BIER P. Thubert, Ed.
Internet-Draft Cisco
Intended status: Standards Track T. Eckert
Expires: September 4, 2018 Huawei
Z. Brodard
Ecole Polytechnique
H. Jiang
Telecom Bretagne
March 3, 2018
BIER-TE extensions for Packet Replication and Elimination Function
(PREF) and OAM
draft-thubert-bier-replication-elimination-03
Abstract
This specification extends Bit Index Explicit Replication - Traffic
Engineering (BIER-TE) forwarding to support in the data plane the
DetNet Packet Replication and Elimination Functions (PREF). It also
provides traceability of links/adjacencies where replication and loss
happen, in a manner that is agnostic to the forwarding information
(OAM).
Status of This Memo
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This Internet-Draft will expire on September 4, 2018.
Copyright Notice
Copyright (c) 2018 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. On BIER - Traffic Engineering . . . . . . . . . . . . . . . . 3
4. BIER-TE-based Replication and Elimination Control . . . . . . 4
5. Elimination Function (Normative) . . . . . . . . . . . . . . 9
6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 12
8. Security considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Deterministic Networking (DetNet) [I-D.ietf-detnet-problem-statement]
provides a capability to carry unicast or multicast data flows for
real-time applications with extremely low data loss rates and known
upper bound maximum latency [I-D.ietf-detnet-architecture].
DetNet applies to multiple environments where there is a desire to
replace a point to point serial cable or a multidrop bus by a
switched or routed infrastucture, in order to scale, lower costs, and
simplify management. One classical use case is found in particular
in the context of the convergence of IT with Operational Technology
(OT), also referred to as the Industrial Internet. But there are
many others use cases [I-D.ietf-detnet-use-cases], for instance in in
professional audio and video, automotive, radio fronthauls, etc..
The DetNet data plane alternatives [I-D.dt-detnet-dp-alt] studies the
applicability of existing and emerging dataplane techniques that can
be leveraged to enable DetNet properties in IP networks. One
critical feature in the dataplane is traceability, the capability to
control the activity of intermediate nodes on a packet. For
instance, if Replication and Elimination is applied to a packet, then
it is desirable to determine which node performed a certain copy of
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that packet that is circulating in the network. Likewise, engineered
paths are required to support redundant transmission across disjoint
paths in support of DetNets PREF functios.
Traceability belongs to Operations, Administration, and Maintenance
(OAM) which is the toolset for fault detection and isolation, and for
performance measurement. More can be found on OAM Tools in "An
Overview of Operations, Administration and Maintenance (OAM) Tools"
[I-D.ietf-opsawg-oam-overview].
This document proposes a new set to mechanisms based on [RFC8279]
(BIER) and more specifically BIER Traffic Engineering
[I-D.ietf-bier-te-arch] (BIER-TE) to control the process or Packet
Replication and Elimination Functions (PREF), and provide
traceability of these operations, in the DetNet dataplane. An
adjacency, which is represented by a bit in the BIER header, can
correspond in the dataplane to an Ethernet hop, a Label Switched
Path, or it can correspond to an IPv6 loose or strict source routed
path.
BIER-TE was primarily designed to carry multicast traffic, but there
is nothing prohibiting for it to be used with unicast traffic, and
the authors of this document think that for networks whose size
requirement match the supportable bitstring length (BSL) in BIER, it
can be a good choice as the forwarding plane specifically for DetNet
type traffic for both multicast and unicast traffic because it would
be a common solution for unicast and multicast (limiting the number
of different technologies a DetNet solution requires) and likely
provides the most flexible support for path engineering, replication
and elimination (PREF) and the novel OAM method described in this
document.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. On BIER - Traffic Engineering
[RFC8279] (BIER) is a network plane replication technique that was
initially intended as a new method for multicast distribution. In a
nutshell, a BIER header includes a bitmap that explicitly signals the
listeners that are intended for a particular packet, which means that
1) the sender is aware of the individual listeners and 2) the BIER
control plane is a simple extension of the unicast routing as opposed
to a dedicated multicast data plane, which represents a considerable
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reduction in OPEX. For this reason, the technology faces a lot of
traction from Service Providers.
The simplicity of the BIER technology makes it very versatile as a
network plane signaling protocol. Already, a new Traffic Engineering
variation is emerging that uses bits to signal segments along a TE
path.
