Traffic and Resource Management

The traffic management features of the ATM switch router provide the following capabilities:

• Integrated support for different types of ATM services

• Flexible policies for bandwidth allocation through controlled link-sharing services

• Effective use of network resources

The congestion control capabilities of the ATM switch router support the following goals:

• Avoid conditions where congestion can occur

• Minimize the effects of congestion when it does occur

• Prevent spreading the congestion state to other parts of the network

Note   The specific resource management capabilities of your ATM switch router are platform dependent. Consult the ATM Switch Router Software Configuration Guide for details.

Because ATM networks are designed to carry many different types of traffic, traffic characteristics and QoS requirements of each virtual connection must be described, and delivery of the contract must be guaranteed within the resource allocation policies defined for the network.

These requirements are carried out in three phases:

1. Define the traffic and service contract.

2. Find an acceptable path for the connections.

3. Use hardware resources to honor the terms of the contract for the life of the connection.

The first of these two steps can be considered the connection setup phase, while the third step represents the data flow phase. These three phases and their supporting mechanisms are discussed in the following sections.

• A connection traffic descriptor, which includes a service category and applicable parameters

• An optional set of QoS parameters applicable to the service category

When establishing the traffic and service contract, target values for QoS parameters can be used as criteria for the connection setup requirements. These values are either metrics (accumulated over multiple hops of a call) or attributes (a gating criterion that is not accumulated, but is checked at each interface). Maximum cell transfer delay (MCTD) and peak-to-peak cell delay variation (ppCDV) are metrics, while cell loss ratio (CLR) is an attribute.

• The connection traffic table rows

• The default QoS objective table

• The default MBS

• The default CDVT

Configuration Overview

Configuring the CTT row requires specifying a row index with parameter values for each of the service categories:

• For VBR-RT and VBR-NRT traffic, you must specify values for PCR and SCR; MBS and CDVT are optional.

• For CBR traffic, you must specify a value for PCR; CDVT is optional.

• For ABR and UBR traffic, you must specify a value for PCR; MCR and CDVT are optional.

Configuration Overview

Configuring the default QoS objective table requires one or more of the following steps; each objective can have a defined or undefined value.

• Specify a MCTD value in microseconds for CBR, VBR-RT, or for both.

• Specify a ppCDV value in microseconds for CBR, VBR-RT, or for both.

• Specify a cell loss ratio exponent for CBR, VBR-RT, VBR-NRT, or for all three.

  You can specify separate loss ratio exponents for CLP0 and CLP0+1 cells.

The default QoS objective table should be configured with the same values for an entire network.

Resource CAC (RCAC) uses the following information provided in the traffic contract for each direction of a requested connection:

1. Parameter values of the source traffic descriptor—PCR, SCR, MCR, MBS, CDVT

2. The requested service category (the service category must be the same for both directions of a connection) or QoS parameters (CLR, CTD, CDV) or both service category and QoS parameters

RCAC is based on a proprietary Equivalent Bandwidth algorithm in which equivalent bandwidths are used as real constant bandwidths in a statistical time-division multiplexing (STM) CAC environment for fast calculation. The equivalent bandwidth (or effective bandwidth by some) of a source is defined to be the minimum capacity required to serve the traffic source so as to achieve a specified steady-state CLR, MCTD, and ppCDV for CBR/VBR and for the nonzero MCR portion of ABR/UBR+ connections.

• SCRM (SCR margin)—extra equivalent bandwidth (over and above SCR) allocated to a VBR connection to account for bursting to PCR

• SCRMF (SCRM factor)—configurable safety margin with default = 1 percent

• R_CLR_CBR:CLR—estimate for CBR connection transiting an interface

• R_CLR_VBR:CLR—estimate for VBR connection transiting an interface

• R_ppCDV_CBR—estimate on maximum transmit ppCDV that might be experienced by a CBR connection transiting an interface

• R_ppCDV_VBR—estimate on maximum transmit ppCDV that might be experienced by a VBR-RT connection transiting an interface

• R_MaxCTD_CBR—estimate on maximum transmit MCTD that may be experienced by a CBR connection transiting an interface

• R_MaxCTD_VBR—estimate on maximum transmit MCTD that may be experienced by a VBR-NRT connection transiting an interface

• MAXCR—Maximum cell rate in a direction on an interface

• MAX_BE_CONNS—user-configured maximum number of best-effort (ABR/UBR) connections allowed on an interface

• MAX_PCR_CBR, MAX_PCR_VBR, MAX_PCR_ABR, MAX_PCR_UBR—user-configured maximum allowed PCR, per service category (default = line rate)

