DNA Working Group S. Narayanan, Ed.
Internet-Draft iTCD/CSUMB
Expires: January 11, 2009 July 10, 2008
Design document for Detecting Network Attachment in IPv6 Networks (DNAv6
Design)
draft-ietf-dna-protocol-08.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
Efficient detection of network attachment in IPv6 needs the following
three components: a method for hosts to detect link change in the
presence of unmodified (non-DNAv6) routers, a method for the host to
query routers on the link to identify the link (Link Identification)
and a method for the routers on the link to consistently respond to
such a query with minimal delay (Fast RA). Solving the link
identification based strictly on RFC 2461 is difficult because of the
flexibility offered to routers in terms of prefixes advertised in a
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router advertisement (RA) message. Similarly, the random delay in
responding to router solicitation messages imposed by RFC 2461 makes
it difficult to receive an RA quickly. In this memo, a mechanism
that was developed for detection of network attachement is documented
for future reference. This memo is an informational document and
thus does not define a new standard. The mechanism proposed in this
informational RFC requires the hosts to monitor all the prefixes
advertised on the link and use it for link identification in the
presence of non-DNAv6 routers is presented. A more efficient link-
identification mechanism requiring the DNAv6 routers to monitor the
link for advertised prefixes to assist the hosts in link
identification combined with a fast router advertisement mechanism
that selects the order of response for the router deterministically
is also presented.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terms and Abbreviations . . . . . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Link Identification . . . . . . . . . . . . . . . . . . . 6
3.2 Fast Router Advertisement . . . . . . . . . . . . . . . . 8
4. Data Structures . . . . . . . . . . . . . . . . . . . . . . 9
4.1 New Flags . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2 Landmark Option . . . . . . . . . . . . . . . . . . . . . 10
4.3 Learned Prefix Option . . . . . . . . . . . . . . . . . . 10
5. DNA Operation . . . . . . . . . . . . . . . . . . . . . . . 10
5.1 DNA Router Operation . . . . . . . . . . . . . . . . . . . 10
5.1.1 Data Structures . . . . . . . . . . . . . . . . . . . 10
5.1.2 Bootstrapping DNA Data Structures . . . . . . . . . . 11
5.1.3 Processing Router Advertisements . . . . . . . . . . . 12
5.1.4 Processing Router Solicitations . . . . . . . . . . . 12
5.1.5 Complete Router Advertisements . . . . . . . . . . . . 14
5.1.6 Inclusion of a common prefixes . . . . . . . . . . . . 14
5.1.7 Scheduling Fast Router Advertisements . . . . . . . . 16
5.1.8 Scheduling Unsolicited Router Advertisements . . . . . 17
5.1.9 Removing a Prefix from an Interface . . . . . . . . . 17
5.1.10 Prefix Reassignment . . . . . . . . . . . . . . . . 18
5.2 DNA Host Operation . . . . . . . . . . . . . . . . . . . . 18
5.2.1 Data Structures . . . . . . . . . . . . . . . . . . . 18
5.2.2 Host Configuration Variables . . . . . . . . . . . . . 19
5.2.3 Detecting Network Attachment Steps . . . . . . . . . . 19
5.2.4 Selection of a Landmark Prefix . . . . . . . . . . . . 20
5.2.5 Sending Router Solicitations . . . . . . . . . . . . . 20
5.2.6 Processing Router Advertisements . . . . . . . . . . . 21
5.2.7 DNA and Address Configuration . . . . . . . . . . . . 26
6. Security Considerations . . . . . . . . . . . . . . . . . . 29
6.1 Attacks on the Token Bucket . . . . . . . . . . . . . . . 29
6.2 Attacks on DNA Hosts . . . . . . . . . . . . . . . . . . . 30
6.3 Tentative options . . . . . . . . . . . . . . . . . . . . 30
6.4 Authorization and Detecting Network Attachment . . . . . . 31
6.5 Addressing . . . . . . . . . . . . . . . . . . . . . . . . 31
7. Constants . . . . . . . . . . . . . . . . . . . . . . . . . 32
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 33
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 33
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10. Informative References . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 36
Intellectual Property and Copyright Statements . . . . . . . 38
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1. Introduction
An efficient but complex mechanism to achieve the goals for detecting
network attachment (DNA) [20] is documented in this memo. As the
community decided to simplify the goals of DNA, this document was
modified to be an informational RFC for archival purpose only.
Hence, this document MUST NOT be considered to be making
recommendation for the behavior of IPv6 hosts or routers. A simplied
solution to achieve detection of network attachment can be found at
[6].
This memo documents the mechanism for an IPv6 host to detect link-
change in the presence of unmodified (non-DNAv6) routers and
proposes new extensions to "IPv6 Neighbor Discovery" [3] to increase
the efficiency of link-change detection in the presence of DNAv6
enabled routers. The proposed mechanism defines the construct that
identifies a link, proposes an algorithm for the routers on the link
to send a quick RA response without randomly waiting for upto
MAX_RA_DELAY_TIME seconds as specified in RFC2461 [3]. This memo
also defines a mechanism to exchange Source Link-Layer Address
without affecting the neighbor caches when the host is performing
Optimistic DAD.
The rest of the document refers to the proposed mechanisms by the
term "DNAv6".
2. Terms and Abbreviations
The term "link" is used as defined in RFC 2460 [2]. NOTE: this is
completely different from the term "link" as used by IEEE 802, etc.
Attachment: The process of establishing a layer-2 connection.
Attachment (and detachment) may cause a link-change.
DNA Hint: An indication from other subsystems or protocol layers that
link-change may have occurred.
Link-Change: Link-Change occurs when a host moves from a point-of-
attachment on a link, to another point-of-attachment where it is
unable to reach devices belonging to the previous link, without
being forwarded through a router.
Point-of-Attachment: A link-layer base-station, VLAN or port through
which a device attempts to reach the network. Changes to a
host's point-of-attachment may cause link-change.
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Reachability Detection: Determination that a device (such as a
router) is currently reachable. This is typically achieved using
Neighbor Unreachability Detection procedure [3].
3. Overview
The DNA protocol presented in this document tries to achieve the
following objectives:
o Eliminate the delays introduced by RFC 2461 in discovering the
configuration.
o Make it possible for the hosts to accurately detect the identity
of their current link from a single RS-RA pair in the presence of
either DNAv6 enabled routers and/or non-DNAv6 routers.
DNAv6 assumes that the host's link interface software and hardware is
capable of delivering a 'link up' event notification when layer 2 on
the host is configured and sufficiently stable for IP traffic. This
event notification acts as a DNA Hint to the layer 3 DNA procedures
to check whether or not the host is attached to the same link as
before. DNAv6 also assumes that an interface on the host is never
connected to two links at the same time. In the case that the layer
2 technology is capable of having multiple attachments (for instance,
multiple layer 2 associations or connections) at the same time, DNAv6
requires the individual layer-2 associations to be represented as
separate (virtual interfaces) to layer 3 and DNAv6 in particular.
3.1 Link Identification
DNAv6 uses the set of prefixes that are assigned to the link to
uniquely identify the link, which is quite natural and doesn't
require introducing any new form of identifier. However, this choice
implies that the protocol needs to be robust against changes in the
set of prefixes assigned to a link, including the case when a link is
renumbered and the prefix is later reassigned to a different link.
