Mitigations and solutions for Broadband

Mitigations versus Solutions of Bufferbloat in Broadband
By gettys
I have distinguished in my writing between what I call “mitigations” and “solutions”.

* mitigations are actions we can take, often immediately, which make the situation better, and improve (possibly greatly) the current grim situation. Since they may only work some of the time, and may require conscious thought tuning and action by network operators and users, or have other limitations that are often far from optimal, they won’t work in some circumstances or necessarily be implemented everywhere. Often these mitigations will come at some cost, as in the case today’s posting below.
* solutions are full solutions for a problem that get behavior to something approximating optimal. Sometimes they may be mitigations that can be widely applied in an ISP, even though though they may require thought there. The “just work” for everyone.

But observed facts (e.g. RED or other AQM is far from universally used; more about this in a future post) shows that anything that does not “just work” is often distrusted and under-used (and seldom enabled by default), so such a solution is seldom the optimal solution we should be looking for: really “solving” the problem once and for all. As good engineers and scientists, we should always be striving for “just works” quality solutions, which we don’t have for bufferbloat in all its forms.

The full “solution” for the entire Internet is going to be hard; we need to solve too many different problems (as you will see) at too many points in all paths your data may traverse, to wave a wand eliminate bufferbloat overnight. Some of the point solutions will actually require replacement of hardware, and time to research and engineer such hardware along with economics will often take time. Does that mean we should do nothing? Of course not: we can immediately make the situation much better than it is, particularly for consumer home Internet service. And remember, your competitor will eventually beat you if you sit on your hands.

Gamers and others have been mitigating bufferbloat in broadband for years. Read on. You’ll suffer much less. Mitigation of home router bufferbloat itself will be tomorrow’s installment.
Mitigating Broadband Bufferbloat in the Home Router

The best solution will be to remove the grossly bloated buffers properly, and not to have to hack around this problem in our home router. ISP’s and their vendors may be able to mitigate existing equipment partially (by cutting buffers to something closer to sane points in the CPE); these mitigations will take time, and are not something you can go do today, yourself. Those are also technology dependent; and what can be done there is probably best taken up by the equipment vendors and standards bodies. As in many mitigations, they may come with costs. In the downstream direction, ISP’s not running RED on their head ends may turn it on. So when your network changes, you will need to repeat this process.

I remind you that bufferbloat can occur elsewhere in your path as you make these tests: some ISP’s and content providers do not run with any queue management enabled, so you may have bottlenecks beyond your broadband connection in your path beyond the last mile broadband connection to help confound you (your wireless hop to your home network, and anywhere beyond your broadband headend. My observations of Comcast’s network beyond the CMTS has been very, very clean, confirming what I was told to expect when I had lunch with Comcast. You may not be so lucky with your ISP. That my test site is on a well run network at MIT, peering directly with Comcast has certainly made my life easier. More on the core Internet topic in the future.

Our quest here is to try to overcome what has already happened: we usually have gross bufferbloat in the broadband provider’s equipment, which may or may not be customer replaceable. If it is customer replaceable (e.g. cable modems) we can hope that in a year or three that the market may start to provide routers and broadband gear that implement some rational queue management and behave better.

Here’s what you can do today, if your router supports it. If not, you can go buy a new home router (or install open source router software) today that can mitigate the problem for little cost (\$100 or less). You’ll see why this is a mitigation at best, rather than a solution: it isn’t something you’re going to ask your aged parents to try.

Many mid-range or high end home routers have traffic shaping features. They may be called traffic shaping, or “QOS” (Quality of Service). Some routers I’ve seen (I’ve seen quite a few over the last years) have a single knob to set bandwidth on both directions; they aren’t particularly useful. You want one which lets you adjust bandwidth in both directions. I’ve experimented with several routers: your mileage will vary. Some commercial routers work really well, some less so. Sometimes these routers are marketed as ”gamer routers.” There is probably some gamer’s web site someplace that goes into this in gory detail, with reviews of different routers. If so, please let me know. Facilities also exist in the open source router projects of various sorts, e.g. OpenWRT, DDWRT, Tomato, and Gargoyle. More on this topic below.

