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Internet Engineering Task Force (IETF)                          B. ChengRequest for Comments: 8629                        MIT Lincoln LaboratoryCategory: Standards Track                                 L. Berger, Ed.ISSN: 2070-1721                                  LabN Consulting, L.L.C.                                                               July 2019Dynamic Link Exchange Protocol (DLEP) Multi-Hop Forwarding ExtensionAbstract   This document defines an extension to the Dynamic Link Exchange   Protocol (DLEP) that enables the reporting and control of multi-hop   forwarding by DLEP-capable modems.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8629.Copyright Notice   Copyright (c) 2019 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://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.Cheng & Berger               Standards Track                    [Page 1]

RFC 8629                DLEP Multi-Hop Extension               July 2019Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Key Words . . . . . . . . . . . . . . . . . . . . . . . .32.  Extension Usage and Identification  . . . . . . . . . . . . .33.  Extension Data Items  . . . . . . . . . . . . . . . . . . . .33.1.  Hop Count . . . . . . . . . . . . . . . . . . . . . . . .33.2.  Hop Control . . . . . . . . . . . . . . . . . . . . . . .53.2.1.  Reset . . . . . . . . . . . . . . . . . . . . . . . .63.2.2.  Terminate . . . . . . . . . . . . . . . . . . . . . .73.2.3.  Direct Connection . . . . . . . . . . . . . . . . . .73.2.4.  Suppress Forwarding . . . . . . . . . . . . . . . . .74.  Security Considerations . . . . . . . . . . . . . . . . . . .85.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .85.1.  Extension Type Value  . . . . . . . . . . . . . . . . . .85.2.  Data Item Values  . . . . . . . . . . . . . . . . . . . .95.3.  Hop Control Actions Registry  . . . . . . . . . . . . . .96.  References  . . . . . . . . . . . . . . . . . . . . . . . . .106.1.  Normative References  . . . . . . . . . . . . . . . . . .106.2.  Informative References  . . . . . . . . . . . . . . . . .10   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .10   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .101.  Introduction   The Dynamic Link Exchange Protocol (DLEP) is defined in [RFC8175].   It provides the exchange of link-related control information between   a modem and a router.  DLEP defines a base set of mechanisms as well   as support for possible extensions.  This document defines one such   extension.   Some modem technologies support mobile ad hoc network (MANET)   forwarding where connectivity to destinations is provided via   forwarding in intermediate modems.  This document refers to   forwarding by intermediate modems as "multi-hop forwarding".  DLEP   Destination Messages can be used to report such reachable   destinations (see [RFC8175]), but do not provide any information   related to the number or capacity of the hops.  The extension defined   in this document enables modems to inform routers when multi-hop   forwarding is being used and allows routers to request that modems   change multi-hop forwarding behavior.  The extension defined in this   document is referred to as "Multi-Hop Forwarding", where each modem   that transmits/sends data to reach a particular destination is   counted as a hop.   It is important to note that the use of the Hop Control mechanism   defined in this document can result in connectivity changes and even   loss of the ability to reach one or more destinations.  The definedCheng & Berger               Standards Track                    [Page 2]

RFC 8629                DLEP Multi-Hop Extension               July 2019   mechanism will report such connectivity changes, but the details of   what a router does or how it reacts to such are out scope of this   document.   This document defines a new DLEP Extension Type Value inSection 2,   which indicates the use of the extension, and three new DLEP Data   Items inSection 3.1.1.  Key Words   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 inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.2.  Extension Usage and Identification   The use of the Multi-Hop Forwarding Extension SHOULD be configurable.   Per [RFC8175], to indicate that the extension is to be used, an   implementation includes the Multi-Hop Forwarding Extension Type Value   in the Extensions Supported Data Item.  The Extensions Supported Data   Item is sent and processed according to [RFC8175].   The Multi-Hop Forwarding Extension Type Value is 1 (seeSection 5).3.  Extension Data Items   Three data items are defined by this extension.  The Hop Count Data   Item is used by a modem to provide the number of modem hops traversed   to reach a particular destination.  The Hop Control Data Item is used   by a router to request that a modem alter connectivity to a   particular destination.  The Suppress Forwarding Data Item is used by   a router to request that a modem disable multi-hop forwarding on   either a device or destination basis.3.1.  Hop Count   The Hop Count Data Item is used by a modem to indicate the number of   modems that transmit/send data to reach a particular destination,   i.e., hops, between the modem and a specific destination.  In other   words, each hop represents a transmission, and the number of hops is   equal to the number of transmissions required to go from a router's   connected modem to the destination's connected modem.  The minimum   number of hops is 1, which represents transmission to destinations   that are directly reachable via the router's locally connected modem.Cheng & Berger               Standards Track                    [Page 3]