The protocol handles this during graceful renumbering (when the valid
lifetime of the prefix is allowed to decrease to zero before it is
removed and perhaps reassigned to a different link), it describes how
to remove and reassign prefixes earlier than this without any
incorrect behaviour, and will also recover in case where a prefix is
reassigned without following the draft recommendations.
DNAv6 is based on using a Router Solicitation/Router Advertisement
exchange to both verify whether the host has changed link, and if it
has, provide the host with the configuration information for the new
link. The base method for detecting link change involves getting
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routers to listen to all of the prefixes that are being advertised by
other routers on the link. They can then respond to solicitations
with complete prefix information. This information consists of the
prefixes a router would advertise itself as per RFC 2461, and also,
the prefixes learned from other routers on the link that are not
being advertised by itself. These learned prefixes are included in a
new Learned Prefix Option in the Router Advertisement.
A host receiving one of these "Complete RAs" - so marked by a flag -
then knows all of the prefixes in use on a link, and by inference all
those that are not. By comparing this with previously received
prefixes the host can correctly decide whether it is connected to the
same link as previously, or whether this Router Advertisement is from
a router on a new link.
If the link contains all non-DNAv6 routers, then the host relies on
the completeness (which the host always keeps track) of its own
prefix list to make a decision; i.e. if its own prefix list is known
to be 'complete', the host can make a decision by comparing the
received prefixes with its prefix list, if its own prefix is not yet
'complete', the host will wait for the completeness criteria to be
met before making the comparison.
Though frequently all routers on a link will advertise the same set
of prefixes and thus experience no cost in making the RAs complete,
there is potential for the RAs to be large when there are many
prefixes advertised. Two mechanisms are defined that allow certain
RAs to be reduced in size. Both these mechanisms use one prefix as a
representative for the set of prefixes on a particular link.
One uses a technique called a "landmark", where the host chooses one
of the prefixes as a landmark prefix, and then includes this in the
Router Solicitation message in the form of a question "Am I on the
link which has this prefix?". The landmark is carried in a new
option, called the Landmark Option.
In the case when the host is still attached to the same link, which
might occur when the host has changed from using one layer 2 access
point to another, but the access points are on the same link, the
Router Advertisement(s) it receives will contain a "yes, that prefix
is on this link" answer by the inclusion of the landmark prefix in
the RA, and no other information. Thus, such RA messages are quite
small.
In the case when the landmark prefix is unknown to the responding
router, the host will receive a "No" answer by non-inclusion of the
landmark prefix in the RA, and also the information it needs to
configure itself for the new link. The routers try to include as
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much information as possible in such messages, so that the host can
be informed of all the prefixes assigned to the new link as soon as
possible.
A second mechanism for reducing packet sizes applies to unsolicited
Router Advertisements. By selecting a common prefix on the link to
be the representative for the entire set of prefixes on the link, and
making sure that it is included in every advertisement, it is
possible to omit some prefixes. The smallest prefix on the link is
selected as the common prefix. Such advertisements will not inform a
host of all of the prefixes at once, but in general these unsolicited
advertisements will not be the first advertisement received on a
link. Inclusion of the smallest prefix simply ensures that there is
overlap in the information advertised by each router on a link and
that hosts will thus not incorrectly interpret one of these
incomplete RAs as an indication of movement. Even though this
document recommends the use of a prefix as the representative of the
link, future specifications can use the Learned Prefix Option to
include a non-prefix identifier as long as this identifier is 128 bit
long to avoid collision with any currently assigned prefix. So, any
future non-prefix link identifier MUST be 128 bits long.
The Router Advertisement messages are, in general, larger than the
solicitations, and with multiple routers on the link there will be
multiple advertisements sent for each solicitation. This
amplification can be used by an attacker to cause a Denial of Service
attack. Such attacks are limited by applying a rate limit on the
unicast Router Advertisements sent directly in response to each
solicitation, and using multicast RAs when the rate limit is
exceeded.
In order for the routers be able to both respond to the landmark
questions and send the complete RAs, the routers need to track the
prefixes that other routers advertise on the link. This process is
initialized when a router is enabled, by sending a Router
Solicitation and collecting the resulting RAs, and then multicasting
a few RAs more rapidly as already suggested in RFC 2461. This
process ensures with high probability that all the routers have the
same notion of the set of prefixes assigned to the link.
3.2 Fast Router Advertisement
According to RFC 2461 a solicited Router Advertisement should have a
random delay between 0 and MAX_RA_DELAY_TIME, to avoid the
advertisements from all the routers colliding on the link causing
congestion and higher probability of packet loss. In addition, RFC
2461 suggests that the RAs be multicast, and multicast RAs are rate
limited to one message every 3 seconds. This implies that the
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response to a RS might be delayed up to 3.5 seconds.
DNAv6 avoids this delay by using a different mechanism to ensure that
two routers will not respond at exactly the same time while allowing
one of the routers on the link to respond immediately. Since the
hosts might be likely to use the first responding router as the first
choice from their default router list, the mechanism also ensures
that the same router doesn't respond first to the RSs from different
hosts.
The mechanism is based on the routers on the link determining (from
the same RAs that are used in Section 3.1 to determine all the
prefixes assigned to the link), the link-local addresses of all the
other routers on the link. With this loosely consistent list, each
router can independently compute some function of the (link-local)
source address of the RS and each of the routers' link-local
addresses. The results of that function are then compared to create
a ranking, and the ranking determines the delay each router will use
when responding to the RS. The router which is ranked as #0 will
respond with a zero delay.
If the routers become out-of-sync with respect to their learned
router lists, two or more routers may respond with the same delay,
but over time the routers will converge on their lists of learned
routers on the link.
4. Data Structures
This memo defines two new flags and three new options.
4.1 New Flags
This document defines two new flags to be exchanged between the
router and hosts. One to indicate that the router sending the
message is participating in the proposed protocol as well as a flag
to indicate the completeness of the set of prefixes included in its
messages.
DNAAware (D)
The DNAAware (D) bit indicates that the router sending the message
is participating in the protocol documented in this memo. Other
routers should include this router in calculating response delay
tokens.
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Complete (C)
The Complete (C) bit indicates that the Router Advertisement
contains PIOs for all prefixes explicitly configured on the
sending router, and, if other routers on the link are advertising
additional prefixes, a Learned Prefix Option containing all
additional prefixes that the router has heard from other routers
on the link.
4.2 Landmark Option
The Landmark Option is used by hosts in a Router Solicitation message
to ask the routers on a link if the specified prefix is being
advertised by some router on the link.
4.3 Learned Prefix Option
The Learned Prefix Option (LPO) is used by a router to indicate
prefixes that are being advertised by other routers on the link, but
not by itself.
5. DNA Operation
5.1 DNA Router Operation
Routers MUST collect information about the other routers that are
advertising on the link.
5.1.1 Data Structures
The routers maintain a set of conceptual data structures for each
interface to track the prefixes advertised by other routers on the
link, and also the set of DNA routers (the routers that will quickly
respond to RSs) on the link.
For each interface, routers maintain a list of all prefixes learned
from other routers on the link but not explicitly configured on the
router's own interface. The list will be referred to in this
document as "DNARouterLearnedPrefixList". Prefixes are learned by
their reception within Prefix Information Options [3] in Router
Advertisements. Prefixes in Learned Prefix Options (see Section 4.3)
MUST NOT update the contents of DNARouterLearnedPrefixList. For each
prefix the router MUST store sufficient information to identify the
prefix and to know when to remove the prefix entry from the list.
