IPv6 Working Group Greg Daley
INTERNET-DRAFT Nick "Sharkey" Moore
Expires: December 2004 Monash University CTIE
Erik Nordmark
Sun Microsystems
9 June 2004
Tentative Source Link-Layer Address Options
for IPv6 Neighbour Discovery
<draft-daley-ipv6-tsllao-00.txt>
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed
and any of which I become aware will be disclosed, in accordance with
RFC 3668 (BCP 79).
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3 of RFC 3667 (BCP 78).
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This document is an individual submission to the IETF. Comments
should be directed to the authors.
Definitions of requirements keywords are in accordance with the IETF
Best Current Practice - RFC2119 [KEYW-RFC]
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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.
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.
1.0 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 [RFC-2461].
Optimistic DAD [OPTIDAD] 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.
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, or a
device is sending Router Solicitations from an unspecified source
address.
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1.1 Tentative Source Link-Layer Address Option Format
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>
Length The length of the option (including the type and
length fields) in units of 8 octets [RFC-2461].
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
[RFC-2461], 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.
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2.0 Sending Solicitations containing TSLLAO
Solicitations sent containing Tentative Source Link-Layer Address
Options need to conform to IPv6 Neighbour Discovery procedures, since
they are send without SLLAOs, and legacy nodes will ignore the new
option.
In a case where it is safe to send a Source Link-Layer Address
Option, a host SHOULD NOT send a TSLLAO, since the message may be
mis-interpreted by legacy nodes.
Importantly, a node MUST NOT send a TSLLAO in the same message where
a Source Link-Layer Address Option is sent.
A node MUST NOT send TSLLAO options except where [RFC-2461] allows
ommision of an SLLAO i.e. in the following cases:
2.1 Sending Neighbour Solicitations with TSLLAO
Except for packets sent from an unspecified source address, Source
Link-Layer Address options are mandatory in Neighbour Solicitation
messages destined to multicast addresses.
Neighbour Solicitations for the unspecified source address are
typically used for Duplicate Address Detection. Any receiver of an
unspecified source addressed Neighbour Solicitation with TSLLAO will
believe the packet to be a DAD attempt if it is unable to interpret
the TSLLAO option.
Since many nodes will halt address configuration if they receive a
DAD NS while an address is tentative, Tentative Source Link-Layer
Address options MUST NOT be sent in Neighbour Solicitation messages
from the unspecified source address.
On the other hand, Neighbour Solicitation packets with unicast source
and destination addresses are not explicitly required to include
SLLAOs. Such packets may instead use a Tentative Source Link-Layer
Address Option, which is safe when undergoing Duplicate Address
Detection[OPTIDAD].
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.
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As discussed in [RFC-2461], 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. If the device is a
legacy [RFC-2461] device, which doesn't recognize TSLLAO options, it
will perform a Neighbour Solicitation back to the tentative node.
Hosts MUST NOT send Neighbour Solicitations with specified source
addresses and TSLLAO to nodes for which there is no pre-existing
neighbour cache entry, or state is INCOMPLETE, unless these nodes are
known to support TSLLAOs.
This is because such Neighbour Solicitations violate IPv6 Neighbour
Discovery specifications since they contain no SLLAO, and may cause
confusion or harm to nodes which receive them.
2.2 Sending Router Solicitations with TSLLAO
Router Solicitations are always able to be sent without Source Link-
Layer Address options.
Some routers may choose to send a multicast response to devices which
send Router Solicitations without SLLAOs when they do not have an
existing neighbour cache entry. If a router does not understand
Tentative Source Link-Layer Address Options, it MAY send a multicast
solicitation in preference to sending Neighbour Solicitation packets
to learn unicast address's link-layer address.
Any router solicitation, including those from the unspecified
address, MAY be sent with a TSLLAO.
Responses from routers depend on existing neighbour cache state, and
their ability to send packets to identified MAC addresses without
using the neighbour cache.
Such issues are discussed in sections 3.4 and 3.5.
3.0 Receiving Tentative Source Link-Layer Address Options
In the case that a node receives a solicitation without a Link-Layer
identifier it will determine if a responding Advertisement will be
sent to a unicast or multicast address.
If the advertisement is to be sent to the solicitor's unicast
address, the node will consult its existing neighbour cache for the
solicitor's information, and if not present, will undertake neighbour
discovery.
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Receiving a Tentative Source Link-Layer Address Option, avoids this
neighbour discovery step, by allowing the host to create or update a
matching entry, setting it to STALE state if it didn't previously
exist.
Additionally, TSLLAO messages may be used to direct advertisements to
particular link-layer destinations without updating neighbour cache
entries. This is described in section 3.4.
3.1 Handling messages with 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, a Tentative Source Link-Layer Address
Option MUST be treated as if it is an unknown option, and processed
appropriately.
It is REQUIRED that the same validation algorithms for Neighbour and
Router Solicitations received with TSLLAO as in the IPv6 Neighbour
Discovery specification [RFC-2461], are used. In the case that a
solicitation containing a TSLLAO is received, this does not mean that
the solicitor needs to be treated differently, except in the updating
of the cache entry and processing of the option. Particularly, there
is no reason to believe that the host will remain tentative after
receiving a responding advertisement.
