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           tc qdisc add ... stab \
               [ mtu BYTES ] [ tsize SLOTS ] \
               [ mpu BYTES ] [ overhead BYTES ] [ linklayer TYPE ] ...
           TYPE := adsl | atm | ethernet
           For  the  description  of  BYTES - please refer to the UNITS section of
               maximum packet size we create size table for, assumed 2048  if  not
               specified explicitly
               required table size, assumed 512 if not specified explicitly
               minimum packet size used in computations
               per-packet size overhead (can be negative) used in computations
               required linklayer adaptation.


           Size  tables allow manipulation of packet size, as seen by whole sched-
           uler framework (of course, the actual packet size  remains  the  same).
           Adjusted  packet  size  is calculated only once - when a qdisc enqueues
           the packet. Initial root enqueue initializes it to  the  real  packet's
           Each  qdisc  can  use  different  size  table, but the adjusted size is
           stored in area shared by whole qdisc hierarchy attached to  the  inter-
           face. The effect is, that if you have such setup, the last qdisc with a
           stab in a chain "wins". For example, consider HFSC  with  simple  pfifo
           attached  to  one  of  its  leaf classes.  If that pfifo qdisc has stab
           defined, it will override lengths calculated during HFSC's enqueue, and
           in  turn,  whenever  HFSC tries to dequeue a packet, it will use poten-
           tially invalid size in its calculations.  Normal  setups  will  usually
           include  stab  defined only on root qdisc, but further overriding gives
           extra flexibility for less usual setups.
           Initial size table is calculated by tc tool using mtu and tsize parame-
           ters.  The  algorithm  sets each slot's size to the smallest power of 2
           value, so the whole mtu is covered by the size  table.  Neither  tsize,
           nor  mtu  have  to  be power of 2 value, so the size table will usually
           support more than is required by mtu.
           Currently  there're  two  methods of creating values stored in the size
           table - ethernet and atm (adsl):
               This is basically 1-1 mapping, so following our example from  above
               (disregarding  mpu  for a moment) slot 0 would have 8, slot 1 would
               have 16 and so on, up to slot 127 with 2048. Note,  that  mpu  >  0
               must  be specified, and slots that would get less than specified by
               mpu, will get mpu instead. If you don't specify mpu, the size table
               will  not  be  created  at  all  (it wouldn't make any difference),
               although any overhead value will be respected during  calculations.
           atm, adsl
               ATM  linklayer  consists  of 53 byte cells, where each of them pro-
               vides 48 bytes for payload. Also all the cells must be  fully  uti-
               lized, thus the last one is padded if/as necessary.
               When  size  table  is  calculated, adjusted size that fits properly
               into lowest amount of cells is assigned to a slot. For  example,  a
               100  byte long packet requires three 48-byte payloads, so the final
               size would require 3 ATM cells - 159 bytes.
               For ATM size tables, 16 bytes sized slots are perfectly enough. The
               default values of mtu and tsize create 4 bytes sized slots.


           The following values are typical for different adsl scenarios (based on
           [1] and [2]):
           LLC based:
               PPPoA - 14 (PPP - 2, ATM - 12)
               PPPoE - 40+ (PPPoE - 8, ATM - 18, ethernet 14, possibly FCS - 4+padding)
               Bridged - 32 (ATM - 18, ethernet 14, possibly FCS - 4+padding)
               IPoA - 16 (ATM - 16)
           VC Mux based:
               PPPoA - 10 (PPP - 2, ATM - 8)
               PPPoE - 32+ (PPPoE - 8, ATM - 10, ethernet 14, possibly FCS - 4+padding)
               Bridged - 24+ (ATM - 10, ethernet 14, possibly FCS - 4+padding)
               IPoA - 8 (ATM - 8)
           There're few important things regarding the above overheads:
           ?   IPoA  in LLC case requires SNAP, instead of LLC-NLPID (see rfc2684)
               - this is the reason, why it actually takes more space than  PPPoA.
           ?   In  rare  cases,  FCS  might be preserved on protocols that include
               ethernet frame (Bridged and PPPoE). In such situation, any ethernet
               specific  padding  guaranteeing  64 bytes long frame size has to be


           It's often forgotten, that modern network cards  (even  cheap  ones  on
           desktop  motherboards)  and/or  their  drivers  often support different
           offloading mechanisms. In context of traffic shaping, 'tso'  and  'gso'
           might cause undesirable effects, due to massive tcp segments being con-
           sidered during traffic shaping (including stab calculations). For  slow
           uplink interfaces, it's good to use ethtool to turn off offloading fea-


           tc(8), tc-hfsc(7), tc-hfsc(8),
           Please direct bugreports and patches to: <>


           Manpage created by Michal Soltys (

    iproute2 31 October 2011 STAB(8)


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