Internet DRAFT - draft-ietf-6lo-mesh-link-establishment
draft-ietf-6lo-mesh-link-establishment
6lo R. Kelsey
Internet-Draft Silicon Labs
Intended status: Experimental December 1, 2015
Expires: June 3, 2016
Mesh Link Establishment
draft-ietf-6lo-mesh-link-establishment-00
Abstract
This document defines the mesh link establishment (MLE) protocol for
establishing and configuring secure radio links in IEEE 802.15.4
radio mesh networks. MLE extends IEEE 802.15.4 for use in multihop
mesh networks by adding three capabilities: 1) dynamically
configuring and securing radio connections between neighboring
devices, 2) enabling network-wide changes to shared radio parameters,
and 3) allowing the determination of radio link quality prior to
configuration. MLE operates below the routing layer, insulating it
from the details of configuring, securing, and maintaining individual
radio links within a larger mesh network.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 3, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Link Configuration . . . . . . . . . . . . . . . . . . . 5
4.2. Parameter Dissemination . . . . . . . . . . . . . . . . . 5
4.3. Link Quality Determination . . . . . . . . . . . . . . . 5
5. Security Formats . . . . . . . . . . . . . . . . . . . . . . 6
6. Command Format . . . . . . . . . . . . . . . . . . . . . . . 7
7. TLV Formats . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Source Address . . . . . . . . . . . . . . . . . . . . . 8
7.2. Mode . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.3. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.4. Challenge . . . . . . . . . . . . . . . . . . . . . . . . 9
7.5. Response . . . . . . . . . . . . . . . . . . . . . . . . 9
7.6. Link-layer Frame Counter . . . . . . . . . . . . . . . . 9
7.7. Link Quality . . . . . . . . . . . . . . . . . . . . . . 9
7.8. Network Parameter . . . . . . . . . . . . . . . . . . . . 11
7.9. MLE Frame Counter . . . . . . . . . . . . . . . . . . . . 12
8. Message transmission . . . . . . . . . . . . . . . . . . . . 12
9. Processing of incoming messages . . . . . . . . . . . . . . . 13
10. Link Configuration . . . . . . . . . . . . . . . . . . . . . 14
11. Parameter Dissemination . . . . . . . . . . . . . . . . . . . 15
12. Neighbor Detection . . . . . . . . . . . . . . . . . . . . . 15
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
14.1. Security Suites . . . . . . . . . . . . . . . . . . . . 16
14.2. Command Types . . . . . . . . . . . . . . . . . . . . . 17
14.3. TLV Types . . . . . . . . . . . . . . . . . . . . . . . 17
14.4. Network Parameters . . . . . . . . . . . . . . . . . . . 17
15. Security Considerations . . . . . . . . . . . . . . . . . . . 18
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
16.1. Normative References . . . . . . . . . . . . . . . . . . 18
16.2. Informative References . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
The configuration of individual links in IEEE 802.15.4 mesh networks
falls into a gap between standards. The IEEE 802.15.4 standard
provides for static point-to-point and star topologies while the
routing (L3) protocols used in multi-hop mesh networks assume that
the L2 links are already up and running. Effective mesh networking
using IEEE 802.15.4 requires identifying, configuring, and securing
usable links to neighboring devices as the network's membership and
physical environment change. Newly usable links need to be
identified and configured automatically, where configuration values
can include link-layer addresses, transmit and receive modes,
security parameters, and so forth.
Security configuration is particularly important, as IEEE 802.15.4's
replay protection applies only between a joining device and the IEEE
802.15.4 coordinator via which it joins the network. Replay
protection with other neighbors requires a synchronization step that
is not specified by IEEE 802.15.4.
MLE can also be used to distribute configuration values that are
shared across a network, such as the channel and PAN ID. Network-
wide configuration uses multicasts and requires some form of multi-
hop multicast forwarding. These messages are sent infrequently, so
forwarding with simple flooding is sufficient.
