[ovs-discuss] Sending UDP traffic in openflow network

Ben Pfaff blp at nicira.com
Thu Oct 27 03:24:13 UTC 2011

On Wed, Oct 26, 2011 at 10:22:18PM -0500, Shan Hu wrote:
> Im trying to test the QoS Rate-Limiting of Kernel vSwitch, i use iperf
> as my measurement tool.  Everything is working fine with TCP part,that
> is, after i limit rate of one port to, say 50Mbps, the rate is limited
> to 50Mbps correctly and packets are tranferred 100%.  But when i turn
> to UDP part,i ran into problems.I have to limit bandwidth to at most
> 4Mbps in order to tranfer 100% packets.And if i use bandwidth more
> than 4Mbps, the lost packets increase, the worst packet lost
> percentage is almost 99%.

Are you using policing?  The documentation says a lot about problems
with policing:

     Ingress Policing:
       These  settings  control  ingress policing for packets received on this
       interface.  On a physical interface, this  limits  the  rate  at  which
       traffic  is  allowed  into  the  system  from the outside; on a virtual
       interface (one connected to a virtual machine), this limits the rate at
       which the VM is able to transmit.

       Policing is a simple form of quality-of-service that simply drops pack-
       ets received in excess of the configured rate.  Due to its  simplicity,
       policing  is  usually  less accurate and less effective than egress QoS
       (which is configured using the QoS and Queue tables).

       Policing is currently implemented only on Linux.  The Linux implementa-
       tion uses a simple ``token bucket'' approach:

              o      The  size  of  the  bucket  corresponds to ingress_polic-
                     ing_burst.  Initially the bucket is full.

              o      Whenever a packet is received,  its  size  (converted  to
                     tokens)  is compared to the number of tokens currently in
                     the bucket.  If the required number of tokens are  avail-
                     able, they are removed and the packet is forwarded.  Oth-
                     erwise, the packet is dropped.

              o      Whenever it is not full,  the  bucket  is  refilled  with
                     tokens at the rate specified by ingress_policing_rate.

       Policing  interacts  badly  with some network protocols, and especially
       with fragmented IP packets.   Suppose  that  there  is  enough  network
       activity to keep the bucket nearly empty all the time.  Then this token
       bucket algorithm will forward a single packet every so often, with  the
       period depending on packet size and on the configured rate.  All of the
       fragments of an IP packets are normally transmitted back-to-back, as  a
       group.   In  such  a  situation, therefore, only one of these fragments
       will be forwarded and the rest will be dropped.  IP  does  not  provide
       any  way for the intended recipient to ask for only the remaining frag-
       ments.  In such a case there are two likely possibilities for what will
       happen next: either all of the fragments will eventually be retransmit-
       ted (as TCP will do), in which case the same problem will recur, or the
       sender  will not realize that its packet has been dropped and data will
       simply be lost (as some UDP-based protocols will do).  Either  way,  it
       is possible that no forward progress will ever occur.

       ingress_policing_rate: integer, at least 0
              Maximum rate for data received on this interface, in kbps.  Data
              received faster than this  rate  is  dropped.   Set  to  0  (the
              default) to disable policing.

       ingress_policing_burst: integer, at least 0
              Maximum  burst  size for data received on this interface, in kb.
              The default burst size if set to 0 is 1000 kb.  This  value  has
              no effect if ingress_policing_rate is 0.

              Specifying  a  larger burst size lets the algorithm be more for-
              giving, which is important for protocols like TCP that react se-
              verely  to  dropped  packets.  The burst size should be at least
              the size of the interface's MTU.  Specifying  a  value  that  is
              numerically  at  least  as large as 10% of ingress_policing_rate
              helps TCP come closer to achieving the full rate.

If you're not using policing, please tell us about your configuration.

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