Re: [EToSat] Packet lost on last mile network?

[Carsten Bormann <[e-mail]>]

Hi Christian,

Thank you for the numbers.

> The simulation sets a satellite link, 250 Mbps down, 3 Mbps up, 300ms
> latency each way. The packet size is 1500 bytes. The 600ms RTT
> translates to a 1sec timer, approximately. In 1 second, the link carries
> more than 20,000 packets. If 1.6% are lost, that means 320 need to be
> retransmitted. Out of those 320, 5 or 6 will need to be retransmitted a
> second time, incurring an additional 2 seconds delay.

Right.  You don’t say what the RTT is on the WiFi part, but I’d assume that is lower than those 1 s.  So local recovery would have had ample time.

> A lot could be done in improving the behavior of Wi-Fi, that's sure. But
> I really don't like the idea of putting proxies everywhere -- the cost
> in management and decreased reliability would be enormous.

I’m not sure where the proxies would come in — a last-mile deployment of LOOPS would look more like a VPN.  (And I don’t want to talk down the deployment work needed here, but we do know how to deploy VPNs.)

> Besides, the
> deployment issues would be daunting -- who is going to set up al these
> proxies in all these Wi-Fi networks?

(People who want to have good WiFi networks?)

> And, if they are only deployed in a
> fraction of the Wi-Fi network, then the transport folks will still need
> an end-to-end solution.

They sure will.  But that never can be as good as local recovery.

> I have not yet explored the end-to-end solutions to the latency problem,
> but I could. FEC would cure the latency issue, but FEC trades bandwidth
> for latency, so you don't want that in the general case. If I had to do
> something, I would explore "selective FEC". Detect when the document is
> almost done, when there is only 1 second of transmission left. During
> that 1 second, apply something like 1/8 FEC for the remaining stream
> frames. That would make the last 1 second 12.5% less efficient, but
> would limit the "cascade' effect to 125ms.

FEC packets as a replacement for tail probe.  Makes a lot of sense to me.

Grüße, Carsten