Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First-Hop Routers
draft-ietf-6man-grand-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 updates [RFC4861] to allow routers to proactively create a Neighbor Cache entry when a new IPv6 address is assigned to a host. It also updates [RFC4862] and recommends hosts to send unsolicited Neighbor Advertisements upon assigning a new IPv6 address. The proposed change will minimize the delay and packet loss when a host initiate connections to off-link destination from a new IPv6 address.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on September 10, 2020.

Copyright Notice

Copyright (c) 2020 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. Proposed Changes to Neighbor Discovery
• 2.1. Hosts Sending Gratuitous Neighbor Advertisements
• 2.2. Routers Creating Cache Entries Upon Receiving Unsolicited Neighbor Advertisements
• 3. Avoiding Disruption
• 3.1. Neighbor Cache Entry Exists in Any State Other That INCOMPLETE
• 3.2. Neighbor Cache Entry Does Not Exist
• 3.3. Neighbor Cache Entry is in INCOMPLETE state
• 4. Modifications to RFC-Mandated Behavior
• 4.1. Modification to RFC4861 Neighbor Discovery for IP version 6 (IPv6)
• 4.1.1. Modification to the section 7.2.5
• 4.1.2. Modification to the section 7.2.6
• 5. IANA Considerations
• 6. Security Considerations
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Author's Address

1. Introduction

The Neighbor Discovery state machine defined in [RFC4861] implies that communications between IPv6 nodes are in most cases bi-directional and if a host A is trying to communicate to its neighbor, host B, the return traffic flows could be expected. So when the host A starts the address resolution process, the target host would also create an entry for the host A address in its neighbor cache. That entry will be used for sending the return traffic to the host A.

However when a host sends traffic to off-link destinations a different scenario is observed. After receiving a Router Advertisement the host populates its neighbor cache with the default router IPv6 and link-layer addresses and is able to send traffic to off-link destinations. At the same time the router does not have any cache entries for the host global addresses yet and only starts address resolution upon receiving the first packet of the return traffic flow. While waiting for the resolution to complete routers only keep a very small number of packets in the queue (as recommended in [RFC4861] Section 7.2.2. All subsequent packets arriving before the resolution process finishes are likely to be dropped. It might cause user-visible packet loss and performance degradation

The detailed problem statement and the various solution approaches could be found in [I-D.ietf-v6ops-nd-cache-init]. This document summarizes the proposed neighbor discovery updates to address the issue.

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].

LLA: Link-Layer Address.

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 Changes to Neighbor Discovery

The following changes are proposed to minimize the delay in creating new entries in a router neighbor cache

• A host SHOULD send unsolicited NAs upon assigning a new IPv6 address to its interface.
• A router SHOULD create a new cache entry upon receiving an unsolicited NA from a host.

The following sections discuss these changes in more detail.

2.1. Hosts Sending Gratuitous Neighbor Advertisements

The section 7.2.6 of [RFC4861] discusses using unsolicited Neighbor Advertisement to inform node neighbors of the new link-layer address quickly. The same mechanism could be used to notify the host neighbors about the new network-layer address as well: the host can send gratuitous unsolicited Neighbor Advertisements upon assigning a new global IPv6 address to its interface.

To minimize the potential disruption in case of duplicate addresses the host SHOULD NOT set the Override flag for a preferred address and MUST NOT set the Override flag if the address is in Optimistic [RFC4429] state.

As the main purpose of sending unsolicited NAs upon configuring a new address is to proactively create a Neighbor Cache entry on the first-hop routers, the gratuitous NAs SHOULD be sent to all-routers multicast address (ff02::2). Limiting the recipients to routers only would help reduce the multicast noise level. If the link-layer devices are performing MLD snooping [RFC4541] then those unsolicited NAs will be only sent to onlink routers instead of being flooded to all nodes.

It should be noted that the proposed mechanism does not cause any significant increase in the multicast traffic. The additional multicast unsolicited NA would proactively create a STALE cache entry on routers as discussed below. When the router receives the return traffic flows it does not need to send multicast NSes to the solicited node multicast address but would be sending unicast NSes instead. Therefore total amount of multicast traffic should not increase.