This may be achieved by storing the following information:
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1. Prefix
2. Prefix length
3. Prefix valid lifetime
4. Expiry time
The expiry time for entries in DNARouterLearnedPrefixList is
LEAST_VALID_LIFETIME after the last received Router Advertisement
affecting the entry, or the scheduled expiry of the prefix valid
lifetime, whichever is earlier.
For each interface, routers also maintain a list of the other routers
advertising on the link. The list will be referred to in this memo
as "DNARouterList". For each router from which a Router
Advertisement is received with the DNAAware flag set, the following
information MUST be stored:
1. Link-local source address of advertising router
2. Token equal to the first 64 bits of an SHA-1 hash of the above
address
3. Expiry time
Each router MUST include itself in the DNARouterList and generate a
token for itself as described above based on the link-local address
used in its RA messages.
The expiry time for entries in DNARouterList is LEAST_VALID_LIFETIME
after the last received Router Advertisement affecting the entry.
5.1.2 Bootstrapping DNA Data Structures
As per RFC 2461 [3], when an interface on a host first starts up, it
SHOULD transmit up to MAX_RTR_SOLICITATIONS Router Solicitations
separated by RTR_SOLICITATION_INTERVAL in order to quickly learn of
the routers and prefixes active on the link. DNAv6 requires the
router to follow the same steps when its interface first starts up.
Upon startup, a router interface SHOULD also send a few unsolicited
Router Advertisements as recommended in Section 6.2.4 of RFC 2461
[3], in order to inform others routers on the link of its presence.
During the bootstrap period ( (MAX_RTR_SOLICITATIONS - 1) *
RTR_SOLICITATION_INTERVAL + RetransTimer [3]), a router interface
both sends unsolicited Router Advertisements and responds to Router
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Solicitations, but the Router Advertisements MUST NOT include any DNA
specific options except for setting the DNAAware flag. The DNAAware
flag is set so that other routers will know to include this router in
their timing calculations for fast RA transmission. Other DNA
options are omitted because the router may have incomplete
information during bootstrap.
During the bootstrap period, the Complete flag in Router
Advertisements MUST NOT be set.
During the bootstrap period, the timing of Router Advertisement
transmission is as specified in RFC 2461.
5.1.3 Processing Router Advertisements
When a router receives a Router Advertisement, it first validates the
RA as per the rules in RFC 2461, and then performs the actions
specified in RFC 2461. In addition, each valid Router Advertisement
is processed as follows:
If the DNAAware flag is set in the RA, the router checks if there is
an entry in its DNARouterList by looking up the source address of the
RA in that list. If not found, a new entry is added to
DNARouterList, including the source address and a token equal to the
first 64 bits of an SHA-1 hash of the source address. The entry's
expiry time is updated.
Regardless of the state of the DNAAware flag, each PIO in the RA is
examined. If the prefix is not in the router's
DNARouterLearnedPrefixList and not in the router's AdvPrefixList [3],
the prefix is added to the DNARouterLearnedPrefixList, and its expiry
time is set.
5.1.4 Processing Router Solicitations
The usual response to a Router Solicitation SHOULD be a unicast RA.
However, to keep control of the rate of unicast RAs sent, a token
bucket is used. The token bucket is filled at one token every
UNICAST_RA_INTERVAL. A maximum of MAX_UNICAST_RA_BURST tokens are
stored.
When a Router Solicitation is received, the router checks if it is
possible to send a unicast response. A unicast response requires
that the following conditions to be met:
o A unicast send token is available.
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o The source address of the Router Solicitation is NOT the
unspecified address (::).
If a unicast response is possible and the Router Solicitation
contains a Landmark Option whose prefix is present in
DNARouterLearnedPrefixList or AdvPrefixList, the router SHOULD send
an abbreviated Router Advertisement.This abbreviated advertisement
includes the Landmark prefix in a PIO if the prefix is in the
AdvPrefixList or in a LPO if the prefix is found in the
DNAROuterLearnedPrefixList, plus the base RA header and any SEND
options as appropriate. The DNAAware flag MUST be set. The Complete
flag MUST NOT be set. This is the one exception where the common
prefix (i.e. the smallest prefix) MAY be omitted.
If there is NO Landmark Option in the received Router Solicitation or
it contains a Landmark Option whose prefix is NOT present in
DNARouterLearnedPrefixList or AdvPrefixList or a unicast response is
not possible, then the router SHOULD generate a Complete RA as
specified in Section 5.1.5. The Router Advertisement MUST include
the common prefix(es), as described in Section 5.1.6.
If a unicast response is possible, then a token is removed and the
Router Advertisement is scheduled for transmission as specified in
Section 5.1.7.
If a unicast response is not possible and there is no multicast RA
already scheduled for transmission in the next MULTICAST_RA_DELAY the
RA MUST be sent to the link-scoped all-nodes multicast address at the
current time plus MULTICAST_RA_DELAY.
If a unicast response is not possible but there is a multicast RA
already scheduled for transmission in the next MULTICAST_RA_DELAY,
then the Router Solicitation MUST be silently discarded.
All Router Advertisements sent by a DNA router MUST have the "D" flag
set so that hosts processing them know that they can interpret the
messages according to this specification.
In order to understand the conditions leading to the different type
of Router Advertisement messages, please refer to the figure below,
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+---------------+--+----+-----+-----+----+-----+----+-----+----+
| RA Message | Unicast | Multicast |
+---------------+--+----+-----+-----+----+-----+----+-----+----+
| Abbreviated RA| Landmark prefix present| Never |
| | on the link | |
+---------------+--+----+-----+-----+----+-----+----+-----+----+
| Complete RA |No LO in RS or Landmark |No token available in|
| |prefix NOT present | the token bucket. |
| | on the link. | |
+---------------+--+----+-----+-----+----+-----+----+-----+----+
5.1.5 Complete Router Advertisements
A CompleteRA is formed as follows:
Starting with a Router Advertisement with all fixed options (MTU,
Advertisement Interval, flags, etc.), the DNAAware flag is set. As
many Prefix Information Options for explicitly configured prefixes as
will fit are added to the Router Advertisement. If there is
sufficient room, a Learned Prefix Option as defined in Section 4.3
containing as many of the learned prefixes as will fit is added.
It may not be possible to include all of the prefixes in use on the
link due to MTU or administrative limitations. If all Prefix
Information Options and a Learned Prefix Option containing all of the
learned prefixes were included in the RA, then the Complete flag in
the Router Advertisement header is set.
If there are known to be prefixes that are not included in the Router
Advertisement, then the Complete flag MUST NOT be set.
Note that although it may not be possible to fit all of the prefixes
into an RA, the smallest prefix(es) MUST be included as discussed in
Section 5.1.6.
5.1.6 Inclusion of a common prefixes
Among the prefixes advertised on a link, all routers selects one
prefix and include that as a common prefix whenever they send an RA,
both solicited and unsolicited.The inclusion of the common prefix
ensures that there always is an overlap in the information advertised
by each router on the link and that hosts will have a common prefix
to correlate all RA messages received from routers on the same link.
This section presents how the routers select the common prefix
without pre-arrangement,advertise it and change the common prefix
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gracefully without causing hosts to mistakenly assume a link change.
