Internet Engineering Task Force (IETF) B. Cheng

Internet Engineering Task Force (IETF) B. Cheng
Request for Comments: 8629 MIT Lincoln Laboratory
Category: Standards Track L. Berger, Ed.
ISSN: 2070-1721 LabN Consulting, L.L.C.
July 2019

Dynamic Link Exchange Protocol (DLEP) Multi-Hop Forwarding Extension

Abstract

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 in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://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 to BCP 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.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Extension Usage and Identification . . . . . . . . . . . . . 3
3. Extension Data Items . . . . . . . . . . . . . . . . . . . . 3
3.1. Hop Count . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Hop Control . . . . . . . . . . . . . . . . . . . . . . . 5
3.2.1. Reset . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2.2. Terminate . . . . . . . . . . . . . . . . . . . . . . 7
3.2.3. Direct Connection . . . . . . . . . . . . . . . . . . 7
3.2.4. Suppress Forwarding . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. Extension Type Value . . . . . . . . . . . . . . . . . . 8
5.2. Data Item Values . . . . . . . . . . . . . . . . . . . . 9
5.3. Hop Control Actions Registry . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10

1. 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 defined

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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 in Section 2,
which indicates the use of the extension, and three new DLEP Data
Items in Section 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 in
BCP 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 (see Section 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.

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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 the

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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 (see Section 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 a

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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 Values

3.2.1. 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.

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3.2.2. 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.

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4. 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 (see Section 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.

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+------+----------------------+
| Code | Description |
+------+----------------------+
| 1 | Multi-Hop Forwarding |
+------+----------------------+

Table 2: Requested Extension Type Value

5.2. 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 Values

5.3. 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 Values

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6. References

6.1. 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 in RFC
2119 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: [email protected]

Lou Berger (editor)
LabN Consulting, L.L.C.

Email: [email protected]

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