Network Working Group B. Cheng
Internet-Draft D. Wiggins
Intended status: Standards Track MIT Lincoln Laboratory
Expires: June 7, 2021 L. Berger
LabN Consulting, L.L.C.
S. Ratliff
December 4, 2020
DLEP Credit-Based Flow Control Messages and Data Items
draft-ietf-manet-dlep-credit-flow-control-07
Abstract
This document defines new Dynamic Link Exchange Protocol (DLEP) Data
Items that are used to support credit-based flow control. Credit
window control is used to regulate when data may be sent to an
associated virtual or physical queue. The Data Items are defined in
an extensible and reusable fashion. Their use will be mandated in
other documents defining specific DLEP extensions.
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 https://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 June 7, 2021.
Copyright Notice
Copyright (c) 2020 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
Cheng, et al. Expires June 7, 2021 [Page 1]
Internet-Draft DLEP Credit-Based Flow Control December 2020
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Credit Window Control . . . . . . . . . . . . . . . . . . . . 3
2.1. Data Plane Considerations . . . . . . . . . . . . . . . . 5
2.2. Credit Window Messages . . . . . . . . . . . . . . . . . 5
2.2.1. Credit Control Message . . . . . . . . . . . . . . . 5
2.2.2. Credit Control Response Message . . . . . . . . . . . 6
2.3. Credit Window Control Data Items . . . . . . . . . . . . 7
2.3.1. Credit Window Initialization . . . . . . . . . . . . 7
2.3.2. Credit Window Associate . . . . . . . . . . . . . . . 9
2.3.3. Credit Window Grant . . . . . . . . . . . . . . . . . 10
2.3.4. Credit Window Status . . . . . . . . . . . . . . . . 12
2.3.5. Credit Window Request . . . . . . . . . . . . . . . . 13
2.4. Management Considerations . . . . . . . . . . . . . . . . 14
3. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 14
4. Security Considerations . . . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
5.1. Message Values . . . . . . . . . . . . . . . . . . . . . 15
5.2. Data Item Values . . . . . . . . . . . . . . . . . . . . 15
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Normative References . . . . . . . . . . . . . . . . . . 16
6.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
The Dynamic Link Exchange Protocol (DLEP) is defined in [RFC8175].
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. DLEP defines Data Items which are sets of information
that can be reused in DLEP messaging. The base DLEP specification
does not include any flow identification beyond DLEP endpoints or
flow control capability. There are various flow control techniques
theoretically possible with DLEP. For example, a credit-window
scheme for destination-specific flow control which provides aggregate
flow control for both modem and routers has been proposed in
[I-D.ietf-manet-credit-window], and a control plane pause based
mechanism is defined in [I-D.ietf-manet-dlep-pause-extension].
Cheng, et al. Expires June 7, 2021 [Page 2]
Internet-Draft DLEP Credit-Based Flow Control December 2020
This document defines DLEP Data Items and Messages which provide a
flow control mechanism for traffic sent from a router to a modem.
Flow control is provided using one or more logical "Credit Windows",
each of which will typically be supported by an associated virtual or
physical queue. Traffic sent by a router will use traffic flow
classification information provided by the modem as defined in
[I-D.ietf-manet-dlep-traffic-classification]. to identify which
traffic is associated with each credit window. In this case, a flow
is identified based on information found in a data plane header and
one or more matches are associated with a single flow. (For general
background on traffic classification see [RFC2475] Section 2.3.)
Credit windows may be shared or dedicated on a per flow basis. The
Data Items are structured to allow for reuse of the defined credit
window based flow control with different traffic classification
techniques.
Note that this document defines common Messages, Data Items and
mechanisms that are reusable. They are expected to be required by
DLEP extensions defined in other documents such as found in
[I-D.ietf-manet-dlep-da-credit-extension].
This document supports credit window control by introducing two new
DLEP messages in Section 2.2, and five new DLEP Data Items in
Section 2.3.
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. Credit Window Control
This section defines additions to DLEP used in credit based flow
control. Two new messages and five Data Items are defined to support
credit window control. The use of credit window control impacts the
data plane.
The credit window control mechanisms defined in this document support
credit based flow control of traffic sent from a router to a modem.
