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rfc:rfc2894

Network Working Group M. Crawford Request for Comments: 2894 Fermilab Category: Standards Track August 2000

                    Router Renumbering for IPv6

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.

IESG Note:

 This document defines mechanisms for informing a set of routers of
 renumbering operations they are to perform, including a mode of
 operation in environments in which the exact number of routers is
 unknown. Reliably informing all routers when the actual number of
 routers is unknown is a difficult problem. Implementation and
 operational experience will be needed to fully understand the
 applicabilty and scalability aspects of the mechanisms defined in
 this document when the number of routers is unknown.

Abstract

 IPv6 Neighbor Discovery and Address Autoconfiguration conveniently
 make initial assignments of address prefixes to hosts.  Aside from
 the problem of connection survival across a renumbering event, these
 two mechanisms also simplify the reconfiguration of hosts when the
 set of valid prefixes changes.
 This document defines a mechanism called Router Renumbering ("RR")
 which allows address prefixes on routers to be configured and
 reconfigured almost as easily as the combination of Neighbor
 Discovery and Address Autoconfiguration works for hosts.  It provides
 a means for a network manager to make updates to the prefixes used by
 and advertised by IPv6 routers throughout a site.

Crawford Standards Track [Page 1] RFC 2894 Router Renumbering for IPv6 August 2000

Table of Contents

 1.  Functional Overview .......................................    2
 2.  Definitions ...............................................    4
     2.1.  Terminology .........................................    4
     2.2.  Requirements ........................................    5
 3.  Message Format ............................................    5
     3.1.  Router Renumbering Header ...........................    7
     3.2.  Message Body -- Command Message .....................    9
         3.2.1.  Prefix Control Operation ......................    9
             3.2.1.1.  Match-Prefix Part .......................    9
             3.2.1.2.  Use-Prefix Part .........................   11
     3.3.  Message Body -- Result Message ......................   12
 4.  Message Processing ........................................   14
     4.1.  Header Check ........................................   14
     4.2.  Bounds Check ........................................   15
     4.3.  Execution ...........................................   16
     4.4.  Summary of Effects ..................................   17
 5.  Sequence Number Reset .....................................   18
 6.  IANA Considerations .......................................   19
 7.  Security Considerations ...................................   19
     7.1.  Security Policy and Association Database Entries ....   19
 8.  Implementation and Usage Advice for Reliability ...........   20
     8.1.  Outline and Definitions .............................   21
     8.2.  Computations ........................................   23
     8.3.  Additional Assurance Methods ........................   24
 9.  Usage Examples ............................................   25
     9.1.  Maintaining Global-Scope Prefixes ...................   25
     9.2.  Renumbering a Subnet ................................   26
 10.  Acknowledgments ..........................................   27
 11.  References ...............................................   28
 12.  Author's Address .........................................   29
 Appendix -- Derivation of Reliability Estimates ...............   30
 Full Copyright Statement ......................................   32

1. Functional Overview

 Router Renumbering Command packets contain a sequence of Prefix
 Control Operations (PCOs).  Each PCO specifies an operation, a
 Match-Prefix, and zero or more Use-Prefixes.  A router processes each
 PCO in sequence, checking each of its interfaces for an address or
 prefix which matches the Match-Prefix.  For every interface on which
 a match is found, the operation is applied.  The operation is one of
 ADD, CHANGE, or SET-GLOBAL to instruct the router to respectively add
 the Use-Prefixes to the set of configured prefixes, remove the prefix
 which matched the Match-Prefix and replace it with the Use-Prefixes,

Crawford Standards Track [Page 2] RFC 2894 Router Renumbering for IPv6 August 2000

