NEMO Working Group P. Thubert
Internet-Draft Cisco
Expires: October 7, 2007 C. Bontoux
Fortinet
N. Montavont
LSIIT - ULP
April 5, 2007
Nested Nemo Tree Discovery
draft-thubert-tree-discovery-05.txt
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Abstract
This paper describes a simple distance vector protocol that exposes
only a default route towards the infrastructure in a nested NEMO
configuration. The draft extends the Neighbor Discovery Protocol [1]
in order to carry information and metrics which will help a Mobile
Router select its Attachment Router(s) in an autonomous fashion and
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provides generic rules which guarantee that the interaction of
different selection processes will not create loops.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 4
3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Multi-Homed nested mobile network . . . . . . . . . . . . 5
3.2. Loops in nested Nemo . . . . . . . . . . . . . . . . . . . 6
4. Router Advertisement extensions . . . . . . . . . . . . . . . 8
4.1. Router Advertisement message . . . . . . . . . . . . . . . 8
4.2. Tree Information Option . . . . . . . . . . . . . . . . . 8
4.3. TIO suboption . . . . . . . . . . . . . . . . . . . . . . 11
4.3.1. Format . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3.2. Pad1 . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3.3. PadN . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3.4. Bandwidth Suboption . . . . . . . . . . . . . . . . . 12
4.3.5. Stable time Suboption . . . . . . . . . . . . . . . . 13
4.3.6. Tree Group ID Suboption . . . . . . . . . . . . . . . 14
4.3.7. Path Free Medium Time Suboption . . . . . . . . . . . 14
5. Tree Discovery . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1. tree selection . . . . . . . . . . . . . . . . . . . . . . 17
5.2. Sub-tree mobility . . . . . . . . . . . . . . . . . . . . 18
5.3. Administrative depth . . . . . . . . . . . . . . . . . . . 18
5.4. DRL entries states and stability . . . . . . . . . . . . . 18
5.4.1. Held-Up . . . . . . . . . . . . . . . . . . . . . . . 19
5.4.2. Held-Down . . . . . . . . . . . . . . . . . . . . . . 20
5.4.3. Collision . . . . . . . . . . . . . . . . . . . . . . 20
5.4.4. Instability . . . . . . . . . . . . . . . . . . . . . 21
5.5. Legacy Routers . . . . . . . . . . . . . . . . . . . . . . 21
6. Directed Acyclic Graph Discovery . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Changes from version 00 to 01 . . . . . . . . . . . . . . 22
9.2. Changes from version 01 to 02 . . . . . . . . . . . . . . 22
9.3. Changes from version 02 to 03 . . . . . . . . . . . . . . 22
9.4. Changes from version 03 to 04 . . . . . . . . . . . . . . 23
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9.5. Changes from version 04 to 05 . . . . . . . . . . . . . . 23
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Normative Reference . . . . . . . . . . . . . . . . . . . 24
11.2. Informative Reference . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . . . 26
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1. Introduction
As per Nemo Basic support [3], a Mobile Router autoconfigures a
single Care of Address (CoA) to register to its Home Agent and
terminate its Mobile Router-Home Agent tunnel. That Care of Address
is the Mobile Router point of attachment to the nested Nemo.
Consequently, if loops are avoided, the nested Nemo assumes the shape
of a tree. The nodes of the tree are Mobile Routers, the root is
either a fixed or a Mobile Router, called in the latter case the root
Mobile Router in NEMO terminology [4]. The leaves are mobile or
fixed hosts, called Local Fixed Nodes, Local Mobile Nodes and
Visiting Mobile Nodes in the NEMO terminology.
This paper provides (1) a minimum extension to IPv6 Neighbor
Discovery Router Advertisements in order to ensure that Mobile
Routers attaching to one another actually avoid loops and end up
forming a tree, and (2) the minimum common part of all Mobile Router
algorithms that is required to ensure that whatever their specific
decisions, loops between Mobile Routers will be avoided.
The method is based on an autonomous decision by each Mobile Router
with no global state convergence such as a MANET proactive routing
protocol. In fact, Mobile Routers may make different decisions from
a same input, based on their own configuration and their own
algorithms.
In order to build trees of Mobile Routers, we propose an extension to
the ICMP Router Advertisement (RA) message, the Tree Information
Option (TIO). The TIO allows Mobile Routers to advertise the tree
they belong to, and to select and move to the best location within
the available trees. Mobile Routers propagate the TIO in RA down the
tree, updating some metrics such as the tree depth, leaving alone
root information such as the tree identifier, and sending the result
in RAs over the ingress interfaces.