The mapping of specific flows of traffic to a particular credit
window is based on the Traffic Classification Data Item defined in
[I-D.ietf-manet-dlep-traffic-classification]. Both types of DLEP
endpoints, i.e., a router and a modem, negotiate the use of extension
during session initialization, e.g., see
[I-D.ietf-manet-dlep-da-credit-extension]. When using credit
Cheng, et al. Expires June 7, 2021 [Page 3]
Internet-Draft DLEP Credit-Based Flow Control December 2020
windows, data traffic is only allowed to be sent by the router to the
modem when there are credits available.
Credits are managed on a per logical "Credit Windows" basis. Each
credit window can be thought of as corresponding to a queue within a
modem. Credit windows may be shared across, or dedicated to,
destinations and data plane identifiers, e.g., DSCPs, at a
granularity that is appropriate for a modem's implementation and its
attached transmission technology. As defined below, there is a
direct one-for-one mapping of credit windows to flows as identified
by FIDs carried within the Traffic Classification Data Item. Modems
pass to the router information on their credit windows and FIDs prior
to a router being able to send data when an extension requiring the
use of credit window control is used. In addition to the traffic
classification information associated with an FID, routers provide an
initial credit window size, as well as the maximum size of the
logical queue associated with each credit window. The maximum size
is included for informative and potential future uses.
Modems provide an initial credit window size at the time of "Credit
Window Initialization". Such initialization can take place during
session initiation or any point thereafter. It can also take place
when rate information changes. Additional "Credit Grants", i.e.,
increments to Credit Window size, are provided using a Destination Up
or the new "Credit Control" Message. A router provides its view of
the Credit Window, which is known as "Status", in Destination Up
Response and the new "Credit Control Response" Messages. Routers can
also request credits using the new "Credit Control" Message.
When modems provide credits to a router, they will need to take into
account any overhead of their attached transmission technology and
map it into the credit semantics defined in this document. In
particular, the credit window is defined below to include per frame
(packet) MAC headers, and this may not match the actual overhead of
the modem attached transmission technology. In that case a direct
mapping, or an approximation will need to be made by the modem to
provide appropriate credit values.
Actual flows of traffic are mapped to credit windows based on flow
identification information provided by modems in the Traffic
Classification Data item defined in
[I-D.ietf-manet-dlep-traffic-classification]. This data item
supports traffic classification on a per destination or more fine
grain level. Routers use the combination of the DLEP identified
destination and flow information associated with a credit window in
order to match traffic they send to specific credit windows.
Cheng, et al. Expires June 7, 2021 [Page 4]
Internet-Draft DLEP Credit-Based Flow Control December 2020
When a destination becomes reachable, a modem "Associates"
(identifies) the appropriate traffic classification information via
the TID to be used for traffic sent by the router to that
destination. As defined, each credit window has a corresponding FID.
This means that the use of FIDs, TIDs and the association of a TID to
a DLEP destination enables a modem to share or dedicate resources as
needed to match the specifics of its implementation and its attached
transmission technology.
The defined credit window control has similar objectives as the
control found in [I-D.ietf-manet-credit-window]. One notable
difference from that credit control is that in this document, credits
are never provided by the router to the modem.
2.1. Data Plane Considerations
When credit windowing is used, a router MUST NOT send data traffic to
a modem for forwarding when there are no credits available in the
associated Credit Window. This document defines credit windows in
octets. A credit window value MUST be larger than the number of
octets contained in a packet, including any MAC headers used between
the router and the modem, in order for the router to send the packet
to a modem for forwarding. The credit window is decremented by the
number of sent octets.
A router MUST identify the credit window associated with traffic sent
to a modem based on the traffic classification information provided
in the Data Items defined in this document. Note that routers will
typically view a DLEP destination as the next hop MAC address.
2.2. Credit Window Messages
Two new messages are defined in support for credit window control:
the Credit Control and the Credit Control Response Message. Sending
and receiving both message types is REQUIRED to support the credit
window control defined in this document.
2.2.1. Credit Control Message
Credit Control Messages are sent by modems and routers. Each sender
is only permitted to have one message outstanding at one time. That
is, a sender (i.e., modem or router) MUST NOT send a second or any
subsequent Credit Control Message until a Credit Control Response
Message is received from its peer (i.e., router or modem).
Credit Control Messages are sent by modems to provide credit window
increases. Modems send credit increases when there is transmission
or local queue availability that exceeds the credit window value
Cheng, et al. Expires June 7, 2021 [Page 5]
Internet-Draft DLEP Credit-Based Flow Control December 2020
previous provided to the router. Modems will need to balance the
load generated by sending and processing frequent credit window
increases against a router having data traffic available to send, but
no credits available.
