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Simulating Network Latency?

Posted by Cliff on from the network-testbed-technology dept.

ixmo asks: "I've just come around an interesting problem: to simulate low-bandwidth network links without buying expensive WAN simulators, I can connect two old Cisco routers back to back with serial cables, and control the bandwidth via the 'clock rate' IOS command, but how can I simulate network latency? Is there some OS tool or patch (for Linux/OpenBSD) that allows for tuning of network delay? Any hints?"

37 of 76 comments (clear)

  1. FreeBSD Dummynet by Inominate · · Score: 3, Insightful

    FreeBSD's dummynet can quite easily do it. I suspect the same of openbsd and linux.

    What kind of question is this for ask slashdot?

    1. Re:FreeBSD Dummynet by Inominate · · Score: 2, Informative

      not even a kernel patch, a loadable module included in the base.

    2. Re:FreeBSD Dummynet by Drakon · · Score: 2, Interesting

      What also may work is lowering the granularity of the timer in the kernel,
      ie HERTZ=1

      --
      Buttsex.
    3. Re:FreeBSD Dummynet by jesup · · Score: 4, Informative

      Dummynet can absolutely do it. Put a PC with BSD & Dummynet and two ethernet interfaces in to simulate delay/loss/BW restrictions/etc. Very configurable. You can chose which packets are affected.

      Worst problem: fixed delay, not bell-curve/whatever. You can roughly approximate delay variance by several rules of varying probabilities. Also, loss is random not bursty. For most testing, this is fine.

      It can take a little while to get used to configuring it. Don't forget to make it act like a network in both directions!

    4. Re:FreeBSD Dummynet by Piquan · · Score: 4, Informative

      Dummynet can absolutely do it. Put a PC with BSD & Dummynet and two ethernet interfaces in to simulate delay/loss/BW restrictions/etc. Very configurable. You can chose which packets are affected.

      You betcha. At a job I previously had, supporting TCP/IP for a large Unix producer, I had a FreeBSD box set up for just this purpose. It only had one ether interface, and was in a different part of the building from the test lab, but that didn't matter; I'd use PPP over TCP to connect to the test machines.

      I'd use it to test how different OSs (including our own) would handle different bandwidth*delay products, packet loss, etc. For instance, one I found out that a customer was having problems with a lossy WAN connection to an NT server. I experimented with high packet loss percentages, and changed the rules to narrow down the problem. Turns out that NT won't retransmit a FIN packet, at least not back then.

      When I left that job, my coworkers insisted that I show them how to set up that box to run those kinds of experiments.

      Quite an educational experience, too. It's one thing to read Richard Stevens describing congestion avoidance algorithms; it's something else to watch them in action.

  2. The solution is obvious. by mayoff · · Score: 5, Funny

    Use a longer cable.

    1. Re:The solution is obvious. by BrynM · · Score: 4, Funny
      Use a longer cable.
      Or do what my last job did: Use CAT-3! Oh, you're just testing... Too bad they weren't.
      --
      US Democracy:The best person for the job (among These pre-selected choices...)
  3. Download a copy of SP2 by Marxist+Hacker+42 · · Score: 3, Interesting

    This may be the oddest use for this yet- but any large file would work >200MB. Set up your Serial port for a slow connection, say 110 baud, then start an ftp transfer in the background of SP2 from one machine to the other. That ought to simulate latency errors just fine.

    --
    SJW: a person who perceives an injustice, and while correcting it, commits a greater injustice.
  4. dummynet by Fweeky · · Score: 2, Informative

    FreeBSD has dummynet; I'd guess OpenBSD has something similar.

  5. We use Dummynet by jshare · · Score: 4, Informative
    We use FreeBSD and its "dummynet" capabilites. (Perhaps other BSDs have this as well?)

    You can get m0n0wall and stick it on random hardware. I think you then have to recompile the kernel to enable dummynet.

    We use a Soekris 4501. It'll only bridge upto about 50mbit of traffic, but if you want to simulate T1 speeds it'll be fine. Beefier hardware (the soekris box is roughly a 133MHz 486) will probably let you max out at wire speed.

