The dictionary is simple, but the actual code to limit the
speed is not part of rp-pppoe. It's only included in a
commercial derivative of rp-pppoe called ServPoET.
Here's the dictionary (not that it will do you much good...)
# $Id: dictionary.roaringpenguin,v 1.2 2001/12/06 18:25:56 dfs Exp $
# Vendor-Specific attributes use the SMI Network Management Private
# Enterprise Code from the "Assigned Numbers" RFC
#
# Roaring Penguin Vendor-Specific Attributes
# Our vendor ID from IANA
VENDOR RoaringPenguin 10055