IEC 62439-3:2010

IEC 62439-3 ®
Edition 1.0 2010-02
INTERNATIONAL
STANDARD
colour
inside
Industrial communication networks – High availability automation networks –
Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless
Redundancy (HSR)
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IEC 62439-3 ®
Edition 1.0 2010-02
INTERNATIONAL
STANDARD
colour
inside
Industrial communication networks – High availability automation networks –
Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless
Redundancy (HSR)
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
XA
ICS 25.040, 35.040 ISBN 978-2-88910-706-3
– 2 – 62439-3 © IEC:2010(E)
CONTENTS
FOREWORD.5
INTRODUCTION.7
1 Scope.8
2 Normative references .8
3 Terms, definitions, abbreviations, acronyms, and conventions.8
3.1 Terms and definitions .8
3.2 Abbreviations and acronyms.9
3.3 Conventions .9
4 Parallel Redundancy Protocol (PRP) .9
4.1 PRP principle of operation.9
4.1.1 PRP network topology .9
4.1.2 PRP LANs with linear or bus topology .10
4.1.3 PRP LANs with ring topology.11
4.1.4 DANP node structure.11
4.1.5 PRP attachment of singly attached nodes.12
4.1.6 Compatibility between singly and doubly attached nodes.12
4.1.7 Network management.12
4.1.8 Implication on configuration .13
4.1.9 Transition to non-redundant networks.13
4.1.10 Duplicate handling.14
4.1.11 Configuration check.18
4.1.12 Network supervision .18
4.1.13 Redundancy management interface.19
4.2 PRP protocol specifications.19
4.2.1 Installation, configuration and repair guidelines .19
4.2.2 MAC addresses .20
4.2.3 Multicast MAC addresses .20
4.2.4 IP addresses .20
4.2.5 Nodes.20
4.2.6 Duplicate accept mode .21
4.2.7 Duplicate discard mode .21
4.3 PRP service specification .27
4.3.1 Arguments .27
4.3.2 NodesTable .28
4.3.3 PRP write .29
4.3.4 PRP read.30
5 High-availability Seamless Redundancy (HSR) .31
5.1 HSR objectives.31
5.2 HSR principle of operation.31
5.2.1 Basic operation with a ring topology .31
5.2.2 DANH node structure.33
5.2.3 Topology .34
5.2.4 RedBox structure.40
5.3 HSR node specifications .42
5.3.1 Host sequence number.42
5.3.2 DANH receiving from its link layer interface .42

62439-3 © IEC:2010(E) – 3 –
5.3.3 DANH receiving from an HSR port .42
5.3.4 DANH forwarding rules .43
5.3.5 CoS .44
5.3.6 Clock synchronization.44
5.3.7 Deterministic medium access .44
5.4 HSR RedBox specifications .44
5.4.1 RedBox properties.44
5.4.2 RedBox receiving from interlink .45
5.4.3 RedBox forwarding on the ring.46
5.4.4 RedBox receiving from an HSR port .46
5.4.5 Redbox proxy node table handling.47
5.4.6 RedBox CoS.47
5.4.7 RedBox clock synchonization .47
5.4.8 RedBox medium access .47
5.5 QuadBox specification.47
5.6 Association definition .47
5.7 Frame format for HSR .47
5.7.1 HSR-tagged frame format.47
5.7.2 HSR_Supervision frame .48
5.7.3 Constants .50
6 Protocol Implementation Conformance Statement (PICS) .51
7 PRP/HSR Management Information Base (MIB) .51
Bibliography.58

