Deep Dive: HTB Rate Limiting (QoS) with Open vSwitch and XenServer

DISCLAIMER: I’m still getting my feet wet with Open vSwitch. This post is just a cleaned up version of my scratchpad.

Open vSwitch has a few ways of providing rate limiting – this deep dive will go into the internals of reverse engineering an existing virtual interface’s egress rate limits applied with tc-htb. Hierarchy Token Bucket (htb) is a standard linux packet scheduling implementation. More reading on HTB can be done on the author’s site – I found the implementation and theory pretty interesting.

This is current as of Open vSwitch 1.9.

The information needed to retrieve htb rate limits mostly lives in the ovsdb:

Open vSwitch Schema (

Things can get complex depending on how your vifs plug into your physical interfaces. In my case, OpenStack Quantum requires an integration bridge which I’ve attempted to diagram:


  1. On instance boot, vifs are plugged into xapi0. xapi0’s controller nodes pull down information including flows and logical queues.
  2. The flows pulled from (1) set the destination queue on all traffic for the source IP address for the interface.
  3. The queue which the traffic gets sent to goes to a linux-htb ring where the packets are scheduled.

Let’s take a look at an example. I want to retrieve the rate limit according to the hypervisor for vif2.1 which connects to xapi0, xenbr1, and the physical interface eth1. The IP address is


  • Find the QoS used by the physical interface:
    # ovs-vsctl find Port name=eth1 | grep qos
    qos : 678567ed-9f71-432b-99a2-2f28efced79c

  • Determine which queue is being used for your virtual interface. The value after set_queue is our queue_id.
    # ovs-ofctl dump-flows xapi0 | grep | grep "set_queue"
    ... ,nw_src= actions=set_queue:13947, ...
  • List the QoS from the first step and its type. NOTE: This command outputs every single OpenFlow queue_id/OVS Queue UUID for the physical interface. The queue_id from the previous step will be the key we’re interested in and the value is our Queue’s UUID
    # ovs-vsctl list Qos 678567ed-9f71-432b-99a2-2f28efced79c | egrep 'queues|type'
    queues : { ... 13947=787b609b-417c-459f-b9df-9fb5b362e815,... }
    type : linux-htb
  • Use the Queue UUID from the previous step to list the Queue:
    # ovs-vsctl list Queue 787b609b-417c-459f-b9df-9fb5b362e815 | grep other_config
    other_config : {... max-rate="614400000" ...}
  • In order to tie it back to tc-htb we have to convert the OpenFlow queue_id+1 to hexadecimal (367c). I think it’s happening here in the OVS code, but I’d love to have a definitive answer.
    # tc -s -d class show dev eth1 | grep 367c | grep ceil # Queue ID + 1 in Hex
    class htb 1:367c ... ceil 614400Kbit