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):
- 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.
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),  http://ace-host.stuart.id.au/russell/files/tc/tc-atm/  http://www.faqs.org/rfcs/rfc2684.html
Please direct bugreports and patches to: <email@example.com>