Dynamic Link Exchange Protocol (DLEP) Credit-Based Flow Control Messages and Data Items

Credit Window Flow Control This section defines additions to DLEP used in credit-based flow control. The additions provide the DLEP mechanisms to control credits. Routers then use this information to regulate when data is sent to a modem. The credit window flow 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 to a particular credit window is based on the Traffic Classification Data Item defined in . Both types of DLEP peers -- router and modem -- negotiate the use of an extension employing this mechanism during session initialization as required; for example, see . When using credit 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 window"' 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 -- for example, DSCPs -- at a granularity that is appropriate for a modem's implementation and its attached transmission technology. As specified in , there is a direct one-to-one mapping of credit windows to flows as identified by Flow Identifiers (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 flow control is used. Note that Traffic Classification Identifier (TID) values and FID values are significant only to the issuing modem. There is no relationship implied by the same TID or FID value being issued by more than one modem. In addition to the traffic classification information associated with a FID, modems 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. An increment to a credit window size, specified in a Credit Window Grant Data Item, is provided in a Destination Up Message () or Credit Control Message (). A router provides its view of the credit window, which is known as "Status", in Destination Up Response Messages () and Credit Control Response Messages (). Routers can also request credits using the 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 (per-packet) Media Access Control (MAC) headers, and this may not match the actual overhead of the modems' 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 . This Data Item supports traffic classification on a per-destination or more fine-grained 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. In some cases, the Traffic Classification Data Item allows the modem to specify a wildcard to match any packets that do not match other Data Items; for example, see . In the absence of a wildcard, a packet may not match any of the Data Items and, in this case, MUST be dropped by the router. 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. This is supported by the Credit Window Association Data Item, which is carried in Destination Up and Destination Update Messages; see . The TID provides the information to support router traffic classification, based on the FIDs contained in the TID; see . As defined, each credit window has a corresponding FID, so traffic is mapped to a credit window by locating a matching FID that is contained in the TID that is associated with the traffic's destination. This means that the use of FIDs and TIDs, and the association of a TID to a DLEP destination, enable a modem to share or dedicate resources as needed to match the specifics of its implementation and its attached transmission technology. Credit window flow control as defined in this document has objectives similar to the control technique described in . One notable difference from that type of credit control is that in this document, credits are never provided by the router to the modem.

Data Plane Considerations When credit windowing is used, a router MUST NOT send data traffic to a modem for forwarding if there is no matching classifier. If a matching classifier is found, a router MUST NOT send data traffic to a modem when there are no credits available in the associated credit window. describes how classifiers are associated with destinations and how credit windows are associated with classifiers. Additionally, a router MUST ensure that sufficient credits are available in the associated credit window for the current data packet before sending that data packet to the modem. The count of octets in the packet includes MAC overhead. Taking Ethernet as an example, framing, header, and trailer are all included in this count. This document defines credit windows in octets, and the credit window is decremented by the number of sent octets. A router MUST identify the credit window associated with traffic to be sent to a modem based on the traffic classification information provided in the Data Items defined in this document.

Credit Window Messages This document defines two new messages that support credit window flow control: Credit Control Messages and Credit Control Response Messages. Sending and receiving both message types is REQUIRED to support the credit window flow control mechanisms defined in this document.

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 (either modem or router) MUST NOT send a subsequent Credit Control Message until a Credit Control Response Message has been received from its peer. Credit Control Messages are sent by modems to provide credit window increases. Modems send credit increases when their transmission or local queue availability exceeds the credit window value previously provided to the router. Modems will need to balance the load generated by sending and processing credit window increases against a router having data traffic available to send but no available credits. 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 credit window requests against having data traffic available to send but no available credits. The Message Type value in the DLEP Message Header is set to 17. A Credit Control Message sent by a modem MUST contain one or more Credit Window Grant Data Items as defined in . A router receiving this message MUST respond with a Credit Control Response Message. A Credit Control Message sent by a router MUST contain one or more Credit Window Request Data Items as defined 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 in , which will typically result in credit window increments being provided. Specific processing associated with each Credit Data Item is provided in .