  6. Stand next to the router and... by stefanlasiewski · · Score: 4, Funny

    Stand next to the router and simulate rough conditions... ... yank out the wire a couple times. ... play kick the router. ... simulate lightening by plugging a network cable into a 220V plug ... paint sunspots on your face and spew out some EMF pulses like the sun.

    --
    "Can of worms? The can is open... the worms are everywhere."
    1. Re:Stand next to the router and... by Piquan · · Score: 4, Funny

      Don't forget the most fun part of cable simulation: the backhoe!

    2. Re:Stand next to the router and... by Piquan · · Score: 2, Informative

      simulate lightening by plugging a network cable into a 220V plug

      Enter the Etherkiller.

  7. In related news by WasterDave · · Score: 4, Funny

    It's good fun telling people who come down here to visit (New Zealand) that the twists in our cat 5 go the other way - because the earth's magnetic field is reversed.

    Made all the better when they discover that southern hemisphere monitors actually *are* different for exactly the same reason.

    Dave

    --
    I write a blog now, you should be afraid.
    1. Re:In related news by menscher · · Score: 2, Insightful
      Made all the better when they discover that southern hemisphere monitors actually *are* different for exactly the same reason.

      I call bull. The Earth's magnetic field is far too weak to have an effect on anything like that. Besides, don't you think someone might notice if it mattered whether your monitor was facing north or south? You really shouldn't believe everything your BOFH tells you.

    2. Re:In related news by SuiteSisterMary · · Score: 3, Interesting

      Go type 'southern hemisphere monitors' into Google.

      --
      Vintage computer games and RPG books available. Email me if you're interested.
    3. Re:In related news by FireFury03 · · Score: 2, Informative

      One of the reasons you have to calibrate your own monitor's picture rotation is because it is affected by the local geomagnetic field.

      --
      http://blog.nexusuk.org
  8. Simple by Anonymous Coward · · Score: 5, Funny

    Sign up with Comcast.

    1. Re:Simple by cfallin · · Score: 2, Informative

      AMEN TO THAT. All my friends in the area and I are on Comcast (still better than DSL, considering price), and a few in particular seem to have outages every week or so. There are random incidents of packet loss every few days that cause Trillian to drop my AIM connection. It's really quite annoying, but for the price (compared to a quality T1) you can't complain much.

    2. Re:Simple by michael_cain · · Score: 2, Informative
      Oddly enough, Comcast has a Linux-based program named NETSIM that does exactly what the original article was looking for. It emulates queueing delays for a variety of access technologies (dial-up, DSL, etc) including custom settings. It emulates network impairments like delay, jitter and loss. It has been used to test a variety of commercial IP-based services under a broad range of impairment conditions. The usability group made extensive use of NETSIM while evaluating different upstream/downstream speed limits for possible tiered service. It has the useful attribute that it runs entirely in user space, although it does need root privileges. It ran on every version of Linux that I ever tried it on. There's a nice Tcl/Tk control interface that works well for certain kinds of demonstrations.

      I know they have it because I wrote it and conducted many of the studies while I was at USWest Advanced Technologies. Ownership transferred to MediaOne Labs when USWest split itself in two. At that time, we licensed it to a variety of other groups under the best terms I could get out of our big-company lawyers: use for whatever you want internally, modify as desired, but no redistribution. There's a patent for it, one that's quite broad and that dummynet probably infringes. MediaOne didn't care, the patent was applied for so that no one else could get one and force us to stop using our own program. For those who care about prior art arguments, TTBOMK NETSIM predates both dummynet and NIST's nistnet software. When AT&T bought MediaOne, the Labs survived but in reduced form. Licensing of NETSIM stopped, although we continued to use it internally. When Comcast bought AT&T Broadband, most of the staff at the Labs were laid off. I turned over the complete documentation, source code, and an operating NETSIM installation on my way out the door. However, I would be surprised if any of that made the trip from Denver to Philadelphia, so in all probability, the program is no longer available.

      Some days I hate big corporations.

  9. Linux QoS by Phacka · · Score: 5, Informative

    Network emulator
    CONFIG_NET_SCH_NETEM:
    Say Y if you want to emulate network delay, loss, and packet re-ordering. This is often useful to simulate networks when testing applications or protocols.

