Internet Area WG R. Kelsey Internet-Draft Ember Corporation Intended status: Standards Track June 28, 2012 Expires: December 30, 2012 Mesh Link Establishment draft-kelsey-intarea-mesh-link-establishment-04 Abstract This document defines the mesh link establishment (MLE) protocol for establishing and configuring secure links in an ad hoc mesh radio network. Effective mesh networking using radio links requires identifying, configuring, and securing usable links to neighboring devices. In an ad hoc mesh network a device's neighbors may come and go 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. MLE includes the option of securing the configuration messages themselves, as link-layer security is typically not available prior to configuration. 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 working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any 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 December 30, 2012. Copyright Notice Copyright (c) 2012 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 Kelsey Expires December 30, 2012 [Page 1] Internet-Draft Mesh Link Establishment June 2012 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Security Formats . . . . . . . . . . . . . . . . . . . . . . . 6 5. Command Format . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6.1. Source Address . . . . . . . . . . . . . . . . . . . . . . 8 6.2. Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6.3. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 8 6.4. Challenge . . . . . . . . . . . . . . . . . . . . . . . . 9 6.5. Response . . . . . . . . . . . . . . . . . . . . . . . . . 9 6.6. Link-layer Frame Counter . . . . . . . . . . . . . . . . . 9 6.7. MLE Frame Counter . . . . . . . . . . . . . . . . . . . . 9 6.8. Link Quality . . . . . . . . . . . . . . . . . . . . . . . 9 6.9. Network Parameter . . . . . . . . . . . . . . . . . . . . 11 6.10. Network Parameter Request . . . . . . . . . . . . . . . . 12 7. Message transmission . . . . . . . . . . . . . . . . . . . . . 13 8. Processing of incoming messages . . . . . . . . . . . . . . . 15 9. Application to 802.15.4 . . . . . . . . . . . . . . . . . . . 16 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 11.1. Security Suites . . . . . . . . . . . . . . . . . . . . . 19 11.2. Command Types . . . . . . . . . . . . . . . . . . . . . . 19 11.3. TLV Types . . . . . . . . . . . . . . . . . . . . . . . . 19 11.4. Network Parameters . . . . . . . . . . . . . . . . . . . . 20 12. Security Considerations . . . . . . . . . . . . . . . . . . . 21 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 13.1. Normative References . . . . . . . . . . . . . . . . . . . 22 13.2. Informative References . . . . . . . . . . . . . . . . . . 22 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23 Kelsey Expires December 30, 2012 [Page 2] Internet-Draft Mesh Link Establishment June 2012 1. Introduction The configuration of individual links in an ad hoc mesh network can fall into a gap between standards. Link layer standards typically deal with individual links and networking standards assume that the links are up and running. In an ad hoc mesh network a device's neighbors may come and go as the network's membership and physical environment change, requiring that new links be configured automatically. The required configuration information can include link layer addressing, transmit and receive modes, wake/sleep cycles, and so on. Security configuration is particularly important, as the link layer may not be able to secure packets until after the link itself has been configured. Existing IETF neighbor discovery protocols, such as IPv6 ND [RFC4861] and NHDP [RFC6130] either require (IPv6 ND) or recommend (NHDP) that their messages be sent over secured links. While there are some similarities between these protocols and MLE, they are actually complementary: MLE is entirely concerned with link- layer configuration, including security, while IPv6 ND and NHDP use already-configured links to discover IP properties of their neighbors. MLE can be used to configure individual links and to distribute configuration values that are shared across a network. Per-link configuration uses one-hop messages with link-local addresses. Network-wide configuration uses multicasts and requires some form of multi-hop multicast forwarding. One of the most important properties of a radio link, how reliably the two neighbors can communicate, often cannot be determined unilaterally by either node. Many radio 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. 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]. Kelsey Expires December 30, 2012 [Page 3] Internet-Draft Mesh Link Establishment June 2012 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. Kelsey Expires December 30, 2012 [Page 4] Internet-Draft Mesh Link Establishment June 2012 3. Overview MLE provides a means of transmitting link configuration values. An MLE message is one of: Link configuration A one-hop unicast sent as part of establishing a link with one particular neighbor. Link configuration messages are either a request that the link be configured, or an acceptance or rejection of such a request. Advertisement A one-hop multicast that advertise a node's link quality estimates to its neighbors. Update Messages that updates a shared configuration value. These may be multicast or unicast. If additional negotiation is required, establishment of a link may require an exchange of two