Credit Control Messages MAY be sent by routers to request credits and
provide window status. Routers will need to balance the load
generated by sending and processing frequent credit window requests
against a having data traffic available to send, but no credits
available.
The Message Type value in the DLEP Message Header is set to TBA2.
A message sent by a modem, MUST contain one or more Credit Window
Grant Data Items as defined below in Section 2.3.3. A router
receiving this message MUST respond with a Credit Control Response
Message.
A message sent by a router, MUST contain one or more Credit Window
Request Data Items defined below in Section 2.3.5 and SHOULD contain
a Credit Window Status Data Item, defined in Section 2.3.4,
corresponding to each credit window request. A modem receiving this
message MUST respond with a Credit Control Response Message based on
the received message and Data Item and the processing defined below,
which will typically result in credit window increments being
provided.
Specific processing associated with each Credit Data Item is provided
below.
2.2.2. Credit Control Response Message
Credit Control Response Messages are sent by routers to report the
current Credit Window for a destination. A message sent by a router,
MUST contain one or more Credit Window Status Data Items as defined
below in Section 2.3.4. Specific receive processing associated with
the Credit Window Status Data Item is provided below.
Credit Control Response Messages sent by modems MUST contain one or
more Credit Window Grant Data Items. A Data Item for every Credit
Window Request Data Item contained in the corresponding Credit
Control Response Message received by the modem MUST be included.
Each Credit Grant Data Item MAY provide zero or more additional
credits based on the modem's transmission or local queue
availability. Specific receive processing associated with each Grant
Data Item is provided below.
The Message Type value in the DLEP Message Header is set to TBA3.
Cheng, et al. Expires June 7, 2021 [Page 6]
Internet-Draft DLEP Credit-Based Flow Control December 2020
2.3. Credit Window Control Data Items
Five new Data Items are defined to support credit window control.
The Credit Window Initialization Data Item is used by a modem to
identify a credit window and set its size. The Credit Window
Association Data Item is used by a modem to identify which traffic
classification identifiers (flows) should be used when sending
traffic to a particular DLEP identified destination. The Credit
Window Grant is used by a modem to provide additional credits to a
router. The Credit Request is used by a router to request additional
credits. The Credit Window Status is used to advertise the sender's
view of number of available credits for state synchronization
purposes.
Any errors or inconsistencies encountered in parsing Data Items are
handled in the same fashion as any other data item parsing error
encountered in DLEP, see [RFC8175]. In particular, the node parsing
the Data Item MUST terminate the session with a Status Data Item
indicating Invalid Data.
2.3.1. Credit Window Initialization
The Credit Window Initialization Data Item is used by a modem to
identify a credit window and set its size. This Data Item SHOULD be
included in any Session Initialization Response Message that also
indicates support for an extension that requires support for the
credit window control mechanisms defined in this document, e.g., see
[I-D.ietf-manet-dlep-da-credit-extension]. Updates to previously
identified credit windows or new credit windows MAY be sent by a
modem by including the Data Item in Session Update Messages. More
than one data item MAY be included in a message to provide
information on multiple credit windows.
The Credit Window Initialization Data Item identifies a credit window
using a Flow Identifier, or FID. It also provides the size of the
identified credit window. Finally, a queue size (in bytes) is
provided for informational purposes. Note that to be used, a FID
must be defined within a Traffic Classification Data Item and the
associated TID must be provided via a Credit Window Association Data
Item.
The format of the Credit Window Initialization Data Item is:
Cheng, et al. Expires June 7, 2021 [Page 7]
Internet-Draft DLEP Credit-Based Flow Control December 2020
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 (16) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow Identifier (FID) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Value :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Credit Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scale | Credit Window Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data Item Type: TBA4
Length: 16
Per [RFC8175] Length is the number of octets in the Data Item. It
MUST be equal to sixteen (16).
Flow Identifier (FID):
A flow identifier as defined by the Traffic Classification Data
Item. The FID also uniquely identifies a credit window.
Reserved:
MUST be set to zero by the sender (a modem) and ignored by the
receiver (a router).
Credit Value:
A 64-bit unsigned integer representing the credits, in octets, to
be applied to the Credit Window. This value includes MAC headers
as seen on the link between the modem and router.
