Ubuntu Manpage: tc-stab - Generic size table manipulations

Provided by: iproute2_5.15.0-1ubuntu2_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)

iproute2                                         31 October 2011                                         STAB(8)