DLEP DiffServ Aware Credit Windowing
ExtensionLincoln LaboratoryMassachusetts Institute of Technology244 Wood StreetLexingtonMA02421-6426bcheng@ll.mit.eduLincoln LaboratoryMassachusetts Institute of Technology244 Wood StreetLexingtonMA02421-6426David.Wiggins@ll.mit.eduLabN Consulting, L.L.C.lberger@labn.net
This document defines an extension to the DLEP protocol that enables a
DiffServ aware credit-windowing scheme for destination-specific and
shared flow control.
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.
The base DLEP specification does not include any flow control
capability. There are various flow control theoretically possible
with DLEP. For example, a credit-windowing scheme for
destination-specific flow control which provides aggregate flow
control for both modem and routers has been proposed in .
This document defines a DLEP extension which provides an alternate
flow control mechanism for traffic sent from a router to a modem. Flow
control is provide 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 classification
information provided by the modem to identify which traffic is
associated with each credit window. (For general background on
traffic classification see Section 2.3.) This
document supports traffic classification based on DLEP destination and
DiffServ DSCPs (differentiated services
codepoints). Credit windows may be shared or dedicated on a per
{destination, DSCP} tuple basis. Said another way, the defined
mechanism allows for credit windows to be shared across traffic sent
to multiple DLEP destinations and DSCPs, or used exclusively for
traffic sent to a particular destination and/or DSCP. The extension
also supports the "wildcard" matching of any DSCP.
The extension defined in this document is referred to as "DiffServ
Aware Credit Windowing" or, more simply, the "DA Credit" extension.
The extension is structured to allow for reuse of the defined credit
window based flow control with traffic classification techniques other
than DiffServ, e.g., IEEE 802.1 Q priority code points or even 5-tuple
IP flows.
This document defines a new DLEP Extension Type Value in which is used to indicate the use of the
extension. Two new messages are defined in , and six 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 when, and
only when, they appear in all capitals, as shown here.
The DA Credit extension can be used to support credit based flow
control of traffic sent from a router to a modem. The extension can
be used to support DiffServ and non-DiffServ based flow control.
Both types of DLEP endpoints, i.e., a router and a modem, negotiate
the use of extension during session initialization, see Section 5.2
. When using this extension,
data traffic is allowed to be sent by the router to the modem only 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. Modems pass to the router information on its credit windows
and identify each via a "Credit Window Identifier", or "CID". In
addition to the CID, credit window information includes an initial
credit window size, as well as the maximum size of the logical queue
associated with each CID. The maximum size is included for
informative and, potential, future uses.
As mentioned above, traffic classification supported by this
document is performed on a per {destination, DSCP} tuple basis.
This means that, routers need the combination of the DLEP identified
destination and DSCP associated with a CID in
order to match traffic it sends to specific credit
windows. Modems provide the mapping of DSCPs to CIDs in sets, each
of which is identified via a "Traffic Classification Identifier" or
"TID". When a destination becomes reachable, a modem "Associates"
(identifies) the TID to be used for traffic sent by the router to
that destination. This use of CIDs, TIDs and association of a TID
to a DLEP destination enables modem to share or dedicate resources
as needed to match the specifics of its implementation and its
attached transmission technology.
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.
The use of the extension defined in this document SHOULD be
configurable. To indicate that the DiffServ Aware Credit Windowing
Extension is to be used, an implementation MUST include the DiffServ
Aware Credit Windowing Type Value in the Extensions Supported Data
Item. The Extensions Supported Data Item is sent and processed
according to .
The DiffServ Aware Credit Windowing Extension Type Value is TBA1, see
.
When the use of the extension defined in this document is agreed upon
per standard DLEP processing, see , 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. The
definitions support identification based on DLEP destination and, at
the modem's discretion, DSCPs. Note that routers will typically view
a DLEP destination as the next hop.
Two new messages are defined by this extension: the Credit Control
and the Credit Control Response Message. Sending and receiving both
message types MUST be supported by any implementation that
advertises use of this extension per .
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 (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 (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 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 . 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 and SHOULD contain a Credit Window
Status Data Item, defined in ,
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.
