Network Working Group L. Qiang, Ed.
Internet-Draft B. Liu
Intended status: Informational T. Eckert, Ed.
Expires: September 9, 2019 Huawei
L. Geng
China Mobile
March 8, 2019
Large-Scale Deterministic Network
draft-qiang-detnet-large-scale-detnet-03
Abstract
This document presents the overall framework and key method for
Large-scale Deterministic Network (LDN). LDN can provide bounded
latency and delay variation (jitter) without requiring precise time
synchronization among nodes, or per-flow state in transit nodes.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology & Abbreviations . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Background . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.1. Deterministic End-to-End Latency . . . . . . . . . . 4
2.2.2. Hop-by-Hop Delay . . . . . . . . . . . . . . . . . . 4
2.2.3. Cyclic Forwarding . . . . . . . . . . . . . . . . . . 5
2.2.4. Co-Existence with Non-Deterministic Traffic . . . . . 6
2.3. System Components . . . . . . . . . . . . . . . . . . . . 6
3. LDN Forwarding Mechanism . . . . . . . . . . . . . . . . . . 7
3.1. Cyclic Queues . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Cycle Mapping . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Cycle Identifier Carrying . . . . . . . . . . . . . . 9
4. Performance Analysis . . . . . . . . . . . . . . . . . . . . 10
4.1. Queueing Delay . . . . . . . . . . . . . . . . . . . . . 10
4.2. Jitter . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
8. Normative References . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This document explores the DetNet forwarding over large-scale
network. In contrast to TSN that deployed in LAN, DetNet is expected
to be deployed in larger scale network that has the following
features:
o a large number of network devices
o the distance between two network devices is long
o a lot of deterministic flows on the network
These above features will bring the following challenges to DetNet
forwarding:
o difficult to achieve precise time synchronization among all nodes
o long link propagation delay may introduce bigger jitter
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o per-flow state is unscalable
Motivated by these challenges, this document presents a Large-scale
Deterministic Network (LDN) mechanism. As
[draft-ietf-detnet-problem-statement] indicates, deterministic
forwarding can only apply on flows with well-defined traffic
characteristics. The traffic characteristics of DetNet flow has been
discussed in [draft-ietf-detnet-architecture], that could be achieved
through shaping at Ingress node or up-front commitment by
application. LDN assumes that DetNet flows follow some specific
traffic patterns accordingly.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology & Abbreviations
This document uses the terminology defined in
[draft-ietf-detnet-architecture].
TSN: Time Sensitive Network
PQ: Priority Queuing
CQF: Cyclic Queuing and Forwarding
LDN: Large-scale Deterministic Network
DSCP: Differentiated Services Code Point
EXP: Experimental
TC: Traffic Class
T: the length of a cycle
H: the number of hops
2. Overview
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2.1. Summary
In LDN, nodes (network devices) have synchronized frequency, and each
node forwards packets in a slotted fashion based on a cycle
identifiers carried in packets. Ingres nodes or senders have a
function called gate to shape/condition traffic flows. Except for
this gate function, the LDN has no awareness of individual flows.
2.2. Background
This section motivates the design choices taken by the proposed
solution and gives the necessary background for deterministic delay
based forwarding plane designs.
2.2.1. Deterministic End-to-End Latency
Bounded delay is delay that has a deterministic upper and lower
bound.
The delay for packets that need to be forwarded with deterministic
delay needs to be deterministic on every hop. If any hop in the
network introduces non-deterministic delay, then the network itself
can not deliver a deterministic delay service anymore.
2.2.2. Hop-by-Hop Delay
Consider a simple example shown in Figure 1, where Node X has 10
receiving interfaces and one outgoing interface I all of the same
speed. There are 10 deterministic traffic flows, each consuming 5%
of a links bandwidth, one from each receiving interface to the
outgoing interface.
Node X sends 'only' 50% deterministic traffic to interface I, so
there is no ongoing congestion, but there is added delay. If the
arrival time of packets for these 10 flows into X is uncontrolled,
then the worst case is for them to all arrive at the same time. One
packet has to wait in X until the other 9 packets are sent out on I,
resulting in a worst case deterministic delay of 9 packets
serialization time. On the next hop node Y downstream from X, this
problem can become worse. Assume Y has 10 upstream nodes like X, the
worst case simultaneous burst of packets is now 100 packets, or a 99
packet serialization delay as the worst case upper bounded delay
incurred on this hop.
