Network Shaofu. Peng
Internet-Draft Bin. Tan
Intended status: Standards Track ZTE Corporation
Expires: July 14, 2022 January 10, 2022
Deadline Based Deterministic Forwarding
draft-peng-detnet-deadline-based-forwarding-00
Abstract
This document describes a deterministic forwarding mechanism based on
deadline. The mechanism enhances strict priority scheduling
algorithm with dynamically adjusting the priority of the queue
according to its deadline attribute.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Deadline Queue . . . . . . . . . . . . . . . . . . . . . . . 3
3. Get Deadline Information of Packets . . . . . . . . . . . . . 6
3.1. Get Planned Deadline . . . . . . . . . . . . . . . . . . 6
3.2. Get Existing Cumulative Planned Deadline . . . . . . . . 7
3.3. Get Existing Accumulated Actual Dwell Time . . . . . . . 7
3.4. Get Existing Accumulated Deadline Deviation . . . . . . . 8
4. Put Packets into the Deadline Queues . . . . . . . . . . . . 8
5. Traffic Regulation and Shaping . . . . . . . . . . . . . . . 11
6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
10. Normative References . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
[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.
Resource reservation refers to the occupation of resources by service
traffic, exclusive or shared in a certain proportion, such as
dedicated physical link, link bandwidth, queue resources, etc;
Explicit routing means that the transmission path of traffic flow in
the network needs to be selected in advance to ensure the stability
of the route and does not change with the real-time change of network
topology, and based on this, the upper bound of end-to-end delay and
delay jitter can be accurately calculated; Service protection refers
to sending multiple service flows along multiple disjoint paths at
the same time to reduce the packet loss rate. In general, a
deterministic path is a strictly explicit path calculated by a
centralized controller, and resources are reserved on the nodes along
the path to meet the SLA requirements of deterministic services.
[I-D.stein-srtsn] describes that the controller calculates the local
deadline time of each node for the traffic to be transmitted in
advance, which is a absolute system time, forms a stack of these
local deadline time, and then carries them in the forwarded data
packets. Each node forwards the packets according to its own local
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deadline. [I-D.stein-srtsn] suggests that FIFO queue can not be used
to realize this function, because the packets stored in the queue are
always first in first out, so a special data structure is recommoned.
The packets in this data structure will be automatically sorted with
the order from emergency to non emergency according to the deadline
of the packets. However, it may be difficult to implement this
structure in hardware, and especially for a large network it may be
challenge to synchronize time.
Considering that the link transmission delay is generally a fixed
value, and we focus on the dwell time of the packets inside the node,
an alternate approach is to make the deadline eliminate the
interference of link delay and avoids relying on time synchronization
between nodes.
This document desrbies an alternate packets scheduling scheme. It
suggests to only use a single deadline time to control the packets
scheduling of all nodes along the path. The single deadline time is
an offset time, which is based on the time when the packet enters the
node and represents the maximum time allowed for the packet to stay
inside the node. However, if each node has obvious differences in
the capability of packets forwarding and scheduling, more offset-time
may be needed.
1.1. 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.
2. Deadline Queue
For nodes in the network, some queues with deadline time (also termed
as TTL) are introduced and maintained for each outgoing port. These
queues are called deadline queue. Deadline queue has the following
characteristics:
The TTL of each deadline queue will decrease with the passage of
time. When it decreases to 0, the scheduling priority of the
queue will be set to the highest, and the scheduling opportunity
will be obtained immediately (note that there may be interference
delay caused by a large packet being sent by a low priority
queue). It will prohibit receiving new packets, in which the
buffered packets will be sent to the outgoing port immediately,
and the maximum duration allowed to send packets is the
authorization time. In principle, all packets buffered in the
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queue shall be sent within this authorization time. If the queue
is sent and the authorization time is still free, other queues
with lower priority can be scheduled during this authorization
time.
The scheduling engine can initiate a cycle timer to decrement the
TTL of all deadline queues, that is, whenever the timer expires,
the deadline values of all queues will be subtracted from the
timer interval. Note that the time interval of the timer must be
greater than or equal to the authorization time of the deadline
queue. For simplicity, they are same.
For a deadline queue whose TTL has been reduced to 0, after a new
round of timer timeout, the TTL will return to the maximum initial
value, allow receiving new packets, and continue to enter the next
round of operation that decreases with the passage of time.
