Operation Manual

NetWAYS/ISDN – Glossary 71
SMTP (Simple Mail Transfer Protocol)
SMTP is a standard protocol for exchanging e-mail between
computers. SMTP implementations listen on TCP port 25. The
protocol structure is simple, supporting only e-mail transmis-
sion over a data network. SMTP is defined in RFC 821.
Spoofing
“Spoofing” in data communication means to send data with
a false source address, pretending to be from a different sys-
tem.
Several network applications are known to exchange data
packets that can cause frequent, unnecessary physical con-
nections when operated over ISDN WAN links. Some packet
types, in particular those used by Windows file and printer
sharing, require acknowledgement from the remote system.
They cannot simply filter such packets out of the data stream
going over the ISDN link, since without the response the
server would consider the client application to be inactive.
The responses are therefore “spoofed”, or generated at the
local end using the remote client's source address. If the
ISDN connection is physically active, the packets can be sent
over the ISDN line. As soon as the physical connection is in-
terrupted by the inactivity timeout, and as long as the logical
ISDN connection persists, the remote access software an-
swers the packets locally, simulating the existence of a phys-
ical connection to the remote site. Once the physical ISDN
connection has been dialed up again due to user data,
spoofing stops and the overhead packets are transported
over ISDN again.
The spoofing mechanisms to be used are negotiated with the
remote client on connection set-up in accordance with the
PSCP Draft. If the remote client does not support spoofing,
the function is not activated.
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