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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?"
http://lartc.org/wondershaper/
If you have a non-shaped asynchronous connection, like most forms of DSL and cable, it's pretty easy to cap out your upstream. When you do that, your downstream goes through the floor because your ACKs don't get through. This just says that if your routers prioritize ACKs, your downstream will still be fine even if your upstream is saturated. This isn't exactly new, my cable ISP already does this.
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
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?'"---
No it doesn't....
It is a differend solution to a different problem caused by the same thing....
The cause is the big cache in the modem, it results in a delay on outgoing traffic.
One problem is that interactive traffic gets, well, less interactive (e.g. the echo characters in a remote shell have a delay). This is solved in the HOWTO you refered to.
Another problem is that the downstream acks get delayed resulting in less downstream data. This is solved in the mentioned article.
A combination of the two would be really great and could probably be done in both linux and openbsd.
Jeroen
Secure messaging: http://quickmsg.vreeken.net/
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.
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
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.
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.
No. If the downstream is flooded, the packets won't be received, and no ACK will be sent. ACKs have higher priority, but even that can't make them appear out of thin air.
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.