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 .
Specific receive processing associated with the Credit Window
Window Status Data Item is provided below.
Credit Control Response Messages sent by modems MUST contain one
or more Credit Window Credit Grant Data Items. A data item
for every Credit Window Request data item contained in the
for Credit Control Response Message sent by the router MUST be
included. Each Grant Data Item MAY provide zero or more additional
credits based on a 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.
Six data items are defined by this extension.
The Credit Window Initialization
Data Item is used by a modem to identify a credit window and set its
size.
The Credit Window Traffic Classification
Data Item is used by a modem to identify a set which identifies which
DSCPs are mapped to a credit window.
The Credit Window Association
Data Item is used by a modem to identify which traffic classification
set 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 dtem parsing error
encountered in DLEP, see . In particular, the
node parsing the data item MUST terminate the session with a
Status Data Item indicating Invalid Data.
The defined data items and operations have similar objectives as those
found in . One notable
difference from this extension is that in this document credits are
never provided by the router to the modem.
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
contains the DiffServ Aware Credit Windowing Extension Type Value in
the Extensions Supported Data Item. 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 Credit Window Identifier, or CID. 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 CID must
be listed in a Credit Window Traffic Classification Data Item.
The format of the Credit Window Initialization 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 (16) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Window Identifier (CID)| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Value :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Credit Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scale | Credit Window Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TBA4 16
Per Length
is the number of octets in the data item, excluding the Type and
Length fields.
A 16-bit unsigned integer identifying a credit window. The value
of 0xFFFFFFFF is reserved and MUST not be used in this data item.
There is no other restriction on values used by a modem, and there
is no requirement for sequential or ordered values.
MUST be set to zero by the sender (a modem) and ignored by the
receiver (a router).
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.
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)
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
locate the credit window that is associated with the CID 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 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.
The Credit Window Traffic Classification Data Item is used by a modem
to provide information to enable router to identify the credit window
associated with traffic the router sends. Each data item includes a
set of credit windows and provides traffic classification for each
window. The data item is defined to support classification based on
DSCPs, but is designed to be extensible for future traffic
classification types. The data item is not required to provide full
traffic classification information. In the context of the
definition included in this document, the DLEP destination address
needed to complete the traffic classification information is provided
by a modem when it identifies the traffic classification set in a
Destination Up Message using the Credit Window Associate Data Item
defined below.
The Credit Window Traffic Classification Data Item SHOULD be
included by a modem in any Session Initialization Response Message
that also contains the DiffServ Aware Credit Windowing Extension
Type Value in the Extensions Supported Data Item. Updates to
previously provided traffic classifications or new traffic
classifications 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 traffic classifiers.
The set of traffic classification information provided in the data
item is identified using a Traffic Classification Identifier, or TID.
Addition information, e.g., the list DSCPs associated with a credit
window, is provided in a variable series of Queue Parameter sub-data
items. Note that to be used, a TID must be listed in a Credit Window
Associate Data Item.
The format of the Credit Window Traffic Classification 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Traffic Class. Identifier (TID)| Num SDIs | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Traffic Classification Sub Data Item 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Traffic Classification Sub Data Item n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TBA5Variable
Per Length
is the number of octets in the data item, excluding the Type and
Length fields.
A 16-bit unsigned integer identifying a traffic classification
set. There is no restriction on values used by a modem, and there
is no requirement for sequential or ordered values.
An 8-bit unsigned integer indicating the number of Traffic
Classification Sub Data Items included in the data item. A value
of zero (0) is allowed and indicates that no traffic should be
matched against this TID.
MUST be set to zero by the sender (a modem) and ignored by the
receiver (a router).
Zero or more Traffic Classification Sub Data Items of the format
defined below MAY be included. The number MUST match the value
carried in the Num SDIs field.
A router receiving the Traffic Classification Data Item MUST locate
the traffic classification information that is associated with the
TID indicated in each received data item. If no associated traffic
classification information is found, the router MUST initialize a
new information set using the values carried in the data item. When
associated traffic classification information is found, the router
MUST update the information using the values carried in the Data
Item. In both cases, a router MUST also ensure that any data plane
state, see , that is associated with the
TID is updated as needed.