Figure 1 – PRP example of general redundant network.10
Figure 2 – PRP example of redundant network as two LANs (bus topology).10
Figure 3 – PRP example of redundant ring with SANs and DANPs.11
Figure 4 – PRP with two DANPs communicating .11
Figure 5 – PRP RedBox, transition from single to double LAN .13
Figure 6 – PRP frame extended by an RCT.15
Figure 7 – PRP VLAN-tagged frame extended by an RCT.15
Figure 8 – PRP constructed, padded frame closed by an RCT .16
Figure 9 – PRP drop window on LAN_A .17
Figure 10 – PRP drop window reduction after a discard .17
Figure 11 – PRP frame from LAN_B was not discarded.18
Figure 12 – PRP synchronized LANs .18
Figure 13 – HSR example of ring configuration for multicast traffic .32
Figure 14 – HSR example of ring configuration for unicast traffic .33
Figure 15 –HSR structure of a DANH .34
Figure 16 – HSR example of topology using two independent networks .35
Figure 17 – HSR example of peer coupling of two rings .36
Figure 18 – HSR example of connected rings .37
Figure 19 – HSR example of coupling two redundant PRP LANs to a ring .38
Figure 20 – HSR example of coupling from a ring node to redundant PRP LANs.39
Figure 21 – HSR example of meshed topology.40
Figure 22 – HSR structure of a RedBox .41

– 4 – 62439-3 © IEC:2010(E)
Figure 23 – HSR frame without VLAN tag .48
Figure 24 – HSR frame with VLAN tag .48

Table 1 – PRP_Supervision frame with VLAN tag .25
Table 2 – PRP constants .27
Table 3 – PRP arguments .28
Table 4 – PRP arguments .29
Table 5 – PRP write.29
Table 6 – PRP read .30
Table 7 – HSR_Supervision frame with optional VLAN tag .49
Table 8 – HSR Constants .50

62439-3 © IEC:2010(E) – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL COMMUNICATION NETWORKS –
HIGH AVAILABILITY AUTOMATION NETWORKS –

Part 3: Parallel Redundancy Protocol (PRP) and
High-availability Seamless Redundancy (HSR)

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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6) All users should ensure that they have the latest edition of this publication.
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
International Standard 62439-3 has been prepared by subcommittee 65C: Industrial Networks,
of IEC technical committee 65: Industrial-process measurement, control and automation.
This standard cancels and replaces IEC 62439 published in 2008. This first edition constitutes
a technical revision.
This edition includes the following significant technical changes with respect to IEC 62439
(2008):
– adding a calculation method for RSTP (rapid spanning tree protocol, IEEE 802.1Q),
– adding two new redundancy protocols: HSR (High-availability Seamless Redundancy)
and DRP (Distributed Redundancy Protocol),
– moving former Clauses 1 to 4 (introduction, definitions, general aspects) and the
Annexes (taxonomy, availability calculation) to IEC 62439-1, which serves now as a
base for the other documents,
– moving Clause 5 (MRP) to IEC 62439-2 with minor editorial changes,

– 6 – 62439-3 © IEC:2010(E)
– moving Clause 6 (PRP) was to IEC 62439-3 with minor editorial changes,
– moving Clause 7 (CRP) was to IEC 62439-4 with minor editorial changes, and
– moving Clause 8 (BRP) was to IEC 62439-5 with minor editorial changes,
– adding a method to calculate the maximum recovery time of RSTP in a restricted
configuration (ring) to IEC 62439-1 as Clause 8,
– adding specifications of the HSR (High-availability Seamless Redundancy) protocol,
which shares the principles of PRP to IEC 62439-3 as Clause 5, and
– introducing the DRP protocol as IEC 62439-6.
The text of this standard is based on the following documents:
FDIS Report on voting
65C/583/FDIS 65C/589/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This International Standard is to be read in conjunction with IEC 62439-1:2010, Industrial
communication networks – High availability automation networks – Part 1: General concepts
and calculation methods.
A list of the IEC 62439 series can be found, under the general title Industrial communication
networks – High availability automation networks, on the IEC website.
This publication has been drafted in accordance with ISO/IEC Directives, Part 2.
The committee has decided that the contents of this amendment and the base publication will
remain unchanged until the stability date indicated on the IEC web site under
"http://webstore.iec.ch" in the data related to the specific publication. At this date, the
publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this standard may be issued at a later date.

IMPORTANT – The “colour inside” logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this publication using a colour printer.

