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QWhere are the passwords and user names?

A Please contact InterWorking Labs support ( maxwell-support@iwl.mojohlepdesk.com  This e-mail address is being protected from spam bots, you need JavaScript enabled to view it ) for the user names and passwords required to control Mini Maxwell.

QWhy are my impairment settings not having any effect?

A The most frequent reasons why a user's settings are not having any apparent effect are these:

  • The impairment was applied to the other direction.  Each of Mini Maxwell's "bands" is composed of a pair of impairment settings.  One of these settings establishes the impairments to be used on packets arriving on LAN A and leaving via LAN B.  The other establishes the impairments to be used on packets arriving on LAN B and leaving via LAN A.
  • The impairment was applied to a different band than the one that the packets are flowing through.  Mini Maxwell uses the user-specified filters to sort incoming packets into the various bands.  There are different impairment settings for each band.
  • A useful trick for isolating these kinds of problems is to go through all of the bands, and for the LAN A to LAN B  and LAN B to LAN A directions and set the drop probability to 100%.  This should have the effect of stopping all traffic.  Then those drop settings can be removed, one by one, until the traffic resumes.  That should give a good indication of which group of settings is affecting (and not affecting) your traffic.

QIf a packet does not match a filter what band does it go into?

A Band 5 is the catch-all band.  Packets that do not match any filters end up in Band 5.

For this reason it is often useful to set some obvious impairment - like a large delay - on both the uphill and downhill sides of band 5 so that any packets that slip past the filter settings will be impaired in a way that is fairly easy to notice.

QHow do I use Mini Maxwell with gigabit Ethernet?

A The easiest way to do this is to use an inexpensive, consumer grade 10/100/1000 switch between Mini Maxwell and the gigabit Ethernet device.

Mini Maxwell does not support gigabit Ethernet jumbo packets, i.e. Ethernet frames larger than 1500 bytes.  However, both Maxwell G and Maxwell Pro support gigabit Ethernet.

QHow do I use Mini Maxwell with wireless?

A The easiest way to do this is to attach a wireless "access point" to either LAN A (eth1) or LAN B (eth2) on Mini Maxwell.

Some wireless base stations act as more than a mere access point - for instance many act as routers, firewalls, or network address translators.  In nearly every case you simply want to attach Mini Maxwell to the wireless base station in the same way you would attach other wired devices that need to communicate with wireless devices.  Often this means attaching Mini Maxwell to the "harmonica" of wired network ports found on many wireless stations that also include a built-in Ethernet switch.

Some wireless base station devices work under the control of a manager.  In these cases please feel free to discuss this with our support staff.

Q What happens to Mini Maxwell if my power goes out?

A After a power failure Mini Maxwell is able to boot up without any additional user configuration.

QHow do I use Mini Maxwell with fiber optic Ethernet?

A InterWorking Labs has used external copper-to-fiber conversion boxes such as the TRENDnet TFC-1000MSC Multi-Mode Fiber Converter with SC-Type Connectorwhich is available from many on-line sellers, such as Newegg.  Other converters will probably work.

Note: Some types of converter may lock link-state on the copper Ethernet side to an "on" state without regard whether optical link state has been achieved.

QCan Mini Maxwell support High Definition Television traffic?

A Yes.

For 1500 byte packets Mini Maxwell can handle 100% of the line rate of a full duplex 100Megabit Ethernet, i.e. 8224 packets/second in each direction.  This is more than adequate to handle most forms of compressed HDTV traffic.

For example, consider a compressed HDTV stream that of about 18,000,000 bits/second.  Most of these bits will be carried in full-sized packets of about 1500 bytes each.  That works out to about 1500 packets per second, well within Mini Maxwell's packet per second capacity.

QWhen I ask for packet corruption, it appears that the number of packets being corrupted is low.  What's going on?

A A random bit offset is calculated each time the corruption mechanism decides that a packet is to be corrupted.  That offset may be anywhere in the entire Ethernet frame, not just the IP part of the packet.

If the bit that is changed is within the source MAC address (48-bits/6-bytes) then the packet will still reach the intended target computer.

If the receiving machine or its software is not overly zealous about checking consistency between source MAC address and source IP address, then the packet will be accepted and the change in the source MAC address may not be noticed.

The chance that the corruption will occur within the source MAC address varies depending on the overall size of the packet.  That chance is greater for small packets and less for larger packets.  For small packets, typical of ICMP Echo (ping), which are typically 60 bytes, then the chance that the bit corruption will occur in the source MAC field can be as high as 10%.  This, in turn, means that the perceived loss of packets due to corruption is less than one would expect.

QCan Mini Maxwell limit bandwidth?

A Yes.  Mini Maxwell uses a rate-limiting token bucket filter of the same kind that are found in many internet routing and switching devices.

Please note that rate limitation is slightly different than emulation of a link.  Rate limitation is concerned with average traffic rates over a span of time; link emulation is concerned with things like the time to serialize the bits onto the link, the packet propagation delay, queue lengths and queue overflows, and the overhead of link level headers.  For most users the difference boils down to this: a rate limiter may sometimes allow a packet to flow through without delay if there has been a prior idle period, a link emulator recognizes that no packet, even if there had been an idle period, can escape serialization and propagation delays.