2. Terms and Abbreviations
This document assumes that the reader is familiar with Mobile IPv6 as
defined in [2] and with the concept of Mobile Router defined in the
Nemo terminology document [4].
For the needs of this paper, the following new definitions are
introduced:
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Nemo clusterhead: The root of a tree of mobile routers. When the
tree of Mobile Routers is attached to the infrastructure, the
fixed Access Router may act as cluster head if it supports the
Tree Information Option described in this document. If it does
not, then the clusterhead coincides with the root Mobile Router in
NEMO terminology. A clusterhead is elected even when the tree is
not attached to the infrastructure. A stand-alone Mobile Router
is a clusterhead.
Floating Tree: A Nested Nemo which clusterhead is a Mobile Router
that is not attached to an Access Router.
Grounded Tree: A Nested Nemo whose clusterhead is attached to the
infrastructure. In other words, the clusterhead is either a fixed
router that supports Router Advertisement - Tree Information
Option or is a Mobile Router which attachment router is a fixed
router that does not support Router Advertisement - Tree
Information Option.
Mobile Access Router: A Mobile Router that provides Access Router
services to other Mobile Routers.
Attachment Router: The Router that is selected as Access Router by a
Mobile Router, making it its parent in the nested NEMO tree.
Propagation: The action by a Mobile Router that consists in
receiving a Router Advertisement Tree Information Option from its
Attachment Router, recomputing a few specific fields, removing
unknown suboptions, and appending the resulting TIO to RAs sent
over the ingress interfaces.
3. Motivations
3.1. Multi-Homed nested mobile network
A nested mobile network that is made of multiple Mobile Routers
having a direct connection to the Internet is said to be multi-homed.
Multihoming in Nemo offers useful properties to Mobile Network Nodes.
The NEMO multihoming issues [7] draft lists potential multi-homed
configurations for Nemo and explains the different problems and
advantages that some configurations may introduce. Multihoming
offers three main abilities to the Nemo: it allows route recovery on
failure, redundancy and load-sharing between Mobile Routers (or
between interfaces of a given Mobile Router). However, for the
moment, there is no requirements nor protocol that would define in
interaction between several egress interfaces inside a Nemo.
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In a nested Nemo, the hierarchy of Mobile Routers increases the
complexity of the route and/or router selection for Mobile Network
Nodes. Each level of a Nemo implies the usage of a new tunnel
between the Mobile Router and its home agent. Thus if a Mobile
Network Node connects to a sub-Nemo which is also a sub-Nemo, packets
from the Mobile Network Node will be encapsulated three times.
When the Nemo where the MN is connected to is multi-homed, the MN may
have the choice between several Attachment Router to be its default
router. Reference [5] introduces new options in Router Advertisement
to allow any node on a link to choose between several routers. This
option mainly consists of a 2-bits flag that indicates the preference
of the router (low, medium or high). Furthermore, the same flag can
be set in the Route Information option indicating the preference of a
specific prefix. Therefore, any node can determine its best default
router(s) according to a given destination and its best router for
default, which will be used by default.
However this preference is only useful in a flat topology; It gives a
way to the node to choose between different attachment routers
advertising prefixes on the node link. But if the node is inside a
hierarchical topology the node can not learn the depth of each
attachment router, and might not select the most efficient path.
One of the usage of the new option introduced in this document is to
distribute information on the hierarchy of Mobile Routers. This
information can be distributed to Attachment Routers, Mobile Routers
and Mobile Network Nodes as well in order to allow better route
selection and to increase the knowledge of the Nemo topology on each
node.
3.2. Loops in nested Nemo
When several Mobile Routers attach to each other to form a nested
Nemo, loops can be created if they are not explicitly avoided. In
the simplest case, when egress and ingress interfaces of A Mobile
Router are all wireless, a mobile router may be listening to Router
Advertisement from its own ingress interface, creating a confliction
problem. In the general case, arbitrary attachment of Mobile Routers
will form graphs that are not exempt of loops. For instance: Assume
a nested Nemo where Mobile Router1 is connected to the
infrastructure, and Mobile Router3 is attached to Mobile Router2.
Say that Mobile Router2 can hear both Mobile Router3 and Mobile
Router1 over its wireless egress interface. If Mobile Router2 select
Mobile Router1, the connectivity to the infrastructure is provided
for all. But if Mobile Router2 selects Mobile Router3, Mobile
Router2 and Mobile Router3 end up forming a loop and are disconnected
from their Home Agents.
