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Better Bandwidth Utilization
jtorin writes "Daniel Hartmeier (of OpenBSD fame) has written a short but interesting article which explains how to better utilize available bandwidth. In short it gives priority to TCP ACKs over other types of traffic, thereby making it possible to max both upload and download bandwidth simultaenously. Be sure to check ot the nice graphs! Also note the article on OpenBSD Journal. OpenBSD 3.3 beta is now stable enough for daily use, so why not download a snapshot from one of the mirrors and try it out?"
How ironic, an article about better utilizing available bandwidth, and already /.-ed with 0 comments.
Guess bandwidth is utilized now.
rule 1: don't put an article in slashdot pointing to your site
Now if only I could find that Linux thing...
http://lartc.org/wondershaper/
Corporate networks are already optimized under 100 or gigabit ethernet with Cisco routers which automatically handle collisions and error corrections.
Yell & scream & rant & rave... it's no use... you need a shaaaave ~ Bugs Bunny
Actually ever since my isp changed from A2000 to Chello we had the same problem as this guy has with his download being killed by his upload, a few months ago me and some friends figured the same solution but we had no idea how to actually do it on a windows based machine, anyone with a idea?
I was able to read up to the results section due to being /.'d but what I have read was quite interesting. I like the idea of prioritizing which packets go out first by what their intent is rather then everything goes out and fights for the bandwidth.
The Linux Advanced Routing & Traffic Control HOWTO discuss how to achieve the same thing on linux using QoS. See section 9.2.2.2(Sample configuration)
Belief is the currency of delusion.
Put lower priorities on p0rn, MP3s, Windows viruses, and Slashdot referrals. That should speed everything else up by about two orders of magnitude.
Sheesh, evil *and* a jerk. -- Jade
A little off topic but I always find it interesting that people with hicap gear (Foundry, Cisco, etc.) are always talking about QOS when it really only makes sense most times on low bandwidth lines. So his work is really important when you look at where it is in scheme of things - out at the end users line.
Ask Slashdot: What percentage of internet traffic is pr0n?
You can find Daniels original email on the subject at:= 10463 0260218727
/.'d graphs
http://marc.theaimsgroup.com/?l=openbsd-pf&m
It contains a little more of the pf rules than the article does, and has all the relevant information you need except for the nice
Ahhh... That bring down memories of the infamous ZModem protocol that was widely used in warez BBSs. There was a rule that said "you need to upload if you want to download". Therefore, the fastest way to get a software would be to upload at the same time as the download. The funny thing is that the synchronization was done by hand. Meaning that you needed to see the download caracter at the screen, to start uploading. If you miss by a couple of seconds, you could not get the synchronization right, and had to start over.
That "Intro to the Internet" class from college is a little hazy now, but I don't recall it being as simple as the "internet" coming out of the pipe like water.
Someone far more knowledgable than myself will get to correct me, but I seem to recall there was a process of-
Send some stuff-wait for ACK.
When you get the ACK, send some more.
By turbocharging the ACKs, you are reducing that lag time
---"What did I say that sounded like 'Tell me about your day?'"---
Was I the only one who was a little apprehensive about clicking on a link from the .cx domain?
"Ask not what your country can do for you." --John F. Kennedy
Isn't there an attack that essentially floods a server with ACK's? Just wondering what effect that would have if ACK had priority over other traffic.
Or maybe it was flooded with SYN's? Damn. I can't remember.
it's a possible way to game the system, however they can also ignore what the packets are marked as and just boost the priority of the smaller packets, which are almost always system messages. if the bump up everything under 64 bytes, then they'd get the same effect, but without the possibility of someone cheating the system like that. though I'm pretty sure someone else has already done that.
The World's Worst Webcomic!
Since the website seems slashdotted now I've set up a mirror. You can see it there.
"OpenBSD 3.3 beta is now stable enough for daily use, so why not download a snapshot from one of the mirrors and try it out?"
Windows XP is now stable enough for daily use, so why not download a snapshot from one of the mirrors and try it out?"
(intended as a joke)
Your correct, bandwidth is fixed. This is about better bandwidth utilisation.