For a deadline queue whose TTL is not reduced to 0, it can receive
packets. In detailed, when a node receives a packet to be
forwarded from a specific outgoing port, it first obtains the
expected deadline of the packet, and then put the packet to the
deadline queue with the relevant TTL value of the outgoing port
for transmission.
For a deadline queue whose TTL is not reduced to 0, its scheduling
priority cannot be set to the highest value. A local policy may
be used to control the transmission of buffered packets. There
are two options: the first option, allowing to be involved in
scheduling, also termed as early sending policy; the second
option, not allowing, also termed as punctual sending policy.
Early sending policy is applicable to the service requirements of
low delay, and punctual sending policy is applicable to low delay
jitter.
At the beginning, all deadline queues have different TTL values,
i.e., staggered from each other, so that the TTL of only one
deadline queue will decrease to 0 at any time.
The above authorization time, timer interval and maximum initial TTL
value shall be specified according to the actual capacity of the
node. In fact, each node in the network can independently use
different timer interval for different outgoing ports. The general
principle is that if an outgoing port has a large bandwidth (such as
100G bps), the timer interval (and the authorization time of the
deadline queue) can be small (such as 1us), because the link with
large bandwidth can send the required bits amount even within a small
time interval; If an outgoing port has a small bandwidth (e.g. 1G
bps), the timer interval (and the authorization time of the deadline
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queue) should be larger (e.g. 10us), because the link with small
bandwidth needs to send the required bit amount within a larger time
interval.
A specific example of the deadline queue is depicted in Figure 1.
+------------------------------+ +------------------------------+
| Deadline Queue Group: | | Deadline Queue Group: |
| queue-1(TTL=60us) ###### | | queue-1(TTL=50us) ###### |
| queue-2(TTL=50us) ###### | | queue-2(TTL=40us) ###### |
| queue-3(TTL=40us) ###### | | queue-3(TTL=30us) ###### |
| queue-4(TTL=30us) ###### | | queue-4(TTL=20us) ###### |
| queue-5(TTL=20us) ###### | | queue-5(TTL=10us) ###### |
| queue-6(TTL=10us) ###### | | queue-6(TTL=0us) ###### |
| queue-7(TTL=0us) ###### | | queue-7(TTL=60us) ###### |
+------------------------------+ +------------------------------+
+------------------------------+ +------------------------------+
| Non-deadline Queue Group: | | Non-deadline Queue Group: |
| queue-8 ############ | | queue-8 ############ |
| queue-9 ############ | | queue-9 ############ |
| queue-10 ############ | | queue-10 ############ |
| ... ... | | ... ... |
+------------------------------+ +------------------------------+
-o----------------------------------o-------------------------------->
T0 T0+10us time
Figure 1: Example of Deadline Queue for outgoing Port
In this example, the timer interval for deadline queue group is
configured to 10us. Queue-1 ~ queue-7 are deadline queues, and other
queues are traditional non-deadline queues. Each deadline queue has
its TTL attribute. The maximum initial TTL is 60us. At the initial
time (T0), the TTL of all deadline queues are staggered from each
other. For example, the TTL of queue-1 is 60us, the TTL of queue-2
is 50uS, the TTL of queue-3 is 40us, and so on. At this time, only
the TTL of queue-7 is 0, which has the highest scheduling priority.
Suppose the scheduling engine initiates a cycle timer with a time
interval of 10us. After each timer timeout, the timer interval will
be subtracted from the TTL of all deadline queues. As shown in the
figure, at T0 + 10us, the timer timeout, the TTL of queue-1 becomes
50uS, the TTL of queue-2 becomes 40us, the TTL of queue-3 becomes
30us, etc. At this time, the TTL of queue-7 returns to the maximum
initial TTL of 60us and is no longer set to the highest scheduling
priority; The TTL of queue-6 becomes 0, which has the highest
scheduling priority.
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For simplicity, set the authorization time of the deadline queue to
be consistent with the time interval of the cycle timer, which is
also 10us. When the TTL of a deadline queue becomes 0, it has a time
limit of 10us to send packets in the queue. During this period, it
will be prohibited to receive new packets (in fact, there can be no
new packets with a deadline of 0). After the 10us time elapses, the
cycle timer will timeout again, The TTL of another deadline queue
will change to 0. It is also feasible to set the authorization time
to be less than the cycle timer interval.