Another option to reduce multicast noises would be sending the gratuitous NAs as unicast to all router addresses. However such approach has a serious disadvantage as it requires the host to have the complete list of routers on link and their link-layaer addresses. If not all routers are kept in the Default Router list ([RFC4861] requires a node to keep at least two entries), the unsolicited NA would reach only subset of routers, not nessesary the routers receiving the return traffic flows. If the network provides a first-hop router redundancy traffic flows can be asymmetrical: the host can send traffic to one router while the return packets enters the network via another one. So the router the host is using as its default gateway (and would send a unicast gratuitous NA to) might not be the router which needs the cache entry to be created. In addition, a race condition may occur, if RAs from some routers are delayed and arrive after the unsolicited NA has been sent.

As number of routers on a link is expected to be quite small, hosts could send the the multicast gratuitous NAs as Ethernet unicasts, mapping the IPv6 all-routers multicast address ff02::2 to routers Ethernet unicast addresses as per [RFC6085]. This approach would also mitigate the risk of informing an on-link attacker about IPv6 addresses assigned to the host. However it has the same disadvantages as sending unicast NAs: the routers the NA is sent to might not be ones routing the return traffic.

2.2. Routers Creating Cache Entries Upon Receiving Unsolicited Neighbor Advertisements

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".

The reasoning behind dropping unsolicited Neighbor Advertisements ("the recipient has apparently not initiated any communication with the target") is valid for onlink host-to-host communication but, as discussed in [I-D.ietf-v6ops-nd-cache-init] it does not really apply for the scenario when the host is announcing its address to routers. Therefore it would be beneficial to allow routers creating new entries upon receiving an unsolicited Neighbor Advertisement.

This document suggests that routers SHOULD create a new Neighbor Cache entry when receive an unsolicited Neighbor Advertisement.

3. Avoiding Disruption

If hosts following the recommendations in this document are using the DAD mechanism defined in [RFC4862], they would send unsolicited NA as soon as the address changes the state from tentative to preferred (after its uniqueness has been verified). However hosts willing to minimize network stack configuration delays might be using optimistic addresses, which means there is a possibility of the address not being unique on the link. The section 2.2 of [RFC4429] discusses measures to ensure that ND packets from the optimistic address do not override any existing neighbor cache entries as it would cause traffic interruption of the rightful address owner in case of address conflict. As hosts willing to speed up their network stack configuration are most likely to be affected by the problem outlined in this document it seems reasonable for such hosts to advertise their optimistic GUAs by sending unsolicited NAs. The main question to consider is the potential risk of overriding the cache entry for the rightful address owner if the optimistic address happens to be duplicated.

3.1. Neighbor Cache Entry Exists in Any State Other That INCOMPLETE

If the router Neighbor Cache entry for the target address already exists in any state other than INCOMPLETE, then as per section 7.2.5 of [RFC4861] an unsolicited NA with the Override flag cleared would change the entry state from REACHABLE to STALE but would not update the entry in any other way. Therefore even if the host sends an unsolicited NA from the its Optimistic address the router cache entry would not be updated with the new Link-Layer address and no impact to the traffic for the rightful address owner is expected.

3.2. Neighbor Cache Entry Does Not Exist

If there is no entry then it would be created/updated with the supplied LLA and its state set to STALE. In that case as soon as the entry is used for sending traffic to the host, the entry state will be changed to DELAY, then PROBE and the unicast NS will be send. If the DAD process has already failed, the host with the duplicated address would not respond to the unicast NSes. The router will then send multicast NSes which would reach the rightful owner of the address and its LLA will be added to the routerND cache. So in the scenario when the rightful owner does not use the address for communication then it might be a short (a few seconds) period of time when the data packets sent from the outside could reach the host with the optimistic address. However it seems likely that hosts using Optimistic DAD would start sending/receiving traffic right away, so the first return packet would trigger the NUD process and rewrite the cache.

3.3. Neighbor Cache Entry is in INCOMPLETE state

Another corner case is the INCOMPLETE cache entry for the address. If the host sends an unsolicited NA from the Optimistic address it would update the entry with the host LLA and set the entry to the STALE state. As the INCOMPLETE entry means that the router has started the ND process for the address and the multicast NS has been sent, the rightful owner is expected to reply with solicited NA with the Override flag set. Upon receiving a solicited NA with the Override flag the cache entry will be updated with the TLLA supplied and (as the NA has the Solicited flag set), the entry state will be set to REACHABLE. It would recover the cache entry and set the LLA to the one of the rightful owner. The only potential impact would be for packets arriving to the router after the unsolicited NA from the host but before the rightful owner responded with the solicited NA. Those packets would be sent to the host with the optimistic address instead of its rightful owner. However those packets would have been dropped anyway as until the solicited NA is received the router can not send the traffic.

