DETNET T. Eckert
Internet-Draft Futurewei Technologies USA
Intended status: Standards Track S. Bryant
Expires: 20 November 2022 University of Surrey ICS
A. G. Malis
Malis Consulting
19 May 2022
Deterministic Networking (DetNet) Data Plane - MPLS TC Tagging for
Cyclic Queuing and Forwarding (MPLS-TC TCQF)
draft-eckert-detnet-mpls-tc-tcqf-02
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
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This Internet-Draft will expire on 20 November 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction (informative) . . . . . . . . . . . . . . . . . 2
2. Using TCQF in the DetNet Archticture and MPLS forwarding plane
(informative) . . . . . . . . . . . . . . . . . . . . . . 3
3. MPLS T-CQF forwarding (normative) . . . . . . . . . . . . . . 6
3.1. Configuration Data model and tag processing for MPLS TC
TCQF . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Packet processing . . . . . . . . . . . . . . . . . . . . 6
3.3. TCQF with label stack operations . . . . . . . . . . . . 7
3.4. Ingres operations . . . . . . . . . . . . . . . . . . . . 8
4. TCQF Pseudocode (normative) . . . . . . . . . . . . . . . . . 8
5. Operational considerations (informative) . . . . . . . . . . 9
5.1. Controller plane computation of cycle mappings . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction (informative)
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.
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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
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 in the DetNet Archticture and MPLS forwarding plane
(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 5.1).
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. MPLS T-CQF forwarding (normative)
3.1. Configuration Data model and tag processing for MPLS TC TCQF
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]
tcqf_tc[oif]
+--uint8 tc[oif_cycle]
Figure 2: TCQF Configuration Data Model
3.2. Packet processing
This section explains the MPLS T-CQF packet processing and through
it, introduces the semantic of the objects in Figure 2
tcqf contains the router/LSR wide configuration of TCQF parameters,
independent of the specific tagging mechanism on any interface. Any
interface can have a different tagging method.
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/LSR may
have multiple TCQF domains each with a set of interfaces disjoint
from those of any other TCQF domain.
tcqf.cycles is the number of cycles used across all interfaces in the
TCQF domain. router/LSR MUST support 3 and 4 cycles. To support
interfaces with MPLS TC tagging, 7 or less cycles MUST be used across
all interfaces in the CQF domain.
The unit of tcqf.cycle_time is 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 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)
)
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The local reference clock of the LSR/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.
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. When
tcqf_tc[oif] is configured, oif will use MPLS TC tagging for TCQF.
This mapping 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.
3.3. TCQF with label stack operations
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.
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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, or no label changes from SID hop-by-hop
forwarding and/or SID/label pop as in the case of SR-MPLS traffic
steering.
3.4. Ingres operations
The ingress LSR of a TCQF domain has to mark packets with an internal
cycle number and ensure that any such marked traffic complies with
the traffic envelope admitted by the controller plane.
The algorithms to map packets of traffic flows into cycles are
outside the scope of this specification, because there can be
multiple ones of varying complexity. In a most simple admission
model, a particular flow is allocated a maximum number of bytes in
every cycle. This can easily be mapped into an appropriate policing
gate.
For the purpose of this specification, such ingress operations is
simply represented as an (internal/virtual) interface from which the
packet is received, complete with a correctly assigned internal cycle
number.
4. TCQF Pseudocode (normative)
The following pseudocode restates the forwarding behavior of
Section 3 in an algorithmic fashion as pseudocode. It uses the
objects of the TCQF configuration data model defined in Section 3.1.
void receive(pak) {
// Receive side TCQF - remember cycle in
// 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
pak.context.tcqf_cycle = map_tc2cycle( tc, tcqf_tc[iif])
} else // other future encap/tagging options for TCQF
}
forward(pak);
}
void inject_tcqf_pak(pak, cycle) {
pak.context.iif = INTERNAL
pak.context.tcqf_cycle = cycle
forward(pak);
}
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void forward(pak) {
// Forwarding including any LSE operations
oif = pak.context.oif = forward_process(pak)
// ... optional DetNet PREOF functions here
// ... if router is DetNet service node
if(pak.context.tcqf_cycle && // non TCQF packets cycle is 0
tcqf.if_config[oif]) { // TCQF enabled
// Map tcqf_cycle iif to oif
cycle = pak.context.tcqf_cycle
= map_cycle(cycle,
tcqf.if_config[oif].cycle_map[[iif])
if(tcqf.mpls_tc_tag[iif]) { // TC-TCQF
pak.mpls_header.lse[tos].tc =
map_cycle2tc(cycle, tcqf_tc[oif])
} else // other future encap/tagging options for TCQF
tcqf_enqueue(pak, oif.cycleq[cycle])
}
}
// Started when TCQF is enabled on an interface
// dequeues packets from oif.cycleq
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) {
while(tnow < nextcyclestart) { }
while(pak = dequeue(oif.cycleq(cycle)) {
send(pak)
}
cycle = (cycle + 1) mod tcqf.cycles + 1
nextcyclestart += tcqf.cycle_time
}
}
Figure 3: TCQF Pseudocode
5. Operational considerations (informative)
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5.1. 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 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.
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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. Security Considerations
TBD.
7. IANA Considerations
This document has no IANA considerations.
8. Changelog
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
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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.
02
Refresh
9. References
9.1. Normative References
[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>.
[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>.
9.2. 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>.
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[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>.
[I-D.ietf-bier-te-arch]
Eckert, 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://www.ietf.org/archive/id/draft-ietf-
bier-te-arch-13.txt>.
[I-D.ietf-detnet-bounded-latency]
Finn, N., Boudec, J. L., Mohammadpour, E., Zhang, J., and
B. Varga, "DetNet Bounded Latency", Work in Progress,
Internet-Draft, draft-ietf-detnet-bounded-latency-10, 8
April 2022, <https://www.ietf.org/archive/id/draft-ietf-
detnet-bounded-latency-10.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>.
[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>.
[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
Eckert, et al. Expires 20 November 2022 [Page 13]
Internet-Draft detnet-mpls-tc-tcqf May 2022
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
Andrew G. Malis
Malis Consulting
Email: agmalis@gmail.com
Eckert, et al. Expires 20 November 2022 [Page 14]