tc-stab

STAB(8)                              Linux                             STAB(8)

NAME
       tc-stab - Generic size table manipulations

SYNOPSIS
       tc qdisc add ... stab
           [ mtu BYTES ] [ tsize SLOTS ]
           [ mpu BYTES ] [ overhead BYTES ]
           [ linklayer { adsl | atm | ethernet } ] ...

OPTIONS
       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 specification.

DESCRIPTION
       Size  tables  allow  manipulation of packet sizes, as seen by the whole
       scheduler framework (of course, the  actual  packet  size  remains  the
       same).  Adjusted packet size is calculated only once - when a qdisc en-
       queues the packet. Initial root enqueue  initializes  it  to  the  real
       packet's size.

       Each  qdisc  can  use  a different size table, but the adjusted size is
       stored in an area shared by whole qdisc hierarchy attached to  the  in-
       terface.  The  effect  is that if you have such a 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  en-
       queue;  and  in  turn, whenever HFSC tries to dequeue a packet, it will
       use a potentially 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.

       The initial size table is calculated by tc tool using mtu and tsize pa-
       rameters.  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
       17 - 32, ..., slot 127 to 2033 - 2048. Sizes assigned to each slot  de-
       pend on linklayer parameter.

       Stab calculation is also safe for an unusual case, when a size assigned
       to a slot would be larger than  2^16-1  (you  will  lose  the  accuracy
       though).

       During  the  kernel  part  of  packet size adjustment, overhead will be
       added to original size, and then slot will be calculated. If  the  size
       would  cause  overflow,  more than 1 slot will be used to get the final
       size. This of course will  affect  accuracy,  but  it's  only  a  guard
       against unusual situations.

       Currently  there  are 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), al-
           though 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 the 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 are a 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  frames (Bridged and PPPoE). In such situation, any Ether-
           net specific padding guaranteeing 64 bytes long frame size  has  to
           be  included  as  well  (see RFC2684).  In the other words, it also
           guarantees that any packet you send will take minimum 2 atm  cells.
           You should set mpu accordingly for that.

       o   When  the  size  table is consulted, and you're shaping traffic for
           the sake of another modem/router, an Ethernet header (without  pad-
           ding)  will already be added to initial packet's length. You should
           compensate for that by subtracting 14 from the above  overheads  in
           this  case.  If you're shaping directly on the router (for example,
           with speedtouch usb modem) using ppp daemon, you're  using  raw  ip
           interface without underlying layer2, so nothing will be added.

           For more thorough explanations, please see [1] and [2].

ETHERNET CARDS CONSIDERATIONS
       It's  often  forgotten  that  modern  network cards (even cheap ones on
       desktop motherboards) and/or their drivers often support different  of-
       floading mechanisms. In the 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: <netdev@vger.kernel.org>

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

iproute2                        31 October 2011                        STAB(8)