Internet DRAFT - draft-eckert-detnet-tcqf
draft-eckert-detnet-tcqf
DETNET T. Eckert, Ed.
Internet-Draft Futurewei Technologies USA
Intended status: Standards Track Y. Li, Ed.
Expires: 9 July 2024 Huawei Technologies
S. Bryant
University of Surrey ICS
A. G. Malis
Malis Consulting
J.-d. Ryoo
ETRI
P. Liu
China Mobile
G. Li
S. Ren
F. Yang
Huawei Technologies
6 January 2024
Deterministic Networking (DetNet) Data Plane - Tagged Cyclic Queuing and
Forwarding (TCQF) for bounded latency with low jitter in large scale
DetNets
draft-eckert-detnet-tcqf-05
Abstract
This memo specifies a forwarding method for bounded latency and
bounded jitter for Deterministic Networks and is a variant of the
IEEE TSN Cyclic Queuing and Forwarding (CQF) method. Tagged CQF
(TCQF) supports more than 2 cycles and indicates the cycle number via
an existing or new packet header field called the tag to replace the
cycle mapping in CQF which is based purely on synchronized reception
clock.
This memo standardizes TCQF as a mechanism independent of the tagging
method used. It also specifies tagging via the (1) the existing MPLS
packet Traffic Class (TC) field for MPLS packets, (2) the IP/IPv6
DSCP field for IP/IPv6 packets, and (3) a new TCQF Option header for
IPv6 packets.
Target benefits of TCQF include low end-to-end jitter, ease of high-
speed hardware implementation, optional ability to support large
number of flow in large networks via DiffServ style aggregation by
applying TCQF to the DetNet aggregate instead of each DetNet flow
individually, and support of wide-area DetNet networks with arbitrary
link latencies and latency variations as well as low accuracy clock
synchronization.
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Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview (informative) . . . . . . . . . . . . . . . . . . . 4
2.1. Cyclic Queuing and Forwarding (CQF) . . . . . . . . . . . 4
2.2. Benefits of CQF with higher speed links . . . . . . . . . 5
2.3. Challenges of CQF with higher latency links . . . . . . . 7
2.4. Review of CQF benefits and challenges for DetNet . . . . 8
2.5. Tagged CQF . . . . . . . . . . . . . . . . . . . . . . . 9
2.5.1. CQF with more than two buffers . . . . . . . . . . . 9
2.5.2. From CQF with multiple buffers to TCQF . . . . . . . 11
2.6. Summary of TCQF benefits and goals for DetNet . . . . . . 14
3. Using TCQF in the DetNet Architecture and MPLS forwarding plane
(informative) . . . . . . . . . . . . . . . . . . . . . . 15
4. TCQF per-flow stateless forwarding (normative) . . . . . . . 17
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4.1. Configuration Data model and tag processing for MPLS TC
tags . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2. Packet processing . . . . . . . . . . . . . . . . . . . . 17
4.3. TCQF for MPLS with TC tagging . . . . . . . . . . . . . . 19
4.4. TCQF for IP/IPv6 with DSCP tagging . . . . . . . . . . . 20
4.5. TCQF for IPv6 with IPv6 Option tagging . . . . . . . . . 20
4.5.1. TCQF Option Format . . . . . . . . . . . . . . . . . 21
4.5.2. TCQF Option Processing . . . . . . . . . . . . . . . 22
4.5.3. Encapsulation of TCQF Option for Deterministic IP (DIP)
data plane . . . . . . . . . . . . . . . . . . . . . 22
4.6. TCQF Pseudocode (normative) . . . . . . . . . . . . . . . 23
5. TCQF Per-flow Ingress forwarding (normative) . . . . . . . . 26
5.1. Ingress Flows Configuration Data Model . . . . . . . . . 26
5.2. Ingress Flows Pseudocode . . . . . . . . . . . . . . . . 26
6. Implementation, Deployment, Operations and Validation
considerations (informative) . . . . . . . . . . . . . . 28
6.1. High-Speed Implementation . . . . . . . . . . . . . . . . 28
6.2. Controller plane computation of cycle mappings . . . . . 29
6.3. Link speed and bandwidth sharing . . . . . . . . . . . . 30
6.4. Controller-plane considerations . . . . . . . . . . . . . 30
6.5. Validation . . . . . . . . . . . . . . . . . . . . . . . 31
7. Security Considerations . . . . . . . . . . . . . . . . . . . 31
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
9. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 32
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 32
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 32
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
12.1. Normative References . . . . . . . . . . . . . . . . . . 34
12.2. Informative References . . . . . . . . . . . . . . . . . 35
Appendix A. CSQF . . . . . . . . . . . . . . . . . . . . . . . . 38
Appendix B. TCQF with multiple priorities . . . . . . . . . . . 39
Appendix C. TSN Multiple Buffer CQF . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
1.1. Terminology
CQF Cyclic Queuing and Forwarding. A queuing mechanism defined by
annex T of [IEEE802.1Q].
DT Dead Time. A term from CQF indicating the time during each cycle
in which no frames can be sent because the the receiving node
could not receive it into the desired cycle buffer.
TCQF Tagged Cyclic Queuing and Forwarding. The mechanism specified
in this memo.
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2. Overview (informative)
2.1. Cyclic Queuing and Forwarding (CQF)
Cyclic Queuing and Forwarding (CQF) is a bounded (guaranteed) per-hop
latency forwarding mechanism standardized for use in ethernet
switched networks by the IEEE TSN working group originally via
[IEEE802.1Qch] (802.1 Qch), which later became annex T of
[IEEE802.1Q]. See also [RFC9320], Section 6.6.
CQF is not a separate forwarding mechanism, but it is simple a
profile of the IEEE Time Aware Shaper (TAS) standard, [IEEE802.1Qbv],
which introduce Time-Gated Queues.
CQF uses a two-queue based forwarding mechanism on every switch along
a path between a sender and receiver. One queue is used to receive
and store frames destined toward a particular outgoing interface on
the switch, the other queue is used simultaneously to send frames to
the same outgoing interface. At every cycle time T_c interval these
two queues are swapped, or in terms of Time-Gated Queus, one is
closed for sending, the other is opened for sending. This operation
is synchronized across all switches in the network by network wide
synchronized clocks, so that all queues open and close at the same
time.
For a path of h hops, the end-to-end latency bound is between (h-1) *
T_c + DT and (h+1) * T_c. DT is the so-called dead time at the end
of a cycle during which no frames can be transmitted from the sending
queue to ensure that the last byte of the last frame will be received
earlier than the end of the same cycle on the receiving switch.
A core contributor to DT is the (physical) link between the sending
and receiving switch. DT needs to be larger than the latency of this
link, including physical propagation latency (speed of light),
possible error correction latencies, and interface serialization
latency.
T_c needs to be choosen carefully: The larger it is, the higher the
bounded latency. The smaller it is, the fewer bytes (and hence
frames) will fit into a cycle.
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To admit flows into a CQF network, the ingress switch uses per-flow
Time-Gated Queues. In the most simple case, such a gate is
configured to admit a maximum amount of bytes from the flow into
every cycle. More advanced admission control can be performed for
bursty flows. For example N bursty flows f_i = 0...(N-1) could share
admitted bandwidth by each having their burst admitted in different
cycles c_i = c % N + c_i, where c is a continuous increasing cycle
number.
2.2. Benefits of CQF with higher speed links
The typical CQF deployments in manufacturing networks with 1Gbps
links uses no less than hundreds of microseconds as a cycle interval.
In a network with a small diameter, say less than 8 hops, it is
sufficiently good to provide an end-to-end latency bound in the order
of several milliseconds.
With the increasing of link speed from 100Mbps to 1Gbps, 10Gbps,
100Gbps or even higher in larger networks, either more bytes can be
transmitted within the same cycle interval or the smaller cycle
interval is required to transmit the same amount of bytes in a cycle
as that in low speed networks. Likewise, the serialization latency
reduces with higher speed links and DT reduces. This overall makes
CQF for higher speed networks more attractive than for lower speed
networks.
Figure 1 shows a simple calculation on the number of bytes that can
be transmitted in a cycle with different cycle intervals and link
speeds. A minimum of 1500 bytes is labeled with * as a baseline
because a typical maximum Ethernet frame is 1500 bytes and a selected
cycle interval should at least allow one such frame size to be
transmitted unless otherwise specified.
TBD: These numbers probbly need to be adjusted to reflect reducing DT
based on serialization latency.