62439-3 © IEC:2010(E) – 7 –
INTRODUCTION
The IEC 62439 series specifies relevant principles for high availability networks that meet the
requirements for industrial automation networks.
In the fault-free state of the network, the protocols of the IEC 62439 series provide
ISO/IEC 8802-3 (IEEE 802.3) compatible, reliable data communication, and preserve
determinism of real-time data communication. In cases of fault, removal, and insertion of a
component, they provide deterministic recovery times.
These protocols retain fully the typical Ethernet communication capabilities as used in the
office world, so that the software involved remains applicable.
The market is in need of several network solutions, each with different performance
characteristics and functional capabilities, matching diverse application requirements. These
solutions support different redundancy topologies and mechanisms which are introduced in
IEC 62439-1 and specified in the other Parts of the IEC 62439 series. IEC 62439-1 also
distinguishes between the different solutions, giving guidance to the user.
The IEC 62439 series follows the general structure and terms of IEC 61158 series.
The International Electrotechnical Commission (IEC) draws attention to the fact that it is
claimed that compliance with this document may involve the use of a patent concerning
detection of redundant frames given in 4.1.10.3, and concerning coupling of PRP and HSR
LANs given in 5.4 (patent pending).
IEC takes no position concerning the evidence, validity and scope of this patent right.
The holder of this patent right has assured the IEC that he/she is willing to negotiate licences
either free of charge or under reasonable and non-discriminatory terms and conditions with
applicants throughout the world. In this respect, the statement of the holder of this patent right
is registered with IEC. Information may be obtained from:
ABB Switzerland Ltd
Corporate Research
Segelhofstr 1K
5405 Baden
Switzerland
Attention is drawn to the possibility that some of the elements of this document may be the
subject of patent rights other than those identified above. IEC shall not be held responsible for
identifying any or all such patent rights.
ISO (www.iso.org/patents) and IEC (http://www.iec.ch/tctools/patent_decl.htm) maintain on-
line data bases of patents relevant to their standards. Users are encouraged to consult the
data bases for the most up to date information concerning patents.

– 8 – 62439-3 © IEC:2010(E)
INDUSTRIAL COMMUNICATION NETWORKS –
HIGH AVAILABILITY AUTOMATION NETWORKS –

Part 3: Parallel Redundancy Protocol (PRP) and
High-availability Seamless Redundancy (HSR)

1 Scope
The IEC 62439 series is applicable to high-availability automation networks based on the
ISO/IEC 8802-3 (IEEE 802.3) (Ethernet) technology.
This part of the IEC 62439 series specifies two redundancy protocols based on the duplication
of the LAN, resp. duplication of the transmitted information, designed to provide seamless
recovery in case of single failure of an inter-switch link or switch in the network.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60050-191:1990, International Electrotechnical Vocabulary – Chapter 191: Dependability
and quality of service
IEC 62439-1:2010, Industrial communication networks – High availability automation networks
– Part 1: General concepts and calculation methods
ISO/IEC 8802-3:2000, Information technology – Telecommunications and information
exchange between systems – Local and metropolitan area networks – Specific requirements –
Part 3: Carrier sense multiple access with collision detection (CSMA/CD) access method and
physical layer specifications
IEEE 802.1D:2004, IEEE standard for local Local and metropolitan area networks Media
Access Control (MAC) Bridges
IEEE 802.1Q, IEEE standards for local and metropolitan area network. Virtual bridged local
area networks
3 Terms, definitions, abbreviations, acronyms, and conventions
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050-191, as well
as in IEC 62439-1, apply, in addition to the following.
3.1.1
extended frame
frame that has been extended by a Redundancy Control Trailer
3.1.2
interlink
link that connects two network hierarchies

62439-3 © IEC:2010(E) – 9 –
3.1.3
RedBox
device allowing to attach single attached nodes to a redundant network
3.1.4
QuadBox
Quadruple port device connecting two peer HSR rings, which behaves as an HSR node in
each ring and is able to filter the traffic and forward it from ring to ring
3.1.5
HSR frame
frame that carries the HSR EtherType
3.2 Abbreviations and acronyms
For the purposes of this document, the following abbreviations and acronyms apply, in
addition to those given in IEC 62439-1:
DANH Double attached node implementing HSR
DANP Double attached node implementing PRP
ICMP Internet Control Message Protocol (part of the Internet protocol suite)
RCT Redundancy Check Tag
SRP Serial Redundancy Protocol
VDAN Virtual Doubly Attached Node (SAN as visible through a RedBox)
3.3 Conventions
This document follows the conventions defined in IEC 62439-1.
4 Parallel Redundancy Protocol (PRP)
4.1 PRP principle of operation
4.1.1 PRP network topology
This redundancy protocol implements redundancy in the devices, through doubly attached
nodes operating according to PRP (DANPs).
A DANP is attached to two independent LANs of similar topology, named LAN_A and LAN_B,
which operate in parallel. A source DANP sends the same frame over both LANs and a
destination DANP receives it from both LANs within a certain time, consumes the first frame
and discards the duplicate.
Figure 1 shows a redundant network consisting of two switched LANs, which can have any
topology, e.g. tree, ring or meshed.

