Watchguard Firebox X20E Reference Manual page 52

PPP initialization strings
mpfto <period>
Specifies how long the PPP session should wait for a valid management
session to begin. If no valid session starts, then PPP will disconnect after this
time-out period. The default is 90 seconds.
mru n
Set the MRU (Maximum Receive Unit) value to n. Pppd will ask the peer to
send packets of no more than n bytes. The minimum MRU value is 128. The
default MRU value is 1,500. A value of 296 is recommended for slow links (40
bytes for TCP/IP header + 256 bytes of data).
mtu n
Set the MTU (Maximum Transmit Unit) value to n. Unless the peer requests a
smaller value via MRU negotiation, pppd will request that the kernel
networking code send data packets of no more than n bytes through the PPP
network interface.
passive
Enables the "passive" option in the LCP. With this option, pppd will attempt
to initiate a connection; if no reply is received from the peer, pppd will then
just wait passively for a valid LCP packet from the peer, instead of exiting, as
it would without this option.
bsdcomp nr,nt
Request that the peer compress packets that it sends, using the BSD-Compress
scheme, with a maximum code size of nr bits, and agree to compress packets
sent to the peer with a maximum code size of nt bits. If nt is not specified, it
defaults to the value given for nr. Values in the range 9 to 15 can be used for nr
and nt; larger values give better compression but consume more kernel
memory for compression dictionaries. Alternatively, a value of 0 for nr or nt
disables compression in the corresponding direction. Use nobsdcomp or
bsdcomp 0 to disable BSD-Compress compression entirely.
debug
Enables connection debugging facilities. If this option is given, pppd will log
the contents of all control packets sent or received in a readable form.
default-asyncmap
Disable asyncmap negotiation, forcing all control.
default-mru
Disable MRU (Maximum Receive Unit) negotiation. With this option, pppd
will use the default MRU value of 1,500 bytes for both the transmit and
receive direction.
deflate nr,nt
Request that the peer compress packets that it sends, using the Deflate
scheme, with a maximum window size of 2**nr bytes, and agree to compress
packets sent to the peer with a maximum window size of 2**nt bytes. If nt is
not specified, it defaults to the value given for nr. Values in the range 8 to 15
can be used for nr and nt; larger values give better compression but consume
more kernel memory for compression dictionaries. Alternatively, a value of 0
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