Traffic control and queuing discipline

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Traffic control and queuing discipline

• Jennifer Hou and Honghai Zhang

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Outline

• Linux traffic control overview
• Queuing discipline
• Queuing functions
• Interaction of kernel with queuing functions
• Interaction of user with queuing functions
• Case Study TCP RED

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Linux traffic control
4
Processing of network data
A non-terminal machine
Upper layers (TCP, UDP, )
Traffic control
Input de-multiplexing
forwarding
Output queuing
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Components of traffic control

• Queuing disciplines
• Classes (within a queuing disciplines)
• Filters
• policing

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Queuing discipline

• Each network device has a queuing discipline
• It controls how packets enqueued on that device
  are treated
• Keep, drop, mark
• A simple one may just consist of a single queue

Queuing discipline
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Complex queuing discipline

• Queuing discipline
• May use filters to distinguish among different
  classes of packets
• Process each class in a specific way
• Two filters can point to one class
• Classes
• do not store packets
• They use another queuing discipline to do that

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Complex queuing discipline (cont)
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Policing

• When enqueuing packets, the corresponding flow
  can be policed
• Letting it go
• Dropping
• Letting it going but marking

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Queuing disciplines data structure

• struct Qdisc_ops
• struct Qdisc_ops next
• struct Qdisc_class_ops cl_ops
• char idIFNAMSIZ
• int priv_size
• int (enqueue)(struct sk_buff , struct Qdisc
  )
• struct sk_buff (dequeue)(struct Qdisc )
• int (requeue)(struct sk_buff , struct Qdisc
  )
• unsigned int (drop)(struct Qdisc )
• int (init)(struct Qdisc , struct rtattr
  arg)
• void (reset)(struct Qdisc )
• void (destroy)(struct Qdisc )
• int (change)(struct Qdisc , struct rtattr
  arg)
• int (dump)(struct Qdisc , struct sk_buff )

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Queuing disciplinesFunctions

• enqueue)()
• Enqueues a packet
• dequeue()
• Returns the next packet eligible for sending
• requeue()
• Puts a packets back into the queue after
  dequeuing it with dequeue
• Drop()
• Drops one packet from the queue

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Queuing disciplinesFunctions (cont)

• Init()
• Initalize and configures the queuing discipline
• reset()
• Reset the queuing discipline to its initial state
• destroy()
• Remove a queuing discipline
• change()
• Change the configurations of a queuing discipline
• dump()
• Returns diagnostic data for maintenance.

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TCP RED
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Dropping probability pa
Linux implementation
pb
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TCP RED implementation I

• struct red_sched_data / Parameters /
• u32 limit / HARD maximal queue length /
• u32 qth_min / Min average length threshold A
  scaled /
• u32 qth_max / Max average length threshold A
  scaled /
• char Wlog / log(W) /
• char Plog / random number bits /
• unsigned long qave / Average queue length A
  scaled /
• int qcount / Packets since last random number
  generation /
• u32 qR / Cached random number /
• psched_time_t qidlestart / Start of idle period
  /
• struct tc_red_xstats st

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TCP RED implementation II Compute average queue
length

• We want
• avg avg (1- w) w backlog
• Code in Linux
• q-gtqave sch-gtstats.backlog - (q-gtqave gtgt
  q-gtWlog)
• Explain
• avg q-gtqave w
• w 2(-wlog)

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TCP RED implementation III

• Ideally avg should be calculated every constant
  clock interval
• In Linux it is updated every packet outgoing
• Care need to be taken for idle period

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TCP RED implementation IVDecide dropping
probability

• We want enqueue if
• Linux code
• if (((q-gtqave - q-gtqth_min)gtgtq-gtWlog)q-gtqcount lt
  q-gtqR) goto enqeue
• max_P (qth_max qth_min)/2Plog
• q-gtqR rnd 2Plog

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Interaction with Kernel
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Interaction with User

• Command tc usage example
• tc qdisc add dev eth0 root red HELP OPTIONS
• tc qdisc show dev eth0
• Source code of tc http//snafu.freedom.org/linux2
  .2/iproute-notes.html
• Directory of the source code for tc
• Iproute2/tc/

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Flow of functions when calltc qdisc add dev eth0
root red
tc
filter
qdisc do_qdisc() in tc.c
class
add tc_qdisc_modify
q-gtparse_qopt() rtnl_open() rtnl_talk()
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Flow of functions when calltc qdisc show (or tc
s qdisc)
tc
filter
qdisc do_qdisc in tc.c
class
show tc_qdisc_list
rt_open
rtnl_dump_request
rtnl_dump_filter
print_qstat print_qopt print_tcstat