 or replace all global-scope prefixes with the Use-Prefixes.  If the
 set of Use-Prefixes in the PCO is empty, the ADD operation does
 nothing and the other two reduce to deletions.
 Additional information for each Use-Prefix is included in the Prefix
 Control Operation: the valid and preferred lifetimes to be included
 in Router Advertisement Prefix Information Options [ND], and either
 the L and A flags for the same option, or an indication that they are
 to be copied from the prefix that matched the Match-Prefix.
 It is possible to instruct routers to create new prefixes by
 combining the Use-Prefixes in a PCO with some portion of the existing
 prefix which matched the Match-Prefix.  This simplifies certain
 operations which are expected to be among the most common.  For every
 Use-Prefix, the PCO specifies a number of bits which should be copied
 from the existing address or prefix which matched the Match-Prefix
 and appended to the use-prefix prior to configuring the new prefix on
 the interface.  The copied bits are zero or more bits from the
 positions immediately after the length of the Use- Prefix.  If
 subnetting information is in the same portion of the old and new
 prefixes, this synthesis allows a single Prefix Control Operation to
 define a new global prefix on every router in a site, while
 preserving the subnetting structure.
 Because of the power of the Router Renumbering mechanism, each RR
 message includes a sequence number to guard against replays, and is
 required to be authenticated and integrity-checked.  Each single
 Prefix Control Operation is idempotent and so could be retransmitted
 for improved reliability, as long as the sequence number is current,
 without concern about multiple processing.  However, non-idempotent
 combinations of PCOs can easily be constructed and messages
 containing such combinations could not be safely reprocessed.
 Therefore, all routers are required to guard against processing an RR
 message more than once.  To allow reliable verification that Commands
 have been received and processed by routers, a mechanism for
 duplicate-command notification to the management station is included.
 Possibly a network manager will want to perform more renumbering, or
 exercise more detailed control, than can be expressed in a single
 Router Renumbering packet on the available media.  The RR mechanism
 is most powerful when RR packets are multicast, so IP fragmentation
 is undesirable.  For these reasons, each RR packet contains a
 "Segment Number".  All RR packets which have a Sequence Number
 greater than or equal to the highest value seen are valid and must be
 processed.  However, a router must keep track of the Segment Numbers
 of RR messages already processed and avoid reprocessing a message

Crawford Standards Track [Page 3] RFC 2894 Router Renumbering for IPv6 August 2000

 whose Sequence Number and Segment Number match a previously processed
 message.  (This list of processed segment numbers is reset when a new
 highest Sequence Number is seen.)
 The Segment Number does not impose an ordering on packet processing.
 If a specific sequence of operations is desired, it may be achieved
 by ordering the PCOs in a single RR Command message or through the
 Sequence Number field.
 There is a "Test" flag which indicates that all routers should
 simulate processing of the RR message and not perform any actual
 reconfiguration.  A separate "Report" flag instructs routers to send
 a Router Renumbering Result message back to the source of the RR
 Command message indicating the actual or simulated result of the
 operations in the RR Command message.
 The effect or simulated effect of an RR Command message may also be
 reported to network management by means outside the scope of this
 document, regardless of the value of the "Report" flag.

2. Definitions

2.1. Terminology

 Address
    This term always refers to a 128-bit IPv6 address [AARCH].  When
    referring to bits within an address, they are numbered from 0 to
    127, with bit 0 being the first bit of the Format Prefix.
 Prefix
    A prefix can be understood as an address plus a length, the latter
    being an integer in the range 0 to 128 indicating how many leading
    bits are significant.  When referring to bits within a prefix,
    they are numbered in the same way as the bits of an address.  For
    example, the significant bits of a prefix whose length is L are
    the bits numbered 0 through L-1, inclusive.
 Match
    An address A "matches" a prefix P whose length is L if the first L
    bits of A are identical with the first L bits of P.  (Every
    address matches a prefix of length 0.)  A prefix P1 with length L1
    matches a prefix P2 of length L2 if L1 >= L2 and the first L2 bits
    of P1 and P2 are identical.

Crawford Standards Track [Page 4] RFC 2894 Router Renumbering for IPv6 August 2000

 Prefix Control Operation
    This is the smallest individual unit of Router Renumbering
    operation.  A Router Renumbering Command packet includes zero or
    more of these, each comprising one matching condition, called a
    Match-Prefix Part, and zero or more substitution specifications,
    called Use-Prefix Parts.
 Match-Prefix
    This is a Prefix against which a router compares the addresses and
    prefixes configured on its interfaces.
 Use-Prefix
    The prefix and associated information which is to be configured on
    a router interface when certain conditions are met.
 Matched Prefix
    The existing prefix or address which matched a Match-Prefix.
 New Prefix
    A prefix constructed from a Use-Prefix, possibly including some of
    the Matched Prefix.
 Recorded Sequence Number
    The highest sequence number found in a valid message MUST be
    recorded in non-volatile storage.
    Note that "matches" is a transitive relation but not symmetric.
    If two prefixes match each other, they are identical.