Even when stateful address configuration (DHCPv6) is used, at least
one router on a link MUST be configured with one prefix, so that the
common prefix can be included in the RA messages.
5.1.6.1 Selecting the common prefix
The router MUST pick the smallest prefix of all the prefixes
configured on the routers on the link as the common prefix. The
selection is made from among the prefixes whose valid lifetime is
greater than LEAST_VALID_LIFETIME, and learned prefixes which were
received within LEAST_VALID_LIFETIME.
For comparing prefixes, they are padded to the right with zeros to
make them 128 bit unsigned integers. Note that this smallest prefix
is the smallest of all the prefixes configured on the routers on the
link and may not be the smallest prefix used in the link through
stateful address configuration.
When a router receives a new prefix in PIO, if the prefix is smaller
than the current common prefix and has valid lifetime greater than
LEAST_VALID_LIFETIME, the router selects that new prefix as a new
common prefix. In case a new prefix smaller than the current common
prefix is advertised in LPIO, the router doesn't change the common
prefix.
5.1.6.2 Advertising the common prefix
Whenever a router sends an RA, whether solicited or unsolicited, it
MUST include the common prefix in it. Hence, all RAs MUST carry the
common prefix except the abbreviated RA message sent in response to a
RS with LO.
When a router advertises the common prefix, if the common prefix is
explicitly configured on the router, it sends it in PIO. If the
prefix was learned from advertisement of another router on the link,
the router sends the common prefix in LPIO.
5.1.6.3 Changing the common prefix gracefully
Basic idea is, when a router changes a common prefix, it MUST send
both the new common prefix and the old common prefix to ensure an
overlapping prefix among RAs for LEAST_VALID_LIFETIME period and then
it can retire the old common prefix.
When either a new prefix is added to a link that is numerically
smaller than the current common prefix or the lifetime of the current
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common prefix falls below LEAST_VALID_LIFETIME, a new common prefix
MUST be determined. In order to ensure that there is overlap between
consecutive RAs on the link, the old common prefix must continue to
be advertised for some time alongside the new common prefix. After
the change, the old common prefix MUST be included in RAs for the
following LEAST_VALID_LIFETIME. If the common prefix changes
multiple times within LEAST_VALID_LIFETIME time window, the RA SHOULD
include all of the previous common prefixes that were advertised
during that time window.
For the purposes of propagating information, it is reasonable to
assume that after three advertisements of the change, all routers
have been made aware of it.
5.1.6.3.1 Using non-prefix identifier as common prefix
Although this memo only discusses the use of prefixes as common
identifier among multiple RA messages, a future specification or
ammendment may describe a mechanism to select a "link identifier"
that is not a prefix.
Sinice information from the Learned Prefix Option is only stored in
DNAHostPrefixList, and is only used for DNA purposes and because a
length field is used in LPIO, it is possible to carry any variable
length identifier less than or equal to 128 bits in an LPIO and store
it in DNAHostPrefixList (Section 5.2.1). To avoid any collision to
prefixes, an future non-prefix link identifier MUST be 128 bits long
and can be included in the LPIO of a RA message.
Future specifications are advised NOT to treat the information in an
LPIO as prefixes such as they would the prefixes found in a Prefix
Information Option. Future specifications are also advised NOT to
assume that the entries in a host's DNAHostPrefixList are actual
prefixes in use on the link.
5.1.7 Scheduling Fast Router Advertisements
RAs may need to be delayed to avoid collisions in the case that there
is more than one router on a link. The delay is calculated by
determining a ranking for the router for the received RS, and
multiplying that by RA_SEPARATION.
A Host Token is needed from the RS to calculate the router's ranking.
The first 64 bits of an SHA-1 hash of the source address of the RS
MUST be used as the RS host token.
A router's ranking is determined by taking the XOR of the RS Host
Token and each of the stored Router Tokens. The results of these XOR
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operations are sorted lowest to highest. The router corresponding to
the first entry in the sorted list is ranked zero, the second, one,
and so on.
Note: it is not necessary for a router to actually sort the whole
list. Each router just needs to determine its own position in the
sorted list.
If Rank < FAST_RA_THRESHOLD, then the RA MUST be scheduled for
transmission in Rank * RA_SEPARATION milliseconds. When the router
is ranked as zero, the resulting delay is zero, thus the RA SHOULD be
sent immediately.
If Rank >= FAST_RA_THRESHOLD, then the RA MUST be replaced with a
Complete RA, if there is not one already, and scheduled for multicast
transmission as in RFC 2461.
5.1.8 Scheduling Unsolicited Router Advertisements
Unsolicited router advertisements MUST be scheduled as per RFC 2461.
The "D" flag in the RA header MUST be set.
They MAY be Complete RAs or MAY include only a subset of the
configured prefixes, but MUST include the common prefix as discussed
in Section 5.1.6.
This ensures that there will be overlap in the sets of prefixes
contained in consecutive RAs on a link from DNA routers, and thus an
absence of that overlap can be used to infer link change.
5.1.9 Removing a Prefix from an Interface
When a prefix is to stop being advertised in a PIO in RAs by an
interface before the expiry of the prefix's valid lifetime, then the
router MUST add the prefix to the DNARouterLearnedPrefixList and set
it to expire in LEAST_VALID_LIFETIME or at the expiry of the last
advertised valid lifetime, whichever is earlier. This ensures that
to hosts there will be overlap in the prefixes in the RAs they see
and prevent them from incorrectly interpreting changed prefixes as
movement.
5.1.9.1 Early Removal of the common Prefix
If the common (the smallest) prefix is to be withdrawn early from a
link, that is before the expiry of its previously advertised valid
lifetime, it MUST be advertised for at least LEAST_VALID_LIFETIME
with a valid lifetime of less than LEAST_VALID_LIFETIME. This
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ensures that all of the other routers are notified to begin the
process of changing the common prefix as well, and hosts will always
see overlap between the prefixes in consecutive RAs and thus not
mistake an RA for an indication of link change.
5.1.10 Prefix Reassignment
A prefix whose lifetime has expired after counting down in real time
for at least LEAST_VALID_LIFETIME may be reassigned to another link
immediately after expiry. If a prefix is withdrawn from a link
without counting down to the expiry of its valid lifetime, it SHOULD
NOT be reassigned to another link for at least LEAST_VALID_LIFETIME
or until the original expiry time, whichever is earlier. This gives
sufficient time for other routers that have learned the prefix to
expire it, and for hosts that have seen advertisements from those
routers to expire the prefix as well.
Earlier reassignment may result in hosts that move from between the
old and new links failing to detect the movement.
When the host is sure that the prefix list is complete, a false
movement assumption may happen due to renumbering when a new prefix
is introduced in RAs at about the same time as the host handles the
'link UP' event. We may solve the renumbering problem with minor
modification as specified below.
When a router starts advertising a new prefix, it includes at least
one old prefix in the same RA. The old prefix assures that the host
doesn't falsely assume a link change because of a new prefix. After
a while, hosts will recognize the new prefix as the one assigned to
the current link and update its prefix list.
In this way, we may provide a fast and robust solution. If a host
can make the Complete Prefix List with certainty, it can check for
link change fast. Otherwise, it can fall back on a slow but robust
scheme. It is up to the host to decide which scheme to use.
5.2 DNA Host Operation
Hosts collect information about the prefixes advertised on the link
to facilitate change detection.