While BIER-TE was like BIER primarily developed for multicast
traffic, the authors think that it can equally be attractive for
unicast traffic requiring the DetNet resilience of multiple
transitions. If the topology of the network can well be represented
by standard BIER-TE bitstring sizes of e.g.: up to 256 bits, then
this would allow for a single technology for both unicast and
multicast.
BIER-TE supports a Traffic Engineered forwarding plane by explicit
hop-by-hop forwarding and loose hop forwarding of packets.
From the BIER-TE architecture, the key differences over BIER are:
o BIER-TE replaces in-network autonomous path calculation by
explicit paths calculated off path for example by a BIER-TE
controller host.
o In BIER-TE every BitPosition of the BitString of a BIER-TE packet
indicates one or more adjacencies - instead of a BFER as in BIER.
processing packets as a destination (BFER) is one of the possible
adjacency types.
o BIER-TE in each BFR has no routing table but only a BIER-TE
Forwarding Table (BIFT) indexed by SI:BitPosition and populated
with only those adjacencies to which the BFR should replicate
packets to.
The generic view of an adjacency can be over a link, a tunnel or
along a path segment.
4. BIER-TE-based Replication and Elimination Control
This document only needs to introduce new functionality to support
the Elimination Function and OAM. Creation of appropriate BIER-TE
packets is subject to to existing work.
In the solution described below, the encapsulation/insertion of flow-
identification and sequence number into packets is performed by a
function on the BFIR outside the scope of this document. A companion
document draft-huang-bier-te-encapsulation defines an encapsulation
for BIER-TE and BIER that can support flow-id and sequence-number ID.
Other encapsulations can be used as well, as long as they provide
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these signaling elements and are supported by the Elimination
Function described in this document (e.g.: that the EF can read these
fields and therefore remove duplicates). In the remainder of this
document we will call this the extended BIER encapsulation and assume
that it is used when describing examples. Unless otherwise noted, we
assume that the BFIR performs encapsulation of some data flow packets
with an extended BIER header, indicates BIER-TE forwarding in it and
fills in flow-id and sequence number. It then fills in the bitstring
with two (or more) alternative paths/DAGs and sends off the packets
into the BIER-TE domain, replicating it itself if so indicated by the
bitstring.
In a nutshell, BIER-TE is used as follows:
o A controller computes a complex path, sometimes called a track,
which takes the general form of a ladder. The steps and the side
rails between them are the adjacencies that can be activated on
demand on a per-packet basis using bits in the BIER header.
===> (A) ====> (C) ====
// ^ | ^ | \\
ingress (I) | | | | (E) egress
\\ | v | v //
===> (B) ====> (D) ====
Figure 1: Ladder Shape with Replication and Elimination Points
o The controller assigns a BIER domain, and inside that domain,
assigns bits to the adjacencies. The controller assigns each bit
to a replication node that sends towards the adjacency, for
instance the ingress router into a segment that will insert a
routing header in the packet. A single bit may be used for a step
in the ladder, indicating the other end of the step in both
directions.
===> (A) ====> (C) ====
// 1 ^ | 4 ^ | 7 \\
ingress (I) |2| |6| (E) egress
\\ 3 | v 5 | v 8 //
===> (B) ====> (D) ====
Figure 2: Assigning Bits
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o The controller activates the replication by deciding the setting
of the bits associated with the adjacencies. This decision can be
modified at any time, but takes the latency of a controller round
trip to effectively take place. Below is an example that uses
Replication and Elimination to protect the A->C adjacency. The
"(EF)" in the following pictures Owner column indicates the fact
that that BFR will perform the "Elimination Function" for received
BIER-TE packets before further processing/copying them. In this
example, only C performs EF. A (1) in the Example Bitstring
indicates that the bit is set, but that the actual adjacency is
not used by packets because this bit is shared with another
adjacency and the overall bitstring will make the packet only use
that other adjacency. This applies to bits 2 and 6.