4. Modifications to RFC-Mandated Behavior

All normative text in this memo is contained in this section.

4.1. Modification to RFC4861 Neighbor Discovery for IP version 6 (IPv6)

4.1.1. Modification to the section 7.2.5

This document proposes the following changes to the section 7.2.5 of [RFC4861]:

------------------------------------------------------------------

OLD TEXT:

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.

NEW TEXT:

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, hosts SHOULD silently discard the advertisement. There is no need to create an entry if none exists, since the recipient has apparently not initiated any communication with the target. Routers SHOULD create a new entry for the target address with the link-layer address set to the Target link-layer address option (if supplied). The entry its reachability state MUST also be set to STALE. If the received Neighbor Advertisement does not contain the Target link-layer address option the advertisement SHOULD be silently discarded.

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4.1.2. Modification to the section 7.2.6

This document proposes the following changes to the section 7.2.6 of [RFC4861]:

OLD TEXT:

In such cases, a node MAY send up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited Neighbor Advertisement messages to the all-nodes multicast address. These advertisements MUST be separated by at least RetransTimer seconds.

NEW TEXT:

In such cases, a node MAY send up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited Neighbor Advertisement messages to the all-nodes multicast address. These advertisements MUST be separated by at least RetransTimer seconds.

A host may also wish to notify its first-hop routers when it configures a new global IPv6 address so the routers can proactively populate their neighbor caches with the corresponding entries. In such cases a host SHOULD send up to MAX_NEIGHBOR_ADVERTISEMENT Neighbor Advertisement messages. If the address is preferred then the Override flag SHOULD NOT be set. If the address is in the Optimistic state then the Override flag MUST NOT be set. The destination address SHOULD be set to the all-routers multicast address. These advertisements MUST be separated by at least RetransTimer seconds. The first advertisement SHOULD be sent as soon as one of the following events happens:

• if Optimistic DAD [RFC4429] 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.

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5. IANA Considerations

This memo asks the IANA for no new parameters.

6. Security Considerations

One of the potential attack vectors to consider is a cache spoofing when the attacker might try to install a cache entry for the victim's IPv6 address and the attacker's Link-Layer address. However it should be noted that this document does not propose any changes for the scenario when the ND cache for the given IPv6 address already exists. Therefore it is not possible for the attacker to override any existing cache entry.

A malicious host could attempt to exhaust the neighbor cache on the router by creating a large number of STALE entries. However this attack vector is not new and this document does not increase the risk of such an attack: the attacker could do it, for example, by sending a NS or RS packet with SLLAO included. All recommendations from [RFC6583] still apply.

Announcing a new address to all-routers multicast address may inform an on-link attacker about IPv6 addresses assigned to the host. However hiding information about the specific IPv6 address should not be considered a security measure as such information is usually disclosed via DAD to all nodes anyway. Network administrators can also mitigate this issue by enabling MLD snooping on the link-layer devices to prevent IPv6 link-local multicast packets being flooded to all onlink nodes. If peer-to-peer onlink communications are not desirable for the given network segment they should be prevented by proper layer2 security mechanisms. Therefore the risk of allowing hosts to send unsolicited Neighbor Advertisements to all-routers multicast address is low. Should the issue needs to be mitigated on the host level, the host can send unsolicited NAs to its routers Ethernet unicast addresses as described in Section 2.1.

It should be noted that the proposed mechanism allows hosts to proactively inform their routers about global IPv6 addresses existing on-link. Routers could use that information to distinguish between used and unused addresses to mitigate ND cache exhaustion DoS attacks described in Section 4.3.2 [RFC3756] and [RFC6583].

7. Acknowledgements

Thanks to the following people (in alphabetical order) for their comments, review and feedback: Lorenzo Colitti, Fernando Gont, Tatuya Jinmei, Erik Kline, Warren Kumari, Erik Nordmark, Michael Richardson, Mark Smith, Dave Thaler, Pascal Thubert, Loganaden Velvindron, Eric Vyncke.

8. References

8.1. Normative References

8.2. Informative References

Author's Address