All Traffic Classification Sub Data Items share a common format
that is patterned after the standard DLEP data item format, see
Section 11.3. There is no requirement
on, or meaning to sub data item ordering. Any errors or
inconsistencies encountered in parsing sub data items are handled
in the same fashion as any other data item parsing error
encountered in DLEP.
The format of the Traffic Classification Sub 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub Data Item Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A 16-bit unsigned integer that indicates the type and
corresponding format of the data item's Value field. Sub Data
Item Types are managed according to the IANA registry described
in .
Variable
Copying , Length a 16-bit unsigned
integer that is the number of octets in the sub data item,
excluding the Type and Length fields.
The DiffServ Traffic Classification Sub Data Item is used to
identify the DSCPs associated with a specific credit
window. Credit windows are identified using CID 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 sub data item. DSCPs are identified in a list of DiffServ
fields. An implementation that does not support DSCPs, and wants
to use credit window flow control for all traffic to a destination
or destinations, would indicate with 0 DSCPs.
The format of the DiffServ Traffic Classification Sub 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must be two (2) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Window Identifier (CID)| Num DSCPs | DS Field 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DS Field 2 | ... | DS Field n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Variable
Length is the number of octets
in the sub data item. It is equal to seven (7) plus the value
of the Num DSCPs field.
A 16-bit unsigned integer identifying a credit window that has
been identified in a Credit Window Initialization Data Item,
see . Unknown CID values SHOULD be
reported and result in associated traffic being dropped.
An 8-bit unsigned integer indicating the number of DSCPs
carried in the sub data item. A zero (0) indicates a (wildcard)
match against any DSCP value.
Each DS Field is an 8-bit whose definition is the same as .
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| DSCP | CU |
+---+---+---+---+---+---+---+---+
DSCP: differentiated services codepoint
CU: currently unused, MUST be zero
A router receiving the DiffServ Traffic Classification Sub Data
Item MUST validate the information on receipt prior to the
information and data plan update described earlier in this
section. The credit window associated with the CID indicated in
each data item must be located. It is important to note that the
CID value may be present in a Credit Window Initialization Data
Item carried in the same message as the sub data item. If the CID
is not located, the it MUST be treated as an error as described
above.
In the case there are no unknown CIDs, the receiver MUST ensure
that each DS Field value is listed only once across the whole
Credit Window Traffic Classification Data Item. Note, this check
is across the data item and not the individual sub data item. If
the same DS Field value is listed more than once within the same
Credit Window Traffic Classification Data Item, the data item MUST
be treated as an error as described above.
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 been previously been sent by the modem or is listed
in a Credit Window 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 use of the extension defined in this document is agreed upon,
see .
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)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TBA6 2
Per Length
is the number of octets in the data item, excluding the Type and
Length fields.
A 16-bit unsigned integer identifying a traffic classification
set that has been identified in a Credit Window Traffic
Classification Data Item, see .
Unknown TID values SHOULD be reported and result in associated
traffic being dropped.
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 , that is associated with the
TID is updated as needed.
The Credit Window Credit 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 window are
identified via using using CID 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Window Identifier (CID)| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Additional Credits :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Additional Credits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TBA712
Per , Length
is the number of octets in the data item, excluding the Type and
Length fields. It will equal 8 plus the number of CID
fields carried in the data item and MUST be at least 9.
A 16-bit unsigned integer identifying a credit window that has
been identified in a Credit Window Initialization Data Item,
see . Unknown CID values SHOULD be
reported and ignored.
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 CID. If the CID 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 CID 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.
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 indicated different sizes of different
credit windows. Similar to the Credit Window Grant, credit windows
are identified using CID 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Window Identifier (CID)| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Window Size :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Credit Window Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TBA812
Per Length
is the number of octets in the data item, excluding the Type and
Length fields. It will equal 8 plus the number of CID
fields carried in the data item and MUST be at least 9.
A 16-bit unsigned integer identifying a credit window that has
been identified in a Credit Window Initialization Data Item,
see .
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 .
When receiving this data item, a modem MUST identify the credit
window indicated by the CID. If the CID 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 CID 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 CID
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.