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With Nemo basic support, a Mobile Router uses a single primary Care
Of Address to attach to the nested structure. As a result, if loops
are avoided, the nested NEMO end up forming a tree. It is beneficial
to be able to form that tree in an optimum fashion for a given set of
metrics such as tree depth.
The shape of a nested Nemo may change rapidly due to Mobile Routers
movement. It is thus impractical to expect each Mobile Router to be
able to maintain states about the whole tree structure in a link
state fashion. On the contrary, it is also beneficial to allow each
Mobile Router to make its own independent selection based on a
minimum information about its immediate neighbors, in order to
reestablish the tree quickly upon erratic movements.
Each Mobile Router should be able to make its own attachment router
selection based on its own condition (eg battery level), its own set
of constraints that may not apply to other Mobile Routers in the
tree, and in general its own algorithm. As a result, the
standardization effort should concentrate on a common minimum set of
rules that must be common to all Mobile Routers in order to prevent
routing loops in the nested NEMO while leaving Mobile Routers
independent in their Attachment Router selection algorithms.
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4. Router Advertisement extensions
New extensions of Router Advertisement are proposed to distribute the
knowledge of the Mobile Router hierarchy inside a nested Nemo. These
extensions are defined in different options/sub-options: a flag bit
from the reserved flag field of Router Advertisement message is used
to indicate whether the sending router is a Mobile Router or not; a
new option is defined to transport minimum information on the tree to
avoid loops generation;
4.1. Router Advertisement message
We propose to use a reserved flag of the Router Advertisement message
to inform whether the sending router is a Mobile Router or not.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cur Hop Limit |M|O|H|N|Reservd| Router Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reachable Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Retrans Timer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 1: Router Advertisement
Nemo enabled router (N)
The Nemo enabled router (N) bit is set when the sending router is a
Mobile Router.
4.2. Tree Information Option
The Tree Information Option carries a number of metrics and other
information that allows a Mobile Router to discover a tree and select
its point of attachment while avoiding loop generation.
The option is a container option, which might contain a number of
suboptions. The base option regroups the minimum information set
that is mandatory in all cases.
A TIO can also be used by Mobile Network Nodes to select their best
default router. If the default router of a non-Mobile Router sends
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Router Advertisements with a Tree Information Option, the non-Mobile
Router MUST set the N flag of its own Router Advertisement to 0 and
copy the Tree Discovery Option in its own Router Advertisement.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |G|H|B| Reserved| Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TreePref. | BootTimeRandom |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MR Preference | TreeDepth | TreeDelay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PathDigest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| TreeID |
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: RA Tree Information Option
Type: 8-bit unsigned integer set to 10 by the clusterhead. Value is
"TBD".
Length: 8-bit unsigned integer set to 4 when there is no suboption.
The length of the option (including the type and length fields and
the suboptions) in units of 8 octets.
Grounded (G): The Grounded (G) flag is set when the clusterhead is
attached to a fixed network infrastructure (such as the Internet).
Home (H): The Home (H) flag is set when the clusterhead is attached
to its home network.
Battery (B): The Battery (B) flag is indicates that a parent in the
tree operates on batteries, an indication of a costly operation.
It is set by a mobile router which operates on battery and when
set, it is left set as it is propagated down the tree.
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Reserved: 13-bit unsigned integer set to 0 by the clusterhead.
Sequence Number: 8-bit unsigned integer set by the clusterhead and
incremented with each new TIO it sends on a link and propagated
with no change down the tree.
TreePreference: 8-bit unsigned integer set by the clusterhead to its
preference and unchanged at propagation. Default is 0 (lowest
preference). The tree preference provides a mechanism to engineer
the mesh of mobile routers, for instance indicating the most
preferred home gateway or the communication ship in a fleet at
sea.
BootTimeRandom: A random value computed at boot time and recomputed
in case of a duplication with another Attachment Router. The
concatenation of the Preference and the BootTimeRandom is a 32-bit
extended preference that is used to resolve collisions. It is set
by each Mobile Router at propagation time.
Preference: The administrative preference of that (mobile) Access
Router. Default is 0. 255 is the highest possible preference.
Set by each Mobile Router at propagation time.
