1. RFC 8968
Internet Engineering Task Force (IETF)                         A. Décimo
Request for Comments: 8968             IRIF, University of Paris-Diderot
Category: Standards Track                                    D. Schinazi
ISSN: 2070-1721                                               Google LLC
                                                           J. Chroboczek
                                       IRIF, University of Paris-Diderot
                                                            January 2021

     Babel Routing Protocol over Datagram Transport Layer Security


   The Babel Routing Protocol does not contain any means to authenticate
   neighbours or provide integrity or confidentiality for messages sent
   between them.  This document specifies a mechanism to ensure these
   properties using Datagram Transport Layer Security (DTLS).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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 Babel over DTLS and Unprotected
           Babel on a Node
     2.7.  Simultaneous Operation of Babel over DTLS and Unprotected
           Babel on a Network
   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
   Authors' Addresses

1.  Introduction

   The Babel routing protocol [RFC8966] 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 that could lead a network to route traffic to an
   attacker or to an under-resourced target, causing denial of service.
   This document 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",
   "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, optional 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 Message
   Authentication Code (MAC) authentication for Babel (Babel-MAC)
   [RFC8967].  Babel-MAC only offers basic features, namely
   authentication, integrity, and replay protection with a small number
   of symmetric keys.  A comparison of Babel security mechanisms and
   their applicability can be found in [RFC8966].

   Note that Babel over DTLS provides a single authentication domain,
   meaning that all nodes that have the right credentials can convey any
   and all routing information.

   DTLS supports several mechanisms by which nodes can identify
   themselves and prove possession of secrets tied to these identities.
   This document does not prescribe which of these mechanisms to use;
   details of identity management are left to deployment profiles of
   Babel over DTLS.

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 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 given UDP
   port and MUST listen for unencrypted Babel traffic on another UDP
   port, which MUST be distinct from the first one.  The default port
   for Babel over DTLS is registered with IANA as the "babel-dtls" port
   (UDP port 6699, see Section 4), and the port exchanging unencrypted
   Babel traffic is registered as the "babel" port (UDP port 6696, see
   Section 5 of [RFC8966]).

   When a Babel node discovers a new neighbour (generally by receiving
   an unencrypted multicast Babel packet), it compares the neighbour's
   IP address with its own, using network byte ordering.  If a node's
   address is lower than the recently discovered neighbour's address, it
   acts as a client and connects to the neighbour.  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

   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 could 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 either an IPv6 link-local address or an IPv4
   address belonging to the local network; if it is neither, it MUST
   reject the connection.  Nodes use mutual authentication
   (authenticating both client and server); clients MUST authenticate
   servers and servers MUST authenticate clients.  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.  The guidance given in
   [BCP195] MUST be followed to avoid attacks on DTLS.  Additionally,
   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 two IHU (I Heard You) intervals, to avoid letting attackers
   make stale information last longer.  If a node receives a new DTLS
   connection from a neighbour to whom it already has a connection, the
   node MUST NOT discard the older connection until it has completed the
   handshake of the new one and validated the identity of the peer.

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 neighbour
   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
   TLVs when converting to unicast in order to remove redundant

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 neighbours, 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 [RFC8966]), 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 Babel over DTLS and Unprotected Babel on
      a Node

   Implementations MAY implement both Babel over DTLS and unprotected
   Babel.  Additionally, a node MAY simultaneously run both Babel over
   DTLS and unprotected Babel.  However, a node running both MUST ensure
   that it runs them on separate interfaces, as the security properties
   of Babel over DTLS rely on ignoring unprotected Babel packets (other
   than Multicast Hellos).  An implementation MAY offer configuration
   options to allow unprotected Babel on some interfaces but not others,
   which effectively gives nodes on that interface the same access as
   authenticated nodes; however, this SHOULD NOT be done unless that
   interface has a mechanism to authenticate nodes at a lower layer
   (e.g., IPsec).