Scale:
An 8-bit unsigned integer indicating the scale used in the Credit
Window Size fields. The valid values are:
Value Scale
------------
0 B - Bytes (Octets)
1 KB - Kilobytes (B/1024)
2 MB - Megabytes (KB/1024)
3 GB - Gigabytes (MB/1024)
Cheng, et al. Expires June 7, 2021 [Page 8]
Internet-Draft DLEP Credit-Based Flow Control December 2020
Credit Window Size:
A 24-bit unsigned integer representing the maximum size, in the
octet scale indicated by the Scale field, of the associated credit
window.
A router that receives a Credit Window Initialization Data Item MUST
ensure that the FID field value has been provided by the modem in a
Traffic Classification Data Item carried in either the current or
previous message. If the FID cannot be found the router SHOULD
report or log this information. Note that no traffic will be
associated with the credit window in this case. After FID
validation, the router MUST locate the credit window that is
associated with the FID indicated in each received Data Item. If no
associated credit window is found, the router MUST initialize a new
credit window using the values carried in the Data Item. When an
associated credit window is found, the router MUST update the credit
window and associated data plane state using the values carried in
the Data Item. It is worth noting, that such updates can result in a
credit window size being reduced, for example, due to a transmission
rate change on the modem.
2.3.2. Credit Window Associate
The Credit Window Associate Data Item is used by a modem to associate
traffic classification information with a destination. The traffic
classification information is identified using a TID value that has
previously been sent by the modem or is listed in a Traffic
Classification Data Item carried in the same message as the Data
Item.
A single Credit Window Associate Data Item MUST be included in all
Destination Up and Destination Update Messages sent by a modem when
the credit window control defined in this document is used. Note
that a TID will not be used unless it is listed in a Credit Window
Associate Data Item.
The format of the Credit Window Associate 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 (2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Traffic Class. Identifier (TID)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data Item Type: TBA5
Cheng, et al. Expires June 7, 2021 [Page 9]
Internet-Draft DLEP Credit-Based Flow Control December 2020
Length: 2
Per [RFC8175] Length is the number of octets in the Data Item. It
MUST be equal to two (2).
Traffic Classification Identifier (TID):
A 16-bit unsigned integer identifying a traffic classification set
that has been identified in a Traffic Classification Data Item,
see [I-D.ietf-manet-dlep-traffic-classification].
A router that receives the Credit Window Associate Data Item MUST
locate the traffic classification information indicated by the
received TID. If no corresponding information can be located, the
Data Item MUST be treated as an error as described above. Once the
traffic classification information is located, it MUST be associated
with the destination and the router MUST ensure that any data plane
state, see Section 2.1, that is associated with the TID and its
corresponding FIDs is updated as needed.
2.3.3. Credit Window Grant
The Credit Window Grant Data Item is used by a modem to provide
credits to a router. One or more Credit Window Grant Data Items MAY
be carried in the DLEP Destination Up, Destination Announce Response,
Destination Update, Credit Control Messages, and Credit Control
Response Messages. Multiple Credit Window Grant Data Items in a
single message are used to indicate different credit values for
different credit windows. In all message types, this Data Item
provides an additional number of octets to be added to the indicated
credit window. Credit windows are identified using FID values that
have been previously been sent by the modem or are listed in a Credit
Window Initialization Data Item carried in the same messages as the
Data Item.
The format of the Credit Window Grant 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 (12) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow Identifier (FID) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Additional Credits :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Additional Credits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cheng, et al. Expires June 7, 2021 [Page 10]
Internet-Draft DLEP Credit-Based Flow Control December 2020
Data Item Type: TBA6
Length: 12
Per [RFC8175], Length is the number of octets in the Data Item.
It MUST be equal to twelve (12).
Flow Identifier (FID):
A flow identifier as defined by the Traffic Classification Data
Item. The FID also uniquely identifies a credit window.
Additional Credit:
A 64-bit unsigned integer representing the credits, in octets, to
be added to the Credit Window. This value includes MAC headers as
seen on the link between the modem and router. A value of zero
indicates that no additional credits are being provided.
When receiving this Data Item, a router MUST identify the credit
window indicated by the FID. If the FID is not known to the router,
it SHOULD report or log this information and discard the Data Item.
It is important to note that while this Data Item can be received in
a destination specific message, credit windows are managed
independently from the destination identified in the message carrying
this Data Item, and the indicated FID MAY even be disjoint from the
identified destination.
Once the credit window is identified, the credit window size MUST be
increased by the value contained in the Additional Credits field. If
the increase results in a window overflow, i.e., the size of the
credit window after the increase is smaller than the original credit
window size, the Credit Window must be set to its maximum
(0xFFFFFFFFFFFFFFFF).