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+----------+------------------------------------------------+
| | Bytes Transmitted in a Cycle |
|Cycle Time+------------------------------------------------+
| | Link Speed |
| (us) | 100Mbps | 1Gbps | 10Gbps | 100Gbps |
+----------+------------+-------------+-----------+---------+
| 1 | 12.5 | 125 | 1250 | 12500*|
+----------+------------+-------------+-----------+---------+
| 1.2 | 15 | 150 | 1500* | 15000 |
+----------+------------+-------------+-----------+---------+
| 2 | 25 | 250 | 2500 | 25000 |
+----------+------------+-------------+-----------+---------+
| 4 | 50 | 500 | 5000 | 50000 |
+----------+------------+-------------+-----------+---------+
| 10 | 125 | 1250 | 12500 | 125000 |
+----------+------------+-------------+-----------+---------+
| 12 | 150 | 1500* | 15000 | 150000 |
+----------+------------+-------------+-----------+---------+
| 120 | 1500* | 15000 | 150000 | 1500000 |
+----------+------------+-------------+-----------+---------+
Figure 1: Bytes transmitted within one cycle interval
When the link speed is at 10Gbps, the cycle interval could be as
small as 1.2 us if a 1500 byte frame needs to be transmitted in one
cycle interval, and with 100Gbps links even 1 usec cycle time allows
for 8 frames of 1500 byte each. These are not accurate calculations
because there are certainly other factors to determine the cycle
interval. However, it shows that as the link speed increases, cycle
interval can be greatly reduced in practice while satisfying the
minimum amount of data transmitted in a single cycle. The end-to-end
latency bound when applying CQF is determined by cycle interval and
number of hops. That is to say, CQFs with a smaller cycle interval
have the potential to meet more strict end-to-end latency
requirements in higher link speed networks or meet the same end-to-
end latency requirement in networks with much larger network diameter
(number of hops).
Industry automation has some typical application period requirement,
e.g. 100 us to 2 ms for isochronous traffic, 500 us to 1 ms for
cyclic-synchronous and 2 to 20 ms for cyclic-asynchronous traffic.
The network cycle interval is usually a fraction of the application
period. When the cycle interval is in the order of tens of
microseconds, CQF can be used to meet the most strict end-to-end
latency requirements. For instance, if we assume the number of hops
is 24, when cycle interval is set to 10us, the end-to-end latency
bound can be around (24+1)*10 = 250 us which has the potential to
meet the latency bound requirement for isochronous traffic.
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In summary a higher speed network makes the shorter cycle interval
feasible because sufficiently large traffic volume can be transmitted
within one cycle interval. A shorter cycle interval further offers
shorter end-to-end latency and jitter bounds which provide CQF with
the potentials to meet more strict latency requirements in wider
deployments while preserving its simplicity of latency calculation
and provisioning. Therefore there is a strong motivation to leverage
CQF and at the same time to make cycle interval as short as possible.
2.3. Challenges of CQF with higher latency links
Unlike the original targets for IEEE TSN work, DetNet not only
targets to support IETF forwarding planes (IP, MPLS,...), but also
wide-area networks with therefore longer physcial propagation
latencies.
As shown in Figure 2 for fundamental (two buffer) CQF, the last byte
sent by node A in cycle (i-1) has to be ready for sending at node B
before the start of cycle i. To realize it, DT or dead time is
imposed. It is a time interval usually at the end of a cycle so that
a node should not send the scheduled CQF packets.
Dead time is at least the sum of the maximum propagation delay to the
next node, the maximum processing delay at the next node and the
maximum other time variations. Therefore either the longer
propagation or longer processing delay makes dead time larger.
Packets from DetNet service is likely to be propagated over long
links in the wider area. It takes around 5us per kilometer to
propagate, i.e. 0.5ms every hundred kilometers. Hence the dead time
can be as large as milliseconds or tens of milliseconds in case of
hundred kilometers of longer links and larger processing delays.
That would make the dead time eat up most of the cycle interval when
cycle interval is short (e.g., at the same order or one order higher
of magnitude in time as dead time). Then the useful time in a cycle
will be much reduced. In some extreme cases, when the link is long
and the cycle interval is set to extremely short, the first packet
sent in a cycle by a node will not be possibly received in the same
cycle interval at the next node. That makes the useful time in a
cycle reaches zero in two buffer CQF. Then two buffer CQF will be no
longer suitable.
In result of these considerations, reasonable limits for the size of
TSN CQF networks are in the order of at most few Km per hop, beyond
which DT exceeds common cycle times and possible through of CQF
traffic is hence 0.
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--------------------------------------------------------> Time
| | | |
Node A | cycle i-1 | cycle i | cycle i+1 |
| | | |
Sending ---------------+----------------------------------
|+------+ |+------+ |+------+ |
||//////| ||//////| ||//////| |
|+------+ |+------+ |+------+ |
| buf_1 | buf_2 | buf_1 |
| | | | | | |
| | DT | | DT | | DT |
Node B | |<--->| |<--->| |<--->|
| | | |
Receiving--------------------------------------------------
| +------+| +------+| +------+|
| |//////|| |//////|| |//////||
| +------+| +------+| +------+|
| buf_1 | buf_2 | buf_1 |
| | | |
| | | |
Node B | | | |
| | | |
Sending --------------------------------------------------
| |+------+ |+------+ |
| ||//////| ||//////| |
| |+------+ |+------+ |
| | buf_1 | buf_2
DT=Dead Time
Figure 2: Fundamental Two Buffer CQF
2.4. Review of CQF benefits and challenges for DetNet
In review, CQF has a range of benefits for DetNet.
1. It provides bounded latency.
2. It provided tightly bounded jitter.
3. It has a very simple and easily standardized calculus for its
bounded latency and jitter.
4. It has very simple per-hop forwarding machinery (cyclic queues)
easily supportable in high-speed network equipment.
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5. Like Diffserv forwarding, it does not use per-hop,per-flow state
in the forwarding plane and therefore does not require per-
hop,per-flow signaling with the DetNet controller-plane, allowing
it to scale to large number of flows.
6. The faster the links are, the lower the per-hop latency impact of
the cyclic queuing mechanism.
The core limitation of CQF, which TCQF intends to solve, lies in its
use of arrival time clock to determine the cycle into which the
packet is to be placed, see
[I-D.eckert-detnet-bounded-latency-problems] for more details.
1. Cycles times should be as short as feasible to support lower end
to end latency (Section 2.2).
2. When networks have longer links, or links with higher propagation
jitter as in Metro and WAN, this increases the dead time, and
hence reduces the possible utilization or need to increase cycle
times.
3. When shorter cycle times are feasible because of higher speed
links, this would require an increase in clock-synchronization
accuracy.
2.5. Tagged CQF
Tagging of CQF packets with cycle identifiers can be used to solve
the dilemma aforementioned with minor changes to the fundamental two
buffer CQF. This section introduces this mechanism with multipl
buffers and CQF cycle identification in the packet header. Note that
we are also now using the term packet (as used for IP, MPLS and other
IETF forwarding planes) and buffers for packets, as opposed to frames
as used by IEEE.
2.5.1. CQF with more than two buffers
CQF can use more than two buffers to minimize the dead time and
increase the useful time in a cycle so as to support long link delay.
Figure 3 shows how a three buffer CQF works in a rotating manner in
general. Node A sends packets in cycle (i-1). The time interval
over which node B receives these packet spans two cycles, cycle (i-1)
and cycle i. Hence a method is needed to make node B send them all
at once in cycle (i+1) in order to ensure packets in a single cycle
from the previous node always being sent out in one cycle at the
current node.
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--------------------------------------------------------> Time
| | | |
Node A | cycle i-1 | cycle i | cycle i+1 |
| | | |
Sending ---------------+----------------------------------
|+----------+ |+----------+ |+----------+ |
||//////////| ||//////////| ||//////////| |
|+----------+ |+----------+ |+----------+ |
| buf_1 | buf_2 buf_3 |
| | | | | | |
| ->| |<- ->| |<- ->| |<-
| DT DT DT
|
-------------------------------------------------
Node B | +-----------+ +-----------+ +-----------+
| |///////////| |///////////| |///////////|
Receiving | +-----------+ +-----------+ +-----------+
| buf_1 | buf_2 | buf_3 |
| | | |
| | | |
| | | |
| | | |
---------------|----------------------------------
Node B | | |+----------+ |+----------+
| | ||//////////| ||//////////|
Sending | | |+----------+ |+----------+
| | buf_1 buf_2
DT=Dead Time
Figure 3: Three Buffer CQF
More than three buffers will be required when the receiving interval
at node B for packets sent in a single cycle interval from node A
spans over more than two cycle interval boundaries. This can happen
when the time variance (jitter) including propagation, processing,
regulation, clock synchronization variance (so called Maximum Time
Interval Error - MTIE) and other factors between two neighbouring
DetNet nodes can become larger than a single cycle tim.
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2.5.2. From CQF with multiple buffers to TCQF
Note that due to the variance in time, the receiving interval at the
downstream node can be much larger than one cycle interval in which
the upstream node transmits. When time variance is large and cycle
interval and dead time are set small, the possible receiving time of
the last few packets from node A’s cycle (i-1) at node B can overlap
with the possible receiving time of the first few packets from node
A’s cycle i in different rounds of buffer rotations. Hence, when the
buffer number is larger than two, if the receiving side still uses
the traditional CQF implicit time borderline to demarcate the
receiving packets from the consecutive cycles of the upstream node,
it may cause the ambiguity in identifying the right sending cycle at
the upstream node and further affect the correctness of the decision
of which output buffer to put the received packets at the current
node.