Rate Limitation is a complex subject and it is often conflated with Link Emulation.  The main difference is that Rate Limitation gives bandwidth credit for bandwidth in the past that was not used.  Link Emulation does not.

Mini Maxwell performs rate limitation rather than link emulation.  This means that if there has been a period of low traffic followed by a burst of over-limit traffic, that over-limit traffic will pass through the Mini Maxwell without limitation for a short period as the credit for the unused prior bandwidth is consumed.  This behavior is consistent with many, perhaps most, internet edge pathways (such as DSL or Cable TV links, or IP over mobile telephone.)

For more information please see the white paper on Rate Limitation vs. Bandwidth Limitation.

QCan Mini Maxwell handle IEEE 802.1q tagged VLANs?

A Mini Maxwell will carry VLAN tagged frames between its to data interfaces.  However, the filters are not able to handle the fact that the tags move the IP headers four bytes deeper into the packet.  Consequently filters do not work properly on Ethernet frames that have IEEE 802.1q headers.

Nonetheless, Mini Maxwell can still be productively used on a VLAN trunk, i.e. an Ethernet segment in which all the Ethernet frames have IEEE 802.1q headers.  When no filters are used then the frames will all be classified into the default band, Band #5.  Impairments specified for Band 5 will be applied to these frames.  In other words, all of the frames on the trunked VLAN will be subject to the same set of impairments.  (Remembering, as always, that Mini Maxwell can have different impairments specified for each of the two directions of packet flow.)

For more advance handling of VLAN tags we recommend Maxwell Pro.

QWhen updating the Maxwell software can I use a Bluetooth serial device rather rather than a serial cable?

A Yes.  InterWorking Labs often uses a Bluetooth serial adapter - such as the IOGear GBS3001 as a way to connect to the serial port on Mini Maxwell from a computer equipped with a Bluetooth interface.

The Bluetooth serial adapter is attached to the console port on Mini Maxwell.  The adapter should be set to be a Bluetooth slave and the bit/baud rate set to 38,400 bits/second.

QWhy don't packets flow immediately when I plug-in the Ethernet cable?

A The bridging code in Linux takes a few seconds to initiate packet bridging after it detects the presence of Ethernet link-state (as indicated by one of the LED's on each of Mini Maxwell's network interface plugs).

QI want to connect to the serial console but I am running Windows and there is no terminal emulator program!

A Microsoft has removed the Hyperterm program from Windows.

You can download a free, open source, alternative called "PuTTY".

The PuTTY home page may be found at http://www.chiark.greenend.org.uk/~sgtatham/putty/

The easiest way to install PuTTY is to download and run the PuTTY installer program.

From PuTTY's "Session" menu select the "Serial" connection type.  That will allow you to enter the "Speed" (use 38400) and "Serial line".  The value for "Serial line" will vary from machine to machine but it will generally be of the form COM# where # is '1', '2', '3', '4', etc.  You may have to try these one at a time until you find the one that works.  Alternatively you may want to take a look at the Windows "Hardware Manager" (often found under the "System Information" link from "My Computer") and look under the Communications port item.

QWhat is a good serial communications program to use for Linux?

A The "kermit" program is a good command line tool, but you may have to fetch it from the download siteand compile it.

A good alternative is PuTTY.  Your Linux distribution may already have it available as a package.  (For example, PuTTY is available as one of the packets in the Fedora distribution.)  Go to the package manager of your Linux and see if it has PuTTY available.

As is typical for serial ports you may have to search through your serial ports to find which logical name (/dev/ttyS# or /dev/ttyUSB#) is hooked to which physical DB-9 connector.

QMy computer has no serial port, what do I do?

A InterWorking Labs uses inexpensive USB to Serial adapters.  These cost about $10 from places such as NewEgg.

QWhat are the transitional side effects when I change an impairment parameter or alter my filter map?

A There can be side effects that are caused by a change to an impairment parameter or a change to the packet classification filter map or filter sequence.

Inside a Mini Maxwell or Maxwell G changes to settings are completed within a few milliseconds (or faster.)  During that short period packets that arrive may be processed without impairment or filter classification.

When changing an impairment parameter the side effect may be the early release or discard of some or all of packets that are being delayed at the time of that change.  Because of buffering of packets that are being delayed due to the prior impairment setting it is possible that several packets may be affected.  These side effects are generally limited to only the band and direction, i.e. a change to Band 5 LAN-A-to-LAN-B generally will have no effect on other bands or on Band 5 LAN-B-to-LAN-A.

When changing the classification filter map or filter sequence there may be the following side effects.  For convenience the classification or map prior to each change will be here called "old" and the classification or map after each change is "new".

