tc-stab


SYNOPSIS
       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
       tc(8).


       mtu
           maximum packet size we create size table for, assumed 2048  if  not
           specified explicitly

       tsize
           required table size, assumed 512 if not specified explicitly

       mpu
           minimum packet size used in computations

       overhead
           per-packet size overhead (can be negative) used in computations

       linklayer
           required linklayer adaptation.

DESCRIPTION
       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
       size.

       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.

       For example, with mtu = 1500 and tsize = 128, a table  with  128  slots
       will  be created, where slot 0 will correspond to sizes 0-16, slot 1 to

       Currently  there're  two  methods of creating values stored in the size
       table - ethernet and atm (adsl):


       ethernet
           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.

TYPICAL OVERHEADS
       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:

       o   IPoA in LLC case requires SNAP, instead of LLC-NLPID (see  rfc2684)
           - this is the reason, why it actually takes more space than PPPoA.

       o   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
           included as well (see rfc2684).  In the other words, it also  guar-
           antees  that any packet you send will take minimum 2 atm cells. You
           should set mpu accordingly for that.

       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-
       tures.

SEE ALSO
       tc(8), tc-hfsc(7), tc-hfsc(8),
       [1] http://ace-host.stuart.id.au/russell/files/tc/tc-atm/
       [2] http://www.faqs.org/rfcs/rfc2684.html

       Please direct bugreports and patches to: <net...@vger.kernel.org>

AUTHOR
       Manpage created by Michal Soltys (sol...@ziu.info)



iproute2                        31 October 2011                        STAB(8)
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