DLEP Multi-Hop Forwarding ExtensionLincoln LaboratoryMassachusetts Institute of Technology244 Wood StreetLexingtonMA02420-9108bcheng@ll.mit.eduLabN Consulting, L.L.C.lberger@labn.net
This document defines an extension to the DLEP protocol that enables a
the reporting and control of Multi-Hop Forwarding by DLEP capable
modems.
The Dynamic Link Event Protocol (DLEP) is defined in . It provides the exchange of link
related control information between DLEP peers. DLEP peers are
comprised of 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 connectivity to destinations via
multi-hop forwarding. DLEP Destination messages can be used to report
such connectivity, see , 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 routers to
request that modems change multi-hop forwarding behavior. The
extension defined in this document is referred to as "Multi-Hop
Forwarding".
This document defines a new DLEP Extension Type Value in which is used to indicate the use of the
extension, and three new DLEP Data Items in .
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, RFC 2119 .
The use of the Multi-Hop Forwarding Extension SHOULD be configurable.
To indicate that the extension is to be used, an implementation MUST
include the Multi-Hop Forwarding Extension Type Value in the
Extensions Supported Data Item. The Extensions Supported Data Item is
sent and processed according to .
The Multi-Hop Forwarding Extension Type Value is TBA1, see .
Three data items are defined by this extension. The Hop Count Data
Item is used by a modem to provide the number of network 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.
The Hop Count Data Item is used by a modem to indicate the number of
physical 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
connected modem to the destination's connected modem. The minimum
number of hops is 1, which represents the router's locally connected
modem.
The data item also contains an indication of when a destination which
currently has a hop count of greater than one (1) could be made
direcly reachable by a modem, e.g., by re-aiming 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 MAY use this information in
its forwarding and routing decisions, and specific use is out of scope
of this document. 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:
TBA24
An unsigned 8-bit integer indicating the number of network 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 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 indicated that processing of a Hop
Control action, see , has resulted
in a destination no longer being reachable. A zero value MUST NOT
be used in any message other then a Destination Announce Response
Message.
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.
MUST be set to zero by the sender (a modem) and ignored by the
receiver (a router).
The Hop Control Data Item is used by a router to request a change in
connectivity to a particular destination, or in multi-hop processing
on a device wide basis. A router can request multi-hop reachable
destination be changed to a single hop. A router can also indicate
that the modem terminate connectivity to a particular destination.
The Hop Control Data Item MAY be carried in the Session Update Message
or Destination Announce Message.
A router that receives the Hop Control in a Session Update Message
SHOULD attempt to make the change indicated by the data item for the
whole device. Results of any changes made are reflected in
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.
A router that receives the Hop Control in a Destination Announce
Message SHOULD attempt to make the change indicated by the data item
for the associated destination MAC address. Once the change is made,
or fails or is rejected, the modem MUST respond with a Destination
Announce Response Message containing an updated Hop Count Data Item.
The format of the Hop Control Data Item is:
TBA34
An unsigned 16-bit value with the following meaning:
ValueAction0Reset1Terminate2Direct Connection3Suppress Forwarding
The Reset Action requests that the default behavior be restored.
When received in a Session Update Message message, a modem SHOULD
clear all control actions that have previously been processed on a
device wide basis, and revert to its configured behavior. When
received in a Destination Announce Message, a modem SHOULD clear all
control actions that have previously been processed for the
destination indicated in the message.
The Terminate Action is only valid on a per destination basis and
MUST NOT be sent in a Session Update Message message. It indicates
that the modem SHOULD attempt to terminate communication
with the destination identified in the message. This request has no
impact for multi-hop destinations and may fail even in a single
hop case, i.e. MAY result in the Hop Count to the destination not
being impacted by the processing of the request
The Direct Connection is only valid on a per destination basis and
MUST NOT be sent in a Session Update Message 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 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. If any other destination
is impacted in the processing of this action, the modem MUST send a
Destination Update Message for each impacted destination.
The Suppress Forwarding Action is used by a router to indicate to
its peer that multi-hop forwarding is to be suppressed. A router may
request that multi-hop forwarding may be suppressed on a device wide
or destination specific basis.
A modem which receives the Suppress Forwarding Data Item in a Session
Update Message MUST suppress multi-hop forwarding on a device wide
basis. Impact to destination hop counts are provided to the router by
the modem as described above.
A modem which receives the Suppress Forwarding Data Item in a
Destination Announce Message MUST suppress multi-hop forwarding for
only the destination indicated in the message. Results are provided as
described above.
The extension introduces a new mechanism for flow control between a
router and modem using the DLEP protocol. The
extension does not inherently introduce any additional threats
above those documented in .
The approach taken to Security in that document applies equally
when running the extension defined in this document.
This document requests the assignment of 3 values by IANA. All
assignments are to registries defined by .
It also requests creation of one new registry.
This document requests 1 new assignment to the DLEP Extensions
Registry named "Extension Tyoe Values" in the range with the
"Specification Required" policy. The requested value is as follows:
CodeDescriptionTBA1Multi-Hop Forwarding
This document requests 2 new assignments to the DLEP Data Item
Registry named "Data Item Values" in the range with the "Specification
Required" policy. The requested values are as follows:
Type CodeDescriptionTBA2Hop CountTBA3Hop Control
Upon approval of this document, IANA is requested to create a new
DLEP registry, named "Hop Control Actions Values".
The following table provides initial registry values and the . defined policies that should apply to the
registry:
ValueAction/Policy0Reset1Terminate2Direct Connection3Suppress Forwarding4-65519Specification Required65520-65534Private Use65535Reserved