  10. single-shot delays by isj · · Score: 2, Interesting

    I have found kill -s SIGSTOP and kill -s SIGCONT on the server process useful for simulating a temporary network congestion / single packet-drop on a TCP connection.

  11. OpenBSD by macx666 · · Score: 3, Informative

    For openbsd you can throttle bandwidth right in PF.

    Just cap whichever queue you want at whatever rate you want.

  12. My Thesis by Xetrov · · Score: 2, Interesting

    My thesis (which I am only just starting) deals with networking, and one major part of it is in simulating different network conditions.

    I was planning to write my own wrapper around the standard socket operations which will add things like latency, unreliability etc etc (for testing the robustness of a protocol).

    However I am looking forward to seeing some of the answers here as maybe I wont have to do as much work as I previously thought ;)

  13. NIST ATM simulator by Anonymous Coward · · Score: 3, Informative

    The NIST ATM simulator (public domain) might be useful. You need to provide some personal info to download it, but that isn't verified.

  14. nistnet by j1m+5n0w · · Score: 4, Informative

    Nistnet is another tool that simulates delay.

    -jim

  15. Shaper by russ_allegro · · Score: 2, Informative

    Howabout using a traffic shaper I should do what you want.

    Shaper is a traffic shaper and a packet filter for a server and for a gateway. With only limited configuration information, that are to be supplied, this script can control which and how information flow through the box.

    http://www.chronox.de/

    There are other ones as well type shapper at freshmeat.

  16. use a traffic limiter (shaper) by wotevah · · Score: 3, Interesting
    Use a bandwidth limiter that throttles your send rate. Someone else mentioned PF for BSD. On linux you can use the shaper module. My old notes go something like this, it might still work. Assuming eth0 is your output interface (the limiter works only on outgoing traffic, so you need the other end to do the same):
    modprobe shaper
    shapecfg attach shaper0 eth0
    shapecfg speed shaper0 9600 # in baud
    ifconfig shaper0 <eth0-address> netmask <eth0-netmask>
    # remote automatic route added by above
    route del -net <eth0-network> netmask <eth0-netmask> dev shaper0
    # add routes to be choked
    route add -host <otherhost> dev shaper0
    # or if going through a router
    route add -net <othernet> gw <router> dev shaper0
    ...you get the idea. I wonder if iptools makes this a bit less hacky.
  17. Use Google? by Anonymous Coward · · Score: 2, Informative

    Google is sometimes a very good tool for finding things like this out, without having to submit a question to Slashdot. A Google for simulating network latency and a click on the first link turns up a thread on a FreeBSD mailing list that provides the answer: Dummynet.

  18. Found a hardware solution for $10, Ricochet. :) by Myself · · Score: 3, Informative

    Connect the machines using PPP over a pair of Ricochet modems, available on eBay for a song. They include a neat little command for developers:

    AT~I13 -- WAN Simulation Command and Information Display
    This command enables the Ricochet modem's WAN simulation feature.
    Syntax:
    AT~I13
    You can use this function to test various transport protocols in the presence of network delay and packet loss. This simulation only affects the modem's transport modes, i.e., LIGHT/PPP/SLIP/STREAM. If you are going to reset the WAN simulation values, then you should reboot the modem because it is not built to reset and process incoming packets at the same time. WAN simulation affects the processing of received packet, therefore, when testing the simulation needs to be set at both ends of the connection.

    The incoming packets are processed in the following order. First, the drop percentage value is checked and the modem drops that N% immediately. Second, the base delay is added to a random percentage of the variable delay. Then the packet is inserted on a time ordered delivery queue. If the variable delay component is great enough, a large number of incoming packets will be reordered.

    Note:
    In WAN simulation, there are fewer (Time to Live) TTL expirations than in an real network because packets ending up on the delivery queue is not expiring based on the TTL value.

  19. !bull by jamesh · · Score: 2, Informative

    I don't know the science behind it, but I did used to work in a manufacturing plant for a certain very large computer manufacturer (whose name has 3 letters in it), and the part numbers for monitors for southern and northern (and in at least 1 case, equatorial) units was in fact different for precisely this reason.

