tc-stab manpage

Search topic Section

STAB(8)				     Linux			       STAB(8)

       tc-stab - Generic size table manipulations

       tc qdisc add ... stab
	   [ mtu BYTES ] [ tsize SLOTS ]
	   [ mpu BYTES ] [ overhead BYTES ]
	   [ linklayer { 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 specification.

       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
       enqueues 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
       interface.  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
       enqueue;	 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
       parameters.  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 usu-
       ally 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
       depend 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

       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):

	   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 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.

       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].

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

       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>

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

iproute2			31 October 2011			       STAB(8)