Movatterモバイル変換

Network Working Group                                         W. SimpsonRequest for Comments: 1333                                    Daydreamer                                                                May 1992PPP Link Quality MonitoringStatus of this Memo   This RFC specifies an IAB standards track protocol for the Internet   community, and requests discussion and suggestions for improvements.   Please refer to the current edition of the "IAB Official Protocol   Standards" for the standardization state and status of this protocol.   Distribution of this memo is unlimited.Abstract   The Point-to-Point Protocol (PPP) [1] provides a standard method of   encapsulating Network Layer protocol information over point-to-point   links.  PPP also defines an extensible Link Control Protocol, which   allows negotiation of a Quality Protocol for continuous monitoring of   the viability of the link.   This document defines a protocol for generating Link-Quality-Reports.   This RFC is a product of the Point-to-Point Protocol Working Group of   the Internet Engineering Task Force (IETF).  Comments on this memo   should be submitted to the ietf-ppp@ucdavis.edu mailing list.Simpson                                                         [Page i]

RFC 1333              PPP Link Quality Monitoring               May 1992Table of Contents1.     Introduction ..........................................12.     Link Quality Monitoring ...............................22.1       Design Motivation ...............................22.2       Counters ........................................22.3       Counting Packets and Octets .....................42.4       Processes .......................................42.5       Configuration Option Format .....................62.6       Packet Format ...................................82.7       Transmission of Reports .........................122.8       Calculations ....................................122.9       Failure Detection ...............................132.10      Policy Suggestions ..............................14     SECURITY CONSIDERATIONS ......................................14     REFERENCES ...................................................14     ACKNOWLEDGEMENTS .............................................14     CHAIR'S ADDRESS ..............................................15     AUTHOR'S ADDRESS .............................................15Simpson                                                        [Page ii]

RFC 1333              PPP Link Quality Monitoring               May 19921.  Introduction   PPP has three main components:      1. A method for encapsulating datagrams over serial links.      2. A Link Control Protocol (LCP) for establishing, configuring,         and testing the data-link connection.      3. A family of Network Control Protocols (NCPs) for establishing         and configuring different network-layer protocols.   In order to establish communications over a point-to-point link, each   end of the PPP link must first send LCP packets to configure the data   link during the Establishment phase.  During the Authentication and   Network-Layer Protocol phases, the link may be tested to determine if   quality is sufficient for operation.  This testing is completely   optional.   If an implementation desires that the peer use some specific link   quality monitoring protocol, then it MUST negotiate the use of that   protocol using the Quality-Protocol Configuration Option during Link   Establishment phase.   The negotiation mechanism is independent in each direction.  However,   if the peer agrees to send Quality-Protocol packets, it MUST   correctly process such packets on reception, even if it does not   request such packets or implement a monitoring policy.Simpson                                                         [Page 1]

RFC 1333              PPP Link Quality Monitoring               May 19922.  Link Quality Monitoring   Data communications links are rarely perfect.  Packets can be dropped   or corrupted for various reasons (line noise, equipment failure,   buffer overruns, etc.).  Sometimes, it is desirable to determine   when, and how often, the link is dropping data.  Routers, for   example, may want to temporarily allow another route to take   precedence.  An implementation may also have the option of   disconnecting and switching to an alternate link.  The process of   determining data loss is called "Link Quality Monitoring".2.1.  Design Motivation   There are many different ways to measure link quality, and even more   ways to react to it.  Rather than specifying a single scheme, Link   Quality Monitoring is divided into a "mechanism" and a "policy".  PPP   fully specifies the "mechanism" for Link Quality Monitoring by   defining the Link-Quality-Report (LQR) packet and specifying a   procedure for its use.  PPP does NOT specify a Link Quality   Monitoring "policy" -- how to judge link quality or what to do when   it is inadequate.  That is left as an implementation decision, and   can be different at each end of the link.  Implementations are   allowed, and even encouraged, to experiment with various link quality   policies.  The Link Quality Monitoring mechanism specification   insures that two implementations with different policies may   communicate and interoperate.   To allow flexible policies to be implemented, the PPP Link Quality   Monitoring mechanism measures data loss in units of packets, octets,   and Link-Quality-Reports.  Each measurement is made separately for   each half of the link, both inbound and outbound.  All measurements   are communicated to both ends of the link so that each end of the   link can implement its own link quality policy for both its outbound   and inbound links.   Finally, the Link Quality Monitoring protocol is designed to be   implementable on many different kinds of systems.  Although it may be   common to implement PPP (and especially Link Quality Monitoring) as a   single software process, multi-process implementations with hardware   support are also envisioned.  The PPP Link Quality Monitoring   mechanism provides for this by careful definition of the Link-   Quality-Report packet format, and by specifying reference points for   all data transmission and reception measurements.2.2.  Counters   Each Link Quality Monitoring implementation maintains counts of the   number of packets and octets transmitted and successfully received,Simpson                                                         [Page 2]

