Network Working Group Shaofu. Peng
Internet-Draft Bin. Tan
Intended status: Standards Track Quan. Xiong
Expires: August 19, 2022 ZTE Corporation
February 15, 2022
IGP Flexible Algorithm with Deterministic Routing
draft-peng-lsr-flex-algo-deterministic-routing-01
Abstract
IGP Flex Algorithm proposes a solution that allows IGPs themselves to
compute constraint based paths over the network, and it also
specifies a way of using Segment Routing (SR) Prefix-SIDs and SRv6
locators, or pure IP prefix to steer packets along the constraint-
based paths. This document describes how to compute deterministic
paths within Flex-algo plane.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Determinisitc Links . . . . . . . . . . . . . . . . . . . . . 4
3.1. Deterministic Link Bound with CQF . . . . . . . . . . . . 4
3.2. Deterministic Link Bound with Deadline . . . . . . . . . 5
3.3. ISIS Advertisement of Deterministic Link . . . . . . . . 6
3.3.1. Advertisement of Forwarding Delay intra Node . . . . 6
3.3.2. Advertisement of CQF Parameters . . . . . . . . . . . 7
3.3.3. Advertisement of Deadline Parameters . . . . . . . . 8
3.4. OSPF Advertisement of Deterministic Link . . . . . . . . 10
4. Deterministic Routes Computation . . . . . . . . . . . . . . 10
4.1. Bind CQF Parameters with Flex-Algo . . . . . . . . . . . 10
4.1.1. ISIS Advertisement of Flex-algo Binding CQF . . . . . 11
4.1.2. OSPF Advertisement of Flex-algo Binding CQF . . . . . 11
4.2. Bind Deadline Parameters with Flex-Algo . . . . . . . . . 11
4.2.1. ISIS Advertisement of Flex-algo Binding Deadline . . 12
4.2.2. OSPF Advertisement of Flex-algo Binding Deadline . . 13
4.3. FAD Flags Extensions . . . . . . . . . . . . . . . . . . 13
4.3.1. ISIS FAD Flags Extensions . . . . . . . . . . . . . . 13
4.3.2. OSPF FAD Flags Extensions . . . . . . . . . . . . . . 14
4.4. CQF based Deterministic Routes Computation . . . . . . . 14
4.5. Deadline based Deterministic Routes Computation . . . . . 14
5. Route Convergence and Redundance Considerations . . . . . . . 16
6. Examples of Deterministic SPF . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
IGP Flex Algorithm [I-D.ietf-lsr-flex-algo] proposes a solution that
allows IGPs themselves to compute constraint based paths over the
network, and it also specifies a way of using Segment Routing
[RFC8402] Prefix-SIDs and SRv6 locators, or pure IP prefix
[I-D.ietf-lsr-ip-flexalgo] to steer packets along the constraint-
based paths. It specifies a set of extensions to ISIS, OSPFv2 and
OSPFv3 that enable a router to send TLVs that identify (a)
calculation-type, (b) specify a metric-type, and (c )describe a set
of constraints on the topology, that are to be used to compute the
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best paths along the constrained topology. A given combination of
calculation-type, metric-type, and constraints is known as an FAD
(Flexible Algorithm Definition).
[RFC8655] describes the architecture of deterministic network and
defines the QoS goals of deterministic forwarding: Minimum and
maximum end-to-end latency from source to destination, timely
delivery, and bounded jitter (packet delay variation); packet loss
ratio under various assumptions as to the operational states of the
nodes and links; an upper bound on out-of-order packet delivery. In
order to achieve these goals, deterministic networks use resource
reservation, explicit routing, service protection and other means. A
deterministic path is typically (but not necessarily) explicit routes
so that it does not normally suffer temporary interruptions caused by
the convergence of routing or bridging protocols.
IGP Flex-algo has the characteristic mentioned in [RFC8655]: under a
single administrative control or within a closed group of
administrative control. IGP Flex-algo supports Min Unidirectional
Link Delay (defined in [RFC8570]) metric type to compute shortest
paths with minimum delay, however, the cumulative delay is
essentially the accumulation of transmission delay of all links,
excluding node delay. In order to make up for this gap, it is
necessary to enhance IGP flex-algo to compute the path with
deterministic delay, i.e., including deterministic node delay and
link transmission delay.
