v6ops J. Linkova
Internet-Draft Google
Intended status: Informational December 10, 2019
Expires: June 12, 2020

Neighbor Cache Entries on First-Hop Routers: Operational Considerations
draft-ietf-v6ops-nd-cache-init-01

Abstract

Neighbor Discovery (RFC4861) is used by IPv6 nodes to determine the link-layer addresses of neighboring nodes as well as to discover and maintain reachability information. This document discusses how the neighbor discovery state machine on a first-hop router is causing user-visible connectivity issues when a new (not being seen on the network before) IPv6 address is being used.

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 June 12, 2020.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1. Introduction

The section 7.2.5 of [RFC4861] states: "When a valid Neighbor Advertisement is received (either solicited or unsolicited), the Neighbor Cache is searched for the target's entry. If no entry exists, the advertisement SHOULD be silently discarded. There is no need to create an entry if none exists, since the recipient has apparently not initiated any communication with the target."

This approach is perfectly suitable for host2host communications which are in most cases bi-directional and it could be expected that if a host A has an ND cache entry for the host B IPv6 address, the host B also has the corresponding ND entry for the host A address in its cache. However when a host communicates to off-link destinations via its first-hop router that logic does not apply. The most typical scenario when the problem may arise is a host joining the network, forming a new address and using that address for accessing the Internet:

  1. A host joins the network and receives a Router Advertisement packet from the first-hop router (either a periodic unsolicited RA or a response to an RS sent by the host). The RA contains information the host needs to perform SLAAC and to configure its network stack. As in most cases the RA also contains the Source link-layer address of the router, the host can populate its Neighbor Cache with the router link-local and link-layer addresses.
  2. The host starts opening connections to off-link destinations. Very common use case is a mobile device sending probes to detect the Internet connectivity and/or the captive portals presence on the network. To speed up that process many implementations use the Optimistic Duplicate Address Detection [RFC4429] which allows them to send probes from their GUA before the DAD process is completed. At that moment the device ND cache contains all information required to send those probes (such as the default gateway LLA and the link-layer address). The router ND cache, however, might contain an entry for the device link-local address (if the device has been performing the ND process for the roiter LLA) but there are no entries for the device GUA.
  3. Return traffic is received by the first-hop router. As the router does not have any ND cache entry for the host GUA yet, the router starts the neighbor discovery process by creating an INCOMPLETE cache entry and then sending an NS to the Solicited Node Multicast Address. Most router implementations buffer only one data packet while performing the neighbor discovery for the packet destination address, so it would drop all subsequent packets for the host GUA, until the address resolution process is completed.
  4. If the host sends multiple probes in parallel it would considered all but one of them failed. It leads to user-visible delay in connecting to the network, especially if the host implements some form of backoff mechanism and does not retransmit the porbes as soon as possible.

This scenario illustrates the problem happening when the device connects to the network for the first time or after a timeout long enough for the device address to be removed from the router neighbor cache. However the same sequence of events happen when the host starts using the new GUA previously unseen by the router, such as a new privacy address [RFC4941] or if the router Neighbor Cache has been flushed.

While in dual-stack networks this problem might hidden by Happy Eyeballs [RFC8305] it manifest quite clearly in IPv6-only environments, especially wireless ones, leading to poor user experience and contributing to negative perception of IPv6-only solutions as unstable and non-deployable.

1.1. 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.

1.2. Terminology

ND: Neighbor Discovery, [RFC4861].

SLAAC: IPv6 Stateless Address Autoconfiguration, [RFC4862].

NS: Neighbor Solicitation, [RFC4861].

NA: Neighbor Advertisement, [RFC4861].

RS: Router Solicitation, [RFC4861].

RA: outer Advertisement, [RFC4861].

SLLA: Source link-layer Address, an option in the ND packets containing the link-layer address of the sender of the packet, [RFC4861].

TLLA: Target link-layer Address, an option in the ND packets containing the link-layer address of the target, [RFC4861].

GUA: Global Unicast Address, [RFC4291].

DAD: Duplicate Address Detection, [RFC4862].

Optimistic DAD: a modification of DAD, [RFC4429].

2. Proposed Solution

2.1. Solution Requirements

It would be highly desirable to improve the Neighbor Discovery mechanics so routers have a usable cache entry for a host address by the time the first packet for that address is received by the router. In particular,

2.2. Solution Overview

The Neighbor Discovery is designed to allow IPv6 nodes to discover neighboring nodes reachability and learn IPv6 to link-layer addresses mapping. Therefore ND seems to be the most appropriate tool to inform the first-hop routers about addresses the host is going to use.

Section 4.4 of [RFC4861] says:

"A node sends Neighbor Advertisements in response to Neighbor Solicitations and sends unsolicited Neighbor Advertisements in order to (unreliably) propagate new information quickly."

Propagating information about new GUA as quickly as possible is exactly what is required to solve the problem outlined in this document. Therefore the host might send an unsolicited NA with the target link-layer address option to advertize its GUA as soon as the said address enters Optimistic or Preferred state.

The proposed solution is discussed in [I-D.linkova-6man-grand]. In summary the follwing changes to [RFC4861] are suggested:

It should be noted that some routing and switching platforms have implemented such behaviour already. Administrators could enable creating neighbor discovery cache entries based on unsolicited NA packets sent from the previously unknown neighbors on that interface.