RFC 1333              PPP Link Quality Monitoring               May 1992   and periodically transmits this information to its peer in a Link-   Quality-Report packet.   These counters are similar to sequence numbers; they are constantly   increasing to give a "relative" indication of the number of packets   and octets communicated across the outbound link.  By comparing the   values in successive Link-Quality-Reports, an LQR receiver can   compute the "delta" number of packets and octets successfully   communicated across the link.  Comparing these absolute numbers then   gives an indication of a link's quality.  Relative numbers, rather   than absolute, are transmitted because they greatly simplify link   synchronization.   The Link-Quality-Report uses the Interface counters defined by SNMP   MIB-II [2].  These counters are not initialized to any particular   value when the LCP enters the Establishment phase.   In addition, the Link-Quality-Report requires the implementation of   the following three unsigned, monotonically increasing counters which   conform to the type and size requirements for SNMP MIB Counters [3].   OutLQRs      OutLQRs is a 32-bit counter which increases by one for each      tranmitted Link-Quality-Report packet.  This counter MUST be set      to zero when the LCP enters the Establishment phase, and MUST NOT      be reset until the LCP leaves the Termination phase.  This counter      is incremented before it is inserted into the LQR packet.   InLQRs      InLQRs is a 32-bit counter which increases by one for each      received Link-Quality-Report packet.  This counter MUST be set to      zero when the LCP enters the Establishment phase, and MUST NOT be      reset until the LCP leaves the Termination phase.  This counter is      incremented before it is inserted (in an implementation dependent      fashion) into the LQR packet.   InGoodOctets      InGoodOctets is a 32-bit counter which increases by the number of      octets in each successfully received Data Link Layer packet.      Unlike the MIB ifInOctets, octets for frames which are counted in      ifInDiscards and ifInErrors MUST NOT be counted.  This counter MAY      be set to any initial value when the LCP enters the Establishment      phase, but MUST NOT be reset until the LCP leaves the Termination      phase.Simpson                                                         [Page 3]

RFC 1333              PPP Link Quality Monitoring               May 19922.3.  Counting Packets and Octets   The intent of the counters is to provide an indication of the amount   of information passing over the link, rather than an actual   measurement of the total bandwidth used.  This specification is   designed to yield the same count in various circumstances, such as   when a separate device provides the framing and escaping mechanisms   invisibly to the implementation, or a synchronous-to-asynchronous   converter in the link changes between mechanisms.   All octets which are included in the FCS calculation MUST be counted,   including the packet header, the information field, and any padding.   The FCS octets MUST also be counted, and one flag octet per frame   MUST be counted.  All other octets (such as additional flag   sequences, and escape bits or octets) MUST NOT be counted.   When inserting the packet and octet counts in the LQR, the counts   MUST include the expected values for the LQR itself.2.4.  Processes   The PPP Link Quality Monitoring mechanism is described using a   "logical process" model.  As shown below, there are five logical   processes duplicated at each end of the duplex link.   +---------+   +-------+   +----+ Outbound   |         |-->|  Mux  |-->| Tx |=========>   | Link-   |   +-------+   +----+   | Manager |   |         |   +-------+   +----+ Inbound   |         |<--| Demux |<--| Rx |<=========   +---------+   +-------+   +----+   Link-Manager      The Link-Manager process transmits and receives Link-Quality-      Reports, and implements the desired link quality policy.  LQR      packets are transmitted at a constant rate, which is negotiated by      the LCP Quality-Protocol Configuration Option.   Mux      The Mux process multiplexes packets from the various protocols      (e.g., LCP, IP, XNS, etc.) into a single, sequential, and      prioritized stream of packets.  Link-Quality-Report packets MUST      be given the highest possible priority to insure that link quality      information is communicated in a timely manner.Simpson                                                         [Page 4]

