Abstract
This white paper provides the guidelines for the QOS implementation in the DSL environment
It also demonstrates the auto-degradation of the Service category/Class of Service parameters or the Service category/Class of Service itself on the PVC using different scenarios when the DSL line trains at a rate lower than the configured guaranteed bandwidth.
Guidelines to be Followed while Configuring Layer 2/Layer 3 QOS on PVC
In the DSL world, PVCs are provisioned by the Service Provider based upon certain service contract for a guaranteed traffic rate and hence provide reliable service to all customers. Service contracts also aid the Service Providers to police the traffic exceeding the requested/granted rate.
DSL flavors of Cisco 800/1800 series Integrated Services Routers fixed configuration models and Cisco DSL High speed WAN interface cards (HWICs) /WAN interface cards (WICs) support PVC with the following service categories to provide the bandwidth guarantee for various applications. (Appendix A provides information on all platforms and DSL HWICs/WICs supporting these features)
Table 1.
The user can choose the appropriate Service category to suffice his network demands and configure the parameters for the Service categories inline with the Service contract of the PVC.
It is important for the user to realize that they are not necessarily restricted to a particular type of service category to carry out their ATM traffic nor they are bound to establish the same service categories on both ends of the link. However, each service category uses certain traffic parameters that best define the transmission characteristic of a type of traffic and will help them to optimize their flow and to meet the requirement of their traffic contract.
Following table provides information on the various traffic parameters available for the different service categories
Table 2.
|
Service Category |
Traffic Parameter Used to Guarantee Cell Rate |
|
CBR |
PCR |
|
nrt-VBR |
SCR |
|
rt-VBR |
SCR |
|
UBR + |
MCR |
|
UBR |
none |
In addition, each service category has a default transmission priority associated with it. Therefore, the user will achieve the best use of the bandwidth and optimize the performance for their traffic if they choose the service category that best represents the type of traffic and applications that will be carried over the PVC. The primary thing to keep in mind is that the ATM service category defines how the ATM network devices and other treat the cells of the VC with respect to bandwidth guarantees, cell delays and cell loss.
The next step would be to configure the QOS on the Layer 3 queues using the MQC (Modular QOS CLI) once the service category along with the required parameters is configured on the PVC.
Following are the list of guidelines that need to be taken care while configuring the QOS on the layer 3 Software Queues
• Service policies providing the Layer 3 QOS using CBWFQ/LLQ have to be applied at the PVC level and should not be done at the interface or sub interface level.
• To provide preferential treatment to voice traffic, the hardware queue length has to be modified in such way that congestion is realized at the output queue on each PVC and hence the software queue (Layer 3 queue) stores the packet in the buffer to be treated by the congestion management mechanism such as (CBWFQ/LLQ).
The length of the hardware queue has to be calculated based on applications in the network that need specialized treatment.
Please refer to Appendix B for guidance on determining the ideal value for the hardware queue-limit
The command to modify the length of the hardware queue on per-pvc basis is
Table 3.
|
Command |
Purpose |
|
tx-ring-limit <specify the length of the queue> |
Modifies the length of the hardware queue |
• Class-Based Traffic shaping is not supported on the DSL interface.
• QOS on Layer 3 queues is not supported on the PVC configured as UBR as there is no bandwidth guarantee provided by the PVC at layer 2. By default all the PVCs are in UBR mode. Hence when the customer requires QOS on Layer 3 queues, the PVC has to be configured as CBR or VBR-rt or VBR-nrt or UBR+ depending on nature of applications in the network.
Deployment Scenarios
Following section focuses on explaining the common deployments done in the field. It also demonstrates the QOS operation at Layer 2 and Layer 3 in each of the scenarios.
Scenario 1:
This is most common scenario, where all the customer traffic flows over single PVC. Bandwidth guarantee at layer two is provided using CBR or VBR or UBR+. Service policy is applied on the PVC to provide layer 3 classifications and bandwidth guarantee for different types of traffic like Voice, Critical data or normal data.
