SSL Performance

SSL performance is determined by a few factors, including hardware, Avi Service Engine scaling, and the ciphers and certificates used. Performance can be broken down into three primary benchmark numbers:

  • Transactions Per Second — Primarily gated by the available CPU
  • Bulk throughput— Dependent upon CPU and the maximum packets per second through Avi Vantage
  • Concurrent connections — Memory is the limiting factor

While each of these benchmark numbers is useful for gauging capacity, it is equally important to understand the impact of real-world scenarios which are not tailored for one metric at a time. This article will attempt to illustrate the real-world performance that can be expected.  

Scale Up Service Engine Hardware

CPU

Intel and AMD have steadily enhanced their processors such as the AES-NI to offload more cryptographic workload to dedicated rather than general-purpose instructions.

Avi Vantage supports x86 based processors including those from AMD and Intel.

Leveraging AMD’s and Intel’s processors with AES-NI and similar enhancements steadily enhances Avi Vantage’s performance with each successive generation of the processor.

vCPUscaleout1

Increasing CPU cores allocated to a Service Engine provides near-linear increases in SSL performance, as CPU is the primary constraint for both transactions per second and bulk throughput. A single SE can scale up to 36 vCPU cores. Within a Service Engine, a vCPU core will be given a dispatcher role. It will interface with a NIC and distribute network flows across the other cores within the system, effectively load balancing traffic to other CPU cores. Each core is then responsible for terminating TCP, SSL, and other processing determined by the virtual service configuration. vCPU 0 in the diagram is acting as the dispatcher and may also handle some percent of SSL traffic if it has available capacity. By using a system of internally load balancing across CPU cores, Avi Vantage can scale linearly across ever-increasing capacity.

Memory

Memory allocation for a Service Engine, which primarily impacts the number of concurrent connections, can be anywhere between 1 and 128 GB. Refer to SE Memory Consumption for a verbose description of expected concurrent SSL connections.  As a general rule, SSL connections consume about twice as much memory as HTTP layer 7 connections, and four times as much memory as layer 4 with TCP proxy.

NIC

Throughput through a Service Engine can be a gating factor for the bulk throughput and sometimes for TPS.  The throughput for an SE is highly dependent upon the platform. For maximum performance on a single SE, Avi recommends bare metal or Linux cloud deployments, using an Intel 10 Gb/s or greater NIC capable of supporting DPDK.

scale illustrated

Scale Out Across Service Engines

Similar to how Avi Vantage scales capacity across CPU cores, Avi Vantage may also scale traffic across multiple Service Engines. This is primarily useful when CPU or memory are the limiting factor for a virtual service, as it allows linearly scaling.  It is beneficial for increasing throughput as well.  Using Avi Vantage’s native autoscale feature allows a virtual service to be striped across four Service Engines.  Using ECMP, a virtual service may be scaled out across up to 32 Service Engines.

Refer to SE Autoscaling for in-depth description.

Transactions Per Second

In addition to the hardware factors outlined above, maximum TPS is dependent on the the negotiated settings of the SSL session between the client and Avi Vantage.  Avi supports both RSA and elliptic curve certificates.  The type of certificate used, along with the cipher selected during negotiation, determines the CPU cost of establishing the session. At a high level, RSA 2k keys are about 4x more computationally expensive compared to EC.  EC using perfect forward secrecy (ECDHE) is about 15% more expensive than EC without PFS (ECDH).  As a general rule, assume 500 TPS per core for RSA and 2000 TPS for EC.

SSL Performance

Avi Networks strongly recommends using EC with PFS, which provides the best performance and the best possible security. RSA certificates may still be used as a backup for clients that do not support current industry standards. Refer to RSA versus EC certificate priority for more on this. Avi Vantage’s default settings prioritize EC over RSA, and PFS over non-PFS.

The table below illustrates the average performance seen on a Service Engine running on a virtual machine in VMware ESX 6, using a Xeon E5-2630 v3 @ 2.4-GHz processor.  These numbers are generated with SSL session reuse disabled.

Type PFS 2 Core 4 Core 8 Core 16 Core
ECDH-ECDSA-AES256-SHA EC N 5107 9352 16091 34543
ECDHE-ECDSA-AES256-SHA EC Y 4011 7950 12173 26040
ECDHE-ECDSA-AES128-GCM-SHA256 EC Y 4543 9237 15979 33994
AES128-GCM-SHA256 RSA N 1476 2740 5314 9586
DHE-RSA-AES128-GCM-SHA256 RSA Y 828 1538 2939 5220

To achieve greater TPS numbers, use faster processors, bare metal servers rather than virtual machines, more CPU cores, and scale across multiple Service Engines.  Real-world performance will likely be higher due to SSL session reuse.

Bulk Throughput

The maximum throughput for an SSL terminated virtual service depends on CPU as well as the NIC or hypervisor.

Using multiple NICs for client and server traffic can reduce the possibility of congestion or NIC saturation. The maximum packets per second for virtualized environments vary dramatically and will be the same limit regardless of the traffic being SSL or unencrypted HTTP. For best throughput, Avi Networks recommends bare metal or Linux cloud servers with Avi-supported Intel NICs capable of DPDK.

Refer to Avi performance datasheet numbers for SSL throughput numbers. These numbers are generated with standard ciphers (such as those listed above). Using more esoteric or expensive ciphers can have a negative impact on throughput. Similarly, using less secure ciphers, such as RC4-MD5, will provide better performance, but are also not recommended by security experts.

The bulk throughput numbers, like any single benchmark metric, are based on large transaction sizes with fewer TPS. To estimate CPU requirements for real world scenarios, add the requirements for bulk throughput plus TPS.

Typically, SSL termination is only performed on the client-to-Avi-Vantage side of the connection, not Avi Vantage to the server. If SSL re-encryption is required, generally the TPS impact is negligible on the Service Engine’s CPU, since most of the CPU cost for establishing a new SSL session is on the server, not the client. For the bulk throughput, the impact to the Service Engine’s CPU will be double for this metric.

Concurrent Connections

Often overlooked when capacity planning for SSL is the impact on concurrent connections. Often administrators hear numbers of tens of millions of concurrent numbers achieved by a load balancer, so no further attention is paid to this metric. The concurrent benchmark numbers floating around are generally for layer 4 in a pass-through, or non-proxy mode. In other words, there are many orders of magnitude greater than what will be achieved. As a rough yet conservative estimate, assume 40kB of memory per SSL terminated connection in the real world. The amount of HTTP header buffering, caching, compression, and other features play a role in the final number.

Refer to SE Memory Consumption for more details, including methods for optimizing for greater concurrency.