RFC 1333              PPP Link Quality Monitoring               May 1992   Tx      The Tx process maintains the MIB counters ifOutUniPackets and      ifOutOctets, and the internal counter OutLQRs, which are used to      measure the amount of data which is transmitted on the outbound      link.  When Tx processes a Link-Quality-Report packet, it inserts      the values of these counters into the corresponding PeerOut...      fields of the packet.  The Tx process MUST follow the Mux process      so that packets are counted in the order transmitted to the link.   Rx      The Rx process maintains the MIB counters ifInUniPackets,      ifInDiscards, ifInErrors and IfInOctets, and the internal counters      InLQRs and InGoodOctets, which are used to measure the amount of      data which is received by the inbound link.  When Rx processes a      Link-Quality-Report packet, it inserts the values of these      counters into the corresponding SaveIn... fields of the packet (in      an implementation dependent manner).   Demux      The Demux process demultiplexes packets for the various protocols.      The Demux process MUST follow the Rx process so that packets are      counted in the order received from the link.Simpson                                                         [Page 5]

RFC 1333              PPP Link Quality Monitoring               May 19922.5.  Configuration Option Format   Description      Implementations MUST be prepared to receive the Quality-Protocol      Configuration Option for the Link-Quality-Report.  However,      negotiation is not required.  Negotiation is only necessary when      the implementation wishes to ensure that the peer transmits Link-      Quality-Reports as opposed to some other Quality-Protocol, or else      to prevent the peer from maintaining its own timer, or else to      establish a maximum time between transmissions of Link-Quality-      Reports.   A summary of the Quality-Protocol Configuration Option format to   negotiate the Link-Quality-Report is shown below.  The fields are   transmitted from left to right.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |        Quality-Protocol       |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                      Reporting-Period                         |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      4   Length      8   Quality-Protocol      c025 (hex) for Link-Quality-Report   Reporting-Period      The Reporting-Period field is four octets and indicates the      maximum time in hundredths of seconds between transmission of      packets.  The peer MAY transmit packets at a faster rate than that      which was negotiated.      A value of zero indicates that the peer does not need to maintain      a timer.  Instead, the peer generates a LQR immediately upon      receiving a LQR.  A value of zero MUST be Nak'd by the peer withSimpson                                                         [Page 6]

RFC 1333              PPP Link Quality Monitoring               May 1992      an appropriate non-zero value when that peer has sent or will send      a Configure-Request packet containing the Quality-Protocol      Configuration Option for the Link-Quality-Report with a zero      Reporting-Period.Simpson                                                         [Page 7]

RFC 1333              PPP Link Quality Monitoring               May 19922.6.  Packet Format   Exactly one Link-Quality-Report packet is encapsulated in the   Information field of PPP Data Link Layer frames where the protocol   field indicates type hex c025 (Link-Quality-Report).  A summary of   the LQR packet format is shown below.  The names of the fields are   relative to the packet receiver, since it is the receiver who   requested the packet in the Configuration Option.  The fields are   transmitted from left to right.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         Magic-Number                          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         LastOutLQRs                           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        LastOutPackets                         |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        LastOutOctets                          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         PeerInLQRs                            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        PeerInPackets                          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        PeerInDiscards                         |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        PeerInErrors                           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        PeerInOctets                           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         PeerOutLQRs                           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        PeerOutPackets                         |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        PeerOutOctets                          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   The following fields are not actually transmitted over the inbound   link.  Rather, they are logically appended (in an implementation   dependent manner) to the packet by the implementation's Rx process.   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                         SaveInLQRs                            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        SaveInPackets                          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        SaveInDiscards                         |Simpson                                                         [Page 8]