As defined in Section 1.2, Tentative Source Link-Layer Address
Options do not overwrite existing neighbour cache entries where the
link-layer addresses differ.
If a solicitation from a unicast source address is received where no
conflict occurs 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,
the receiving node MAY send a direct advertisement to the soliciting
host by responding with an appropriate advertisement, where the link-
layer address contained in the TSLLAO is copied into the destination
address of the link-layer frame.
This is described further in sections 3.4 and 3.5.
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
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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.
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 [RFC-2461].
If there is a conflicting neighbour cache entry, a node processes the
solicitation in a fashion defined in in sections 3.4 and 3.5.
3.3 Receiving a Router Solicitation containing TSLLAO
In IPv6 Neighbour Discovery [RFC-2461], 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 conflicting neighbour cache entry. In this case, Router
Advertisement continues as in section 6.2.6 of [RFC-2461].
For received solicitations with a conflicting neighbour cache entry
or those containing a TSLLAO with an unspecified source address,
responses can be generated in accordance with sections 3.4 and 3.5
below.
3.4 Sending Directed Advertisements without the Neighbour Cache.
In the case where a received solicitation has a link-layer address
specified in a TSLLAO which conflicts with an existing with a
neighbour cache entry, modification of the neighbour cache MUST NOT
occur.
Router Solicitations with the unspecified source address MUST NOT
create neighbour cache entires.
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In both situations, it may be valuable to send a direct message to
the soliciting nodes.
In these cases a node MAY generate a responding advertisement
according to the rules in [RFC-2461].
The responding packets MAY then have the unicast link-layer address
from the TSLLAO inserted into the destination address of the link-
layer frame used to transport this advertisement, without consulting
the neighbour or destination caches for this entry.
Such packets SHOULD scheduled as if they were unicast advertisements
as specified in [RFC-2461].
3.5 Alternatives to Sending Directed Advertisements.
Some implementations will be unable to generate directed
advertisements by copying the tentative source link-layer address
into a packet.
Also, some nodes will be unable to send unicast packets without
consulting their neighbour caches. Alternative mechanisms for such
nodes SHOULD perform at least as well as implementations where TSLLAO
is not understood.
For such implementations as these, it is not possible to send a
response for Neighbour Solicitation messages without modifying the
neighbour cache.
Such nodes MAY send a responding NA message as if it did not
understand the TSLLAO message.
This will deliver the NA to the soliciting host if it has both the
tentative link-layer address and the entry in the neighbour cache
configured on the same link. Otherwise, the message will be sent to
the originator of the neighbour cache entry, and the solicitor will
receive no response.
For Router Solicitations where no neighbour cache entry is able to be
created, a multicast response SHOULD be sent in accordance with
[RFC-2461]. This includes those solicitations sent from unicast
sources, for which a conflicting neighbour cache entry was found.
4.0 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.
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Potential details of the allocation process for these options is
detailed in the expired draft [IPv6-ALLOC].
5.0 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.
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. [SEND] 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. [SEND] 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
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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 two hours in
extreme cases [SEND].
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.
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.
Normative References
[KEYW-RFC] S. Bradner. Key words for use in RFCs to Indicate
Requirement Levels. Request for Comments (Best Current Practice)
2119 (BCP 14), Internet Engineering Task Force, March 1997
[OPTIDAD] N. Moore. Optimistic Duplicate Address Detection. Internet
Draft (work in progress), March 2004.
[RFC-2461] T. Narten, E.Nordmark, W. Simpson. Neighbour Discovery
for IP Version 6 (IPv6). Request for Comments (Draft Standard)
2461, Internet Engineering Task Force, December 1998.
[SEND] J. Arkko (Editor) et al. SEcure Neighbour Discovery (SEND).
Internet Draft (work in progress), April 2004.
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INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004
Non-Normative References
[IPv6-ALLOC] T. Narten. "IANA Allocation Guidelines for Values in
IPv6 and Related Headers", Internet Draft (work in progress),
October 2002. www.watersprings.org/pub/id/draft-narten-
ipv6-iana-considerations-00.txt
Acknowledgments
Erik Nordmark coined a proposal for TSLLAO during a conversation with
JinHyeock Choi and Greg Daley.
Authors' Addresses
Greg Daley
greg.daley@eng.monash.edu.au
Nick "Sharkey" Moore
nick.moore@eng.monash.edu.au
Centre for Telecommunications and Information Engineering
Department of Electrical and Computer Systems Engineering
Monash University
Clayton 3800
Victoria
Australia
Erik Nordmark
erik.nordmark@sun.com
Sun Microsystems, Inc.
17 Network Circle
Mountain View, CA
USA
phone: +1 650 786 2921
fax: +1 650 786 5896
Intellectual Property Rights
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004
Copyright Statement
Copyright (C) The Internet Society (2004). This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their
rights.
This document and the information contained herein are provided
on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
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EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.
This document expires in December 2004.
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