+-------+-----------+--------+-------------------+
| Bit # | Adjacency | Owner | Example Bitstring |
+-------+-----------+--------+-------------------+
| 1 | I->A | I | 1 |
| 2 | A->B | A | 1 |
| | B->A | B | (1) |
| 3 | I->B | I | 0 |
| 4 | A->C | A | 1 |
| 5 | B->D | B | 1 |
| 6 | C->D | C (EF) | (1) |
| | D->C | D | |
| 7 | C->E | C (EF) | 1 |
| 8 | D->E | D | 0 |
+-------+-----------+--------+-------------------+
Replication and Elimination Protecting A->C
Table 1: Controlling Replication
o The BIER header with the controlling BitString , flow-id and
sequence number is injected in the packet by the ingress node I
(BFIR). That node may act as a replication point, in which case
it may issue multiple copies of the packet, but for the purpose of
this example it will not do it, so that the two paths used in this
example only go from A to C, and therefore require explicit path
engineering. For example, bandwidth I-A and I-B may be more
limited and those paths being non long-haul may not warrant the
dual transmission.
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====> Repl ===> Elim ====
// | ^ \\
ingress | | egress
v |
Fwd ====> Fwd
Figure 3: Enabled Adjacencies
o For each of its bits that is set in the BIER header, the owner
replication point resets the bit used for a copy and transmits
towards the associated adjacency; to achieve this, the replication
point copies the packet and inserts the relevant data plane
information, such as next-hop label, MAC-address or source route
header (for a BIER-TE routed adjacency), towards the adjacency
that corresponds to the bit
+-----------+----------------+
| Adjacency | BIER BitString |
+-----------+----------------+
| I->A | 01011110 |
| A->B | 00011110 |
| B->D | 00010110 |
| D->C | 00010010 |
| A->C | 01001110 |
+-----------+----------------+
BitString in BIER Header as Packet Progresses
Table 2: BIER-TE in Action
o Adversely, an elimination node on the path performs the
Elimination Function which will remove duplicate packets (same
flow-id, same sequence number) and performs a bitwise AND on the
BitStrings from the various copies of the packet that it has
received, before it forwards the packet with the resulting
BitString. Details of the Elimination Function are described
below.
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+-----------+----------------+
| Operation | BIER BitString |
+-----------+----------------+
| D->C | 00010010 |
| A->C | 01001110 |
| | -------- |
| AND in C | 00000010 |
| | |
| C->E | 00000000 |
+-----------+----------------+
BitString Processing at Elimination Point C
Table 3: BIER-TE in Action (cont.)
o In this example, all the transmissions succeeded and the BitString
at arrival has all the bits reset - note that the egress may be an
Elimination Point in which case this is evaluated after this node
has performed its AND operation on the received BitStrings).
+-------------------+-----------------------+
| Failing Adjacency | Egress BIER BitString |
+-------------------+-----------------------+
| I->A | Frame Lost |
| I->B | Not Tried |
| A->C | 00010000 |
| A->B | 01001100 |
| B->D | 01001100 |
| D->C | 01001100 |
| C->E | Frame Lost |
| D->E | Not Tried |
+-------------------+-----------------------+
BitString indicating failures
Table 4: BIER-TE in Action (cont.)
o But if a transmission failed along the way, one (or more) bit is
never cleared. Table 4 provides the possible outcomes of a
transmission. If the frame is lost, then it is probably due to a
failure in either I->A or C->E, and the controller should enable
I->B and D->E to find out. A BitString of 00010000 indicates
unequivocally a transmission error on the A->C adjacency, and a
BitString of 01001100 indicates a loss in either A->B, B->D or
D->C; enabling D->E on the next packets may provide more
information to sort things out.
In more details:
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The BIER header is of variable size, and a DetNet network of a
limited size can use a model with 64 bits if 64 adjacencies are
enough, whereas a larger deployment may be able to signal up to 256
adjacencies for use in very complex paths. The format of this header
is common to BIER and BIER-TE.
For the DetNet data plane, a replication point is an ingress point
for more than one adjacency, and an elimination point is an egress
point for more than one adjacency.
A pre-populated state in a replication node indicates which bits are
served by this node and to which adjacency each of these bits
corresponds. With DetNet, the state is typically installed by a
controller entity such as a PCE. The way the adjacency is signaled
in the packet is fully abstracted in the bit representation and must
be provisioned to the replication nodes and maintained as a local
state, together with the timing or shaping information for the
associated flow.
The DetNet data plane uses BIER-TE to control which adjacencies are
used for a given packet. This is signaled from the path ingress,
which sets the appropriate bits in the BIER BitString to indicate
which replication must happen.
The replication point clears the bit associated to the adjacency
where the replica is placed, and the elimination points perform a
logical AND of the BitStrings of the copies that it gets before
forwarding.