No response is sent by the router to a modem after processing a
Credit Window Grant Data Item received in a Credit Control Response
Message. In other cases, the receiving router MUST send a Credit
Window Status Data Item or items reflecting the resulting Credit
Window value of the updated credit window. When the Credit Grant
Data Item is received in a Destination Up Message, the Credit Window
Status Data Item(s) MUST be sent in the corresponding Destination Up
Response Message. Otherwise, a Credit Control Message MUST be sent.
Cheng, et al. Expires June 7, 2021 [Page 11]
Internet-Draft DLEP Credit-Based Flow Control December 2020
2.3.4. Credit Window Status
The Credit Window Status Data Item is used by a router to report the
current credit window size to its peer modem. One or more Credit
Window Status Data Items MAY be carried in a Destination Up Response
Message or a Credit Control Response Message. As discussed above,
the Destination Up Response Message is used when the Data Item is
sent in response to a Destination Up Message, and the Credit Control
Response Message is sent in response to a Credit Control Message.
Multiple Credit Window Status Data Items in a single message are used
to indicate different sizes of different credit windows. Similar to
the Credit Window Grant, credit windows are identified using FID
values that have been previously been sent by the modem.
The format of the Credit Window Status 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 (12) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow Identifier (FID) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Window Size :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Credit Window Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data Item Type: TBA7
Length: 12
Per [RFC8175] Length is the number of octets in the Data Item. It
MUST be equal to twelve (12).
Flow Identifier (FID):
A flow identifier as defined by the Traffic Classification Data
Item. The FID also uniquely identifies a credit window.
Credit Window Size:
A 64-bit unsigned integer, indicating the current number of
credits, in octets, available for the router to send to the modem.
This is referred to as the Modem Receive Window in
[I-D.ietf-manet-credit-window].
Cheng, et al. Expires June 7, 2021 [Page 12]
Internet-Draft DLEP Credit-Based Flow Control December 2020
When receiving this Data Item, a modem MUST identify the credit
window indicated by the FID. If the FID is not known to the modem,
it SHOULD report or log this information and discard the Data Item.
As with the Credit Window Grant Data Item, the FID MAY be unrelated
to the Destination indicated in the message carrying the Data Item.
Once the credit window is identified, the modem SHOULD check the
received Credit Window Size field value against the outstanding
credit window's available credits at the time the most Credit Window
Initialization or Grant Data Item associated with the indicated FID
was sent. If the values significantly differ, i.e., greater than can
be accounted for based on observed data frames, then the modem SHOULD
send a Credit Window Initialization Data Item to reset the associated
credit window size to the modem's current view of the available
credits. As defined above, Credit Window Initialization Data Items
are sent in Session Update Messages. When multiple Data Items need
to be sent, they SHOULD be combined into a single message when
possible. Alternatively, and also in cases where there are small
differences, the modem MAY adjust the values sent in Credit Window
Grant Data Items to account for the reported Credit Window.
2.3.5. Credit Window Request
The Credit Window Request Data Item is used by a router to request
additional credits for particular credit windows. Credit Window
Request Data Items are carried in Credit Control Messages, and one or
more Credit Window Request Data Items MAY be present in a message.
Credit windows identified using a FID as defined above in
Section 2.3.1. Multiple FIDs MAY be present to allow for the case
where the router identifies that credits are needed in multiple
credit windows. A special FID value, as defined below, is used to
indicate that a credit request is being made across all queues.
The format of the Credit Window Request 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow Identifier (FID) | ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... | Flow Identifier (FID) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data Item Type: TBA8
Cheng, et al. Expires June 7, 2021 [Page 13]
Internet-Draft DLEP Credit-Based Flow Control December 2020
Length: Variable
Per [RFC8175] Length is the number of octets in the Data Item,
excluding the Type and Length fields. It will equal the number of
FID fields carried in the Data Item times 2 and MUST be at least
2.
Flow Identifier (FID):
A flow identifier as defined by the Traffic Classification Data
Item. The FID also uniquely identifies a credit window. The
special value of 0xFFFF indicates that the request applies to all
FIDs.
A modem receiving this Data Item MUST provide a Credit Increment for
the indicated credit windows via Credit Window Grant Data Items
carried in a new Credit Control Message. Multiple values and queue
indexes SHOULD be combined into a single Credit Control Message when
possible. Unknown FID values SHOULD be reported or logged and then
ignored by the modem.
2.4. Management Considerations
This section provides several network management guidelines to
implementations supporting the credit window mechanisms defined in
this document.
