Internet DRAFT - draft-ietf-6man-grand
draft-ietf-6man-grand
IPv6 Maintenance J. Linkova
Internet-Draft Google
Updates: 4861 (if approved) July 5, 2021
Intended status: Standards Track
Expires: January 6, 2022
Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First-
Hop Routers
draft-ietf-6man-grand-07
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 initiates connections to an 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 January 6, 2022.
Copyright Notice
Copyright (c) 2021 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
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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 . . . . . . . . . . . . . . . . . . . . . . . 4
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Solution Requirements . . . . . . . . . . . . . . . . . . . . 6
4. Changes to Neighbor Discovery . . . . . . . . . . . . . . . . 6
4.1. Nodes Sending Gratuitous Neighbor Advertisements . . . . 7
4.2. Routers Creating Cache Entries Upon Receiving Unsolicited
Neighbor Advertisements . . . . . . . . . . . . . . . . . 7
5. Avoiding Disruption . . . . . . . . . . . . . . . . . . . . . 8
5.1. Neighbor Cache Entry Exists in Any State Other Than
INCOMPLETE . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Neighbor Cache Entry is in INCOMPLETE state . . . . . . . 9
5.3. Neighbor Cache Entry Does Not Exist . . . . . . . . . . . 10
5.3.1. The Rightful Owner Is Not Sending Packets From The
Address . . . . . . . . . . . . . . . . . . . . . . . 11
5.3.2. The Rightful Owner Has Started Sending Packets From
The Address . . . . . . . . . . . . . . . . . . . . . 12
6. Modifications to RFC-Mandated Behavior . . . . . . . . . . . 13
6.1. Modification to RFC4861 Neighbor Discovery for IP version
6 (IPv6) . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1.1. Modification to the section 7.2.5 . . . . . . . . . . 13
6.1.2. Modification to the section 7.2.6 . . . . . . . . . . 14
7. Solution Limitations . . . . . . . . . . . . . . . . . . . . 15
8. Solutions Considered but Discarded . . . . . . . . . . . . . 16
8.1. Do Nothing . . . . . . . . . . . . . . . . . . . . . . . 16
8.2. Change to the Registration-Based Neighbor Discovery . . . 16
8.3. Host Sending NS to the Router Address from Its GUA . . . 17
8.4. Host Sending Router Solicitation from its GUA . . . . . . 17
8.5. Routers Populating Their Caches by Gleaning From Neighbor
Discovery Packets . . . . . . . . . . . . . . . . . . . . 18
8.6. Initiating Hosts-to-Routers Communication . . . . . . . . 18
8.7. Making the Probing Logic on Hosts More Robust . . . . . . 19
8.8. Increasing the Buffer Size on Routers . . . . . . . . . . 20
8.9. Transit Dataplane Traffic From a New Address Triggering
Address Resolution . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
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12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 24
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 containing A's IPv6 and link-layer addresses 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]. Any additional packets
arriving before the resolution > process finishes are likely to
result in dropped packets It can cause packet loss and performance
degradation that can be user-visible.
This document updates the Neighbor Discovery protocol [RFC4861] to
avoid packet loss in the scenario described above. Section 4
discusses the changes and analyses the potential impact, while
normative changes to [RFC4861] are specified in Section 6.
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
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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].
ND: Neighbor Discovery, [RFC4861].
NC: Neighbor Cache, [RFC4861]. The Neighbor Cache entry can be in
one of five states, as described in section 7.3.2 of [RFC4861]:
INCOMPLETE, REACHABLE, STALE, DELAY, PROBE.
SLAAC: IPv6 Stateless Address Autoconfiguration, [RFC4862].
NS: Neighbor Solicitation, [RFC4861].
NA: Neighbor Advertisement, [RFC4861].
RS: Router Solicitation, [RFC4861].
RA: Router Advertisement, [RFC4861].
SLLAO: Source link-layer Address Option, an option in the ND packets
containing the link-layer address of the sender of the packet
[RFC4861].
TLLAO: Target link-layer Address Option, 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].
Preferred Address: an address assigned to an interface whose
uniqueness has been verified using DAD and whose use by upper-layer
protocols is unrestricted, [RFC4862]. Preferred addresses may be
used as the source address of packets sent from the interface.
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:
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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. The RA is sent from the router's
link-local address to a link-local destination address and may
contain the link-layer address of the router. As a result 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 and link-layer addresses).
