#webperf As we continue to find new ways to make connections more efficient, capacity planning must look to other metrics to ensure scalability without compromising performance.

big picture cps dps

Infrastructure metrics have always been focused on speeds and feeds. Throughput, packets per second, connections per second, etc… These metrics have been used to evaluate and compare network infrastructure for years, ultimately being used as a critical component in data center design.

This makes sense. After all, it's not rocket science to figure out that a firewall capable of handling 10,000 connections per second (CPS) will overwhelm a next hop (load balancer, A/V scanner, etc… ) device only capable of 5,000 CPS.

Or will it? The problem with old skool performance metrics is they focus on ingress, not egress capacity. With SDN pushing a new focus on both northbound and southbound capabilities, it makes sense to revisit the metrics upon which we evaluate infrastructure and design data centers.


As we've progressed from focusing on packets to sessions, from IP addresses to users, from servers to applications, we've necessarily seen an evolution in the intelligence of network components. It's not just application delivery that's gotten smarter, it's everything. Security, access control, bandwidth management, even routing (think NAC), has become much more intelligent. But that intelligence comes at a price: processing. That processing turns into latency as each device takes a certain amount of time to inspect, evaluate and ultimate decide what to do with the data.

And therein lies the key to our conundrum: it makes a decision. That decision might be routing based or security based or even logging based. What the decision is is not as important as the fact that it must be made. SDN necessarily brings this key differentiator between legacy and next-generation infrastructure to the fore, as it's just software-defined but software-deciding networking. When a switch doesn't know what to do with a packet in SDN it asks the controller, which evaluates and makes a decision. The capacity of SDN – and of any modern infrastructure – is at least partially determined by how fast it can make decisions.

Examples of decisions:

  1. URI-based routing (load balancers, application delivery controllers)
  • Virus-scanning 
  • SPAM scanning
  • Traffic anomaly scanning (IPS/IDS)
  • SQLi / XSS inspection (web application firewalls)
  • SYN flood protection (firewalls)
  • BYOD policy enforcement (access control systems)
  • Content scrubbing (web application firewalls)
  • The DPS capacity of a system is not the same as its connection capacity, which is merely the measure of how many new connections a second can be established (and in many cases how many connections can be simultaneously sustained). Such a measure is merely determining how optimized the networking stack of any given solution might be, as connections – whether TCP or UDP or SMTP – are protocol oriented and it is the networking stack that determines how well connections are managed.

    The CPS rate of any given device tells us nothing about how well it will actually perform its appointed tasks. That's what the Decisions Per Second (DPS) metric tells us.


    Reality is that most systems will have a higher CPS compared to its DPS. That's not necessarily bad, as evaluating data as it flows through a device requires processing, and processing necessarily takes time. Using both CPS and DPS merely recognizes this truth and forces it to the fore, where it can be used to better design the network.

    A combined metric helps design the network by offering insight into the real capacity of a given device, rather than a marketing capacity. When we look only at CPS, for example, we might feel perfectly comfortable with a topological design with a flow of similar CPS capacities. But what we really want is to make sure that DPS –> CPS (and vice-versa) capabilities were matched up correctly, lest we introduce more latency than is necessary into a given flow.


    What we don't want is to end up with is a device with a high DPS rate feeding into a device with a lower CPS rate. We also don't want to design a flow in which DPS rates successively decline. Doing so means we're adding more and more latency into the equation. The DPS rate is a much better indicator of capacity than CPS for designing high-performance networks because it is a realistic measure of performance, and yet a high DPS coupled with a low CPS would be disastrous. Luckily, it is almost always the case that a mismatch in CPS and DPS will favor CPS, with DPS being the lower of the two metrics in almost all cases.

    What we want to see is as close a CPS:DPS ratio as possible. The ideal is 1:1, of course, but given the nature of inspecting data it is unrealistic to expect such a tight ratio. Still, if the ratio becomes too high, it indicates a potential bottleneck in the network that must be addressed. For example, assume an extreme case of a CPS:DPS of 2:1. The device can establish 10,000 CPS, but only process at a rate of 5,000 DPS, leading to increasing latency or other undesirable performance issues as connections queue up waiting to be processed. Obviously there's more at play than just new CPS and DPS (concurrent connection capability is also a factor) but the new CPS and DPS relationship is a good general indicator of potential issues.

    Knowing the DPS of a device enables architects to properly scale out the infrastructure to remediate potential bottlenecks. This is particularly true when TCP multiplexing is in play, because it necessarily reduces CPS to the target systems but in no way impacts the DPS. On the ingress, too, are emerging protocols like SPDY that make more efficient use of TCP connections, making CPS an unreliable measure of capacity, especially if DPS is significantly lower than the CPS rating of the system.

    Relying upon CPS alone – particularly when using TCP connection management technologies - as a means to achieve scalability can negatively impact performance. Testing systems to understand their DPS rate is paramount to designing a scalable infrastructure with consistent performance.