Deterministic Networking (DetNet) Data Plane - MPLS TC Tagging for Cyclic Queuing and Forwarding (MPLS-TC TCQF)
draft-eckert-detnet-mpls-tc-tcqf-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Toerless Eckert , Stewart Bryant | ||
| Last updated | 2021-09-08 | ||
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draft-eckert-detnet-mpls-tc-tcqf-00
DETNET T. Eckert
Internet-Draft Futurewei Technologies USA
Intended status: Standards Track S. Bryant
Expires: 12 March 2022 University of Surrey ICS
8 September 2021
Deterministic Networking (DetNet) Data Plane - MPLS TC Tagging for
Cyclic Queuing and Forwarding (MPLS-TC TCQF)
draft-eckert-detnet-mpls-tc-tcqf-00
Abstract
This memo defines the use of the MPLS TC field of MPLS Label Stack
Entries (LSE) to support cycle tagging of packets for Multiple Buffer
Cyclic Queuing and Forwarding (TCQF). TCQF is a mechanism to support
bounded latency forwarding in DetNet network.
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 12 March 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Using TCQF with DetNet MPLS (informative) . . . . . . . . . . 3
3. Data model and tag processing for MPLS TC TCQF (normative) . 6
4. TCQF with labels stack operations (normative) . . . . . . . . 8
5. TCQF Pseudocode (normative) . . . . . . . . . . . . . . . . . 8
6. TCQF YANG Model (normative) TBD . . . . . . . . . . . . . . . 9
7. Computing cycle mappings (informative) . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Informative References . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Cyclic Queuing and Forwarding [CQF], is an IEEE standardized queuing
mechanism in support of deterministic bounded latency. See also
[I-D.ietf-detnet-bounded-latency], Section 6.6.
CQF benefits for Deterministic QoS include the tightly bounded jitter
it provides as well as the per-flow stateless operation, minimizing
the complexity of high-speed hardware implementations and allowing to
support on transit hops arbitrary number of DetNet flow in the
forwarding plane because of the absence of per-hop, per-flow QoS
processing. In the terms of the IETF QoS architecture, CQF can be
called DiffServ QoS technology, operating only on a traffic
aggregate.
CQFs is limited to only limited-scale wide-area network deployments
because it cannot take the propagation latency of links into account,
nor potential variations thereof. It also requires very high
precision clock synchronization, which is uncommon in wide-area
network equipment beyond mobile network fronthaul. See
[I-D.eckert-detnet-bounded-latency-problems] for more details.
This specification introduces and utilizes an enhanced form of CQF
where packets are tagged with a cycle identifier, and a limited
number of cycles, e.g.: 3...7 are used to overcome these distance and
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clock synchronization limitations. Because this memo defines how to
use the TC field of MPLS LSE as the tag to carry the cycle
identifier, it calls this scheme TC Tagged multiple buffer CQF (TC
TCQF). See [I-D.qiang-DetNet-large-scale-DetNet] and
[I-D.dang-queuing-with-multiple-cyclic-buffers] for more details of
the theory of operations of TCQF. Note that TCQF is not necessarily
limited to deterministic operations but could also be used in
conjunction with congestion controlled traffic, but those
considerations are outside the scope of this memo.
TCQF is likely especially beneficial when MPLS networks are designed
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 and/or BIER-TE [I-D.ietf-bier-te-arch] for tree
engineering of MPLS multicast traffic. In these networks, it is
specifically undesirable to require per-flow signaling to P-LSR
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 (e.g. In RSVP-TE). Note that the DetNet architecture
[RFC8655] does not include full support for this DiffServ model,
which is why this memo describes how to use MPLS TC TCQF with the
DetNet architecture per-hop, per-flow processing as well as without
it.
2. Using TCQF with DetNet MPLS (informative)
This section gives an overview of how the operations of T-CQF relates
to the DetNet architecture. We first revisit QoS with DetNet in the
absence of T-CQF.
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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 1: A DetNet MPLS Network
The above Figure 1, 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, such as in [TSN-ATS], this requires the use of a per-detnet
flow F-Label across the network from S-PE1 to S-PE2, for example
through RSVP-TE [RFC3209] enhanced as necessary with QoS parameters
matching the underlying bounded latency mechanism (such as
[TSN-ATS]).
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With TC TCQF, a sequence of LSR and DetNet service node implements TC
TCQF, ideally from T-PE1 (ingress) to T-PE2 (egress). The ingress
node needs to perform per-DetNet-flow per-packet "shaping" to assign
each packet of a flow to a particular TCQF cycle. This ingress-edge-
function is currently out of scope of this document (TBD), but would
be based on the same type of edge function as used in CQF.