To avoid the problem of high upper bound end-to-end delay, traffic
needs to be conditioned/interleaved on every hop. This allows to
create solutions where the per-hop-delay is bounded purely by the
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physics of the forwarding plane across the node, but not the
accumulated characteristics of prior hop traffic profiles.
+--+ +--+ --- ---
|A1| |A0| - - - -
+--+ +--+ - - - -
---------------->- - - -
+--+ +--+ - - - -
|B1| |B0| - - - -
+--+ +--+ - -Interface I - -
---------------->-Node X ---------------> -Node Y ----->
+--++--+ - - - -
|C1||C0| - -
+--++--+ - - - -
---------------->- - - -
. - - - -
. - - - -
. - - - -
--- ---
Figure 1: Micro-burst and micro-burst iteration
2.2.3. Cyclic Forwarding
The common approach to solve that problem is that of a cyclic hop-by-
hop forwarding mechanism. Assume packets forwarded from N1 via N2 to
N3 as shown in Figure 2. When N1 sends a packet P to interface I1
with a Cycle X, it must be guaranteed by the forwarding mechanism
that N2 will forward P via I2 to N3 in a cycle Y.
The cycle of a packet can either be deduced by a receiving node from
the exact time it was received as is done in SDN/TDMA systems, and/or
it can be indicated in the packet. This document solution relies on
such markings because they allow to reduce the need for synchronous
hop-by-hop transmission timings of packets.
In a packet marking based slotted forwarding model, node N1 needs to
send packets for cycle X before the latest possible time that will
allow for N2 to further forward it in cycle Y to N3. Because of the
marking, N1 could even transmit packets for cycle X before all
packets for the previous cycle (X-1) have been sent, reducing the
synchronization requirements between across nodes.
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P sent in P sent in P sent in
cycle(N1,I1,X) cycle(N2,I2,Y) cycle(N3,I3,Z)
+--------+ +--------+ +--------+
| Node N1|------->| Node N2|-------->| Node N3|------>
+--------+I1 +--------+I2 +--------+I3
Figure 2: Cyclic Forwarding
2.2.4. Co-Existence with Non-Deterministic Traffic
Traffic with deterministic delay requirements can co-exist with
traffic only requiring non-deterministic delay by using packet
scheduling where the delay incurred by non-deterministic packets is
deterministic for the deterministic traffic (and low). If LDN is
deployed together with such non-deterministic delay traffic than such
a scheme must be supported by the forwarding plane. A simple
approach for the delay incurred on the sending interface of a
deterministic node due to non-deterministic traffic is to serve
deterministic traffic via a strict, highest-priority queue and
include the worst case delay of a currently serialized non-
deterministic packet into the deterministic delay budget of the node.
Similar considerations apply to the internal processing delays in a
node.
2.3. System Components
The Figure 3 shows an overview of the components considered in this
document system and how they interact.
A network topology of nodes, Ingress, Core and Egress support a
method for cyclic forwarding to enable LDN. This forwarding requires
no per-flow state on the nodes, and tolerates loss time
synchronization.
Ingress edge nodes may support the (G)ate function to shape traffic
from sources into the desired traffic characteristics, unless the
source itself has such function. Per-flow state is required on the
ingress edge node. LDN should work with some resource reservation
methods, that will be not discussed in this document.
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/--\. +--+ +--+ +--+ +--+. /--\
| (G)+-----+GS+--------+ S+------+ S+--------+ S+-----+ |
\--/ +--+ +--+ +--+ +--+ \--/
Sender Ingress Core Core Egress Receiver
Edge Node Node Node Edge Node
Figure 3: Overview of LDN
3. LDN Forwarding Mechanism
DetNet aims at providing deterministic service over large scale
network. In such large scale network, it is difficulty to get
precise time synchronization among numerous devices. To reduce
requirements, the forwarding mechanism described in this document
assumes only frequency synchronization but not time synchronization
across nodes: nodes maintain the same clock frequency 1/T, but do not
require the same time as shown in Figure 4.
<-----T-----> <-----T----->
| | | | | |
Node A +-----------+-----------+ Node A +-----------+-----------+
T0 T0
| | | | | |
Node B +-----------+-----------+ Node B +-----------+-----------+
T0 T0
(i) time synchronization (ii) frequency synchronization
T: length of a cycle
T0: timestamp
Figure 4: Time Synchronization & Frequency Synchronization
IEEE 802.1 CQF is an efficient forwarding mechanism in TSN that
guarantees bounded end-to-end latency. CQF is designed for limited
scale networks. Time synchronization is required, and the link
propagation delay is required to be smaller than a cycle length T.