RFC 1333              PPP Link Quality Monitoring               May 1992   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        SaveInErrors                           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                        SaveInOctets                           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Magic-Number      The Magic-Number field is four octets and aids in detecting links      which are in the looped-back condition.  Unless modified by a      Configuration Option, the Magic-Number MUST be transmitted as zero      and MUST be ignored on reception.  If Magic-Numbers have been      negotiated, incoming LQR packets SHOULD be checked to ensure that      the local end is not seeing its own Magic-Number and thus a      looped-back link.  See the Magic-Number Configuration Option for      further explanation.   LastOutLQRs      The LastOutLQRs field is four octets, and is copied from the most      recently received PeerOutLQRs on transmission.   LastOutPackets      The LastOutPackets field is four octets, and is copied from the      most recently received PeerOutPackets on transmission.   LastOutOctets      The LastOutOctets field is four octets, and is copied from the      most recently received PeerOutOctets on transmission.   PeerInLQRs      The PeerInLQRs field is four octets, and is copied from the most      recently received SaveInLQRs on transmission.      Whenever the PeerInLQRs field is discovered to be zero, the      LastOut... fields are indeterminate, and the PeerIn... fields      contain the initial values for the peer.   PeerInPackets      The PeerInPackets field is four octets, and is copied from the      most recently received SaveInPackets on transmission.Simpson                                                         [Page 9]

RFC 1333              PPP Link Quality Monitoring               May 1992   PeerInDiscards      The PeerInDiscards field is four octets, and is copied from the      most recently received SaveInDiscards on transmission.   PeerInErrors      The PeerInErrors field is four octets, and is copied from the most      recently received SaveInErrors on transmission.   PeerInOctets      The PeerInOctets field is four octets, and is copied from the most      recently received SaveInOctets on transmission.   PeerOutLQRs      The PeerOutLQRs field is four octets, and is copied from OutLQRs      on transmission.  This number MUST include this LQR.   PeerOutPackets      The PeerOutPackets field is four octets, and is copied from the      current MIB ifOutUniPackets and ifOutNUniPackets on transmission.      This number MUST include this LQR.   PeerOutOctets      The PeerOutOctets field is four octets, and is copied from the      current MIB ifOutOctets on transmission.  This number MUST include      this LQR.   SaveInLQRs      The SaveInLQRs field is four octets, and is copied from InLQRs on      reception.  This number MUST include this LQR.   SaveInPackets      The SaveInPackets field is four octets, and is copied from the      current MIB ifInUniPackets and ifInNUniPackets on reception.  This      number MUST include this LQR.   SaveInDiscards      The SaveInDiscards field is four octets, and is copied from the      current MIB ifInDiscards on reception.  This number MUST include      this LQR.Simpson                                                        [Page 10]

RFC 1333              PPP Link Quality Monitoring               May 1992   SaveInErrors      The SaveInErrors field is four octets, and is copied from the      current MIB ifInErrors on reception.  This number MUST include      this LQR.   SaveInOctets      The SaveInOctets field is four octets, and is copied from the      current InGoodOctets on reception.  This number MUST include this      LQR.      Note that InGoodOctets is not the same as the MIB ifInOctets      counter, as InGoodOctets does not include octets for packets which      are discards or errors.Simpson                                                        [Page 11]

RFC 1333              PPP Link Quality Monitoring               May 19922.7.  Transmission of Reports   When the PPP Link Control Protocol has reached the Opened state, the   Link Quality Monitoring process MAY commence sending Link-Quality-   Reports.  If a Protocol-Reject is received specifying a LQR packet,   the LQM process MUST cease sending LQRs.   Usually, the LQR is transmitted when the LQR timer for the link   expires.  If no LQR timer is used, a LQR is generated upon receipt of   an incoming LQR.  The negotiation process ensures that at least one   side of the link is using a LQR timer.   In addition, a LQR is generated whenever two successive LQRs are   received which have the same PeerInLQRs value.  This may indicate   that a LQR has been missed, or that the implementation is sending at   a significantly slower rate than the peer, or that the peer has   accelerated LQR generation to better quantify errors on the link.   Whenever a LQR is sent, the LQR timer MUST be restarted.2.8.  Calculations   Each time a Link-Quality-Report packet is received from the inbound   link, the Link-Manager can compare the associated fields.  The fields   of the previous LQR can be subtracted from the current LQR values to   obtain an absolute "delta", which allows comparision of the changes   seen by each end of the link.   If the received PeerInLQRs field is zero, the LastOut... fields are   indeterminate, and the PeerIn... fields contain the initial values   for the peer.  No calculations using these fields can be performed at   this time.   Implementation Note:      The following counters wrap to zero when their maximum value is      reached.  Care must be taken to ensure that correct "delta"      calculations are performed at that time.   The LastOutLQRs field may be directly compared with the PeerInLQRs   field to determine how many outbound LQRs have been lost.   The LastOutLQRs field may be directly compared with the OutLQRs   counter to determine how many outbound LQRs are still in the   pipeline.   The change in PeerInPackets may be compared with the change in   LastOutPackets to determine the number of lost packets over theSimpson                                                        [Page 12]