or more unicast messages. The only multiple-message exchange in MLE protocol as described in this document performs liveness determination (frame counter initialization). MLE messages are sent using UDP. A node maintains two boolean values for each 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 accepts an incoming link request from the neighbor, and set to false when the 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 received from the neighbor. When an advertisement message is received from the neighbor the Transmit State is set to the neighbor's Receive State as reported, either implicitly or explicitly, in that message. These states are advisory only; a node may send a message to a neighbor regardless of the Transmit State of 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. Kelsey Expires December 30, 2012 [Page 5] Internet-Draft Mesh Link Establishment June 2012 4. 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 the link layer. This document describes two security suites, one with no security and the other using Advanced Encryption Standard 128 [AES] in Counter with CBC-MAC Mode [CCM] as described in [IEEE802154]. Later extensions may include other security suites for use with other types of links. 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 as described in [IEEE802154] (all codes are to be confirmed by IANA; see Section 11). 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 5. MIC Message Integrity Code as described in [IEEE802154]. 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]. MLE security MUST NOT use any key that is being used by the link (or any other) layer. Other than the above requirements, the distribution or derivation of the key(s) used for MLE security is outside the scope of this document. Kelsey Expires December 30, 2012 [Page 6] Internet-Draft Mesh Link Establishment June 2012 5. 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 (all codes are to be confirmed by IANA, see Section 11): 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. 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 6. Kelsey Expires December 30, 2012 [Page 7] Internet-Draft Mesh Link Establishment June 2012 6. Values 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. There are no alignment requirements and no padding. 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. 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. 6.1. 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. 6.2. 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 and interpretation of the mode value are specific to the link layer in use. Mode information can include such things as the sender's listening schedule, whether it will poll for messages, and so forth. 6.3. Timeout The Timeout TLV (TLV Type 2) has a Value containing a 32-bit unsigned integer, most significant byte first. 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. Kelsey Expires December 30, 2012 [Page 8] Internet-Draft Mesh Link Establishment June 2012 6.4. 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. A new value MUST be chose 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. 6.5. Response The Response TLV (TLV Type 4) has a Value containing a byte string copied from a Challenge TLV. 6.6. 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-bit unsigned integer, most significant byte first. The size of the Frame Counter is determined the link layer in use. 6.7. MLE Frame Counter The MLE Frame Counter TLV (TLV Type 9) has a Value containing the sender's current outgoing MLE Frame Counter, encoded as an N-bit unsigned integer, most significant byte first. The size of the Frame Counter is determined the security suite in use. 6.8. 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 ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Kelsey Expires December 30, 2012 [Page 9] Internet-Draft Mesh Link Establishment June 2012 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 of "0". Res Reserved. Size The size in bytes of the included neighbor link- layer addresses, minus 1. This supports addresses of lengths 1 to 16. 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. An MLE message SHOULD NOT contain more than one Link Quality TLV. 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's 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 Kelsey Expires December 30, 2012 [Page 10] Internet-Draft Mesh Link Establishment June 2012 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 regular multicast Advertisement or a unicast to that neighbor containing only a single Neighbor Data record. 6.9. 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, most significant byte first. 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 SHOULD contain only Network Parameter TLVs. Update messages with new parameter settings SHOULD be 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 Kelsey Expires December 30, 2012 [Page 11] Internet-Draft Mesh Link Establishment June 2012 information. The defined Network Parameters are: 0 Channel 1 PAN ID 2 Permit Joining 3 Beacon Payload (values to be confirmed by IANA) 6.10. Network Parameter Request The Parameter Request TLV (TLV Type 9) requests that the recipient send the current network parameter values to the sender of the request. This is sent by devices that have just joined a network or are otherwise unaware of the current network parameter values. It has no Value. Kelsey Expires December 30, 2012 [Page 12] Internet-Draft Mesh Link Establishment June 2012 7. Message transmission Messages SHOULD be sent using UDP port X (value to be assigned by IANA). 