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 CID as defined above in . Multiple CIDs MAY be present to allow for the
case where the router identifies that credits are needed in multiple
credit windows. The special CID value of 0xFFFFFFFF 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Credit Window Identifier (CID)| ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... | Credit Window Identifier (CID)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[LB note: a list of CIDs is now supported as is special value
255.]
TBA9Variable
Per Length
is the number of octets in the data item, excluding the Type and
Length fields. It will equal the number of CID
fields carried in the data item and MUST be at least 1.
One or more 16-bit fields used to indicate a credit window that
has been identified in a Credit Window Initialization Data Item,
see . The special value of 0xFFFFFFFF
indicates that the request applies to all CIDs.
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 CID values SHOULD be reported or logged and then
ignored by the modem.
Sessions established with both peers identified as supporting the
DiffServ Aware Credit Windowing Extension Type, see , MUST NOT use the defined Credit data items.
If a node supporting the extension defined in this document, receives a
defined data item,
the recipient MUST ignore the received information.
The extension introduces a DiffServ awareness to the mechanisms
defined in . The
extension does not inherently introduce any additional threats above
those documented in . The
approach taken to Security in that document and apply equally when running
the extension defined in this document.
This document requests the assignment of 5 values by IANA. All
assignments are to registries defined by .
This document requests 1 new assignment to the DLEP Extensions
Registry named "Extension Type Values" in the range with the
"Specification Required" policy. The requested value is as follows:
CodeDescriptionTBA1DiffServ Aware Credit Windowing
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 CodeDescriptionTBA2Credit ControlTBA3Credit Control Response
This document requests the following 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 CodeDescriptionTBA4Credit Window InitializationTBA5Credit Window Traffic ClassificationTBA6Credit Window AssociationTBA7Credit Window GrantTBA8Credit Window StatusTBA9Credit Window Request
Upon approval of this document, IANA is requested to create a new
DLEP registry, named "Sub Data Item Type Values". The registry
shall identify the type code value, the Data Item which may use the
value, and a description of the value. While the same value may be
reused in different data items, this is not recommended at this time.
The following table provides initial registry values and the
defined policies that should apply to the registry:
Type CodeValid Data ItemsDescription0N/AReserved1N/AReserved (for pause sub-DI)2(TBD4) Credit Window Traffic ClassificationDiffServ Traffic Classification2-65407Specification Required65408-65534Private Use65535Reserved
The sub data item format was inspired by Rick Taylor's "Data Item
Containers". He also proposed the separation of credit windows from
traffic classification at IETF98.
This section captures issues that are open or have been addressed
since the document has become a WG draft. This section will be
removed before submission for publication.
There has been discussion within the WG that this document should
be merged with .
The timing of this is TBD. The authors would like to reach
closure on some of the remaining open issues before embarking on
this merge, but are open to discussion with the WG and other authors.
Routers may have limits on the number of queues that they can
support and, perhaps, if per destination queues can even be
supported at all. There is no current way for a router to
communicate these limits to a modem or for a router to indicate
that the identified queue information cannot be supported.
Support for these cases clearly needs to be addressed.
This issue was reported by Stan Ratliff <ratliffstan@gmail.com>.
Stan Ratliff <ratliffstan@gmail.com> suggested an approach to
simplify credit synchronization and re-synchronization by always
passing the credit window size rather than credit window
increments. This is an interesting idea that needs to be explored
and perhaps experimented with.
One of the key differences between this draft and is that this document only
supports flow control in one direction, i.e., for traffic sent
from a router to a modem, while the other credit-window draft
supports it in both directions.
This was a conscious choice, made out of the desire to keep the
extension as simple as possible and to provide what is really
expected to be used.
Clearly the reason for being able to control traffic that is sent from
the router to the modem/radio is pretty easily understood. Also, while
bidirectional flow control existed in pre-dlep solutions, it wasn't
really used. There is also no reason why router to modem flow
control can't be defined at a later time - once there is an actual
need.
Based on a brief discussion on the WG list, and the absence of any
advocates for bidirectional flow control, there is no current plan
to add bidirectional flow control to this document.