What's "maximum packet size"?

[der Mouse <mouse%Rodents.Montreal.QC.CA@localhost>]

There's a "maximum packet size" in CHANNEL_OPEN and
CHANNEL_OPEN_CONFIRMATION messages.  4254 says that this is "the
maximum size of an individual data packet that can be sent to the
sender".  I've interpreted this to mean that I should send, there, the
maximum payload length I support, where "paylaod" is as in section 6 of
4253 - the part of a binary-protocol packet after MAC and padding are
stripped and decryption and uncompression are done.  My value for this
is 35000.  But when talking to other implementations, I not uncommonly
get crashes where my code complains about a payload overflow, getting a
35009-byte payload from the peer.

Have I misinterpreted 4254?  If so, what *is* that the maximum size of?
The only other thing that makes any sense to me is that it's the "data"
field of CHANNEL_DATA packets (see 4254 5.2); is that what it's
supposed to be?

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