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 any of the device's 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
(Section 7.3.2 of [RFC4861]). As per Section 7.2.2 of [RFC4861]
Routers MUST buffer at least one data packet and MAY buffer more,
while resolving the packet destination address. However, most
router implementations limit the buffer size to a few packets
only, and some implementations are known to buffer just one
packet. So any subsequent packets arriving before the address
resolution process is completed are causing packet loss by
replacing older packets in the buffer.
4. If the host sends multiple probes in parallel, in the worst case,
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 an inactivity
period 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
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the router, such as a new privacy address [RFC8981] 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 proactively, prior to
arrival of the first packet intended for that address.
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
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 should 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 needs to be compatible with the recommendations
provided in [RFC6583].
4. Changes to Neighbor Discovery
The following changes are required 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.
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The following sections discuss these changes in more detail.
Normative changes are specified in Section 6.
4.1. Nodes Sending Gratuitous Neighbor Advertisements
The section 7.2.6 of [RFC4861] discusses using unsolicited Neighbor
Advertisements 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 the 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 routers on the given network segment/link, instead of
being flooded to all nodes.
It should be noted that the proposed mechanism does not cause any
significant increase in 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 this procedure would only produce an increase in
the overall amount of multicast traffic if no return traffic arrives
for the address that sent the unsolicited NA or if the router does
not create a STALE entry upon receiving such NA. The increase would
be negligible as that additional traffic is a few orders of magnitude
less than the usual level of Neighbor Discovery multicast traffic.
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".
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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 to create 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 for
an address that does not already have a Neighbor Cache entry. . The
proposed changes do not modify routers behaviour specified in
[RFC4861] for the scenario when the corresponding Neighbor Cache
entry already exists.
The next section analyses various scenarios of duplicated addresses
and discusses the potential impact of creating a STALE entry for a
duplicated IPv6 address.
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. 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 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 discuss the address collision scenario when a
node sends an unsolicited NA for an address in the Optimistic state,
while another node (the rightful owner) has the same address assigned
already. This document uses the term "the rightful owner" as the
same terminology is used in [RFC4429]. The analysis assumes that the
host performs Duplicate Address Detection, as section 5.4 of
[RFC4862] requires that DAD MUST be performed on all unicast
addresses prior to assigning them to an interface.
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5.1. Neighbor Cache Entry Exists in Any State Other Than 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 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.
The return traffic intended for the host with the Optimistic address
would be sent to the rightful owner. However, this is unavoidable
with or without the unsolicited NA mechanism.
5.2. Neighbor Cache Entry is in INCOMPLETE state
Another corner case is the INCOMPLETE cache entry for the address.
1. The router receives a packet for the rightful owner of the
address.
2. The router starts the address resolution process by creating an
INCOMPLETE entry and sends the multicast NS.
3. More packets arrive at the router for the address in question.
4. The host configures an Optimistic address and sends an
unsolicited NA.
5. The router creates a STALE entry and sends the buffered packet(s)
to the host (while at least some of those packets are actually
intended for the rightful owner).
6. As the STALE entry was used to send packets, the router changes
the entry state to DELAY and waits up to DELAY_FIRST_PROBE_TIME
([RFC4861], 5 secs) before sending unicast NS.
7. The rightful owner responds to the multicast NS sent at Step 2
with a solicited NA with the Override flag set.
8. The router updates the entry with the TLLAO supplied (the
rightful owner link-layer address) and sets the entry state to
REACHABLE (as the NA has the Solicited flag set).
As a result some packets (ones in the buffer at Step 6 and all
packets arriving between Step 6 and Step 8) are delivered to the host
with the Optimisitc address, while some of them, if not all, are
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intended for the rightful owner. Without the unsolicited NA, packet
which are in the buffer at Step 8 (usually just one packet but some
routers may buffer a few) would have been delivered to the rightful
owner and the rest of the packets would have been dropped. However,
the probability of such scenario is rather low as it would require
the following things to happen almost simultaneously (within tens of
milliseconds in most cases):
o One host starts using a new IPv6 address and sending traffic
without sending an unsolicited NA first.
o Another host configures the same IPv6 address in Optimistic mode
before the router completes the address resolution for the
rightful owner.