5.2.1 Data Structures
Hosts MUST maintain a list of prefixes advertised on the link. This
is separate from the RFC 2461 "Prefix List" and will be referred to
here as the "DNAHostPrefixList". All prefixes SHOULD be stored,
however an upper bound MUST be placed on the number stored to prevent
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overflow. For each prefix stored the host MUST store the following
information:
1. Prefix
2. Prefix length
3. Expiry time
If a host is not able to store this information for every prefix,
there is a risk that the host will incorrectly decide that it has
moved to a new link, when it receives advertisements from a non-DNA
router.
Prefix entries in the DNAHostPrefixList expire and MUST be removed
LEAST_VALID_LIFETIME after they are last seen in a received Router
Advertisement (in either a PIO or LPIO) or at the expiry of the valid
lifetime of the prefix, whichever is earlier.
Hosts SHOULD also maintain a "Landmark Prefix" as described in
Section 5.2.4.
5.2.2 Host Configuration Variables
Hosts MUST make use of the following conceptual variables and they
SHOULD be configurable:
DNASameLinkDADFlag
Boolean value indicating whether or not a host should re-run DAD
when DNA indicates that link change has not occurred.
Default: False
5.2.3 Detecting Network Attachment Steps
An IPv6 host SHOULD follow the following steps when they receive a
DNA Hint indicating the possibility of link change.
1. Mark all the preferred IPv6 addresses in use as optimistic. See
Section 5.2.7.2.
2. Set all Neighbor Cache entries for routers on its Default Router
List to STALE.
3. Send router solicitation. (See Section 5.2.5).
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4. Receive router advertisement(s).
5. Mark the router Neighbor Cache Entry [3] as REACHABLE for the
router from which RA(s) arrived, or add a new Neighbor Cache
Entry for the router in the REACHABLE state if one does not
currently exist.
6. Process received router advertisement. (See Section 5.2.6).
7. If the link has changed
Change the IP configuration parameters of the host (see
Section 5.2.7).
8. If the link has NOT changed
Restore the address configuration state of all the IPv6
addresses known to be on the link. See Section 5.2.7.2.
9. Update default routers list and their reachability information
(see Section 5.2.6.3).
5.2.4 Selection of a Landmark Prefix
For each interface, hosts SHOULD choose a prefix to use as a Landmark
Prefix in Router Solicitations. The following rules are used in
selecting the landmark prefix:
The prefix MUST have a non-zero valid lifetime. If the valid
lifetime of a previously selected Landmark Prefix expires, a new
Landmark Prefix MUST be selected.
The prefix MUST be one of those that the hosts has used to assign
a non-link-local address to itself.
The prefix SHOULD be chosen as the one with the longest preferred
lifetime, but it is not necessary to switch to different prefix if
the preferred lifetime of the current landmark prefix changes.
5.2.5 Sending Router Solicitations
Upon the occurrence of a Layer 2 link-up event notification, hosts
SHOULD send a Router Solicitation. Hosts SHOULD apply rate limiting
and/or hysteresis to this behaviour as appropriate to the link
technology subject to the reliability of the DNA Hints.
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Editor's note: The following two paragraph are talking about behavior
specified by 2461. Do we want to keep these?
The host also uses this to trigger sending an RS, subject to the rate
limitations in [3]. Since there is no natural limit on how
frequently the link UP notifications might be generated, we take the
conservative approach that even if the host establishes new link
layer connectivity very often, under no circumstances should it send
Router Solicitations more frequently than RTR_SOLICITATION_INTERVAL
as specified by RFC 2461 [3].
If the RS does not result in the host receiving at least one RA with
at least one valid prefix, then the host can retransmit the RS. It
is allowed to multicast up to MAX_RTR_SOLICITATIONS RS messages
spaced RTR_SOLICITATION_INTERVAL apart as per RFC 2461 [3].
Note that if link-layer notifications are reliable, a host can reset
the number of sent Router Solicitations to 0, while still maintaining
RTR_SOLICITATION_INTERVAL between RSs. Resetting the count is
necessary so that after each link up notification, the host is
allowed to send MAX_RTR_SOLICITATIONS to reliably discover the,
possibly new, prefix list.
Hosts SHOULD include a Landmark Option (LO) in the RS message with
the landmark prefix chosen based on the rules in Section 5.2.4.
Hosts SHOULD include a tentative source link layer address option
(TO) in the RS message. The router solicitation message is sent to
the All_Routers_Multicast address and the source address MUST be the
link local address of the host.
The host MUST consider its link local address to be in the
"Optimistic" state for duplicate address detection [5] until either
the returned RA confirms that the host has not switched to a new link
or, if an link change has occurred, until the host has performed
optimistic duplicate address detection for the address.
5.2.6 Processing Router Advertisements
When the host receives a Router Advertisement, the host checks for
the following conditions in the given order and derives the
associated conclusions given below:
If the RA includes a prefix that matches an entry in its
DNAHostPrefixList, then the host SHOULD conclude that no link
change has occurred and the current configuration can be assumed
to still be current.
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If the RA is a Complete RA, as indicated by the "Complete" flag in
the RA header, and there are no prefixes included in it in either
a PIO or LPIO that are also in the host's DNAHostPrefixList, then
the host MUST conclude that it has changed link and MUST initiate
re-configuration using the information in the received Router
Advertisement.
If the host has the complete prefix list (CPL) and the RA does NOT
include any prefixes in either a PIO or LPIO that matches a prefix
in CPL then the host MUST conclude that link change has occurred
and use the information in the received RA to configure itself.
If the host doesn't have the complete prefix list (CPL), the
received RA is not complete, contains no prefixes that are stored
in DNAHostPrefixList, then the host SHOULD execute RS/RA exchange
until num_RS_RA is equal to NUM_RS_RA_COMPLETE to create a new CPL
and compare it with the already known prefixes. If after
NUM_RS_RA_COMPLETE exchanges still no prefix received in either a
PIO or LPO of the RAs match known prefixes, the host MUST conclude
link change. If a matching prefix is received in the RAs, then
the host SHOULD conclude that no link change has occured.
5.2.6.1 Pseudocode
IF (Receive RA contains a prefix matching a prefix in
DNAHostPrefixList) THEN
{
/* This case covers the landmark prefix being included in the RA,
smallest prefix included in RA or CompleteRA message containg all
prefixes*/
No link change has occured.
RETURN; // Don't have to do the following checks.
}
IF (Receive RA is a CompleteRA) THEN
{
/* We already checked if there are any matching prefix before.
Since this is a CompleteRA, implies link-change.*/
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Link change has occured.
RETURN; // Don't have to do the following checks.
}
{
Link change has occured.
RETURN; // Don't have to do the following checks.
} -->
IF (DNAHostPrefixList is marked as complete (i.e. the completeness
criteria is already met)) THEN
{
/* We already checked if there are any matching prefix before.
Since the DNAHostPrefixList is complete, implies link-change.*/
Link change has occured.
RETURN; // Don't have to do the following checks.
}
Wait for NUM_RS_RA_COMPLETE exchanges of RS/RA message to be done
since the previous link UP event (Previous link UP event here refers
to the link UP received before the current link UP event that lead to
this processing).
IF (One of the received RA contains a prefix matching a prefix in
DNAHostPrefixList from before the current link UP event), THEN No
link change has occured ELSE link change has occured.
