Support for Adjustable Maximum Router Lifetimes per Link

Abstract

The IPv6 Neighbor Discovery protocol specifies the maximum time allowed between sending unsolicited multicast Router Advertisements (RAs) from a router interface as well as the maximum router lifetime. It also allows the limits to be overridden by documents that are specific to the link layer. This document allows for overriding these values on a per-link basis.

This document specifies updates to the IPv6 Neighbor Discovery Protocol (RFC 4861) to increase the maximum time allowed between sending unsolicited multicast RAs from a router interface as well as to increase the maximum router lifetime.

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 https://www.rfc-editor.org/info/rfc8319.

Copyright Notice

Copyright © 2018 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Relationship between AdvDefaultLifetime and MaxRtrAdvInterval   3
   4.  Updates to RFC 4861 . . . . . . . . . . . . . . . . . . . . .   4
   5.  Host Behavior . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1. Introduction

IPv6 Neighbor Discovery relies on IP multicast based on the expectation that multicast makes efficient use of available bandwidth and avoids generating interrupts in the network nodes. On some data link layers, multicast may not be natively supported. On such links, any possible reduction of multicast traffic will be highly beneficial. Unfortunately, due to the fixed protocol constants specified in [RFC4861], it is difficult to relax the multicast timers for Neighbor Discovery. There are already clarifications specific to the link technology about how to tune the Neighbor Discovery Protocol (NDP) constants for certain systems in order to reduce excess NDP traffic. For example, [RFC6459] and [RFC7066] contain such clarifications for 3GPP cellular links.

This document specifies updates to the IPv6 Neighbor Discovery Protocol [RFC4861] to increase the maximum time allowed between sending unsolicited multicast RAs from a router interface as well as to increase the maximum router lifetime.

2. Requirements Language

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.

3. Relationship between AdvDefaultLifetime and MaxRtrAdvInterval

MaxRtrAdvInterval is an upper bound on the time between which two successive Router Advertisement messages are sent. Therefore, one might reason about the relationship between these two values in terms of a ratio K = AdvDefaultLifetime / MaxRtrAdvInterval, which expresses how many Router Advertisements are guaranteed to be sent before the router lifetime expires.

Assuming unicast Solicited Router Advertisements or a perfectly stable network, on a theoretically perfect link with no losses, it would be sufficient to have K just above 1, so that the sent Router Advertisement refreshes the router entry just before it expires. On the real links that allow for some loss, one would need to use K > 2 in order to minimize the chances of a single Router Advertisement loss causing a loss of the router entry.

The exact calculation will depend on the packet loss probability. An example: if we take a ballpark value of 1% probability of a packet loss, then K = 2 will give 0.01% chance of an outage due to a packet loss, K = 3 will give 0.0001% chance of an outage, and so forth. To reverse the numbers, with these parameters, K ~= 1 gives 99% reliability, K ~= 2 gives 99.99% reliability, and K ~= 3 gives 99.9999% reliability -- which should be good enough for a lot of scenarios.

In a network with higher packet loss probabilities or if higher reliability is desired, the K might be chosen to be even higher. On the other hand, some of the data link layers provide reliable delivery at Layer 2, so there one might even consider using the "theoretical" value of K just above 1. Since the choice of these two parameters does not impact interoperability per se, this document does not impose any specific constraints on their values other than providing the guidelines in this section. Therefore, each individual link can optimize according to its use case.

Also, AdvDefaultLifetime MUST be set to a value greater than or equal to the selected MaxRtrAdvInterval. Otherwise, a router lifetime is guaranteed to expire before the new Router Advertisement has a chance to be sent, thereby creating an outage.

4. Updates to RFC 4861

This document updates Sections 4.2 and 6.2.1 of [RFC4861] to change the following router configuration variables.

In Section 4.2, inside the paragraph that defines Router Lifetime, change 9000 to 65535 seconds.

In Section 6.2.1, inside the paragraph that defines MaxRtrAdvInterval, change 1800 to 65535 seconds.

In Section 6.2.1, inside the paragraph that defines AdvDefaultLifetime, change 9000 to 65535 seconds.

As explained in Section 3, the probability of packet loss must be considered when choosing the relationship between MaxRtrAdvInterval and AdvDefaultLifetime.

5. Host Behavior

Legacy hosts on a link with updated routers may have issues with a Router Lifetime of more than 9000 seconds. In the few implementations we have tested with general-purpose operating systems, there does not seem to be any issue with setting this field to more than 9000, but there might be implementations that incorrectly reject such RAs (since RFC 4861 requires receivers to handle any value).

6. Security Considerations

On a link where Router Advertisements are few and far between, the detrimental effects of a rogue router that sends an unsolicited RA are greatly increased. These rogue RAs can be prevented by using approaches like RA-Guard [RFC6105] and SEcure Neighbor Discovery (SEND) [RFC3971].

7. IANA Considerations

This document has no IANA actions.

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,
              <https://www.rfc-editor.org/info/rfc2119>.
   
   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.
   
   [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>.

8.2. Informative References

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <https://www.rfc-editor.org/info/rfc3971>.
   
   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              DOI 10.17487/RFC6105, February 2011,
              <https://www.rfc-editor.org/info/rfc6105>.
   
   [RFC6459]  Korhonen, J., Ed., Soininen, J., Patil, B., Savolainen,
              T., Bajko, G., and K. Iisakkila, "IPv6 in 3rd Generation
              Partnership Project (3GPP) Evolved Packet System (EPS)",
              RFC 6459, DOI 10.17487/RFC6459, January 2012,
              <https://www.rfc-editor.org/info/rfc6459>.
   
   [RFC7066]  Korhonen, J., Ed., Arkko, J., Ed., Savolainen, T., and S.
              Krishnan, "IPv6 for Third Generation Partnership Project
              (3GPP) Cellular Hosts", RFC 7066, DOI 10.17487/RFC7066,
              November 2013, <https://www.rfc-editor.org/info/rfc7066>.

Acknowledgements

The authors would like to thank the members of the 6MAN efficient ND design team for their comments that led to the creation of this document. The authors would also like to thank Lorenzo Colitti, Erik Kline, Jeena Rachel John, Brian Carpenter, Tim Chown, Fernando Gont, Warren Kumari, and Adam Roach for their comments and suggestions that improved this document.

Authors' Addresses

Suresh Krishnan
Kaloom
335 Rue Peel
Montreal, QC
Canada

Email:

          suresh@kaloom.com

Jouni Korhonen
Nordic Semiconductor ASA
Metsanneidonkuja 10
02130 Espoo
Finland

   Email: jouni.nospam@gmail.com

Samita Chakrabarti
Verizon
United States of America

   Email: samita.chakrabarti@verizon.com

Erik Nordmark
Zededa
Santa Clara, CA
United States of America

Email:

          nordmark@acm.org
   
   Andrew Yourtchenko
   Cisco
   6b de Kleetlaan
   Diegem  1831
   Belgium

Email:

          ayourtch@cisco.com