One of the most important properties of a radio link, how reliably
the two neighbors can communicate, often cannot be determined
unilaterally by either neighbor. Many 802.15.4 links are asymmetric,
where messages traveling one way across the link are received more or
less reliably than messages traveling in the opposite direction.
There is a chicken and egg problem here. It is a waste of effort to
configure a link that does not have sufficient two-way reliability to
be useful, but the two-way reliability cannot be determined without
exchanging messages over the link. MLE resolves this by allowing a
node to periodically multicast an estimate of the quality of its
links. This allows a node to determine if it has a usable radio link
to a neighbor without first configuring that link.
MLE was developed as part of the ZigBee IP networking standard
[ZigBeeIP].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
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2. Terminology
ETX Expected Transmission Count [RFC6551]; the number
of transmission attempts required to send a
packet over a particular link. Defined to be the
product of the IDR values for both directions. A
perfect link has an ETX of 1, less than perfect
links have higher ETX values.
Frame counter A value that is incremented with each new secured
message and used to detect replayed messages.
IDR Inverse Delivery Ratio; the number of
transmission attempts divided by the number of
successful transmissions in a given direction
over a link. Used in computing the ETX value for
a link.
3. Applicability
This protocol provides configuration and management mechanisms for
using IEEE 802.15.4 links in IP-based multi-hop mesh networks. The
protocol is designed to be easily extended to add additional
features. It could also be adapted for use with other single-hop
link protocols that have some of the same features (message
encryption, one-hop multicast) and omissions (listed at the start of
Section 4) as IEEE 802.15.4.
4. Overview
MLE adds three capabilities to IEEE 802.15.4:
o Dynamically configuring and securing radio links.
o Enabling network-wide changes to radio parameters.
o Determining link quality, prior to link configuration.
The first two are mutually independent; either one can be used
without the other. The purpose of the third, determining link
quality, is to make link management more efficient by detecting
unreliable links before any effort is spent configuring them.
All MLE messages are sent using UDP. While UDP is not an obvious
choice for a protocol used for L2 configuration, it was chosen to
simplify integration of MLE into existing systems.
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4.1. Link Configuration
Link configuration is done using link-local unicasts to exchange IEEE
802.15.4 radio parameters (addresses, node capabilities, and frame
counters) between neighbors. Link configuration messages are either
a request that the link be configured, or an acceptance or rejection
of such a request.
IEEE 802.15.4 security uses frame counters to detect replayed
messages. Each sender maintains an outgoing frame counter that is
included in outgoing secured messages and is incremented for each
message sent. Receivers store the highest frame counter yet seen
from each neighbor. Messages that arrive with a frame counter less
than or equal to the highest previously-seen frame counter are
discarded.
In a mesh network neighbors come and go as the mesh topology changes.
When a message arrives from a previously unheard neighbor, the
receiver has no stored frame counter that can be used to determine if
the message has been replayed by an attacker. MLE solves this by
providing a simple handshake mechanism that allows the receiver to
securely obtain the sender's current outgoing frame counter.
4.2. Parameter Dissemination
Network-wide changes to radio parameters, such as moving the network
to a new channel, is done by multicasting the new value(s) to all
devices in the network. Along with the values themselves, the
multicast messages include a delay value indicating when the new
value takes effect. The delay avoids having the parameters change
while the multicast is still propagating.
In addition to network wide dissemination, a device that does not
have the current network values, either because it has just joined
the network or for any other reason, can send a unicast request to a
neighbor. The neighbor will respond by sending the current network
values.
4.3. Link Quality Determination
802.15.4 links can be asymmetric in that a link between neighboring
devices may be much more reliable in one direction than in the other.
This limits the usefulness of unilateral link quality detection: a
link that looks strong to one device may not be usable because it
works poorly in the other direction. To avoid wasting effort
configuring unusable links, devices can use MLE to send link-local
multicasts containing their local link quality estimates.
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Neighboring nodes can then form an estimate of the two-way quality of
their link to the sender.
