To me, it would be natural to specify an expiration time as the
time-to-expiration, measured in slot times.
draft-lijo-6lo-expiration-time-00 specifies expiration time as an
absolute time, measured from ASN=0 in microseconds. The latter approach
seems to require larger headers, without making processing much simpler.
It seems to me that measuring expriation time in microseconds is no more
useful than measuring it in slot times, because there is no functional
difference between any of the 10,000 microseconds within a single 10 ms
slot time. But counting microseconds requires an additional 13 bits
(over 1.5 octets) in the header.
Specifying time-to-expiration means that every time the packet is
forwarded, the expiration header must be updated. But it can make the
expiration header shorter. For instance, in the example I mentioned in
my previous message,
Example: In a 6TiSCH network let the time-slot length be 10ms. If
the network has been operational for 2 years, the
packet_origination_time = Current ASN is 6,307,200,000, and the
max_allowable_delay is 1 second, then:
expiration_time = packet_origination_time + max_allowable_delay
= 6,307,200,000*10 ms + 1 second
= 63,072,001,000,000 microseconds
or
expiration_time = ASN 6,307,200,100
Expressing the absolute expiration time in slot times requires 33 bits/5
octets. But the time-to-expiration is only 100 and can be expressed in
7 bits/1 octet.
Dale
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