| Quality of Service Guideline |
Last Updated: 7 May 02
Contact: Ronald Olton
PURPOSE:
The purpose of this guideline is to provide the information needed to implement Quality of Service (QoS) on campus infrastructures.
SCOPE:
This guideline covers the strategy and components used to deliver campus level QoS for inbound and outbound campus traffic.
This document will be updated to adapt to changes in technology, as they occur.
APPLICABILITY:
This guideline is applicable to all colleges and campuses within VCCS.
GUIDELINE:
This guideline is based on using the Ethernet 802.1Q and P Standards for delivering Quality of Service (QoS) in the switched portions of the campus infrastructure and Differentiated Services (Diffserv) for the IP portions of the infrastructure and the wide area network. It defines a framework for implementing a subset of Diffserv on VCCS campuses. This will simplify the migration to NWVng and Internet2 QoS offerings because both networks plan to use Diffserv as their QoS technology. It uses the backward compatible values defined in Diffserv to match current IP Precedence prioritiy implementations.
A network that carries voice, video, and data traffic over a single connection is referred to as a “converged” network. In a converged network, certain types of application traffic need “special” handling to ensure that the application operates properly. The most demanding of these are called “time dependent” applications. They are called time dependent because their traffic must be delivered with a minimum of delay in order for them to operate properly. Among them are two applications currently being implemented at many VCCS colleges, IP telephony and H.323 video.
The strategy described here creates an end-to-end Quality of Service (QoS) environment. Providing end-to-end support is the only way to ensure proper operation of these applications across both the WAN and LAN environments. End-to-end is defined as being from the source end point to the destination end point. It includes the wide area network (WAN) and the campus local area network (LAN) environments.
Unfortunately, Differentiated Services (Diffserv) technology is not currently available in either NWVng or Internet2. This paper provides a campus level subset of Diffserv (IP Precedence) that can be used on the campus now, and that can be easily migrated to NWVng and Internet2 as they implement DiffServ in the wide area network.
QoS can, and should be implemented for both outbound and inbound traffic. “Outbound” is defined as any traffic sent to an end point that is not on the same LAN segment as the sending end point. “Inbound” is defined as any traffic received from an end point that is not on the same LAN segment as the receiving end point. Outbound traffic needs to be controlled to minimize the delay introduced in time dependent application traffic by the LAN and WAN networks. Inbound traffic needs to be controlled to ensure that campus network traffic priorities are enforced. All campus routers and switches must provide inbound and outbound traffic management to ensure end-to-end performance. This document provides the QoS goals for both the campus edge devices and the campus local area network devices.
IP QoS Strategy:
There are two environments to be considered in any end-to-end QoS implementation. The wide area network and the local campus switched infrastructure. This strategy uses Diffserv for the WAN (when it is implemented in NWVng and/or Internet2) and IP Precedence for the current IP campus infrastructures. It uses the Ethernet 802.1q and 802.1p standards for the campus switched Ethernet LAN. Certain IP QoS values have been reserved in the Diffserv specification specifically to provide backward compatibility with current uses of IP Precedence. This QoS strategy uses these same reserved values to minimize the campus impact of the eventual migration to Diffserv.
All end points create IP packets as the first step in their outbound traffic. This provides a field in the IP packet header, the Type of Service (TOS) field that is used to establish the packet priority in the IP environment. IP Precedence uses the first 3 bits of the field to indicate packet priority in IP Precedence based QoS implementations. Diffserv uses the first 6 bits of this same field to establish the packet priority in Diffserv implementations. This QoS strategy uses the first 3 bits to implement IP Precedence based QoS as the first step. The second step will add the remaining 3 bits to convert to Diffserv.
All end points create Ethernet packets as the second step in their outbound traffic. They should all conform to the 802.1q and 802.1p standards. This provides a field in the Ethernet packet header, the Class of Service (COS) field that is used to establish the packet priority in the switched Ethernet infrastructure.
Every device that a packet passes through may ignore, use, or change the contents of the TOS and/or the COS fields. Since all packets have the fields and all network devices can use them for their own purposes, the fields are the core elements of all types of QoS implementations.
Wide Area Network (WAN) Implementation Plan:
Campus ATM Switches:
The current implementation of FIFO queuing in the ATM switches allows the campus edge router to control the actual transmission order of inbound and outbound traffic. No configuration changes are required in the ATM switches to implement QoS.
Both inbound and outbound campus traffic (relative to the campus) is managed at the campus edge router. Outbound traffic is managed at the ATM port. Inbound traffic is managed at the Ethernet port(s).
The IP Precedence value in the TOS field of outbound traffic is used to determine the transmission order of the traffic (highest priority first). The campus edge router must be configured to check all traffic from the campus LAN and set the TOS field value to conform to the VCCS wide area network priority policies.
The IP Precedence value of the TOS field of all inbound traffic is used to set the traffic priority in the receiving campus infrastructure. The campus edge router must be configured to check all traffic from the WAN and set the TOS field value to conform to the campus priority policy of the receiving campus.
See Appendix A for the VCCS IP Precedence TOS field priority policy recommendations.
