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   tc-hfcs    ( 7 )

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REALTIME CRITERION

RT criterion ignores class hierarchy and guarantees precise bandwidth and delay allocation. We say that a packet is eligible for sending, when the current real time is later than the eligible time of the packet. From all eligible packets, the one most suited for sending is the one with the shortest deadline time. This sounds simple, but consider the following example:

Interface 10Mbit, two classes, both with two-piece linear service curves:

• 1st class - 2Mbit for 100ms, then 7Mbit (convex - 1st slope < 2nd slope)

• 2nd class - 7Mbit for 100ms, then 2Mbit (concave - 1st slope > 2nd slope)

Assume for a moment, that we only use D() for both finding eligible packets, and choosing the most fitting one, thus eligible time would be computed as D^(-1)(w) and deadline time would be computed as D^(-1)(w+l). If the 2nd class starts sending packets 1 second after the 1st class, it's of course impossible to guarantee 14Mbit, as the interface capability is only 10Mbit. The only workaround in this scenario is to allow the 1st class to send the packets earlier that would normally be allowed. That's where separate E() comes to help. Putting all the math aside (see HFSC paper for details), E() for RT concave service curve is just like D(), but for the RT convex service curve - it's constructed using only RT service curve's 2nd slope (in our example 7Mbit).

The effect of such E() - packets will be sent earlier, and at the same time D() will be updated - so the current deadline time calculated from it will be later. Thus, when the 2nd class starts sending packets later, both the 1st and the 2nd class will be eligible, but the 2nd session's deadline time will be smaller and its packets will be sent first. When the 1st class becomes idle at some later point, the 2nd class will be able to "buffer" up again for later active period of the 1st class.

A short remark - in a situation, where the total amount of bandwidth available on the interface is larger than the allocated total realtime parts (imagine a 10 Mbit interface, but 1Mbit/2Mbit and 2Mbit/1Mbit classes), the sole speed of the interface could suffice to guarantee the times.

Important part of RT criterion is that apart from updating its D() and E(), also V() used by LS criterion is updated. Generally the RT criterion is secondary to LS one, and used only if there's a risk of violating precise realtime requirements. Still, the "participation" in bandwidth distributed by LS criterion is there, so V() has to be updated along the way. LS criterion can than properly compensate for non-ideal fair sharing situation, caused by RT scheduling. If you use UL service curve its F() will be updated as well (UL service curve is an extension to LS one - see UPPERLIMIT CRITERION section).

Anyway - careless specification of LS and RT service curves can lead to potentially undesired situations (see CORNER CASES for examples). This wasn't the case in HFSC paper where LS and RT service curves couldn't be specified separately.