– 10 – 62439-3 © IEC:2010(E)
DADANPNP DADANPNP
SASANN
A1A1
switch switch
switched local switched local
area network area network
(ring) LAN_A (tree) LAN_B
switch
switch switch switch
SASANN
A2A2
SASANN SASANN
B1B1 B2B2
RedBox
DADANNPP DADANNPP DANP
SASANN SASANN
R1R1 R2R2
IEC  356/10
Figure 1 – PRP example of general redundant network
The two LANs are identical in protocol at the MAC-LLC level, but they can differ in
performance and topology. Transmission delays may also be different, especially if one of the
networks reconfigures itself, e.g. using RSTP, to overcome an internal failure.
The two LANs follow configuration rules that allow the network management protocols such as
Address Resolution Protocol (ARP) to operate correctly.
The two LANs have no connection between them and are assumed to be fail-independent.
Redundancy can be defeated by single points of failure, such as a common power supply or a
direct connection whose failure brings both networks down. Installation guidelines in this
document provide guidance to the installer to achieve fail-independence.
4.1.2 PRP LANs with linear or bus topology
As an example of a simpler configuration,
Figure 2 draws a PRP network as two LANs in linear topology, which may also be a bus
topology.
DADANPNP DADANPNP DADANNPP DADANPNP DADANNPP DADANPNP
LAN_A
LAN_B
IEC  357/10
62439-3 © IEC:2010(E) – 11 –
Figure 2 – PRP example of redundant network as two LANs (bus topology)
4.1.3 PRP LANs with ring topology
The two LANs can have a ring topology, as Figure 3 shows.
DADANPNP DADANNPP
switch
switch
switch switch switch
switch switch switch
DADANNPP
DANP DADANNPP
. . .
RedBox
SANSAN
DANP DADANNPP
SANSAN SANSAN
DANP
IEC  358/10
Figure 3 – PRP example of redundant ring with SANs and DANPs
4.1.4 DANP node structure
Each node has two ports that operate in parallel and that are attached to the same upper
layers of the communication stack through the Link Redundancy Entity (LRE), as Figure 4
shows.
DANP 1 DANP 2
hard hard
UDP TCP UDP TCP
upper layers
real-time
real-time
network layer network layer
stack stack
Link Redundancy Entity Link Redundancy Entity
port A port B port A port B
network
adapters
Tx Rx Tx Rx Tx Rx Tx Rx
transceivers
LAN_A
LAN_B
IEC  359/10
Figure 4 – PRP with two DANPs communicating

