How to absolutely guarantee QoS with network traffic

Perhaps this is an oxymoron.

Below, network performance measuring vendor - Plixer International, gives the Cisco Subnet its How-To Tutorial on guaranteeing QOS with network traffic: "As timely transmission of data across the network becomes increasingly important, so too does the science of QoS. How do we know for sure that millisecond sensitive packets are getting the priority we want them to receive as they traverse the IT infrastructure?" Cisco DiffServ to the rescue?

Plixer International: Not so fast. DiffServ (Differentiated Services) is merely an architecture for marking the ToS field of datagrams with a DSCP value. The switches and routers in the path can be configured to prioritize packets with specific DSCP values. Certainly it is a good idea to take advantage of DiffServ if you are utilizing VoIP and the like, however, how do we know for sure that all frames are marked correctly?

According to CCIE# 15255 - Chuck Cahoon - a solutions architect for Cisco online partner - CDW: "Setting up DiffServ for VoIP and video can be both a science and an art. You don’t need to use all the DSCP values; many times starting off using three or four is plenty. If nothing else, make sure VoIP has a DSCP value of EF. "You have to be careful because if you don’t assign the DSCP values needed by the business, anything undefined is given a DSCP value of 00. I often point our customers to the Classifying VoIP Signaling and Media with DSCP for QoS page on Cisco’s web site." IP SLA does the trick?

Plixer International: IP SLA (IP Service Level Agreements) is helpful as a measurement tool, but it doesn't provide definitive data. IP SLA is a simulation utility found in the Cisco IOS that can make TCP connections to other IP devices and record the time it takes to receive a reply. IP SLA can be used for VoIP Jitter probes and other routine time checks. Yes it's very useful, but it's not a solution to wipe out QoS issues and guarantee delivery. IP SLA in Plixer's opinion does provide evidence that things are good or bad, however, it's best recognized as useful when paired up with other measurement technologies.

CBQoS is the answer?

Plixer International: CBQoS (Class Based Quality of Service) is headed in the right direction. CBQoS is used to mark datagrams with an assigned ToS or DSCP value before they leave the router or switch. CBQoS is a Cisco feature set that is part of IOS 12.4(4)T and above, as well as available at no additional cost. The QoS statistics provided by CBQoS are gathered via SNMP polling and give detailed information about the defined QoS policies applied to interfaces and class-based traffic patterns. How does the router know what to assign? The administrator sets up the policies on the router. This technology is especially important to configure when congestion occurs. Why? Well, if any of the traffic dropped during congestion was because of the rules defined in a policy, CBQoS reports the amount of traffic discarded. This metric is viewed hop-by-hop which can be tedious as it really wasn’t designed as an end-to-end measurement.

Use egress NetFlow exports?

Plixer International: If the router only supports NetFlow v5, NetFlow reporting tools have to display outbound traffic using inbound ‘ingress’ flows. This ‘cheat’ is required in NetFlow v5 because flows are only collected when traffic comes in on an interface. Because of this, outbound traffic is displayed with the same DSCP value it came in with. This could report a lie! If you are using NetFlow v9 it is still a lie unless ‘egress’ flows are enabled. Make sure your NetFlow Reporting tool can handle an amalgamation of ingress and egress being enabled. Egress NetFlow with Cisco WAAS (Best Practices for Cisco WAAS Reporting using NetFlow) is especially important. Enabling Egress NetFlow exports ensures that the flows are sent out with the actual DSCP value that may or may not have changed. Loaded with this information in the database you can report on the data as it comes out the other end of the network or you can monitor the chain of events as the flow traverses each hop in the network. But there is a big problem with NetFlow; it doesn’t report on latency.

What does this all mean?

Plixer International: You need several different tools to monitor and measure the above services. Using good proactive mechanisms such as NetFlow Reporting thresholds, you can help ensure QoS to the critical business applications. Without the above, you will generally be forced to rely on the science of traditional TCP or ECT (Explicit Congestion Notification) to get the data delivered or possibly take the 'buy more bandwidth' approach.

So, how can we absolutely guarantee traffic?

Plixer International: You can’t guarantee delivery in networking because it isn’t a guaranteed delivery technology. It can however be measured and monitored carefully. DiffServ domains can be configured via CBQoS, IP SLA probes can be setup, egress NetFlow can be collected and thresholds can be setup. As a network professional, you'll have the great job of being able to stay on top of this issue and hopefully guarantee delivery the best way you can.

View more Cisco How-To Tutorials.


What's your take, do you agree with CCIE# 15255 - Chuck Cahoon and Plixer International?

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