RFC 8629                DLEP Multi-Hop Extension               July 2019   The data item also contains an indication of when a destination that   currently has a hop count of greater than one (1) could be made   directly reachable by a modem, e.g., by reaiming an antenna.   The Hop Count Data Item SHOULD be carried in the Destination Up,   Destination Update, Destination Announce Response, and Link   Characteristics Response Messages when the Hop Count to a destination   is greater than one (1).   A router receiving a Hop Count Data Item can use this information in   its forwarding and routing decisions, but specific use is out of   scope of this document.  When using this extension, the absence of   the Hop Count Data Item MUST be interpreted by the router as a Hop   Count value of one (1).   The format of the Hop Count Data Item is:        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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       | Data Item Type                | Length                        |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |P|  Reserved   |   Hop Count   |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Data Item Type:  21   Length:  2   P:      The P-bit indicates that a destination is potentially directly      reachable.  When the P-bit is set, the router MAY request a direct      link to the associated destination using the Hop Control Data Item      described below.  This field MUST be ignored when the value      contained in the Hop Count field is one (1).   Reserved:      The Reserved field MUST be set to zero by the sender (a modem) and      ignored by the receiver (a router).   Hop Count:      The Hop Count is an unsigned 8-bit integer indicating the number      of modem hops required (i.e., number of times a packet will be      transmitted) to reach the destination indicated in the message.      The special value of 255 (0xFF) is used to indicate that theCheng & Berger               Standards Track                    [Page 4]

RFC 8629                DLEP Multi-Hop Extension               July 2019      number of hops is an unknown number greater than one (1).  This      field MUST contain a value of at least one (1) if the associated      destination is reachable.      A value of zero (0) is used to indicate that the processing of a      Hop Control action (seeSection 3.2) has resulted in the      destination no longer being reachable.  A zero value MUST NOT be      used in any message other than a Link Characteristics Response      Message.3.2.  Hop Control   The Hop Control Data Item is used by a router to request a change in   connectivity to a particular destination or to perform multi-hop   processing on a device-wide basis.  A router can request that a   multi-hop-reachable destination be changed to a single-hop   destination.  A router can also indicate that the modem terminates a   previous direct connectivity request to a particular destination.   The Hop Control Data Item MAY be carried in a Session Update Message   sent by a router when the control applies to the whole device, or a   Link Characteristics Request Message when the control applies to a   particular destination.   A modem that receives the Hop Control Data Item in a Link   Characteristics Request Message SHOULD take whatever actions are   needed to make the change indicated by the data item for the   associated destination Media Access Control (MAC) address.  Once the   change is made, fails, or is rejected, the modem MUST respond with a   Link Characteristics Response Message containing an updated Hop Count   Data Item.  Note that other destinations can be impacted as a result   of the change, and such changes are reported in Destination Down and   Destination Update Messages.  The modem MUST notify the router of   each destination that is not identified in the Link Characteristics   Response Message and is no longer reachable via a Destination Down   Message.  The modem MUST also notify the router of each impacted   destination that is not identified in the Link Characteristics   Response Message via a Destination Update Message.   Failures may occur for multiple reasons, for example, the   transmission characteristics of the link don't support the one-hop   connection at the time of the request.  Requests can be rejected by   local policy.   A modem that receives the Hop Control Data Item in a Session Update   Message SHOULD take whatever actions are needed to make the change   indicated by the data item for all known destinations.  Once the   change is made, fails, or is rejected, the modem MUST respond with aCheng & Berger               Standards Track                    [Page 5]

