Neighbor Cache Entries on First-Hop Routers: Operational Considerations
draft-ietf-v6ops-nd-cache-init-00

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 April 30, 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
• 1.1. Requirements Language
• 1.2. Terminology
• 2. Potential Solutions
• 2.1. Do Nothing
• 2.1.1. Pros
• 2.1.2. Cons
• 2.2. Change to the Registration-Based Neighbor Discovery
• 2.3. Hosts Explicitly Advertizing Their GUAs Using Existing ND Mechanisms
• 2.3.1. Host Sending Unsolicited NA
• 2.3.1.1. Pros
• 2.3.1.2. Cons
• 2.3.2. Host Sending NS to the Router Address from Its GUA
• 2.3.2.1. Pros
• 2.3.2.2. Cons
• 2.3.3. Host Sending Router Solicitation from its GUA
• 2.3.3.1. Pros
• 2.3.3.2. Cons
• 2.4. Routers Populating Their Caches by Gleaning From Neighbor Discovery Packets
• 2.4.1. Pros
• 2.4.2. Cons
• 2.5. Initiating Hosts2Routers Communication
• 2.5.1. Pros
• 2.5.2. Cons
• 2.6. Tweaking Probing Algorithms
• 2.7. Routers Buffering More Packets
• 2.7.1. Pros
• 2.7.2. Cons
• 3. Recommendations
• 4. IANA Considerations
• 5. Security Considerations
• 6. Acknowledgements
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Author's Address

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. Here is the most typical scenario when the problem may arise:

1. When a host joins the network it receives an RA 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. Among other things the host populates its ND cache with the router link-local address and potentially link-layer address (if included in the RA Source Link-Layer Address option).
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 are using the Optimistic Duplicate Address Detection ([RFC4429]) which allows them to send probes from their GUA before the DAD process is completed. Imprortant point here is that 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. Response packets for the probes (or any other traffic sent by the host) are received by the first-hop router. As the router does not have any ND cache entry for the host GUA, the router starts the neighbor discovery process by creating an INCOMPLETE cache entry and then sending an NS to the Solicited Node Multicast Address. Apparently most of the router implementations buffer only one data packet while performing the ND process for its destination. Therefore all packets for the host GUA, except for the very first one are dropped until the address resolution process is completed.
4. As many implementations send multiple probes in parallel it's very likely that all probes ex. the first one would be considered failed. If the host implements an exponential backoff for probing it leads to user-noticeable delay in detecting network connectivity/reporting the network as usable.

The above-mentioned scenario illustrates the problem happening when the device connects to the network for the first time/after a long timeout. However the same sequence of events happen when the host starts using the new (previously unseen by the router or ) GUA (e.g. 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 manifests itself quite clearly in IPv6-only networks, 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: Router 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. Potential Solutions

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

• Just do nothing.
• Migrate from the "reactive" Neighbor Discovery ([RFC4861]) to the registration-based mechanisms ([RFC8505]).
• The host explicitly advertizes its GUAs using Neighbor Discovery mechanisms.
• The router creates new entries in its Neighbor Cache by gleaning from Neighbor Discovery DAD messages.
• The host initiates bidirectional communication to the router using the host GUA.
• Making the probing logic on hosts more robust.
• Increasing the buffer size on routers.

The following sections discuss those approaches in more detail.

2.1. Do Nothing

One of the possible approaches might be to declare that everything is working as intended.

2.1.1. Pros

• No work required.

2.1.2. Cons

• Unhappy users.
• Many support tickets.
• More resistance to deploy IPv6 and IPv6-Only networks.

2.2. 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..

2.3. Hosts Explicitly Advertizing Their GUAs Using Existing ND Mechanisms

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. The following sections discuss potential apptoaches in more detail.

2.3.1. Host Sending Unsolicited NA

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 to advertize its GUA as soon as the said address enters Optimistic or Preferred state. The NA should include the target link-layer address option. To ensure that all first-hop routers receive the advertisement it could be sent to all-routers multicast address (ff02::2).

As it's been mentioned, [RFC4861] explicitly states that receiving a NA should not create a new NC entry. However the justification for that requirement ("There is no need to create an entry if none exists, since the recipient has apparently not initiated any communication with the target.") clearly does not apply for the case discussed. As per [RFC2119] "there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.". Therefore routers creating a new NC entry upon receiving an unsolicited NA would still be compliant with [RFC4861].

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.

2.3.1.1. Pros

• Already implemented on some platforms.
• In accordance with [RFC4861].

2.3.1.2. Cons

• Allows a malicious host to execute an ND cache exhaustion attack. It's recommended that this functionality is configurable and recommendations from [RFC6583] are taken into account.
• Requires hosts to send unsolicited NA (changes to the hosts).
• Some wireless devices are known to fiddle with ND packets and perform various non-obvious forms of ND proxy actions. In some cases unsolicited NAs might not even reach the routers.

2.3.2. 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 NS source address is the host GUA.
• The Source Link-Layer Address option contains the host link-layer address.
• The target address is the host default gateway address (the default router address the host received in the RA).

