In this episode of PING, Shumon Huque from Salesforce discusses how protocols with extensible flag fields can benefit from regular testing of the values possible in the packet structure. This technique is known as "greasing" and has a strong metaphorical meaning of "greasing the wheels" to ensure future uses aren't blocked by mistaken beliefs about the possible values.

Intermediate systems (so-called "middleboxes") have to try and determine "risky" packetflows, and one of the mechanisms they use is to consider unexpected values in the known packetflows as possibly dangerous. This is an over-simplistic approach, and risks "ossifying" a protocol into the range of values which are actively in use now. Protocols usually include extra potential values for flag-fields, settings, options and the like, and these frequently have a large range of "reserved" values which are held in trust in an IANA registry, for future use. Greasing is a proposed mechanism to test out some of these values, and see what happens "on the wire" for the protocol in question.

Shumon and his co-author and collaborator Mark Andrews from ISC have been applying the greasing model to the DNS, and we talked about it's history in other protocols, and how in practice greasing can be applied on the global internet.

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