IPv6 Maintenance J. Linkova Internet-Draft Google Updates: 4861 (if approved) September 13, 2020 Intended status: Standards Track Expires: March 17, 2021 Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First- Hop Routers draft-ietf-6man-grand-02 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 node. It also updates RFC4861 and recommends nodes to send unsolicited Neighbor Advertisements upon assigning a new IPv6 address. The proposed change will minimize the delay and packet loss when a node 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/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on March 17, 2021. Copyright Notice Copyright (c) 2020 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 Linkova Expires March 17, 2021 [Page 1]
Internet-Draft Gratuitous ND September 2020 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 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4 3. Solution Requirements . . . . . . . . . . . . . . . . . . . . 5 4. Proposed Changes to Neighbor Discovery . . . . . . . . . . . 6 4.1. Nodes Sending Gratuitous Neighbor Advertisements . . . . 6 4.2. Routers Creating Cache Entries Upon Receiving Unsolicited Neighbor Advertisements . . . . . . . . . . . . . . . . . 7 5. Avoiding Disruption . . . . . . . . . . . . . . . . . . . . . 7 5.1. Neighbor Cache Entry Exists in Any State Other That INCOMPLETE . . . . . . . . . . . . . . . . . . . . . . . 8 5.2. Neighbor Cache Entry is in INCOMPLETE state . . . . . . . 8 5.3. Neighbor Cache Entry Does Not Exist . . . . . . . . . . . 8 5.3.1. The Rightful Owner Is Not Sending Packets From The Address . . . . . . . . . . . . . . . . . . . . . . . 9 5.3.2. The Rightful Owner Has Started Sending Packets From The Address . . . . . . . . . . . . . . . . . . . . . 10 6. Modifications to RFC-Mandated Behavior . . . . . . . . . . . 11 6.1. Modification to RFC4861 Neighbor Discovery for IP version 6 (IPv6) . . . . . . . . . . . . . . . . . . . . . . . . 11 6.1.1. Modification to the section 7.2.5 . . . . . . . . . . 11 6.1.2. Modification to the section 7.2.6 . . . . . . . . . . 12 7. Solutions Considered but Discarded . . . . . . . . . . . . . 12 7.1. Do Nothing . . . . . . . . . . . . . . . . . . . . . . . 13 7.2. Change to the Registration-Based Neighbor Discovery . . . 13 7.3. Host Sending NS to the Router Address from Its GUA . . . 13 7.4. Host Sending Router Solicitation from its GUA . . . . . . 14 7.5. Routers Populating Their Caches by Gleaning From Neighbor Discovery Packets . . . . . . . . . . . . . . . . . . . . 15 7.6. Initiating Hosts-to-Routers Communication . . . . . . . . 15 7.7. Transit Dataplane Traffic From a New Address Triggering Address Resolution . . . . . . . . . . . . . . . . . . . 16 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 9. Security Considerations . . . . . . . . . . . . . . . . . . . 16 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 11.1. Normative References . . . . . . . . . . . . . . . . . . 17 11.2. Informative References . . . . . . . . . . . . . . . . . 18 Linkova Expires March 17, 2021 [Page 2]
Internet-Draft Gratuitous ND September 2020 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19 1. Introduction The Neighbor Discovery state machine defined in [RFC4861] assumes that communications between IPv6 nodes are in most cases bi- directional and if a node A is trying to communicate to its neighbor, node B, the return traffic flows could be expected. So when the node A starts the address resolution process, the target node B would also create an entry for A address in its neighbor cache. That entry will be used for sending the return traffic to A. In particular, 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." While this approach is perfectly suitable for host-to-host on-link communications, it does not work so well when a host sends traffic to off-link destinations. After joining the network and 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 Section 7.2.2 [RFC4861]. All subsequent packets arriving before the resolution process finishes are likely to be dropped. It might cause user-visible packet loss and performance degradation. 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 Node: a device that implements IP, [RFC4861]. Host: any node that is not a router, [RFC4861]. Linkova Expires March 17, 2021 [Page 3]
Internet-Draft Gratuitous ND September 2020 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. Problem Statement 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 (RA) packet from the first-hop router (either a periodic unsolicited RA or a response to a Router Solicitation 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 link-layer address of the router, the host can populate its Neighbor Cache with the router's link-local and link-layer addresses. 2. The host starts opening connections to off-link destinations. A very common use case is a mobile device sending probes to detect the Internet connectivity and/or the presence of a captive portal on the network. To speed up that process many implementations use Optimistic DAD which allows them to send probes before the DAD process is completed. At that moment the device neighbor cache contains all information required to send those probes (such as the default router link-local the link-layer addresses). Linkova Expires March 17, 2021 [Page 4]