Figure 4 shows such an ambiguity when time based cycle demarcation is
used. The packet sent by node A in its cycle (i-1) can be received
at any time in the receiving interval indicated as “receiving window
for A’s buf_1” in Figure 4. The receiving window refers to the time
interval between the earliest time that the first packet sent in a
given cycle from an upstream node is processed and enqueued in an
output buffer and the latest time that the last packet of the cycle
is processed and enqueued in an output buffer. Network operators may
configure the size of the receiving window, taking the time variance
of their networks into account. It can be seen that the spanning
time period of receiving window is longer than the cycle interval.
This is because there is a large time variance experienced between A
and B, e.g. varying processing time for different packets in
different cycles. It does not mean the receiving interval for every
cycle always constantly span over such a large receiving window. The
receiving window time interval indeed is determined by the worst case
time variance value and that should be used for regular time cycle
demarcation.
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--------------------------------------------------------> Time
| | | | |
Node A | cycle | cycle | cycle | cycle |
| i-1 | i | i+1 | i+2 |
Sending ----------+--------+--------+--------+
|+-----+ |+-----+ |+-----+ |+-----+ |
||/////| ||/////| ||/////| ||/////| |
|+-----+ |+-----+ |+-----+ |+-----+ |
| buf_1 | buf_2 | buf_3 | buf_4 |
| | | | | | | | |
| ->| |<- ->| | ->| | ->| |
| DT DT DT DT
|
--------------------------------------
| +-----------+receiving window
Node B | |///////////|for A's buf_1
| +-----------+
Receiving | put to B's buf_1
|
| ->| |<- ambiguity window 1
|
| +-----------+receiving window
| |///////////|for A's buf_2
| | +-----------+
| | put to B's buf_2
| |
| | ->| |<- ambiguity window 2
| | |
| | | +-----------+receiving window
| | | |///////////|for A's buf_3
| | | +-----------+
| | | put to B's buf_3
| | |
| | | ..........
| | |
-|--------|--------|--------|---------------
Node B | | | | | |
| | | +-----+|+-----+ |+-----+ |+-----+
Sending | | | |/////|||/////| ||/////| ||/////|
| | | +-----+|+-----+ |+-----+ |+-----+
| | | buf_4 | buf_1 | buf_2 | buf_3
DT=Dead Time
Figure 4: Three Buffer ambiguity
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When a packet is received in ambiguity window 1 in Figure 4, node B
is not able to use the receiving time to determine which buffer is
the correct one to put the packet in because it cannot tell if the
packet is sent from cycle (i-1) or cycle i on node A. If node B puts
the packet to the wrong output buffer, the packet may experience the
unexpected delay. At the same time, the packet occupying the non-
designated buffer may break the contracts between the end hosts and
DetNet networks and then cause the unpredictable consequences.
It has been noted that the DT can be greatly increased to beat the
time variance in order to make the receiving windows do not overlap
so as to remove such ambiguity. However, it is not always practical
and usually not desired because large DT will eat useful cycle time
and bring the low utilization issue as illustrated in Section 2.3.
Therefore, it would be desired to keep DT as small as possible and at
the same time identify the cycle interval correctly.
With tagged CQF, the sending router A encodes the sending cycle
identification in some existing or new packet header field as
specified later in this document. This allows the receiving router B
to determine the correct output port cycle buffer to place the data
packet into. Except for the need for the operator to pre-configure
this mapping on router B, based on the above described latency and
jitter of the link (and processing between the sending and receiving
router, tagging does not change the fundamental mechanism and
benefits of CQF. makes no change from the fundamental CQF.
Compared to CQF with multiple buffers, Tagged CQF allows to operate
with clock synchronization at significantly reduced accuracy
requirements than CQF. In CQF, the MTIE is an addend determing DT
and should hence typically be less than 1% of the cycle time. In
TCQF it is an addent in the permitted receive window and can hence be
for example as large as the cycle time, and such 100 times larger. A
network using TCQF with 100Gbps interfaces can hence can hence use
the same or less expensive clock synchronization setup than a CQF
network with 1Gbps interfaces. In addition, when conditions of the
network connections change, the mappings can dynamically changed from
network operations.
CQF with multiple buffers but without tagging has been proposed to
IEEE TSN in [multipleCQF], but has not been adopted. Instead of
relying on a cycle tag in a packet header, it still relies solely on
the arrival time of packet, and can hence not equally resolve arrival
time ambiguities as TCQF can, because it does not know the cycle from
which the packet was sent.
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2.6. Summary of TCQF benefits and goals for DetNet
TCQF inherits the benefits of CQF for DetNet as outlined in
Section 2.4, and byusing a configurable number of three or more
cycles, and signaling the cycle as part of a packet header, it
resolves these problems as follows.
1. With three cycles, TCQF can support arbitrary latency links at
arbitrary speeds without reduction of utilization because of
longer links or higher link speeds (same cycle time, same clock
accuracy, only change in lengths and speeds).
2. With four or more cycles, TCQF can also eliminate Dead Time
caused by variation of clock synchronization inaccuracies (MTIE)
as well as jitter caused by link propagation and processing
variation. The sum of cycles times needs to be larger than the
total jitter to achieve this.
Prior documents describing the concept of TCQF (without using that
name) include [I-D.qiang-detnet-large-scale-detnet] and
[I-D.dang-queuing-with-multiple-cyclic-buffers]. TCQF does not
depend on other elements of [RFC8655], so it can also be used stand
alone in otherwise non-deterministic IP/IPv6 or MPLS networks to
achieve bounded latency and low jitter.
TCQF is likely especially beneficial when networks are architected to
avoid per-hop, per-flow state even for traffic steering, which is the
case for networks using SR-MPLS [RFC8402] for traffic steering of
MPLS unicast traffic, SRv6 [RFC8986] for traffic steeering of IPv6
unicast traffic and/or BIER-TE [I-D.ietf-bier-te-arch] for tree
engineering of MPLS multicast traffic by using the TC and/or DSCP
header fields of BIER packets according to [RFC8296].
In these networks, it is specifically undesirable to require per-flow
signaling to non-edge forwarders (such as P-LSR in MPLS networks)
solely for DetNet QoS because such per-flow state is unnecessary for
traffic steering and would only be required for the bounded latency
QoS mechanism and require likely even more complex hardware and
manageability support than what was previously required for per-hop
steering state (such as in RSVP-TE, [RFC4875]). Note that the DetNet
architecture [RFC8655] does not include full support for this
DiffServ model, which is why this memo describes how to use TCQF with
the DetNet architecture per-hop, per-flow processing as well as
without it.
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3. Using TCQF in the DetNet Architecture and MPLS forwarding plane
(informative)
This section gives an overview of how the operations of TCQF relates
to the DetNet architecture. We first revisit QoS with DetNet in the
absence of TCQF using an MPLS network as an example.
DetNet MPLS Relay Transit Relay DetNet MPLS
End System Node Node Node End System
T-PE1 S-PE1 LSR-P S-PE2 T-PE2
+----------+ +----------+
| Appl. |<------------ End-to-End Service ----------->| Appl. |
+----------+ +---------+ +---------+ +----------+
| Service |<--| Service |-- DetNet flow --| Service |-->| Service |
+----------+ +---------+ +----------+ +---------+ +----------+
|Forwarding| |Fwd| |Fwd| |Forwarding| |Fwd| |Fwd| |Forwarding|
+-------.--+ +-.-+ +-.-+ +----.---.-+ +-.-+ +-.-+ +---.------+
: Link : / ,-----. \ : Link : / ,-----. \
+........+ +-[ Sub- ]-+ +......+ +-[ Sub- ]-+
[Network] [Network]
`-----' `-----'
|<- LSP -->| |<-------- LSP -----------| |<--- LSP -->|
|<----------------- DetNet MPLS --------------------->|
Figure 5: A DetNet MPLS Network
The above Figure 5, is copied from [RFC8964], Figure 2, and only
enhanced by numbering the nodes to be able to better refer to them in
the following text.
Assume a DetNet flow is sent from T-PE1 to T-PE2 across S-PE1, LSR,
S-PE2. In general, bounded latency QoS processing is then required
on the outgoing interface of T-PE1 towards S-PE1, and any further
outgoing interface along the path. When T-PE1 and S-PE2 know that
their next-hop is a service LSR, their DetNet flow label stack may
simply have the DetNet flows Service Label (S-Label) as its Top of
Stack (ToS) LSE, explicitly indicating one DetNet flow.
On S-PE1, the next-hop LSR is not DetNet aware, which is why S-PE1
would need to send a label stack where the S-Label is followed by a
Forwarding Label (F-Label), and LSR-P would need to perform bounded
latency based QoS on that F-Label.
For bounded latency QoS mechanisms relying on per-flow regulator
state (aka: per-flow packet scheduling), such as in [TSN-ATS], this
requires the use of a per-detnet flow F-Labels across the network
from S-PE1 to S-PE2. These could for for example be assigned/managed
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through RSVP-TE [RFC3209] enhanced as necessary with QoS parameters
matching the underlying bounded latency mechanism (such as
[TSN-ATS]).
