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Re: Standard GDB Remote Protocol


>>>>> "Andrew" == Andrew Cagney <ac131313@cygnus.com> writes:
>> If we change the lower levels, this becomes easy.  Each packet
>> would have a 'protocol' field which would indicate which upper
>> level stack would handle the packet.  One value would indicate be a
>> GDB command/ response protocl.  Another would be a Link Control
>> Protocol used to establish/negotiate link layer options (8 vs. 7
>> bit, hex vs. base64 encoding, RLE compression, etc.).

Andrew> This is starting to sound like IP:-) Just as long as it isn't
Andrew> as heavy :-)

I was thinking more along the lines of something between SL/IP and
PPP.

WRS uses SL/IP with a shim IP and UDP layer, on top of which they run
their SunRPC based protocol.  It's not a full stack by any means, but
it's enough of one that you can connect a target to a terminal server
(or configure your host to run SL/IP) and connect to it over the
network.  The disadvantage of this is that it adds substantial packet
overhead (IP and UDP headers) and target code to implement the shim
network layers.

SL/IP by itself doesn't buy us much, since it's really just a packet
framing mechanism.  In many ways, it is inferior to the existing RDP
framing.  For example, packet integrity (checksums) is left to upper
network layers.

PPP is more complicated.  Although it should be possible to have RDP
as one of PPP's encapsualted protocols (like IP, IPX, etc.), I don't
think we'd actually want to do so.  I suspect that GDB would have to
implement a host side stack as well, even on hosts that support PPP,
in order for GDB to tunnel RDP.  Of course, the target would need a
PPP stack as well.

Even without those problems, It's not clear that PPP is appropriate.
PPP leaves reliable transport to the upper layers; and that's one of
the problems we're having with the existing RDP.  

So, what we want is something with the simplicity and efficiency of
SL/IP, with link layer configuration/negotiation and multi protocol
support of PPP, plus a reliable transport shared by all protcols (RDP
commands, virtual I/O, etc.)

        --jtc

-- 
J.T. Conklin
RedBack Networks

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