As is apparent in the examples above, clearing the bits enables to
trace a packet to the replication points that made any particular
copy. BIER-TE also enables to detect the failing adjacencies or
sequences of adjacencies along a path and to activate additional
replications to counter balance the failures.
Finally, using the same BIER-TE bit for both directions of the steps
of the ladder enables to avoid replication in both directions along
the crossing adjacencies. At the time of sending along the step of
the ladder, the bit may have been already reset by performing the AND
operation with the copy from the other side, in which case the
transmission is not needed and does not occur (since the control bit
is now off).
5. Elimination Function (Normative)
This section defines the normative behavior of the Elimination
Function with optional OAM sub-function.
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The Elimination Function is performed logically on reception of BIER-
TE packets. It is therefore not part of the adjacencies or otherwise
assigned to a specific bit. "Logically" means that this
specification does not constrain implementations, especially on
multi-linecard/multi-chassis systems to perform EF on a physcial
egres module. It just implies that it has to happen before
replication to the bits in the bitstring.
TBD: In addition to being an ingres, EF could as well be modelled as
a new adjacency asigned to bits. The full adjacency of a bit could
then be a sequence of EF followed by one (or more) of existing
adjacencies. This is currently not considered by this document due
to the lack of identified need to support this option - e.g.:
problems that can not be equally/better be solved with EF logically
on ingres.
The Elimination Function is more formally written as EF(OAM, BIFT,
{flows}/*), and is configured like BIFTs from the BIER-TE controller
host and/or other future mechanisms.
OAM is boolean and indicates whether OAM function of bitwise AND of
received packet copies is performed. This OAM function requires
additional memory/processing over EF without OAM. Note that the OAM
function does not change the effect of the Elimination Function for
BFR/receivers - they will continue to just receive the first copy of
a packet. Instead, it will continue to track further copies solely
for the purpose of providing OAM information. This also requires
some timout or sequence number advancement to decide when to
terminate waiting for further copies of packets before considering
the OAM analysis of this packet to be complete. BFR supporting this
document SHOULD support the OAM sub-function.
BIFT indicates the <SD,SI,BSL> for which to perform EF. Devices
SHOULD support enabling per EF. {flows}/* indicates the set of flows
for which EF operates (using the specified BIFT). Duplicate
elimination has to create per-flow state to remember which sequence
number packets for this flow where already received. In the case of
OAM also what bits where set in that received prior copy of the
packet.
When a device supports "*", then it will automatically allocate such
a flow-state for every new recognized flow and expire such flow state
after an operator determined timeout of activity - for example with a
default of 10 seconds. Dynamic allocation of flow-state may cause
some inital duplicates before this state is working and it makes the
BFR more vulnerable to state DOS attacks, but it will allows BIER
applications to send flows with the benefit of EF without the help of
the controller having to know and program every flow.
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In the {flows} option, control procedures (e.g.: BIER-TE controller
host) indicate to the BFR explicitly the set of flows for which it
should install/operate the EF function. Note that the flow-id in the
extended BIER encapsulation is the combination of BFIR-ID and entropy
field of the BIER header.
BFR supporting this document MUST support the {flows} option and MAY
support the "*" option.
The following picture explains the results of EF being performed on
ingres in a typical example:
I1
|
v
/---------- B1 ------------\
| |
\-- B3 -- B4 -- B5 -- B6 --/
| | | |
| | | |
O1 O2 O3 O4
Figure 4: EF with Rings
Consider a simple ring where BFIR I1 generates BIER-TE packets. The
bitstring indicates that the packet is sent hop-by-hop
counterclockwise B1->B3->B4->B6 and counterclockwise
B1->B6->B5->B4->B3. Bits for BFER O1, O2, O3 and O4 are also set.
B3,B4,B5,B6,B7 perform EF. The result of this setup is that B2
creates two copies of the packets received from I1, one going to B6,
the other to B3. Assume B4 first received the counter-clockwise copy
from B3 and B5 the clockwise copy from B6. They will both forward
these packets to each other because those where the first copies they
saw, but the would block these second copies. Therefore only the
link B4<->B5 will have carried the packet copy twice (once in each
direction). All the other ring links will only carry one copy of the
packet.
