Babel Routing Protocol over Datagram Transport Layer Security
draft-ietf-babel-dtls-02

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

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 18, 2019.

Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

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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 both Babel over DTLS and unprotected Babel
• 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
• Appendix B. Acknowledgments
• Authors' Addresses

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 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 very 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

All Babel over DTLS nodes MUST act as DTLS servers on the "babel-dtls" port (UDP port TBD), and MUST listen for 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. 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 the source IP address, and reject the connection if the address is not an IPv6 link-local address. Nodes MUST use mutual authentication (authenticating both client and server); servers MUST request client authentication by sending a CertificateRequest message. If either node fails to verify the peer's authentication, it MUST abort the DTLS handshake. Nodes MUST only negotiate DTLS version 1.2 or higher.

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 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 SHOULD send a DTLS close_notify alert to the neighbour if it believes the link is still viable.

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).

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.

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.

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

6.1. Normative References

6.2. Informative References

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 Thomas Fossati, Gabriel Kerneis, Antoni Przygienda, Markus Stenberg, Dave Taht, and Martin Thomson for their input and contributions. The performance considerations in this document were inspired from the ones for DNS over DTLS [RFC8094].

Authors' Addresses