• MAX_SCR—user-configured maximum allowed SCR (default = line rate)

• MAX_MCR_ABR, MAX_MCR_ABR— user-configured maximum allowed MCR for ABR, UBR+ (default = line rate)

• MAX_MBS—user-configured maximum allowed MBS (default = none)

• MAX_CDVT_CBR, MAX_CDVT_VBR, MAX_CDVT_ABR, MAX_CDVT_UBR:—
  user-configured maximum allowed CDVT, per service category (default = no limit)

Parameters used in the algorithm are defined as follows:

Input:
Traffic contract input:   service_category, PCR, SCR, SCRMF, MBS, CDVT, MCR
			   tx_MaxCTD_obj, tx_MaxCTD_acc, tx_ppCDV_obj, tx_ppCDV_acc, CLR 
 
output:CAC_accept - true,  connection is accepted
                     false, connection is rejected 
 
algorithm:
 
Based on service category of the proposed connection:
 
service_category == CBR:
 
   if ((CLR >= R_CLR_CBR) && 
       (tx_MaxCTD_obj >= R_MaxCTD_CBR + tx_MaxCTD_acc) && 
       (tx_ppCDV_obj  >= R_ppCDV_CBR + tx_ppCDV_acc) && 
       (PCR <= MAXCR) && 
       (PCR <= MAX_PCR_CBR) &&
       (CDVT <= MAX_CDVT_CBR) {
 
          CBR_EQ_BW = PCR
          if (CBR_EQ_BW >  ACR_CBR) 
             CAC_accept = false 
          else 
             CAC_accept = true  
   } else
      CAC_accept = false 
 
service_category == VBR-RT:
 
   if ((CLR >= R_CLR_VBR) && 
       (tx_MaxCTD_obj >= R_MaxCTD_VBR + tx_MaxCTD_acc) &&
       (tx_ppCDV_obj  >= R_ppCDV_VBR  + tx_ppCDV_acc) && 
       (PCR <= MAXCR) && 
       (PCR <= MAX_PCR_VBR) && 
       (SCR <= MAX_CR) && 
       (SCR <= MAX_SCR) && 
       (MBS <= MAX_MBS) &&
       (CDVT <= MAX_CDVT_VBR) {
 
          SCRM = SCRMF * (PCR - SCR) /* SCRMF = [0,...,1] w. default=0.01 */
          VBR_BW = SCR + SCRM
          if (VBR_BW >  ACR_VBR)
             CAC_accept = false
          else 
             CAC_accept = true 
   } else
      CAC_accept = false 
 
service_category == VBR-NRT:
 
   if ((CLR >= R_CLR_VBR) &&
       (PCR <= MAXCR) &&
       (PCR <= MAX_PCR_VBR) &&
       (SCR <= MAX_CR) &&
       (SCR <= MAX_SCR) &&
       (MBS <= MAX_MBS) &&
       (CDVT <= MAX_CDVT_VBR) {
 
          SCRM = SCRMF * (PCR - SCR) /* SCRMF = [0,...,1] w. default=0.01 */
          VBR_BW = SCR + SCRM
          if (VBR_BW >  ACR_VBR)
             CAC_accept = false
          else 
             CAC_accept = true 
   } else
      CAC_accept = false 
 
service_category == ABR:
    
   if ((PCR <= MAXCR) &&
       (PCR <= MAX_PCR_ABR) &&
       (MCR <= MAXCR) &&
       (MCR <= MAX_MCR_ABR) &&
       (CDVT <= MAX_CDVT_ABR) &&
       (ABR_count + UBR_count + 1  <= MAX_BE_CONNS)){
 
          CBR_EQ_BW = PCR
          if (CBR_EQ_BW >  ACR_CBR) 
             CAC_accept = false 
          else 
             CAC_accept = true  
   } else
      CAC_accept = false 
 
service_category == UBR:
 
   if ((PCR <= MAXCR) &&
       (PCR <= MAX_PCR_UBR) &&
       (MCR <= MAXCR) &&
       (MCR <= MAX_MCR_UBR) &&
       (CDVT <= MAX_CDVT_UBR) &&
       (ABR_count + UBR_count + 1  <= MAX_BE_CONNS)){
 
          CBR_EQ_BW = PCR
          if (CBR_EQ_BW >  ACR_CBR)
             CAC_accept = false
          else
             CAC_accept = true
   } else
      CAC_accept = false
 

Note that the above algorithm does not describe the derivation of available cell rate (ACR) per service category. In the absence of controlled link sharing, this algorithm applies to each direction:

(.95 * MAXCR - sum of equivalent bw allocated to all connections on interface)
 

If controlled link sharing is configured, it then establishes limits on ACR for all guaranteed bandwidth or a service type (the maximum case) and limits on the encroachment of other service types (the minimum case).

