[Cake] More overhead keywords

[Jonathan Morton <chromatix99@gmail.com>]

This is entirely userspace (ie. iproute2) stuff:

The initial cake-overhead patch included only “raw” and “conservative” shortcut keywords, alongside the numeric “overhead” parameter for experts.  I’ve now worked out an extended set of keywords which, I think, takes care of all the normal cases.

There are eight new keywords which deal with the basic ADSL configurations.  These switch on ATM cell-framing compensation, and set the overhead based on the raw IP packet as a baseline.

ipoa-vcmux (8)
ipoa-llcsnap (16)
bridged-vcmux (24)
bridged-llcsnap (32)
pppoa-vcmux (10)
pppoa-llc (14)
pppoe-vcmux (32)
pppoe-llcsnap (40)

Note that “pppoa-llc” is not a typo - it really doesn’t involve SNAP, and is thus a little more compact than if it did.

Two more new keywords deal with the basic VDSL2 configurations.  Again, the overheads use IP as a baseline, but this time ATM cell-framing is turned off.  Apparently PTM does have a small additional overhead on the order of 1/128, due to HDLC framing which attaches special meaning to 0x7D and 0x7E bytes; I might need to add approximate handling for that, kernel-side.

pppoe-ptm (27)
bridged-ptm (19)

The final three keywords are not for standalone use, but act as modifiers to some previous keyword.  They can be specified more than once, which is probably only useful for “ether-vlan”.

via-ethernet (-14)
ether-fcs (+4)
ether-vlan (+4)

Does all this look reasonable?  In particular, have I missed anything regarding PTM/VDSL2?  I’m using http://www.ethernetinthefirstmile.com/articles/WTC2002.pdf as a reference, but it isn’t as clear as it might be.

 - Jonathan Morton