Following scenarios depict the layer 2 and layer 3 QOS functioning on single PVC with different Class of Service at layer 2.
Figure 1. Single PVC with "CBR" Class of Service
In this scenario, PVC is defined with CBR service category having the PCR value of 1500 Kbps. There are two different classes defined to classify the Voice and Critical data information and then bandwidth guarantee is provided to those classes using Service Policy. All other non-classified traffic would fall into the default class which uses Fair-queue for congestion management.
Following are the details on the bandwidth allocation for different applications:
• Class RT: Strict Priority bandwidth of 400 Kbps using LLQ
• Class MC: Assured bandwidth of 200 Kbps using CBWFQ
• Class-Default: Fair-queue
Running Configuration
877(CPE):
Building configuration...
Current configuration : 1623 bytes
!
version 12.4
no service pad
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname ADSL_877
!
boot-start-marker
boot-end-marker
!
no aaa new-model
!
resource policy
!
ip cef
!
class-map match-any RT
match ip dscp ef
class-map match-all MC
match ip dscp af43
!
policy-map QOS
class RT
priority 400
class MC
bandwidth 200
class class-default
fair-queue
!
interface ATM0
no ip address
no atm ilmi-keepalive
dsl operating-mode auto
!
interface ATM0.1 point-to-point
ip address 20.1.1.2 255.255.255.0
no snmp trap link-status
pvc 1/99
protocol ip 20.1.1.1 broadcast
cbr 1500
tx-ring-limit 3
service-policy output QOS
!
interface FastEthernet0
duplex full
speed 100
!
interface FastEthernet1
!
interface FastEthernet2
!
interface FastEthernet3
!
interface Dot11Radio0
no ip address
shutdown
speed basic-1.0 basic-2.0 basic-5.5 6.0 9.0 basic-11.0 12.0 18.0 24.0 36.0 48.0
54.0
station-role root
!
interface Vlan1
ip address 10.1.1.2 255.255.255.0
ip route 0.0.0.0 0.0.0.0 20.1.1.1
!
no ip http server
no ip http secure-server
control-plane
!
line con 0
exec-timeout 0 0
no modem enable
line aux 0
line vty 0 4
login
!
scheduler max-task-time 5000
end
But as per the setting on the DSLAM profile the line trains-up at only 832 Kbps. Due to this change the CPE automatically downgrades the CBR PVCs PCR rate to 832 Kbps.
Following is the snapshot of the notification sent on the Console:
ADSL_877#
*Mar 21 10:11:56.715: %LINK-3-UPDOWN: Interface ATM0, changed state to up
*Mar 21 10:11:57.715: %LINEPROTO-5-UPDOWN: Line protocol on Interface ATM0, changed state to up
*Mar 21 10:12:02.447: %DSLSAR-1-DOWNGRADEDBW: PCR and SCR for VCD 1 (1/99) has been reduced to 832k 832k due to insufficient upstream bandwidth
To confirm the Congestion management mechanism after the downgrade of the PCR rate, three different streams with the following specification were sent:
Traffic matching the RT class (Voice): 370 PPS (Resultant Layer 3 Throughput sent = PPS*8*Packet size = 370*8*121 = 358 Kbps)
Traffic matching the MC class (Data): 200 PPS (Resultant Layer 3 Throughput sent = PPS*8*Packet size = 200*8*121 = 193 Kbps)
Unclassified Traffic: 1500 PPS (Resultant Layer 3 Throughput sent = PPS*8*Packet size = 1500*8*121 = 1.45 Mbps)
The output confirms that the voice and data traffic are received without any drops and only the excess traffic in the default-class is dropped.