5. Security Formats
One of the main functions of MLE is to initialize link-layer
security. This means that MLE itself cannot rely on link-layer
security. To avoid the cost and complexity of adding a second
security suite, MLE reuses that of 802.15.4: [AES] in Counter with
CBC-MAC Mode [CCM] as described in [IEEE802154]. Later extensions
may include other security suites for use with other radio standards.
An MLE message begins with single byte indicating the security suite
used in that message. If that initial byte is "255" no security is
used and the messages has no additional security data. An initial
byte of "0" indicates that the message is secured (encrypted and
authenticated) as described in [IEEE802154]; these messages MUST NOT
also be secured at the link layer. MLE messages thus have one of the
two following formats:
+-----+------------+---------+-----+
| 0 | Aux Header | Command | MIC |
+-----+------------+---------+-----+
+-----+---------+
| 255 | Command |
+-----+---------+
Aux Header Auxiliary Security Header as described in [IEEE802154].
Command MLE command; see Section 6.
MIC Message Integrity Code as described in [IEEE802154].
MLE security MUST NOT use any key that is being used by the link (or
any other) layer. [CCM] requires that each key and nonce pair be
used exactly once, which is most easily achieved by using different
keys.
If MLE security is in use each device MUST maintain an outgoing MLE
frame counter for use in securing outgoing packets in compliance with
[CCM]. This MAY be the same frame counter used for securing 802.15.4
frames. Other than the above requirements, the distribution or
derivation of the key(s) used for MLE security is outside the scope
of this document. The outgoing MLE frame counter MUST be handled as
required by [CCM]. In particular, frame counters MUST NOT be reused
for any given key; if the outgoing MLE frame counter reaches its
maximum value (0xFFFFFFFF), secured MLE messages MUST NOT be sent
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until a new key is available, at which point the outgoing MLE frame
counter MAY be set back to zero.
6. Command Format
MLE messages consist of a command type and a series of type-length-
value parameters.
+--------------+-----+-----+-----+
| Command Type | TLV | ... | TLV |
+--------------+-----+-----+-----+
Command Type An eight-bit unsigned integer identifying the type of
message. This document defines the following commands:
0 Link Request. A request to establish a link to a
neighbor.
1 Link Accept. Accept a requested link.
2 Link Accept and Request. Accept a requested link
and request a link with the sender of the original
request.
3 Link Reject. Reject a link request.
4 Advertisement. Inform neighbors of a device's link
state.
5 Update. Informs of changes to link parameters
shared by all nodes in a network.
6 Update Request. Request that an Update message be
sent.
The first four (Link Request, Link Accept, Link Accept
and Request, and Link Reject) are collectively referred
to as link configuration messages.
TLVs Zero or more TLV frames. These are described in
Section 7.
7. TLV Formats
Values are encoded using a type-length-value format, where the type
and length are one byte each and the length field contains the length
of the value in bytes. TLVs are stored serially with no padding
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between them. They are byte-aligned but are not aligned in any other
way such as on 2 or 4 byte boundaries. All values in TLVs are in
network byte order.
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 | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type An eight-bit unsigned integer giving the type of
the value, from IANA registry Section 14.3.
Length An eight-bit unsigned integer giving the length
of the Value field in bytes.
Value Length bytes of value, formatted as defined for
the Type.
With the exceptions of the Source Address TLV and Parameter TLV, an
MLE message MUST NOT contain two or more TLVs of the same type. To
allow devices to have multiple source addresses, an MLE message MAY
contain two or more Source Address TLVs.
7.1. Source Address
The Source Address TLV (TLV Type 0) has a Value containing a byte
string representing a link-layer address assigned to the source of
the message. A given radio interface may have multiple link-layer
addresses. This TLV is used to communicate any source address(es)
that is not included in the message by the link layer itself.