If the deadline queue with the highest priority is still free after
sending packets within the authorized time, the scheduling engine
will visit other queues with the second highest priority during the
rest of the authorized time.
3. Get Deadline Information of Packets
3.1. Get Planned Deadline
The planned deadline of the packet is an offset time, which is based
on the time when the packet enters the node and represents the
maximum time allowed for the packet to stay inside the node. There
are many ways to obtain the planned deadline of the packet.
Carried in the packet. The ingress PE node, when encapsulating
the deterministic service flow, can explicitly insert the planned
deadline into the packet according to SLA. The intermediate node,
after receiving the packet, can directly obtain the planned
deadline from the packet. Generally, only a single planned
deadline needs to be carried in the packet, which is applicable to
all nodes along the path; Or insert a stack composed of multiple
deadlines, one for each node. How to carry planned deadline in
the packets will be defined in other documents.
Included in the FIB entry. Each node in the network can maintain
the deterministic FIB entry. After the packet hits the
deterministic FIB entry, the planned deadline is obtained from the
forwarding information contained in the FIB entry.
Included in the policy entry. Configure local policies on each
node in the network, and then set the corresponding planned
deadline according to the matched specific characteristics of the
packet, such as 5-tuple.
For a deterministic delay path based on deadline queue scheduling,
the path it passes through has deterministic end-to-end delay
requirements. It includes two parts, one is the cumulative node
delay and the other is the cumulative link transmission delay. The
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end-to-end delay requirement is subtracted from the cumulative link
transmission delay to obtain the cumulative node delay. A simple
method is that the accumulated node delay is shared equally by each
intermediate node along the path to obtain the planning deadline of
each node.
3.2. Get Existing Cumulative Planned Deadline
The existing cumulative planned deadline of the packet refers to the
sum of the planned deadline of all upstream nodes before the packet
is transmitted to this node. This information needs to be carried in
the packet. Every time the packet passes through a node, the node
accumulates its corresponding planned deadline to the existing
cumulative planned deadline field in the packet. How to carry
existing cumulative planned deadline in the packets will be defined
in other documents.
The setting of "existing cumulative planned deadline" in the packet
needs to be friendly to the chip for reading and writing. For
example, it should be designed as a fixed position in the packet.
The chip may support flexible configuration for that position.
3.3. Get Existing Accumulated Actual Dwell Time
The existing cumulative actual dwell time of the packet, refers to
the sum of the actual dwell time of all upstream nodes before the
packet is transmitted to this node. This information needs to be
carried in the packet. Every time the packet passes through a node,
the node accumulates its corresponding actual dwell time to the
existing cumulative actual dwell time field in the packet. How to
carry existing cumulative actual dwell time in the packets will be
defined in other documents.
The setting of "existing cumulative actual dwell time" in the packet
needs to be friendly to the chip for reading and writing. For
example, it should be designed as a fixed position in the packet.
The chip may support flexible configuration for that position.
Although other methods can also be, for example, carrying the
absolute system time of receiving and sending in the packet to
compute the actual dwell time indirectly, that has a low
encapsulation efficiency and require strict time synchronization
between nodes.
A possible method to get the actual dwell time in the node is that,
the receiving and sending time of the packet can be recorded in the
auxiliary data structure (note that is not packet itself) of the
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packet, and the actual dwell time of the packet in the node can be
calculated according to these two times.
3.4. Get Existing Accumulated Deadline Deviation
The existing accumulated deadline deviation equals existing
cumulative planned deadline minus existing cumulative actual dwell
time. This value can be positive or negative.
If the existing cumulative planned deadline and the existing
cumulative actual dwell time are carried in the packet, it is not
necessary to carry the existing accumulated deadline deviation.
Otherwise, it is necessary. The advantage of the former is that it
can be applied to more scenarios.
4. Put Packets into the Deadline Queues
The lifetime of the packet inside the node mainly includes two parts:
the first part is to lookup the forwarding table when the packet is
received from the incoming port (or generated by the control plane)
and deliver the packet to the line card where the outgoing port is
located; the second part is to store the packet in the queue of the
outgoing port for transmission. These two parts contribute to the
actual dwell time of the packet in the node. The former can be
called forwarding delay and the latter can be called queuing delay.