All LSR/Service node 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 therefore 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 an 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 7).
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
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.
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3. Data model and tag processing for MPLS TC TCQF (normative)
module ietf-detnet-tcqf
augment TBD
+--rw tcqf-config
| +--rw cycles uint16
| +--rw cycle-time uint16
| +--rw cycle-clock-offset uint32
| +--rw tcqf-if-config* [oif-name]
| +--rw oif-name if:interface-ref
| +--rw cycle-clock-offset int32
| +--rw tcqf-iif-cycle-map* [iif-name]
| +--rw iif-name if:interface-ref
| +--rw iif-cycle-map* [iif-cycle]
| +--rw iif-cycle uint8
| +--rw oif-cycle uint8
|
+--rw tcqf-mpls-tc-tag* [name]
+--rw name if:interface-ref
+--rw cycle* [cycle]
+--rw cycle uint8
+--rw tc uint8
Figure 2: TCQF Data Model
tcqf-config is the router/LSR wide configuration of TCQF parameters,
independent of the tagging of the method with which cycles are tagged
on any interface. This YANG 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.
When multiple independent instances or TCQF domains are used, they
can have separate parameters.
cycles is the number of cycles used across all interfaces. router/
LSR MUST support 3 and 4 cycles. To support interfaces with MPLS TC
tagging, 7 or less cycles must be used.
The cycle time is cycle-time in units of micro-seconds. router/LSR
MUST support configuration of cycle-times of
20,50,100,200,500,1000,2000 usec.
Cycles start at an offset of 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:
cycle-clock-offset = (clock1 mod (cycle-time * cycles) )
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The local reference clock is expected to be synchronized with the
neighboring nodes. cycle-clock-offset can be configurable, or it may
be derived from immutable properties of the implementation, in which
case it is read-only.
tcqf-if-config is the optional per-interface configuration of TCQF
parameters.
The cycle-clock-offset in tcqf-oif-config may be different from the
router/LSR wide 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.
If cycle-clock-offset is unused and therefore the router/LSR wide
cycle-clock-offset is used, the value MUST be -1. This is the only
permitted negative number.
tcqf-iif-cycle-map is defining how to map the cycle iif-cycle of a
packet received from an incoming interface (iif-name) once the LSR
has determined that the packet needs to be sent to oif-name and sent
with TCQF. The packet is then assigned to cycle oif-cycle on oif-
name.
Note that all parameters so far allow for different methods of
tagging the cycle in the packet across different interfaces and
allowing TCQF to operate across them, even if future work would
introduce different tagging methods than the following MPLS TC
mapping.
tcqf-mpls-tc-tag defines the mapping of cycle number to MPLS TC tag.
This mapping is configured for all interfaces that use MPLS TC
tagging. When a packet is received with a ToS LSE indicating a TC
for which there is a mapping to a cycle in tcqf-mpls-tc-tag, then
this packet is assigned to the configured cycle.
If the packet is forwarded to another interface with tcqf configured,
the cycle number derived from mapping the received ToS LSE TC field
to the cycle number when receiving the packet will be mapped
according to tcqf-oif-config after all label stack changes are
applied and the packet is to be sent. If that outgoing interface is
also using MPLS TC TCQF tagging, then the TC value of the ToS LSE
will be rewritten according to the tcqf-mpls-tc-tag configuration of
that outgoing interface.
tc in tcqf-mpls-tc-tag MUST NOT use values to be used for non-TCQF
traffic, most commonly 0 for Best Effort (BE) traffic.
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4. TCQF with labels stack operations (normative)
TCQF QoS as defined here is in the terminology of [RFC3270] a TC-
Inferred-PSC LSP (E-LSP) behavior. Packets are determined to belong
to the TCQF PSC solely based on the TC of he received packet.
Packets originated into the TCQF PSC on the ingress LSR are assumed
for the purpose of this specification to be received from an internal
interface for which the cycle mapping table on every interface is
1:1. This allows to distinguish the case of originated TCQF packets
from those received from another LSR.
Note that this ingress mapping rule does not represent the shaping
necessary on an ingress TCQF router. TBD.
Label swap in the case of LDP or RSVP-TE LSP, or label pop in the
case of SR-MPLS traffic steering, or any other operation may result
in a different label to become the ToS LSE. Whenever a packet has an
associated TCQF cycle and is sent to an interface with TCQF, the
cycle is mapped to that outgoing interfaces cycle space and the TC of
the ToS LSE accordingly updated.