2.7.  Simultaneous Operation of Babel over DTLS and Unprotected Babel on
      a Network

   If Babel over DTLS and unprotected Babel are both operated on the
   same network, the Babel over DTLS implementation will receive
   unprotected Multicast Hellos and attempt to initiate a DTLS
   connection.  These connection attempts can be sent to nodes that only
   run unprotected Babel, who will not respond.  Babel over DTLS
   implementations SHOULD therefore rate-limit their DTLS connection
   attempts to avoid causing undue load on the network.

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
   [RFC8966] 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 ciphersuites 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.

   Note that distinct DTLS connections can use different ciphers, which
   can have different amounts of per-packet overhead.  Therefore, the
   MTU to one neighbour can be different from the MTU to another
   neighbour on the same link.

4.  IANA Considerations

   IANA has registered a UDP port number, called "babel-dtls", for use
   by Babel over DTLS:

      Service Name:  babel-dtls

      Port Number:  6699

      Transport Protocols:  UDP only

      Description:  Babel Routing Protocol over DTLS

      Assignee:  IESG, iesg@ietf.org

      Contact:  IETF Chair, chair@ietf.org

      Reference:  RFC 8968

      Service Code:  None

5.  Security Considerations

   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 until the handshake completes and can be obfuscated
   with IPv6 temporary addresses, a server needs to mitigate the impact
   of such an attack.  Note that attackers might attempt to keep in-
   progress handshakes open for as long as possible by using variants on
   the attack commonly known as Slowloris [SLOWLORIS].  Mitigating these
   attacks might involve limiting the rate of handshakes from a given
   subnet or more advanced denial of service avoidance techniques beyond
   the scope of this document.

   Babel over DTLS allows sending Multicast Hellos unprotected;
   attackers can therefore tamper with them.  For example, an attacker
   could send erroneous values for the Seqno and Interval fields,
   causing bidirectional reachability detection to fail.  While
   implementations MAY use Multicast Hellos for link quality estimation,
   they SHOULD also emit protected Unicast Hellos to prevent this class
   of denial-of-service attack.

   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.
   Additionally, an attacker could save some packets and replay them
   later in hopes of propagating stale routing information at a later
   time.  This can be mitigated by discarding received packets that have
   been reordered by more than two IHU intervals.

6.  References

6.1.  Normative References

   [BCP195]   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, May 2015,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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>.

   [RFC8966]  Chroboczek, J. and D. Schinazi, "The Babel Routing
              Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,

6.2.  Informative References

   [DTLS-CID] Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
              Identifiers for DTLS 1.2", Work in Progress, Internet-
              Draft, draft-ietf-tls-dtls-connection-id-08, 2 November
              2020, <https://tools.ietf.org/html/draft-ietf-tls-dtls-

   [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>.

   [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,

   [RFC8967]  Dô, C., Kolodziejak, W., and J. Chroboczek, "MAC
              Authentication for the Babel Routing Protocol", RFC 8967,
              DOI 10.17487/RFC8967, January 2021,

              Hansen, R., "Slowloris HTTP DoS", June 2009,

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.


   The authors would like to thank Roman Danyliw, Donald Eastlake,
   Thomas Fossati, Benjamin Kaduk, Gabriel Kerneis, Mirja Kühlewind,
   Antoni Przygienda, Henning Rogge, Dan Romascanu, Barbara Stark,
   Markus Stenberg, Dave Taht, Martin Thomson, Sean Turner, and Martin
   Vigoureux for their input and contributions.  The performance
   considerations in this document were inspired from the ones for DNS
   over DTLS [RFC8094].

Authors' Addresses

   Antonin Décimo
   IRIF, University of Paris-Diderot

   Email: antonin.decimo@gmail.com

   David Schinazi
   Google LLC
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America

   Email: dschinazi.ietf@gmail.com

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

   Email: jch@irif.fr
  1. RFC 8968