IPv6 Neighbor Discovery Optional Unicast RS/RA Refresh
draft-nordmark-6man-rs-refresh-00
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| Author | Erik Nordmark | ||
| Last updated | 2014-10-24 | ||
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draft-nordmark-6man-rs-refresh-00
6man WG E. Nordmark
Internet-Draft Arista Networks
Updates: 4861 (if approved) October 24, 2014
Intended status: Standards Track
Expires: April 27, 2015
IPv6 Neighbor Discovery Optional Unicast RS/RA Refresh
draft-nordmark-6man-rs-refresh-00
Abstract
IPv6 Neighbor Discovery relies on periodic multicast Router
Advertisement messages to update timer values and to distribute new
information (such as new prefixes) to hosts. On some links the use
of periodic multicast messages to all host becomes expensive, and in
some cases it results in hosts waking up frequently. Many
implementations of RFC 4861 also use multicast for solicited Router
Advertisement messages, even though that behavior is optional.
This specification provides an optional mechanism for hosts and
routers where instead of periodic multicast Router Advertisements the
hosts are instructed (by the routers) to use unicast Router
Solicitations to request refreshed Router Advertisements. This
mechanism is enabled by configuring the router to include a new
option in the Router Advertisement in order to allow the network
administrator to choose host behavior based on whether periodic
multicast are more efficient on their link or not. The routers can
also tell whether the hosts are capable of the new behavior through a
new flag in the Router Solicitations.
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 http://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 April 27, 2015.
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Copyright Notice
Copyright (c) 2014 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Goals and Requirements . . . . . . . . . . . . . . . . . . . . 4
3. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . 4
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
5. New Neighbor Discovery Flags and Options . . . . . . . . . . . 5
5.1. Introducing a Router Solicitation Flag . . . . . . . . . . 5
5.2. Refresh Time option . . . . . . . . . . . . . . . . . . . 5
6. Conceptual Data Structures . . . . . . . . . . . . . . . . . . 6
7. Host Behavior . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Sleep and Wakeup . . . . . . . . . . . . . . . . . . . . . 7
7.2. Movement . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Router Behavior . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Router and/or Interface Initialization . . . . . . . . . . 8
8.2. Periodic Multicast RA for unmodified hosts . . . . . . . . 8
8.3. Unsolicited RAs to share new information . . . . . . . . . 8
9. Router Advertisement Consistency . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 9
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
13. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 9
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
14.1. Normative References . . . . . . . . . . . . . . . . . . . 10
14.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
IPv6 Neighbor Discovery [RFC4861] was defined at a time when local
area networks had different properties than today. A common link was
the yellow-coax shared wire Ethernet, where a link-layer multicast
and unicast worked the same - send the packet on the wire and the
interested receivers will pick it up. Thus the network cost
(ignoring any processing cost on the receivers that might not
completely filter out Ethernet multicast addresses that they did not
want) and the reliability of sending a link-layer unicast and
multicast was the same. Furthermore, the hosts at the time was
always on and connected. Powering on and off the workstation/PC
hosts at the time was slow and disruptive process.
Under the above assumptions it was quite efficient to maintain the
shared state of the link such as the prefixes and their lifetimes
using periodic multicast Router Advertisement messages. It was also
efficient to use multicast Neighbor Solicitations for address
resolution as a slight improvement over the broadcast use in ARP.
And finally, checking for a potential duplicate IPv6 address using
broadcast was efficient and natural.
There are still links, such a satellite links, where periodic
multicast advertisements is the most efficient and reliable approach
to keep the hosts up to date. However other links have different
performance and reliability for multicast than for unicast (see for
instance [I-D.vyncke-6man-mcast-not-efficient] which discusses WiFi
links). Cellular networks which employ paging and support sleeping
hosts have different issues (see e.g.,
[I-D.garneij-6man-nd-m2m-issues] that would benefit from having the
hosts wake up and request information from the routers instead of the
routers periodically multicasting the information.
Since different links types and deployments have different needs,
this specification provides mechanism by which the routers can
determine whether all the hosts support the RS refresh, and the hosts
only employ the RS refresh when instructed by the routers using an
option in the Router Advertisement.
The operator retains the option to use unsolicited multicast Router
Advertisement to announce new or removed information. That can be
useful for uncommon cases while allowing using a higher refresh time
for normal network operations.
The specification does not assume that all hosts on the link
implement the new capability. As soon as there are router(s) on a
link which supports these optimizations, then the updated hosts on
the link can sleep better, while co-existing on the same link with
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unmodified hosts.
2. Goals and Requirements
The key goal is to allow the operator to choose whether unicast RS
refresh is more efficient than periodic multicast RAs, while
preserving the timely and scalable reconfiguration capabilities that
a periodic RA model provides.
In addition, an operator might want to be notified whether the link
includes hosts that do not support the new mechanism. Potential
router implementations can react dynamically to that information, or
can log events to system management when hosts appear which do not
implement this new capability.
The assumption is that host which implement this specification also
implement [I-D.ietf-6man-resilient-rs] as that ensures resiliency to
packet loss that host initialization.
3. Definition Of Terms
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
4. Protocol Overview
The hosts include a new flag in the Router Solicitation message,
which allows the routers to report to system management whether there
are hosts that do not support the RS refresh on the link.
If the network administrator has configured the routers to send the
new Refresh Timer option, then the option will be included in all the
Router Advertisements. This option includes the time interval when
the hosts should unicast Router Solicitations.
The host maintains the value of the Refresh Timer option (RTO) by
recording it in the default router list. A value of zero can be used
to indicate that a router did not include a Refresh Timer option.
The host calculates a timeout after it has sent a RTO - either per
router or per link. If it is maintained per link then the host
SHOULD use the minimum Refresh Timer it has received from the routers
on the link. The timeout is a random value uniformly distributed
between 0.5 and 1.5 times the Refresh Timer value (in order to avoid
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synchronization of the timers across hosts. [TBD: Add SYNC reference
from RFC 4861.] When this timer fires the host sends one unicast
Router Solicitation to the router (if maintained per router) or to
all the routers on the link (if maintained per link.)
5. New Neighbor Discovery Flags and Options
This specification introduces a option used in the RAs which both
indicates that the router can handle RS refresh using unicast RA, and
a flag for the RS that indicates to the router that the host will do
RS refresh if the router so wishes.
5.1. Introducing a Router Solicitation Flag
A node which implements this specification sets the R flag in all the
Router Solicitation messages it sends. That allows the router to
determine whether there are legacy hosts on the link.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
New fields:
R-flag: When set indicates that the sending node is capable of
doing unicast RS refresh.
Reserved: Field is reduced from 32 bits to 31 bits. It MUST be
initialized to zero by the sender and MUST be ignored
by the receiver.
5.2. Refresh Time option
A router which implements this specification can be configured to
operate without periodic multicast Router Advertisements. When the
operator configures this mode of operation, then the router MUST
include this new option in the RA.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length=1 | Refresh Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: TBD ND option code value (IANA)
Length: 8-bit unsigned integer. The length of the option
(including the type and length fields) in units of 8
bytes. The value 0 is invalid. Value is 1 for this
option.
Refresh Time: 16-bit unsigned integer. Units is seconds. The all-
ones value (65535) means infinite.
Reserved: 32 bits. This field is unused. It MUST be
initialized to zero by the sender and MUST be ignored
by the receiver.
6. Conceptual Data Structures
In addition to the Conceptual Data structures in [RFC4861] a host
records the received RTO value in the default router list. It also
maintains a timeout - either per link or per default router.
7. Host Behavior
See Protocol Overview section above.
A host implementing this specification SHOULD also implement
[I-D.ietf-6man-resilient-rs]. That ensures that a router that has
been configured to not send periodic RA messages will always receive
an RS from the host as the host initializes.
If there is no RTO in the received Router Advertisements, then the
host behavior does not change. However, if RTOs start appearing in
RAs after the initial RAs, the host SHOULD start performing RS
refresh. As the last router that included RTO options time out from
the default router list, the host SHOULD stop sending RS refresh
messages.
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The host MUST join the all-nodes multicast address as in [RFC4861]
since the routers MAY send multicast RAs for important changes.
Some links might have routers with different configuration where some
router includes RTO in the RA and others do not. Hosts MAY make the
simplifying assumption that if any router on the link includes RTO
then the host can use RS refresh to all the routers on the link.
Also, the routers might advertise different refresh time, and hosts
MAY use the minimum of the time received from any router that remains
in the default router list. Note that setion (xref
target='consistency'/> says that routers SHOULD report such
inconsistences to system management.
7.1. Sleep and Wakeup
The protocol allows the sleepy nodes to complete its sleep schedule
without waking up due to multicast Router Advertisement messages and
the host is not required to wake up solely for the purposes of
performing RS refresh. This assumes that sleepy nodes perform a RS
refresh when they wake up. If hosts do wake up due to multicast RAs,
then the host only needs to perform a refresh on wakeup if the
Refresh timeout has expired while the host was sleeping.
7.2. Movement
When a host wakes up it can combine movement detecting (DNA), NUD,
and refreshing its prefixes etc by sending a unicast RS to each of
its existing default router(s). If it receives unicast RA from a
router, then it can mark the router as REACHABLE.
Note that DNA [RFC6059] specifies using NS messages since many IPv6
routers delay (and multicast) solicited RAs and DNA wants to avoid
that delay. Routers which implement this specification SHOULD
unicast solicited RAs, hence if a router included the RTO then the
host can use RS for DNA. For non-RTO routers the host MAY choose to
use NS for DNA as in [RFC6059].
8. Router Behavior
See Protocol Overview section.
A router implementing this specification (and including RTO in the
RAs) SHOULD also respond to unicast RS messages (that do not have an
unspecified source address) with unicast RAs. If a RS message has an
unspecified source address then the host MAY respond with a RA
unicast at layer 2 (sent to the link-layer address in the SLLAO in
the RS, or the link-layer source address of the RS), or it MAY follow
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the rate-limited multicast RA procedure in [RFC4861].
The RECOMMENDED default configuration for routers is to have RTO
disabled.
8.1. Router and/or Interface Initialization
This specification does not change the initialization procedure.
Thus a router multicasts some initial Router Advertisements
(MAX_INITIAL_RTR_ADVERTISEMENTS) at system startup or interface
initialization as specified in [RFC4861] and its updates.
8.2. Periodic Multicast RA for unmodified hosts
By default a router MUST send periodic multicast RAs as specified in
[RFC4861]. A router can be configured to omit those, which can be
used in particular deployments. If they are omitted, then there MUST
be a mechanism to prevent or detect the existence of unmodified hosts
on the link. That be be performed at deployment time (e.g., only
hosts which are known to support RTO are configured with the layer 2
security keys), or the routers detect any RSs which do not include
the R-flag and report this to system management, or dynamically
enable periodic multicast RAs when observing at least one RS without
the R-flag.
Note that such dynamic detection is not bullet proof. If a host does
not implement RS refresh nor implements resilient RS
[I-D.ietf-6man-resilient-rs], then the host might receive a multicast
RA (from router initialization or the periodic multicast RAs) without
the router ever receiving a RS from the host. Such a host would
function as long as the routers are sending periodic multicast RAs.
8.3. Unsolicited RAs to share new information
When a router has new information to share (new prefixes, prefixes
that should be immediately deprecated, etc) it MAY multicast up to
MAX_INITIAL_RTR_ADVERTISEMENTS number of Router Advertisements.
On links where multicast is expensive the router MAY instead unicast
up to MAX_INITIAL_RTR_ADVERTISEMENTS number of Router Advertisements
to the hosts in its neighbor cache.
. Note that such new information is not likely to reach sleeping
hosts until those hosts refresh by sending a RS.
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9. Router Advertisement Consistency
The routers follows section 6.2.7 in [RFC4861] by receiving RAs from
other routers on the link. In addition to the checks in that
section, the routers SHOULD verify that the RTO have the same Refresh
Time, and report to system management if they differ. While the host
will pick the lowest time and operate correctly, it is not useful to
use different Refresh Times for different routers.
10. Security Considerations
These optimizations are not known to introduce any new threats
against Neighbor Discovery beyond what is already documented for IPv6
[RFC3756].
Section 11.2 of [RFC4861] applies to this document as well.
The mechanisms in this document work with SeND [RFC3971].
11. IANA Considerations
A new flag (R-flag) in the Router Solicitation message has been
introduced by carving out a bit from the Reserved field. There is
currently no IANA registry for RS flags. Perhaps one should be
created?
This document needs a new Neighbor Discovery option type for the RTO.
12. Acknowledgements
The original idea came up in a discussion with Suresh Krishnan.
Comments from Erik Kline, Samita Chakrabarti, and Andrew Yourtchenko
have helped improve the document.
This document has discussed in the efficient-nd design team.
13. Open Issues
Should we update the DNA procedures [RFC6059]? We can use a
unicast RS with this approach since that will result in an
immediate unicast RA which would include any updated prefixes.
Would it be worth-while to try to remove unchanged information
from the refreshed RAs? If so it could be done by including some
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epoch number in the RS and RA, and if the RS contains the current
epoch then the RA would not need to include any options except the
epoch number indicating that none of the options are the same as
before.
14. References
14.1. Normative References
[I-D.ietf-6man-resilient-rs]
Krishnan, S., Anipko, D., and D. Thaler, "Packet loss
resiliency for Router Solicitations",
draft-ietf-6man-resilient-rs-04 (work in progress),
October 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
14.2. Informative References
[I-D.garneij-6man-nd-m2m-issues]
Garneij, F., Chakrabarti, S., and S. Krishnan, "Impact of
IPv6 Neighbor Discovery on Cellular M2M Networks",
draft-garneij-6man-nd-m2m-issues-00 (work in progress),
July 2014.
[I-D.vyncke-6man-mcast-not-efficient]
Vyncke, E., Thubert, P., Levy-Abegnoli, E., and A.
Yourtchenko, "Why Network-Layer Multicast is Not Always
Efficient At Datalink Layer",
draft-vyncke-6man-mcast-not-efficient-01 (work in
progress), February 2014.
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
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[RFC5175] Haberman, B. and R. Hinden, "IPv6 Router Advertisement
Flags Option", RFC 5175, March 2008.
[RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for
Detecting Network Attachment in IPv6", RFC 6059,
November 2010.
Author's Address
Erik Nordmark
Arista Networks
Santa Clara, CA
USA
Email: nordmark@acm.org
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