Credit Control Response Message Credit Control Response Messages are sent by routers to report the current credit window for a destination. A Credit Control Response Message sent by a router MUST contain one or more Credit Window Status Data Items as defined in . Specific receive processing associated with the Credit Window Status Data Item is provided in . 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 Message received by the modem MUST be included. Each Credit Window 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 Credit Window Grant Data Item is provided in . The Message Type value in the DLEP Message Header is set to 18.

Data Items for the Control of Credit Windows Five new Data Items are defined to support the control of credit windows:

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 TIDs (flows) should be used when sending traffic to a particular DLEP-identified destination.
The Credit Window Grant Data Item () is used by a modem to provide additional credits to a router.
The Credit Window Status Data Item () is used to advertise the sender's view of the number of available credits for state synchronization purposes.
The Credit Window Request Data Item () is used by a router to request additional credits.

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 . In particular, the node parsing the Data Item MUST terminate the session with a Status Data Item indicating "Invalid Data".

Credit Window Initialization As noted above, the Credit Window Initialization Data Item is used by a modem to identify a credit window and set its size. In order to avoid errors caused by the use of undefined FIDs or uninitialized credit windows, this Data Item SHOULD be included in any Session Initialization Response Message that indicates support for any such 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 FID. It also provides the size of the identified credit window. 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 as follows:

Data Item Type:

Length:

16 As specified in , Length is the number of octets in the Data Item. It MUST be equal to sixteen (16). If it is some other value, the Data Item is corrupt, so the message in which it appears cannot be reliably parsed and is ignored.

Flow Identifier (FID):

A 2-octet flow identifier as defined by the Traffic Classification Data Item . The FID also uniquely identifies a credit window for a specific DLEP session.

Reserved:

For the Credit Window Initialization Data Item, this reserved field is currently unused. It MUST be set to all zeros for this version of the Data Item and is currently ignored on reception. This allows for future extensions of the Data Item if needed.

Credit Value:

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.

Scale:

An 8-bit unsigned integer indicating the scale used in the Credit Window Max Size field. The valid values are as follows: Valid Scale Field Values

Value	Scale
0	B: Bytes (Octets)
1	KB: Kilobytes (B/1024)
2	MB: Megabytes (KB/1024)
3	GB: Gigabytes (MB/1024)

Credit Window Max 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 message or a previous message. If the FID cannot be found, the router SHOULD log this information. The method of logging is left to the router implementation. 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. If the resultant credit value results in the credit window exceeding the represented Credit Window Max Size, the Credit Window Max Size field value is used as the new credit window size. 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. After sending the Session Update Message with one or more Credit Window Initialization Data Items that decrease the Credit Window Max Size, the modem SHOULD continue processing received packets that match the indicated FIDs, fit within the window for the unmodified Credit Window Max Size, and arrive before the modem receives the corresponding Session Update Response Message. The modem SHOULD NOT issue additional credits for any affected FID until that FID's associated window has drained to be less than the new Credit Window Max Size, regardless of when the corresponding Session Update Response Message is received.

Credit Window Association The Credit Window Association 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 sent by the modem or is listed in a Traffic Classification Data Item carried in the same message as the Credit Window Association Data Item. TIDs in different credit windows must not overlap. A single Credit Window Association Data Item MUST be included in every Destination Up and Destination Update Message sent by a modem when a credit window flow control mechanism defined in this document is used. Note that a TID will not be used unless it is listed in a Credit Window Association Data Item. The format of the Credit Window Association Data Item is as follows:

Data Item Type:: 31
Length:: 2 As specified in , Length is the number of octets in the Data Item. It MUST be equal to two (2). If it is some other value, the Data Item is corrupt, so the message in which it appears cannot be reliably parsed and is ignored.
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 .

A router that receives a Credit Window Association Data Item MUST locate the traffic classification information indicated by the received TID. If no corresponding information is found, the Credit Window Association Data Item MUST be treated as an error as described above. If the traffic classification information is located, the router MUST ensure that any data plane state that is associated with the TID and its corresponding FIDs is updated as needed (per ). If a router determines that a newly received Data Item results in credit windows with overlapping TIDs, the Data Item MUST be treated as an error as described above.

