
Appendix B. Acknowledgments
The authors would like to thank Donald Eastlake, Thomas Fossati,
Gabriel Kerneis, Antoni Przygienda, Barbara Stark, Markus Stenberg,
Dave Taht, Martin Thomson, and Sean Turner for their input and
contributions. The performance considerations in this document were
inspired from the ones for DNS over DTLS [RFC8094].
Authors' Addresses
Antonin Decimo
IRIF, University of Paris-Diderot
Paris
France
Email: antonin.decimo@gmail.com
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David Schinazi
Google LLC
1600 Amphitheatre Parkway
Mountain View, California 94043
USA
Email: dschinazi.ietf@gmail.com
Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
Email: jch@irif.fr
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