With TCQF, a sequence of LSR and DetNet service node implements TCQF
with MPLS TC, ideally from T-PE1 (ingress) to T-PE2 (egress). The
ingress node needs to perform per-DetNet-flow per-packet
"shaping"/"regulating" to assign each packet of a flow to a
particular TCQF cycle. This is specified in Section 5.
All LSR/Service nodes after the ingress node only have to map a
received TCQF tagged DetNet packet to the configured cycle on the
output interface, not requiring any per-DetNet-flow QoS state. These
LSR/Service nodes do therefore also not require per-flow interactions
with the controller plane for the purpose of bounded latency.
Per-flow state therefore is only required on nodes that are DetNet
service nodes, or when explicit, per-DetNet flow steering state is
desired, instead of ingress steering through e.g.: SR-MPLS.
Operating TCQF per-flow stateless across a service node, such as
S-PE1, S-PE2 in the picture is only one option. It is of course
equally feasible to Have one TCQF domain from T-PE1 to S-PE2, start a
new TCQF domain there, running for example up to S-PE2 and start
another one to T-PE2.
A service node must act as an egress/ingress edge of a TCQF domain if
it needs to perform operations that do change the timing of packets
other than the type of latency that can be considered in
configuration of TCQF (see Section 6.2).
For example, if T-PE1 is ingress for a TCQF domain, and T-PE2 is the
egress, S-PE1 could perform the DetNet Packet Replication Function
(PRF) without having to be a TQCF edge node as long as it does not
introduce latencies not included in the TCQF setup and the controller
plane reserves resources for the multitude of flows created by the
replication taking the allocation of resources in the TCQF cycles
into account.
Likewise, S-PE2 could perform the Packet Elimination Function without
being a TCQF edge node as this most likely does not introduce any
non-TCQF acceptable latency - and the controller plane accordingly
reserves only for one flow the resources on the S-PE2->T-PE2 leg.
If on the other hand, S-PE2 was to perform the Packet Reordering
Function (PRF), this could create large peaks of packets when out-of-
order packets are released together. A PRF would either have to take
care of shaping out those bursts for the traffic of a flow to again
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conform to the admitted CIR/PIR, or else the service node would have
to be a TCQF egress/ingress, performing that shaping itself as an
ingress function.
4. TCQF per-flow stateless forwarding (normative)
4.1. Configuration Data model and tag processing for MPLS TC tags
The following data model summarizes the configuration parameters as
required for TCQF and discussed in further sections. 'tcqf' includes
the parameters independent of the tagging on an interface. 'tcqf_*'
describes the parameters for interfaces using MPLS TC and IP DSCP
tagging.
# Encapsulation agnostic data
tcqf
+-- uint16 cycles
+-- uint16 cycle_time
+-- uint32 cycle_clock_offset
+-- if_config[oif] # Outgoing InterFace
+-- uint32 cycle_clock_offset
+-- cycle_map[iif] # Incoming InterFace
+--uint8 oif_cycle[iif_cycle]
Figure 6: Encapsulation independent TCQF Configuration Data Model
4.2. Packet processing
This section explains the TCQF packet processing and through it,
introduces the semantic of the objects in Figure 6
tcqf contains the router wide configuration of TCQF parameters,
independent of the specific tagging mechanism on any interface. Any
interface can have a different tagging method. This document uses
the term router when it is irrelevant whether forwarding is for IP or
MPLS packet, and the term Label Switched Router (LSR) to indicate
MPLS is used, or IP router to indicate IP or IPv6 are used -
independent of the specific encapsulation used for IP or MPLS to
carry the cycle identification.
The model represents a single TQCF domain, which is a set of
interfaces acting both as ingress (iif) and egress (oif) interfaces,
capable to forward TCQF packets amongst each other. A router may
have multiple TCQF domains each with a set of interfaces disjoint
from those of any other TCQF domain.
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tcqf.cycles is the number of cycles used across all interfaces in the
TCQF domain. routers MUST support 3 and 4 cycles. The maximum number
of supportable cycles depends on the encapsulation. For example, to
support interfaces with MPLS TC tagging, 7 or fewer cycles MUST be
used across all interfaces in the CQF domain. See Section 4.3.
The unit of tcqf.cycle_time is micro-seconds. routers MUST support
configuration of cycle-times of 20,50,100,200,500,1000,2000 usec.
Cycles start at an offset of tcqf.cycle_clock_offset in units of nsec
as follows. Let clock1 be a timestamp of the local reference clock
for TCQF, at which cycle 1 starts, then:
tcqf.cycle_clock_offset = (clock1 mod (tcqf.cycle_time * tcqf.cycles)
)
The local reference clock of the router is expected to be
synchronized with the neighboring LSR/router in TCQF domain.
tcqf.cycle_clock_offset can be configurable by the operator, or it
can be read-only. In either case will the operator be able to
configure working TCQF forwarding through appropriately calculated
cycle mapping.
tcqf.if_config[oif] is optional per-interface configuration of TCQF
parameters. tcqf.if_config[oif].cycle_clock_offset may be different
from tcqf.cycle_clock_offset, for example, when interfaces are on
line cards with independently synchronized clocks, or when non-
uniform ingress-to-egress propagation latency over a complex router/
LSR fabric makes it beneficial to allow per-egress interface or line
card configuration of cycle_clock_offset. It may be configurable or
read-only.
The value of -1 for tcqf.if_config[oif].cycle_clock_offset is used to
indicate that the domain wide tcqf.cycle_clock_offset is to be used
for oif. This is the only permitted negative number for this
parameter.
When a packet is received from iif with a cycle value of iif_cycle
and the packet is routed towards oif, then the cycle value (and
buffer) to use on oif is
tcqf.if_config[oif].cycle_map[iif].oif_cycle[iif_cycle]. This is
called the cycle mapping and is must be configurable. This cycle
mapping always happens when the packet is received with a cycle tag
on an interface in a TCQF domain and forwarded to another interface
in the same TCQF domain.
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This encapsulation independent data model only defines how to map
from a received packets cycle to a sending interface cycle buffer and
hence sent packet cycle. It does not specify how the cycle
identifier is encoded in the received or sent packet. This is
amended by the specification in the following sections.
This data model does therefore also not determine whether interfaces
use IP/IPv6, MPLS or any other encapsulation. This is determined by
the configuration of the DetNet domain. A mixed use of MPLS and IP/
IPv6 interfaces is possible with this data model, but at the time of
writing this document not supported by DetNet.
4.3. TCQF for MPLS with TC tagging
This section describes operation of TCQF for MPLS packets using the
Traffic Class (TC) field of MPLS label to carry the cycle-id. To
support this encapsulation, the TCQF Data Model as defined in
Figure 6 is expanded as follows.
# MPLS TC tagging specific data
tcqf_tc[oif]
+--uint8 tc[oif_cycle]
Figure 7: TCQF Configuration Data for MPLS TC
tcqf_tc[oif].tc[oif_cycle] defines how to map from the internal cycle
number oif_cycle to an MPLS TC value on interface oif. tcqf_tc[oif]
MUST be configured, when oif uses MPLS. This oif_cycle <=> tc
mapping is not only used to map from internal cycle number to MPLS TC
tag when sending packets, but also to map from MPLS TC tag to the
internal cycle number when receiving packets.
In the terminology of [RFC3270], TCQF QoS as defined here, is TC-
Inferred-PSC LSP (E-LSP) behavior: Packets are determined to belong
to the TCQF PSC solely based on the TC of the received packet.
The internal cycle number SHOULD be assigned from the Top of Stack
(ToS) MPLS label TC bits before any other label stack operations
happens. On the egress side, the TC value of the ToS MPLS label
SHOULD be assigned from the internal cycle number after any label
stack processing.
With this order of processing, TCQF can support forwarding of packets
with any label stack operations such as label swap in the case of LDP
or RSVP-TE created LSP, Penultimate Hop Popping (PHP), or no label
changes from SID hop-by-hop forwarding and/or SID/label pop as in the
case of SR-MPLS traffic steering.
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4.4. TCQF for IP/IPv6 with DSCP tagging
This section describes operation of TCQF for IP/IPv6 packets using
the Differentiated Services Code Point (DSCP) field of IP/IPv6
packets to carry the cycle-id. To support this encapsulation, the
TCQF Data Model as defined in Figure 6 is expanded as follows.
# IP/IPv6 DSCP tagging specific data
tcqf_dscp[oif]
+--uint8 dscp[oif_cycle]
Figure 8: TCQF Configuration Data for IP/IPv6 DSCP
tcqf_dscp[oif].dscp[oif_cycle] defines how to map from the internal
cycle number oif_cycle to an IP/IPv6 DSCP value on interface oif.
tcqf_dscp[oif] MUST be configured, when oif uses DSCP tagging of IP/
IPv6 packets for TCQF. This oif_cycle <=> idscp mapping is not only
used to map from internal cycle number to the DSCP tag when sending
packets, but also to map from IP/IPv6 DSCP to the internal cycle
number when receiving packets.