• Sustained cell rate margin factor

• Controlled link sharing

• Outbound link distance

• Limits of best-effort connections

• Interface maximum of individual traffic parameters

• Service category support per interface

• Interface overbooking

• Framing overhead

Note   CAC can also be affected by the threshold group configuration for SVCs. See the "Threshold Groups" section.

The sustained cell rate margin factor is configured globally. The remaining CAC-related features are configured on a per-interface basis.

Note   CAC is bypassed for tag switching virtual connections; see the "Resource Management CAC" section.

bandwidth = (SCRMF * (PCR-SCR))/100 + SCR

Configuration Overview

You can change the default sustained cell rate margin factor for admitting VBR connections using a global configuration command. Configuring this value as 100 causes CAC to treat VBR like CBR.

Configuration Overview

Configuring controlled link sharing on an interface requires the following steps:

Step 2   Take one or more of the following steps:

   a. Specify a maximum percent of interface bandwidth to be used for guaranteed bandwidth connections. You can configure values for both the receive and transmit directions.

   b. Specify the maximum percent of interface bandwidth to be used for any of the service categories. You can configure values individually for each service category and for both the receive and transmit directions.

   c. Specify the minimum percent of interface bandwidth to be used for any of the service categories. You can configure values individually for each service category and for both the receive and transmit directions.

Changing the outbound link distance from its default of zero can affect the SVC requests accepted. For example, you might want to discourage use of a transcontinental link by configuring a higher propagation delay.

Configuration Overview

Configuring the outbound link distance requires the following steps:

Step 2   Specify an outbound link distance in kilometers.

Configuration Overview

Configuring the limits of best-effort connections requires the following steps:

Step 2   Specify the maximum number of best-effort connections to allow.

Configuration Overview

Configuring the maximum traffic parameters on an interface requires the following steps:

Step 2   Do one or more of the following steps for the receive direction, transmit direction, or both, on the interface:

   a. Specify a maximum PCR value in kbps for any of the CBR, VBR, ABR, and UBR service categories.

   b. Specify a maximum SCR value in kbps.

   c. Specify a maximum CDVT value (expressed in 2.72 microsecond cell times) for any of the CBR, VBR, ABR, and UBR service categories.

   d. Specify a maximum MBS value in number of cells.

   e. Specify a maximum MCR value in kbps for the best-effort service categories (ABR and UBR).

The default for physical interfaces and hierarchical VP tunnels is to allow virtual connections of any service category to transit the interface. However, interface service category configuration can be used explicitly to allow or prevent virtual connections of specified service categories to migrate across the interface.

Configuration Overview

The restrictions that apply to interface service category support are summarized as follows:

• Configuration is allowed on physical interfaces and shaped and hierarchical VP tunnel logical interfaces.

• On shaped VP tunnel logical interfaces, only one service category is permitted at a time. To replace CBR with another service category on these interfaces, you must first deny the CBR service category, then permit the chosen service category. To deny a service category, you must delete all user VCCs of that service category on the interface.

• For ABR and UBR, only zero MCR is supported for VCCs on a shaped VP tunnel.

Configuring interface service category support requires the following steps:

Step 2   Specify which traffic categories to deny on the interface.

Step 3   Specify which traffic categories to permit on the interface.

You can deny or permit any of the CBR, VBR-RT, VBR-NRT, UBR, and ABR traffic categories.

The following restrictions apply to interface overbooking:

• Regular (simple) VP tunnels do not support interface overbooking.

• You cannot add new hierarchical VP tunnels on a physical interface if the interface's bandwidth guarantees exceed the MaxCR regardless of any overbooking configured on that interface.

• On IMA interfaces, the available equivalent bandwidth for PVCs differs from the available equivalent bandwidth for SVCs. The available equivalent bandwidth for PVCs is based on the number of interfaces configured as part of the IMA group. The available equivalent bandwidth for SVCs on an IMA interface is based on the number of interfaces that are active in the IMA group. Overbooking increases both the available equivalent bandwidth values by the same configured percentage.

• The MaxCR for transmit and receive flows might differ on output-paced physical interfaces. Configuring overbooking on such interfaces results in different maximum guaranteed services bandwidth values and available cell rates for service categories for transmit and receive flows. Maximum guaranteed services bandwidth is the maximum equivalent bandwidth allocated for guaranteed services on the interface.