TreeDepth: 8-bit unsigned integer. The tree depth of the
clusterhead is 0 if it is a fixed router and 1 if it is a Mobile
Router. The tree Depth of a tree Node is the depth of its
attachment router as received in a TIO, incremented by at least
one. All nodes in the tree advertise their tree depth in the Tree
Information Options that they append to the RA messages over their
ingress interfaces as part of the propagation process.
TreeDelay: 16-bit unsigned integer set by the clusterhead indicating
the delay before changing the tree configuration, in milliseconds.
A default value is 128ms. It is expected to be an order of
magnitude smaller than the RA-interval so if the clusterhead has a
sub-second RA-interval, the Tree delay may be shorter than 100ms.
It is also expected to be an order of magnitude longer than the
typical propagation delay inside the nested Nemo.
PathDigest: 32-bit unsigned integer CRC, updated by each Mobile
Router. This is the result of a CRC-32c computation on a bit
string obtained by appending the received value and the Mobile
Router Care of Address. clusterheads use a 'previous value' of
zeroes to initially set the PathDigest.
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TreeID: 128-bit unsigned integer which uniquely identify a tree.
This value is set by the clusterhead. The global IPv6 home
address of the clusterhead can be used.
The following values MUST not change during the propagation of the
TIO down the tree: Type, Length, G, H, TreePreference, TreeDelay and
TreeID. All other fields of the TIO are updated at each hop of the
propagation.
4.3. TIO suboption
In addition to the minimum options presented in the base option, a
number of suboptions are defined for the TIO:
4.3.1. Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subopt. Type | Subopt Length | Suboption Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: TIO suboption generic format
Suboption Type: 8-bit identifier of the type of mobility option.
When processing a TIO containing a suboption for which the
suboption Type value is not recognized by the receiver, the
receiver MUST silently ignore and skip over the suboption,
correctly handling any remaining options in the message.
Suboption Length: 8-bit unsigned integer, representing the length in
octets of the suboption, not including the suboption Type and
Length fields.
Suboption Data: A variable length field that contains data specific
to the option.
The following subsections specify the TIO suboptions which are
currently defined for use in the Mobility Header.
Implementations MUST silently ignore any TIO suboptions options that
they do not understand.
TIO suboptions may have alignment requirements. Following the
convention in IPv6, these options are aligned in a packet so that
multi-octet values within the Option Data field of each option fall
on natural boundaries (i.e., fields of width n octets are placed at
an integer multiple of n octets from the start of the header, for n =
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1, 2, 4, or 8).
4.3.2. Pad1
The Pad1 suboption does not have any alignment requirements. Its
format is as follows:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
Figure 4: Pad 1
NOTE! the format of the Pad1 option is a special case - it has
neither Option Length nor Option Data fields.
The Pad1 option is used to insert one octet of padding in the TIO to
enable suboptions alignment. If more than one octet of padding is
required, the PadN option, described next, should be used rather than
multiple Pad1 options.
4.3.3. PadN
The PadN option does not have any alignment requirements. Its format
is as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Type = 1 | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
Figure 5: Pad N
The PadN option is used to insert two or more octets of padding in
the TIO to enable suboptions alignment. For N (N > 1) octets of
padding, the Option Length field contains the value N-2, and the
Option Data consists of N-2 zero-valued octets. PadN Option data
MUST be ignored by the receiver.
4.3.4. Bandwidth Suboption
This suboption carries the maximum bandwidth available up the tree
via a specific parent. It is the lowest speed of the links on the
way and does not reflect the actual use of those links in run time.
The value is expressed in the log base 2 of the speed, expressed in
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bps. The Bandwidth suboption does not have any alignment
requirements. Its format is as follows:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| Type = 2 | Length = 1 | Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
Figure 6: Bandwidth Suboption
Type: Set to 2 for the Bandwidth suboption.
Length: Set to 1 for the Bandwidth suboption.
Bandwidth: 8-bit unsigned integer. The Log2 of the speed of the
path expressed in bps. The clusterhead initializes that field
using the speed of the link to the Access Router to which it is
attached or 0xFF if it is floating. An attached MR propagates it
as the minimum of the Bandwidth as received in the TIO from the
parent and the access speed between the MR and the parent. As a
result, the value received from a candidate AR is that of the
bottleneck between that AR and the wire access.
4.3.5. Stable time Suboption
This suboption carries an indicator of the stability of a network.
