Network Working Group A. Decimo
Internet-Draft IRIF, University of Paris-Diderot
Updates: 6126bis (if approved) D. Schinazi
Intended status: Standards Track Apple Inc.
Expires: April 11, 2019 J. Chroboczek
IRIF, University of Paris-Diderot
October 8, 2018
Babel Routing Protocol over Datagram Transport Layer Security
draft-ietf-babel-dtls-01
Abstract
The Babel Routing Protocol does not contain any means to authenticate
neighbours or protect messages sent between them. This documents
describes 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 . . . . . . . . . . . . . . . . . . . . . . 2
2. Operation of the Protocol . . . . . . . . . . . . . . . . . . 3
2.1. DTLS Connection Initiation . . . . . . . . . . . . . . . 3
2.2. Protocol Encoding . . . . . . . . . . . . . . . . . . . . 4
2.3. Transmission . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Reception . . . . . . . . . . . . . . . . . . . . . . . . 4
2.5. Neighbour table entry . . . . . . . . . . . . . . . . . . 4
3. Interface Maximum Transmission Unit Issues . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Performance Considerations . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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 describes 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 current two main mechanisms for securing Babel are Babel over
DTLS (as described in this document) and Babel Cryptographic
Authentication [BabelHMAC]. The latter has the advantages of being
simpler and not requiring a dependency on DTLS, therefore
implementers are encouraged to consider it in preference to the
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mechanism defined in this document whenever both are applicable to a
given deployment. Both mechanisms ensure integrity of messages and
prevent message replay.
However, DTLS offers several features that are not provided by Babel
Cryptographic Authentication, therefore Babel over DTLS is applicable
in cases where those features are needed. Examples of such features
include:
o Asymmetric keys. DTLS allows authentication via asymmetric keys,
which allows a finer granularity of trust per-peer, and allows for
revocation.
o Confidentiality of data. DTLS encrypts payloads, preventing an
on-link attacker from observing the routing table.
2. Operation of the Protocol
Babel over DTLS requires changes to how Babel is operated, for two
reasons. Firstly, because DTLS introduces the concepts of client and
server, while Babel is a peer-to-peer protocol. Secondly, DTLS can
only protect unicast, while Babel TLVs can be sent over both unicast
and multicast.
2.1. DTLS Connection Initiation
All Babel over DTLS nodes MUST act as DTLS servers on the "babel-
dtls" port (UDP port TBD), and MUST listen for multicast traffic on
the unencrypted "babel" port (UDP port 6696). 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 connections use different ephemeral ports from recently
used connections to allow servers to differentiate between the new
and old DTLS connections. When a node receives a new DTLS
connection, it MUST verify the source IP address, and reject the
connection if the address is not an IPv6 link-local address.
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2.2. Protocol Encoding
Babel over DTLS sends all unicast Babel packets encrypted 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 packet. 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 on unprotected multicast Hellos, or also send protected unicast
Hellos.
Since Babel over DTLS only protects unicast packets, implementors may
implement Babel over DTLS by modifying an unprotected implementation
of Babel, and replacing any TLV sent over multicast with a separate
TLV sent over unicast for each neighbour.
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 MAY send a DTLS
close_notify alert to the neighbour.
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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. 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 lower layers (IP, UDP
and DTLS). This helps reduce the likelihood of lower-layer
fragmentation which would negatively impact performance and
reliability. 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. Babel packets MUST NOT be sent in
IPv6 Jumbograms.
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.
5. Security Considerations
The 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. The DTLS client SHOULD use the TLS
Certificate Status Request extension (Section 8 of [RFC6066]).
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 IPv4 address sharing and
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.
6. References
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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-05,
May 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
[BabelHMAC]
Do, C., Kolodziejak, W., and J. Chroboczek, "Babel
Cryptographic Authentication", Internet Draft draft-ietf-
babel-hmac-00, August 2018.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[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>.
[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>.
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[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.
These performance considerations 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
David Schinazi
Apple Inc.
One Apple Park Way
Cupertino, California 95014
USA
Email: dschinazi@apple.com
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Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
Email: jch@irif.fr
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