[HECnet] Old protocols in new ones

[Keith Halewood]

Hi,

I might have posted this to just Paul and Johnny but it's probably good for a bit of general discussion and it might enlighten me because I often have a lot of difficulty in separating the layers and functionality around tunnels of various types, carrying one protocol on top of another.

I use Paul's excellent PyDECnet and about half the circuits I have connecting to others consist of DDCMP running over UDP. I feel as though there's something missing but that might be misunderstanding. A DDCMP packet is encapsulated in a UDP one and sent. The receiver gets it or doesn't because that's the nature of UDP. I'm discovering it's often the latter. A dropped HELLO or its response brings a circuit down. This may explain why there's a certain amount of flapping between PyDECnet's DDCMP over UDP circuits. I notice it a lot between area 31 and me but but much less so with others.

In the old days, DDCMP was run over a line protocol (sync or async) that had its own error correction/retransmit protocol, was it not? So a corrupted packet containing a HELLO would be handled at the line level and retransmitted usually long before a listen timer expired?

Are we missing that level of correction and relying on what happens higher up in DECnet to handle missing packets?

I'm having similar issues (at least on paper) with an implementation of the CI packet protocol over UDP having initially and quite fatally assumed that a packet transmitted over UDP would arrive and therefore wouldn't need any of the lower level protocol that a real CI needed. TCP streams are more trouble in other ways.

Just some thoughts

Keith


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