/.ed but the gist is that if you consider a full duplex conection, and you max out one side of that, say uploads, then the ACK packets get swamped, so your downstrem bandwidth is spent re-transmitting, or empty whilst the other end is waiting for ACKs.
The article is
The bandwidth is there, it;s just under utilised. By prioritisng the ACK's, so that they get boosted through, it becomes possible to saturate both upstream and downstream pipes at once, at peak efficency, rather than one of the coasting along, waithing for the other.
Note that this only applies for TCP/IP and similar, reliable, protocols. If you had a UDP app (e.g. media streaming done properly), then this trick won't affect it at all, as it doesn't wait for an ACK.
If the acks are sped up, this interferes with TCP keeping track of the statistical average Round Trip Time.
So if the network is congested and an ACK SHOULD time out but doesn't, TCP will keep on flooding the network, ruining the pool for everyone.(see: Tragedy of the commons)
Yes, I agree that this is a big-O style worse case scenario, but its something to consider.
In the future, I would want to not be isolated from my friends in the Space Station.
It seems to me that a great many
Reading that book will give you a foundation to understanding how a single endpoint behaves in an IP network. If you want some understanding of the guts of a large scale internetwork I'd suggest the Cisco Press IP Quality of Service book.
There are a great many things near and dear to
If you're impatient you can look at my journal - I've covered some of the issues there.
I am very easy to get along with, but I don't have time to waste being nice to people who are being stupid. -Theo
The technique you suggest is one of several proposed by Stefan Savage in TCP Congestion Control with a Misbehaving Receiver. He called it TCP Daytona. :)
http://www.benzedrine.cx/ackpri-norm.jpg
http://www.benzedrine.cx/ackpri-priq.jpg
benzedrine.cx - Prioritizing empty TCP ACKs with pf and ALTQ Prioritizing empty TCP ACKs with pf and ALTQ
Introduction ALTQ is a framework to manage queueing disciplines on network interfaces. It manipulates output queues to enforce bandwidth limits and priorize traffic based on classification.
While ALTQ was part of OpenBSD and has been enabled by default since several releases, the next release will merge the ALTQ and pf configuration into a single file and let pf assign packets to queues. This both simplifies the configuration and greatly reduces the cost of queue assignment.
This article presents a simple yet effective example of what the pf/ALTQ combination can be used for. It's meant to illustrate the new configuration syntax and queue assignment. The code used in this example is already available in the -current OpenBSD source branch.
Problem I'm using an asymmetric DSL with 512 kbps downstream and 128 kbps upstream capacity (minus PPPoE overhead). When I download, I get transfer rates of about 50 kB/s. But as soon as I start a concurrent upload, the download rate drops significantly, to about 7 kB/s.
Explanation Even when a TCP connection is used to send data only in one direction (like when downloading a file through ftp), TCP acknowledgements (ACKs) must be sent in the opposite direction, or the peer will assume that its packets got lost and retransmit them. To keep the peer sending data at the maximum rate, it's important to promptly send the ACKs back.
When the uplink is saturated by other connections (like a concurrent upload), all outgoing packets get delayed equally by default. Hence, a concurrent upload saturating the uplink causes the outgoing ACKs for the download to get delayed, which causes the drop in the download throughput.
Solution The outgoing ACKs related to the download are small, as they don't contain any data payload. Even a fast download saturating the 512 kbps downstream does not require more than a fraction of upstream bandwidth for the related outgoing ACKS.
Hence, the idea is to priorize TCP ACKs that have no payload. The following pf.conf fragment illustrates how to set up the queue definitions and assign packets to the defined queues:
ext_if="kue0"
altq on $ext_if priq bandwidth 100Kb queue { q_pri, q_def }
queue q_pri priority 7
queue q_def priority 1 priq(default)
pass out on $ext_if proto tcp from $ext_if to any flags S/SA \
keep state queue (q_def, q_pri)
pass in on $ext_if proto tcp from any to $ext_if flags S/SA \
keep state queue (q_def, q_pri)
First, a macro is defined for the external interface. This makes it easier to adjust the ruleset when the interface changes.
Next, altq is enabled on the interface using the priq scheduler, and the upstream bandwidth is specified.