Traffic matching the RT class (Voice): 370 PPS (Resultant Layer 3 Throughput received = PPS*8*Packet size = 370*8*121 = 358 Kbps)
Traffic matching the MC class (Data): 200 PPS (Resultant Layer 3 Throughput received = PPS*8*Packet size = 200*8*121 = 193 Kbps)
Unclassified Traffic: 1500 PPS (Resultant Layer 3 Throughput received = PPS*8*Packet size = 89*8*121 = 86 Kbps)
Following is the snapshot taken from the Traffic Generator and Traffic Reflector:
Traffic Details of PVC with PCR rate of 832 Kbps (requested 1500 Kbps as PCR rate):
Generator(TGN:OFF,Fa0/0:3/3)#show send
Summary of sending traffic streams on FastEthernet0/0
ts# template state interval/rate send-amount/left-to-send total-sent
1 IP on 370 pps 0 0 3805
2 IP on 200 pps 0 0 2057
3 IP on 1500 pps 0 0 15424
Reflector(Fast Counting)#show fast-count
Fast-count counts count pps or sec/packet
Interface: FastEthernet0/0
Filter: dscpef incoming 3805 370.002 pps
Filter: dscpaf43 incoming 2057 199.902 pps
Filter: dscpnone incoming 932 89.614 pps
Following snapshot shows the output of the Policy map command on the router while providing the congestion management
Policy Map Output with CBWFQ and LLQ
ADSL_877#show policy-map interface atm 0.1
ATM0.1: VC 1/99 -
Service-policy output: QOS
Class-map: RT (match-any)
3805 packets, 506065 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: ip dscp ef (46)
3805 packets, 506065 bytes
5 minute rate 0 bps
Queueing
Strict Priority
Output Queue: Conversation 72
Bandwidth 400 (kbps) Burst 10000 (Bytes)
(pkts matched/bytes matched) 3805/506065
(total drops/bytes drops) 0/0
Class-map: MC (match-all)
2057 packets, 273581 bytes
5 minute offered rate 0 bps, drop rate 0 bps
Match: ip dscp af43 (38)
Queueing
Output Queue: Conversation 73
Bandwidth 200 (kbps)Max Threshold 64 (packets)
(pkts matched/bytes matched) 2057/273581
(depth/total drops/no-buffer drops) 0/0/0
Class-map: class-default (match-any)
15424 packets, 2051392 bytes
5 minute offered rate 36000 bps, drop rate 34000 bps
Match: any
Queueing
Flow Based Fair Queueing
Maximum Number of Hashed Queues 64
(total queued/total drops/no-buffer drops) 0/14492/0
ADSL_877#
Figure 2. Single PVC with "VBR-rt" Class of Service
In this scenario, PVC is defined with VBR-rt service category having PCR value of 1500 Kbps and SCR value of 1500 Kbps. There are two different classes defined to classify the Voice and Critical data information and then bandwidth guarantee is provided to those classes using Service Policy. All other non-classified traffic would fall into the default class which uses Fair-queue for congestion management.
Following are the details on the bandwidth allocation for different applications:
• Class RT: Strict Priority bandwidth of 400 Kbps using LLQ
• Class MC: Assured bandwidth of 200 Kbps using CBWFQ
• Class-Default: Fair-queue
Running Configuration
877(CPE):
Building configuration...
Current configuration : 1393 bytes
!
version 12.4
no service pad
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname ADSL_877
!
boot-start-marker
boot-end-marker
!
no aaa new-model
!
resource policy
!
ip cef
!
class-map match-any RT
match ip dscp ef
class-map match-all MC
match ip dscp af43
!
policy-map QOS
class RT
priority 400
class MC
bandwidth 200
class class-default
fair-queue
!
interface ATM0
no ip address
no atm ilmi-keepalive
dsl operating-mode auto
!
interface ATM0.1 point-to-point
ip address 20.1.1.2 255.255.255.0
no snmp trap link-status
pvc 1/99
protocol ip 20.1.1.1 broadcast
vbr-rt 1500 1500
tx-ring-limit 3
service-policy output QOS
!