It should be noted that in this scenario the rigthful owner does not
send any unsolicited NAs before sending packets. If the rightful
owner implements the functionality described in this document and
sends unsolicited NAs upon configuring its address, then the router
creates a STALE entry for the address, causing all packets are
delivered to the rightful owner (see Section 5.1). The rightful
owner would experience no disruption but might receive some packets
intended for the host with Optimistic address.
This section focuses on the scenario when the solicited NA from the
rightful owner arrives after the unsolicited one sent from the
Optimistic address (Step 7 and Step 4 respectively). If the
solicited NA arrives first it changes the NC entry state from
INCOMPLETE to REACHABLE. As discussed in Section 5.1, there will be
no disruption for the rightful owner if the router already has a
REACHABLE entry for the address when an unsolicited NA is received.
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:
1. The rightful owner of the address has not been using it for off-
link communication recently or has never used it at all.
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.
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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 waits 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. (It is unlikely that the host has
not completed the DAD process at this stage, as
DELAY_FIRST_PROBE_TIME (5 seconds) is much higher than the DAD
duration (DupAddrDetectTransmits*RetransTimer*1000 +
MAX_RTR_SOLICITATION_DELAY secs, section 5.4 of [RFC4862]). The
default value for the DAD process would be 1*1*1000 + 1 = 2 secs,
[RFC4861]. If the host has completed DAD but did not detect the
address conflict then there are two hosts with the same address
in the Preferred state and the disruption is inevitable anyway.
8. As the router receives no response for the unicast NSes, it
deletes the NC entry.
9. If return packets for communication initiated at step 2 are still
arriving, the router buffers a small number of those packets and
starts the address resolution again by sending 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 buffered packet(s) are sent to
that address. Any packets still arriving after the address
resolution still completed are sent to the rightful address owner
as well.
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The rightful owner is not experiencing any disruption as it does not
send any traffic. It would only start receiving packets intended for
another host after Step 8 is completed and only if return packets for
the communication initiated at step 2 are still arriving.
However, the same behaviour would be observed if changes proposed in
this document are not implemented. If the host starts sending
packets from its Optimistic address but then changes the address
state to Duplicated, the first return packet would trigger the
address resolution process and would be buffered until the resolution
is completed. The buffered packet(s) and any packets still arriving
after the address is resolved would be forwarded to the rightful
owner of the address. So the rightful owner might still receive one
or more packets from the flows intended for another host. Therefore,
it's safe to conclude that the proposed changes do introduce any
disruption for the rightful owner of the duplicated address.
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 for the IPv6 address in
question. Some flows intended for the rightful owner of the
duplicated address, while some are for the new host. As the NC
entry is STALE it sends traffic using that entry, changes it to
DELAY and waits 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.
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8. As the host has detected the address conflict already it does
not respond to the unicast NSes.
9. As the router receives no response for the unicast NSes, it
deletes the NC entry.
10. The next packet re-creates the entry and triggers the resolution
process. The router buffers the packet and 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.
However, it should be noted that the probability of such scenario is
rather low. Similary to the scenario discussed in Section 5.2, it
would require the following things to happen almost simultaneously
(within tens of milliseconds in most cases):
o One host starts using a new IPv6 address and sending traffic
without sending an unsolicited NA first.
o Another host configures the same IPv6 address in Optimistic mode
before the router receives the return traffic for the first host.
As discussed in Section 5.2, the disruption to the rightful owner can
easily be prevent if that node implements the mechanism described in
the document. Sending unsolicited NAs before initiatining off-link
communication would create a STALE entry in the router NC and prevent
any tarffic to that address to be sent to the host with the
Optimistic address (see Section 5.1).
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 makes the following changes to the section 7.2.5 of
[RFC4861]:
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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.
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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:
o 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.
o 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's reachability state MUST 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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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:
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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.
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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.
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7. Solution Limitations
The solution described in this document provides some improvement for
a node configuring a new IPv6 address and starting sending traffic
from it. However, that approach does not completely eliminate the
scenario when a router receives some transit traffic for an address
without the corresponding Neighbor Cache entry. For example:
o If the host starts using an already configured IPv6 address after
a long period of inactivity, the router might not have the NC
entry for that address anymore, as old/expired entries are
deleted.
o Clearing the router Neighbor Cache would trigger the packet loss
for all actively used addresses removed from the cache.
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8. 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]).
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.
8.1. Do Nothing
One of the possible approaches might be to declare that everything is
working as intended and let the upper-layer protocols 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.