This indicator is the time since the branch to which the MR is
attached has remained unchanged. The value is expressed in the log
base 2 of that duration, expressed in milliseconds. The Stable time
suboption does not have any alignment requirements. Its format is as
follows:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
| Type = 3 | Length = 1 | Stable time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---------------+
Figure 7: Stable time
Type: Set to 3 for the Stable time suboption.
Length: Set to 1 for the Stable time suboption.
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Stable time: 8-bit unsigned integer. The Log2 of the time since the
last change in the attachment branch, expressed in milliseconds.
This is set by the MR as it propagates the TIO down the tree,
indicating for how long the PathDigest in the TIO from its parent
has remained unchanged.
4.3.6. Tree Group ID Suboption
This suboption carries the Group ID for the tree. It is set by the
clusterhead and is left unchanged by the MR that propagates the TIO
down the tree. The Tree Group ID Suboption has an alignment
requirement of 8n+6. Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Length = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Tree |
+ Group ID +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Tree Group ID Suboption
Type: 8-bit unsigned integer. Its value is 4 for the Tree Group ID
suboption.
Length: 8-bit unsigned integer. Its value is 16 for the Tree Group
ID suboption.
Tree Group ID: 128-bit unsigned integer which identify a group for a
tree. This value is set by the clusterhead. It can be set
administratively, for instance to an IPv6 multicast group.
4.3.7. Path Free Medium Time Suboption
This suboption carries the Free Medium Time available up the tree via
a specific parent at a given point of time. It is an indication of
whether bandwidth is available to place VoIP calls for instance. As
defined by the Quality of Service (QoS) Task Group of the Wi-Fi
Alliance, the Medium Time describes the amount of time admitted to
access the medium, in units of 32 microsecond periods per second.
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The Free Medium Time is the amount of time left the medium, in other
words ((1000000/32) - SIGMA(MT)). The Path Free Medium Time is the
lowest available Free Medium Time along the way and it reflects the
actual use of those links in run time.
The Path Free Medium Time suboption does not have any alignment
requirements. Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 5 | Length = 2 | Path Free Medium Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Path Free Medium Time Suboption
Type: Set to 5 for the Path Free Medium Time Suboption.
Length: Set to 2 for the Path Free Medium Time Suboption.
Path Free MT: 16-bit unsigned integer. The amount of Medium Time
that is available along the path to the clusterhead in units of 32
microsecond periods per second. The clusterhead initializes that
field to the Free MT on the link where the TIO is issued. An
attached MR propagates it as the minimum of the Path Free MT as
received in the TIO from the parent and the Path Free MT on the
link on which the TIO is propagated. As a result, the value
received from a candidate AR is that of the bottleneck between
that AR and the clusterhead.
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5. Tree Discovery
Here follows a set of rules and definitions that MUST be followed by
all Mobile Routers:
1. A Mobile Router that is not attached to an Attachment Router is
the Nemo clusterhead of its own floating tree. It's depth is 1.
A Mobile Router will end up in that situation when it looses its
current parent and there is no alternate parent that it can
attach to. In that case, the MR remembers the treeID and the
sequence counter in the TIO of the lost parent for a period of
time which covers multiple TIO.
2. A Mobile Router that is attached to an Attachment Router that
does not support TIO, is the clusterhead of its own grounded
tree. It's depth is 1.
3. A router sending a RA without TIO is considered a grounded
Attachment Router at depth 0.
4. The Nemo clusterhead of a tree exposes the tree in the Router
Advertisement Tree Information Option and Mobile Routers
propagate the TIO down the tree with the RAs that they forward
over their ingress links.
5. A Mobile Router that is already part of a tree MAY move at any
time and with no delay in order to get closer to the clusterhead
of its current tree - i.e. in order to reduce its own tree depth.
But A Mobile Router MUST NOT move down the tree that it is
attached to. Mobile Routers MUST ignore RAs that are received
from other routers located deeper within the same tree.
6. A Mobile Router may move from its current tree into any different
tree at any time and whatever the depth it reaches in the new
tree, but it may have to wait for a Tree Hop timer to elapse in
order to do so. If the MR was clusterhead of its own floating
tree, it may not join its previous identified by the last parent
treeID tree unless the sequence number is the TIO was
incrememented since the MR left that tree, indicating that the
candidate parent was not attached behind this MR and kept getting
subsequent TIOs from the same tree. The Mobile Router will join
that other tree if it is more preferable for reasons of
connectivity, configured preference, free Medium Time, size,
security, bandwidth, tree depth, or whatever metrics the Mobile
Router cares to use.