Considering the large scale network deployment, the proposed LDN
Forwarding mechanism permits frequency synchronization and link
propagation delay may exceed T. Besides these two points, CQF and
the asynchronous forwarding of LDN are very similar.
Figure 5 compares CQF and LDN through an example. Suppose Node A is
the upstream node of Node B. In CQF, packets sent from Node A at
cycle x, will be received by Node B at the same cycle, then further
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be sent to downstream node by Node B at cycle x+1. Due to long link
propagation delay and frequency synchronization, Node B will receive
packets from Node A at different cycle denoted by y in the LDN, and
Node B swaps the cycles carried in packets with y+1, then sends out
those packets at cycle y+1. The cycle mapping relationship (e.g.,
x->y+1) exists between any pair of neighbor nodes. With this kind of
cycle mapping, the receiving node can easily figure out when the
received packets should be sent out, the only requirement is to carry
the cycle identifier of sending node in the packets.
| cycle x | cycle x+1 | | cycle x | cycle x+1 |
Node A +-----------+-----------+ Node A +-----------+-----------+
\ \
\packet \packet
\receiving \receiving
\ \
| V | cycle x+1 | | V | cycle y+1|
Node B +-----------+-----------+ Node B +-----------+-----------+
cycle x \packets cycle y \packets
\sending \sending
\ \
\ \
V V
(i) CQF (ii) LDN
Figure 5: CQF & LDN
3.1. Cyclic Queues
In CQF each port needs to maintain 2 (or 3) queues: 1 receiving queue
is used to buffer newly received packets, 1 sending queue is used to
store the packets that are going to be sent out, one more queue may
be needed to avoid output starvation [scheduled-queues]. In LDN, at
least 3 cyclic queues (2 receiving queues and 1 sending queue) are
maintained for each port on a node. A cyclic queue corresponds to a
cycle.
As Figure 6 illustrated, the downstream Node B may receive packets
sent at two different cycles from Node A due to the absence of time
synchronization. Following the cycle mapping (i.e., x --> y+1),
packets that carry cycle identifier x should be sent out by Node B at
cycle y+1, and packets that carry cycle identifier x+1 should be sent
out by Node B at cycle y+2. Therefore, two receiving queues are
needed to store the received packets, one is for the packets that
carry cycle identifier x, another one is the packets that carry cycle
identifier x+1. Plus one sending queue, each port needs at least 3
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cyclic queues in LDN. In order to absorb more link delay variation
(such as on radio interface), more queues may be necessary.
| cycle x | cycle x+1 |
Node A +-----------+-----------+
\ \
\ \packet
\ \receiving
| V V | |
Node B +-----------+-----------+
cycle y cycle y+1
Figure 6: An example illustrates for 2 receiving queue in LDN
3.2. Cycle Mapping
When this packet is received by Node B, some methods are possible how
the forwarding plane could operate. In one method, Node B has a
mapping determined by the control plane. Packets from (the link
from) Node A indicating cycle x are mapping into cycle y+1. This
mapping is necessary, because all the packets from one cycle of the
sending node need to get into one cycle of the receiving node. This
is called "configured cycle mapping".
Instead of configuring an explicit cycle mapping such as cycle x ->
cycle y+1, the receiving Node B could also have the intelligence in
the forwarding plane to recognize the first packet from (the link
from) Node A that has a new cycle x number, and map this cycle x to a
cycle after the current cycle y. We call this option "self
synchronized cycle mapping".
3.2.1. Cycle Identifier Carrying
In self synchronized cycle mapping, cycle identifier needs to be
carried in the LDN packets, so that an appropriate queue can be
selected accordingly. That means 2 bits are needed in the three
queues model of LDF, in order to identify different cycles between a
pair of neighboring nodes. There are several ways to carry this 2
bits cycle identifier. This document does not yet aim to propose
one, but gives an (incomplete) list of ideas:
o DSCP of IPv4 Header
o Traffic Class of IPv6 Header
o TC of MPLS Header (used to be EXP)
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o EtherType of Ethernet Header
o IPv6 Extension Header
o UDP Option
o SID of SRv6
o Reserved of SRH
o TLV of SRv6
o TC of SR-MPLS Header (used to be EXP)
o Three labels/adjacency SIDs for SR-MPLS
4. Performance Analysis
4.1. Queueing Delay
Figure 7 describes one-hop packet forwarding delay, that mainly
consisted of A->B link propagation delay and queuing delay in Node B.
|cycle x |
Node A +-------\+
\
\
\
|\ cycle y|cycle y+1|
Node B +V--------+--------\+
: \
: Queueing Delay :\
:...=2*T ............ V
Figure 7: Single-Hop Queueing Delay
As Figure 7 shows, cycle x of Node A will be mapped into cycle y+1 of
Node B as long as the last packet sent from A->B is received within
the cycle y. If the last packet is re-sent out by B at the end of
cycle y+1, then the largest single-hop queueing delay is 2*T.