As how DetNet domains are currently assumed to be single
administrative network operator domains, this document does not ask
for standardization of the DSCP to use with TCQF. Instead,
deployments wanting to use TCQF with IP/IPv6 encapsulation and DSCP
tagging need to assign within their domain DSCP from the xxxx11 "EXP/
LU" Codepoint space according to [RFC2474], Section 6. This allows
up to 16 DSCP for intradomain use and hence up to 16 cycle
identifiers.
4.5. TCQF for IPv6 with IPv6 Option tagging
This section describes operation of TCQF for IPv6 packets without
having to rely on DSCP by defining a new IPv6 option for DetNet.
This option is to be placed in the IPv6 HbH (Hop-by-Hop) Options or
DOH (Destination Option Header) header. To support this
encapsulation, the TCQF Data Model as defined in Figure 6 is expanded
as follows.
# IPv6 TCQF Option tagging specific data
tcqf_ipv6oh[oif]
+--uint8 ipv6oh[oif_cycle]
Figure 9: TCQF Configuration Data for IPv6 TCQF Option Header
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4.5.1. TCQF Option Format
The TCQF Option helps the receiving port to identify in which time
cycle interval the packet is sent from the upstream router. It can
be used to determine the output port cycle buffer to enqueue the
packet.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len |E| Flags | Cycle Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
~ (64-bit extension if flag E-bit is 1) ~
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: TCQF Option Format
TCQF-Option fields:
* Option Type: 8-bit identifier of the type of option. Value TBD by
IANA. If the processing IPv6 node does not recognize the Option
Type it must discard the packet and return an ICMPv6 message (the
highest-order 2 bits = 10). The Option Data of this option may
change en route to the packet's final destination (the third-
highest-order bit=1).
* Opt Data Len: 8-bit length of the option data.
* Flags: 8-bit field to indicate what TCQF Option information
follows. The leftmost bit is called E-bit. When E-bit set to 1,
there is a 64-bit extension in length after Cycle Id.
* Cycle Id: 8-bit field to indicate the time cycle ID at output port
of the upstream node when the packet is sent out. This is the
packet header field name for the data model ipv6oh[oif_cycle]
element.
* 64-bit extension: This field contains values required for a
possible additional options, such as timestamp. This field exists
only when E-bit in Flags field is set to one. [Editor's Note:
Text will be modified or added as specific uses for this field are
identified]
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4.5.2. TCQF Option Processing
A packet carrying the TCQF Option with Cycle Id does not change the
fundamental cyclic queuing and forwarding behaviors of TCQF over the
encapsulation independent forwarding behavior described above
(Section 4.2).
Compared to DSCP it does not introduce a limited number of cycle-ids,
and eliminates the possible operation consideration to use multiple
DSCP for effectively a single per-hop forwarding behavior, which
otherwise would be a novel aspect that could cause issues for example
with diagnostics or other operational standards. It also allows
easier extensions with other potentially beneficial DetNet features
in the same Option header.
As part of the packet processing of Section 4.2, the Cycle ID field
of the option heade is rewritting from tcqf.ipv6oh[oif_cycle], in the
same way as DSCP wold be rewritten from tcqf.dscp[oif_cycle].
4.5.3. Encapsulation of TCQF Option for Deterministic IP (DIP) data
plane
When used in IPv6 ([RFC8200]) networks, the TCQF Option can be placed
in an HbH extension header or Destination Option Header (DOH).
+-----------------------------------+
| DetNet IP Packet |
+-----------------------------------+
| other EHs |
+-----------------------------------+
| IPv6 Hop-by-Hop Ex Hdr |
| (DIP-TCQF Option) |
+-----------------------------------+
| IPv6 Header |
+-----------------------------------+
| Data-Link |
+-----------------------------------+
| Physical |
+-----------------------------------+
Figure 11: TCQF Option Encapsulated in HbH for Deterministic IP
data plane
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Figure 11 shows the encapsulation of TCQF option in HbH extension
header for deterministic IP (DIP）data plane. When every DetNet
forwarding node along the path is provisioned to use TCQF as the
queuing mechanism, this option should be placed here. If a router
does not support this option, it discards the packet and returns an
ICMP message.
In some deployments the path selection is indicated using IPv6
routing header (RH) by specifying a set of nodes that must be
traversed by the packet along its path to the destination. When such
a source routing mechanism is used, TCQF Option is placed in DOH
(Destination Option Header) as shown in Figure 12 for Deterministic
IP data plane. Then the TCQF Option will be processed by the
specified in-path routers.
+-----------------------------------+
| DetNet IP Packet |
+-----------------------------------+
| other EHs including RH |
+-----------------------------------+
| IPv6 Destination Ex Hdr |
| (DIP-TCQF Option) |
+-----------------------------------+
| IPv6 Header |
+-----------------------------------+
| Data-Link |
+-----------------------------------+
| Physical |
+-----------------------------------+
Figure 12: TCQF Option Encapsulated in DOH for Deterministic IP
data plane
(TBD: Should and how TCQF Option be used in SRv6 ?)
4.6. TCQF Pseudocode (normative)
The following pseudocode restates the forwarding behavior of
Section 4 in an algorithmic fashion as pseudocode. It uses the
objects of the TCQF configuration data model defined in Section 4.1.
void receive(pak) {
// Receive side TCQF - retrieve cycle of received packet
// from packet internal header
iif = pak.context.iif
if (tcqf.if_config[iif]) { // TCQF enabled on iif
if (tcqf_tc[iif]) { // MPLS TCQF enabled on iif
tc = pak.mpls_header.lse[tos].tc
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pak.context.tcqf_cycle = map_tc2cycle( tc, tcqf_tc[iif])
} else
if (tcqf_ipv6oh[iif]) { // IPv6 Option Header used on iif
cycle_id = pak.ipv6_header.tcqf_oh[cycle_id]
pak.context.tcqf_cycle =
map_ipv6oh2cycle( cycle_id, tcqf_ipv6oh[iif])
} else
if (tcqf_dscp[iif]) { // IP DSCP TCQF used on iif
dscp = pak.ip_header.dscp
pak.context.tcqf_cycle = map_dscp2cycle( dscp, tcqf_dscp[iif])
} else // ... other encaps
}
forward(pak);
}
// ... Forwarding including any label stack operations
void forward(pak) {
oif = pak.context.oif = forward_process(pak)
if(ingres_flow_enqueue(pak))
return // ingress packets are only enqueued here.
if(pak.context.tcqf_cycle) // non TCQF packets cycle is 0
if(tcqf.if_config[oif]) { // TCQF enabled on OIF
// Map tcqf_cycle iif to oif - encap agnostic
cycle = pak.context.tcqf_cycle
= map_cycle(cycle,
tcqf.if_config[oif].cycle_map[[iif])
// MPLS TC-TCQF
if(tcqf.tc[oif]) {
pak.mpls_header.lse[tos].tc = map_cycle2tc(cycle, tcqf_tc[oif])
} else
if (tcqf_ipv6oh[oif]) { // IPv6 Option Header used on iif
pak.ipv6_header.tcqf_oh[cycle_id] =
map_cycle2ipv6oh(cycle, tcqf_ipv6oh[oif])
} else
// IP DSCP TCQF enabled on iif
if (tcqf_dscp[oif]) {
pak.ip_header.dscp = map_cycle2dscp(cycle, tcqf_dscp[oif])
} // else... other future encap/tagging options for TCQF
tcqf_enqueue(pak, oif.cycleq[cycle,iif]) // [3]
return
} else {
// Forwarding of egress TCQF packets [1]
}
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}
// ... non TCQF OIF forwarding [2]
}
// Started when TCQF is enabled on an interface
// dequeues packets from oif.cycleq
// independent of encapsulation
void send_tcqf(oif) {
cycle = 1
cc = tcqf.cycle_time *
tcqf.cycle_time
o = tcqf.cycle_clock_offset
nextcyclestart = floor(tnow / cc) * cc + cc + o
while(1) {
ingress_flow_2_tcqf(oif,cycle) // [5]
wait_until(tnow >= nextcyclestart); // wait until next cycle
nextcyclestart += tcqf.cycle_time
forall(iif) {
forall(pak = tcqf_dequeue(oif.cycleq[cycle,iif]) {
schedule to send pak on oif before nextcyclestart; // [4]
}
}
cycle = (cycle + 1) mod tcqf.cycles + 1
}
}
Figure 13: TCQF Pseudocode
Processing of ingress TCQF packets is performed via
ingres_flow_enqueue(pak) and ingress_flow_2_tcqf(oif,cycle) as
explained in Section 5.2.
Packets in a cycle buffer can be sent almost arbitrarily within the
time period of the cycle. They also do not need to be sent as soon
as possible, as long as all will be sent within that period. There
is no need to send them in the order of their arrival except that
packets from the same ingres flow that end up in the same cycle must
not be reordered across any number of tcqf hops. The pseudocode
describes this by using a queue oif.cycleq[cycle,iif] ([3]) for all
packets from the same iif. The pseudocode describes the oterwise
arbitrary scheduling of all packets within the cycle time via the
statement shown in [4].