Internet-Draft Gratuitous ND September 2020 The router neighbor cache, however, might contain an entry for the device link-local address (if the device has been performing the address resolution for the router link-local address), but there are no entries for the device global addresses. 3. Return traffic is received by the first-hop router. As the router does not have any cache entry for the host global address yet, the router starts the neighbor discovery process by creating an INCOMPLETE cache entry and then sending a Neighbor Solicitation to the Solicited Node Multicast Address. Most router implementations buffer only one data packet while resolving the packet destination address, so it would drop all subsequent packets for the host global address, until the address resolution process is completed. 4. If the host sends multiple probes in parallel, it would consider all but one of them failed. That 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 probes as soon as possible. This scenario illustrates the problem occurring 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's neighbor cache. However, the same sequence of events happen when the host starts using a new global address previously unseen by the router, such as a new privacy address [RFC4941] or if the router's Neighbor Cache has been flushed. While in dual-stack networks this problem might be hidden by Happy Eyeballs [RFC8305] it manifests quite clearly in IPv6-only environments, especially wireless ones, leading to poor user experience and contributing to a negative perception of IPv6-only solutions as unstable and non-deployable. 3. 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 router receives the first packet for that address. In particular: o If the router does not have a Neighbor Cache entry for the address, a STALE entry needs to be created. o The solution needs to work for Optimistic addresses as well. Devices implementing the Optimistic DAD usually attempt to minimize the delay in connecting to the network and therefore are Linkova Expires March 17, 2021 [Page 5]
Internet-Draft Gratuitous ND September 2020 more likely to be affected by the problem described in this document. o In case of duplicate addresses present in the network, the proposed solution MUST NOT override the existing entry. o In topologies with multiple first-hop routers the cache needs to be updated on all of them, as traffic might be asymmetric: outgoing flows leaving the network via one router while the return traffic enters the segment via another one. In addition the solution MUST NOT exacerbate issues described in [RFC6583] and MUST be compatible with the recommendations provided in [RFC6583]. 4. Proposed Changes to Neighbor Discovery The following changes are proposed to minimize the delay in creating new entries in a router neighbor cache o A node sends unsolicited NAs upon assigning a new IPv6 address to its interface. o A router creates a new cache entry upon receiving an unsolicited NA from a host. The following sections discuss these changes in more detail. 4.1. Nodes 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 node neighbors about the new network-layer address as well: the node can send gratuitous unsolicited Neighbor Advertisements upon assigning a new IPv6 address to its interface. To minimize the potential disruption in case of duplicate addresses the node 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 are 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 Linkova Expires March 17, 2021 [Page 6]
Internet-Draft Gratuitous ND September 2020 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. 4.2. 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 above, 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 updates [RFC4861] so that routers create a new Neighbor Cache entry upon receiving an unsolicited Neighbor Advertisement. The proposed changes do not modify routers behaviour specified in [RFC4861] for the scenario when the corresponding Neighbor Cache entry already exists. 5. Avoiding Disruption If nodes 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 nodes 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 Linkova Expires March 17, 2021 [Page 7]
Internet-Draft Gratuitous ND September 2020 conflict. As nodes 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 addresses 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. The following sections are discussing the address collision scenario when a node sends an unsolicited NA for an address in the Optimistic state, while another node has the same address assigned already. 5.1. 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. 5.2. 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 link-layer address 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 link-layer address 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. 5.3. Neighbor Cache Entry Does Not Exist There are two distinct scenarios which can lead to the situation when the router does not have a NC entry for the IPv6 address: Linkova Expires March 17, 2021 [Page 8]
Internet-Draft Gratuitous ND September 2020 1. The rightful owner of the address has not been using it for communication. 2. The rightful owner just started sending packets from that address but the router has not received any return traffic yet. The impact on the rightful owner's traffic flows would be different in those cases. 5.3.1. The Rightful Owner Is Not Sending Packets From The Address In this scenario the following events are expected to happen: 1. The host configures the address and sets its state to Optimistic. 2. The host sends an unsolicited NA with the Override flag set to zero and starts sending traffic from the Optimistic address. 3. The router creates a STALE entry for the address and the host link-layer address. 4. The host starts DAD and detects the address duplication. 5. The router receives the return traffic for the duplicated address. As the NC entry is STALE it sends traffic using that entry, changes it to DELAY and wait up to DELAY_FIRST_PROBE_TIME ([RFC4861]) seconds. 6. The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT ([RFC4861]) unicast NSes separated by RetransTimer milliseconds ([RFC4861]) to the host link-layer address. 7. As the host has detected the address conflict already it does not respond to the unicast NSes. 8. The router sends a multicast NS to the solicited node multicast address, the rightful owner responds and the router NC entry is updated with the rightful owner link-local address. The rightful owner is not experiencing any disruption as it does not send/receive any traffic. If after step 7 the router keeps receiving any return traffic for communication initiated at step 2, those packets would be forwarded to the rightful owner. However the same behaviour would be observed if changes proposed in this document are implemented: if the host starts sending packets from its Optimistic address but then changed the address state to Duplicated, almost all return traffic would be forwarded to the rightful owner of the said Linkova Expires March 17, 2021 [Page 9]
