Why packet loss is important
Some degree of packet loss is almost always present in every wide area network. At times it is due to link quality, and at other times it is intended: core routers avoid congestion and buffer bloat by randomly dropping packets when queues grow too large.
- Loss has a severe impact on TCP performance: TCP throughput over a 100 Mbps link is limited to 1 Mbps at 80 ms and 2% loss.
- More businesses connect their offices via low-cost VPN links. These typically have a 1% loss rate, which is 10 times higher than MPLS links.
- SLAs provide a false sense of security. A 3%+ loss for an hour a day means providers can still meet a very low monthly average loss rate.
- Packet loss is higher in emerging and developing countries: on average 2% or more.
How packet loss recovery works
Wanos provides three layers of loss recovery.
- Fast retransmit replay: loss recovery monitors packet sequences and the real-time round-trip time of the link between two Wanos peers. When a packet is lost, a timer expires and the packet is retransmitted. Packet order correction ensures the end hosts are unaware of the losses on the WAN, and the TCP layer does not reduce the transfer rate.
- Fast retransmit request: when a lost packet is detected, a replay request is sent to the peer, so the lost packet is detected faster and replayed sooner.
- Forward error correction: a special parity packet is sent for every group of 5, 10 or 20 packets. If a packet in the group is lost, it is reconstructed in real time. FEC adds an acceleration benefit in conjunction with packet loss recovery and TCP acceleration, and can be enabled when required.
Packet loss recovery demo
In this demo compression, deduplication and the TCP Accelerator are disabled. The link has 100 ms latency and a 2% packet loss rate. With packet loss recovery and FEC enabled, throughput improves up to 20x.
Configuration
From version 4 onward packet loss recovery is configured automatically. Set the global WAN Tx rate to the WAN upstream speed for optimal packet loss recovery. Higher recovery levels suit higher-latency links.