7. If a Mobile Router has selected a new attachment router but has
not moved yet (because it is waiting for Tree Hop timer to
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elapse), the Mobile Router is unstable and refrains from sending
Router Advertisement - Tree Information Options.
8. When A Mobile Router joins a tree, moves within its tree, or when
it receives a modified TIO from its current attachment router,
the Mobile Router sends an unsolicited Router Advertisement
message on all its mobile networks (i.e. all its ingress
interfaces). The RA contains a TIO that propagates the new tree
information. At the same time, the Mobile Router MAY send a
Binding Update to its home agent or a local proxy of some sort,
because the tree it is attached to has changed. If the Mobile
Router fails to reach its Home Agent, it MAY attempt to roll back
the movement or to retry the Home Agent discovery procedure.
9. This allows the new higher parts of the tree to take place first
eventually dragging their sub-tree with them, and allowing
stepped sub-tree reconfigurations, limiting relative movements.
5.1. tree selection
The tree selection is implementation and algorithm dependent. In
order to limit erratic movements, and all metrics being equal, Mobile
Routers SHOULD stick to their previous selection. Also, Mobile
Routers SHOULD provide a mean to filter out candidate Attachment
Routers whose availability is detected as fluctuating, at least when
more stable choices are available. For instance, the Mobile Router
MAY place the failed Attachment Router in a Hold Down mode that
ensures that the Attachment Router will not be reused for a given
period of time.
The known trees are associated with the Attachment Router that
advertises them and kept in a list by extending the Default Router
List. DRL entries are extended to store the information received
from the last TIO. These entries are managed by states and timers
described in the next section.
When connection to a fixed network is not possible or preferable for
security or other reasons, scattered trees should aggregate as much
as possible into larger trees in order to allow inner connectivity.
How to balance these trees is implementation dependent, and MAY use a
specific visitor-counter suboption in the TIO.
A Mobile Router SHOULD verify that bidirectional connectivity is
available with a candidate Attachment Router before it attaches to
that candidate. Some layer 2 such as 802.11 infrastructure mode will
provide for this, while others such as 802.11 adhoc mode will not.
If the layer 2 does not guarantee the bidirectional connectivity,
then the MR needs to make sure that it can reach the AR. This can be
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achieved using Neighbor Sollicitation and refraining from attaching
to an AR for which no neighbor cache exists, or the state is still
INCOMPLETE.
5.2. Sub-tree mobility
It might be perceived as beneficial for a sub-tree to move as a
whole. The way it would work is for a Mobile Router to stay
clusterhead even if itself is attached into a parent tree. But the
loop avoidance is based on the knowledge of the tree that the Mobile
Router visit, preventing a Mobile Router to move down a same tree.
So without additional support, tree-level loops could form.
To avoid this, it is possible to add a path vector suboption to the
TIO that reflects the nesting of trees. If a root-Mobile Router
joins a parent tree, then it needs to add its treeID to the path
vector, but it can not join if the treeID is already listed.
A specific case is the root-Mobile Router of a tree that attaches to
a fixed Access Router. That root-Mobile Router might omit to
consider a TIO that comes from the new Attachment Router and decide
to stay root, in order to keep the tree consistency from the nested
Mobile Routers standpoint. This does not create loops, even if the
path vector is not present
5.3. Administrative depth
When the tree is formed under a common administration, or when a
Mobile Router performs a certain role within a community, it might be
beneficial to associate a range of acceptable depth with that MR.
For instance, a MR that has limited battery should be a leaf unless
there is no other choice, and thus expose an exagerated depth. On
the other hane, a MR that is designed for backhaul should operate in
a low range of depth.
With Tree Discovery, a MR has to expose a depth that is incremented
from its parent's depth as receive in the TIO. In particular, a MR
might expose a depth which is incremented by more than one from its
parent's depth, in order to fit in its own administrative range. So
a depth of N does not mean that there is precisely N Mobile Routers
on the way, but at most N.
5.4. DRL entries states and stability
Attachment routers in the DRL may or may not be usable for roaming
depending on runtime conditions. The following states are defined:
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Current This Attachment Router is used for roaming
Candidate This Attachment Router can be used for roaming.
Held-Up This Attachment Router can not be used till tree hop timer
elapses. This does not occur for a fixed Attachment Router that
does not send a TIO since the tree delay is null in that case.
Held-Down This Attachment Router can not be used till hold down
timer elapses. At the end of the hold-down period, the router is
removed from the DRL, and will be reinserted if it appears again
with a RA.
