общие манипуляции с таблицей размеров (Generic size table manipulations)
Имя (Name)
tc-stab - Generic size table manipulations
Синопсис (Synopsis)
tc qdisc add ... stab
[ mtu BYTES ] [ tsize SLOTS ]
[ mpu BYTES ] [ overhead BYTES ]
[ linklayer { adsl | atm | ethernet } ] ...
Параметры (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.
Описание (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.
Смотри также (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>