Ingress packets are passed from their ingress queues to the next
cycle queue via [5].
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Processing of egres TCQF packets is out-of-scope. It can performed
by any non-TCQF packet forwarding mechanism such as some strict
priority queuing in step [2], and packets could accordingly be marked
with an according packet header traffic class indicator for such a
traffic class in step [1].
5. TCQF Per-flow Ingress forwarding (normative)
Ingress flows in the context of this text are packets of flows that
enter the router from a non-TCQF interface and need to be forwarded
to an interface with TCQF.
In the most simple case, these packets are sent by the source and the
router is the first-hop router. In another case, the routers ingress
interface connects to a hop where the previous router(s) did perform
a different bounded latency forwarding mechanism than TCQF.
5.1. Ingress Flows Configuration Data Model
# Extends above defined tcqf
tcqf
...
| Ingress Flows, see below (TBD:
+-- iflow[flowid]
+-- uint32 csize # in bits
Figure 14: TCQF Ingress Configuration Data Model
The data model shown in Figure 14 expands the tcqf data model from
Figure 6. For every DetNet flow for which this router is the TCQF
ingress, the controller plane has to specify a maximum number of bits
called csize (cycle size) that are permitted to go into each
individual cycle.
Note, that iflow[flowid].csize is not specific to the sending
interface because it is a property of the DetNet flow.
5.2. Ingress Flows Pseudocode
When a TCQF ingress is received, it first has to be enqueued into a
per-flow queue. This is necessary because the permitted burst size
for the flow may be larger than what can fit into a single cycle, or
even into the number of cycles used in the network.
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bool ingres_flow_enqueue(pak) {
if(!pak.context.tcqf_cycle &&
flowid = match_detnetflow(pak)) {
police(pak) // according to RFC9016 5.5
enqueue(pak, flowq[oif][flowid])
return true
}
return false
}
Figure 15: TCQF Ingress Enqueue Pseudocode
ingres_flow_enqueue(pak) as shown in Figure 15 performs this
enqueuing of the packet. Its position in the DetNet/TCQF forwarding
code is shown in Figure 13.
police(pak): If the router is not only the TCQF ingress router, but
also the first-hop router from the source, ingres_flow_enqueue(pak)
will also be the place where policing of the flows packet according
to the Traffic Specification of the flow would happen - to ensure
that packets violating the Traffic Specification will not be
forwarded, or be forwarded with lower priority (e.g.: as best
effort). This policing and resulting forwarding action is not
specific to TCQF and therefore out of scope for this text. See
[RFC9016], section 5.5.
void ingress_flow_2_tcqf(oif, cycle) {
foreach flowid in flowq[oif][*] {
free = tcqf.iflow[flowid].csize
q = flowq[oif][flowid]
while(notempty(q) &&
(l = head(q).size) <= free) {
pak = dequeue(q)
free -= l
tcqf_enqueue(pak, oif.cycleq[cycle,internal])
}
}
}
Figure 16: TCQF Ingress Pseudocode
ingress_flow_2_tcqf(oif, cycle) as shown in Figure 16 transfers
ingress DetNet flow packets from their per-flow queue into the queue
of the cycle that will be sent next. The position of
ingress_flow_2_tcqf() in the DetNet/TCQF forwarding code is shown in
Figure 13.
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6. Implementation, Deployment, Operations and Validation considerations
(informative)
6.1. High-Speed Implementation
High-speed implementations with programmable forwarding planes of
TCQF packet forwarding require Time-Gated Queues for the cycle
queues, such as introduced by [IEEE802.1Qbv] and also employed in CQF
[IEEE802.1Qch].
Compared to CQF, the accuracy of clock synchronization across the
nodes is reduced as explained in Section 6.2 below.
High-speed forwarding for ingress packets as specified in Section 5
above would require to pass packets first into a per-flow queue and
then re-queue them into a cycle queue. This is not ideal for high
speed implementations. The pseudocode for ingres_flow_enqueue() and
ingress_flow_2_tcqf(), like the rest of the pseudocode in this
document is only meant to serve as the most compact and hopefully
most easy to read specification of the desired externally observable
behavior of TCQF - but not as a guidance for implementation,
especially not for high-speed forwarding planes.
High-speed forward could be implemented with single-enqueueing into
cycle queues as follows:
Let B[f] be the maximum amount of data that the router would need to
buffer for ingress flow f at any point in time. This can be
calculated from the flows Traffic Specification. For example, when
using the parameters of [RFC9016], section 5.5.
B[f] <= MaxPacketsPerInterval*MaxPayloadSize*8
maxcycles = max( ceil( B[f] / tcqf.iflow[f].csize) | f)
Maxcycles is the maximum number of cycles required so that packets
from all ingress flows can be directly enqueued into maxcycles
queues. The router would then not cycle across tcqf.cycles number of
queues, but across maxcycles number of queues, but still cycling
across tcqf.cycles number of cycle tags.
Calculation of B[f] and in result maxcycles may further be refined
(lowered) by additionally known constraints such as the bitrates of
the ingress interface(s) and TCQF output interface(s).
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6.2. Controller plane computation of cycle mappings
The cycle mapping is computed by the controller plane by taking at
minimum the link, interface serialization and node internal
forwarding latencies as well as the cycle_clock_offsets into account.
Router . O1
R1 . | cycle 1 | cycle 2 | cycle 3 | cycle 1 |
. .
. ............... Delay D
. .
. O1'
. | cycle 1 |
Router . | cycle 1 | cycle 2 | cycle 3 | cycle 1 |
R2 . O2
CT = cycle_time
C = cycles
CC = CT * C
O1 = cycle_clock_offset router R1, interface towards R2
O2 = cycle_clock_offset router R2, output interface of interest
O1' = O1 + D
Figure 17: Calculation reference
Consider in Figure 17 that Router R1 sends packets via C = 3 cycles
with a cycle_clock offset of O1 towards Router R2. These packets
arrive at R2 with a cycle_clock offset of O1' which includes through
D all latencies incurred between releasing a packet on R1 from the
cycle buffer until it can be put into a cycle buffer on R2:
serialization delay on R1, link delay, non_CQF delays in R1 and R2,
especially forwarding in R2, potentially across an internal fabric to
the output interface with the sending cycle buffers.
A = ( ceil( ( O1' - O2 ) / CT) + C + 1) mod CC
map(i) = (i - 1 + A) mod C + 1
Figure 18: Calculating cycle mapping
Figure 18 shows a formula to calculate the cycle mapping between R1
and R2, using the first available cycle on R2. In the example of
Figure 17 with CT = 1, (O1' - O2) =~ 1.8, A will be 0, resulting in
map(1) to be 1, map(2) to be 2 and map(3) to be 3.
The offset "C" for the calculation of A is included so that a
negative (O1 - O2) will still lead to a positive A.
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In general, D will be variable [Dmin...Dmax], for example because of
differences in serialization latency between min and max size
packets, variable link latency because of temperature based length
variations, link-layer variability (radio links) or in-router
processing variability. In addition, D also needs to account for the
drift between the synchronized clocks for R1 and R2. This is called
the Maximum Time Interval Error (MTIE).
Let A(d) be A where O1' is calculated with D = d. To account for the
variability of latency and clock synchronization, map(i) has to be
calculated with A(Dmax), and the controller plane needs to ensure
that that A(Dmin)...A(Dmax) does cover at most (C - 1) cycles.
If it does cover C cycles, then C and/or CT are chosen too small, and
the controller plane needs to use larger numbers for either.
This (C - 1) limitation is based on the understanding that there is
only one buffer for each cycle, so a cycle cannot receive packets
when it is sending packets. While this could be changed by using
double buffers, this would create additional implementation
complexity and not solve the limitation for all cases, because the
number of cycles to cover [Dmin...Dmax] could also be (C + 1) or
larger, in which case a tag of 1...C would not suffice.
6.3. Link speed and bandwidth sharing
TCQF hops along a path do not need to have the same bitrate, they
just need to use the same cycle time. The controller plane has to
then be able to take the TCQF capacity of each hop into account when
admitting flows based on their Traffic Specification and TCQF csize.
TCQF does not require to be allocated 100% of the link bitrate. When
TCQF has to share a link with other traffic classes, queuing just has
to be set up to ensure that all data of a TCQF cycle buffer can be
sent within the TCQF cycle time. For example by making the TCQF
cycle queues the highest priority queues and then limiting their
capacity through admission control to leave time for other queues to
be served as well.
6.4. Controller-plane considerations
TCQF is applicable to both centralized as well as decentralized/
distributed controller-plane models. From the perspective of the
controller plane. If the controller-plane is centralized, then it is
logically very simple to perform admission control for any additional
flow by checking that there is sufficient bandwidth for the amount of
bits required for the flow on every cycle along the intended path.
Likewise, path computation can be done to determine on which non-
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shortest path those resources are available.