– 12 – 62439-3 © IEC:2010(E)
The Link Redundancy Entity (LRE) has two tasks: handling of duplicates and management of
redundancy. This layer presents toward its upper layers the same interface as the network
adapter of a non-redundant adapter.
When receiving a frame from the node’s upper layers, the LRE sends the frame through both
its ports at nearly the same time.
The two frames transit through the two LANs with different delays, ideally they arrive at the
same time at the destination node.
When receiving frames from the network, the LRE forwards the first received frame of a pair
to the node’s upper layers and discards the duplicate frame (if it arrives).
For management of redundancy, the LRE can append a Redundancy Check Trailer (RCT)
including a sequence number to the frames it sends to keep track of duplicates. In addition,
the LRE periodically sends PRP_Supervision frames and evaluates the PRP_Supervision
frames of the other DANPs.
4.1.5 PRP attachment of singly attached nodes
Singly attached nodes (SANs) can be attached in two ways:
• SANs can be attached directly to one LAN only. SANs can only communicate with other
SANs on the same LAN. For instance, in Figure 1, SAN A1 can communicate with SAN A2,
but not with SAN B1 or SAN B2. SANs can communicate with all DANPs.
• SANs can be attached over a RedBox (redundancy box) to both LANs, as Figure 1 shows
for R1 and R2 (see also 4.1.9). Such SANs can communicate with all SANs, for instance
SAN A1 and SAN R1 can communicate.
NOTE SANs do not need to be aware of PRP, they can be off-the-shelf computers.
In some applications, only availability-critical devices need a double attachment, for instance
the operator workplaces, while the majority of the devices are SANs. Taking advantage of the
basic infrastructure of PRP, a DANP can be attached to two different switches of the same
LAN (e.g. a ring) and use protocols different from PRP to reconfigure the network in case of
failure. The DANP then behaves as a switch element according to IEEE 802.1D. For instance,
the switch element may implement the MRP protocol, the RSTP protocol, or a subset of RSTP,
where there is no forwarding of traffic between the ports. These abilities are optional and not
detailed in this International Standard. The supported mode is specified in the PICS (see 6) .
4.1.6 Compatibility between singly and doubly attached nodes
Singly attached nodes (SAN), for instance maintenance laptops or printers that belong to one
LAN, can be connected to any LAN. A SAN connected to one LAN cannot communicate
directly to a SAN connected to the other LAN. Switches are always SANs. These SANs are
not aware of PRP redundancy, so DANPs generate a traffic that these SANs understand. The
condition is however that the SANs ignore the RCT in the frames, which should be the case
since a SAN cannot distinguish the RCT from ISO/IEC 8802-3 (IEEE 802.3) padding.
Conversely, DANPs understand the traffic generated by SANs, since these do not append a
RCT. They only forward one frame to their upper layers since the SAN traffic uses one LAN
only. If a DANP cannot positively identify that the remote device is a DANP, it considers it as
a SAN.
4.1.7 Network management
A node has the same MAC address on both ports, and only one set of IP addresses assigned
to that address. This makes redundancy transparent to the upper layers. Especially, this
allows the Address Resolution Protocol (ARP) to work the same as with a SAN. Switches in a
LAN are not doubly attached devices, and therefore all managed switches have different IP
addresses. A network management tool is preferably a DANP and can access nodes and

62439-3 © IEC:2010(E) – 13 –
switches as if they all belong to the same network. Especially, network management
implemented in a DANP is able to see SANs connected to either LAN.
Some applications require different MAC addresses on the redundant ports, and these MAC
addresses may be different from the default MAC address of that node. This involves address
substitution mechanisms which are not specified in this International Standard. However, the
basic protocol and the frame format are prepared for such extension. Nodes that support MAC
address substitution are indicated as supporting PICS_SUBS.
4.1.8 Implication on configuration
Since the same frame can come from the two ports with significant time difference, the period
of cyclic time-critical data must be chosen so that it considers the difference between worst
case and best case path latency between publisher and subscriber.
4.1.9 Transition to non-redundant networks
The mechanism of duplicate rejection can be implemented by the RedBox that does the
transition between a SAN and the doubled LANs, as Figure 5 shows. The RedBox mimics the
SANs connected behind it (called VDA or virtual DANs) and multicasts supervision frames on
their behalf, appending its own information. The RedBox is itself a DANP and has its own IP
address for management purposes, but it may also perform application functions.
switch
non-redundant network
SASANN SASANN SASANN
S1S1 S2S2 S3S3
singly attached nodes
local
application
Tx Rx
C
TCP/IP
network
SNMP
adapter
RedBox
switching logic
network network
adapter adapter
A B
Tx Rx Tx Rx
transceivers
LAN_A
LAN_B
IEC  360/10
Figure 5 – PRP RedBox, transition from single to double LAN