• When an interface is overbooked with traffic, cell flow to the well-known virtual connections might be reduced.

• Although overbooking increases the available cell rates for various service categories on an interface, various traffic parameters of a connection are still limited by the MaxCR.

• If the overbooking configuration results in a maximum guaranteed services bandwidth that is below the currently allocated bandwidth guarantees on an interface, the configuration is rejected.

Caution Overbooking can cause interface traffic to exceed the guaranteed bandwidth that the switch can provide.

Note   Interface overbooking configuration is not supported on systems with FC-PCQ installed.

Configuration Overview

Configuring interface overbooking requires the following steps:

Step 2   Shut down the interface.

Step 3   Configure interface overbooking for CAC as a percentage of the maximum equivalent bandwidth.

Step 4   Reenable the interface.

• Policing—Usage Parameter Control (UPC)

• Cell queuing

• Congestion notification

• Output scheduling

For most applications, when one or more cells are dropped by the network, the corresponding packet becomes corrupted and useless. This results in the need to retransmit the many cells that comprise that packet, and leads to exacerbated congestion. For example, loss of a cell from an IP over ATM packet (RFC 1577) might require resending 192 ATM cells, given an MTU of 9 KB.

To maximize the number of complete delivered packets, the ATM switch router implements a unique tail packet discard and early packet discard (TPD/EPD) scheme that intelligently and selectively discards cells belonging to the same packet. These congestion control mechanisms reduce the effects of fragmentation and make the ATM switch router essentially emulate a packet switch, which discards entire packets.

UPC on the ATM switch router checks the following parameters:

• PCR and CDVT for CBR, UBR, and ABR connections

• SCR and MBS for VBR connections

On systems equipped with hardware to support the dual leaky bucket, there are two policers for VBR connections. One policer uses PCR and CDVT, while the other policer uses SCR and MBS.

Configuration Overview

Configuring the default UPC behavior requires the following steps:

Step 2   For SVCs, select an interface, enter interface configuration mode, and specify tag or drop.

You can specify CDVT or MBS for PVCs through a connection traffic table row. If no CDVT or MBS is specified in the row, then a per-interface, per-service category default is applied for purposes of UPC on the connection.

Note   CDVT cannot be signaled. Therefore, the defaults specified on the interface apply for SVCs and the destination leg of a soft PVC.

Configuration Overview

Configuring the default CDVT and MBS on an interface requires the following steps:

Step 2   Specify a CDVT default value for a service category. You can repeat this step for additional service categories you want to configure.

Step 3   Specify an MBS default value for a service category. You can repeat this step for additional service categories you want to configure.

The specific features available depend upon the hardware:

• For systems with the FC-PCQ

  • Oversubscription factor

  • Service category limit

  • Maximum queue size per interface

  • Interface queue thresholds per service category

• For systems with the FC-PFQ and the Catalyst 8540 MSR, threshold groups can be configured.

The size of the VBR-NRT queue and ABR/UBR queues is determined by using the oversubscription factor (OSF) in the following formula:

size (vbr-nrt) = .25 * ((osf * 2048) - DefaultSize (cbr) - DefaultSize (vbr-rt))
size (abr-ubr) = .75 * ((osf * 2048) - DefaultSize (cbr) - DefaultSize (vbr-rt))
 

When you configure the service category limit requirements, you are specifying a new value to use rather than the default. To do so requires just one global configuration command in which you specify the limit in number of cells for a queue type. You repeat this command for each additional queue type for which you want to configure a maximum.

Configuration Overview

Configuring the maximum queue size for an interface requires the following steps:

Step 2   Specify an output queue maximum size for a service category. Repeat for additional service categories you want to configure.

The following queue thresholds can be configured per interface queue:

• Threshold at which the EFCI bit is set

• Threshold at which cells are eligible for CLP discard or EPD

• Threshold for relative rate (RR) marking on ABR connections

Configuration Overview

Configuring the interface queue threshold per service category requires the following steps:

Step 2   Specify an output percent for the EFCI marking threshold for a service category. You can repeat this step for additional service categories on the interface.

Step 3   Specify an output percent for the discard threshold for a service category. You can repeat this step for additional service categories on the interface.

Step 4   Specify an output threshold percent for RR marking on ABR connections.

Note   The threshold groups feature depends upon the hardware model and feature card installed in the ATM switch router. In addition, the total number of threshold groups available on the ATM switch router is platform dependent. For details, refer to the ATM Switch Router Software Configuration Guide.

Each threshold group has a set of eight regions, and each region has a set of thresholds. When these thresholds are exceeded, cells are dropped to maintain the integrity of the shared memory resource.