This document describes how to compute distributed shortest paths
with deterministic delay metric within Flex-algo plane, as the basis
of the whole distributed deterministic scheme. It should be noted
that relying on this enhancement alone does not guarantee complete
determinacy, it needs to be used in conjunction with other tools,
such as creating additional backup explicit path with consistent
delay metric for PREOF (Packet Replication, Elimination, and Ordering
Functions), smoothing the delay jitter during route convergence,
providing deterministic forwarding mechanism, etc.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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3. Determinisitc Links
When a packet is forwarded to a link, the delay produced includes two
parts: the first part is the dwell delay of the packet in the node,
and the second part is the transmission delay of the packet on the
link. In packet switching networks, priority based queuing scheme is
generally used. It may give better average latency, but may have
worst case latency. We call those links bound with a queue mechanism
that can not guarantee node delay are non-determinisitc links.
On the contrary, those links bound with a queue mechanism that can
provide deterministic node delay are called deterministic links.
Typical queue mechanisms are:
o IEEE 802.1 WG has specified IEEE802.1Qch [CQF] which uses cyclic
queuing and forwarding (CQF) mechanism and relies on time
synchronization. According to CQF, the maximum delay experienced
by a given packet is (H+1)*D, the minimum delay experienced by a
given packet is (H-1)*D, and the delay jitter is 2*D, where H is
the number of hops and D is cycle duration. Other variants based
on CQF can avoid relying on time synchronization, but only the
same cycle duration for all nodes. Basically, the packet received
in the current sending window (i.e., cycle) will ensure that it
can be sent in the next sending window, then the deterministic
node delay, on average, is one cycle duration, or seveval cycle
durations if the forwarding delay intra node (from incoming port
to outgoing port) inside the node can't be ignored.
o [I-D.peng-detnet-deadline-based-forwarding] introduced a deadline
based forwarding mechanism that allow packet to control its
expected dwell time in the node according to the planned deadline.
There are two policies for deadline queue to schedule packets.
For early sending policy, the end-to-end delay is H*(F~D), jitter
is H*Q, where, H is the number of hops, F is the forwarding delay
intra node, D is the planned deadline; For punctual sending
policy, the end-to-end delay is H*D, jitter is a single
authorization time. That is, the packet received at any time will
ensure that it can be sent in offset time F~D or D respectively
for these two policies.
3.1. Deterministic Link Bound with CQF
A node may configure the CQF based packet scheduling parameter
information for its local link, including CQF scheduling enable/
disable, one or more cycle durations. Accordingly, for each cycle
duration, the node delay/jitter attributes of the link will be
obtained. The meanings of these parameters or attributes of the link
are as follows:
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o CQF scheduling enable/disable: the CQF scheduling algorithm can be
enabled for a link, then the packets sent to that link will be
scheduled by the CQF scheduling algorithm.
o Cycle duration: the duration of the cycle of CQF, which is also
called cycle_size. One or more cycle_size with different lengths
can be configured for a link, such as 10us, 20us, 30us, and so on.
o Node delay/jitter:
* According to classical TSN CQF, for a given cycle_size, it can
be deduced that the minimum delay in the node of the packet is
0, the maximum delay in the node is 2*cycle_size, the average
delay in the node is one cycle_size, and the delay jitter in
the node is 2*cycle_size. The detailed reasons for these data
are as follows: if a node receives a packet at the tail end of
cycle i and sends that packet at the head end of cycle i+1, the
resulting node delay, i.e., the minimum node delay, is 0; if a
node receives a packet at the head end of cycle i and sends
that packet at the tail end of cycle i+1, the resulting node
delay, i.e., the maximum node delay, is 2*cycle_ size; the
average node delay is one cycle_size, and the node delay jitter
is 2*cycle_size. Each cycle_size corresponds to a different
set of delay/jitter attributes.
* However, for some variants based on TSN CQF, if the forwarding
delay intra node can't be ignored, e.g, wasting 2 cycle
duration, then the minimum node delay, the maximum node delay,
and the average node delay need to add 2 cycle_size
respectively, but the node delay jitter is still 2*cycle_size.