!
interface FastEthernet0
duplex full
speed 100
!
interface FastEthernet1
!
interface FastEthernet2
!
interface FastEthernet3
switchport access vlan 2
!
interface Dot11Radio0
no ip address
shutdown
speed basic-1.0 basic-2.0 basic-5.5 6.0 9.0 basic-11.0 12.0 18.0 24.0 36.0 48.0
54.0
station-role root
!
interface Vlan1
ip address 10.1.1.2 255.255.255.0
!
ip route 30.1.1.0 255.255.255.0 20.1.1.1
!
no ip http server
no ip http secure-server
!
control-plane
!
line con 0
exec-timeout 0 0
no modem enable
line aux 0
line vty 0 4
login
!
scheduler max-task-time 5000
end
ADSL_877#
But as per the setting on the DSLAM profile the line trains-up at only 832 Kbps. Due to this change the CPE automatically downgrades the VBR-rt PVCs PCR rate and SCR rate to 832 Kbps.
Following is the snapshot of the notification sent on the Console:
ADSL_877#
*Mar 21 10:11:56.715: %LINK-3-UPDOWN: Interface ATM0, changed state to up
*Mar 21 10:11:57.715: %LINEPROTO-5-UPDOWN: Line protocol on Interface ATM0, changed state to up
*Mar 21 10:12:02.447: %DSLSAR-1-DOWNGRADEDBW: PCR and SCR for VCD 1 (1/99) has been reduced to 832k 832k due to insufficient upstream bandwidth
To confirm the Congestion management mechanism after the downgrade of the PCR and SCR, three different streams with the following specification were sent:
Traffic matching the RT class (Voice): 370 PPS (Resultant Layer 3 Throughput sent = PPS*8*Packet size = 370*8*121 = 358 Kbps)
Traffic matching the MC class (Data): 200 PPS (Resultant Layer 3 Throughput sent = PPS*8*Packet size = 200*8*121 = 193 Kbps)
Unclassified Traffic: 1500 PPS (Resultant Layer 3 Throughput sent = PPS*8*Packet size = 1500*8*121 = 1.45 Mbps)
The output confirms that the voice and data traffic are sent without any drops and only the excess traffic in the default-class is dropped.
Traffic matching the RT class (Voice): 370 PPS (Resultant Layer 3 Throughput received = PPS*8*Packet size = 370*8*121 = 358 Kbps)
Traffic matching the MC class (Data): 200 PPS (Resultant Layer 3 Throughput received = PPS*8*Packet size = 200*8*121 = 193 Kbps)
Unclassified Traffic: 1500 PPS (Resultant Layer 3 Throughput received = PPS*8*Packet size = 87*8*121 = 84 Kbps)
Following is the snapshot taken from the Traffic Generator and Traffic Reflector:
Traffic Details of PVC with PCR and SCR rate of 832 Kbps (requested 1500 Kbps as PCR and SCR rate):
Generator(TGN:OFF,Fa0/0:3/3)#show send
Summary of sending traffic streams on FastEthernet0/0
ts# template state interval/rate send-amount/left-to-send total-sent
1 IP on 370 pps 0 0 8660
2 IP on 200 pps 0 0 4681
3 IP on 1500 pps 0 0 35107
Reflector(Fast Counting)#show fast-count
Fast-count counts count pps or sec/packet
Interface: FastEthernet0/0
Filter: dscpef incoming 8660 370.011 pps
Filter: dscpaf43 incoming 4681 200.017 pps
Filter: dscpnone incoming 2033 86.460 pps
Following snapshot shows the output of the Policy map command on the router while providing the congestion management
Policy Map Output with CBWFQ and LLQ
ADSL_877#show policy-map interface atm0.1
ATM0.1: VC 1/99 -
Service-policy output: QOS
Class-map: RT (match-any)
8660 packets, 1151780 bytes
5 minute offered rate 32000 bps, drop rate 0 bps
Match: ip dscp ef (46)
8660 packets, 1151780 bytes
5 minute rate 32000 bps
Queueing
Strict Priority
Output Queue: Conversation 72