Internet DRAFT - draft-ietf-manet-credit-window
draft-ietf-manet-credit-window
Mobile Ad hoc Networks Working Group S. Ratliff
Internet-Draft VT iDirect
Intended status: Standards Track November 13, 2016
Expires: May 17, 2017
Credit Windowing extension for DLEP
draft-ietf-manet-credit-window-07
Abstract
This draft describes an extension to the DLEP protocol to provide a
credit-windowing scheme for destination-specific flow control.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 17, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
6. DLEP Messages for Credit-Window Extension . . . . . . . . . . 5
7. DLEP Status Codes for Credit-Window Extension . . . . . . . . 5
8. DLEP Data Items for Credit-Window Extension . . . . . . . . . 5
9. Credit Window Data Item Definitions . . . . . . . . . . . . . 6
9.1. Credit Grant . . . . . . . . . . . . . . . . . . . . . . 6
9.2. Credit Window Status . . . . . . . . . . . . . . . . . . 7
9.3. Credit Request . . . . . . . . . . . . . . . . . . . . . 8
10. Credit Data Item Use in DLEP Messages . . . . . . . . . . . . 9
10.1. DLEP Destination Up Message . . . . . . . . . . . . . . 9
10.2. DLEP Destination Announce Message . . . . . . . . . . . 9
10.3. DLEP Destination Up Response Message . . . . . . . . . . 9
10.4. DLEP Destination Announce Response Message . . . . . . . 10
10.5. DLEP Destination Update Message . . . . . . . . . . . . 11
10.6. DLEP Link Characteristics Request Message . . . . . . . 11
11. Security Considerations . . . . . . . . . . . . . . . . . . . 11
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
12.1. Registrations . . . . . . . . . . . . . . . . . . . . . 12
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
14. Normative References . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
In the world of radio-based networking, there are modems that need
fine-grained flow control over traffic ingressing from a LAN
connection, bound for transmission over a Radio Frequency (RF) link.
The need for such fine-grained control can exist for multiple
reasons. For example, radio devices are typically connected to the
network by Ethernet. The capacity of an Ethernet link is normally
far superior to that of the wireless medium, leading to the
possibility of overruns and dropped traffic. This is exacerbated by
the fact that RF link capacity can vary from moment to moment, for an
indeterminate amount of time. Additionally, the capacity of the link
can vary greatly depending on the destination, due to factors such as
obstructions or multipath fading.
These challenges motivate the requirement for a fine-grained flow
control in radio-based communications - one that can support
different window sizes for each destination accessed across the RF
network. To address this requirement, this document describes an
extension to the Dynamic Link Exchange Protocol ([DLEP]), allowing
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for a Credit windowing scheme to be implemented on a destination-by-
destination basis.
2. Requirements
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 [RFC2119].
3. Overview
This protocol extension to DLEP describes a credit windowing scheme
for flow control of data over the RF network. In this scheme, data
plane traffic flowing between the router and modem is controlled by
the availability of credits. Credits are expressed as if two
unidirectional windows exist between the modem and router. This
document identifies these windows as the 'Modem Receive Window', or
MRW, and the 'Router Receive Window', or RRW. The responsibility of
granting credits lies with the receiver on a window - that is, on the
MRW, the modem is responsible for granting credits to the router,
allowing it (the router) to send data plane traffic to the modem.
Likewise, the router is responsible for granting credits on the RRW,
which allows the modem to send data plane traffic to the router.
Credits represent the number of data plane octets, or an increment in
the number of data plane octets, that can be sent on a given window
at the MAC layer to the receiver.
4. Terminology
In general, the document uses the same terminology as specified in
the core DLEP document [DLEP]. In addition, the document uses the
following terms:
o Modem Receive Window, or MRW. The MRW represents a logical,
unidirectional window for traffic flowing from the router to the
modem.
o Router Receive Window, or RRW. The RRW represents a logical,
unidirectional window for traffic flowing from the modem to the
router.
5. Operation
DLEP peers supporting this extension MUST include a DLEP 'Extensions
Supported' data item, including the value TBD1 representing this
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extension in the appropriate DLEP Session Initialization and Session
Initialization Response messages.
Credits are managed on a destination-specific basis - separate credit
counts MUST be maintained for each destination requiring the service.
Credits MUST NOT be applied to the DLEP session that exists between
routers and modems; they are applied only to the data plane traffic.
There are no default values for either the initial credit window or
the credit increments.
When DLEP peers desire to employ the credit-windowing extension, the
peer originating the Destination Up or Destination Announce message
MUST supply a Credit Grant data item with an initial, non-zero value
as the increment of the window the originator controls (i.e., the
MRW, or RRW).
