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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: TCP limitations (was Re: ISCSI: Urgent Flag requirement violates TCP.)> > But the Urgent pointer only helps when you happen to have a sequence > > hole. So how in practice can it be worth the effort? > > It will permit a high speed endpoint to deliver full pipe aggregate > throughput across multiple streams. Then I still have another question/concern. What happens with out-of-sequence data that arrives that doesn't have an Urgent pointer within it, either due to coalescence or because there just doesn't happen to be an ISCSI header in the packet? I would hope it's buffered even though the receiver doesn't know where it should ultimately wind up. Indeed, if it isn't buffered, then the TCP exhibits the "Failure to retain above-sequence data" problem documented in RFC 2525, and is not unconditionally complaint with the TCP standard. But if there's sufficient buffer for this case, then there pretty much needs to be sufficient buffer for the general case (all data in flight, whether or not it contains an ISCSI header), and also a mechanism to copy the data from its initial buffer to its final location. In which case, what the Urgent framing gains is some performance (avoiding a buffer copy), but not a savings in memory. However, you only get this performance gain for a stream that is likely slowing down, so it still seems to be a corner case and not a significant win. Vern
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