< draft-ietf-6lo-fragment-recovery-15.txt		draft-ietf-6lo-fragment-recovery-16.txt >
	6lo P. Thubert, Ed.		6lo P. Thubert, Ed.
	Internet-Draft Cisco Systems		Internet-Draft Cisco Systems
	Updates: 4944 (if approved) 9 March 2020		Updates: 4944 (if approved) 9 March 2020
	Intended status: Standards Track		Intended status: Standards Track
	Expires: 10 September 2020		Expires: 10 September 2020
	6LoWPAN Selective Fragment Recovery		6LoWPAN Selective Fragment Recovery
	draft-ietf-6lo-fragment-recovery-15		draft-ietf-6lo-fragment-recovery-16
	Abstract		Abstract
	This draft updates RFC 4944 with a simple protocol to recover		This draft updates RFC 4944 with a simple protocol to recover
	individual fragments across a route-over mesh network, with a minimal		individual fragments across a route-over mesh network, with a minimal
	flow control to protect the network against bloat.		flow control to protect the network against bloat.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	skipping to change at page 14, line 23 ¶		skipping to change at page 14, line 23 ¶
	to a bitmap that acknowledges only the received fragments to let the		to a bitmap that acknowledges only the received fragments to let the
	intermediate nodes know that the datagram is fully received. As the		intermediate nodes know that the datagram is fully received. As the
	timer runs, the reassembling endpoint absorbs the fragments that were		timer runs, the reassembling endpoint absorbs the fragments that were
	still in flight for that datagram without creating a new state,		still in flight for that datagram without creating a new state,
	acknowledging the ones that that bear an Ack-Request with an FRAG		acknowledging the ones that that bear an Ack-Request with an FRAG
	Acknowledgment and the FULL bitmap. The reassembling endpoint aborts		Acknowledgment and the FULL bitmap. The reassembling endpoint aborts
	the communication if fragments with matching source and Datagram-Tag		the communication if fragments with matching source and Datagram-Tag
	continue to be received after the timer expires.		continue to be received after the timer expires.
	Note that acknowledgments might consume precious resources so the use		Note that acknowledgments might consume precious resources so the use
	of unsolicited acknowledgments should be configurable and not enabled		of unsolicited acknowledgments SHOULD be configurable and not enabled
	by default.		by default.
	An observation is that streamlining forwarding of fragments generally		An observation is that streamlining forwarding of fragments generally
	reduces the latency over the LLN mesh, providing room for retries		reduces the latency over the LLN mesh, providing room for retries
	within existing upper-layer reliability mechanisms. The fragmenting		within existing upper-layer reliability mechanisms. The fragmenting
	endpoint protects the transmission over the LLN mesh with a retry		endpoint protects the transmission over the LLN mesh with a retry
	timer that is configured for a use case and may be adapted		timer that is configured for a use case and may be adapted
	dynamically, e.g., according to the method detailed in [RFC6298]. It		dynamically, e.g., according to the method detailed in [RFC6298]. It
	is expected that the upper layer retries obey the recommendations in		is expected that the upper layer retries obey the recommendations in
	[RFC8085], in which case a single round of fragment recovery should		[RFC8085], in which case a single round of fragment recovery should
	fit within the upper layer recovery timers.		fit within the upper layer recovery timers.
	Fragments are sent in a round-robin fashion: the fragmenting endpoint		Fragments MUST be sent in a round-robin fashion: the sender MUST send
	sends all the fragments of the datagram for a first time before it		all the fragments for a first time before it retries any lost
	retries any lost fragment; lost fragments are retried in sequence,		fragment; lost fragments MUST be retried in sequence, oldest first.
	oldest first through the whole datagram. This mechanism enables the		This mechanism enables the receiver to acknowledge fragments that
	reassembling endpoint to acknowledge fragments that were delayed in		were delayed in the network before they are retried.
	the network before they are retried.