– 14 – 62439-3 © IEC:2010(E)
4.1.10 Duplicate handling
4.1.10.1 Methods for handling duplicates
Since a DANP receives the same frame over both adapters, when both are operational, it
should keep one and ignore the duplicate.
There are two methods for handling duplicates:
a) duplicate accept, in which the sender LRE uses the original frames and the receiver LRE
forwards both frames it receives to its upper protocol layers;
b) duplicate discard, in which the sender LRE appends a redundancy control trailer to both
frames it sends and the receiver LRE uses that redundancy control trailer to send only the
first frame of a pair to its upper layers and filter out duplicates.
4.1.10.2 Duplicate accept
This method does not attempt to discard duplicates at the link layer. The sender LRE sends
the same frame as it would in the non-redundant case over both LANs. The receiver’s LRE
forwards both frames of a pair (if both arrive) to its upper layers, assuming that well-designed
network protocols and applications are able to withstand duplicates – indeed IEEE 802.1D
explicitly states that it cannot ensure freedom of duplicates.
The internet stack, consisting of a network layer with an UDP and a TCP transport layer, is
assumed to be resilient against duplicates. The TCP protocol is designed to reject duplicates,
so it discards the second frame of a pair. The UDP layer is by definition connectionless and
unacknowledged. All applications that use UDP are assumed to be capable of handling
duplicates, since duplication of frames can occur in any network. In particular, a UDP frame is
assumed to be idempotent, i.e. sending it twice has the same effect as sending it once.
Administrative protocols of the internet such as ICMP and ARP are not affected by duplicates,
since they have their own sequence numbering.
Real-time stack that operate on the publisher-subscriber principle are not affected by
duplicates, since only the latest value is kept. Duplicate reception increases robustness since
a sample that gets lost on one LAN is usually received from the other LAN.
Therefore, one can assume that handling of duplicates is taken care of by the usual network
protocols, but one has to check if each application complies with these assumptions.
This simple duplicate accept method does not provide easy redundancy supervision, since it
does not keep track of correct reception of both frames. The receiver would need hash tables
to know that a frame is the first of a pair of a duplicate, and could for this effect store the CRC
and length of each frame as a hash code. Such redundancy supervision method is however
not specified in this International Standard, but it is not excluded.
4.1.10.3 Duplicate discard in the link layer
4.1.10.3.1 Principle
It is advantageous to discard duplicates already at the link layer.
Without duplicate discard, the processor receives twice as many interrupt requests as when
only one LAN is connected. To offload the application processor, the LRE can perform
Duplicate Discard, possibly with an independent pre-processor or an intelligent Ethernet
controller. This allows at the same time to improve the redundancy supervision.
The duplicate discard protocol uses an additional four-octet field in the frame, the
Redundancy Control Trailer (RCT), which the LRE inserts into each frame that it receives from
the upper layers before sending, as Figure 6 shows. The RCT consists of the following
parameters:
62439-3 © IEC:2010(E) – 15 –
a) 16-bit sequence number (SequenceNr);
b) 4-bit LAN identifier (Lan);
c) 12 bit frame size (LSDU_size).
Sequence LSDU
preamble destination source LT LSDU FCS
Nr _size
octet position 0 6 12 14
time
frame without redundancy control Redundancy Control Trailer