  • Older packets that have been accumulated into the old rate limiter or delay queue will trickle out according to the rate limit of that old limiter or delay queue.
  • Newly arriving packets that encounter a new rate limiter will receive the benefit of any unused bandwidth credit, thus there can be a short period when rate limitation will not be apparent as that credit is consumed.
  • Because of the interaction of the two points above it is possible that:
    • Some number of new packets will be sent by the Mini Maxwell before older packets.  The net effect of this that for a few seconds packets may be received in a different sequence than they were sent.
    • For a short period of time after the transition the burst data rate on the link may reach or even exceed the sum of the old and new rate limiters.

QWhat should be my IPv4 MTU?

A The IPv4 MTU should be the standard value of 1500.  That number allows for the 14 bytes of a standard (non tagged-VLAN) Ethernet header as well as the Ethernet CRC.

QWhat about bufferbloat?

A Rate limitation and delay both cause packets to be held for a period of time.  That time, which is perceived as latency across the network can cause what is called "bufferbloat."

Bufferbloat is not a flaw in the Mini Maxwell or Maxwell G - rather it is a common internet situation that must be anticipated and handled by protocol software in vendor devices.

Users should expect that when rate limiters or long delays have been configured that bufferbloat effects may be observed in the devices under test.

In particular users should anticipate that the delay due to rate limitation can grow so that several seconds could elapse between the time a packet is sent and the time it arrives.  The effect is the same if a large delay is configured.

Bufferbloat may have an amplified impact on TCP streams because of the various algorithms found in TCP stacks to handle slow start, congestion detection, congestive back off, and recovery.

See http://en.wikipedia.org/wiki/Bufferbloat for more information on bufferbloat causes and effects.

QHow do I coerce the LAN-A and LAN-B interfaces to 10Megabits clocking?

A By default the LAN-A and LAN-B interfaces announce (using low level IEEE 802 Ethernet signaling) that they are able to operate at clock rates of either 10megabits/second or 100megabits/second in either half or full duplex mode.  There are sometimes reasons to coerce the interface clock rate so that it is fixed at 10megabits/second.  This is typically done when there is a risk that the traffic load that might be presented would exceed the capacity limitations of a Mini Maxwell.  This is not typically an issue on Maxwell G.

On more recent Mini Maxwell units there is a script file that will cause the LAN-A and LAN-B interfaces to run at a fixed 10megabit/second clock rate starting with the next reboot.  This file is /etc/sysconfig/tenmeg.sh and it must be run with root privilege.

Note that the interface clock speed pertains only to the hardware rate at which bits are electrically encoded onto the Ethernet cable.  One should not conflate this clocking rate with any rate limitation setting that might be imposed by the Mini Maxwell.  The interface clock rate is a fixed metronome that ticks at its given rate no matter how much or how little data the user might try to push through the interface.  The Mini Maxwell rate limiter is a kind of speed-limit that applies only to the data that is contained in packets that are actually flowing on the network.

To obtain status of the 10 megabit setting do:

  sh /etc/sysconfig/tenmeg.sh status

To activate the 10 megabit setting do:

  sudo sh /etc/sysconfig/tenmeg.sh on

To deactivate the 10 megabit setting do:

  sudo sh /etc/sysconfig/tenmeg.sh off

QI changed my IP address but it did not change?

A There is a bug in certain releases, including Revision 17, in which a requested reboot does not occur.  Because IP address changes in Mini Maxwell are put into effect only upon a reboot the effect of this problem is that users make changes to IP address settings but those settings never seem to be activated.

The offending code has been found and fixed but will not appear until Revision 18.

In the meantime it will be necessary to perform a physical power cycle on the unit to force the Mini Maxwell to reboot.

QWhat is the interaction between rate limitation and delay?

A Suppose you set a rate limit of a million bits per second and a fixed delay of 250 milliseconds.  And then suppose that you present a load that is higher than that rate limit setting.

At first you may observe that your packets are being delayed by the anticipated 250 milliseconds.

However after a while you may observe that your packets are being delayed substantially longer than 250 milliseconds.

The reason for this is that if packets are arriving faster than the rate limitation setting then those packets enter a delay queue.  That queue drains at the configured rate limitation value.  As a consequence each packet spends some amount of time - delay - in that queue.  The amount of that delay depends on how much data is waiting in the queue ahead of that packet.

The rate limitation queue can grow to be quite long - it can hold as much as ten seconds worth of data.

So it is possible, and in fact it is very likely, that if the presented traffic rate is greater than the configured rate limit that the limitation queue will grow and grow and grow - and the rate limitation packet delay will grow correspondingly.  (The queue will grow up to the point where the rate limitation queue reaches its maximum limit and new packets are discarded because there is no where to put them.)

The overall effect of this rate limitation delay is that the delay component created by the rate limiter can become much larger than the value configured in the explicit delay setting.

So, returning to our example, that 250 milliseconds of configured delay can be overwhelmed by the delay caused by the rate limiter.  It isn't that that 250 milliseconds has disappeared.  Rather it can be hard to perceive that 250 milliseconds after it is added to a significantly larger delay caused by the the rate limitation queue.

QCan I drive Mini Maxwell or Maxwell G without using the web interface?

A Yes.  We have created some Python programs that may be used to control a Mini Maxwell or Maxwell G.  Please contact Sales for more information.


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