Collision This Attachment Router can not be used till its next RA.
5.4.1. Held-Up
This state is managed by the tree Hop timer, it serves 2 purposes:
Delay the reattachment of a sub-tree that has been forced to
detach. This allows to make sure that when a sub-tree has
detached, the Router Advertisement - Tree Information Option that
is initiated by the new clusterhead has spread down the sub-tree
so that two different trees have formed.
Limit Router Advertisement - Tree Information Option storms when
two trees collide. The idea is that between the nodes from tree A
that wish to move to tree B, those that see the highest place in
tree B will move first and advertise their new locations before
other nodes from tree A actually move.
A new tree is discovered upon a router advertisement message with or
without a Router Advertisement - Tree Information Option. The Mobile
Router joins the tree by selecting the source of the RA message as
its attachment router (default gateway) and propagating the TIO
accordingly.
When a new tree is discovered, the candidate Attachment Router that
advertises the new tree is placed in a held up state for the duration
of a Tree Hop timer. If the new Attachment Router is more preferable
than the current one, the Mobile Router expects to jump and becomes
unstable.
A Mobile Router that is unstable may discover other Attachment
Routers from the same new tree during the instability phase. It
needs to start a new Tree Hop timer for all these. The first timer
that elapses for a given new tree clears them all for that tree,
allowing the Mobile Router to jump to the highest position available
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in the new tree.
The duration of the Tree Hop timer depends on the tree delay of the
new tree and on the depth of Attachment Router that triggers it:
(AR's depth + random) * AR's tree_delay (where 0 <= random < 1). It
is randomized in order to limit collisions and synchronizations.
5.4.2. Held-Down
When a router is 'removed' from the Default Router List, it is
actually held down for a hold down timer period, in order to prevent
flapping. This happens when an Attachment Router disappears (upon
expiration timer), and when an Attachment Router is tried but can not
reach the Home Agent (upon expiration of another Attachment Router,
or upon tree hop for that Attachment Router).
An Attachment Router that is held down is not considered for the
purpose of roaming. When the hold down timer elapses, the Attachment
Router is removed from the DRL.
5.4.3. Collision
A race condition occurs if 2 Mobile Routers send Router Advertisement
- Tree Information Option at the same time and wish to join each
other. This might happen between routers at a same depth, or routers
which act as clusterhead of their own tree. In order to detect the
situation, Mobile Routers time stamp the sending of Router
Advertisement - Tree Information Option. Any Router Advertisement -
Tree Information Option received within a short media-dependant
period introduces a risk. To divide the risk, A 32bits extended
preference is added in the TIO. The first byte is the clusterhead
(tree) preference, the remaining 24 bits is a boot time computed
random.
A Mobile Router that decides to join an Attachment Router will do so
between (Attachment Router depth) and (Attachment Router depth + 1)
times the Attachment Router tree delay. But since a Mobile Router is
unstable as soon as it receives the Router Advertisement - Tree
Information Option from the preferred Attachment Router, it will
restrain from sending a Router Advertisement - Tree Information
Option between the time it receives the RA and the time it actually
jumps. So the crossing of RA may only happen during the propagation
time between the Attachment Router and the Mobile Router, plus some
internal queuing and processing time within each machine. It is
expected that one tree delay normally covers that interval, but
ultimately it is up to the implementation and the configuration of
the Attachment Router to define the duration of risk window.
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There is risk of a collision when a Mobile Router receives an RA, for
an other mobile router that is more preferable than the current
Attachment Router, within the risk window. In the face of a
potential collision, the Mobile Router with the lowest extended
preference processes the Router Advertisement - Tree Information
Option normally, while the router with the highest preference places
the other in collision state, does not start the tree hop timer, and
does not become instable. It is expected that next RAs between the
two will not cross anyway.
5.4.4. Instability
A Mobile Router is instable when it is prepared to move shortly to
another Attachment Router. This happens typically when the Mobile
Router has selected a more preferred candidate Attachment Router and
has to wait for the tree hop timer to elapse before roaming.
Instability may also occur when the current Attachment Router is lost
and the next best is still held up. Instability is resolved when the
tree hop timer of all the Attachment Router (s) causing instability
elapse. Such Attachment Router is changes state to Current or Held-
Down.
