On Packet Reordering in Time-Sensitive Networks
release_eingzlulvfd6pjf2i36gbildoi
by
Ehsan Mohammadpour, Jean-Yves Le Boudec
2021
Abstract
Time-sensitive networks (IEEE TSN or IETF DetNet) may tolerate some packet
reordering. Re-sequencing buffers are then used to provide in-order delivery,
the parameters of which (timeout, buffer size) may affect worst-case delay and
delay jitter. There is so far no precise understanding of per-flow reordering
metrics nor of the dimensioning of re-sequencing buffers in order to provide
worst-case guarantees, as required in such networks. First, we show that a
previously proposed per-flow metric, reordering late time offset (RTO),
determines the timeout value. If the network is lossless, another previously
defined metric, the reordering byte offset (RBO), determines the required
buffer. If packet losses cannot be ignored, the required buffer may be larger
than RBO, and depends on jitter, an arrival curve of the flow at its source,
and the timeout. Then we develop a calculus to compute the RTO for a flow path;
the method uses a novel relation with jitter and arrival curve, together with a
decomposition of the path into non order-preserving and order-preserving
elements. We also analyse the effect of re-sequencing buffers on worst-case
delay, jitter and propagation of arrival curves. We show in particular that, in
a lossless (but non order-preserving) network, re-sequencing is "for free",
namely, it does not increase worst-case delay nor jitter, whereas in a lossy
network, re-sequencing increases the worst-case delay and jitter. We apply the
analysis to evaluate the performance impact of placing re-sequencing buffers at
intermediate points and illustrate the results on two industrial test cases.
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