This is notably different from schemes where EF is not performed
before replication, but afterwards. In those schemes, both copies of
the packets would flow counterclockwise around (most of) the ring,
ocupying more bandwidth.
6. Summary
With the addition of the functions of this document, BIER-TE becomes
a potential option for the DetNet dataplane specifically beneficial
when PREF (replication and elimination) is required for resilience
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(to reduce packet loss). For DetNet multicast but also DetNet
unicast. The unique capabilities of this approach areare:
o Explicit per-packet path selection for packet. Multicast and
Unicast.
o Control which replication take place on a per packet basis, so
that replication points can be configured but not actually
utilized
o Trace the replication activity and determine which node replicated
a particular packet
o Measure the quality of transmission of the actual data packet
along the replication segments and use that in a control loop to
adapt the setting of the bits and maintain the reliability.
7. Implementation Status
A research-stage implementation of the forwarding plane fir a 6TiSCH
IOT use case was developed at Cisco's Paris Innovation Lab (PIRL) by
Zacharie Brodard. It was implemented on OpenWSN Open-source firmware
and tested on the OpenMote-CC2538 hardware. It implements the header
types 15,16, 17, 18 and 19 (bit-by-bit encoding without group ID) in
order to allow a BIER-TE protocol over IEE802.15.4e.
This work was complemented with a Controller-based control loop by
Hao Jiang. The controller builds the complex paths (called Tracks in
6TiSCH) and decides the setting oif the BitStrings in real time in
order to optimize the delivery ratio within a minimal energy budget.
Links:
github: https://github.com/zach-b/openwsn-fw/tree/BIER
OpenWSN firmware: https://openwsn.atlassian.net/wiki/pages/
viewpage.action?pageId=688187
OpenMote hardware: http://www.openmote.com/
8. Security considerations
TBD.
9. IANA Considerations
This document has no IANA considerations.
10. Acknowledgements
The method presented in this document was discussed and worked out
together with the DetNet Data Plane Design Team:
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Jouni Korhonen
Janos Farkas
Norman Finn
Olivier Marce
Gregory Mirsky
Pascal Thubert
Zhuangyan Zhuang
The authors also like to thank the DetNet chairs Lou Berger and Pat
Thaler, as well as Thomas Watteyne, 6TiSCH co-chair, for their
contributions and support to this work.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
11.2. Informative References
[I-D.dt-detnet-dp-alt]
Korhonen, J., Farkas, J., Mirsky, G., Thubert, P.,
Zhuangyan, Z., and L. Berger, "DetNet Data Plane Protocol
and Solution Alternatives", draft-dt-detnet-dp-alt-04
(work in progress), September 2016.
[I-D.ietf-bier-te-arch]
Eckert, T., Cauchie, G., Braun, W., and M. Menth, "Traffic
Engineering for Bit Index Explicit Replication (BIER-TE)",
draft-ietf-bier-te-arch-00 (work in progress), January
2018.
[I-D.ietf-detnet-architecture]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", draft-ietf-
detnet-architecture-04 (work in progress), October 2017.
[I-D.ietf-detnet-problem-statement]
Finn, N. and P. Thubert, "Deterministic Networking Problem
Statement", draft-ietf-detnet-problem-statement-02 (work
in progress), September 2017.
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[I-D.ietf-detnet-use-cases]
Grossman, E., "Deterministic Networking Use Cases", draft-
ietf-detnet-use-cases-14 (work in progress), February
2018.
[I-D.ietf-opsawg-oam-overview]
Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
Weingarten, "An Overview of Operations, Administration,
and Maintenance (OAM) Tools", draft-ietf-opsawg-oam-
overview-16 (work in progress), March 2014.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.
Authors' Addresses
Pascal Thubert (editor)
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com
Toerless Eckert
Huawei USA - Futurewei Technologies Inc.
2330 Central Expy
Santa Clara 95050
USA
Email: tte+ietf@cs.fau.de
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Zacharie Brodard
Ecole Polytechnique
Route de Saclay
Palaiseau 91128
FRANCE
Phone: +33 6 73 73 35 09
Email: zacharie.brodard@polytechnique.edu
Hao Jiang
Telecom Bretagne
2, rue de la Chataigneraie
Cesson-Sevigne 35510
FRANCE
Phone: +33 7 53 70 97 34
Email: hao.jiang@telecom-bretagne.eu
Thubert, et al. Expires September 4, 2018 [Page 15]