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 site-local all-MLE-nodes multicast address (to be assigned by IANA). Outgoing messages SHOULD be secured using the procedure specified in [AES] and [CCM] using the auxiliary security header as described in [IEEE802154]. Key choice is outside the scope of this document. 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. 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. MAX_RESPONSE_DELAY_TIME 1 second If no response is received to a request, the request MAY be retransmitted. Because MLE messages do not require complex processing and are not relayed, a simple timeout scheme is used for retransmitting. This is based on the retransmission mechanism used in DHCPv6 RFC 3315 [RFC3315], simplified to use a single, fixed timeout. Parameter Default Description ------------------------------------------------------- URT 1 sec Unicast Retransmission timeout. MRT 5 sec Multicast Retransmission timeout. MRC 3 Maximum retransmission count. Kelsey Expires December 30, 2012 [Page 13] Internet-Draft Mesh Link Establishment June 2012 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. The randomization factor is included to minimize synchronization of messages transmitted. Kelsey Expires December 30, 2012 [Page 14] Internet-Draft Mesh Link Establishment June 2012 8. 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 update messages that contain TLVs other than Network Parameter TLVs SHOULD be ignored. Unsecured incoming messages SHOULD be ignored. 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 SHOULD maintain a separate incoming MLE frame counter for each neighbor. 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 are not discarded and the counter value SHOULD be saved for comparison with later messages. Kelsey Expires December 30, 2012 [Page 15] Internet-Draft Mesh Link Establishment June 2012 9. Application to 802.15.4 This section describes how MLE could be used in an 802.15.4-based LoWPAN. This section is not normative. The values that may need to be communicated to configure an 802.15.4 include: o Long (64-bit) and short (16-bit) addresses. o Capability data byte, 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 would send a Link Request message containing the following: o Source Address TLV, containing the sender's short (16-bit) MAC address. It is assumed that the sender's long (64-bit) MAC address is 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. If the neighbor has sufficient resources to maintain an additional link, it would 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 an added Link-layer Frame Counter TLV. If the neighbor also required a liveness check, it would 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. Nodes could also send out periodic advertisements containing the incoming IDR values for their neighbors. These would be used to choose likely candidates for link establishment and to determine if existing links continued to provide sufficient two-way reliability. Because link configuration and advertisement messages are used to establish 802.15.4 security they should not be secured at the 802.15.4 layer. Kelsey Expires December 30, 2012 [Page 16] Internet-Draft Mesh Link Establishment June 2012 Update messages may be sent to change the channel, PAN ID, and/or permit joining flags on all nodes. Kelsey Expires December 30, 2012 [Page 17] Internet-Draft Mesh Link Establishment June 2012 10. Acknowledgements TODO. Kelsey Expires December 30, 2012 [Page 18] Internet-Draft Mesh Link Establishment June 2012 11. IANA Considerations TODO: UDP port 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 . 11.1. 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 Values 1-254 are currently unassigned. 11.2. 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 Values 6-255 are currently unassigned. 11.3. TLV Types IANA is requested to create a subregistry, called "TLV Types". Values range from 0 to 255. Kelsey Expires December 30, 2012 [Page 19] Internet-Draft Mesh Link Establishment June 2012 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 Network Parameter Request This document 9 MLE Frame Counter This document Values 8-255 are currently unassigned. 11.4. Network Parameters IANA is requested to create a subregistry, called "Network Parameters". Values range from 0 to 255. 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. Kelsey Expires December 30, 2012 [Page 20] Internet-Draft Mesh Link Establishment June 2012 12. 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 all received packets 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]. Kelsey Expires December 30, 2012 [Page 21] Internet-Draft Mesh Link Establishment June 2012 13. References 13.1. 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. [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. 13.2. 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. [RFC6130] Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)", RFC 6130, April 2011. [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. Kelsey Expires December 30, 2012 [Page 22] Internet-Draft Mesh Link Establishment June 2012 Author's Address Richard Kelsey Ember Corporation 25 Thomson Place Boston, Massachusetts 02210 USA Phone: +1 617 951 1225 Email: richard.kelsey@ember.com Kelsey Expires December 30, 2012 [Page 23]