Campus edge routers are also the source of inbound campus Ethernet traffic. As such, they must also set the value in the Ethernet COS field to set the proper packet priority in the campus LAN infrastructure. Note that the Ethernet headers are automatically stripped from outgoing packets by the campus edge routers, so there is no need to deal with them.
See Appendix A for the VCCS Ethernet COS field priority policy recommendations.
Campus LAN Implementation Plan:
In order to support end-to-end QoS, every campus router and Ethernet switch that the traffic passes through must also support IP Precedence or the Ethernet standards.
There are two elements in an IP Precedence implementation. The first is ensuring that the IP Precedence field contains the proper IP Precedence bit settings. The second is to ensure that all of the routers that the traffic passes through are IP Precedence enabled.
There are also two elements in an Ethernet QoS implementation. The first is ensuring that the Ethernet COS field contains the proper Ethernet bit settings. The second is to ensure that all of the Ethernet switches that the traffic passes through are Ethernet QoS enabled.
Internal Campus Routers:
Internal campus routers are those routers in the campus infrastructure that are not directly connected to NWVng.
The campus edge router sets the value in the IP Precedence field of all traffic received from NWVng and Internet2. It can be assumed to be correct for campus traffic. However, it may have to be changed by an internal campus router to support forwarding the traffic over a slow speed connection (such as over a T1 link to an off campus location).
Outbound traffic must have the IP Precedence field value set by the first router that the traffic encounters to ensure that the traffic gets the proper IP priority while traveling through the campus infrastructure. If an internal campus router is the first router encountered by the traffic, it must perform this task. The router will strip the Ethernet header from the packet, so there is no need to deal with the Ethernet COS field at this point.
See Appendix A for the VCCS IP Precedence TOS field priority policy recommendations.
Campus Ethernet Switches:
In order to provide true end-to-end QoS, the first Ethernet switch encountered by traffic generated by all campus end points must set the value of the COS field. Use the appropriate commands for the Ethernet switches to set the correct values. This must be done for all traffic, to ensure that both inter and intra campus traffic is properly prioritized.
Particular attention must be paid to the queuing mechanisms used by the switches. Some models can be configured to queue traffic in multiple output queues. This prioritizes the Ethernet traffic across the switched portion of the campus infrastructure.
The minimum number of queues is one. In this case, all traffic is sent in the order it is received from the end device. There is no mechanism to prioritize traffic in this environment.
When there are two queues available, they are generally allocated to high priority traffic and low priority traffic. In this case, IP telephone and H.323 video used for scheduled distance learning should be placed in the high priority queue and the remainder of the traffic should be placed in the low priority queue.
When there are three or more queues, IP telephone traffic should be placed in the highest priority queue, H.323 video used for scheduled distance learning should be placed in the second highest priority queue and the remainder of the traffic should be placed in the low priority queue.
See Appendix A for the VCCS Ethernet COS field priority policy recommendations.
Appendix A
QoS Field Value Recommendations
Ethernet
COS Field VCCS
Recommended Uses
Bit Setting 111 - Highest priority, reserved for network control traffic
Bit Setting 110 - Reserved for inter network control traffic
Bit Setting 101 - Recommended for VCCS VoIP telephone traffic
Bit Setting 100 - Recommended for VCCS H.323 scheduled instruction traffic
Bit Setting 011 - Recommended for H.323 desktop video traffic
Bit Setting 010 - Not used in this recommendation
Bit Setting 001 - Not used in this recommendation
Bit Setting 000 - Lowest priority, used for all other traffic
The current Cisco recommendation is to set the COS field value to same value that is in the TOS field. This gives traffic the same relative priority in the Ethernet and IP environments. The above recommendations comply with the Cisco recommendation.
Note that only devices authorized to use bit settings 111 and 110 should be allowed use them. The value for any other device using them should be changed to the default value of 000.
IP Precedence
TOS Field VCCS Recommended Uses
Bit Setting 111 - Highest priority, reserved for network control traffic
Bit Setting 110 - Reserved for network control traffic
Bit Setting 101 - Recommended for VCCS VoIP telephone traffic
Bit Setting 100 - Recommended for VCCS H.323 scheduled instruction traffic
Bit Setting 011 - Recommended for H.323 desktop video traffic
Bit Setting 010 - Not used in this recommendation
Bit Setting 001 - Not used in this recommendation
Bit Setting 000 - Lowest priority, used for all other traffic
These values are compatible with existing IP Precedence standards and are the reserved values in the Diffserv standard for backward compatibility.
Diffserv
TOS Field VCCS
Recommended Uses
Bit Setting 111000 - Highest priority, reserved for network control traffic
Bit Setting 110000 - Reserved for network control traffic
Bit Setting 101000 - Recommended for VCCS VoIP telephone traffic
Bit Setting 100000 - Recommended for VCCS H.323 scheduled instruction traffic
Bit Setting 011000 - Recommended for H.323 desktop video traffic
Bit Setting 010000 - Not used in this recommendation
Bit Setting 001000 - Not used in this recommendation
Bit Setting 000000 - Lowest priority, used for all other traffic
These values are backward compatible with existing IP Precedence standards and map to the default traffic priorities defined in Diffserv.