IEC  361/10
Figure 6 – PRP frame extended by an RCT
4.1.10.3.2 Use of SequenceNr
Each time a LRE sends a frame to a particular destination, it increases the sequence number
corresponding to that destination and sends both (nearly identical) frames over both LANs.
The receiving LRE can then detect duplicates based on the RCT.
This method considers that SANs also exist on the network, and that frames sent by SANs
could be wrongly rejected as duplicates because they happen to have a trailing field with the
same sequence number and the same size. However, SANs send on one LAN only, and the
source will not be the same as that of another frame, so a frame from a SAN will never be
discarded.
4.1.10.3.3 Use of LAN
The field LAN can take one of two values: 1 010 indicating that the frame has been sent over
LAN_A and 1 011 indicating that the frame has been sent over LAN_B. This allows detecting
installation errors.
4.1.10.3.4 Use of LSDU_size
To allow the receiver LRE to distinguish easily frames coming from nodes that obey to the
PRP from the non-redundant ones, the sender LRE appends to the frame the length of the link
service data unit (LSDU) in octets in a 12-bit field.
EXAMPLE If the frame carries a 100-octets LSDU, the size field equals LSDU+RCT: 104 = 100 + 4.
In VLANs, frame VLAN tags may be added or removed during transit through a switch. To
make the length field independent of VLAN tagging, only the LSDU and the RCT are
considered in the LSDU_size, as Figure 7 shows.
LSDU
destination source time
preamble tag ET LSDU SequenceNr
FCS
_size
octet position016 124
IEC  362/10
Figure 7 – PRP VLAN-tagged frame extended by an RCT
The receiver scans the frames, preferably starting from the end. If it detects that the 12 bits
before the end correspond to the LSDU size, and that the LAN identifier matches the identifier
of the LAN it is attached to (see 4.1.11), the frame is a candidate for rejection.
Lan
Lan
– 16 – 62439-3 © IEC:2010(E)
Since short frames need padding to meet the minimum frame size of 64 octets, the sender
already includes the padding to speed up scanning from behind, as Figure 8 shows.
LSDU
preamble destination source LT LSDU padding SequenceNr FCS
_size
octet position 0 6 12 14
time
IEC  363/10
Figure 8 – PRP constructed, padded frame closed by an RCT
NOTE A VLAN-tagged frame can pass several switches which may remove or insert VLAN tags. If the sender
observes the ISO/IEC 8802-3 (IEEE 802.3) rule to send a minimum frame size of 68 octets for a VLAN-tagged
frame and of 64 for a VLAN-untagged frame, there should never be a situation in which there is padding before and
after the RCT. Scanning from behind is specified as a matter of precaution.
4.1.10.3.5 Frame size restriction
Appending the RCT could generate oversize frames that exceed the maxValidSize foreseen
by ISO/IEC 8802-3 (IEEE 802.3).
To maintain compliance with IEEE 802.3:2005, the communication software in a DANP using
duplicate discard is configured for a maximum payload size of 1 496 octets.
NOTE Longer payloads would work in most cases, but this requires previous testing. Many switches are
dimensioned for double-VLAN-tagged (non-IEEE 802.3 compliant) frames that have a maximum size of 1 526
octets. Most Ethernet controllers are certified up to 1 528 octets. Most switches would forward correctly frames of
up to 1 536 octets, but this cannot be relied upon.
4.1.10.3.6 Discard algorithm
The following algorithm is optional, other methods such as hash table can be used.
The receiver assumes that frames coming from a DANP are sent in sequence with increasing
sequence numbers. The sequence number expected for the next frame is kept in the variables
ExpectedSeqA, respectively ExpectedSeqB.
At reception, the correct sequence can be checked by comparing ExpectedSeqA with the
received sequence number in the RCT, CurrentSeqA. Regardless of the result, ExpectedSeqA
is set to one more than CurrentSeqA to allow checking the next expected sequence number
on that line. The same applies to ExpectedSeqB and CurrentSeqB on LAN_B.
Both LANs thus maintain a sliding drop window of contiguous sequence numbers, the upper
bound being ExpectedSeqA (the next expected sequence number on that LAN), excluding that
value, the lower bound being StartSeqA (the lowest sequence number that leads to a discard
on that LAN) as Figure 9 shows for LAN_A. The same applies to ExpectedSeqB and
StartSeqB on LAN_B.
Lan
62439-3 © IEC:2010(E) – 17 –
dropWindow
CurrentSeqA
ExpectedSeqA
StartSeqA
„B is late“ „B is early“
LAN_A
s c e
keepB dropB keepB
LAN_B
IEC  364/10
Figure 9 – PRP drop window on LAN_A
After checking the correct sequence number, the receiver decides whether to discard the
frame or not. Assuming that LAN_A has established a non-void drop window (as in Figure 9),
a frame from LAN_B whose sequence number CurrentSeqB fits into the drop window of A is
discarded (dropB in Figure 9). In all other cases, the frame is kept and forwarded to the upper
protocol layers (keepB in Figure 9).
Discarding the frame (dropB in Figure 9) shrinks the drop window size on LAN_A since no
more frames from B with an earlier sequence number are expected, thus StartSeqA is
increased to one more than the received CurrentSeqB. Also, the drop window on B is reset to
a size of 0 (StartSeqB = ExpectedSeqB), since obviously B lags behind A and no frames from
A should be discarded, as Figure 10 shows.
CurrentSeqA
ExpectedSeqA
StartSeqA
LAN_A
s c e
StartSeqB
s
LAN_B c
e
...