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
       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
       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
       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
       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),
	   although any overhead value will be respected during calculations.

       atm, adsl
	   ATM linklayer consists of 53 byte cells, where each of them
	   provides 48 bytes for payload. Also all the cells must be fully
	   utilized, 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:

       •   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 frames (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
	   guarantees that any packet you send will take minimum 2 atm cells.
	   You should set mpu accordingly for that.

       •   When the size table is consulted, and you're shaping traffic for
	   the sake of another modem/router, an Ethernet header (without
	   padding) 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
       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.

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)