Lossless and transparent flow control

Acceleration operates on any TCP connection passing through two appliances (one at the sending site and one at the receiving site), or a Citrix SD-WAN WANOP appliance and a Citrix SD-WAN WANOP Plug-in. Although the above figure shows a network of two appliances, any appliance can accelerate connections between any number of other appliance-equipped sites simultaneously. This allows a single appliance to be used per site, rather than two per link.

Like any gateway, the Citrix SD-WAN WANOP appliance meters packets onto the link. Unlike ordinary gateways, however, it imposes transparent, lossless flow control on each link segment, including:

  • The LAN segment between the sender and the sending appliance

  • The WAN segment between the sending and receiving appliances

  • The LAN segment between the receiving appliance and the receiver

Flow control can be managed independently for each of these three segments. The segments are partly decoupled, so each can have its speed controlled independently. This is important when a connection’s speed needs to be ramped up or down quickly to its fair bandwidth share, and is also important as a means of supporting enhanced WAN algorithms and compression.

The TCP protocol is designed to make every TCP connection attempt to increase its bandwidth usage continuously. However, the link bandwidth is limited. The result is that the links become overrun. Citrix SD-WAN WANOP flow control keeps the TCP connections flowing at just the right speed. The link is filled but is never overrun, so queuing latency and packet losses are minimized, while throughput is maximized.

With ordinary TCP, long-running connections (which have had time to seize all the bandwidth) tend to squeeze out short-running connections. This problem, which ruins interactive responsiveness, does not occur with flow control .

Flow control is a standard feature on all appliances in the Citrix SD-WAN WANOP family.

Figure 1. Acceleration Enhances Performance Transparently

Localized image

Lossless and transparent flow control