7.2. Mode
The Mode TLV (TLV Type 1) has a Value containing a byte string
representing the mode in which this link is used by the source of the
message. The format of the value is that of the Capability
Information field in the 802.15.4 Associate command as described in
[IEEE802154].
7.3. Timeout
The Timeout TLV (TLV Type 2) has a Value containing a 32-bit unsigned
integer. The value is the expected maximum interval between
transmissions by the sender, in seconds. This allows the receiver to
more accurately timeout a link to a neighbor that polls for its
incoming messages.
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7.4. Challenge
The Challenge TLV (TLV Type 3) has a Value containing a randomly-
chosen byte string that is used to determine the freshness of any
reply to this message. The recommendations in [RFC4086] apply with
regard to generation of the challenge value. The byte string MUST be
at least 4 bytes in length and a new value MUST be chosen for each
Challenge TLV transmitted. An important part of replay protection is
determining if a newly-heard neighbor is actually present or is a set
of recorded messages. This is done by sending a random challenge
value to the neighbor and then receiving that same value in a
Response TLV sent by the neighbor.
7.5. Response
The Response TLV (TLV Type 4) has a Value containing a byte string
copied from a Challenge TLV.
7.6. Link-layer Frame Counter
The Link-layer Frame Counter TLV (TLV Type 5) has a Value containing
the sender's current outgoing link-layer Frame Counter, encoded as an
N-byte unsigned integer. For 802.15.4 this is a 4-byte value.
7.7. Link Quality
The Link Quality TLV (TLV Type 6) reports the sender's measured link
quality for messages received from its neighbors. The format of the
Link Quality value 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Res | Size | Neighbor Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C Complete: "1" if the message includes all
neighboring routers for which the source has link
quality data. Multicast Link Quality TLVs
normally contain complete information; a unicast
to a particular neighbor would normally contain
only that neighbor's link quality and would have
the C flag set to "0".
Res Reserved; MUST be set to 000 and SHOULD be
ignored on receipt.
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Size The size in bytes of the included neighbor link-
layer addresses, minus 1. This supports
addresses of lengths 1 to 16 bytes.
Neighbor Data A sequence of neighbor records, each containing
receive and transmit state flags, the estimated
incoming link reliability (IDR), and the
neighbor's link-layer address.
The neighbor data in a Link Quality TLV is formatted 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|I|O|P|reserved | Incoming IDR | Neighbor Address ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
I(ncoming) "1" if the sender's Receive State for this
neighbor is true, "0" if not.
O(utgoing) "1" if the sender's Transmit State for this
neighbor is true, "0" if not.
P(riority) "1" if the sender expects to use this link for
sending messages, "0" if not. Given limited
resources, the P flag MAY be used in deciding
which links should be maintained.
Incoming IDR The estimated inverse delivery ratio of messages
sent by the neighbor to the source of this
message. This is an eight-bit unsigned integer.
To allow for fractional IDR, the value encoded is
multiplied by 32. A perfect link, with an actual
IDR of 1, would have an Incoming IDR of 0x20. A
value of 0xFF indicates that the link is
unusable.
Address A link-layer address of a neighbor.
The I and O flags are used to facilitate the two-way use of links
between neighboring routers.
A node that does not have a link configured to a neighbor but
receives a Link Quality TLV from that neighbor with the node's O flag
set to "1" SHOULD send an MLE message with a Link Quality TLV with
that neighbor's I bit set to "0". This message may either be a
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regular multicast Advertisement or a unicast to that neighbor
containing only a single Neighbor Data record.
7.8. Network Parameter
The Parameter TLV (TLV Type 7) specifies the value of a link-layer
parameter shared across the network (as opposed to a parameter
specific to a particular link). The Value contains three fields:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter ID | Delay
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Parameter ID The ID of the parameter to be changed.
Delay The delay before setting the parameter, in
milliseconds. This is a four-byte unsigned
integer. Having a delay gives time for the new
value to propagate throughout the network. It
may also be used for limiting the time a
particular parameter setting is in use, by
including two different values for a single
parameter, with two different delays.