If the credit-windowing extension is used on a destination, credits
MUST be employed in both directions (e.g., both the MRW and RRW MUST
be initialized and managed).
When receiving a Credit Grant data item on a Destination Up or
Destination Announce message, the receiver MUST take one of the
following actions:
1. If the receiver of the Credit Grant data item determines that use
of credits is not supported for the destination, the reciver MUST
reject the use of credits for this destination, via the
Destination Up Response or Destination Announce Response message
containing a Status data item with a status code of 'Credit Use
Rejected'. The reasons that a device might reject use of credits
are proprietary in nature, but could include situations like
conflict with existing quality of service algorithms already in
use, or perceived infrequency of traffic to the destination, such
that the credit scheme induces more overhead than is desired.
2. If the receiver supports use of credits for the destination, it
MUST initialize the appropriate window value to zero, then apply
the increment specified in the Credit Grant data item. The
receiver then MUST issue the corresponding response message
(either Destination Up Response or Destination Announce Response)
with a Credit Grant Data Item to complete bi-directional window
initialization.
If credit-windowing initialization is successfully completed, Data
plane traffic would then flow between the DLEP peers, with said peers
accounting for the traffic sent/received by decrementing the
appropriate credit counts.
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The number of credits needed for a given transmission is the length
of the data portion of the packet at the MAC layer. When sending
data to a credit enabled peer, the sender MUST decrement the
appropriate window by the size of the data being sent, prior to
encapsulation at the MAC layer. When traffic is received, the
receiver MUST decrement its own window after decapsulation at the MAC
layer.
When the number of available credits to the destination reaches 0,
the sender MUST stop sending data plane traffic to the destination,
until additional credits are granted by the receiver.
6. DLEP Messages for Credit-Window Extension
The credit-windowing extension does not introduce any additional DLEP
signals or messages.
7. DLEP Status Codes for Credit-Window Extension
The credit-windowing extension introduces two additional DLEP status
code:
+------------+--------+-------------+-------------------------------+
| Status | Value | Failure | Reason |
| Code | | Mode | |
+------------+--------+-------------+-------------------------------+
| Credit | TBD2 | Continue | Credit counts are out-of-sync |
| Window Out | | | between sender and receiver |
| of Sync | | | on the destination. |
| Credit Use | TBD3 | Continue | Credit counts cannot be used |
| Rejected | | | for the destination. |
+------------+--------+-------------+-------------------------------+
8. DLEP Data Items for Credit-Window Extension
The extension introduces 3 DLEP data items:
+------------+------------------------------------------------------+
| Type Code | Description |
+------------+------------------------------------------------------+
| TBD4 | Credit Grant (Section 9.1) |
| TBD5 | Credit Window Status (Section 9.2) |
| TBD6 | Credit Request (Section 9.3) |
+------------+------------------------------------------------------+
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9. Credit Window Data Item Definitions
9.1. Credit Grant
The Credit Grant data item is sent from a DLEP participant to grant
an increment to credits on a window. The value in a Credit Grant
Data Item represents an increment to be added to any existing credits
available on the window.
If credits are to used on a destination, the sender of the DLEP
Destination Up or Destination Announce messages MUST include a Credit
Grant Data Item to initialize the credit window. If present in
Destination Up or Destination Announce, the receiver MUST either
place a Credit Grant Data Item in the Destination Up Response or
Destination Announce Response message, or reject use of credits by
sending the response message with a Status Data Item containing the
'Credit Use Rejected' status code.
If credits are used on a destination, the Credit Grant Data Item MAY
appear in a Destination Update message. Upon successful receipt and
processing of a Credit Grant data item received in Destination
Update, the receiver MUST respond with another Destination Update
message containing a Credit Window Status data item to report the
updated aggregate values for synchronization purposes.
The Credit Grant data item contains the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Item Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Increment |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Increment (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data Item Type: TBD4
Length: 8
Credit Increment: A 64-bit unsigned integer representing the
additional credits, in octets, to be assigned to the credit
window.
Since credits can only be granted by the receiver on a window, the
applicable credit window (either the MRW or the RRW) is derived from
the sender of the grant. The Credit Increment MUST NOT cause the
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window to overflow; if this condition occurs, implementations MUST
set the credit window to the maximum value contained in a 64-bit
quantity.