5.2.6.2 Maintaining the DNAHostPrefixList
The host should maintain a current DNAHostPrefixList with the
prefixes learned after the current link UP event and a previous
DNAHostPrefixList with prefixes learned prior to the link UP event.
These data structures are maintained per interface.
If the Router Advertisement has the C flag set, then the host SHOULD
make the current DNAHostPrefixList match the contents of the
advertisement and mark it as complete (i.e. it becomes CPL). Any new
prefixes are added and any prefixes in the list that are absent in
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the advertisement are removed. Expiry times on prefixes are updated
if the prefix was contained in a PIO, but not if it was contained in
an LPO.
If the Router Advertisement does not have the C flag set, then the
host SHOULD add any new prefixes and update expiry times as above,
but SHOULD NOT remove any entries from DNAHostPrefixList.
If the host decides that a link change has occurred after processing
the received RA message, it uses the information available in the
current DNAHostPrefixList to configure itself as specified in
Section 5.2.7. If the host decides that it is on the same link, then
the current DNAHostPrefixList and the previous DNAHostPrefixList are
merged as specified in the next sub-section and the merged list
becomes the current DNAHostPrefixList.
For each interface, the host also maintains a counter (called
num_RS_RA) which counts how many successful RS/RA exchanges have been
accomplished since the last time the host moved to a different link.
Note that this is not necessarily since the last link UP event as a
link UP event may not correspond to an actual link change. The host
declares "one successful RS/RA exchange" is accomplished after it
sends an RS, waits for MIN_RA_WAIT seconds and receives a positive
number of resulting RAs. At least one RA (with at least one prefix)
should be received. After the RS, if a link UP event occurs before
MIN_RA_WAIT seconds expire, the host should not assume that a
successful RS/RA exchange is accomplished. This counter is used to
determine when DNAHostPrefixList is considered to be complete. The
host SHOULD conclude that the prefix list is complete when
NUM_RS_RA_COMPLETE number of RS/RA exchanges have been completed or a
RA message with the complete bit set is received. The complete
DNAHostPrefixList is also refered to as CPL ( Complete Prefix List).
After NUM_RS_RA_COMPLETE RS/ RA exchange, the host will generate the
Complete Prefix List if there is no packet loss.
5.2.6.2.1 Merging DNAHostPrefixList
When a host has been collecting information about a potentially
different link in its Current DNAHostPrefixList, and it discovers
that it is in fact the same link as another DNAHostPrefixList, then
it needs to merge the information in the two objects to produce a
single new object. Since the DNAHostPrefixList contains the most
recent information, any information contained in it will override the
information in the old DNAHostPrefixList, for example the remaining
lifetimes for the prefixes. When the two objects contain different
pieces of information, for instance different prefixes or default
routers, the union of these are used in the resulting merged object.
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5.2.6.3 Router Reachability Detection and Default Router Selection
The receipt of a unicast RA from a router in response to a multicast
RS indicates that the host has bi-directional reachability with the
routers that responded. Such reachability is necessary for the host
to use a router as a default router, in order to have packets routed
off the host's current link. It is notable that the choice of
whether the messages are addressed to multicast or unicast address
will have different reachability implications. The reachability
implications from the hosts' perspective for the four different
message exchanges defined by RFC 2461 [3] are presented in the table
below. The host can confirm bi-directional reachability from the
neighbor discovery or router discovery message exchanges except when
a multicast RA is received at the host for its RS message. In this
case, using IPv6 Neighbour Discovery procedures, the host cannot know
whether the multicast RA is in response to its solicitation message
or whether it is a periodic un-solicited transmission from the router
[3].
+-----------------+----+----+----+-----+
| Exchanges: |Upstream |Downstream|
+-----------------+----+----+----+-----+
| multicast NS/NA | Y | Y |
+-----------------+----+----+----+-----+
| unicast NS/NA | Y | Y |
+-----------------+----+----+----+-----+
| RS/multicast RA | N | Y |
+-----------------+----+----+----+-----+
| RS/unicast RA | Y | Y |
+-----------------+----+----+----+-----+
If the destination address of the received RA is a unicast address,
the host knows the router heard its RS, and therefore that the host
has reachability with the router.
Prior to sending a DNA RS in response to an indication of link
change, the host SHOULD set all Neighbor Cache entries for routers on
its Default Router List to STALE. When the host receives an RA in
reply to the RS, the host SHOULD mark that router's Neighbor Cache
Entry [3] as REACHABLE, or add a Neighbor Cache Entry in the
REACHABLE state if one does not currently exist.
The host SHOULD also update its Default Router List in the following
fashion. If any of the routers returning RAs are already on the
default router list, the host SHOULD use the information in the RA to
update the Default Route List entry with the new information. The
host SHOULD add entries to the Default Router List for any routers
returning RAs that are not on the list. The host SHOULD confine
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selection of a router from the Default Router List to those routers
whose Neighbor Cache entries are in the REACHABLE state. Note that
the Default Router List SHOULD be updated as described here
regardless of whether the RA indicates that the host has changed to a
new IP link, since changes in router reachability are possible on
some link types even if the host remains on the same IP link.
Note that this procedure does not prevent a MN from sending packets
to its current default router while the RA solicitation is in
progress and if reachability with the current default router is
unchanged, there should be no change in default router after the RA
solicitation completes. If the current default router is still
reachable, it will forward the packets.
5.2.7 DNA and Address Configuration
When a host moves to a new point of attachment, a potential exists
for a change in the validity of its unicast and multicast addresses
on that network interface. In this section, host processing for
address configuration is specified. The section considers both
statelessly and statefully configured addresses.
5.2.7.1 Duplicate Address Detection
A DNA host MUST support optimistic Duplicate Address Detection [5]
for autoconfiguring unicast link local addresses. If a DNA host uses
address autoconfiguration [8] for global unicast addresses, the DNA
host MUST support optimistic Duplicate Address Detection for
autoconfiguring global unicast addresses.
5.2.7.2 DNA and the Address Autoconfiguration State Machine
When a link level event occurs on a network interface indicating that
the host has moved from one point of attachment to another, it is
possible that a change in the reachability of the addresses
associated with that interface may occur. Upon detection of such a
link event and prior to sending the RS initiating a DNA exchange, a
DNA host MUST change the state of addresses associated with the
interface in the following way (address state designations follow RFC
2461):
o Addresses in the "Preferred" state are moved to the "Optimistic"
state, but the host defers sending out an NS to initiate Duplicate
Address Detection.
o Addresses in the "Optimistic" state remain in the "Optimistic"
state, but the host defers sending out an NS to initiate Duplicate
Address Detection.
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o Addresses in the "Deprecated" state remain in the "Deprecated"
state.
o No addresses should be in the "Tentative" state, since this state
is unnecessary for nodes that support optimistic Duplicate Address
Detection.
A host MUST keep track of which "Preferred" addresses are moved to
the "Optimistic" state, so it is possible to know which addresses
were in the "Preferred" state and which were in the "Optimistic"
state prior to the change in point of attachment.
