[Cerowrt-devel] Ideas on how to simplify and popularize bufferbloat control for consideration.

[Sebastian Moeller]

Hi David,

On Jul 26, 2014, at 22:21 , David Lang <david at lang.hm> wrote:

> On Sat, 26 Jul 2014, Sebastian Moeller wrote:
> 
>> Hi David,
>> 
>> 
>> On Jul 25, 2014, at 22:57 , David Lang <david at lang.hm> wrote:
>> 
>>> On Fri, 25 Jul 2014, Wes Felter wrote:
>>> 
>>>> The Netgear stock firmware measures bandwidth on every boot or link up (not sure which) and I would suggest doing the same for CeroWRT.
>>>> 
>>>> Do you need to measure Internet bandwidth or last mile bandwidth? For link bandwidth it seems like you can solve a lot of problems by measuring to the first hop router. Does the packer pair technique work on TDMA link layers like DOCSIS?
>>> 
>>> The trouble is that to measure bandwidth, you have to be able to send and receive a lot of traffic.
>> 
>> 	Well that is what you typically do, but you can get away with less measurement traffic: in an ideal quiescent network sending two packets back to back should give you the bandwidth (packet size / incoming time difference of both packets), or send two packets of different size (needs synchronized clocks, then difference of packet sizes / difference of transfer times).
> 
> Except that your ideal network doesn't exist in the real world. You are never going to have the entire network quiescent, the router you are going to be talking to is always going to have other things going on, which can affect it's timing.

	Sure, the two packets a required per measurement,  guess I would calculate the average and confidence interval over several of these (potentially by a moving window) to get a handle on the variability. I have done some RTT measurements on a ADSL link and can say that realistically one needs in the hundreds data points per packet size. This sounds awe full, but at least it does not require to saturate the link and hence works without dedicated receivers on the other end...

> 
>>> unless the router you are connecting to is running some sort of service to support that,
>> 
>> 	But this still requires some service on the other side. You could try to use ICMP packets, but these will only allow to measure RTT not one-way delays (if you do this on ADSL you will find the RTT dominated by the typically much slower uplink path). If network equipment would be guaranteed to use NTP for decent clock synchronization and would respond to timestamp ICMP messages with timestamp reply measuring bandwidth might be “cheap” enough to keep running in the background, though.
>> 	Since this looks too simple there must be a simple reason why this would fail. (It would be nice if ping packets with timestamps would have required the echo server top also store its incoming timestamp in the echo, but I digress)
>> 	I note that gargoyle uses a sparse stream of ping packets to a close host and uses increases in RTT as proxy for congestion and signal to throttle down stream link…
> 
> As you say, anything that requires symmetrical traffic (like ICMP isn't going to work, and routers do not currently offer any service that will.

	Well I think the gargoyle idea is feasible given that there is a reference implementation out in the wild ;). 

> 
> you also can't count on time being synced properly.

	Quick testing today drove him that message (ICMP time requests showing receive time before originating times, quite sobering). Naive me had thought that NTP would guarantee <1ms deviation from reference time, but I just figured it is rather low ms to 100ms, so basically useless for one-way delay measurements for close hosts….

> Top Tier companies have trouble doing that in their dedicated datacenters, depending on it for this sort of testing is a non-starter

	Agreed.

> 
>>> you can't just test that link, you have to connect to something beyond that.
>> 
>> 	So it would be sweet if we could use services that are running on the machines anyway, like ping. That way the “load” of all the leaf nodes of the internet continuously measuring their bandwidth could be handled in a distributed fashion avoiding melt-downs by synchronized measurement streams…
> 
> Well, let's talk about what we would like to have on the router
> 
> As I see it, we want to have two services
> 
> 1. a service you send a small amount of data to and it responds by sending you a large amount of data (preferrably with the most accurate timestamps it has and the TTL of the packets it received)
> 
> 2. a service you send a large amount of data to and it responds by sending you small responses, telling you how much data it has received (with a timestamp and what the TTL of the packets it received were)
> 
> questions:
> 
> A. Protocol: should these be UDP/TCP/SCTP/raw IP packets/???
> 
> TCP has the problem of slow start so it would need substantially more traffic to flow to reach steady-state.
> 
> anything else has the possibility of taking a different path through the router/switch software and so the performance may not be the same.

	You thing UDP would not work out?

> 
> B. How much data is needed to be statistically accurate?
> 
> Too many things can happen for 1-2 packets to tell you the answer. The systems on both ends are multi-tasking, and at high speeds, scheduling jitter will throw off your calculations with too few packets.

	Yeah, but you can (to steal an I idea from Rick Jones netperf) just keep measuring until the confidence interval around the mean of the data falls below a set magnitude. But for the purpose of traffic shaping you do not need the exact link bandwidth anyway just a close enough proxy to start the search for a decent set point from a reasonable position. I think that the actual shaping rates need to be iteratively optimized.

> 
> C. How can this be prevented from being used for DoS attacks, either against the thing running the service or against someone else via a reflected attack if it's a forgable protocol (i.e. UDP)

	Well, if it only requires a sparse packet stream it is not going to be to useful for DOS attacks, 

> 
> One thought I have is to require a high TTL on the packets for the services to respond to them. That way any abuse of the service would have to take place from very close on the network.
> 
> Ideally these services would only respond to senders that are directly connected, but until these services are deployed and enabled by default, there is going to be a need to be the ability to 'jump over' old equipment. This need will probably never go away completely.

	But if we need to modify DSLAMs and CMTSs it would be much nicer if we could just ask nicely what the current negotiated bandwidths are ;)

> 
> 
> Other requirements or restrictions?

	I think the measurement should be fast and continuous…

Best Regards
	Sebastian

> 
> David Lang