9.2. Credit Window Status
During normal operation, DLEP session peers may disagree about the
number of available credits. Operational credit mismatches can occur
due to packets in transit on the wire, or sequencing of sending and
receiving packets (e.g. packets are sent prior to processing DLEP
control information). DLEP session peers SHOULD make every effort to
keep credit counts in sync, and implementations MAY use the Credit
Window Status data item to maintain that synchronization. This data
item is informational only; it is used to inform the receiving peer
of the current credit counts for both the MRW and RRW, from the
perspective of the sender.
Upon receipt of a Credit Window Status data item, an implementation
SHOULD compare its own credit counts with that of the originator. If
the receiver of Credit Window Status detects that the local credit
counts are not synchronized with the originator, the receiving
implementation is free to employ various algorithms to re-establish
close synchrnoization, such as:
o Allow the DLEP peer to completely exhaust its credits prior to re-
supplying them via a Credit Grant Data Item, or
o Immediately attempt resynchronization using an additional Credit
Grant, if applicable, or
o Issue a DLEP Destination Down message, to clear credit counts on
the session, and then re-establish the destination via a
Destination Up or Destination Announce message.
o Other re-synchronization algorithms as deemed appropriate.
Implementations issuing Destinaton Down due to credit count
mismatches MUST supply a DLEP Status item, with the status code of
'Credit Window Out of Sync', as defined in this document.
If a DLEP message contains both the Credit Grant (Section 9.1) data
item and the Credit Window Status (Section 9.2) data item,
implementations MUST first apply the Credit Grant (Section 9.1) data
item before comparing the credit counts contained in Credit Window
Status (Section 9.2).
It is RECOMMENDED that implementations issue a DLEP Destination
Update with a Credit Window Status data item at a configurable
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multiple of the DLEP Heartbeat timer, to serve as a continuing check
on synchronization of the credit windows for a destination.
The Credit Window Status data item contains the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Item Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Modem Receive Window Value :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Modem Receive Window Value (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router Receive Window Value :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Router Receive Window Value (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data Item Type: TBD5
Length: 16
Modem Receive Window Value: A 64-bit unsigned integer, indicating
the current number of credits, in octets, available on the Modem
Receive Window, for the destination referred to by the message.
Router Receive Window Value: A 64-bit unsigned integer, indicating
the current number of credits, in octets, available on the Router
Receive Window, for the destination referred to by the message.
9.3. Credit Request
The Credit Request data item MAY be sent from either DLEP
participant, as a data item in a DLEP Destination Update message, or
a Link Characteristics Request message, to indicate the desire for
the partner to grant additional credits in order for data transfer to
proceed on the session. If the corresponding DLEP Destination Up or
Destination Announce message for this session did not contain a
Credit Grant data item, indicating that credits are to be used on the
session, then receipt of the Credit Request data item MUST be
considered as an error by the receiver, requiring a response message
that includes a Status Data Item with the code set to "Credit Window
out of Sync'.
If credits are supported on the destination, then the receiver of a
Credit Request Data Item SHOULD issue a Destination Update message,
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with a Credit Grant Data Item, giving the peer an increment of
credits for the destination.
The Credit Request data item contains the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Item Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data Item Type: TBD6
Length: 0
10. Credit Data Item Use in DLEP Messages
10.1. DLEP Destination Up Message
If use of credits is desired for the destination, then the
Destination Up message MUST contain one Credit Grant (Section 9.1)
data item. The value of the credit increment is at the discretion of
the implementation. If use of credits is accepted by the receiver,
the value in the Credit Grant data item in the Destination Up message
MUST be used as the initial value for the appropriate window.
If the Destination Up message does not contain the Credit Grant data
item, credits MUST NOT be used for that destination.
10.2. DLEP Destination Announce Message
If use of credits is required for the destination, then the
Destination Announce message MUST contain one Credit Grant
(Section 9.1) data item. The value of the credit increment is at the
discretion of the implementation. The receiver of the Destination
Announce message MUST use the value in Credit Grant as the initial
value for the appropriate window.
If the Destination Announce message does not contain the Credit Grant
data item, credits MUST NOT be used for that destination.
10.3. DLEP Destination Up Response Message
If the corresponding Destination Up message contained a Credit Grant
(Section 9.1) data item, the Destination Up Response message MUST
contain either:
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o a Credit Grant (Section 9.1) data item, to initialize the
receiver's window, or
o a Status Data Item, containing the 'Credit Use Rejected' status
code. Optional text MAY be included with the Status Data Item to
further clarify the reason for the rejection.
Likewise, if the corresponding Destination Up message did not contain
a Credit Grant (Section 9.1) data item, the receiver MUST conclude
that credits are NOT to be used for the destination, and therefore,
the Destination Up Response message MUST NOT contain a Credit Grant
(Section 9.1) data item.