The WISEciti research project is also researching the behavior of home routers: if you have an old router, they may be interested in giving it a home.

Our goal is to keep buffers in your upstream broadband link from filling and turn them back into “dark buffers”. We can try to avoid bufferbloat in the broadband device this by transmitting data to it slightly less fast than the broadband device will accept, and ensuring the router forwards data slightly less fast than the broadband device will transmit it. Formally, this is called “traffic shaping”. Gamers have been doing this for years, as they are very latency sensitive and empirically discovered that limiting your bandwidth in this way will have good effects on observed latency. Note you should do traffic shaping before you worry about classifying data (e.g. ensuring your voip gets priority over TCP flows), as the goal here is to mitigate the upstream broadband device’s faults as much as possible.

Some ISP’s provide a home router as part of their service that their wires plug into directly. I have no idea if these routers are usable for the following process. I presume not in the discussion below. In either case, you need the ability to perform traffic shaping.

Plug your router into the ethernet on broadband gear, or at worst, into the ethernet jack of your home router if that is included in your broadband service. We’re trying to mitigate the broadband link problem here, not fix the router’s bufferbloat, which is a later topic.

I recommend monitoring your home connection via smokeping while you try this process. It isn’t clear to me that the bandwidth you get from a broadband carrier is a constant over time, as load occurs. I haven’t explored carefully what happens when my ISP’s network gets busy.

Start “pinging” some nearby site (best is not an ISP router, if only because they may be loaded at times and process ping on the slow path). Note the latency. Saturate the link in an upstream direction (e.g. by copying a file someplace, or uploading a video somewhere; you will probably be able to figure out some way to do so. Note it’s behavior: you’ll very likely find that the latency grows to some value of hundreds of milliseconds or even seconds. You’ll see the latency climb gradually, and then start varying (that’s the behavior you see in my TCP traces).

By using traceroute and ping on the path traceroute exposes, you can figure out which hop is the bottleneck. If it is not the broadband hop, then you need to find some other site to work against.

Next, find out what your provisioned bandwidth is for both directions, nominally. This is what you pay for.

Enter half these values into your home router in the the bandwidth shaping or QOS form, as per your router, having enabled this feature. You may or may not have to reboot your router whenever you adjust the values. Some routers attempt to determine the available bandwidth in some fashion automatically; I have no idea how successful they may be, and expect that features like Comcast’s PowerBoost will confuse them, so manual use is recommended unless you find the router “does the right thing” automatically . I also expect that some routers work better than others in this area.

Again load your link.

Your latency should be only slightly higher than when your line is idle. Exactly how much seems to depend on the router.

You can try approaching or even exceeding your provisioned bandwidth by binary search; when you exceed the available bandwidth, you’ll see the latencies start to rise (slowly). Since the rate at which the buffers will fill is determined by the difference between the broadband bandwidth and your router’s bandwidth to the router, patience is in order to tune the value. Complicating this testing is that some ISP’s dynamically change the available bandwidth (e.g. Comcast’s PowerBoost). You actually do often have more bandwidth temporarily available (if it is available) early in a connection, requiring yet further patience. Did I tell you that you need patience?

Do the same process for downstream bandwidth. There may or may not be similar buffers in the downstream direction in the broadband plant (head end and CPU), and ISP’s may or may not be running RED to control queues in the broadband “head end” equipment itself. Your mileage may vary.

This process works better on some routers than on others. What value you should try is not clear. With one router I tried, the behavior on Comcast was exactly what I would want (low latency) when the router was set to the provisioned bandwidth (Comcast claims they slightly over-provision their customer’s accounts); on another router, I have to reduce the values used by more than 30% from my provisioned bandwidth (which may or may not reflect reality). Even so, I end up on the router I am using today with 20ms latency (I get less than 10ms when idle). Contrast this smokeping with the one in the previous posting: during this one today, I was performing the same kind rdist to MIT that I performed when I found the smoking gun. Not perfect, but way more than an order of magnitude improvement, and also note the packet loss has stopped.
Smokeping of my house after broadband bufferbloat mitigation

Mitigated broadband smokeping

Different routers may not shape the bandwidth to the values you nominally set; before complaining to an ISP that you are not getting what you pay for, please do your homework and verify the actual bandwidth you get out of your router (this is easier said than done: but Dualcomm Technologies makes a cheap port mirroring switch you can afford). The router may not have computed the transformation from the UI to the operating system correctly, and/or forgotten to compensate for packet overhead, or bandwidth shaping may just be broken, and remember, your ISP’s bandwidth includes your packet overhead; your “goodput” should be slightly lower than the marketing BPS of the ISP.