    To be fair though, I did try out a nothern hemisphere monitor once (I am in Australia) and don't remember noticing any difference.

    There are variations between the magnetic lines of the earth all over the place though (look at a map of variations between 'true north' and 'magnetic north'), maybe a northern hemisphere monitor in the southern hemisphere would start to show discoloration at the extremes of these.

    I think the degauss function of a monitor is to neutralise any magnetic field built up in the grid or mesh of the monitor, rather than do anything about the magnetic field of the earth itself.

  20. Nistnet by gproux · · Score: 2, Informative

    http://snad.ncsl.nist.gov/itg/nistnet/

    nuff said...

  21. Here's something that may be useful... by johnnys · · Score: 3, Informative
    I know dummynet has already been mentioned, but here's some detailed info on how to set it up from comp.unix.bsd.freebsd.misc.

    Article: http://groups.google.ca/groups?hl=en&lr=&ie=UTF-8& selm=ucggua1ghi9ic1%40corp.supernews.com&rnum= 12

    --
    Sometimes the "writing on the wall" is blood spatter...
  22. config NET_SCH_DELAY by Kevin+Burtch · · Score: 5, Informative


    I'm shocked no-one has posted this!

    It's been in the kernel for while, though I don't know much about using it. I never bothered even looking at it (had no need) until a coworker wanted to use it (on Thursday) to do some testing and asked me about it.

    Here's the chunk of Kconfig:

    config NET_SCH_DELAY
    tristate "Delay simulator"
    depends on NET_SCHED
    help
    Say Y if you want to delay packets by a fixed amount of
    time. This is often useful to simulate network delay when
    testing applications or protocols.

    To compile this driver as a module, choose M here: the module
    will be called sch_delay.

    Please reply to this if you have been able to get this working... the tuning parameters to tc we found give errors (and yes, we built installed the latest iproute2 tarball).
    Then again, we only spent a few minutes playing with it (he had to leave).

    --
    - Preferences: Solaris 10 (servers), Ubuntu (desktops), Solaris 11 (personal servers) -
  23. VPN out and back in by sparty · · Score: 2, Informative

    If you've got an offsite broadband connection you can stick a PPTP server on (I use PopTop, http://www.poptop.org or http://poptop.sf.net ), try making a VPN connection out to that server and then back in. I've found that operating stuff over VPN at work tends to introduce significant latency (more than running over an 802.11b wireless bridge, which is also quite noticable compared to a hardwired Ethernet connection), so if you can VPN out and back in, you should have a fair amount of latency involved. If out-and-back-in doesn't work, just in (i.e. test machine A offsite, operating via VPN, talks to test machine B which is onsite) should still introduce noticable latency.

    (This does, of course, assuming that you're testing with routable protocols)

  24. Do what I do by laejoh · · Score: 3, Funny

    1. I put a linux box acting as router, firewall, apache server and internet connection on the network.

    2. Then I post a story on slashdot with a link to the apache server.

    3. ???

    4. Instant latency!

    (And by carefully choosing the size of the documents/jpegs/mpegs I can actually simulate different kinds of latency!!!)

  25. Cisco Routers can do this too... but... by agristin · · Score: 2, Informative

    It is kind of a pain. You need to adjust QoS settings to cause congestion or delay. 2 ways: One mark the traffic you want to be delayed and then pump a bunch of traffic through the pipe at the same time. Apply QoS to the unmatched traffic (say a ttcp stream is good). This will provide a real world latency and will be hard to predict how much latency. The second way is to manually mess with the low latency queing settings for an interface (decrease queue size is one way), beware this will foobar your performace so keep a backup config or don't accidentally put the router into production. This will let you do more predictable latency tests, but you still may need a stream of data to really bork up the latency. You can always do the bandwidth command on an interface and introduce pinhole congestion as well (may add latency), but that may not be what you are looking for... and I'm sure occured to you already. -Andrew more info available on www.cisco.com http://www.cisco.com/en/US/tech/tk39/tk824/technol ogies_configuration_example09186a008009461f.shtml and search for QoS as well, and then use the techniques in an inverse manner.