Instability is transient (in the order of tree hop timers). When a
Mobile Router is unstable, it MUST NOT send RAs with TIO. This
avoids loops when Mobile Router A wishes to attach to Mobile Router B
and Mobile Router B wishes to attach to Mobile Router A. Unless RA
cross (see Collision section), a Mobile Router receives TIO from
stable Attachment Routers, which do not plan to attach to itself, so
the Mobile Router can safely attach to them.
5.5. Legacy Routers
A legacy router sends its Router Advertisements without a TIO.
Consequently, a legacy router can be mistaken for a fixed Access
Router when it is placed within a nested NEMO structure, and defeat
the loop avoidance mechanism. Consequently, it is important for the
administrator to prevent address autoconfiguration by visiting Mobile
Routers from such a legacy router.
6. Directed Acyclic Graph Discovery
Tree Discovery builds trees, which are a specific form of a Directed
Acyclic Graph (DAG). In a more general Fashion, TD can be adapted to
form DAGs, oriented towards the clusterhead. This is DAG Discovery.
In Section 5.3, TD enables a given MR to expose a depth that is
incremented by more than one with regards to its parent. When it
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does so, a MR can elect a number of alternate parents as feasible
successors. A feasible successor belongs to the same tree as the MR
parent, and has a depth that is less than that of the MR.
The links MR to feasible successors complete the tree as built by TD
into a DAG towards the clusterhead. The DAG enables alternate exit
paths for a multihomed Mobile Router.
7. IANA Considerations
Section 4.2. requires the definition of a new option to Neighbor
discovery [1] messages, the Router Advertisement - Tree Information
Option (RA-TIO). The Router Advertisement - Tree Information Option
has been assigned the value TBD within the numbering space for IPv6
Neighbor Discovery Option Formats.
8. Security Considerations
At the current level of this draft, the TIO bears the security level
of the RA and the link. Nothing is added to it. A deeper threat
analysis would be required to eventually propose additional security.
9. Changes
9.1. Changes from version 00 to 01
Added text on sub-tree mobility from the discussion with Marcelo.
Added text on nested legacy routers from the discussion with
Marcelo.
9.2. Changes from version 01 to 02
Improved text on instability
Changed the formula for the 4 bytes number used in collision
avoidance
9.3. Changes from version 02 to 03
Added suboptions for tree group, stable time and bandwidth.
Added administrative depth and increment by more than 1.
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Added words on bidirectional check using ND.
Added DAG discovery.
9.4. Changes from version 03 to 04
Added suboptions for Path Free Medium Time.
9.5. Changes from version 04 to 05
Added a sequence counter which provides additional loop protection
based on a comment by Christipher Dearlove. For the sake of the
discussion, note that if a loop were to occur, the count to
infinity would actually cause the MR taht reaches the max depth to
detach and that would resolve the issue anyway.
10. Acknowledgments
The authors wish to thank Marco Molteni and Patrick Wetterwald
(cisco) for their participation to this design and the review of the
document, Massimo Villari (university of Messina), for his early work
on simulation and research on the subject and Julien Abeille for his
advanced participation in simulation and real testing. Also the
authors wish to thank Christopher Dearlove for his suggestion to add
a sequence counter which provides additional protection against loop
formation. This work is also based on prior publications, in
particular HMRA [6] by Hosik Cho and Eun-Kyoung Paik from Seoul
National University and other non IETF publications coauthored with
Thierry Ernst and Thomas Noel. Finally, thanks to Marcelo Bagnulo
Braun for his constructive review.
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11. References
11.1. Normative Reference
[1] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[2] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[3] Devarapalli, V., Wakikawa, R., Petrescu, A., and P. Thubert,
"Network Mobility (NEMO) Basic Support Protocol", RFC 3963,
January 2005.
[4] Ernst, T. and H. Lach, "Network Mobility Support Terminology",
draft-ietf-nemo-terminology-06 (work in progress),
November 2006.
[5] Draves, R. and D. Thaler, "Default Router Preferences and More-
Specific Routes", RFC 4191, November 2005.
11.2. Informative Reference
[6] Cho, H., "Hierarchical Mobile Router Advertisement for nested
mobile networks", draft-cho-nemo-hmra-00 (work in progress),
January 2004.
[7] Ng, C., "Analysis of Multihoming in Network Mobility Support",
draft-ietf-nemo-multihoming-issues-06 (work in progress),
June 2006.
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Authors' Addresses
Pascal Thubert
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com
Caroline Bontoux
Fortinet
Sophia Antipolis
Biot 06410
FRANCE
Email: cbontoux@fortinet.com
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/
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