Therefore the end-to-end queueing delay's upper bound is 2*T*H, where
H is the number of hops.
If A did not forward the LDN packet from a prior LDN forwarder but is
the actual traffic source, then the packet may have been delayed by a
gate function before it was sent to B. The delay of this function is
outside of scope for the LDN delay considerations. If B is not
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forwarding the LDN packet but the final receiver, then the packet may
not need to be queued and released in the same fashion to the
receiver as it would be queued/released to a downstream LDN node, so
if a path has one source followed by N LDN forwarders followed by one
receivers, this should be considered to be a path with N-1 LDN hops
for the purpose of latency and jitter calculations.
4.2. Jitter
Considering the simplest scenario one hop forwarding at first,
suppose Node A is the upstream node of Node B, the packet sent from
Node A at cycle x will be received by Node B at cycle y as Figure 8
shows.
- The best situation is Node A sends packet at the end of cycle x,
and Node B receives packet at the beginning of cycle y, then the
delay is denoted by w;
- The worst situation is Node A sends packet at the beginning of
cycle x, and Node B receives packet at the end of cycle y, then
the delay= w + length of cycle x + length of cycle y= w+2*T;
- Hence the jitter's upper bound of this simplest scenario= worst
case-best case=2*T.
|cycle x | |cycle x |
Node A +-------\+ Node A +\-------+
:\ \ :
: \ -------------\
: \ : \
:w |\ | :w| \ |
Node B : +V--------+ Node B : +--------V+
cycle y cycle y
(a) best situation (b) worst situation
Figure 8: Jitter Analysis for One Hop Forwarding
Next considering two hops forwarding as Figure 9 shows.
- The best situation is Node A sends packet at the end of cycle x,
and Node C receives packet at the beginning of cycle z, then the
delay is denoted by w';
- The worst situation is Node A sends packet at the beginning of
cycle x, and Node C receives packet at the end of cycle z, then
the delay= w' + length of cycle x + length of cycle z= w'+2*T;
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- Hence the jitter's upper bound = worst case-best case=2*T.
|cycle x |
Node A +-------\+
\
:\| cycle y |
Node B : \---------+
: \
: \--------\
: \ |
Node C ......w'......+V--------+
cycle z
(a) best situation
|cycle x |
Node A +\-------+
\ :
\ : | cycle y |
Node B \ : +---------+
\ :
---:--------------------\
: | \ |
Node C :......w'.....+--------V+
cycle z
(b) worst situation
Figure 9: Jitter Analysis for Two Hops Forwarding
And so on. For multi-hop forwarding, the end-to-end delay will
increase as the number of hops increases, while the delay variation
(jitter) still does not exceed 2*T.
5. IANA Considerations
This document makes no request of IANA.
6. Security Considerations
Security issues have been carefully considered in
[draft-ietf-detnet-security]. More discussion is TBD.
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7. Acknowledgements
TBD.
8. Normative References
[draft-ietf-detnet-architecture]
"DetNet Architecture", .
[draft-ietf-detnet-dp-sol]
"DetNet Data Plane Encapsulation",
.
[draft-ietf-detnet-problem-statement]
"DetNet Problem Statement",
.
[draft-ietf-detnet-security]
"DetNet Security Considerations",
.
[draft-ietf-detnet-use-cases]
"DetNet Use Cases", .
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
[scheduled-queues]
"Scheduled queues, UBS, CQF, and Input Gates",
.
Authors' Addresses
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Li Qiang (editor)
Huawei
Beijing
China
Email: qiangli3@huawei.com
Bingyang Liu
Huawei
Beijing
China
Email: liubingyang@huawei.com
Toerless Eckert (editor)
Huawei USA - Futurewei Technologies Inc.
2330 Central Expy
Santa Clara 95050
USA
Email: tte+ietf@cs.fau.de
Liang Geng
China Mobile
Beijing
China
Email: gengliang@chinamobile.com
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