RFC 8629                DLEP Multi-Hop Extension               July 2019   Session Update Response Message with an appropriate Status Code.  The   destination-specific impact of processing a Hop Control Data Item in   a Session Update Message is provided via Destination Down and   Destination Update Messages.  The modem MUST notify the router of   each destination that is no longer reachable via a Destination Down   Message.  The modem MUST notify the router of any changes in Hop   Counts via Destination Update Messages.   The format of the Hop Control Data Item is:        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       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       | Data Item Type                | Length                        |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |       Hop Control Actions     |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Data Item Type:  22   Length:  2   Hop Control Actions:      The Hop Control Actions field is an unsigned 16-bit value with the      following meaning:                      +-------+---------------------+                      | Value | Action              |                      +-------+---------------------+                      | 0     | Reset               |                      | 1     | Terminate           |                      | 2     | Direct Connection   |                      | 3     | Suppress Forwarding |                      +-------+---------------------+                    Table 1: Hop Control Actions Values3.2.1.  Reset   The Reset Action requests that the default behavior be restored.   When received in a Session Update Message, a modem MUST clear all   control actions that have previously been processed on a device-wide   basis and revert to its configured behavior.  When received in a Link   Characteristics Request Message, a modem MUST clear all control   actions that have previously been processed for the destination   indicated in the message.Cheng & Berger               Standards Track                    [Page 6]

RFC 8629                DLEP Multi-Hop Extension               July 20193.2.2.  Terminate   The Terminate Action is only valid on a per-destination basis and   MUST NOT be sent in a Session Update Message.  It indicates that a   direct connection is no longer needed with the destination identified   in the message.  This request has no impact on multi-hop destinations   and may fail even in a single-hop case, i.e., it can result in the   Hop Count to the destination not being impacted by the processing of   the request.3.2.3.  Direct Connection   The Direct Connection Action is only valid on a per-destination basis   and MUST NOT be sent in a Session Update Message.  It indicates that   the modem SHOULD attempt to establish a direct connection with the   destination identified in the message.  This action SHOULD only be   sent for destinations for which the Hop Count is both greater than 1   and has the P-Bit set in the previously received Hop Count Data Item.   Results of the request for the destination identified in the message   are provided as described above.3.2.4.  Suppress Forwarding   The Suppress Forwarding Action is used by a router to indicate to its   peer that multi-hop forwarding performed by the modem is to be   suppressed.  A router can request that multi-hop forwarding be   suppressed on a device-wide or destination-specific basis.   A modem that receives the Suppress Forwarding Data Item in a Session   Update Message MUST suppress multi-hop forwarding on a device-wide   basis.  This means that data traffic originating from the modem's   peer router SHALL only be sent by the modem to destinations that are   one modem hop away, and that any data traffic received by the modem   from another modem that is not destined to the peer router SHALL be   dropped.  The impact on destination hop counts are provided to the   router by the modem as described above.   A modem that receives the Suppress Forwarding Data Item in a Link   Characteristics Request Message MUST suppress multi-hop forwarding   for only the destination indicated in the message.  This means that   data traffic originating from the modem's peer router SHALL be sent   by the modem to the destination indicated in the Link Characteristics   Request Message only when it is one modem hop away.  Notably, data   traffic received by the modem from another modem can be forwarded by   the modem per its normal processing.  Results are provided as   described above.Cheng & Berger               Standards Track                    [Page 7]