I'm using 100 kbps instead of 128 kbps as this is the real maximum I can reach (due to PPPoE encapsulation overhead). Some experimentation might be needed to find the best value. If it's set too high, the priority queue is not effective, and if it's set too low, the available bandwidth is not fully used.
Then, two queues are defined with (arbitrary) names q_pri and q_def. The queue with the lower priority is made the default.
Finally, the rules passing the relevant connections (statefully) are extended to specify what queues to assign the matching packets to. The first queue specified in the parentheses is used for all packets by default, while the second (and optional) queue is used for packets with ToS (type of service) 'lowdelay' (for instance interactive ssh sessions) and TCP ACKs without payload.
Both incoming and outgoing TCP connections will pass by those two rules, create state, and all packets related to the connections will be assigned to either the q_def or q_pri queues. Packets assigned to the q_pri queue will have priority and will get sent before any pending packets in the q_def queue.
Result The following test was performed first without and then with the ALTQ rules explained above:
The first graphs shows the results of the test without ALTQ, and the second one with ALTQ:
Image 1, ACK PRI Normal
Image 2, ACK PRI PRIq
The improvement is quite significant, the saturated uplink no longer delays the outgoing empty ACKs, and the download rate doesn't drop anymore.
This effect is not limited to asymmetric links, it occurs whenever one direction of the link is saturated. With an asymmetric link this occurs more often, obviously.
Related links
Good security is based upon reality and common sense. Common sense is a function of having common knowledge.
Though you might see more effects of this on a low bandwidth link, it is not just for low bandwidth.
A fair number of protocols do transmit windows of a certain size. They'll send a certain amount of data, and not send more data until the oldest packest in the window gets an ACK back. You therefore only have so much data "in-flight" at any one time. Strongly asynchronous link (like aDSL and cablemodems) can require strikingly different window sizes than synchronous links.
The right amount of in-flight data is dependent on the speed of your pipe, obviously, but a lot of applications still use defaults set for low-bandwidth pipes. You can argue that the proper solution for this is to change the defaults, but if you just give ACKs priority, you don't need to worry about it, and the less you force users to change, the better. (The transmit window size has to be a user setting, directly or indirectly, either by asking a window size, or by asking "what kind of pipe do you have?" and guessing a window size from that.)
This is dependent on the protocol, true, but giving ACKs priority is actually a decent generic solution to what many consider an application-specific problem.
QOS is also often about bandwidth guarantees, not necessarily throughput. You have a 155mbit link shared among several applications, and an application that *requires* 45mbit. So you use QOS to guarantee that application gets 45mbit if it wants it, and everything else shares the remainder. If the app isn't going, then that 45mbit it requires can be made available to other apps until it is required.
This is my sig. There are many like it but this one is... Oops. Frank, I've got your sig again! Where's mine?
Windows XP uses a DDR Fairness technique to solve the same problem, I wonder how the two techniques compare?
See "QoS for Modems and Remote Access" at this KB article.
This is what is known today as "ACK traffic shaping". First on the market, I believe, was packeteer (www.packeteer.com) with their PacketShaper.
Unlike most conventional traffic shapers which queue and control the data rate on the outgoing channel, PacketShaper controls the rate of acknowledgements on the reverse channel.
This is usually used to *slow* traffic. I.e., instead of having the router drop packets (thereby wasting resources until the source TCP understands that the net is congested and reduces load) it just slows the ACKs and the sender automatically reduces its sending rate.
Anyway, the real nice thing about the OpenBSD implementation is that they merge their packet filter (pf) with the ALTQ queuing code. Now this is really powerful.
Sounds like a good time for all BSDs to adopt this new combination instead of relying on less-capable mechanisms. E.g. FreeBSD has ipfw for filtering and dummynet for queue management. I don't know how pf compares with ipfw but ALTQ is definitely better than dummynet.
Nimrod.
Just in case you don't run openbsd or linux (wondershaper) and are looking for ack packet priority, you can get throttled from http://www.intrarts.com/throttled.html and have the same functionality for Mac OS X and freebsd. It is great to see this information finally getting out to the public, as it does offer significant improvements in network performance.