Network Working Group G. Daley
Internet-Draft Monash University CTIE
Expires: January 19, 2006 E. Nordmark
Sun Microsystems
N. Moore
Monash University CTIE
July 18, 2005
Tentative Source Link-Layer Address Options for IPv6 Neighbour Discovery
draft-daley-ipv6-tsllao-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
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
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is tentative.
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 Source Link-Layer Address Option Format . . . . 3
1.2 Tentative Source Link-Layer Address Option Semantics . . . 4
2. Sending Solicitations containing TSLLAO . . . . . . . . . . . 4
2.1 Sending Neighbour Solicitations with TSLLAO . . . . . . . 5
2.2 Sending Router Solicitations with TSLLAO . . . . . . . . . 5
3. Receiving Tentative Source Link-Layer Address Options . . . . 5
3.1 Handling Tentative Source Link-Layer Address Options . . . 6
3.2 Receiving Neighbour Solicitations containing TSLLAO . . . 6
3.3 Receiving a Router Solicitation containing TSLLAO . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1 Normative References . . . . . . . . . . . . . . . . . . . 9
7.2 Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
A. Constraints imposed by IPv6 Neighbour Discovery . . . . . . . 10
A.1 Constraints on Neighbour Solicitations . . . . . . . . . . 11
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 . 13
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.
Tentative Source Link-Layer Address Options are 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 Source Link-Layer Address 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 33 (0x21)
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 Source Link-Layer Address 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 Source Link-Layer Address Option Semantics
The Tentative Source Link-Layer Address option (TSLLAO) 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 Source Link-Layer Address 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 TSLLAO.
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 TSLLAO
Tentative Source Link-Layer Address 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
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Option, a host SHOULD NOT send a TSLLAO, since the message may be
misinterpreted by legacy nodes.
Importantly, a node MUST NOT send a TSLLAO in the same message where
a Source Link-Layer Address Option is sent.
2.1 Sending Neighbour Solicitations with TSLLAO
Neighbour Solicitations sent to unicast addresses MAY contain a
TSLLAO.
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 TSLLAO 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 TSLLAO may
create a neighbour cache entry, without the need for neighbour
discovering the original solicitor.
2.2 Sending Router Solicitations with TSLLAO
Any Router Solicitation from a Preferred, Deprecated or Optimistic
address MAY be sent with a TSLLAO [4].
An extension which allows Router Solicitations to be sent with a
TSLLAO from the unspecified address is described in Appendix C.
3. Receiving Tentative Source Link-Layer Address Options
Receiving a Tentative Source Link-Layer Address 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, TSLLAO messages may be used to direct advertisements to
particular link-layer destinations without updating neighbour cache
entries. This is described in Appendix C.
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3.1 Handling Tentative Source Link-Layer Address Options
Use of Tentative Source Link-Layer Address Options is only defined
for Neighbour and Router 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 TSLLAO as in the IPv6 Neighbour
Discovery specification [2], are used.
In the case that a solicitation containing a TSLLAO 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 Source Link-Layer Address
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 TSLLAO 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 TSLLAO
The TSLLAO option is only allowed in Neighbour Solicitations with
specified source addresses for which SLLAO is not required.
A Neighbour Solicitation message received with TSLLAO and an
unspecified source address MUST be silently discarded.
Upon reception of a Tentative Source Link-Layer Address 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.
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If no such entry exists, the neighbour cache of the receiver SHOULD
be updated, as if the Tentative Source Link-Layer Address Option was
a SLLAO.
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 a Router Solicitation containing TSLLAO
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 TSLLAO 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 TSLLAO 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 Source Link-Layer Address Option (Section
1.1) from the IPv6 Neighbour Discovery options for IPv6.
Current experimental implementations have used the value 0x11 (17)
for the Tentative Source Link-Layer Address Option.
Potential details of the allocation process for these options is
detailed in the expired draft [5].
5. Security Considerations
The use of the TSLLAO in Neighbour and Router Solicitation messages
acts in a similar manner to SLLAO, updating neighbour cache entries,
in a way which causes packet transmission.
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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.
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 all-nodes multicast. [3] specifies guidelines to hosts
receiving unsolicited advertisements in order to mitigate such
attacks.
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
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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.
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 TSLLAO 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 TSLLAO 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., Sommerfeld, B., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", draft-ietf-send-ndopt-06
(work in progress), July 2004.
[4] Moore, N., "Optimistic Duplicate Address Detection for IPv6",
draft-ietf-ipv6-optimistic-dad-03 (work in progress),
January 2005.
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7.2 Informative References
[5] Narten, T., "IANA Allocation Guidelines for Values in IPv6 and
Related Headers", draft-narten-ipv6-iana-considerations-00 (work
in progress), October 2002.
[6] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
Authors' Addresses
Greg Daley
Centre for Telecommunications and Information Engineering
Department of Electrical and Computer Systems Engineering
Monash University
Clayton, Victoria 3800
Australia
Phone: +61 3 9905 4655
Email: greg.daley@eng.monash.edu.au
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
Centre for Telecommunications and Information Engineering
Department of Electrical and Computer Systems Engineering
Monash University
Clayton, Victoria 3800
Australia
Email: nick.moore@eng.monash.edu.au
Appendix A. Constraints imposed by IPv6 Neighbour Discovery
Hosts which send and receive Tentative Source Link Layer Address
Options may be interacting with legacy nodes which support IPv6
Neighbour Discovery procedures, but do not understand the new option.
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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 TSLLAO 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
TSLLAO
Neighbour Solicitations to unicast addresses SHOULD include a link-
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 TSLLAO, 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 TSLLAO.
Appendix B. Interactions with legacy nodes
Devices which do not implement Tentative Source Link Layer address
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 TSLLAO.
Appendix B.1 Legacy Neighbour Solicitation processing
A node can include the TSLLAO option in a unicast NS (and no SLLAO
option) when the transmitter's address is either tentative or
optimistic.
An RFC 2461 host receiving such a packet will "see" a packet
without an SLLAO option, which is allowed in RFC2461.
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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 TSLLAO 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.
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 TSLLAO 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 TSLLAO 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 (such options are not allowed in RFC2461).
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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 TSLLAO. Also, RS packets sent without a specificed
source address may potentially contain a TSLLAO.
In this case the unicast link-layer address from the solicitation MAY
be extracted from the TSLLAO option and used as the destination of
the link-layer frame for a responding Router Advertisment.
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 TSLLAO i.e, without consulting the neighbour
cache, then it SHOULD behave as if the TSLLAO option was not present
in the solicitation message.
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