Internet-Draft Gratuitous ND September 2020 address. Therefore it's safe to conclude that the proposed changes do not cause any disruption for the rightful owner. 5.3.2. The Rightful Owner Has Started Sending Packets From The Address In this scenario the following events are happening: 1. The rightful owner starts sending traffic from the address (e.g. the address has just been configured or has not been recently used). 2. The host configures the address and sets its state to Optimistic. 3. The host sends an unsolicited NA with the Override flag set to zero and starts sending traffic from the Optimistic address. 4. The router creates a STALE entry for the address and the host link-layer address. 5. The host starts DAD and detects the address duplication. 6. The router receives the return traffic flows for both the rightful owner of the duplicated address and the new host. As the NC entry is STALE it sends traffic using that entry, changes it to DELAY and wait up to DELAY_FIRST_PROBE_TIME ([RFC4861]) seconds. 7. The router changes the NC entry state to PROBE and sends up to MAX_UNICAST_SOLICIT ([RFC4861]) unicast NSes separated by RetransTimer milliseconds ([RFC4861]) to the host link-layer address. 8. As the host has detected the address conflict already it does not respond to the unicast NSes. 9. The router sends a multicast NS to the solicited node multicast address, the rightful owner responds and the router NC entry is updated with the rightful owner link-local address. As a result the traffic for the address rightful owner would be sent to the host with the duplicated address instead. The duration of the disruption can be estimated as DELAY_FIRST_PROBE_TIME*1000 + (MAX_UNICAST_SOLICIT - 1)*RetransTimer milliseconds. As per the constants defined in Section 10 of [RFC4861] this interval is equal to 5*1000 + (3 - 1)*1000 = 7000ms or 7 seconds. Linkova Expires March 17, 2021 [Page 10]
Internet-Draft Gratuitous ND September 2020 However it should be noted that the probability of such scenario is rather low as it would require the following things to happen almost simultaneously (within tens of milliseconds): o One host starts using a new IPv6 address and sending traffic. o Another host configures the same IPv6 address in Optimistic mode before the router receives the return traffic for the first host. 6. Modifications to RFC-Mandated Behavior All normative text in this memo is contained in this section. 6.1. Modification to RFC4861 Neighbor Discovery for IP version 6 (IPv6) 6.1.1. 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. ------------------------------------------------------------------ Linkova Expires March 17, 2021 [Page 11]
Internet-Draft Gratuitous ND September 2020 6.1.2. Modification to the section 7.2.6 This document proposes the following changes to the section 7.2.6 of [RFC4861]: OLD TEXT: Also, a node belonging to an anycast address MAY multicast unsolicited Neighbor Advertisements for the anycast address when the node's link-layer address changes. NEW TEXT: Also, a node belonging to an anycast address MAY multicast unsolicited Neighbor Advertisements for the anycast address when the node's link-layer address changes. A node 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 node 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: o if Optimistic DAD [RFC4429] is used: a new Optimistic address is assigned to the node interface. o if Optimistic DAD is not used: an address changes the state from tentative to preferred. ------------------------------------------------------------------ 7. Solutions Considered but Discarded There are other possible approaches to address the problem, for example: o Just do nothing. o Migrating from the "reactive" Neighbor Discovery ([RFC4861]) to the registration-based mechanisms ([RFC8505]). Linkova Expires March 17, 2021 [Page 12]
Internet-Draft Gratuitous ND September 2020 o Creating new entries in routers Neighbor Cache by gleaning from Neighbor Discovery DAD messages. o Initiates bidirectional communication from the host to the router using the host GUA. o Making the probing logic on hosts more robust. o Increasing the buffer size on routers. o Transit dataplane traffic from an unknown address (an address w/o the corresponding neighbor cache entry) triggers an address resolution process on the router. It should be noted that some of those options are already implemented by some vendors. The following sections discuss those approaches and the reasons they were discarded. 7.1. 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: o Unhappy users. o Many support tickets. o More resistance to deploy IPv6 and IPv6-Only networks. 7.2. 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 requires some investigation and discussions and seems to be excessive for the problem described in this document. 7.3. 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: o The NS source address is the host GUA. o The destination address is the default router IPv6 address. Linkova Expires March 17, 2021 [Page 13]