The forwarding delay is related to the chip implementation and is
generally constant; The queuing delay is unstable.
When a node receives a packet from an upstream node, it can first get
the existing accumulated deadline deviation, and then add it to the
planned deadline of the packet at this node to obtain the deadline
adjustment value, and then on the basis of the deadline adjustment
value, deducting the forwarding delay of the packet in the node, the
allowable queuing delay value is obtained, and then the packet will
be put to the deadline queue with TTL as the above allowable queuing
delay value for sending.
Under normal circumstances, if each hop strictly controls the
scheduling of the packet according to its planned deadline, the
actual dwell time of the packet will be very close to the planned
deadline, and the absolute value of the existing accumulated deadline
deviation will be very small.
More generally, assume that the local node in a deterministic path is
i, all upstream nodes are from 1 to i-1, and downstream nodes are i +
1, the planned deadline is D, the actual dwell time is R, the
deadline adjustment value is M, the forwarding delay inside the node
is P, the existing accumulated deadline deviation is E, and the
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allowable queuing delay is Q, then the allowable queuing delay (Q) of
the packet on this node i is calculated as follows:
E(i-1) = D(1) + D(2) + ... + D(i-1) - R(1) - R(2) - ... - R(i-1)
M(i) = D(i) + E(i-1)
Q(i) = M(i) - P(i)
Consider some extreme cases. For example, many upstream nodes adopt
the early sending policy to send packets quickly. Packets almostly
need not queue in these nodes, but only depend on the forwarding
delay. Then the existing accumulated deadline deviation (E) may be a
very large positive value, resulting in a large allowable queuing
delay (Q). If this value exceeds the maximum initial TTL of the
deadline queue maintained by the node, the allowable queuing delay
(Q) should be modified to the maximum initial TTL.
For another example, if some upstream nodes are abnormal and have a
very large actual dwell time (R), the existing accumulated deadline
deviation (E) may be a negative number, resulting in the allowable
queuing delay (Q) may be less than or equal to 0, which is smaller
than the cycle timer interval of the deadline queue maintained in the
node, then the allowable queuing delay (Q) should be modified to the
cycle timer interval value.
Figure 2 depicts an example of packets buffered to the deadline
queue.
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packet-2 packet-1 +------------------------------+
+--------+ +--------+ | Deadline Queue Group: |
| D=20us | | D=30us | | queue-1(TTL=60us) ###### |
| E=10us | | E=-10us| +--+ | queue-2(TTL=50us) ###### |
+--------+ +--------+ |\/| | queue-3(TTL=40us) ###### |
------incoming port-1------> |/\| | queue-4(TTL=30us) ###### |
|\/| | queue-5(TTL=20us) ###### |
packet-4 packet-3 |/\| | queue-6(TTL=10us) ###### |
+--------+ +--------+ |\/| | queue-7(TTL=0us) ###### |
| | | D=30us | |/\| +------------------------------+
+--------+ | E=-30us| |\/|
+--------+ |/\|
------incoming port-2------> |\/| +------------------------------+
|/\| | Non-deadline Queue Group: |
packet-6 packet-5 |\/| | queue-8 ############ |
+--------+ +--------+ |/\| | queue-9 ############ |
| | | D=40us | |\/| | queue-10 ############ |
+--------+ | E=40us | |/\| | ... ... |
+--------+ +--+ +------------------------------+
------incoming port-2------> ---------outgoing port---------->
-o----------------------------------o-------------------------------->
receiving-time base +P time
Figure 2: Time Sensitive Packets Buffered to Deadline Queue
As shown in Figure 2, the node successively receives six packets from
three incoming ports, among which packet 1, 2, 3 and 5 have
corresponding deadline information, while packet 4 and 6 are
traditional packets. These packets need to be forwarded to the same
outgoing port according to the forwarding table entries. It is
assumed that they arrive at the line card where the outgoing port is
located at almost the same time after the forwarding delay in the
node (P = 10us). At this time, the queue status of the outgoing port
is shown in the figure. Then:
The allowable queuing delay (Q) of packet 1 in the node is 30 - 10
-10 = 10us, and it will be put into the deadline queue-6 (its TTL
is 10us).
The allowable queuing delay (Q) of packet 2 in the node is 20 + 10
-10 = 20us, and it will be put into the deadline queue-5 (its TTL
is 20us).