Value A byte string containing the new value of the
parameter. The format of this value is
determined by the particular parameter
Update messages MUST contain only Network Parameter TLVs. Update
messages with new parameter settings are normally multicast to the
entire MLE domain. They may also be unicast to nodes that have just
joined the network or otherwise do not have up-to-data parameter
information.
The defined Network Parameters are:
0 Channel
1 PAN ID
2 Permit Joining
3 Beacon Payload
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7.9. MLE Frame Counter
The MLE Frame Counter TLV (TLV Type 8) has a Value containing the
sender's current outgoing MLE Frame Counter, encoded as an 32-bit
unsigned integer.
8. Message transmission
MLE messages SHOULD be sent using the assigned UDP port number
(19788) as both the source and destination port. Link configuration
and advertisement messages MUST be sent with an IP Hop Limit of 255,
either to a link-local unicast address or to the link-local all-nodes
(FF02::1) or all-routers (FF02::2) multicast addresses. Update
messages MAY be sent as above, or MAY be sent to a realm-local all-
MLE-nodes multicast address (to be assigned by IANA).
Outgoing link configuration and advertisement messages SHOULD be
secured using the procedure specified in [AES] and [CCM] using the
auxiliary security header as described in [IEEE802154]. The one
exception to this is messages sent to or from a device that is
joining the network and does not yet have the necessary keys; such
unsecured messages MUST NOT contain Challenge, Response, or Link-
Layer Frame Counter TLVs.
The authenticated data consists of the following three values
concatenated together:
IP source address
IP destination address
auxiliary security header
The secured data consists of the messages body following the
auxiliary security header (the command ID and TLVs). The security
suite identifier is not included in either the authenticated data or
the secured data. Key choice is outside the scope of this document.
In order to allow update messages to be forwarded multiple hops,
outgoing update messages, MUST be secured at the link layer, if link
layer security is in use, and MUST NOT be secured by MLE.
A message sent in response to a multicast request, such as a
multicast Link Request, MUST be delayed by a random time between 0
and MAX_RESPONSE_DELAY_TIME seconds, with a resolution of at least
1ms.
MAX_RESPONSE_DELAY_TIME 1 second
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If no response is received to a unicast request, the request MAY be
retransmitted using a simple timeout mechanism. This is based on the
retransmission mechanism used in DHCPv6 RFC 3315 [RFC3315],
simplified to use a single, fixed timeout. Unicast requests are not
relayed, which avoids the need for a more elaborate mechanism.
Parameter Default Description
-------------------------------------------------------
URT 1 sec Unicast Retransmission timeout.
MRT 5 sec Multicast Retransmission timeout.
MRC 3 Maximum retransmission count.
For each transmission the appropriate URT or MRT value is multiplied
by a random number chosen with a uniform distribution between 0.9 and
1.1 with a resolution of at least 1ms. The randomization factor is
included to minimize synchronization of messages transmitted.
9. Processing of incoming messages
Any incoming link configuration or advertisement message, or an
incoming update sent to a link-local address, whose IP Hop Limit is
not 255 may have been forwarded by a router and MUST be discarded.
Incoming messages whose Command Type is a reserved value MUST be
ignored. Any TLVs in an incoming message whose TLV Type has a
reserved value MUST be ignored.
Incoming messages that are not secured with either MLE or link-layer
security SHOULD be ignored. The one exception to this is messages
sent to or from a device that is joining the network and does not yet
have the necessary keys. Secured incoming messages are decrypted and
authenticated using the procedures specified in [AES] and [CCM], with
security material obtained from the auxiliary security header as
described in [IEEE802154]. The key source may be obtained either
from the link layer source address or from the auxiliary security
header.
A device MUST maintain a separate incoming MLE frame counter for each
neighbor with which it establishes a link. Any MLE message received
with a frame counter the same or lower than that of a previously
received and authenticated message from the same source MUST be
discarded. Messages for which no previous frame counter are
available MAY be processed, but their counter value MUST be saved for
comparison with later messages.