• A service category cannot be mapped to more than one threshold group.

• If you configure a service category to a threshold group more than once, only the last configuration is in effect.

• The default assigns each service category to a different threshold group. However, you can assign more than one service category to a threshold group.

Note   The configuration of threshold groups is static, not dynamic.

How It Works

Note   If the max- and min-queue-limits are equal, the queue size does not reduce as the group congests.

When congestion is in the range of 0 cells (uncongested) to 1/8th full, the connection queues are limited to max-queue-size. When congestion is in the range of 7/8ths full to completely full, the connection queues are limited to min-queue-size.

Configuration Overview

Configuring the threshold group parameters requires the following steps:

Step 2   Specify the maximum number of cells queued for all connections that are members of the threshold group.

Step 3   Specify the percent at which the per-connection queue is to be considered full for purposes of CLP discard and EPD.

Step 4   Specify the maximum per-connection queue limit (in number of cells) for the threshold group.

Step 5   Specify the minimum per-connection queue limit (in number of cells) for the threshold group.

Step 6   Specify a name to associate with the threshold group (optional).

Step 7   Specify the percent at which the per-connection queue is considered full for EFCI (on all connections) and RR marking (on ABR connections).

Note   RR marking is not supported on all platforms.

You can repeat these steps for any additional service category thresholds you want to configure.

• EFCI marking (for non-ABR connections)—In this mode the ATM switch router sets the EFCI state in the headers of forward data cells to indicate congestion. When the destination receives an EFCI flag, it marks the congestion indication bit in the resource management cells to indicate congestion and sends the resource management cells back to the destination.

• Relative rate (RR) marking mode (for ABR connections)—In this mode an ATM switch router that experiences congestion can set the congestion indication bit directly in forward and backward resource management cells. RR marking mode is used only for ABR connections and is not supported on all hardware platforms.

EFCI and RR marking involve two important functions: detecting incipient congestion and providing selective feedback to the source. As with any feedback mechanism, congestion control schemes operate best when the latency of the feedback path is minimized. Thus RR mode, because of its ability to use backward RM cells to send the congestion indicator rather than relying on the destination end system to reflect it back, can greatly reduce feedback delays and deliver better performance than the EFCI mode.

The two modes can be used independently or in combination to support ABR traffic, and thresholds can be set to indicate when EFCI or RR marking should occur.

Configuration Overview

Interface output pacing is disabled by default on the ATM switch router. Configuring the interface output pacing requires the following steps:

Note   There are restrictions on which interface types can be configured for output pacing; refer to the ATM Switch Router Command Reference publication for details.

Step 2   Specify the output pacing limit as a bit rate in kbps.

This configuration does not take effect if the amount of bandwidth allocated to CBR and VBR connections in the transmit direction on the interface is greater than the configured output pacing value.

In scheduling the next cell to be transmitted from a port, RS has first call on supplying an eligible cell. If RS does not have one, then the WRR scheduler chooses a service class with an output virtual connection ready to transmit, and finally a virtual connection within the service class is selected.

Note   Scheduler and service class configuration depends upon the feature card installed in the ATM switch router. On systems that do not support scheduler and service class configuration, output scheduling is done on a strict priority basis; refer to the ATM Switch Router Software Configuration Guide for details.

If the timewheel RS does not have an output virtual circuit ready to transmit, the WRR scheduler becomes active to pick out a virtual circuit to transmit a cell. The WRR scheduler uses the interface bandwidth left over after guaranteed cell service to transmit cells. Thus, an output virtual circuit of a service category other than CBR can be serviced by both the rate scheduler and the WRR scheduler. A CBR output virtual circuit cannot be serviced by the WRR scheduler because its PCR is already guaranteed by the rate scheduler. (Any additional cell transmission by the WRR scheduler out of that output virtual circuit is likely to arrive too soon at the next switch and might be policed.)

The following service categories can be serviced by the WRR scheduler:

• VBR-RT

• VBR-NRT

• ABR

• UBR

Each service class is assigned a weight. These weights are configurable, in the range of 1 to 15. The default weighting is {15,2,2,2} for classes {2,3,4,5}, respectively. The weighting is not modified dynamically.

Within service classes, individual output virtual circuits are also weighted, again in the range of
1 to 15. A standard weight (2) is assigned to all PVCs in a service class. Optionally, PVCs can be configured with a specific weight per half-leg (applying to the transmit output virtual circuit weight). SVCs take the value 2.

Configuration Overview

Configuring the service class weights on an interface requires the following steps:

Step 2   Specify a service class and a weight value. You can repeat this step for additional service classes.