3.2. Deterministic Link Bound with Deadline
A node may configure the deadline based packet scheduling parameter
information for its local link, including deadline scheduling enable/
disable, one or more deadline scheduling delays, and the scheduling
policy supported for each deadline scheduling delay. Accordingly,
for each deadline scheduling delay, the node delay/jitter attributes
of the link will be obtained. The meanings of these parameters or
attributes of the link are as follows:
o Deadline scheduling enable/disable: the deadline scheduling
algorithm can be enabled for a link, then the packet forwarded to
the link will be scheduled by the deadline based packet scheduling
algorithm. The dwell time of the packet in the node does not
exceed the maximum allowable dwell time D, where, D = forwarding
delay intra node (F) + specific deadline scheduling delay (Q).
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o Supported deadline scheduling delay set: the set composed of one
or more deadline scheduling delays <Q1, Q2, ..., Qn>, assuming
that Q1 is the minimum and Qn is the maximum in the set.
Generally, the difference between two adjacent elements in the set
is fixed, for example, a fixed interval (I).
o Scheduling policy: for each scheduling delay Q, there are two
scheduling policies: early sending policy and punctual sending
policy. In case of early sending policy, the scheduling delay of
the packet may be sent to the outgoing port when it does not reach
Q; In the punctual sending policy, the packet is sent to the
outgoing port only when the scheduling delay of the packet is
equal to Q. Therefore, for early sending policy, the actual dwell
time of the packet in the node is within the range [F, F+Q], i.e.,
the minimum node delay is F, the maximum node delay is F+Q, and
the node delay jitter is Q; For punctual sending policy, the
actual dwell time of the packet in the node is equal to F+Q, i.e.,
the minimum node delay is F+Q, the maximum node delay is also F+Q,
and the node delay jitter is 0 (however, there may be a deviation
equal to one authorization time).
3.3. ISIS Advertisement of Deterministic Link
3.3.1. Advertisement of Forwarding Delay intra Node
The forwarding delay intra node is related to the chip implementation
and is generally constant.
A new IS-IS sub-TLV is defined: the Forwarding Delay sub-TLV, which
is advertised within TLV-22, 222, 23, 223, 141. At most only one
Forwarding Delay sub-TLV can be included.
The following format is defined for the Forwarding Delay sub-TLV:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Forwarding Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1
where:
Type: TBD
Length: 2
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Forwarding Delay: The latency of packet from the incoming port (or
generated from control plane) to the outgoing port, in units of
microseconds. If the forwarding delay intra node can be ignored,
it is set to 0. If this sub-TLV is not advertised, the forwarding
delay intra node can be regarded as 0.
NOTE: for all links of a specific node, it may be possible that
they have the same forwarding delay intra node, therefore the
forwarding delay intra node can also be advertised by a unified
node attribute. This would be considered in future versions.
3.3.2. Advertisement of CQF Parameters
A new IS-IS sub-TLV is defined: the Cycle Durations sub-TLV, which is
advertised within TLV-22, 222, 23, 223, 141. At most only one Cycle
Durations sub-TLV can be included.
The following format is defined for the Cycle Durations sub-TLV:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Cycle_size 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cycle_size 2 | ... ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cycle_size N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2
where:
Type: TBD
Length: 2*N, depending on the count of the cycle_size.
Cycle_size: The length of cycle duration, in units of
microseconds. A link can support multiple cycle durations, for
example, 10us, 20us, 30us, etc, each for a specific service
requirement.
Only those links that enable CQF scheduling algorithm need to
advertise the Cycle Durations sub-TLV, otherwise there is no need to
advertise.
Note that the advertised cycle_size must be consistent with the CQF
queue scheduling mechanism actually instantiated by the link in the
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forwarding plane. If the forwarding plane does not instantiate a CQF
queue scheduling supporting a certain cycle_size, which is however
advertised in the Cycle Durations sub-TLV, the subsequent route
computation may get wrong results.
For a given cycle_size, it can deduce the corresponding node delay
and jitter attributes, so these attributes can no longer be
explicitly included in the Cycle Durations sub-TLV. As mentioned
earlier, if the forwarding delay intra node (assuming F) is not 0,
the minimum node delay, the maximum node delay, and the average node
delay need to take F into account respectively. F is replaced by
((F/cycle_size)+1)*cycle_size for deducing. That is:
o If F is 0, for a given cycle_size, the minimum node delay is 0,
the maximum node delay is 2*cycle_size, the average node delay is
cycle_size, and the node delay jitter is 2*cycle_size.
o If F is not 0, for a given cycle_size, the minimum node delay is
((F/cycle_size)+1)*cycle_size, the maximum node delay is ((F/
cycle_size)+3)*cycle_size, the average node delay is ((F/
cycle_size)+2)*cycle_size, and the node delay jitter is
2*cycle_size.