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10. Link Configuration
The values that may need to be communicated to configure an 802.15.4
link are:
o Long (64-bit) and short (16-bit) addresses.
o Capability Information, as in the 802.15.4 Association command in
[IEEE802154], especially the Device Type and Receiver On When Idle
fields.
o Initialization of AES-CCM frame counters.
A device wishing to establish a link to a neighbor MUST send a Link
Request message containing the following:
o Source Address TLV, containing the sender's short (16-bit) MAC
address. The sender's long (64-bit) MAC address MUST used as the
MAC source address of the message.
o Mode TLV, containing the sender's Capability data byte.
o Timeout TLV, if the sender is an rxOffWhenIdle device.
o Challenge TLV, whose size is determined by the network
configuration.
The neighbor SHOULD respond with a Link Accept message containing the
same TLVs (with its own values), but with a Response TLV in place of
the Challenge TLV and with added Link-layer Frame Counter and MLE
Frame Counter TLVs. If large numbers of Link Request messages arrive
a device MAY reduce or completely suspend sending Link Accept
messages, and MAY send Link Reject messages instead. The MLE Frame
Counter TLV MAY be omitted if the sender uses the same counter for
both MLE and 802.15.4 messages. If the neighbor also requires a
liveness check, it MAY include its own challenge, and use the Link
Accept And Request message type.
If a node receives a secured 802.15.4 unicast from a neighbor for
whom it does not have link configuration data, the receiving node
SHOULD respond with a Link Reject message to inform the neighbor that
the link is not configured. If large numbers of such messages arrive
a device MAY reduce or completely suspend sending Link Reject
messages.
Link Configuration messages are used to establish 802.15.4 security
and so MUST NOT be secured at the 802.15.4 layer.
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11. Parameter Dissemination
Update messages may be sent to change the channel, PAN ID, and/or
permit joining flags on all nodes. Determining when these values
should be changed is beyond the scope of this document.
To make a network-wide change to one of these parameters, an MLE
update messages SHOULD be sent to an appropriate multicast address,
such as the realm-local all-node, all-routers or all-MLE-nodes
multicast address (to be assigned by IANA). Alternatively, MLE
update messages MAY be unicast to individual devices, either to avoid
the cost of a multicast or to have the parameter change apply to only
a subset of devices. This requires some form of multi-hop multicast
forwarding; these messages are sent infrequently, so forwarding with
simple flooding is sufficient.
A single update message MAY contain multiple values for the same
parameter with different time delays. In particular, the permit
joining flag can be enabled for a limited time by including both on
and off values in a single update message.
A device that does not have the current network values, either
because it has just joined the network or for any other reason, MAY
send a unicast Update Request to a neighbor. The neighbor responds
by sending an Update message containing the current values of the
parameters.
12. Neighbor Detection
Nodes MAY send out periodic advertisements containing the incoming
IDR values for their neighbors. The primary purpose of these
messages is to allow nodes to choose likely candidates for link
establishment. They can also be used to determine if existing links
continue to provide sufficient two-way reliability.
A node maintains two boolean values for each known neighbor:
Receive State True if the node will accept incoming non-MLE messages
from that neighbor.
Transmit State A local cache of the neighbor's Receive State
corresponding to this node.
Both values default to false.
The Receive State is set to true when the node receives a valid
incoming link accept from the neighbor, and set to false when the
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link configuration information is discarded for any reason (link
failure or timeout, for example).
The Transmit State is set to true when a link accept message is sent
to the neighbor. When an advertisement message is received from the
neighbor the Transmit State is set to the Receive State as reported
in the advertisement. If the advertisement's C flag is 1 and the
receiving node's address is not included in the advertisement, the
recipient's Transmit State for the sender is set to false.
These states are advisory only; a node may send a message to a
neighbor regardless of its Transmit State for that neighbor.
