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Re: [ndt-dev] Allowing protocol changes without losing backward compatibility

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  • From: Aaron Brown <>
  • To: Sebastian Kostuch <>
  • Cc: "<>" <>
  • Subject: Re: [ndt-dev] Allowing protocol changes without losing backward compatibility
  • Date: Tue, 1 Apr 2014 13:11:36 +0000
  • Accept-language: en-US

It might be cleaner in the long run to introduce a new message type (MSG_EXTENDED_LOGIN or something). The client could try logging in with that extended login message, and if it fails, it could fall back to MSG_LOGIN if applicable. We could probably also move to a JSON structure as the message content in the process. If we think through that route, it might make it easier down the road to do something that works over HTTP as well.


On Mar 31, 2014, at 8:56 AM, Sebastian Kostuch <> wrote:

while working on Issue136 I have encountered some difficulties with making little changes in middlebox protocol
without making old client not working with new server. So after some investigation I would like to propose
following changes in order to solve this problem:

To make protocol changes possible in future versions without losing backward compatibility both server and client
need to know versions of each other. At this moment server has no information about client one. To fix this we
need our clients to sent such information before tests are being run but we can't either change currently sent messages
(such as the login message which could contain client version) either pass some new type of message from client to
server (old server would know nothing about it).

So probably it would be good to create some new test running before any other for only this versions exchange.
This test should be obligatory (when server will not receive id of this one in client's request then it will treat client as the
one with older versions, before proposed changes). On the other  hand  when new client will send such test request to
old server, then it would be just ignored (marked as not supported) and all will work ok also. This way backward
compatibility should be assured.

Having both server and client know about their versions we can then perform some changes in tests protocol. And
here goes the solution for Issue136: instead of always sending additional RTT and TCP buffer size values server will
just check which client he is connected to. If it is older one which does not support newest middlebox test protocol then
these values will not be attached to final results.

Also instead of checking only client/server version we could introduce numbering of individual tests protocols and server
would sent such information to client before tests (i.e. string containing entries such as "[testID]-[protocolVersion]").

What do you think about such solution? Is this good way to assure backward compatibility and making introducing
some protocol changes in future easier? Also if you have any questions feel free to ask them :).

Sebastian Kostuch

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