The main disadvantage of this approach is that it would not work if the GUA the host needs to advertise is still in the Optimistic state. The section 2.2 of [RFC4429] explicitly prohibits sending Neighbor Solicitations from an Optimistic Address.

2.3.2.1. Pros

• Router implementations which follow recommendations made in [RFC6583] might prioritize responding to NS packets to own addresses.

2.3.2.2. Cons

• Does not work for Optimistic addresses (see section 2.2 of [RFC4429]).
• If first-hop redundancy is deployed in the network, the NS would reach the active router only, so all backup routers (or all active routers ex. one) would not get their neighbor cache updated.
• Some wireless devices are known to fiddle with ND packets and perform various non-obvious forms of ND proxy actions. In some cases unsolicited NAs might not even reach the routers.

2.3.3. 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 (see 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).

2.3.3.1. Pros

• Unlike NS packets, RS packets would reach all routers on link, allowing all routers to update their neighbor caches and preventing packet loss in case of asymmetric routing.

2.3.3.2. Cons

• As for the Optimistic addresses SLLAO can not be included into RS packets, the cache entry for the optimistic address would be created only if the router sends solicited RAs as unicast. In addition, there might be a random delay between receiving an RS and sending a unicast RA back (and creating a cache entry) which might undermine the idea of creating the cache entry proactively.
• Some wireless devices are known to fiddle with ND packets and perform various non-obvious forms of ND proxy actions. In some cases RSes might not even reach the routers.

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

If hosts do not send unsolicited NAs upon configuring new addresses as described above the routers may be able to learn about new address 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 for and set it's reachability state to 'INCOMPLETE'. Then the router can start the address resolution for the new entry.

2.4.1. Pros

• No changes required on hosts.

2.4.2. Cons

• Routers would receive all multicast Neighbor Discovery packets which might negatively impact the routers CPU.
• If the router starts the address resolution as soon as it receives the DAD Neighbor Solicitation the host might be still performing the DAD and the target address might be tentative. In that case the host SHOULD silently ignore the received Neighbor Solicitation from the router as per the Section 5.4.3 of [RFC4862]. Such race condition scenario would prevent the router to learn the new address.

2.5. Initiating Hosts2Routers Communication

Every time the host configures a new GUA (when the address enters the Optimistic state or, if the optimistic DAD is not used, as soon as it changes the state from tentative to preferred) the host can a ping or traceroute packet to the default gateway LLA. 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. There are pretty good chances that the process would be completed before the actual data traffic reaches the router.

2.5.1. Pros

• As data packets are involved, there is no potential impact caused by smart wireless infrastructure performing ND proxy.
• Full compliance with existing standards.

2.5.2. Cons

• Data packets to the router LLA could be blocked by security policy or control plane protection mechanism.
• Maximum overhead for routers control plane (in addition to processing ND packets, the data packet needs to be processed as well).
• If the first hop redundancy is implemented in the network the host ping/traceroute packet would reach the active router only. All backup routers would not receive it and therefore would not start populating the cache. So in the case of asymmetric traffic flow (packets leave the network via one router while the return flow is going via another) the backup router(s) still would not have the cache entry. (A hacky way to overcome this limitation would be sending ping/traceroute packet to 'all routers' ff02::2 multicast address).

2.6. Tweaking Probing Algorithms

While tweaking the probing logic on devices might make the problem less visible it would be still desirable to avoid packet loss everytime the new GUA is used by a host. It would be quite tricky to adjust every probing algorithm to find the right balance between prompt detection of network connectivity and false positives in IPv6-only mode.

2.7. Routers Buffering More Packets

Another way to mitigate the issue, at least partially, would be increasing the number of packets the router could buffer while performing the neighbor discovery process for the INCOMPLETE cache entry. However it would be against recommendations made in the section 7.2.2 of [RFC4861] and [RFC6583].

2.7.1. Pros

• Does not require changes on hosts.

2.7.2. Cons

• This approach makes the routers even more vulnerable to attack vectors described in [RFC6583]. In particular, it would amplify the impact of any scanning attack.
• Against the recommendations from the section 7 of [RFC6583].
• Requires router vendors support.

3. Recommendations

• Hosts SHOULD send at least one unsolicited NA packet to all-routers multicast address (ff02::2) as soon as one of the following events happens:
  • (if Optimistic DAD is used): a new Optimistic GUA is assigned to the host interface.
  • (if Optimistic DAD is not used): a GUA changes the state from tentative to preferred.

• Routers SHOULD have a configuration knob to enable creating ND cache entry upon receiving unsolicited NAs on a specific interface. This document does not change the behavior if the ND cache entry already exists when receiving an unsolicited NA.

As the recommendations include modification to Neighbor Discovery state machine defined in [RFC4861] and hosts behaviour, they are discussed in a separate Standart track document draft-linkova-6man-grand.

4. IANA Considerations

This memo asks the IANA for no new parameters.

5. Security Considerations

See the Security Considerations section of draft-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

7.2. Informative References

Author's Address