Ubuntu Manpage: tc-stab - Generic size table manipulations

Provided by: iproute2_6.1.0-1ubuntu2_amd64

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.

AUTHOR

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