Internet-Draft Gratuitous ND September 2020 o The Source Link-Layer Address option contains the host link-layer address. o The target address is the host default router address (the default router address the host received in the RA). The main disadvantages of this approach are: o Would not work for Optimistic addresses as section 2.2 of [RFC4429] explicitly prohibits sending Neighbor Solicitations from an Optimistic Address. o 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 except one) would not get their neighbor cache updated. o Some wireless devices are known to alter ND packets and perform various non-obvious forms of ND proxy actions. In some cases, unsolicited NAs might not even reach the routers. 7.4. 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: 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: o If the address is in the Optimistic state the RS can not contain SLLAO. As a result the router would only create a cache entry if solicited RAs are sent as unicast. Routers sending solicited RAs as multicast would not create a new cache entry as they do not need to send a unicast packet back to the host. o 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. Linkova Expires March 17, 2021 [Page 14]
Internet-Draft Gratuitous ND September 2020 o Some wireless devices are known to intercept ND packets and perform various non-obvious forms of ND proxy actions. In some cases the RS might not even reach the routers. 7.5. 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: o Routers need to receive all multicast Neighbor Discovery packets which might negatively impact the routers CPU. o If the router starts the address resolution as soon as it receives the DAD Neighbor Solicitation the host might be still performing 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]. As a result the router might not be able to complete the address resolution before the return traffic arrives. 7.6. Initiating Hosts-to-Routers Communication The host may force the router to start 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 router 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. This approach has the following drawbacks: o Data packets to the router link-local address could be blocked by security policy or control plane protection mechanism. Linkova Expires March 17, 2021 [Page 15]
Internet-Draft Gratuitous ND September 2020 o It introduces an additional overhead for routers control plane (in addition to processing ND packets, the data packet needs to be processed as well). o Unless the data packet is sent to 'all routers' ff02::2 multicast address, if the network provides a first-hop redundancy then only the active router would create a new cache entry. 7.7. 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 approach has a number of limitations, in particular: o If traffic flows are asymmetrical the return traffic might not transit the same router as the original traffic which triggered the address resolution. So the neighbor cache entry is created on the "wrong" router, not the one which actually needs the neighbor cache entry for the host address. o The 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. o Implementing such functionality is much more complicated than all other solutions as it would involve complex data-control planes interaction. 8. IANA Considerations This memo asks the IANA for no new parameters. 9. 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. Linkova Expires March 17, 2021 [Page 16]
Internet-Draft Gratuitous ND September 2020 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. 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]. 10. Acknowledgements Thanks to the following people (in alphabetical order) for their comments, review and feedback: Mikael Abrahamsson, Stewart Bryant, Lorenzo Colitti, Owen DeLong, Igor Gashinsky, Fernando Gont, Tatuya Jinmei, Erik Kline, Warren Kumari, Barry Leiba, Jordi Palet Martinez, Erik Nordmark, Michael Richardson, Mark Smith, Dave Thaler, Pascal Thubert, Loganaden Velvindron, Eric Vyncke. 11. References 11.1. 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>. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006, <https://www.rfc-editor.org/info/rfc4291>. Linkova Expires March 17, 2021 [Page 17]
Internet-Draft Gratuitous ND September 2020 [RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD) for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006, <https://www.rfc-editor.org/info/rfc4429>. [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>. [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, September 2007, <https://www.rfc-editor.org/info/rfc4862>. [RFC6775] Shelby, Z., Ed., 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, <https://www.rfc-editor.org/info/rfc6775>. [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>. [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Better Connectivity Using Concurrency", RFC 8305, DOI 10.17487/RFC8305, December 2017, <https://www.rfc-editor.org/info/rfc8305>. [RFC8505] Thubert, P., Ed., 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, <https://www.rfc-editor.org/info/rfc8505>. 11.2. Informative References [I-D.halpern-6man-nd-pre-resolve-addr] Chen, I. and J. Halpern, "Triggering ND Address Resolution on Receiving DAD-NS", draft-halpern-6man-nd-pre-resolve- addr-00 (work in progress), January 2014. [RFC3756] Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6 Neighbor Discovery (ND) Trust Models and Threats", RFC 3756, DOI 10.17487/RFC3756, May 2004, <https://www.rfc-editor.org/info/rfc3756>. Linkova Expires March 17, 2021 [Page 18]
Internet-Draft Gratuitous ND September 2020 [RFC4541] Christensen, M., Kimball, K., and F. Solensky, "Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006, <https://www.rfc-editor.org/info/rfc4541>. [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, <https://www.rfc-editor.org/info/rfc4941>. [RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational Neighbor Discovery Problems", RFC 6583, DOI 10.17487/RFC6583, March 2012, <https://www.rfc-editor.org/info/rfc6583>. Author's Address Jen Linkova Google 1 Darling Island Rd Pyrmont, NSW 2009 AU Email: furry@google.com Linkova Expires March 17, 2021 [Page 19]
