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Does size Matter? Capacity considerations when selecting a DDoS mitigation service

Internet pipes have gotten fatter in the last decade. We have gone from expensive 1 Mbps links to 1 Gbps links, which are available at a relatively low cost. Most enterprises have at least a 1 Gbps ISP link to their data center, many have multiple 1 Gbps links at each data center. In the past, QoS, packet shaping, application prioritization, etc., used to be a big deal, but now we just throw more capacity to solve any potential performance problems.

However, when it comes to protecting your infrastructure from DDoS attacks, 1 Gbps, 10Gbps or even 40Gbps is not enough capacity. This is because in 2019, even relatively small DDoS attacks are a few Gbps in size, and the larger ones are greater than 1 Tbps.

For this reason, when security professionals design a DDoS mitigation solution, one of the key considerations is the capacity of the DDoS mitigation service. That said, it isn’t easy to figure out which DDoS mitigation service actually has the capacity to withstand the largest DDoS attacks. This is because there are a range of DDoS mitigation solutions to pick from, and capacity is a parameter most vendors can spin to make their solution appear to be flush with capacity.

Let us examine some of the solutions available and understand the difference between their announced capacity and their real ability to block a large bandwidth DDoS attack.

On-premises DDoS Mitigation Appliances 

First of all, be wary of any Router, Switch, or Network Firewall which is also being positioned as a DDoS mitigation appliance. Chances are it does NOT have the ability to withstand a multi Gbps DDoS attack.

There are a handful of companies that make purpose built DDoS mitigation appliances. These devices are usually deployed at the edge of your network, as close as possible to the ISP link. Many of these devices canmitigate attacks which are in the 10s of Gbps, however, the advertised mitigation capacity is usually based on one particular attack vector with all attack packets being of a specific size.

Irrespective of the vendor, don’t buy into 20/40/60 Gbps of mitigation capacity without quizzing the device’s ability to withstand a multi-vector attack, the real-world performance and its ability to pass clean traffic at a given throughput while also mitigating a large attack. Don’t forget, pps is sometimes more important than bps, and many devices will hit their pps limit first. Also be sure to delve into the internals of the attack mitigation appliance, in particular if the same CPU is used to mitigate an attack while passing normal traffic. The most effective devices have the attack “plane” segregated from the clean traffic “plane,” thus ensuring attack mitigation without affecting normal traffic.

Finally, please keep in mind that if your ISP link capacity is 1 Gbps and you have a DDoS mitigation appliance capable of 10Gbps of mitigation, you are NOT protected against a 10Gbps attack. This is because the attack will fill your pipe even before the on-premises device gets a chance to “scrub” the attack traffic.

Cloud-based Scrubbing Centers

The second type of DDoS mitigation solution that is widely deployed is a cloud-based scrubbing solution. Here, you don’t install a DDoS mitigation device at your data center. Rather, you use a DDoS mitigation service deployed in the cloud. With this type of solution, you send telemetry to the cloud service from your data center on a continuous basis, and when there is a spike that corresponds to a DDoS attack, you “divert” your traffic to the cloud service.

There are a few vendors who provide this type of solution but again, when it comes to the capacity of the cloud DDoS protection service, the devil is in the details. Some vendors simply add the “net” capacity of all the ISP links they have at all their data centers. This is misleading because they may be adding the normal daily clean traffic to the advertised capacity — so ask about the available attack mitigation capacity, excluding the normal clean traffic.

Also, chances are the provider has different capacities in different scrubbing centers and the net capacity across all the scrubbing centers may not be a good reflection of the scrubbing center attack mitigation capacity  in the geography of your interest (where your data center is located).

Another item to inquire about is Anycast capabilities, because this gives the provider the ability to mitigate the attack close to the source. In other words, if a 100 Gbps attack is coming from China, it will be mitigated at the scrubbing center in APAC.

Finally, it is important that the DDoS mitigation provider has a completely separate data path for clean traffic and does not mix clean customer traffic with attack traffic.

Content Distribution Networks

A third type of DDoS mitigation architecture is based upon leveraging a content distribution network (CDN) to diffuse large DDoS attacks. When it comes to the DDoS mitigation capacity of a CDN however, again, the situation is blurry.

Most CDNs have 10s, 100s, or 1000s of PoPs geographically distributed across the globe. Many simply count the net aggregate capacity across all of these PoPs and advertise that as the total attack mitigation capacity. This has two major flaws. It is quite likely that a real world DDoS attack is sourced from a limited number of geographical locations, in which case the capacity that really matters is the local CDN PoP capacity, not the global capacity at all the PoPs.

Second, most CDNs pass a significant amount of normal customer traffic on all of the CDN nodes, so if a CDN service claims its attack mitigation capacity is 40 Tbps , it may be counting in 30Tbps of normal traffic. The question to ask is what is the total unused capacity, both on a net aggregate level as well as within a geographical region.

ISP Provider-based DDoS Mitigation

Many ISP providers offer DDoS mitigation as an add-on to the ISP pipe. It sounds like a natural choice, as they see all traffic coming into your data center even before it comes to your infrastructure, so it is best to block the attack within the ISP’s infrastructure – right?

Unfortunately, most ISPs have semi-adequate DDoS mitigation deployed within their own infrastructure and are likely to pass along the attack traffic to your data center. In fact, in some scenarios, some ISPs could actually black hole your traffic when you are under attack to protect their other customers who might be using a shared portion of their infrastructure. The question to ask your ISP is what happens if they see a 500Gbps attack coming towards your infrastructure and if there is any cap on the maximum attack traffic.

All of the DDoS mitigation solutions discussed above are effective and are widely deployed. We don’t endorse or recommend one over the other. However, one should take any advertised attack mitigation capacity from any provider with a grain of salt. Quiz your provider on local capacity, differentiation between clean and attack traffic, any caps on attack, and any SLAs. Also, carefully examine vendor proposals for any exclusions.

This blog was written by Dileep Mishra, Systems Engineering Manager at Radware.

Dileep Mishra - May 8 2019

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