Similarly, a node may unilaterally change its Receive State (and
discard any link configuration data) without first informing the
neighbor of its intention. The change in Receive State will be
reflected in the next advertisement sent by the node.
Advertisement messages are used prior to establishing 802.15.4
security and thus SHOULD NOT be secured at the 802.15.4 layer.
13. Acknowledgements
The author would like to acknowledge the helpful comments of Thomas
Clausen, Robert Cragie, Colin O'Flynn, Edward Hill, Matteo Paris,
Kundok Park, Joseph Reddy, and Dario Tedeschi, which greatly improved
the document.
14. IANA Considerations
IANA has assigned UDP port 19788 to MLE.
IANA is requested to establish a new top-level registry, called "MLE:
Mesh Link Establishment", to contain all MLE objects, codepoints, and
sub-registries.
The allocation policy for each new registry is by IETF review: new
values are assigned through the IETF review process .
14.1. Security Suites
IANA is requested to create a subregistry, called "Security Suites".
Values range from 0 to 255.
Value Meaning Reference
0 802.15.4 Security This document
255 No Security This document
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Values 1-254 are currently unassigned.
14.2. Command Types
IANA is requested to create a subregistry, called "Command Types".
Values range from 0 to 255.
Value Meaning Reference
0 Link Request This document
1 Link Accept This document
2 Link Accept and Request This document
3 Link Reject This document
4 Advertisement This document
5 Update This document
6 Update Request This document
Values 7-255 are currently unassigned.
14.3. TLV Types
IANA is requested to create a subregistry, called "TLV Types".
Values range from 0 to 255.
Value Meaning Reference
0 Source Address This document
1 Mode This document
2 Timeout This document
3 Challenge This document
4 Response This document
5 Link-layer Frame Counter This document
6 Link Quality This document
7 Network Parameter This document
8 MLE Frame Counter This document
Values 9-255 are currently unassigned.
14.4. Network Parameters
IANA is requested to create a subregistry, called "Network
Parameters". Values range from 0 to 255.
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Value Meaning Reference
0 Channel This document
1 PAN ID This document
2 Permit Joining This document
3 Beacon Payload This document
Values 4-255 are currently unassigned.
15. Security Considerations
In general MLE has the strengths and weaknesses of the link layer
security that it inherits. The one exception is that MLE's operation
requires accepting and acting on incoming Advertisements and Link
Requests messages for which the receiver has no prior knowledge of
the sender's MLE frame counter. Because of this, implementers must
be careful in how they use information obtained from these possibly-
replayed messages. For example, information from unsecured messages
should not be used to modify any stored information obtained from
secured messages.
The Hop Limit field of received packets other than multihop update
messages is verified to contain 255, the maximum legal value.
Because routers decrement the Hop Limit on all packets they forward,
received packets containing a Hop Limit of 255 must have originated
from a neighbor. This technique is borrowed from IPv6 ND [RFC4861].
16. References
16.1. Normative References
[AES] National Institute of Standards and Technology,
"Specification for the Advanced Encryption Standard
(AES)", FIPS 197, November 2001.
[CCM] National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation: The
CCM Mode for Authentication and Confidentiality", SP
800-38C, May 2004.
[IEEE802154]
Institute of Electrical and Electronics Engineers,
"Wireless Personal Area Networks", IEEE Standard
802.15.4-2006, 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
16.2. Informative References
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
Barthel, "Routing Metrics Used for Path Calculation in
Low-Power and Lossy Networks", RFC 6551, March 2012.
[ZigBeeIP]
ZigBee Alliance, "ZigBee IP Specification", 2014,
<http://www.zigbee.org/non-menu-pages/zigbee-ip-download>.
Author's Address
Richard Kelsey
Silicon Labs
343 Congress St
Boston, Massachusetts 02210
USA
Phone: +1 617 951 1225
Email: richard.kelsey@silabs.com
Kelsey Expires June 3, 2016 [Page 19]