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, and Credit Control Response Messages. Multiple Credit Window Grant Data Items may be present in a single message. Each item grants credits to a different credit window and therefore references a different FID. 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 sent by the modem or are listed in a Credit Window Initialization Data Item carried in the same message as the Data Item. The format of the Credit Window Grant Data Item is as follows:

Data Item Type:: 32
Length:: 12 As specified in , Length is the number of octets in the Data Item. It MUST be equal to twelve (12). If it is some other value, the Data Item is corrupt, so the message in which it appears cannot be reliably parsed and is ignored.
Flow Identifier (FID):: A 2-octet flow identifier as defined by the Traffic Classification Data Item. The FID also uniquely indicates a credit window.
Reserved:: For the Credit Window Grant Data Item, this reserved field is currently unused. It MUST be set to all zeros for this version of the Data Item and is currently ignored on reception. This allows for future extensions of the Data Item if needed.
Additional Credits:: 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 log this information and discard the Data Item. The method of logging is left to the router implementation. It is important to note that while this Data Item can be received in a destination-specific message, credit windows are managed independently of 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, the credit window must be set to its maximum as defined by the Credit Window Max Size carried in the Credit Window Initialization Data Item. 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. For each Credit Window Grant Data Item received in other message types, the receiving router MUST send a Credit Window Status Data Item reflecting the resultant credit window value of the updated credit windows. When the Credit Window 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.

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 in , 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 Data Item, credit windows are identified using FID values that have been previously sent by the modem. The format of the Credit Window Status Data Item is as follows:

Data Item Type:: 33
Length:: 12 As specified in , Length is the number of octets in the Data Item. It MUST be equal to twelve (12). If it is some other value, the Data Item is corrupt, so the message in which it appears cannot be reliably parsed and is ignored.
Flow Identifier (FID):: A 2-octet flow identifier as defined by the Traffic Classification Data Item. The FID also uniquely identifies a credit window.
Reserved:: For the Credit Window Status Data Item, this reserved field is currently unused. It MUST be set to all zeros for this version of the Data Item and is currently ignored on reception. This allows for future extensions of the Data Item if needed.
Current 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 .

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 log this information and discard the Data Item. The method of logging is left to the modem implementation. 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 Current Credit Window Size field value against the outstanding credit window's available credits at the time the most recent Credit Window Initialization or Credit Window Grant Data Item associated with the indicated FID was sent. If the difference in values is greater than what 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 specified in , 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.

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 are identified using a FID as defined in . Multiple FIDs MAY be present to allow for the case where the router determines that credits are needed in multiple credit windows. A special FID value, as defined below, is used to indicate that a credit window request is being made across all queues. The format of the Credit Window Request Data Item is as follows:

Data Item Type:: 34
Length:: Variable As specified in , Length is the number of octets in the Data Item, excluding the Data Item Type and Length fields. It is equal to the number of FID fields carried in the Data Item times 2 and MUST be at least 2. If it is less than 2, the Data Item is corrupt, so the message in which it appears cannot be reliably parsed and is ignored.
Flow Identifier (FID):: A 2-octet flow identifier as defined by the Traffic Classification Data Item. The FID also uniquely identifies a credit window. The special value 0xFFFF indicates that the request applies to all FIDs. When this special value is included, all other FID values included in the Data Item are redundant, as the special value indicates all FIDs.

A modem receiving this Data Item MUST provide a credit window 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 logged and then ignored by the modem. The method of logging is left to the modem implementation.

Management Considerations This section provides several network management guidelines for implementations supporting the credit window flow control 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. They may even have limits on supported credit window combinations. For example, per-destination queues may not be supported at all. When credit window information provided by a modem exceeds the capabilities of a router, the router SHOULD 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, any mismatch in capabilities SHOULD be reported to the user via normal network management mechanisms, such as the user interface or error logging. In all cases, if credit windows are in use, traffic for which credits are not available MUST NOT be sent to the modem by the router.

Type Code	Description
30	Credit Window Initialization
31	Credit Window Association
32	Credit Window Grant
33	Credit Window Status
34	Credit Window Request