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


   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.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 11, 2019.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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",
   "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

   o  Asymmetric keys.  DTLS allows authentication via asymmetric keys,
      which allows a finer granularity of trust per-peer, and allows for

   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

   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,

              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, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

6.2.  Informative References

              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,

   [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, <>.

   [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,

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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,

   [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


   David Schinazi
   Apple Inc.
   One Apple Park Way
   Cupertino, California  95014


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   Juliusz Chroboczek
   IRIF, University of Paris-Diderot
   Case 7014
   75205 Paris Cedex 13


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