More efficient use of resources can be achieved by considering that
flows with low bit rates would not need bits reserved in every cycle,
but only in every N'th cyce. This requires different gates on ingres
to admit packets from such flows than shown in this document and more
complex admission control that attempts for example to interleave
multiple flows across different set of cycles to as best as possible
utilize all cycles. This is the same complexity as possible in TSN
technologies. Beside the admission control and different ingres
policing, such enhancements have no impact on the per-hop TCQF
forwarding and can thus potentially be added incrementally.
Decentralized or distributed controller planes including on-path,
per-flow signaling, such as one using the mechanisms of RSVP-TE,
[RFC3209] is equally feasible with TCQF. In this case one of the
potential benefits of TCQ is not leveraged, which is the complete
removal of per-hop,per-flow awarenes on each router. Nevertheless,
the controller-plane only introduces the need for this state
maintenance into the control-plane of each router, but does not
change the TCQF forwarding plane, but maintains its per-hop, per-flow
non-stateful nature and resulting performance/cost benefits.
6.5. Validation
[LDN] describes an accurate simualtion based validation of TCQF and
provides further details on the mathematical models.
[CENI] is a report summary of a 100Gbps link speed commercial router
validation implementation of TCQF deployed and measured in a research
testbed with a range of up to 2000km across China, operated by the
China Environment for Network Innovations (CENI). The report also
provides a reference to a more detailled version of the report. Note
that both reports are in chinese. TCQF is called DIP in these
reports.
7. Security Considerations
TBD.
8. IANA Considerations
This document defines a new TCQF-Variant Option for the “Destination
Options and Hop-by-Hop Options” under the “Internet Protocol Version
6 (IPv6) Parameters” registry [IPV6-PARMS] with the suggested values
in Figure 19.
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+------+-----+-----+-------+--------------------+-------------+
| Hexa | act | chg | rest | Description | Reference |
+------+-----+-----+-------+--------------------+-------------+
| 0xB1 | 10 | 1 | 10001 | TCQF Option |this document|
+------+-----+-----+-------+--------------------+-------------+
Figure 19: TCQF Option Code in Destination Options and Hop-by-Hop
Options
9. Acknowledgement
Many thanks for review by David Black (DetNet techadvisor).
10. Contributors
The following co-authors have contributed to this document.
Xiaoliang Zheng Huawei Email: zhengxiaoliang@huawei.com
11. Changelog
[RFC-editor: please remove]
Initial draft name: draft-eckert-detnet-mpls-tc-tcqf
00
Initial version
01
Added new co-author.
Changed Data Model to "Configuration Data Model",
and changed syntax from YANG tree to a non-YANG tree, removed empty
section targeted for YANG model. Reason: the configuration
parameters that we need to specify the forwarding behavior is only a
subset of what likely would be a good YANG model, and any work to
define such a YANG model not necessary to specify the algorithm would
be scope creep for this specification. Better done in a separate
YANG document. Example additional YANG aspects for such a document
are how to map parameters to configuration/operational space, what
additional operational/monitoring parameter to support and how to map
the YANG objects required into various pre-existing YANG trees.
Improved text in forwarding section, simplified sentences, used
simplified configuration data model.
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02
Refresh
03
Added ingress processing, and further implementation considerations.
New draft name: draft-eckert-detnet-tcqf
00
Added text for DSCP based tagging of IP/IPv6 packets, therefore
changing the original, MPLS-only centric scope of the document,
necessitating a change in name and title.
This was triggered by the observation of David Black at the IETF114
DetNet meeting that with DetNet domains being single administrative
domains, it is not necessary to have standardized (cross
administrative domain) DSCP for the tagging of IP/IP6 packets for
TCQF. Instead it is sufficient to use EXP/LU DSCP code space and
assignment of these is a local matter of a domain as is that of TC
values when MPLS is used. Standardized DSCP in the other hand would
have required likely work/oversight by TSVWG.
In any case, the authors feel that with this insight, there is no
need to constrain single-domain definition of TCQF to only MPLS, but
instead both MPLS and IP/IPv6 tagging can be easily specified in this
one draft.
01
Added new co-author.
02
Attempt to resolve issues from https://github.com/toerless/detnet/
issues/1.
* Review from David Black, refine queueing/scheduling of pseudocode/
explanation to highlight the non-sequential requirements.
* Comment from Lou Berger re. applicability of controller-plane
resulting in new section about controller-plane.
* Reference to CENI chinese validation deployment.
03
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Merged draft with draft-yizhou-detnet-ipv6-options-for-cqf-variant-
02.
Changed specification to be independent of encapsulation/forwarding
plane and moved MPLS and IP/DSCP (from old TCQF draft) and IPv6 with
extension header into separate seconds.
Human translation of CENI report, uploaded CENI report with
permission from CENI onto web page accessible from outside chinese
firewall.
04
Added appendix sections on comparison with CSQF and multi class TCQF
05
Refresh.
12. References
12.1. Normative References
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/rfc/rfc2474>.
[RFC3270] Le Faucheur, F., Ed., Wu, L., Davie, B., Davari, S.,
Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen,
"Multi-Protocol Label Switching (MPLS) Support of
Differentiated Services", RFC 3270, DOI 10.17487/RFC3270,
May 2002, <https://www.rfc-editor.org/rfc/rfc3270>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/rfc/rfc8200>.
[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/rfc/rfc8655>.
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[RFC8964] Varga, B., Ed., Farkas, J., Berger, L., Malis, A., Bryant,
S., and J. Korhonen, "Deterministic Networking (DetNet)
Data Plane: MPLS", RFC 8964, DOI 10.17487/RFC8964, January
2021, <https://www.rfc-editor.org/rfc/rfc8964>.
12.2. Informative References
[CENI] China Environment for Network Innovations (CENI), "CENI
DIP Networking Test Report", 2020,
<https://raw.githubusercontent.com/network2030/
publications/main/CENI_DIP_Networking_Test_Report.pdf>.
Translated with permission from chinese version at:
https://ceni.org.cn/406.html
[I-D.chen-detnet-sr-based-bounded-latency]
Chen, M., Geng, X., Li, Z., Joung, J., and J. Ryoo,
"Segment Routing (SR) Based Bounded Latency", Work in
Progress, Internet-Draft, draft-chen-detnet-sr-based-
bounded-latency-03, 7 July 2023,
<https://datatracker.ietf.org/doc/html/draft-chen-detnet-
sr-based-bounded-latency-03>.
[I-D.dang-queuing-with-multiple-cyclic-buffers]
Liu, B. and J. Dang, "A Queuing Mechanism with Multiple
Cyclic Buffers", Work in Progress, Internet-Draft, draft-
dang-queuing-with-multiple-cyclic-buffers-00, 22 February
2021, <https://datatracker.ietf.org/doc/html/draft-dang-
queuing-with-multiple-cyclic-buffers-00>.
[I-D.eckert-detnet-bounded-latency-problems]
Eckert, T. T. and S. Bryant, "Problems with existing
DetNet bounded latency queuing mechanisms", Work in
Progress, Internet-Draft, draft-eckert-detnet-bounded-
latency-problems-00, 12 July 2021,
<https://datatracker.ietf.org/doc/html/draft-eckert-
detnet-bounded-latency-problems-00>.
[I-D.eckert-detnet-flow-interleaving]
Eckert, T. T., "Deterministic Networking (DetNet) Data
Plane - Flow interleaving for scaling detnet data planes
with minimal end-to-end latency and large number of
flows.", Work in Progress, Internet-Draft, draft-eckert-
detnet-flow-interleaving-01, 5 January 2024,
<https://datatracker.ietf.org/doc/html/draft-eckert-
detnet-flow-interleaving-01>.
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[I-D.ietf-bier-te-arch]
Eckert, T. T., Menth, M., and G. Cauchie, "Tree
Engineering for Bit Index Explicit Replication (BIER-TE)",
Work in Progress, Internet-Draft, draft-ietf-bier-te-arch-
13, 25 April 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-bier-te-arch-13>.
[I-D.qiang-detnet-large-scale-detnet]
Qiang, L., Geng, X., Liu, B., Eckert, T. T., Geng, L., and
G. Li, "Large-Scale Deterministic IP Network", Work in
Progress, Internet-Draft, draft-qiang-detnet-large-scale-
detnet-05, 2 September 2019,
<https://datatracker.ietf.org/doc/html/draft-qiang-detnet-
large-scale-detnet-05>.
[IEEE802.1Q]
IEEE 802.1 Working Group, "IEEE Standard for Local and
Metropolitan Area Network — Bridges and Bridged Networks
(IEEE Std 802.1Q)", doi 10.1109/ieeestd.2018.8403927,
2018, <https://doi.org/10.1109/ieeestd.2018.8403927>.
[IEEE802.1Qbv]
IEEE Time-Sensitive Networking (TSN) Task Group., "IEEE
Standard for Local and metropolitan area networks --
Bridges and Bridged Networks - Amendment 25: Enhancements
for Scheduled Traffic", 2015.
[IEEE802.1Qch]
IEEE Time-Sensitive Networking (TSN) Task Group., "IEEE
Std 802.1Qch-2017: IEEE Standard for Local and
Metropolitan Area Networks - Bridges and Bridged Networks
- Amendment 29: Cyclic Queuing and Forwarding", 2017.
