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: February 16, 2019 J. Chroboczek
IRIF, University of Paris-Diderot
August 15, 2018

Babel Routing Protocol over Datagram Transport Layer Security


This documents describes how to use Datagram Transport Layer Security (DTLS) to secure the Babel Routing Protocol.

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Table of Contents

1. Introduction

Babel over DTLS is a security protocol for the Babel routing protocol [RFC6126bis], that uses Datagram Transport Layer Security (DTLS) [RFC6347]. This document describes how to protect Babel with Babel over DTLS.

The motivation for proposing Babel over DTLS is that DTLS provides a sub-layer of security that is well-defined, whose security has been shown, and that has multiple implementations. Babel over DTLS has the following properties, inherited from DTLS:

The main change to the Babel protocol is that Babel over DTLS requires most packets to be sent over unicast.

A malicious entity in range of a non-secured deployment of Babel can learn properties of the network, but also reroute legitimate traffic by advertising routes with a low metric.

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.

2. Operation of the Protocol

At first sight, there are incompatibilities between Babel and DTLS. Babel is a pure peer-to-peer protocol, while DTLS is a two parties client-server protocol. A Babel implementation typically uses unicast and multicast, while DTLS can only protect unicast.

The problem of assigning a client of server role for a DTLS handshake to Babel nodes is solved by a simple arbitrary choice. The addresses of the two nodes are compared, and the node with the lowest address acts as the server.

Babel is sufficiently flexible to work almost without multicast. In Babel over DTLS, almost all packets are sent via unicast. Packets that would have been sent via unicast needs to be duplicated and sent to all of the original recipients via unicast. The cost of duplication is balanced by the fact that on networks with low throughput, unicast is often far more effective than multicast. Only neighbour discovery packets are sent via multicast, as they do not represent a security threat.

3. Handling protected and unprotected data

A Babel node needs to receive unprotected data for bootstrapping reasons, as well as protected data. Protected and unprotected traffic needs to be differentiated.


3.1. Cleartext and DTLS on the same port

In this approach, Babel and Babel over DTLS traffic is received on the same port. The DTLS client port, the DTLS server port, and the Babel port (6696) are equal. When a packet is received, it is unconditionally treated as a DTLS packet and decrypted.

Since the source port is fixed as 6696, a node that loses its DTLS state (e.g. if it reboots), will reuse the same source and destination ports for the new session. In order to avoid discarding these new packets, nodes receiving an unexpected DTLS ClientHello MUST proceed with a new handshake and MUST NOT destroy the existing session until the new session's handshake completes to avoid denial of service attacks (Section 4.2.8 of [RFC6347]).

3.2. Cleartext and DTLS on separate ports

In this approach, a different port (number TBD) is allocated by IANA for Babel over DTLS traffic. The Babel over DTLS server listens on this port and Babel over DTLS clients use an ephemeral source port to initiate outbound DTLS connections. Unprotected Babel messages are sent and received over the standard Babel port (6696). When parsing unprotected packets, all Babel TLVs except Hello and IHU MUST be silently ignored.

4. Establishing and handling Babel over DTLS sessions

4.1. Session Initiation

When a node A acquires a new neighbour B (e.g. when A first receives a Babel packet from B, see Section 3.4 of [RFC6126bis]),

Once the handshake succeeds and a DTLS session is established, nodes send all unicast Babel messages over DTLS.

4.2. Transmission

Since DTLS cannot secure multicast, nodes SHOULD send all TLVs over unicast DTLS, if possible. All TLVs that are not Hello nor IHU MUST be sent over unicast DTLS. Hello and IHU TLVs MAY be sent either over unicast DTLS or unprotected multicast. Nodes MUST NOT send any unprotected packets over unicast.

4.3. Reception

Packets received over unicast DTLS are parsed the same way as any packets in the original specification of Babel. Nodes MUST parse unprotected packets received over multicast, however they MUST silently ignore any TLV that is not Hello or IHU. Unprotected packets received over unicast MUST be silently ignored.

4.4. Neighbour flush

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.

5. Interface MTU Issues

Compared to normal Babel, DTLS adds at least 13 octets of header, plus cipher and authentication overhead to every packet. This reduces the size of the Babel payload that can be carried.

As stated in Section 4 of [RFC6126bis], in order to minimise the number of packets being sent while avoiding lower-layer fragmentation, a Babel node SHOULD attempt to maximise the size of the packets it sends, up to the outgoing interface's MTU adjusted for lower-layer headers (28 octets for UDP over IPv4, 48 octets for UDP over IPv6). It MUST NOT send packets larger than the attached interface's MTU adjusted for lower-layer headers 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 lower-layer headers or 512 octets, whichever is larger. Babel packets MUST NOT be sent in IPv6 Jumbograms.

Theses requirements are retained by this specification, but are extended to take DTLS overhead into account as follows. The Babel node MUST ensure that the DTLS datagram size does not exceed the interface MTU, i.e., each DTLS record MUST fit within a single datagram, as required by [RFC6347]. A Babel node MUST consider the amount of record expansion expected by the DTLS processing when calculating the maximum size of Babel packet that fits within the interface MTU. The overhead can be computed as DTLS overhead of 13 octets + authentication overhead of the negotiated DTLS cipher suite + block padding (Section of [RFC6347]).

6. IANA Considerations

If the final version of this specification uses the standard Babel port for unprotected packets and DTLS Section 3.1, no actions are required from IANA.

If the final version of this specification uses separate ports for unprotected packets and DTLS Section 3.2, IANA is requested to assign a UDP port with label "Babel_DTLS".

7. 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 DoS/DDoS avoidance techniques beyond the scope of this document.

8. References

8.1. 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.
[RFC6126bis] Chroboczek, J. and D. Schinazi, "The Babel Routing Protocol", Internet Draft draft-ietf-babel-rfc6126bis-05, October 2017.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

8.2. Informative References

[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011.
[RFC7250] Wouters, P., Tschofenig, H., 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.
[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.
[RFC7918] Langley, A., Modadugu, N. and B. Moeller, "Transport Layer Security (TLS) False Start", RFC 7918, DOI 10.17487/RFC7918, August 2016.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security (TLS) Cached Information Extension", RFC 7924, DOI 10.17487/RFC7924, July 2016.
[RFC8094] Reddy, T., Wing, D. and P. Patil, "DNS over Datagram Transport Layer Security (DTLS)", RFC 8094, DOI 10.17487/RFC8094, February 2017.

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:
David Schinazi Apple Inc. One Apple Park Way Cupertino, California 95014 USA EMail:
Juliusz Chroboczek IRIF, University of Paris-Diderot Case 7014 75205 Paris Cedex 13, France EMail: