Network Working Group G. Daley
Internet-Draft Panasonic
Expires: August 29, 2006 E. Nordmark
Sun Microsystems
N. Moore
February 25, 2006
Tentative Options for Link-Layer Addresses in IPv6 Neighbour Discovery
draft-ietf-dna-tentative-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
The proposed IPv6 Duplicate Address Detection (DAD) Optimization
"Optimistic DAD" defines a set of recoverable procedures which allow
a node to make use of an address before DAD completes. Essentially,
Optimistic DAD forbids usage of certain Neighbour Discovery options
which could pollute active neighbour cache entries, while an address
is tentative.
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This document defines a new option and procedures to replace cache
polluting options, in a way which is useful to tentative nodes.
These procedures are designed to be to backward compatible with
existing devices which support IPv6 Neighbour Discovery.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Tentative Option format . . . . . . . . . . . . . . . . . 3
1.2 Tentative Option semantics . . . . . . . . . . . . . . . . 4
2. Sending solicitations containing Tentative Options . . . . . . 4
2.1 Sending Neighbour Solicitations with Tentative Options . . 5
2.2 Sending Router Solicitations with Tentative Options . . . 5
3. Receiving Tentative Options . . . . . . . . . . . . . . . . . 5
3.1 Handling Tentative Options . . . . . . . . . . . . . . . . 5
3.2 Receiving Neighbour Solicitations containing Tentative
Options . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3 Receiving Router Solicitations containing Tentative
Options . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1 Normative References . . . . . . . . . . . . . . . . . . . 9
7.2 Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
A. Constraints imposed by IPv6 Neighbour Discovery . . . . . . . 10
A.1 Constraints on Neighbour Solicitations . . . . . . . . . . 10
A.2 Constraints on Router Solicitations . . . . . . . . . . . 11
B. Interactions with legacy nodes . . . . . . . . . . . . . . . . 11
B.1 Legacy Neighbour Solicitation processing . . . . . . . . . 11
B.2 Legacy Router Solicitation processing . . . . . . . . . . 12
C. Sending directed advertisements without the neighbour cache . 12
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Introduction
Source Link-Layer Address Options (SLLAOs) are sent in Neighbour
discovery messages in order to notify neighbours of a mapping between
a specific IPv6 Network layer address and a link-layer (or MAC)
address. Upon reception of a neighbour discovery message containing
such an option, nodes update their neighbour cache entries with the
IP to link-layer address mapping in accordance with procedures
defined in IPv6 Neighbour Discovery [2].
Optimistic DAD [4] prevents usage of these options in Router and
Neighbour Solicitation messages from a tentative address (while
Duplicate Address Detection is occurring). This is because receiving
a Neighbour Solicitation (NS) or Router Solicitation (RS) containing
an SLLAO would otherwise overwrite an existing cache entry, even if
the cache entry contained the legitimate address owner, and the
solicitor was a duplicate address.
Neighbour Advertisement (NA) messages don't have such an issue, since
the Advertisement message contains a flag which explicitly disallows
overriding of existing cache entries, by the target link-layer
address option carried within.
The effect of preventing SLLAOs for tentative addresses is that
communications with these addresses are sub-optimal for the tentative
period. Sending solicitations without these options causes an
additional round-trip for neighbour discovery if the advertiser does
not have an existing neighbour cache entry for the solicitor. In
some cases, multicast advertisements will be scheduled, where
neighbour discovery is not possible on the advertiser.
This document proposes Tentative Options which designed to replace
the existing Source Link-Layer Address Options available in IPv6
Neighbour Discovery, when a device is performing Optimistic DAD.
1.1 Tentative Option format
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 5 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Link-Layer Address ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type TBD (Requires IANA Allocation) suggest 17 (0x11)
Length The length of the option (including the type and
length fields) in units of 8 octets.
Link-Layer Address
The variable length link-layer address.
Description
The Tentative option contains the link-layer
address of the sender of the packet. It is used
in the Neighbour Solicitation and Router
Solicitation packets.
1.2 Tentative Option semantics
The Tentative Option (TO) functions in the same role as the Source
Link-Layer Address option defined for [2], but it MUST NOT override
an existing neighbour cache entry.
The differing neighbour cache entry MUST NOT be affected by the
reception of the Tentative Option. This ensures that tentative
addresses are unable to modify legitimate neighbour cache entries.
In the case where an entry is unable to be added to the neighbour
cache, a node MAY send responses direct to the link-layer address
specified in the TO.
For these messages, no Neighbour Cache entry may be created, although
response messages may be directed to a particular unicast address.
These procedures are discussed further in Section 3.3.
2. Sending solicitations containing Tentative Options
Tentative Options may be sent in Router and Neighbour Solicitations,
as described below.
In a case where it is safe to send a Source Link-Layer Address
Option, a host SHOULD NOT send a TO, since the message may be
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misinterpreted by legacy nodes.
Importantly, a node MUST NOT send a Tentative Option in the same
message where a Source Link-Layer Address Option is sent.
2.1 Sending Neighbour Solicitations with Tentative Options
Neighbour Solicitations sent to unicast addresses MAY contain a
Tentative Option.
Since delivery of a packet to a unicast destination requires prior
knowledge of the destination's hardware address, unicast Neighbour
Solicitation packets may only be sent to destinations for which a
neighbour cache entry already exists.
For example, if checking bidirectional reachability to a router, it
may be possible to send a Neighbour Solicitation with Tentative
Option to the router's advertised address.
As discussed in [2], the peer device may not have a cache entry even
if the soliciting host does, in which case reception of the Tentative
Option may create a neighbour cache entry, without the need for
neighbour discovering the original solicitor.
2.2 Sending Router Solicitations with Tentative Options
Any Router Solicitation from a Preferred, Deprecated or Optimistic
address MAY be sent with a Tentative Option [4].
An extension which allows Router Solicitations to be sent with a TO
from the unspecified address is described in Appendix C.
3. Receiving Tentative Options
Receiving a Tentative Option allows nodes to unicast responses to
solicitations without performing neighbour discovery.
It does this by allowing the solicitation to create STALE neighbour
cache entries if one doesn't exist, but only update an entry if the
link-layer address in the option matches the entry.
Additionally, messages containing TO may be used to direct
advertisements to particular link-layer destinations without updating
neighbour cache entries. This is described in Appendix C.
3.1 Handling Tentative Options
Use of Tentative Options is only defined for Neighbour and Router
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Solicitation messages.
In any other received message, the presence of the option is silently
ignored, that is, the packet is processed as if the option was not
present.
It is REQUIRED that the same validation algorithms for Neighbour and
Router Solicitations received with TO as in the IPv6 Neighbour
Discovery specification [2], are used.
In the case that a solicitation containing a Tentative Option is
received, The only processing differences occur in checking and
updating the neighbour cache entry. Particularly, there is no reason
to believe that the host will remain tentative after receiving a
responding advertisement.
As defined in Section 1.1, Tentative Options do not overwrite
existing neighbour cache entries where the link-layer addresses of
the option and entry differ.
If a solicitation from a unicast source address is received where no
difference exists between the TO and an existing neighbour cache
entry, the option MUST be treated as if it were an SLLAO after
message validation, and processed accordingly.
In the case that a cache entry is unable to be created or updated due
to existence of a conflicting neighbour cache entry, it MUST NOT
update the neighbour cache entry.
An extension which allows a direct advertisement to the soliciting
host without modifying the neighbour cache entry is described in
Appendix C.
3.2 Receiving Neighbour Solicitations containing Tentative Options
The Tentative Option is only allowed in Neighbour Solicitations with
specified source addresses for which SLLAO is not required.
A Neighbour Solicitation message received with a TO and an
unspecified source address MUST be silently discarded.
Upon reception of a Tentative Option in a Neighbour Solicitation for
which the receiver has the Target Address configured, a node checks
to see if there is a neighbour cache entry with conflicting link-
layer address.
If no such entry exists, the neighbour cache of the receiver SHOULD
be updated, as if the Tentative Option was a SLLAO.
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Sending of the solicited Neighbour Advertisement then proceeds
normally, as defined in section 7.2.4 of [2].
If there is a conflicting neighbour cache entry, the node processes
the solicitation as defined in Section 7.2.4 of [2], except that the
Neighbour Cache entry MUST NOT be modified.
3.3 Receiving Router Solicitations containing Tentative Options
In IPv6 Neighbour Discovery [2], responses to Router Solicitations
are either sent to the all-nodes multicast address, or may be sent to
the solicitation's source address if it is a unicast address.
Including a Tentative Option in the solicitation allows a router to
choose to send a packet directly to the link-layer address even in
situations where this would not normally be possible.
For Router Solicitations with unicast source addresses, neighbour
caches SHOULD be updated with the link-layer address from a Tentative
Option if there is no differing neighbour cache entry. In this case,
Router Advertisement continues as in Section 6.2.6 of [2].
For received solicitations with a differing link-layer address to
that stored in the neighbour cache, the node processes the
solicitation as defined in Section 6.2.6 of [2], except that the
Neighbour Cache entry MUST NOT be modified.
4. IANA Considerations
For standardization, it would be required that the IANA provide
allocation of the Tentative Option for link-layer addressing (Section
1.1) from the IPv6 Neighbour Discovery options for IPv6.
Current experimental implementations have used the value 0x11 (17)
for the Tentative Option.
IANA action requires either IESG Approval or Standards Action [5].
5. Security Considerations
The use of the Tentative Option in Neighbour and Router Solicitation
messages acts in a similar manner to SLLAO, updating neighbour cache
entries, in a way which causes packet transmission.
Particular care should be taken that transmission of messages
complies with existing IPv6 Neighbour Discovery Procedures, so that
unmodified hosts do not receive invalid messages.
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An attacker may cause messages may be sent to another node by an
advertising node (a reflector), without creating any ongoing state on
the reflector.
This is attack requires one solicitation for each advertisement and
the advertisement has to go to a unicast MAC destination. That said,
the size of the advertisement may be significantly larger than the
solicitation, or the attacker and reflector may be on a medium with
greater available bandwidth than the victim.
For link-layers where it isn't possible to spoof the link-layer
source address this allows a slightly increased risk of reflection
attacks from nodes which are on-link.
Additionally, since a SEND host must always advertise using SEND
options and signatures, a non-SEND attacker may cause excess
computation on both a victim node and a router by causing SEND
advertisement messages to be transmitted to a particular MAC address
and the lall-nodes multicast. SEND specifies guidelines to hosts
receiving unsolicited advertisements in order to mitigate such
attacks [3].
While this is the same effect as experienced when accepting SLLAO
from non-SEND nodes, the lack of created neighbour cache entries on
the advertiser may make such attacks more difficult to trace.
Modification of Neighbour Discovery messages on the network is
possible, unless SEND is used. [3] provides a protocol specification
in which soliciting nodes sign ND messages with a private key and use
addresses generated from this key.
Even if SEND is used, the lifetime of a neighbour cache entry may be
extended by continually replaying a solicitation message to a
particular router or hosts. Since this may be achieved for any
Neighbour or Router Solicitation message, corresponding
advertisements to the original transmitters of these solicitation
messages may occur.
SEND defines use of Timestamp values to protect a device from attack
through replay of previously sent messages. Although this applies to
Neighbour and Router Solicitation messages, granularity of the
timestamp allows the messages to be used for up to five minutes [3].
All Router and Neighbour Solicitations using SEND contain a Nonce
option, containing a random identifier octet string. Since SEND
messages are digitally signed, and may not be easily modified, replay
attacks will contain the same Nonce option, as was used in the
original solicitation.
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While the Nonce Option included in a transmission to another node may
not vary within one short solicitation period (the host may itself
replay solicitations in the case of packet loss), the presence of the
timestamp option ensures that for later solicitations, a different
Timestamp and Nonce will be used.
Therefore, a receiver seeing a solicitation with the same Timestamp
and Nonce (and signature) for more than either of
MAX_RTR_SOLICITATIONS (for Router Solicitations), MAX_UNICAST_SOLICIT
or MAX_MULTICAST_SOLICIT (for Neighbour Solicitations), SHOULD ignore
further solicitations with this (Nonce,Timestamp,Source) triple,
ensuring that no modification is made to neighbour cache entries.
This applies to any solicitation packet capable of carrying a SEND
payload, whether they use a Tentative Option or SLLAO.
Stations noticing such an attack SHOULD notify their administrator of
the attempt at Denial-of-service.
6. Acknowledgments
Erik Nordmark coined a proposal for Tentative version of the SLLAO
during a conversation with JinHyeock Choi and Greg Daley.
7. References
7.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[3] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[4] Moore, N., "Optimistic Duplicate Address Detection for IPv6",
draft-ietf-ipv6-optimistic-dad-07 (work in progress),
December 2005.
7.2 Informative References
[5] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
Values In the Internet Protocol and Related Headers", BCP 37,
RFC 2780, March 2000.
[6] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
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Authors' Addresses
Greg Daley
Panasonic Digital Networking Laboratory
2 Research Way
Princeton, New Jersey 08540
USA
Phone: +1 609 734 7334
Email: gregd@research.panasonic.com
Erik Nordmark
Sun Microsystems, Inc.
17 Network Circle
Mountain View, CA
USA
Phone: +1 650 786 2921
Email: erik.nordmark@sun.com
Nick "Sharkey" Moore
Email: sharkey@zoic.org
Appendix A. Constraints imposed by IPv6 Neighbour Discovery
Hosts which send and receive Tentative Options may be interacting
with legacy nodes which support IPv6 Neighbour Discovery procedures,
but do not understand the new option.
For these nodes, the presence of the option is silently ignored, that
is, the packet is processed as if the option was not present.
Therefore all messages sent with Tentative Options MUST be compliant
with the existing requirements for options and addressing specified
in the IPv6 Neighbour Discovery RFC [2].
A.1 Constraints on Neighbour Solicitations
As described in Section 7.2.2 of [2], packets sent to solicited
nodes' multicast addresses MUST contain Source Link-Layer Address
options.
Neighbour solicitations to multicast addresses MUST NOT contain
Tentative Options
Neighbour Solicitations to unicast addresses SHOULD include a link-
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layer address (if the sender has one has one) as a Source Link-Layer
Address option.
Unicast neighbour solicitations without Source Link-Layer Address
Options MAY contain Tentative Options, if the solicitor has a
Link-Layer address.
A.2 Constraints on Router Solicitations
As described in Section 6.3.7 of [2], Router Solicitations SHOULD
contain Source Link-Layer Address Options.
Router Solicitations without Source Link-Layer Address options MAY
contain a Tentative Option.
Appendix B. Interactions with legacy nodes
Devices which do not implement Tentative Options will act as if no
option was placed within the Neighbour Discovery message. The
following sections summarize how legacy hosts will interact with
messages containing Tentative Options.
Appendix B.1 Legacy Neighbour Solicitation processing
A node can include the Tentative Option in a unicast NS (and no SLLAO
option) when the transmitter's address is either preferred, tentative
or optimistic.
An RFC 2461 host receiving such a packet will "see" a packet
without an SLLAO option, which is allowed in RFC2461.
If the recipient host has an existing neighbour cache entry for
the transmitter, it can then send a Neighbour Advertisement.
Where no neighbour cache entry exists, the recipient will send a
multicast NS (containing its own SLLAO) in order for the original
transmitter to respond with an NA. Upon reception of the original
transmitter's NA, an NA is sent back to the origin.
The Tentative Option MUST NOT be included in an NS message which has
no source address.
An RFC 2461 host sees an NS without a source address as a
Duplicate Address Detection message.
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Reception of duplicate address detection messages may cause side-
effects on other hosts, which may cause them to treat addresses as
invalid.
Appendix B.2 Legacy Router Solicitation processing
A node can include the Tentative Option in an RS with a unicast
source address (and no SLLAO option) when the transmitter's address
is either tentative or optimistic.
An RFC 2461 router receiving such a packet will "see" a packet
without an SLLAO option, which is allowed in RFC2461.
If the router has an existing neighbour cache entry for this host,
it may send a Unicast RA in response, but may send a multicast in
preference.
If no neighbour cache entry exists, some routers will not be able
to provide a unicast response. These routers will schedule a
multicast response.
Other routers may attempt to perform neighbour discovery (by
sending a multicast NS), and unicast a response when a neighbour
cache entry has been created.
A node can include the Tentative Option in an RS with an unspecified
source address (and no SLLAO option) when the transmitter's address
is tentative. This is described in Appendix C.
RFC 2461 routers receiving this solicitation will "see" a message
without a SLLAO ( SLLAOs are not allowed in RFC2461 for messages
with unspecified source).
These routers will schedule a multicast RA response.
Appendix C. Sending directed advertisements without the neighbour cache
In the case where an entry is unable to be added to the neighbour
cache, a node MAY send responses direct to the link-layer address
specified in the Tentative Option. Also, RS packets sent without a
specificed source address may potentially contain a Tentative Option.
In this case the unicast link-layer address from the solicitation MAY
be extracted from the Tentative Option and used as the destination of
the link-layer frame for a responding Router Advertisment.
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Sending such a packet MUST NOT consult the neighbour or destination
caches for address.
Such packets SHOULD scheduled as if they were unicast advertisements
as specified in [2].
If an implementation can not send a Router Advertisement using
information from the Tentative Option i.e, without consulting the
neighbour cache, then it SHOULD behave as if the Tentative Option was
not present in the solicitation message.
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Acknowledgment
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