Internet DRAFT - draft-eckert-detnet-tcqf
draft-eckert-detnet-tcqf
DETNET T. Eckert
Internet-Draft Futurewei Technologies USA
Intended status: Standards Track S. Bryant
Expires: 14 September 2023 University of Surrey ICS
A. G. Malis
Malis Consulting
G. Li
Huawei Network Technology Laboratory
13 March 2023
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-02
Abstract
This memo specifies a forwarding method for bounded latency for
Deterministic Networks. It uses cycle tagging of packets for cyclic
queuing and forwarding with multiple buffers (TCQF). This memo
standardizes tagging via the MPLS packet Traffic Class (TC) field for
MPLS links and the IP/IPv6 DSCPfield for IP/IPv6 links. The short-
hand for this mechanism is Tagged Cyclic Queuing and Forwarding
(TCQF).
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
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 14 September 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction (informative) . . . . . . . . . . . . . . . . . 3
2. Using TCQF in the DetNet Architecture and MPLS forwarding plane
(informative) . . . . . . . . . . . . . . . . . . . . . . 4
3. TCQF per-flow stateless forwarding (normative) . . . . . . . 6
3.1. Configuration Data model and tag processing for MPLS TC
tags . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Packet processing . . . . . . . . . . . . . . . . . . . . 6
3.3. TCQF with MPLS label stack operations . . . . . . . . . . 8
3.4. TCQF with IP operations . . . . . . . . . . . . . . . . . 9
3.5. TCQF Pseudocode (normative) . . . . . . . . . . . . . . . 9
4. TCQF Per-flow Ingress forwarding (normative) . . . . . . . . 11
4.1. Ingress Flows Configuration Data Model . . . . . . . . . 11
4.2. Ingress Flows Pseudocode . . . . . . . . . . . . . . . . 12
5. Implementation, Deployment, Operations and Validation
considerations (informative) . . . . . . . . . . . . . . 13
5.1. High-Speed Implementation . . . . . . . . . . . . . . . . 13
5.2. Controller plane computation of cycle mappings . . . . . 14
5.3. Link speed and bandwidth sharing . . . . . . . . . . . . 15
5.4. Controller-plane considerations . . . . . . . . . . . . . 16
5.5. Validation . . . . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 17
9. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction (informative)
Cyclic Queuing and Forwarding (CQF), [IEEE802.1Qch], 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 cycle identifiers for a limited number
of cycles (such as 3...7) and hop-by-hop forwarded through the use of
per-cycle buffers. This multiple buffer forwarding overcome the
distance and clock synchronization limitations of CQF.
[I-D.qiang-DetNet-large-scale-DetNet] and
[I-D.dang-queuing-with-multiple-cyclic-buffers] provide additional
details about the background of TCQF. TCQF does not depend on other
elements of [RFC8655], so it can also be used in otherwise non-
deterministic IP 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 (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
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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.
2. 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 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.
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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
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 4.
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 5.2).
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.
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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.
3. TCQF per-flow stateless forwarding (normative)
3.1. 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]
# MPLS TC tagging specific data
tcqf_tc[oif]
+--uint8 tc[oif_cycle]
# IP/IPv6 DSCP tagging specific data
tcqf_dscp[oif]
+--uint8 dscp[oif_cycle]
Figure 2: TCQF Configuration Data Model
3.2. Packet processing
This section explains the TCQF packet processing and through it,
introduces the semantic of the objects in Figure 2
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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.
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.
tcqf.cycles is the number of cycles used across all interfaces in the
TCQF domain. routers 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. 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 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.
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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. 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. Likewise,
tcqf_dscp[oif] MUST be configured, when oif uses IP/IPv6.
This data model does not determine whether interfaces use MPLS or IP/
IPv6 encapsulation. This is determined by the setup 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.
3.3. TCQF with MPLS 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.
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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3.4. TCQF with IP operations
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 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.
3.5. 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 - 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
pak.context.tcqf_cycle = map_tc2cycle( tc, tcqf_tc[iif])
} else
if (tcqf_dscp[iif]) { // IP TCQF enabled 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])
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// MPLS TC-TCQF
if(tcqf.tc[oif]) {
pak.mpls_header.lse[tos].tc = map_cycle2tc(cycle, tcqf_tc[oif])
} else
// IP 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]
}
}
// ... 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 3: 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 4.2.
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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].
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].
4. 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.
4.1. Ingress Flows Configuration Data Model
# Extends above defined tcqf
tcqf
...
| Ingress Flows, see below (TBD:
+-- iflow[flowid]
+-- uint32 csize # in bits
Figure 4: TCQF Ingress Configuration Data Model
The data model shown in Figure 4 expands the tcqf data model from
Figure 2. 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.
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Note, that iflow[flowid].csize is not specific to the sending
interface because it is a property of the DetNet flow.
4.2. 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.
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 5: TCQF Ingress Enqueue Pseudocode
ingres_flow_enqueue(pak) as shown in Figure 5 performs this enqueuing
of the packet. Its position in the DetNet/TCQF forwarding code is
shown in Figure 3.
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])
}
}
}
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Figure 6: TCQF Ingress Pseudocode
ingress_flow_2_tcqf(oif, cycle) as shown in Figure 6 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 3.
5. Implementation, Deployment, Operations and Validation considerations
(informative)
5.1. 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 5.2 below.
High-speed forwarding for ingress packets as specified in Section 4
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.
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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).
5.2. 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 7: Calculation reference
Consider in Figure 7 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 8: Calculating cycle mapping
Figure 8 shows a formula to calculate the cycle mapping between R1
and R2, using the first available cycle on R2. In the example of
Figure 7 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.
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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).
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.
5.3. 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.
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5.4. 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-
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.
5.5. Validation
[LDN] describes an accurate simualtion based validation of TCQF and
provides further details on the mathematical models.
https://ceni.org.cn/406.html (https://ceni.org.cn/406.html) 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 deteilled version of the report. Note that
both reports are in chinese. TCQF is called DIP in these reports.
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6. Security Considerations
TBD.
7. IANA Considerations
This document has no IANA considerations.
8. Acknowledgement
Many thanks for review by David Black (DetNet techadvisor).
9. 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.
02
Refresh
03
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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.
10. References
10.1. Normative References
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[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/info/rfc2474>.
[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>.
10.2. Informative References
[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.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>.
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[I-D.ietf-detnet-bounded-latency]
Finn, N., Le Boudec, J., Mohammadpour, E., Zhang, J., and
B. Varga, "Deterministic Networking (DetNet) Bounded
Latency", Work in Progress, Internet-Draft, draft-ietf-
detnet-bounded-latency-10, 8 April 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-detnet-
bounded-latency-10>.
[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>.
[IEEE802.1Qbv]
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.
[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.
[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>.
[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/info/rfc4875>.
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[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/info/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/info/rfc8402>.
[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/info/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/info/rfc9016>.
[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
Andrew G. Malis
Malis Consulting
Email: agmalis@gmail.com
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Guangpeng Li
Huawei Network Technology Laboratory
Email: liguangpeng@huawei.com
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