Educating all vendors and network operators about bufferbloat is in order, and exercising your pocket book when selecting hardware and services is essential to recovery from bufferbloat. But let’s only complain about the right problem, in the right directions, and politely please; the mistake is so widespread we are all Bozos on this bus. Please report problems to the router vendor if they are at fault, and only bug the ISP they aren’t giving you what you pay for if you determine they aren’t actually providing what you pay for. No one appreciates angry support calls, and ones that aren’t people’s fault and over which they have no control are very frustrating. I am hoping and presuming my audience is primarily technical, and will be a part of bufferbloat mitigation and solution, rather than creating a support nightmare problem for all involved.

Note that this mitigation may also be partial; congestion on the network interconnecting the broadband head-ends might be reflected into the broadband hop itself at times of congestion.

This mitigation has come at a cost: you have defeated any PowerBoost style bandwidth boost your ISP has been kind enough to give you, and possibly a fraction of your rated bandwidth. This hurts, as the Internet tradition is to share when resources are available, and be fair when to everyone when there is not enough resources available. Short of some attempts (which I haven’t had time to try), such as Paul Bixel’s active QOS control implementation found in the Gargoyle open source router, you are out of luck. I’ll report back on my experiences with Gargoyle when I have time. Alternative mitigations, such as Remote Active Queue Management as mentioned in Nick Weaver‘s comments to a previous entry here, may become feasible with time.

For me, the mitigation is a no-brainer: the network th home actually works even when others are using it in my house. With no mitigation, we would periodically be stepping on each other. Additional bandwidth at the cost of tolerating a broken network that I can’t use for some of my essential services is a very poor trade. And if you are a gamer, it may save your life ;-) .
QOS and Telephony

If you succeed at mitigating bufferbloat in your broadband connection, you have further challenges. You may have bufferbloat in the home router itself, particularly over wireless hops (as I have observed and noted earlier). Running an open source router may allow further mitigation of problems in your home router; but this post is long enough as it is and dinner time is fast approaching, so I’ll leave discussing mitigation of bufferbloat in home routers for another day.

Let’s first talk about QOS for telephony for a moment. Note that all this is essentially what Ooma is doing: they put their box in ahead of your home network, reserve bandwidth for VOIP, and classify VOIP traffic ahead of other traffic. I used one of these before it was repetitively damaged by lightning.

Before you have mitigated broadband bufferbloat, any QOS policy you may set in the router may very well (almost certainly is) ineffective when your broadband connection is saturated. And the router itself may also suffer from bufferbloat. (which is why this all can be so confusing; this bear of little brain has often been very confuzed in this quest). But once you have successfully mitigated broadband bufferbloat by bandwidth shaping the broadband hop, you can hope that you to enable QOS for your non-carrier provided VOIP and Skype might work OK (when the home router itself is not feeling bloated). I expect it is wise to do so even though it should not be necessary, for reasons alluded to in a previous entry, that I will elaborate on in a future blog entry. Browsers can cause serious jitter, much more than in past years, and are so worrying they are part of what I lose sleep over. I’ll circle back to the browser problem in a week or so.

Some of the open source routers (and Linux itself) have very fancy traffic classification, queue management and allocation facilities; these may not be enabled even in the open source routers, or properly set up (depending on the distro). Go wild. Have fun. Find and fix bufferbloat bugs with and in the open source routers (since I’ve found that they have the same problems I found on my laptop as covered in fun with your switch, and fun with wireless, particularly since they seem to have only worried about the broadband link). Show everyone what can be done, so the industry catches up faster (and more are free software converts!).