	When a single radio frequency is used by contiguous hops, the		When a single radio frequency is used by contiguous hops, the
	fragmenting endpoint should insert a delay between the frames (e.g.,		fragmenting endpoint SHOULD insert a delay between the frames (e.g.,
	carrying fragments) that are sent to the same next hop. The delay		carrying fragments) that are sent to the same next hop. The delay
	should cover multiple transmissions so as to let a frame progress a		SHOULD cover multiple transmissions so as to let a frame progress a
	few hops and avoid hidden terminal issues. This precaution is not		few hops and avoid hidden terminal issues. This precaution is not
	required on channel hopping technologies such as Time Slotted Channel		required on channel hopping technologies such as Time Slotted Channel
	Hopping (TSCH) [RFC6554], where nodes that communicate at Layer-2 are		Hopping (TSCH) [RFC6554], where nodes that communicate at Layer-2 are
	scheduled to send and receive respectively, and different hops		scheduled to send and receive respectively, and different hops
	operate on different channels.		operate on different channels.
	6.1. Forwarding Fragments		6.1. Forwarding Fragments
	It is assumed that the first fragment is large enough to carry the		It is assumed that the first fragment is large enough to carry the
	IPv6 header and make routing decisions. If that is not so, then this		IPv6 header and make routing decisions. If that is not so, then this
	skipping to change at page 17, line 4 ¶		skipping to change at page 17, line 4 ¶
	RFRAG-ACK flows back to the fragmenting endpoint.		RFRAG-ACK flows back to the fragmenting endpoint.
	If the reverse LSP is not found, the router MUST silently drop the		If the reverse LSP is not found, the router MUST silently drop the
	RFRAG-ACK message.		RFRAG-ACK message.
	Either way, if the RFRAG-ACK indicates that the fragment was entirely		Either way, if the RFRAG-ACK indicates that the fragment was entirely
	received (FULL bitmap), it arms a short timer, and upon timeout, the		received (FULL bitmap), it arms a short timer, and upon timeout, the
	VRB and all the associated state are destroyed. Until the timer		VRB and all the associated state are destroyed. Until the timer
	elapses, fragments of that datagram may still be received, e.g. if		elapses, fragments of that datagram may still be received, e.g. if
	the RFRAG-ACK was lost on the path back and the source retried the		the RFRAG-ACK was lost on the path back and the source retried the
	last fragment. In that case, the router forwards the fragment		last fragment. In that case, the router generates an RFRAG-ACK with
	according to the state in the VRB.		a FULL bitmap back to the fragmenting endpoint if an acknowledgement
			was requested, else it silently drops the fragment.
	This specification does not provide a method to discover the number		This specification does not provide a method to discover the number
	of hops or the minimal value of MTU along those hops. In a typical		of hops or the minimal value of MTU along those hops. In a typical
	case, the MTU is constant and the same across the network. But		case, the MTU is constant and the same across the network. But
	should the minimal MTU along the path decrease, it is possible to		should the minimal MTU along the path decrease, it is possible to
	retry a long fragment (say Sequence of 5) with several shorter		retry a long fragment (say Sequence of 5) with several shorter
	fragments with a Sequence that was not used before (e.g., 13 and 14).		fragments with a Sequence that was not used before (e.g., 13 and 14).
	Fragment 5 is marked as abandoned and will not be retried anymore.		Fragment 5 is marked as abandoned and will not be retried anymore.
	Note that when this mechanism is in place, it is hard to predict the		Note that when this mechanism is in place, it is hard to predict the
	total number of fragments that will be needed or the final shape of		total number of fragments that will be needed or the final shape of
	skipping to change at page 18, line 29 ¶		skipping to change at page 18, line 30 ¶
	RFRAG Acknowledgment is flooded on that same subset in the reverse		RFRAG Acknowledgment is flooded on that same subset in the reverse
	direction. Intermediate RFRAG Acknowledgments can be flooded on any		direction. Intermediate RFRAG Acknowledgments can be flooded on any
	sub-subset of that reverse subset that reach back to the source.		sub-subset of that reverse subset that reach back to the source.