The allowable queuing delay (Q) of packet 3 in the node is 30 - 30
-10 = -10us, and it will be modified to the minimum positive value
10 us then put into the deadline queue-6 (its TTL is 10us). Note
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that the minimum positive value is a timer interval that is a
local parameter per port based on port's bandwidth.
The allowable queuing delay (Q) of packet 5 in the node is 40 + 40
-10 = 70us, and it will be modified to the maximum positive value
60 us then put into the deadline queue-1 (its TTL is 60us). Note
that the maximum positive value is an empirical value that can be
configured according to the maximum delay requirements of
deterministic services.
Packets 4 and 6 will be put into the non-deadline queue in the
traditional way.
5. Traffic Regulation and Shaping
On the ingress PE node, traffic regulation is performed on UNI port,
so that the service traffic does not exceed its reserved bandwidth.
Suppose there are N sources, and the packets they send carry the same
deadline. These packets may arrive at an intermediate node at the
same time and put into the same deadline queue. If the reserved
bandwidth of deadline queue at N sources is M0, and the reserved
bandwidth of deadline queue at intermediate nodes is Mx, then it
needs to meet: N * M0 < = Mx.
On the ingress PE node, traffic shaping is performed on NNI port.
Multiple continuous packets of the specific service flow are stored
in the deadline queue with corresponding remaining time according to
the planned deadline of the service flow. Note that these packets
are not stored in the same queue over time. The amount of bits that
can be stored in one queue is equal to the reserved bandwidth *
authorization time, however, at least one whole packet shall be
loaded. For example, if the allowable queuing delay is 20us, then
within the current timer interval, the first sequence of the packets
will be put into the current deadline queue with TTL = 20us until the
reserved bandwidth limit is reached; Then, within the next timer
interval, the next sequence of packets will be put into the current
TTL = 20us queue until the reserved bandwidth limit is reached; and
so on, until the total service bits are loaded.
Figure 3 depicts an example of deadline based traffic shaping on the
ingress PE node. It is assumed that the packets loaded in each timer
interval do not exceed the reserved bandwidth of the service.
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1st packet
|
v
+-+ +-+ +----+ +-+ +--+ +------+
|1| |2| | 3 | |4| |5 | | 6 | <= packet sequence
+-+ +-+ +----+ +-+ +--+ +------+
| | | | | |
~+P ~+P ~+P ~+P ~+P ~+P
| | | | | |
UNI v v v v v v
ingress PE -+--------+--------+--------+--------+--------+--------+---->
NNI ^ ^ ^ ^ ^ ^ ^
| | | | | | |
|interval|interval|interval|interval|interval|interval|
| | | | | | |
~+Q ~+Q ~+Q ~+Q ~+Q ~+Q ~+Q
| | | | | | |
+-+ +-+ +----+ +-+ +--+ +------+
|1| |2| | 3 | |4| |5 | | 6 |
+-+ +-+ +----+ +-+ +--+ +------+
Figure 3: Deadline Based Traffic Shaping
6. Benefits
The mechanism described in this document has the following benefits:
Time synchronization is not required between network nodes. Each
node can flexibly set the authorization time length of the
deadline queue according to its own outgoing port bandwidth.
Packet multiplexing based, it is an enhancement of PQ scheduling
algorithm, friendly to the upgrade of packet switching network.
All nodes in the network can independently use cycle timers with
different timeout intervals to traverse the deadline queues.
The packet can control its expected dwell time in the node. A
single set of deadline queues supports multiple levels of dwell
time.
For early sending policy, the end-to-end delay is H*(P~D), jitter
is H*Q; For punctual sending policy, the end-to-end delay is H*D,
jitter is a just single authorization time.
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7. IANA Considerations
There is no IANA requestion for this document.
8. Security Considerations
TBD
9. Acknowledgements
TBD
10. Normative References
[I-D.stein-srtsn]
Stein, Y. (., "Segment Routed Time Sensitive Networking",
draft-stein-srtsn-01 (work in progress), August 2021.
[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>.
[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>.
Authors' Addresses
Shaofu Peng
ZTE Corporation
China
Email: peng.shaofu@zte.com.cn
Bin Tan
ZTE Corporation
China
Email: tan.bin@zte.com.cn
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