3.3.3. Advertisement of Deadline Parameters
A new IS-IS sub-TLV is defined: the Deadline Scheduling sub-TLV,
which is advertised within TLV-22, 222, 23, 223, 141. At most only
one Deadline Scheduling sub-TLV can be included.
The following format is defined for the Deadline Scheduling sub-TLV:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | P | Q1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| P | Q2 | P | Q3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... ... | P | Qn |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3
where:
Type: TBD
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Length: 2*N, depending on the count of the supported deadline
scheduling delay.
Q: Indicates the scheduling delay set, <Q1, Q2, ..., Qn>,
supported by the link, in units of microseconds. For each
supported scheduling delay, the highest two bits represent the
scheduling policy P. The value of scheduling policy P can be:
0, not defined yet;
1, indicates that it supports the early sending policy;
2, indicates that it supports the punctual sending policy;
3, indicates that it supports both early sending policy and
punctual sending policy.
As mentioned earlier, given the scheduling delay Q and its scheduling
policy, combined with the forwarding delay intra node (F), the
corresponding delay and jitter attributes in the node can be derived.
Therefore, these attributes can no longer be explicitly included in
the Deadline Scheduling sub-TLV.
Note that the scheduling delay Q advertised in the Deadline
Scheduling sub-TLV must be consistent with the deadline queue
scheduling mechanism actually instantiated by the link in the
forwarding plane. If the forwarding plane does not instantiate the
deadline queue scheduling supporting a certain scheduling delay Q,
which is however advertised in the Deadline Scheduling sub-TLV, the
subsequent route computation may get wrong results.
3.3.3.1. Another Simplified Extension
If the set <Q1, Q2, ..., Qn> to be advertised contains many elements,
and the difference between two adjacent elements in the set is a
fixed interval (I), and the scheduling policy supported for all
elements are same, another more simplified extension, the Deadline
Scheduling Simplified Sub-TLV, can be defined as below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Q1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Qn | P | I |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4
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where:
Type: TBD
Length: 6.
Q1: the minimum scheduling delay, in units of microseconds.
Qn: the maximum scheduling delay, in units of microseconds.
I: the fixed interval between any two adjacent elements in the
set, in units of microseconds. The highest two bits represent the
scheduling policy P. The value of scheduling policy P can be:
0, not defined yet;
1, indicates that it supports the early sending policy;
2, indicates that it supports the punctual sending policy;
3, indicates that it supports both early sending policy and
punctual sending policy.
3.4. OSPF Advertisement of Deterministic Link
To be defined in next version.
4. Deterministic Routes Computation
4.1. Bind CQF Parameters with Flex-Algo
The binding relationship <algorithm, cycle_size> can be configured on
one or more nodes participating in the same IGP Flex-algo plane, and
then advertised in the IGP domain. If there are multiple binding
relationship advertised for the same algorithm, it should choose to
use the binding cycle_size contained in the FAD with the highest
priority.
If a Flex-algo plane eventually uses a binding cycle_size, all links
participated to the Flex-algo plane must be configured with CQF
scheduling enabled and corresponding cycle_size, otherwise, links
that do not meet the conditions must be excluded from the Flex-algo
plane.
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4.1.1. ISIS Advertisement of Flex-algo Binding CQF
The Flexible Algorithm definition can specify the binding cycle_size
that are used to determine the deterministic delay metric for the
computed path within the Flex-algo plane.
A new IS-IS sub-TLV is defined: the FAD Binding Cycle-size Sub-TLV,
which is advertised within IS-IS Flexible Algorithm Definition Sub-
TLV. At most only one FAD Binding Cycle-size Sub-TLV can be
included.
The following format is defined for the FAD Binding Cycle-size Sub-
TLV:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Binding Cycle_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5
where:
Type: TBD
Length: 2
Binding Cycle_size: Cycle_size of CQF scheduling bound by Flex-
algo, in units of microseconds.