RFC 1333              PPP Link Quality Monitoring               May 1992   outgoing link.   The change in PeerInOctets may be compared with the change in   LastOutOctets to determine the number of lost octets over the   outgoing link.   The change in SaveInPackets may be compared with the change in   PeerOutPackets to determine the number of lost packets over the   incoming link.   The change in SaveInOctets may be compared with the change in   PeerOutOctets to determine the number of lost octets over the   incoming link.   The change in the PeerInDiscards and PeerInErrors fields may be used   to determine whether packet loss is due to congestion in the peer   rather than physical link failure.2.9.  Failure Detection   When the link is operating well in both directions of the link, the   LQR is superfluous.  The maximum time interval for transmitting LQRs   SHOULD be chosen to minimally interfere with active traffic.   When there is a measurable loss of data in either direction, if the   overall throughput is adequate, conditions are not severe enough to   warrant dropping the link.  Sending LQRs faster will gain nothing,   except to measure peaks in the loss rate.  The time interval MUST be   chosen to be long enough to have a good smoothing effect on the data,   while short enough to ensure fast enough response to complete   failure.   When the link is good incoming, but very bad outgoing, incoming LQRs   indicate a high loss on the outgoing side of the link.  Sending LQRs   faster won't help, because they are probably lost on the way to the   peer.   When the link is good outgoing, but very bad incoming, incoming LRQs   will be frequently lost.  In this case, LQRs SHOULD be sent at a   faster rate.  This primarily relies on the peer to make an informed   policy decision.  The peer will also send LQRs in response (due to   the duplicate PeerInLQRs field), and some of those LQRs may   successfully arrive.   When a LQR does not arrive within the time expected, or the LQR   received indicates that the links are truly bad, at least one   additional LQR SHOULD be sent.  An algorithmic decision requires at   least 2 round trip intervals.  The loss rate could be transient, dueSimpson                                                        [Page 13]

RFC 1333              PPP Link Quality Monitoring               May 1992   to a heavily loaded link, or a lost outgoing LQR.2.10.  Policy Suggestions   Link-Quality-Report packets provide a mechanism to determine the link   quality, but it is up to each implementation to decide when the link   is usable.  It is recommended that this policy implement some amount   of hysteresis so that the link does not bounce up and down.  One   policy is to use a K out of N algorithm.  In such an algorithm, there   must be K successes out of the last N periods for the link to be   considered of good quality.   Procedures for recovery from poor quality links are unspecified and   may vary from implementation to implementation.  A suggested approach   is to immediately close all other Network-Layer protocols (i.e.,   cause IPCP to transmit a Terminate-Request), but to continue   transmitting Link-Quality-Reports.  Once the link quality again   reaches an acceptable level, Network-Layer protocols can be   reconfigured.Security Considerations   Security issues are not discussed in this memo.References   [1]   Simpson, W., "The Point-to-Point Protocol",RFC 1331, May 1992.   [2]   McCloghrie, K., and M. Rose, "Management Information Base for         Network Management of TCP/IP-based internets: MIB-II",RFC1213, March 1991.   [3]   Rose, M., and K. McCloghrie, "Structure and Identification of         Management Information for TCP/IP-based Internets",RFC 1155,         May 1990.Acknowledgments   Some of the text in this document is taken fromRFC 1172, by Drew   Perkins of Carnegie Mellon University, and by Russ Hobby of the   University of California at Davis.   Special thanks to Craig Fox (Network Systems), and Karl Fox (Morning   Star Technologies), for design suggestions based on implementation   experience.Simpson                                                        [Page 14]

RFC 1333              PPP Link Quality Monitoring               May 1992Chair's Address   The working group can be contacted via the current chair:      Brian Lloyd      Lloyd & Associates      3420 Sudbury Road      Cameron Park, California 95682      Phone: (916) 676-1147      EMail: brian@ray.lloyd.comAuthor's Address   Questions about this memo can also be directed to:      William Allen Simpson      Daydreamer      Computer Systems Consulting Services      P O Box 6205      East Lansing, MI  48826-6025      EMail: bsimpson@ray.lloyd.comSimpson                                                        [Page 15]