8.2. 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 discussion. However, significant
changes to the existing IPv6 implementations would be needed, so
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unclear adoption timeline makes this approach less preferable than
one proposed in this document.
8.3. 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.
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.
8.4. 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 cannot 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:
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o If the address is in the Optimistic state the RS cannot 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.
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.
8.5. 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.
8.6. 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
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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.
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.
8.7. Making the Probing Logic on Hosts More Robust
Theoretically the probing logic on hosts might be modified to deal
better with initial packet loss. For example, only one probe can be
sent or probes retransmit intervals can be reduced. However, this
approach has a number of drawbacks:
o It would require updating all possible applications performing
probing, while the proposed solution is implemented on operating
systems level.
o Some implementations need to send multiple probes. Examples
include but not limited to:
* Sending AAAA and A records DNS probes in parallel.
* Detecting captive portals often require sending multiple
packets.
o While it would increase the probability of the probing to complete
successfully, there are multiple cases when packet loss would
still occur:
* The probe response consists of multiple packets, so all but the
first one are dropped.
* There are multiple applications on the same host sending
traffic and return packets arrive simultaneously.
* There are multiple first-hop routers in the network. The first
probe packet creates the NC entry on one of them. The
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subsequent return traffic flows might cross other routers and
still experience the issue.
o Reducing the probe retransmit interval unnecessary increases the
network utilization and might cause the network congestion.
8.8. Increasing the Buffer Size on Routers
Increasing the buffer size and buffering more packets would
exacerbate issues described in [RFC6583] and make the router more
vulnerable to ND-based denial of service attacks.
8.9. Transit Dataplane Traffic From a New Address Triggering Address
Resolution
When a router receives a transit packet sourced by a on-link neighbor
node, 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. The proactive
address resolution must only be triggered by packets from the
prefixes known to be on-link. Otherwise, traffic from spoofed
source addresses or any transit traffic could lead to neighbor
cache exhaustion.
o Implementing such functionality is much more complicated than all
other solutions as it would involve complex data-control planes
interaction.
9. IANA Considerations
This memo asks the IANA for no new parameters.
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10. 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, there are no new vectors for an attacker to
override an existing cache entry.
Section 5 describes some corner cases when a host with the duplicated
Optimistic address might get some packets intended for the rightful
owner of the address. However such scenarios do not introduce any
new attack vectors: even without the proposed changes, an attacker
can easily override the routers neighbor cache and redirect the
traffic by sending NAs with the Solicited flag set. As discussed in
Section 5.3.2 the worst case scenario might cause a disruption for up
to 7 seconds. This risk is considered acceptable due to very low
probability of that scenario. More importantly, for all cases
described in Section 5 the rightful owner can prevent disruption
caused by an accidental address duplication just by implementing the
mechanism described in this document. If the rightful owner sends
unsolicited NAs before using the address, the STALE entry would be
created on the router NC and any subsequent unsolicited NAs sent from
the host with an Optimistic address would not override the NC 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 if MLD snooping is not enabled.
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 layer-2 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
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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].
11. Acknowledgements
Thanks to the following people (in alphabetical order) for their
comments, review and feedback: Mikael Abrahamsson, Stewart Bryant,
Lorenzo Colitti, Roman Danyliw, Owen DeLong, Martin Duke, Igor
Gashinsky, Carles Gomez, Fernando Gont, Tatuya Jinmei, Benjamin
Kaduk, Scott Kelly, Erik Kline, Warren Kumari, Barry Leiba, Jordi
Palet Martinez, Erik Nordmark, Michael Richardson, Dan Romascanu,
Zaheduzzaman Sarker, Michael Scharf, John Scudder, Mark Smith, Dave
Thaler, Pascal Thubert, Loganaden Velvindron, Eric Vyncke.
12. References
12.1. 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>.
[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>.
[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>.
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12.2. 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>.
[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>.
[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>.
[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>.
[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>.
[RFC8981] Gont, F., Krishnan, S., Narten, T., and R. Draves,
"Temporary Address Extensions for Stateless Address
Autoconfiguration in IPv6", RFC 8981,
DOI 10.17487/RFC8981, February 2021,
<https://www.rfc-editor.org/info/rfc8981>.
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Author's Address
Jen Linkova
Google
1 Darling Island Rd
Pyrmont, NSW 2009
AU
Email: furry@google.com
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