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   tc-cake    ( 8 )

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OVERHEAD COMPENSATION PARAMETERS

The size of each packet on the wire may differ from that seen by Linux. The following parameters allow CAKE to compensate for this difference by internally considering each packet to be bigger than Linux informs it. To assist users who are not expert network engineers, keywords have been provided to represent a number of common link technologies.

Manual Overhead Specification overhead BYTES Adds BYTES to the size of each packet. BYTES may be negative; values between -64 and 256 (inclusive) are accepted.

mpu BYTES Rounds each packet (including overhead) up to a minimum length BYTES. BYTES may not be negative; values between 0 and 256 (inclusive) are accepted.

atm Compensates for ATM cell framing, which is normally found on ADSL links. This is performed after the overhead parameter above. ATM uses fixed 53-byte cells, each of which can carry 48 bytes payload.

ptm Compensates for PTM encoding, which is normally found on VDSL2 links and uses a 64b/65b encoding scheme. It is even more efficient to simply derate the specified shaper bandwidth by a factor of 64/65 or 0.984. See ITU G.992.3 Annex N and IEEE 802.3 Section 61.3 for details.

noatm Disables ATM and PTM compensation.

Failsafe Overhead Keywords These two keywords are provided for quick-and-dirty setup. Use them if you can't be bothered to read the rest of this section.

raw (default) Turns off all overhead compensation in CAKE. The packet size reported by Linux will be used directly.

Other overhead keywords may be added after "raw". The effect of this is to make the overhead compensation operate relative to the reported packet size, not the underlying IP packet size.

conservative Compensates for more overhead than is likely to occur on any widely-deployed link technology. Equivalent to overhead 48 atm.

ADSL Overhead Keywords Most ADSL modems have a way to check which framing scheme is in use. Often this is also specified in the settings document provided by the ISP. The keywords in this section are intended to correspond with these sources of information. All of them implicitly set the atm flag.

pppoa-vcmux Equivalent to overhead 10 atm

pppoa-llc Equivalent to overhead 14 atm

pppoe-vcmux Equivalent to overhead 32 atm

pppoe-llcsnap Equivalent to overhead 40 atm

bridged-vcmux Equivalent to overhead 24 atm

bridged-llcsnap Equivalent to overhead 32 atm

ipoa-vcmux Equivalent to overhead 8 atm

ipoa-llcsnap Equivalent to overhead 16 atm

See also the Ethernet Correction Factors section below.

VDSL2 Overhead Keywords ATM was dropped from VDSL2 in favour of PTM, which is a much more straightforward framing scheme. Some ISPs retained PPPoE for compatibility with their existing back-end systems.

pppoe-ptm Equivalent to overhead 30 ptm

PPPoE: 2B PPP + 6B PPPoE + ETHERNET: 6B dest MAC + 6B src MAC + 2B ethertype + 4B Frame Check Sequence + PTM: 1B Start of Frame (S) + 1B End of Frame (Ck) + 2B TC- CRC (PTM-FCS)

bridged-ptm Equivalent to overhead 22 ptm ETHERNET: 6B dest MAC + 6B src MAC + 2B ethertype + 4B Frame Check Sequence + PTM: 1B Start of Frame (S) + 1B End of Frame (Ck) + 2B TC- CRC (PTM-FCS)

See also the Ethernet Correction Factors section below.

DOCSIS Cable Overhead Keyword DOCSIS is the universal standard for providing Internet service over cable-TV infrastructure.

In this case, the actual on-wire overhead is less important than the packet size the head-end equipment uses for shaping and metering. This is specified to be an Ethernet frame including the CRC (aka FCS).

docsis Equivalent to overhead 18 mpu 64 noatm

Ethernet Overhead Keywords ethernet Accounts for Ethernet's preamble, inter-frame gap, and Frame Check Sequence. Use this keyword when the bottleneck being shaped for is an actual Ethernet cable. Equivalent to overhead 38 mpu 84 noatm

ether-vlan Adds 4 bytes to the overhead compensation, accounting for an IEEE 802.1Q VLAN header appended to the Ethernet frame header. NB: Some ISPs use one or even two of these within PPPoE; this keyword may be repeated as necessary to express this.