Subject: SDP> Proposal posted

From: Roger K Debry <[email-masked]>
Date: Wed, 29 Apr 1998 11:46:58 -0400

Harry Lewis and I have put together an SDP proposal for your review.
We believe that this approach meets all of the main SDP requirements,
while preserving the IPPv1 encoding and the work done to define
Printer and Job Monitoring MIBs. The white paper is at

ftp.pwg.org/pub/pwg/sdp/sdp-proposal.txt
ftp.pwg.org/pub/pwg/sdp/sdp-proposal.pdf

We would appreciate your reading this proposal and providing us
with comments. Harry will present this proposal at the upcoming
PWG meeting in Washington.

Roger K deBry
Senior Technical Staff Member
Architecture and Technology
IBM Printing Systems
email: [email-masked]
phone: 1-303-924-4080

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From: Carl Kugler <[email-masked]>
Date: Fri, 1 May 1998 16:21:46 -0400

Regarding ftp://ftp.pwg.org/pub/pwg/sdp/sdp-proposal.pdf:

I think a synch BYTE (Start of Packet Byte) is too small to be useful.  One
could reasonably expect to encounter 0x25 every 256 bytes, on average, in a
binary PDL file.  Why not a synch WORD or DWORD, say 0x3f2c59d1?  You'd only
see that randomly every 4 GB or so.  The additional 3 bytes would be
insignificant compared to IPP's verbosity.

What does an acknowledgment (ACK) look like?  Is there such a thing as a NACK?
What happens if a required ACK is not received?  Retries?

How many packet length bytes are there?  Is there any restriction on packet
size?

   -Carl Kugler

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From: Carl Kugler <[email-masked]>
Date: Fri, 1 May 1998 18:59:31 -0400

More comments about ftp://ftp.pwg.org/pub/pwg/sdp/sdp-proposal.pdf:

The requirements for synchronization and acknowledgments seem to imply that
this protocol is intended to be suitable for use over unreliable transports.
>From that, one could derive two additional requirements:

1.  Must be able to detect data corruption.
2.  Must be able to recover from data loss.

One way to address these requirements would be to borrow some ideas from TCP:
specifically, checksums and sequence numbers.  The SDP Packet  Header could
include a checksum for the packet that a recipient could use to verify data
integrity.  Packets that failed checksum would be dropped and not
acknowledged.  Acknowledgments could return the sequence number of the last
contiguously received good packet.  Then, if synch were lost or packets were
dropped, the sender would back up and resend starting with the packet following
the last acknowledged.

 -Carl