Using Only Link-Local Address in Network Core

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Network Working Group                                       M. Behringer
Internet-Draft                                                 E. Vyncke
Intended status: BCP                                               Cisco
Expires: September 6, 2012                                 March 5, 2012

             Using Only Link-Local Address in Network Core


   This document proposes to use only IPv6 link-local addresses on
   infrastructure links between routers, wherever possible.  It
   discusses the advantages and disadvantages of this approach to aide
   the decision process for a given network,

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   This Internet-Draft will expire on September 6, 2012.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 3
   2.  Using Link-Local Address on Infrastructure Links  . . . . . . . 3
     2.1.  The Suggested Approach  . . . . . . . . . . . . . . . . . . 3
     2.2.  Advantages  . . . . . . . . . . . . . . . . . . . . . . . . 4
     2.3.  Caveats and Possible Workarounds  . . . . . . . . . . . . . 5
     2.4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     5.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

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1.  Introduction

   An infrastructure link between a set of routers typically does not
   require global or even unique local addressing [RFC4193].  Using
   link-local addressing on such links has a number of advantages, for
   example that routing tables do not need to carry link addressing, and
   can therefore be significantly smaller.  This helps to decrease
   failover times in certain routing convergence events.  An interface
   of a router is also not reachable beyond the link boundaries,
   therefore reducing the attack horizon.

   We propose to configure neither globally routable IPv6 addresses nor
   unique local addresses on infrastructure links of routers, wherever
   possible.  We recommend to use exclusively link-local addresses on
   such links.

   This document discusses the advantages and caveats of this approach.

1.1.  Requirements Language

   In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
   "RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in RFC2119 [RFC2119].

2.  Using Link-Local Address on Infrastructure Links

   This document proposes to use only link-local addresses (LLA) on all
   router interfaces on infrastructure links.  Routers typically do not
   need to be reached from from users of the network, nor from outside
   the network.  For an network operator there may be reasons to send
   packets to an infrastructure link for certain monitoring tasks; we
   suggest that many of those tasks could also be handled differently,
   not requiring routable address space on infrastructure links.

2.1.  The Suggested Approach

   Neither global IPv6 addresses nor unique local addresses are
   configured on infrastructure links.  In the absence of specific
   global or unique local address definitions, the default behavior of
   routers is to use link-local addresses.

   The effect on specific traffic types is as follows:

   o  Control plane protocols, such as BGP, ISIS, OSPFv3, RIPng, PIM
      work by default or can be configured to work with link-local

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   o  Management plane traffic, such as SSH, or ICMPv6 echo requests can
      be addressed to loopback addresses of routers with a global scope
      address.  Router management can also be done over out-of-band

   o  Data plane traffic is forwarded independently of the link address

   o  Neighbor discovery (neighbor solicitation and neighbor
      advertisement) is done by using link-local unicast and multicast
      addresses, therefore neighbor discovery is not affected.

   ICMPv6 [RFC4443] error messages (packet-too-big...) are required for
   routers, therefore a loopback interface must be configured with a
   global scope IPv6 address.  This global scope IPv6 address will be
   used as the source IPv6 address for all generated ICMPv6 messages.

   We therefore conclude that it is possible to construct a working
   network in this way.

2.2.  Advantages

   Smaller routing tables: Since the routing protocol only needs to
   carry one loopback address per router, it is smaller than in the
   traditional approach where every infrastructure link addresses are
   carried in the routing protocol.  This reduces memory consumption,
   and increases the convergence speed in some routing failover cases.

   Reduced attack surface: Every globally routable address on a router
   constitutes a potential attack point: a remote attacker can send
   traffic to that address, for example a TCP SYN flood, or he can
   intent SSH brute force password attacks.  If a network only uses
   loopback addresses for the routers, only those loopback addresses
   need to be protected from outside the network.  This significantly
   eases protection measures, such as infrastructure access control

   Lower configuration complexity: LLAs require no specific
   configuration, thereby lowering the complexity and size of router
   configurations.  This also reduces the likelihood of configuration

   Less address space: The proposal uses less address space than when
   infrastructure links carry global address space.

   Simpler DNS: Less address space in use also means less DNS mappings
   to maintain.

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2.3.  Caveats and Possible Workarounds

   Interface ping: If an interface doesn't have a globally routable
   address, it can only be pinged from a node on the same link.
   Therefore it is not possible to ping a specific link interface
   remotely.  A possible workaround is to ping the loopback address of a
   router instead.  In most cases today it is not possible to see which
   link the packet was received on; however, RFC5837 [RFC5837] suggests
   to include the interface identifier of the interface a packet was
   received on in the ICMP response.  With this approach it would be
   possible to ping a router on the loopback address, yet see which
   interface the packet was received on.  To check liveliness of a
   specific interface it may be necessary to use other methods, for
   example to connect to the router via SSH and to check locally.

   Traceroute: Similar to the ping case, a reply to a traceroute packet
   would come from a loopback address with a global address.  Today this
   does not display the specific interface the packets came in on.  Also
   here, RFC5837 [RFC5837] provides a solution.

   Hardware dependency: LLAs are usually EUI-64 based, hence, they
   change when the MAC address is changed.  This could pose problem in a
   case where the routing neighbor must be configured explicitly (e.g.
   BGP) and a line card needs to be physically replaced hence changing
   the EUI-64 LLA and breaking the routing neighborship.  But, LLAs can
   be statically configured such as fe80::1 and fe80::2 which can be
   used to configure any required static routing neighborship.

2.4.  Summary

   Using link-local addressing only on infrastructure links has a number
   of advantages, such as a smaller routing table size and a reduced
   attack surface.  It also simplifies router configurations.  However,
   the way certain network management tasks are carried out has to be
   adapted to provide the same level of detail, for example interface
   identifiers in traceroute.

3.  Security Considerations

   Using LLAs only on infrastructure links reduces the attack surface of
   a router: Loopback addresses with globally routed addresses are still
   reachable and must be secured, but infrastructure links can only be
   attacked from the local link.  This simplifies security of control
   and management planes.  The proposal does not impact the security of
   the data plane.  This proposal does not address control plane
   [RFC6192] attacks generated by data plane packets (such as hop-limit

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   As in the traditional approach, also this approach relies on the
   assumption that all routers can be trusted due to physical and
   operational security.

4.  IANA Considerations

   There are no IANA considerations or implications that arise from this

5.  References

5.1.  Normative References

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

5.2.  Informative References

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC5837]  Atlas, A., Bonica, R., Pignataro, C., Shen, N., and JR.
              Rivers, "Extending ICMP for Interface and Next-Hop
              Identification", RFC 5837, April 2010.

   [RFC6192]  Dugal, D., Pignataro, C., and R. Dunn, "Protecting the
              Router Control Plane", RFC 6192, March 2011.

Authors' Addresses

   Michael Behringer
   400 Avenue Roumanille, Bat 3
   Biot,   06410


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   Eric Vyncke
   De Kleetlaan, 6A
   Diegem,   1831


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