In order to perform the DNA transaction, the DNA host SHOULD select
one of the unicast link local addresses that was in the "Preferred"
state prior to switching to "Optimistic" and utilize that as the
source address on the DNA RS. If the host had no "Preferred" unicast
link local address but did have an address in the "Optimistic" state,
it MUST utilize such an address as the source address. If the host
currently has no unicast link local addresses, it MUST construct one
and put it into the "Optimistic" state and note this address as
having been in the "Optimistic" state previously, but defer sending
the NS to confirm. Note that the presence of a duplicate unicast
link local address on the link will not interfere with the ability of
the link to route a unicast DNA RA from the router back to the host
nor will it result in corruption of the router's neighbor cache,
because the TO is included in the RS and is utilized by the router on
the RA frame without changing the neighbor cache.
When the host receives unicast or multicast RAs from the router, if
the host determines from the received RAs that it has moved to a new
link, the host MUST immediately move all unicast global addresses to
the "Deprecated" state and configure new addresses using the subnet
prefixes obtained from the RA. For all unicast link local addresses,
the host MUST initiate NS signaling for optimistic Duplicate Address
Detection to confirm the uniqueness of the unicast link local
addresses on the new link.
If the host determines from the received RAs that it has not moved to
a new link (i.e. the link has not changed) and the previous state of
an address was "Optimistic", then the host MUST send an NS to confirm
that the address is unique on the link. This is required because
optimistic Duplicate Address Detection may not have completed on the
previous point of attachment, so the host may not have confirmed
address uniqueness. If the previous state of an address was
"Preferred", whether or not the host initiates optimistic Duplicate
Address Detection depends on the configurable DNASameLinkDADFlag
flag. A host MUST forgo sending an NS to confirm uniqueness if the
value of the DNASameLinkDAD flag is False. If, however, the
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DNASameLinkDAD flag is True, the host MUST perform optimistic
duplicate address detection on its unicast link local and unicast
global addresses to determine address uniqueness.
5.2.7.3 DNA and Statefully Configured Addresses
The DHCPv6 specification [17] requires hosts to send a DHCPv6 CONFIRM
message when a change in point of attachment is detected. Since the
DNA protocol provides the same level of movement detection as the
DHCPv6 CONFIRM, it is RECOMMENDED that DNA hosts not utilize the
DHCPv6 CONFIRM message when a DNA RA is received, to avoid excessive
signaling. If, however, a non-DNA RA is received, the host SHOULD
use the DHCPv6 CONFIRM message as described in RFC 3315 [17] rather
than wait for additional RAs to perform CPL, since this will reduce
the amount of time required for the host to confirm whether or not it
has moved to a new link. If the CONFIRM message validates the
addresses, the host can continue to use them.
When a DNA RA is received and the received RA indicates that the host
has not moved to a new link, the host SHOULD apply the same rules to
interpreting the 'M' flag in the received RA and any subsequently
received RAs as in Section 5.5.3 of RFC 2461 [3]. That is, if an RA
is received with the 'M' flag set, then the 'M' flag value is copied
into the ManagedFlag, and if the ManagedFlag changes from False to
True the host should run DHCPv6, but if the ManagedFlag changes from
True to False, the host should continue to run DHCPv6. If, however,
the value of the ManagedFlag remains the same both before and after
the change in point of attachment on the same link has been
confirmed, it is NOT RECOMMENDED that the host run DHCPv6 to obtain
new addresses, since the old addresses will continue to be valid.
If the DNA RA indicates that the host has moved to a new link or the
DHCPv6 CONFIRM indicates that the addresses are invalid, the host
MUST move its old addresses to the "Deprecated" state and MUST run
DHCPv6 to obtain new addresses. Normally, the DHCPv6 operation is
4-message exchange, however, this exchange allows for redundancy
(multiple DHCPv6 servers) without wasting addresses, as addresses are
only provisionally assigned to a host until the host chooses and
requests one of the provisionally assigned addresses. If the DNA
host supports the Rapid Commit Option [17], the host SHOULD use the
Rapid Commit Option in order to shorten the exchange from 4 messages
to 2 messages.
5.2.7.4 Packet Delivery During DNA
The specification of packet delivery before, during, and immediately
after DNA when a change in point of attachment occurs is out of scope
for this document. The details of how packets are delivered depends
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on the mobility management protocols (if any) available to the host's
stack.
5.2.7.5 Multicast Address Configuration
Multicast routers on a link are aware of which groups are in use
within a link. This information is used to undertake initiation of
multicast routing for off-link multicast sources to the link
[10][18].
If the returning RAs indicate that the host has not moved to a new
link, no further action is required for multicast addresses to which
the host has subscribed using MLD Report [18]. In particular, the
host MUST NOT perform MLD signaling for any multicast addresses
unless such signaling was not performed prior to movement to the new
point of attachment. For example, if an address is put into the
"Optimistic" state prior to movement but the MLD Report for the
Solicited_Node_Multicast_Address is not sent prior to movement to a
new point of attachment, the host MUST send the MLD Report on the new
point of attachment prior to performing optimistic Duplicate Address
Detection. The host SHOULD use the procedure described below for
sending an MLD Report.
If, on the other hand, the DNA RA indicates that the host has moved
to a new link, the host MUST issue a new MLD Report to the router for
subscribed multicast addresses. MLD signaling for the
Solicited_Node_Multicast_Addresses [8] MUST be sent prior to
performing signaling for optimistic DAD.
To avoid lengthy delays in address reconfiguration, it is RECOMMENDED
that the host send the MLD Report for newly configured addresses
immediately, as soon as the addresses have been constructed, rather
than waiting for a random backoff.
Hosts MUST defer MLD signaling until after the results of DNA have
confirmed whether or not a link change has occurred.
6. Security Considerations
6.1 Attacks on the Token Bucket
A host on the link could easily drain the token bucket(s) of the
router(s) on the link by continuously sending RS messages on the
link. For example, if a host sends one RS message every
UNICAST_RA_INTERVAL, and send a additional RS every third
UNICAST_RA_INTERVAL, the token bucket in the router(s) on the link
will drain within MAX_UNICAST_RA_BURST * UNICAST_RA_INTERVAL * 3
time-units. For the recommended values of UNICAST_RA_INTERVAL and
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MAX_UNICAST_RA_BURST, this value is 3000 milliseconds. It is not
clear whether arrival of such RS messages can be recognized by the
router as a DoS attack. This attack can also be mitigated by
aggregating responses. Since only one aggregation is possible in
this interval due to MIN_DELAY_BETWEEN_RAS restriction, the routers
may not be able protect the tokens in the bucket.
6.2 Attacks on DNA Hosts
RFC 3756 outlines a collection of threats involving rouge routers.
Since DNAv6 requires a host to obtain trustworthy responses from
routers, such threats are relevant to DNAv6. In order to counter
such threats, DNAv6 hosts SHOULD support RFC 3971 [4](SEND) secure
router discovery.
6.3 Tentative options
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.
An attacker may cause messages be sent to another node by an
advertising node (a reflector), without creating any ongoing state on
the reflector.
This 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 [4].
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
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possible, unless SEND is used. [4] 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 [4].
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.
6.4 Authorization and Detecting Network Attachment
When a host is determining if link change has occurred, it may
receive messages from devices with no advertised security mechanisms
purporting to be routers, nodes sending signed router advertisements
but with unknown delegation, or routers whose credentials need to be
checked [13]. Where a host wishes to configure an unsecured router,
it SHOULD confirm bidirectional reachability with the device, and it
MUST mark the device as unsecured as described in [4].