The receiver of Destination Up Response MUST use the received Credit
Grant value to initialize the appropriate window (e.g., the MRW value
for routers, the RRW value for modems).
When an implementation detects a mismatch in the presence or absence
of credit window data items between the DLEP Destination Up and
Destination Up Response messages (e.g, the Destination Up message was
sent using credits but the received Destination Up Response message
does NOT include credits), the implementation detecting the credit-
use mismatch MUST terminate the destination by issuing a Destination
Down message with a status code of 'Credit Window Out of Sync', and
continue processing on other DLEP destinations.
10.4. DLEP Destination Announce Response Message
If the corresponding Destination Announce message contained a Credit
Grant (Section 9.1) data item, the Destination Announce Response
message MUST contain either:
o a Credit Grant (Section 9.1) data item, to initialize the
receiver's window, or
o a Status Data Item, containing the 'Credit Use Rejected' status
code. Optional text MAY be included with the Status Data Item to
further clarify the reason for the rejection
Likewise, if the corresponding Destination Announce message did not
contain a Credit Grant (Section 9.1) data item, the receiver MUST
conclude that credits are NOT to be used for the destination, and
therefore, the Destination Announce Response message MUST NOT contain
a Credit Grant (Section 9.1) data item.
The receiver of Destination Announce Response MUST use the received
Credit Grant value to initialize the appropriate window (e.g., the
MRW value for routers, the RRW value for modems).
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When an implementation detects a mismatch in the presence or absence
of credit window data items between the DLEP Destination Announce and
Destination Announce Response messages (e.g, the Destination Announce
message was sent using credits but the received Destination Up
Response message does NOT include credits), the implementation
detecting the credit-use mismatch MUST terminate the destination by
issuing a Destination Down message with a status code of 'Credit
Window Out of Sync', and continue processing on other DLEP
destinations.
10.5. DLEP Destination Update Message
If the corresponding Destination Up or Destination Announce message
contained the Credit Grant data item, the Destination Update message
MAY contain one of each of the following data items:
o Credit Grant (Section 9.1)
o Credit Window Status (Section 9.2)
o Credit Request (Section 9.3)
DLEP peers supporting the extension MAY format and send a DLEP
Destination Update message solely for the purposes of maintaining the
credit windows. In cases where a peer already has information
requiring a Destination Update message, (e.g., a change in Latency on
the link), the credit data items MAY be included in addition to that
information.
10.6. DLEP Link Characteristics Request Message
If the corresponding Destination Up or Destination Announce message
contained the credit Grant data item, the Link Characteristics
Request message MAY contain the following data item:
o Credit Request (Section 9.3)
DLEP peers supporting the extension MAY format and send a DLEP Link
Characteristics Request message solely for the purposes of
maintaining the credit windows. In cases where a peer already has
information requiring a Link Characteristics Request message, the
Credit Request data MAY be included in addition to that information.
11. Security Considerations
The extension introduces a mechanims for destination-specific flow
control between a router and modem supporting the DLEP protocol. The
extension does not introduce any additional threats above those
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documented in [DLEP]. The approach taken to security in that
document applies when implementing this extension.
12. IANA Considerations
This section specifies requests to IANA.
12.1. Registrations
This specification defines three (3) new entries in the repository
entitled "Data Item Type Values for the Dynamic Link Exchange
Protocol (DLEP)". Assignments from that registry are requested for:
o Credit Grant
o Credit Request
o Credit Window Status
The specification also defines an extension to the DLEP protocol. An
assignment from the repository entitled "Extension Type Values for
the Dynamic Link Exchange Protocol (DLEP)" is requested for:
o Credit Windowing
In addition, the specification defines two (2) new DLEP status codes.
Assignments from the repository entitled "Status Code Values for the
Dynamic Link Exchange Protocol (DLEP)" are requested for:
o Credit Window Out of Sync
o Credit Use Rejected
13. Acknowledgements
The author would like to acknowledge and thank the members of the
MANET working group, who have provided valuable insight.
Specifically, the author would like to thank Lou Berger, David
Wiggins, Justin Dean, Brian Amundson, Rick Taylor, John Dowdell,
Shawn Jury, and Darryl Satterwhite.
14. Normative References
[DLEP] Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B.
Berry, "Dynamic Link Exchange Protocol (DLEP)", draft-
ietf-manet-dlep-25 IETF draft, October 2016.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
Author's Address
Stan Ratliff
VT iDirect
13861 Sunrise Valley Drive, Suite 300
Herndon, VA 20171
USA
Email: sratliff@idirect.net
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