5. TCQF Pseudocode (normative)
The following pseudocode restates the prior two section text in an
algorithmic fashion. It uses the objects of the TCQF YANG data model
defined in Section 3.
tcqf = ietf-detnet-tcqf
void receive(pak) {
// Receive side TCQF - remember cycle in
// packet internal header
iif = pak.context.iif
if(tcqf.tcqf-if-config[iif]) { // TCQF enabled
if(tcqf.tcqf-mpls-tc-tag[iif]) { // TC-TCQF
pak.context.tcqf_cycle =
map_tc2cycle(pak.mpls_header.lse[tos].tc,
tcqf.tcqf-mpls-tc-tag[iif])
} else // other future encap/tagging options for TCQF
}
// Forwarding including any LSE operations
oif = pak.context.oif = forward_process(pak)
// ... optional DetNet PREOF functions here
// ... if router is DetNet service node
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if(pak.context.tcqf_cycle && // non TCQF packets value is 0
tcqf.tcqf-if-config[oif]) { // TCQF enabled
// Map tcqf_cycle for iif to oif mapping table
cycle = pak.context.tcqf_cycle =
map_cycle(cycle,
tcqf.tcqf-if-config[oif].tcqf-iif-cycle-map[[iif])
if(tcqf.tcqf-mpls-tc-tag[iif]) { // TC-TCQF
pak.mpls_header.lse[tos].tc =
map_cycle2tc(cycle, tcqf.tcqf-mpls-tc-tag[oif])
} else // other future encap/tagging options for TCQF
tcqf_enqueue(pak, oif.cycleq[cycle])
}
}
// Started when TCQF is enabled on an interface
void send_tcqf(oif) {
cycle = 1
cc = tcqf.tcqf-config.cycle-time *
tcqf.tcqf-config.cycle-time
o = tcqf.tcqf-config.cycle-clock-offset
nextcyclestart = floor(tnow / cc) * cc + cc + o
while(1) {
while(tnow < nextcyclestart) { }
while(pak = dequeue(oif.cycleq(cycle)) {
send(pak)
}
cycle = (cycle + 1) mod tcqf.tcqf-config.cycles + 1
nextcyclestart += tcqf.tcqf-config.cycle-time
}
}
Figure 3: TCQF Pseudocode
6. TCQF YANG Model (normative) TBD
TBD - according to Section 3.
7. Computing cycle mappings (informative)
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.
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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 4: Calculation reference
Consider in {#Calc1} 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 5: Calculating cycle mapping
{#Calc2} shows a formula to calculate the cycle mapping between R1
and R2, using the first available cycle on R2. In the example of
{#Calc1} 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.
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).
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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.
8. Security Considerations
TBD.
9. IANA Considerations
This document has no IANA considerations.
10. Informative References
[CQF] 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.
[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://www.ietf.org/archive/id/draft-dang-queuing-
with-multiple-cyclic-buffers-00.txt>.
[I-D.eckert-detnet-bounded-latency-problems]
Eckert, 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://www.ietf.org/archive/id/draft-eckert-detnet-
bounded-latency-problems-00.txt>.
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[I-D.ietf-bier-te-arch]
Eckert, T., Cauchie, G., and M. Menth, "Tree Engineering
for Bit Index Explicit Replication (BIER-TE)", Work in
Progress, Internet-Draft, draft-ietf-bier-te-arch-10, 9
July 2021, <https://www.ietf.org/archive/id/draft-ietf-
bier-te-arch-10.txt>.
[I-D.ietf-detnet-bounded-latency]
Finn, N., Boudec, J. L., Mohammadpour, E., Zhang, J.,
Varga, B., and J. Farkas, "DetNet Bounded Latency", Work
in Progress, Internet-Draft, draft-ietf-detnet-bounded-
latency-07, 1 September 2021,
<https://www.ietf.org/archive/id/draft-ietf-detnet-
bounded-latency-07.txt>.
[I-D.qiang-DetNet-large-scale-DetNet]
Qiang, L., Geng, X., Liu, B., Eckert, 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://www.ietf.org/archive/id/draft-qiang-DetNet-large-
scale-DetNet-05.txt>.
[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/info/rfc3209>.
[RFC3270] Le Faucheur, F., 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/info/rfc3270>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[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>.
[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/info/rfc8964>.
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Internet-Draft detnet-mpls-tc-tcqf September 2021
[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/>.
Authors' Addresses
Toerless Eckert
Futurewei Technologies USA
2220 Central Expressway
Santa Clara, CA 95050
United States of America
Email: tte@cs.fau.de
Stewart Bryant
University of Surrey ICS
Email: s.bryant@surrey.ac.uk
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