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CompuServe

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c) Packet Type `N': Data.
This is a data packet. The data being transferred is broken
up into a series of <N> packets. There may be zero to the
negotiated data block size. A count of zero does not imply
end of file.
d) Packet Type `F': Failure.
This is the general Failure Packet. A Failure Packet may be
sent at any time; the value of the <Sequence> field is
ignored. The first character of the <Body> indicates the
general nature of the failure, and the remainder may be a
printable ASCII message further describing the condition.
The following standard failures must be supported:
`A': Abort. Usually sent if the user requests that the
terminal program cease tranferring data.
`C': Capacity failure. Out of memory or disk.
`E': Processing error. Any error other than described
by another Failure Packet.
`I': I/O error occured.
`M': File requested for Uploading is Missing (not
found).
`N': Unimplemented Packet Type was received.
`S': Protocol Sequence Number failure.
`r`: Transfer Resume failure.
When a Failure Packet is received, it must be acknowledged and
the transfer aborted. Similarly, when a Failure Packet is sent,
the sending entity must wait for the acknowledge to arrive. In
addition, the entity which sends the Failure Packet must ignore
all other packets while waiting for the acknowledgement to
arrive. Once the Failure Packet is successfully transfered,
both entities are expected to leave protocol mode.
11
e) Optional `T' Packets.
Several "T" Packets may optionally be supported. The
support for these packets is specified by the DR, UR, and FI
Transport Parameters.
1) Packet `Tr' : Download Resume.
When the Responder receives the "TD" (Download Request)
packet and the specified file already exits, it may attempt
to resume the download which was aborted during a prior
session. In order to do this, both entities must specify
Transport Parameter DR > 0x00. If the Responder attempts
the resumption, a `Tr' Packet is returned to the Initiator
as follows:
<DLE> B <Sequence> T r <Length> <CRC-16> <ETX> <Trailer>
where:
<Length> = current number of bytes in Responder's
existing file.
<CRC-16> = XMODEM CRC-16, initialized to 0xffff, of all
existing data in Responder's file.
Both <Length> and <CRC-16> are transmitted as an ASCII
decimal string and are both followed by at least a single
space.
The Initiator will perform the same CRC-16 calculation over
<Length> bytes and compare the resulting CRC-16 values. If
the calculated and `Tr' values agree, downloading continues
with `N` Packets and Responder appends the data to the
existing file. If the files match exactly, a `TC' (Transfer
Close) packet is sent.
If the CRC-16 values do not match, or the Initiator's file
be shorter than <Length> bytes, the Initiator's response