3. Solutions Considered but Discarded

The problem could be addressed from different angles. Possible approaches are:

The following sections discuss those approaches in more detail.

3.1. Do Nothing

One of the possible approaches might be to declare that everything is working as intended and let the upper-layer protocols to deal with packet loss. The obvious drawbacks include:

3.2. Change to the Registration-Based Neighbor Discovery

The most radical approach would be to move away from the reactive ND as defined in [RFC4861] and expand the registration-based ND ([RFC6775], [RFC8505]) used in Low-Power Wireless Personal Area Networks (6LoWPANs) to the rest of IPv6 deployments. This option required some investigation and discussions and seems to be an overkill for the problem described in this document.

3.3. Host Sending NS to the Router Address from Its GUA

The host could force creating a STALE entry for its GUA in the router ND cache by sending the following Neighbor Solicitation message:

The main disadvantages of this approach are:

3.4. Host Sending Router Solicitation from its GUA

The host could send a router solicitation message to 'all routers' multicast address, using its GUA as a source. If the host link-layer address is included in the Source Link-Layer Address option, the router would create a STALE entry for the host GUA as per the section 6.2.6 of [RFC4861]. However this approach can not be used if the GUA is in optimistic state: the section 2.2 of [RFC4429] explicitly prohibits using an Optimistic Address as the source address of a Router Solicitation with a SLLAO as it might disrupt the rightful owner of the address in the case of a collision. So for the optimistic addresses the host can send an RS without SLLAO included. In that case the router may respond with either a multicast or a unicast RA (only the latter would create a cache entry).

This approach has the following drawbacks:

3.5. Routers Populating Their Caches by Gleaning From Neighbor Discovery Packets

Routers may be able to learn about new addresses by gleaning from the DAD Neighbor Solicitation messages. The router could listen to all solicited node multicast address groups and upon receiving a Neighbor Solicitation from the unspecified address search its Neighbor Cache for the solicitation's Target Address. If no entry exists the router may create an entry, set its reachability state to 'INCOMPLETE' and start the address resolution for that entry.

The same solution was proposed in [I-D.halpern-6man-nd-pre-resolve-addr]. Some routing vendors support such optimization already. However this approach has a number of drawbacks and therefore should not be used as the only solution:

3.6. Initiating Hosts2Routers Communication

The host may trigger the router to start the address resolution by sending a data packet such as ping or traceroute to its default router link-local address, using the GUA as a source address. As the RTT to the default gateway is lower than RTT to any off-link destinations it's quite likely that the router would start the neighbor discovery process for the host GUA before the first packet of the returning traffic arrives.

The downside of this approach includes:

3.7. Transit Dataplane Traffic From a New Address Triggering Address Resolution

When a router receives a transit packet it might check the presence of the neighbor cache entry for the packet source address and if the entry does not exist start address resolution process. This approach does ensure that a Neighbor Cache entry is proactively created every time a new, previously unseen GUA is used for sending offlink traffic. However this functionality needs to be limited to explicitly configured networks/interfaces, as the router needs to distinguish between onlink addresses (ones the router needs to have Neighbor Cache entries for) and the rest of the address space. In addition, implementing such functionality is much more complicated than all other solutions as it would involve complex data-control planes interaction.

4. IANA Considerations

This memo asks the IANA for no new parameters.

5. Security Considerations

This memo documents the operational issue and does not introduce any new security considerations. Security considerations of the proposed solution are discussed in the corresponding section of [I-D.linkova-6man-grand].

6. Acknowledgements

Thanks to the following people (in alphabetical order) for their review and feedback: Lorenzo Colitti, Igor Gashinsky, Tatuya Jinmei, Erik Kline, Warren Kumari, Michael Richardson, Pascal Thubert, Loganaden Velvindron, Eric Vyncke.

7. References

7.1. Normative References

[I-D.linkova-6man-grand] Linkova, J., "Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First-Hop Routers", Internet-Draft draft-linkova-6man-grand-01, November 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006.
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD) for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006.
[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.
[RFC4862] Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, September 2007.
[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E. and C. Bormann, "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC 6775, DOI 10.17487/RFC6775, November 2012.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Better Connectivity Using Concurrency", RFC 8305, DOI 10.17487/RFC8305, December 2017.
[RFC8505] Thubert, P., Nordmark, E., Chakrabarti, S. and C. Perkins, "Registration Extensions for IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018.

7.2. Informative References

[I-D.halpern-6man-nd-pre-resolve-addr] Chen, I. and J. Halpern, "Triggering ND Address Resolution on Receiving DAD-NS", Internet-Draft draft-halpern-6man-nd-pre-resolve-addr-00, January 2014.
[RFC4941] Narten, T., Draves, R. and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007.
[RFC6583] Gashinsky, I., Jaeggli, J. and W. Kumari, "Operational Neighbor Discovery Problems", RFC 6583, DOI 10.17487/RFC6583, March 2012.

Author's Address

Jen Linkova Google 1 Darling Island Rd Pyrmont, NSW 2009 AU EMail: furry@google.com