Lantronix ETS Reference Manual - page 59
Server Configuration
TCP/IP Server Parameters
4-7
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Until the port is actively running SLIP, it can be used normally as a service port or a login port. If a
modem is attached to the port, users can connect to the port and dial out. The port changes to SLIP
interpretation only after receiving the Set SLIP command.
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After the SLIP session is started, the Local mode of the ETS is not reachable again until the port is
logged out, thus terminating the SLIP session. The port can be logged out via the Logout Port
command or via de-assertion of DSR on the serial line.
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A SLIP session’s characteristics, such as IP address and MTU, cannot be modified once SLIP is
started.
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Modem control has to be enabled (see Define Port Modem Control) for SLIP connections running
across a modem. Otherwise the SLIP session will not be terminated even though the user might have
hung up the phone line. When modem control is enabled, the SLIP session will be terminated when
the locally-attached modem hangs up.
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SLIP data is “binary” in the sense that any 8-bit character can occur in the SLIP data stream. Because
of this, attached modems cannot be set to use XON/XOFF or any other character-based flow control
method. If they do, the session will hang and/or lose data.
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All devices between the SLIP host and the ETS must pass characters unchanged, including XON/
XOFF flow control characters. Many devices can be configured turn flow control off, but will still
strip XON/XOFF characters out of the data stream. This is not acceptable.
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The ETS will force the serial port’s flow control to None for the duration of the SLIP session, and
will restore it when the SLIP session ends.
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Hardware flow control is unaffected by SLIP operation.
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Seven-bit characters cannot be used; the SLIP session will force 8-bit mode.
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MTU is the size of the largest packet that can be sent unfragmented on a particular media. Packets
larger than that size must be fragmented into two or more packets and then reassembled by the remote
host. The MTU value and IP fragmentation can have a large effect on both the interoperability and
performance of SLIP connections.
In general, the ETS MTU for SLIP should be as high as possible without exceeding the remote host’s
MTU values. If the remote SLIP device does not support IP packet reassembly, the MTU should be
set as high as possible to prevent any fragmentation.
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The Ethernet MTU is 1536 bytes. The SLIP guidelines (RFC 1055) suggest an MTU of 1006 bytes
for SLIP connections. Some SLIP implementations, however, have a lower MTU. In those cases the
SLIP MTU on the ETS must be set lower, which increases IP fragmentation, which in turn decreases
performance and can increase network traffic as more packet headers are needed for the fragmented
packets.
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Maximum packet size (MTU) and whether or not to pass IP broadcast packets to the SLIP lines can
be configured via the Set/Define SLIP commands.