The binding cycle_size contained in the FAD with the highest priority
will take effect. If the FAD with the highest priority does not
contain the FAD Binding Cycle-size Sub-TLV, assuming the Metric-Type
is Min Unidirectional Link Delay, the traditional path considering
only link transmission delay will be calculated, otherwise, the path
will consider both node delay and link delay.
4.1.2. OSPF Advertisement of Flex-algo Binding CQF
To be defined in next version.
4.2. Bind Deadline Parameters with Flex-Algo
The binding relationship <algorithm, scheduling delay, scheduling
policy> can be configured on one or more nodes participating in the
same IGP Flex-algo plane, and then advertised in the IGP domain. If
there are multiple binding relationship advertised for the same
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algorithm, it should choose to use the binding scheduling delay and
scheduling policy contained in the FAD with the highest priority.
If a Flex-algo plane eventually uses a binding deadline parameter,
all links participated to the Flex-algo plane must be configured with
deadline scheduling enabled and corresponding scheduling delay and
scheduling policy, otherwise, links that do not meet the conditions
must be excluded from the Flex-algo plane.
4.2.1. ISIS Advertisement of Flex-algo Binding Deadline
The Flexible Algorithm definition can specify the binding deadline
scheduling delay and scheduling policy that are used to determine the
deterministic delay metric for the computed path within the Flex-algo
plane.
A new IS-IS sub-TLV is defined: the FAD Deadline Scheduling Sub-TLV,
which is advertised within IS-IS Flexible Algorithm Definition Sub-
TLV. At most only oneFAD Deadline Scheduling Sub-TLV can be
included.
The following format is defined for the FAD Deadline Scheduling Sub-
TLV:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Flags |U| P | Q //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Q |
+-+-+-+-+-+-+-+-+
Figure 6
where:
Type: TBD
Length: 3
Flags: Two flags are currently defined.
U-flag: 1 bit, indicating whether the value of scheduling delay
Q is known or unknown. 0 indicates known and 1 indicates
unknown.
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P-flag: 2 bits, indicating scheduling policy. The value can
be: 0, not defined yet; 1, indicates the early sending policy;
2, indicates the punctual sending policy; 3, not defined yet.
Q: Indicates the deadline scheduling delay Q bound by flex
algorithm, in units of microseconds. Note that if the U-flag is
1, the value of Q must be ignored and set to 0.
The binding deadline parameter contained in the FAD with the highest
priority will take effect. If the FAD with the highest priority does
not contain the FAD Deadline Scheduling Sub-TLV, assuming the Metric-
Type is Min Unidirectional Link Delay, the traditional path
considering only link transmission delay will be calculated,
otherwise, the path will consider both node delay and link delay.
Note that the FAD Binding Cycle-size Sub- TLV and the FAD Deadline
Scheduling Sub-TLV MUST not appear in FAD at the same time,
otherwise, the first one is selected.
4.2.2. OSPF Advertisement of Flex-algo Binding Deadline
To be defined in next version.
4.3. FAD Flags Extensions
4.3.1. ISIS FAD Flags Extensions
A new flag, Deterministic flag (D-flag), is introduced to ISIS
Flexible Algorithm Definition Flags Sub-TLV, to indicate to compute
deterministic SPF path when Metric-Type is Min Unidirectional Link
Delay. In other words, it will compute shortest path with minimum
deterministic end-to-end delay, which contains accumulated node delay
and accumulated link transmission delay.
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M|D| | ...
+-+-+-+-+-+-+-+-+...
Figure 7
where:
D-flag: introduced by this document. When set, deterministic SPF
path is computed.
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4.3.2. OSPF FAD Flags Extensions
To be defined in next version.
4.4. CQF based Deterministic Routes Computation
This document reuse the existing Metric-Type, Min Unidirectional Link
Delay, combined with the C-flag, to compute CQF based shortest path
with minimum deterministic end-to-end delay, which contains
accumulated node delay provided by CQF and accumulated link
transmission delay.
NOTE: Whether new metric type need to be introduced needs to be
discussed in the WG.
For a Flex-algo plane that bound to a specific cycle_size, the delay
metric of a candidate path within the Flex-algo plane equals:
H * node delay, where H is the number of hops, and node delay can
be deduced by the cycle_size and forwarding delay intra node; plus
Accumulated link transmission delay;
From the source node to the destination node, the candidate path with
minimum deterministic delay metric is the best one. This calculation
result may be different from the traditional calculation result
considering only link transmission delay, depending on the proportion
of node delay. If the number of intermediate nodes included in the
two candidate paths is different, the node delay will be different.