RFC 8629                DLEP Multi-Hop Extension               July 20194.  Security Considerations   The extension defined in this document enables the reporting and   control of forwarding information by DLEP-capable modems.  The   extension does not inherently introduce any additional   vulnerabilities above those documented in [RFC8175].  The approach   taken to security in that document applies equally when running the   extension defined in this document.   The extension does define one mechanism that is worth particular   note.  It includes a Hop Control mechanism (seeSection 3.2) that is   similar to the Link Characteristics Request Message defined in   [RFC8175] in that it can impact the set of destinations reported as   reachable.  With the Link Characteristics Request Message, this risk   is implicit.  With the Hop Control mechanism defined in this   document, it is more likely.  From a security perspective,   implementations should be aware of this increased risk and may choose   to implement additional configuration control mechanisms to ensure   that the Hop Control mechanism is only used under conditions intended   by the network operator.   Implementations of the extension defined in this document MUST   support configuration of TLS usage, as described in [RFC8175], in   order to protect configurations where injection attacks are possible,   i.e., when the link between a modem and router is not otherwise   protected.   Note that this extension does allow a compromised or impersonating   modem to suppress transmission by the router or a switch that   interconnects the modem and router.  Similar attacks are generally   possible for DLEP, for example, an impersonating modem may cause a   session reset or cause a compromised modem to simply drop all traffic   destined to, or sent by, a router.  [RFC8175] defines the use of TLS   to protect against the impersonating attacker.5.  IANA Considerations   As described below, IANA has assigned 3 values to registries defined   by [RFC8175] and created a new registry.5.1.  Extension Type Value   IANA has registered the following new value in the Specification   Required range of the "Extension Type Values" registry within the   "Dynamic Link Exchange Protocol (DLEP) Parameters" registry.Cheng & Berger               Standards Track                    [Page 8]

RFC 8629                DLEP Multi-Hop Extension               July 2019                      +------+----------------------+                      | Code | Description          |                      +------+----------------------+                      | 1    | Multi-Hop Forwarding |                      +------+----------------------+                  Table 2: Requested Extension Type Value5.2.  Data Item Values   IANA has registered the following 2 values in the Specification   Required range of the "Data Item Type Values" registry within the   "Dynamic Link Exchange Protocol (DLEP) Parameters" registry.                        +-----------+-------------+                        | Type Code | Description |                        +-----------+-------------+                        | 21        | Hop Count   |                        | 22        | Hop Control |                        +-----------+-------------+                    Table 3: Requested Data Item Values5.3.  Hop Control Actions Registry   IANA has created the "Hop Control Actions Values" registry within the   "Dynamic Link Exchange Protocol (DLEP) Parameters" registry.  The   following table provides initial registry values and the registration   procedures [RFC8126] that apply:                 +-------------+------------------------+                 | Value       | Action/Policy          |                 +-------------+------------------------+                 | 0           | Reset                  |                 | 1           | Terminate              |                 | 2           | Direct Connection      |                 | 3           | Suppress Forwarding    |                 | 4-65519     | Specification Required |                 | 65520-65534 | Private Use            |                 | 65535       | Reserved               |                 +-------------+------------------------+                    Table 4: Hop Control Actions ValuesCheng & Berger               Standards Track                    [Page 9]

RFC 8629                DLEP Multi-Hop Extension               July 20196.  References6.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.   [RFC8175]  Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B.              Berry, "Dynamic Link Exchange Protocol (DLEP)",RFC 8175,              DOI 10.17487/RFC8175, June 2017,              <https://www.rfc-editor.org/info/rfc8175>.6.2.  Informative References   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for              Writing an IANA Considerations Section in RFCs",BCP 26,RFC 8126, DOI 10.17487/RFC8126, June 2017,              <https://www.rfc-editor.org/info/rfc8126>.Acknowledgments   Helpful comments were received from members of the MANET working   group, including Henning Rogge, Victoria Pritchard, and David   Wiggins.Authors' Addresses   Bow-Nan Cheng   MIT Lincoln Laboratory   Massachusetts Institute of Technology   244 Wood Street   Lexington, MA  02421-6426   Email: bcheng@ll.mit.edu   Lou Berger (editor)   LabN Consulting, L.L.C.   Email: lberger@labn.netCheng & Berger               Standards Track                   [Page 10]

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