In any case, a secured router SHOULD be preferred over an unsecured
one, except where other factors (unreachability) make the router
unsuitable. Since secured routers' advertisement services may be
subject to attack, alternative (secured) reachability mechanisms from
upper layers, or secured reachability of other devices known to be on
the same link may be used to check reachability in the first
instance.
6.5 Addressing
While a DNA host is checking for link-change, and observing DAD, it
may receive a DAD defense NA from an unsecured source.
SEND says that DAD defenses MAY be accepted even from non SEND nodes
for the first configured address [4].
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While deconfiguring the address is a valid action in the case where a
host collides with another address owner after arrival on a new link,
In the case that the host returns immediately to the same link, such
a DAD defense NA message may be a denial-of-service attempt.
7. Constants
NUM_RS_RA_COMPLETE
Definition: Number of RS/RA exchange messages necessary to declare
the prefix list to be complete.
Value: 2
MIN_RA_WAIT
Definition: Minimum time the host will have to wait before
assuming receipt of all possible RAs.
Default: 4 seconds
UNICAST_RA_INTERVAL
Definition: The interval corresponding to the maximum average rate
of Router Solicitations that the router is prepared to service
with unicast responses. This is the interval at which the token
bucket controlling the unicast responses is replenished.
Value: 50 milliseconds
MAX_UNICAST_RA_BURST
Definition: The maximum size burst of Router Solicitations that
the router is prepared to service with unicast responses. This is
the maximum number of tokens allowed in the token bucket
controlling the unicast responses.
Value: 20
RA_SEPARATION
Definition: The separation between responses from different
routers on the same link to a single Router Solicitation.
Value: 20 milliseconds
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MULTICAST_RA_DELAY
Definition: The delay to be introduced when scheduling a multicast
RA in response to a RS message when the token bucket is empty.
Value: 3000 milliseconds
FAST_RA_THRESHOLD
Definition: The maximum number of fast responses that a host
should receive when soliciting for Router Advertisements.
Value: 3
LEAST_VALID_LIFETIME
Definition: The time for which received prefix can be considered
valid for use in link indentification.
Value: LEAST_VALID_LIFETIME
8. Contributors
This document is the result of merging four different working group
documents. The draft-ietf-dna-protocol-01.txt authored by James
Kempf, Sathya Narayanan, Erik Nordmark, Brett Pentland and JinHyeock
Choi was used as the base for the merger. The draft-ietf-dna-cpl-02
authored by JinHyeock Choi and Erik Normark provided the idea/text
for the complete prefix list mechanism described in this document.
The best current practice for hosts draft (draft-ietf-dna-hosts-03)
authored by Sathya Narayanan, Greg Daley and Nicolas Montavont, and
the tentative options (draft-ietf-dna-tentative-00) authored by Greg
Daley, Erik Normark and Nick Moore were also adopted into this
document.
9. Acknowledgments
The design presented in this document grew out of discussions among
the members of the DNA design team (JinHyeock Choi, Tero Kauppinen,
James Kempf, Sathya Narayanan, Erik Nordmark and Brett Pentland).
The spirited debates on the design, and the advantages and dis-
advantages of various DNA solutions helped the creation of this
document.
Thanks to Syam Madanapalli who co-authored
draft-jinchoi-dna-protocol2 from which this draft draws ideas, as
well as providing feedback on draft-pentland-dna-protocol from which
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most of the text for this draft comes.
Thanks to Greg Daley for much feedback on draft-pentland-dna-protocol
and for helping to work out how to merge the two drafts into this
one.
Thanks to Jari Arkko, Jim Bound, Tero Kauppinen, Syam Madanapalli,
Mohan Parthasarathy, Subba Reddy, and Christian Vogt for their review
of draft-ietf-dna-protocol-01.
Thanks to Gabriel Montenegro for his review of
draft-pentland-dna-protocol.
Thanks also to other members of the DNA working group for their
comments that helped shape this work.
10. Informative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[3] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[4] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[5] Moore, N., "Optimistic Duplicate Address Detection (DAD) for
IPv6", RFC 4429, April 2006.
[6] Krishnan, S. and SG. Daley, "Simple procedures for Detecting
Network Attachment in IPv6", draft-ietf-dna-simple-01 (work in
progress), July 2008.
[7] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[8] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC2462 2462, December 1998.
[9] Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472,
December 1998.
[10] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
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[11] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC2463 2463, December 1998.
[12] Christensen, M., Kimball, K., and F. Solensky, "Considerations
for IGMP and MLD Snooping Switches", draft-ietf-magma-snoop-12
(work in progress), February 2005.
[13] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[14] Liebsch, M., Singh, A., Chaskar, H., Funato, D., and E. Shim,
"Candidate Access Router Discovery (CARD)", RFC 4066,
July 2005.
[15] Koodli, R., "Fast Handovers for Mobile IPv6", RFC 4068,
July 2005.
[16] O'Hara, B. and G. Ennis, "Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications", ANSI/IEEE
Std 802.11, 1999.
[17] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[18] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2
(MLDv2) for IPv6", RFC 3810, June 2004.
[19] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC 3513, April 2003.
[20] Choi, JH. and G. Daley, "Goals of Detecting Network Attachment
in IPv6", RFC 4135, August 2005.
[21] Yegin, A., "Link-layer Event Notifications for Detecting
Network Attachments", draft-ietf-dna-link-information-00 (work
in progress), September 2004.
[22] Manner, J. and M. Kojo, "Mobility Related Terminology",
draft-ietf-seamoby-mobility-terminology-06 (work in progress),
February 2004.
[23] Choi, J. and E. Nordmark, "DNA with unmodified routers: Prefix
list based approach", draft-ietf-dna-cpl-00 (work in progress),
April 2005.
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Authors' Addresses
Sathya Narayanan (editor)
School of Information Technology and Communications Design
California State University, Monterey Bay
3110, Inter-Garrison Road, Building 18, Room 150
Seaside, CA 93955
USA
Phone: +1 (831) 582-33411
Email: sathya_narayanan@csumb.edu
James Kempf
DoCoMo Communications Labs USA
USA
Phone:
Email: kempf@docomolabs-usa.com
Erik Nordmark
Sun Microsystems, Inc.
17 Network Circle
Mountain View, CA
USA
Phone: +1 650 786 2921
Email: erik.nordmark@sun.com
Brett Pentland
Centre for Telecommunications and Information Engineering
Department of Electrical and Computer Systems Engineering
Monash University
Clayton, Victoria 3800
Australia
Phone: +61 3 9905 5245
Email: brett.pentland@eng.monash.edu.au
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JinHyeock Choi
Samsung Advanced Institute of Technology
PO Box 111
Suwon 440-600
Korea
Phone: +82-31-280-8194
Email: jinchoe@samsung.com
Greg Daley
Centre for Telecommunications and Information Engineering
Department of Electrical adn Computer Systems Engineering
Monash University
Clayton, Victoria 3800
Australia
Phone: +61 3 9905 4655
Email: greg.daley@eng.monash.edu.au
Nicolas Montavont
LSIIT - Univerity Louis Pasteur
Pole API, bureau C444
Boulevard Sebastien Brant
Illkirch 67400
FRANCE
Phone: (33) 3 90 24 45 87
Email: montavont@dpt-info.u-strasbg.fr
URI: http://www-r2.u-strasbg.fr/~montavont/
Nick 'Sharkey' Moore
Email: sharkey@zoic.org
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