[IPV6-PARMS]
"Internet Protocol Version 6 (IPv6) Parameters", IANA ,
n.d., <https://www.iana.org/assignments/ipv6-parameters/
ipv6-parameters.xhtml>.
[LDN] Liu, B., Ren, S., Wang, C., Angilella, V., Medagliani, P.,
Martin, S., and J. Leguay, "Towards Large-Scale
Deterministic IP Networks", IEEE 2021 IFIP Networking
Conference (IFIP Networking),
doi 10.23919/IFIPNetworking52078.2021.9472798, 2021,
<https://dl.ifip.org/db/conf/networking/
networking2021/1570696888.pdf>.
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[multipleCQF]
Finn, N., "Multiple Cyclic Queuing and Forwarding",
October 2021,
<https://www.ieee802.org/1/files/public/docs2021/new-finn-
multiple-CQF-0921-v02.pdf>.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/rfc/rfc3209>.
[RFC4875] Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
Yasukawa, Ed., "Extensions to Resource Reservation
Protocol - Traffic Engineering (RSVP-TE) for Point-to-
Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
DOI 10.17487/RFC4875, May 2007,
<https://www.rfc-editor.org/rfc/rfc4875>.
[RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication (BIER) in MPLS and Non-
MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
2018, <https://www.rfc-editor.org/rfc/rfc8296>.
[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/rfc/rfc8402>.
[RFC8938] Varga, B., Ed., Farkas, J., Berger, L., Malis, A., and S.
Bryant, "Deterministic Networking (DetNet) Data Plane
Framework", RFC 8938, DOI 10.17487/RFC8938, November 2020,
<https://www.rfc-editor.org/rfc/rfc8938>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/rfc/rfc8986>.
[RFC9016] Varga, B., Farkas, J., Cummings, R., Jiang, Y., and D.
Fedyk, "Flow and Service Information Model for
Deterministic Networking (DetNet)", RFC 9016,
DOI 10.17487/RFC9016, March 2021,
<https://www.rfc-editor.org/rfc/rfc9016>.
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[RFC9320] Finn, N., Le Boudec, J.-Y., Mohammadpour, E., Zhang, J.,
and B. Varga, "Deterministic Networking (DetNet) Bounded
Latency", RFC 9320, DOI 10.17487/RFC9320, November 2022,
<https://www.rfc-editor.org/rfc/rfc9320>.
[TSN-ATS] Specht, J., "P802.1Qcr - Bridges and Bridged Networks
Amendment: Asynchronous Traffic Shaping", IEEE , 9 July
2020, <https://1.ieee802.org/tsn/802-1qcr/>.
Appendix A. CSQF
[I-D.chen-detnet-sr-based-bounded-latency] (CSQF) describes a
variation of the cyclic queuing mechanism in which the cycle
identifier is not mapped by a mapping table in each node (as in
TCQF), instead the packet header carries the sequence of cycles for
every cyclic queuing hop. In the draft, this is proposed
specifically for networks using Segment Routing and can therefore
allocate for N cycles N SIDs, each one for a different cycle to allow
indicating in a SID sequence header for each hop, which cycle to use.
The core new functionality enabled with eliminating the cycle mapping
table on the routers and moving the sequence of cycles into the
header is the ability to utilize in a flexible fashion more than a
fixed number of cycles, independently on each hop.
Assume a minimum of N (e.g.: N = 3) cycles would be required in a
particular deployment with TCQF. If CSQF is then set up with e.g.: N
+ 4 = 7 cycles, then it would be possible for the controller-plane to
delay packets of a flow on every hop by 1,2,3 or 4 more cycles than
necessary at minimum. This can lead to an easier ability to achieve
higher utilization in the face of controller-plane operations that
manages large number of flows in large scale DetNets, and does not
allocate to every flow bandwidth in every cycle. This naturally
leads to uneven utilization of cycles and the problem of managing
distribution of traffic load across cycles.
[I-D.eckert-detnet-flow-interleaving] discusses this overall advanced
controller-plane traffic management and how different queuing options
can be used in such a setup. It also describes the necessary ingress
processing to allow forwarding traffic flows only in such well
engineered specific cycles.
While such advanced cycle engineering may look at first quite
complex, it should simply be compared to the mechanisms that already
are standard in service provider networks to manage bandwidth/
capacity by engineering per-flow paths across topologies with non-
equal cost paths. In that overall complex problem space, managing
distribution of traffic across cycles is but a minor extension.
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Note that TCQF can of course also benefit from such advanced cycle
engineering at the controller plane, albeit less flexibly than CSQF.
Given how CQSF and TCQF share all the forwarding behavior except for
where the cycle Identifier is retrieved from and how it is mapped, it
would also be a very useeful consideration to consider both
approaches options of a single target standard. It seems unlikely
though, that an implementation that can support TCQF could not
support CSQF - or vice versa.
Appendix B. TCQF with multiple priorities
TSN CQF [IEEE802.1Qch] does permit to establish multiple independent
cyclic queuing instances and therefore create more flexbility.
Consider likewise, that in DetNet, there are separate packet headers
for a packet priority and a cycle identifier. For each priority, a
separate instance of TCQF is established, and the priority decides
which instance of CQF the packet gets processed by, whereas the cycle
identifier determines the cycle within the TCQF instance.
Consider for example a setup with 4 priorities 1..4. The cycle time
for the highest priority 1 is C. The cycle time for priority 2 is 2
* C, for priority 3 3 * C and for priority 4 4 * C. In queuing,
strict priority queuing is used, packets from a priority 1 cycle
queue will always be sent over those from priorities 2...4, and so
on. In result, a flow can now be given one out of 4 priorities, each
with an increasing per-hop latency: C (prio 1), 2C (prio 2), 3C (prio
3), 4C (prio 4). This does of course also require for admission
control to not allow full utilization of the capacity of cycles in
each class. In a simple static splitting of capacity across classes,
each cycle of of each priority could for example be allowed to be
utilized up to 25%.
This multi-priority "extension" to TCQF is in this version of the
document only mentioned as an appendix, because it is not clear if
this degree of flexibility is desired in a first-generation target
standard for TCQF. Given how both priority and cycle identifiers are
needed, this mechanism would certainly require for both MPLS and IP/
IPv6 a new extension header, such as the one proposed in this
document to carry the Cycle IDentifierm and then the priority could
be indicated by the IP header DSCP.
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Appendix C. TSN Multiple Buffer CQF
CQF with multiple buffers but without tagging has been proposed to
IEEE TSN in [multipleCQF], but has not been adopted. Instead of
relying on a cycle tag in a packet header as proposed in this memo,
it still relies solely on the arrival time of packet, and can hence
not equally resolve arrival time ambiguities as TCQF can, because it
does not know the cycle from which the packet was sent, or the cycle
for which it is intended.
Consider that multiple buffer CQF is like TCQF, except the cycle id
is missing from the packet that is sent. Upon arrival at the
receiving router, the sending cycle ID has to be determined solely by
the time the packet is received (reception timestamp) because this
time is an indicator of the sending timestamp and hence the sending
cycle. The sum of MTIE, processing variation link propagation
latency and other variations from layer 1 and layer 2 processing
(forward error correction, retransmissions) is the erorr of the
sending time that the receiving router can determine. As soon as
this error is so large, that the receiving router can not
unambiguously determine a sending cycle, the mechanism does not work
anymore. The receiving router can also not simply assume for a
packet to be sent by one of the possible cycles, because when this is
not the actual sending cycle, then such an assumption will cause
possible overruns of cycle buffers and hence failure of admission
control and pckets drop or congestion. In result, multiple buffer
CQF without carrying a target cycle in a packet header seems not
feasible to actually solve the issue or real propagation latency
variation in transmission, or the perceived variation in propagation
due to jitter in clocks between adjacend nodes.
https://www.iana.org/assignments/ipv6-parameters/
ipv6-parameters.xhtml
Authors' Addresses
Toerless Eckert (editor)
Futurewei Technologies USA
2220 Central Expressway
Santa Clara, CA 95050
United States of America
Email: tte@cs.fau.de
Yizhou Li (editor)
Huawei Technologies
Nanjing
China
Eckert, et al. Expires 9 July 2024 [Page 40]
Internet-Draft detnet-tcqf January 2024
Email: liyizhou@huawei.com
Stewart Bryant
University of Surrey ICS
United Kingdom
Email: s.bryant@surrey.ac.uk
Andrew G. Malis
Malis Consulting
United States of America
Email: agmalis@gmail.com
Jeong-dong Ryoo
ETRI
South Korea
Email: ryoo@etri.re.kr
Peng Liu
China Mobile
China
Email: liupengyjy@chinamobile.com
Guangpeng Li
Huawei Technologies
Beijing
China
Email: liguangpeng@huawei.com
Shoushou Ren
Huawei Technologies
Beijing
China
Email: renshoushou@huawei.com
Fan Yang
Huawei Technologies
Beijing
China
Email: shirley.yangfan@huawei.com
Eckert, et al. Expires 9 July 2024 [Page 41]