Modems MAY support the configuration of the number of credit windows
(queues) to advertise to a router.
Routers may have limits on the number of queues that they can support
and, perhaps, even limits in supported credit window combinations,
e.g., if per destination queues can even be supported at all. When
modem-provided credit window information exceeds the capabilities of
a router, the router MAY use a subset of the provided credit windows.
Alternatively, a router MAY reset the session and indicate that the
extension is not supported. In either case, the mismatch of
capabilities SHOULD be reported to the user via normal network
management mechanisms, e.g., user interface or error logging.
3. Compatibility
The data items defined in this document will only be used when
extensions require their use.
Cheng, et al. Expires June 7, 2021 [Page 14]
Internet-Draft DLEP Credit-Based Flow Control December 2020
4. Security Considerations
This document introduces credit window control and flow mechanisms to
DLEP. These mechanisms do not inherently introduce any additional
vulnerabilities above those documented in [RFC8175]. The approach
taken to Security in that document applies equally to the mechanism
defined in this document.
5. IANA Considerations
This document requests the assignment of several values by IANA. All
assignments are to registries defined by [RFC8175].
5.1. Message Values
This document requests 2 new assignments to the DLEP Message Registry
named "Message Values" in the range with the "Specification Required"
policy. The requested values are as follows:
+-----------+-------------------------+
| Type Code | Description |
+-----------+-------------------------+
| TBA2 | Credit Control |
| | |
| TBA3 | Credit Control Response |
+-----------+-------------------------+
Table 1: Requested Message Values
5.2. Data Item Values
This document requests the following new assignments to the DLEP Data
Item Registry named "Data Item Type Values" in the range with the
"Specification Required" policy. The requested values are as
follows:
Cheng, et al. Expires June 7, 2021 [Page 15]
Internet-Draft DLEP Credit-Based Flow Control December 2020
+-----------+------------------------------+
| Type Code | Description |
+-----------+------------------------------+
| TBA4 | Credit Window Initialization |
| | |
| TBA5 | Credit Window Association |
| | |
| TBA6 | Credit Window Grant |
| | |
| TBA7 | Credit Window Status |
| | |
| TBA8 | Credit Window Request |
+-----------+------------------------------+
Table 2: Requested Data Item Values
6. References
6.1. Normative References
[I-D.ietf-manet-dlep-traffic-classification]
Cheng, B., Wiggins, D., and L. Berger, "DLEP Traffic
Classification Data Item", August 2018.
[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
[I-D.ietf-manet-credit-window]
Ratliff, S., "Credit Windowing extension for DLEP", draft-
ietf-manet-credit-window-07 (work in progress), November
2016.
Cheng, et al. Expires June 7, 2021 [Page 16]
Internet-Draft DLEP Credit-Based Flow Control December 2020
[I-D.ietf-manet-dlep-da-credit-extension]
Cheng, B., Wiggins, D., and L. Berger, "DLEP DiffServ
Aware Credit Window Extension", draft-ietf-manet-dlep-da-
credit-extension-09 (work in progress), June 2020.
[I-D.ietf-manet-dlep-pause-extension]
Cheng, B., Wiggins, D., and L. Berger, "DLEP Control Plane
Based Pause Extension", draft-ietf-manet-dlep-pause-
extension-08 (work in progress), June 2019.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
Appendix A. Acknowledgments
We morn the loss of Stan Ratliff who passed away on October 22, 2019.
His guidance, leadership and personal contributions were critical in
the development of this work and DLEP as a whole. His leadership and
friendship shall be missed.
Many useful comments were received from contributors to the MANET
working group, notably Rick Taylor. This document was derived from
[I-D.ietf-manet-dlep-da-credit-extension] as a result of discussions
at IETF101.
Authors' Addresses
Bow-Nan Cheng
MIT Lincoln Laboratory
Massachusetts Institute of Technology
244 Wood Street
Lexington, MA 02421-6426
Email: bcheng@ll.mit.edu
David Wiggins
MIT Lincoln Laboratory
Massachusetts Institute of Technology
244 Wood Street
Lexington, MA 02421-6426
Email: David.Wiggins@ll.mit.edu
Cheng, et al. Expires June 7, 2021 [Page 17]
Internet-Draft DLEP Credit-Based Flow Control December 2020
Lou Berger
LabN Consulting, L.L.C.
Email: lberger@labn.net
Stan Ratliff
Cheng, et al. Expires June 7, 2021 [Page 18]