packet loss causes

+3 votes
What would cause high packet loss?
asked Feb 24, 2015 in ADSL by anonymous

2 Answers

0 votes
The reasons for packet loss are many. Below are a few:

Congestion: The Internet Standards treat packet loss and congestion as synonyms. Routers discard incoming packets that can’t be stored or transmitted.  Imagine an Ethernet (10 megabit/sec) pipe feeding a T1 (1.54 megabit) router.  Anytime the average feed from the Ethernet exceeds 1.54 megabits/sec, packets will be lost. This is normal congestion, (ie. packets lost) because the average sum of the inputs to a router exceeds the capacity of its output.

Bit errors: As information packets move from place to place, there is always a chance that some bits will be modified. Each packet has a mathematical sum of the bits it contains appended to it. When a receiving router receives a packet whose contents and the appended sum don’t agree, that packet is discarded.  This can occur anyplace in the journey from source to destination.

What is causing the delay?
        A) Speed of Light:.  The speed of light in a fiber optic cable works out to 10 milliseconds per thousand miles, for a ping time (due to the speed of light) of 60 milliseconds on a coast-to-coast (US) fiber link.  Remember that long distance routes are not normally as-the-crow-flies, so the distance may be much further than you think, especially with overseas connections.
        B) Router in and out time: Routers receive packets before forwarding them.  If a router is sending a 1500 byte packet at T1(1.536 megabits/sec) the time from the first bit to the last bit is 7.8 milliseconds.  If the router is storing packets waiting to send them, then the delay time increases.  We believe this is the reason the measured Global Internet ping time varies.

Congestion Avoidance:  TCP assumes that all packet loss is caused by congestion and responds by reducing the transmission rate.

Slow Start: When a TCP connection starts (or re-starts if more than one packet has been lost) it sends one packet, waits for the acknowledgment, then sends 2, then 4…and ramps up its transmission pace. Each step in the ramp consumes a round trip delay.

Data Acknowledgments:  The TCP receiver sends an acknowledgment to the sender whenever a segment of information is received. The sender does not assume any data is lost until a multiple of round trip time has elapsed without receiving an acknowledgment, or until it has received multiple duplicate acknowledgments.

Window Size:  TCP can only send a certain amount of data before it must stop transmitting and wait for an acknowledgment. That amount of data is called the window size. The standard window size in TCP is limited to 64 kilobytes.  RFC1323 allows larger windows, but it is not yet usable by applications running on Microsoft platforms.

When you factor all of the above, it gives a new perspective on why VPN over the internet is subject to such variability in performance.  If you can experience zero packet loss, your performance rises.  This is what makes MPLS networks so attractive for any applications where performance is important.
answered Feb 24, 2015 by AfriDude (43,950 points)
0 votes

Packet loss as reported by ping represents packets that didn't make the complete round trip of ...

  • Out of your computer and onto your local network
  • Across your local network
  • Over your ADSL line to the DSLAM at the exchange
  • Over the Telkom network to the IPC
  • Over the IPC to the ISP
  • From us to the folks you are trying to get a response from
  • The remote side actually deigned to respond to your packet
  • The reply was transmitted over that long path

So of that, what causes high packet loss?  Just one thing, usually:

  • Line quality

Usually it's your own line.  The components of the path that are relatively low quality components is everything downstream from the exchange --

  • the network link at the DSLAM that only you use
  • the flimsy copper line that carries only your signal
  • that crummy little ADSL/voice splitter that your noisy wireless phone plugs into
  • the cheap and nasty ASDL router that carries your signal (unless you got a good one :)
  • the wireless network in your house which competes for bandwidth with your neighbour's CCTV signals

If your ADSL link to the DSLAM is flaky, then resetting the power on your router will cause your ADSL connection to be renegotiated.  Problematic frequencies will be avoided, and new parameters for communication will be negotiated.  Everything will be nice for a bit.  When you see the packet loss again, you can bet that time has passed, and the noise has again started to interfere with the signal.

Packet loss can also be caused when ...

  • Your own line is saturated: there are only so many packets that can cross the link at a time.  When you go above that, you first get latency (extra delay) and then you get packet loss (it's not going to get there, so it gets dropped and sent again).
  • The local exchange link that you share with others is saturated (there isn't time to transmit all the packets that want to be sent).  This is a realisation that will grow slowly on you when you realise that resetting your router makes no temporary or permanent difference to your link quality.
  • The thing you are testing against might be grumpy - most devices have a limited tolerance for pings, and beyond a certain (fairly high) rate of pings per second they just won't respond.  If you and others are testing against a single host, it can get grumpy.
  • Sometimes the network you are connecting to has trouble (cables not working, load balancers not balancing)
  • If you are using a VPN you need to examine the packet loss (or lack thereof) to the VPN server.
  • Sometimes your bandwidth is limited by the funny network stuff on the path.  When that happens, everything is weird.
  • Sometimes high capacity links have complex failures which nobody should pretend to understand - and if you have the bad luck to be the guy that keeps using the one faulty cable out of the 100 available cables, then you are going to be very confused.
answered Feb 24, 2015 by Bananaman (5,390 points)
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