A traditional optimal low latency path only considering the link
transmission delay may contain more hops, resulting in not being
recognized as the optimal deterministic latency path.
The deterministic delay jitter of a candidate path within the Flex-
algo plane equals:
node delay jitter, which is 2*cycle_size; plus
Accumulated link delay jitter, which is almost 0;
4.5. Deadline based Deterministic Routes Computation
This document reuse the existing Metric-Type, Min Unidirectional Link
Delay, combined with the D-flag and the FAD Deadline Scheduling Sub-
TLV, to compute deadline based shortest path with minimum
deterministic end-to-end delay, which contains accumulated node delay
provided by deadline and accumulated link transmission delay.
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For a Flex-algo plane that bound to a specific deadline scheduling
parameter, the delay metric of a candidate path within the Flex-algo
plane equals:
H * node delay, where H is the number of hops, and node delay can
be deduced by the scheduling delay, scheduling policy and
forwarding delay intra node; plus
Accumulated link transmission delay;
Assuming that the bound scheduling delay Q and scheduling policy P
are obtained from the FAD Deadline Scheduling Sub-TLV (note that if
the bound scheduling delay Q is an unknown value, the scheduling
delay Q is temporarily replaced by 0 during path compuation), the
node delay contributed by any intermediate node i in the candidate
path is:
o For early sending policy, the node delay is in the range of [F(i),
F(i)+Q], where F(i) represents the forwarding delay intra node i.
Because the node delay value in this case is a range, and we need
to get a specific value for SPF computation, thus there are
several options to select a specific value as node delay, i.e.,
select F(i), or F(i)+Q, or the average of F(i) and F(i)+Q. This
document take F(i)+Q as the default option.
o For punctual sending policy, the node delay is equal to F(i)+Q.
It should be noted that the above calculation process is used to
select the optimal deterministic delay path from multiple candidate
paths. However, once the deterministic SPF path is obtained, the
deterministic delay metric of the deterministic SPF path should
reflect the actual delay. Especially, when the bound scheduling
delay Q is an unknown value, the deterministic delay metric of the
deterministic SPF path is an expression containing Q. In this case,
the value of scheduling delay Q needs to be given through other
methods, such as carried in the forwarded data packet. This means
that the same path can provide different delays for different
services.
The deterministic delay jitter of a candidate path within the Flex-
algo plane equals:
o Accumulated node delay jitter, which is H*Q for early sending
policy and 0 for punctual sending policy; plus
o Accumulated link delay jitter, which is almost 0;
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5. Route Convergence and Redundance Considerations
To be described in next version.
6. Examples of Deterministic SPF
To be described in next version.
7. IANA Considerations
TBD
8. Security Considerations
TBD.
9. Acknowledgements
TBD
10. References
10.1. Normative References
[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
algo-18 (work in progress), October 2021.
[I-D.ietf-lsr-ip-flexalgo]
Britto, W., Hegde, S., Kaneriya, P., Shetty, R., Bonica,
R., and P. Psenak, "IGP Flexible Algorithms (Flex-
Algorithm) In IP Networks", draft-ietf-lsr-ip-flexalgo-04
(work in progress), December 2021.
[I-D.peng-detnet-deadline-based-forwarding]
Peng, S. and B. Tan, "Deadline Based Deterministic
Forwarding", draft-peng-detnet-deadline-based-
forwarding-00 (work in progress), January 2022.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8570] Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
2019, <https://www.rfc-editor.org/info/rfc8570>.
[RFC8655] Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", RFC 8655,
DOI 10.17487/RFC8655, October 2019,
<https://www.rfc-editor.org/info/rfc8655>.
10.2. Informative References
[CQF] "IEEE802.1Qch", 2017,
<https://ieeexplore.ieee.org/document/7961303>.
Authors' Addresses
Shaofu Peng
ZTE Corporation
China
Email: peng.shaofu@zte.com.cn
Bin Tan
ZTE Corporation
China
Email: tan.bin@zte.com.cn
Quan Xiong
ZTE Corporation
China
Email: xiong.quan@zte.com.cn
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