	7. Management Considerations		7. Management Considerations
	This specification extends "On Forwarding 6LoWPAN Fragments over a		This specification extends "On Forwarding 6LoWPAN Fragments over a
	Multihop IPv6 Network" [FRAG-FWD] and requires the same parameters in		Multihop IPv6 Network" [FRAG-FWD] and requires the same parameters in
	the reassembling endpoint and on intermediate nodes. There is no new		the reassembling endpoint and on intermediate nodes. There is no new
	parameter as echoing ECN is always on. These parameters typically		parameter as echoing ECN is always on. These parameters typically
	include the reassembly time-out at the reassembling endpoint and an		include the reassembly timeout at the reassembling endpoint and an
	inactivity clean-up timer on the intermediate nodes, and the number		inactivity clean-up timer on the intermediate nodes, and the number
	of messages that can be processed in parallel in all nodes.		of messages that can be processed in parallel in all nodes.
	The configuration settings introduced by this specification only		The configuration settings introduced by this specification only
	apply to the fragmenting endpoint, which is in full control of the		apply to the fragmenting endpoint, which is in full control of the
	transmission. LLNs vary a lot in size (there can be thousands of		transmission. LLNs vary a lot in size (there can be thousands of
	nodes in a mesh), in speed (from 10 Kbps to several Mbps at the PHY		nodes in a mesh), in speed (from 10 Kbps to several Mbps at the PHY
	layer), in traffic density, and in optimizations that are desired		layer), in traffic density, and in optimizations that are desired
	(e.g., the selection of a RPL [RFC6550] Objective Function [RFC6552]		(e.g., the selection of a RPL [RFC6550] Objective Function [RFC6552]
	impacts the shape of the routing graph).		impacts the shape of the routing graph).
	skipping to change at page 20, line 43 ¶		skipping to change at page 20, line 43 ¶
	time that a fragmenting endpoint should wait for an RFRAG		time that a fragmenting endpoint should wait for an RFRAG
	Acknowledgment before it takes the next action. It is greater		Acknowledgment before it takes the next action. It is greater
	than or equal to MinARQTimeOut. It is less than or equal to		than or equal to MinARQTimeOut. It is less than or equal to
	MaxARQTimeOut. See Appendix C for recommendations on computing		MaxARQTimeOut. See Appendix C for recommendations on computing
	the round-trip time. By default a value of 3 times the maximum		the round-trip time. By default a value of 3 times the maximum
	expected round-trip time in the respective network is RECOMMENDED.		expected round-trip time in the respective network is RECOMMENDED.
	MaxARQTimeOut: The maximum amount of time a node should wait for the		MaxARQTimeOut: The maximum amount of time a node should wait for the
	RFRAG Acknowledgment before it takes the next action. It must		RFRAG Acknowledgment before it takes the next action. It must
	cover the longest expected round-trip time, and be several times		cover the longest expected round-trip time, and be several times
	less than the time-out that covers the recomposition buffer at the		less than the timeout that covers the recomposition buffer at the
	reassembling endpoint, which is typically on the order of the		reassembling endpoint, which is typically on the order of the
	minute. An upper bound can be estimated to ensure that the		minute. An upper bound can be estimated to ensure that the
	datagram is either fully transmitted or dropped before an upper		datagram is either fully transmitted or dropped before an upper
	layer decides to retry it.		layer decides to retry it.
	MaxFragRetries: The maximum number of retries for a particular		MaxFragRetries: The maximum number of retries for a particular
	fragment. A default value of 3 is RECOMMENDED. An upper bound		fragment. A default value of 3 is RECOMMENDED. An upper bound
	can be estimated to ensure that the datagram is either fully		can be estimated to ensure that the datagram is either fully
	transmitted or dropped before an upper layer decides to retry it.		transmitted or dropped before an upper layer decides to retry it.
End of changes. 8 change blocks.
	14 lines changed or deleted		14 lines changed or added
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