2.2. Requirements

 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 [KWORD].

3. Message Format

 There are two types of Router Renumbering messages: Commands, which
 are sent to routers, and Results, which are sent by routers.  A third
 message type is used to synchronize a reset of the Recorded Sequence
 Number with the cancellation of cryptographic keys.  The three types
 of messages are distinguished the ICMPv6 "Code" field and differ in
 the contents of the "Message Body" field.

Crawford Standards Track [Page 5] RFC 2894 Router Renumbering for IPv6 August 2000

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 /                IPv6 header, extension headers                 /
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 /                 ICMPv6 & RR Header (16 octets)                /
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 /                       RR Message Body                         /
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Router Renumbering Message Format
 Router Renumbering messages are carried in ICMPv6 packets with Type =
 138.  The RR message comprises an RR Header, containing the ICMPv6
 header, the sequence and segment numbers and other information, and
 the RR Message Body, of variable length.
 All fields marked "reserved" or "res" MUST be set to zero on
 generation of an RR message, and ignored on receipt.
 All implementations which generate Router Renumbering Command
 messages MUST support sending them to the All Routers multicast
 address with link and site scopes, and to unicast addresses of link-
 local and site-local formats.  All routers MUST be capable of
 receiving RR Commands sent to those multicast addresses and to any of
 their link local and site local unicast addresses.  Implementations
 SHOULD support sending and receiving RR messages addressed to other
 unicast addresses.  An implementation which is both a sender and
 receiver of RR commands SHOULD support use of the All Routers
 multicast address with node scope.
 Data authentication and message integrity MUST be provided for all
 Router Renumbering Command messages by appropriate IP Security
 [IPSEC] means.  The integrity assurance must include the IPv6
 destination address and the RR Header and Message Body.  See section
 7, "Security Considerations".
 The use of authentication for Router Renumbering Result messages is
 RECOMMENDED.

Crawford Standards Track [Page 6] RFC 2894 Router Renumbering for IPv6 August 2000

3.1. Router Renumbering Header

  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      |     Code      |            Checksum           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        SequenceNumber                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | SegmentNumber |     Flags     |            MaxDelay           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           reserved                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Type        138 (decimal), the ICMPv6 type value assigned to Router
             Renumbering
 Code          0 for a Router Renumbering Command
               1 for a Router Renumbering Result
             255 for a Sequence Number Reset.
             The Sequence Number Reset is described in section 5.
 Checksum    The ICMPv6 checksum, as specified in [ICMPV6].  The
             checksum covers the IPv6 pseudo-header and all fields of
             the RR message from the Type field onward.
 SequenceNumber
             An unsigned 32-bit sequence number.  The sequence number
             MUST be non-decreasing between Sequence Number Resets.
 SegmentNumber
             An unsigned 8-bit field which enumerates different valid
             RR messages having the same SequenceNumber.  No ordering
             among RR messages is imposed by the SegmentNumber.
 Flags       A combination of one-bit flags.  Five are defined and
             three bits are reserved.
                                +-+-+-+-+-+-+-+-+
                                |T|R|A|S|P| res |
                                +-+-+-+-+-+-+-+-+

Crawford Standards Track [Page 7] RFC 2894 Router Renumbering for IPv6 August 2000

            The flags T, R, A and S have defined meanings in an RR
            Command message.  In a Result message they MUST be
            copied from the corresponding Command.  The P flag is
            meaningful only in a Result message and MUST be zero in
            a transmitted Command and ignored in a received Command.
            T   Test command --
                0 indicates that the router configuration is to be
                  modified;
                1 indicates a "Test" message: processing is to be
                  simulated and no configuration changes are to be
                  made.
            R   Result requested --
                0 indicates that a Result message MUST NOT be sent
                  (but other forms of logging are not precluded);
                1 indicates that the router MUST send a Result
                  message upon completion of processing the Command
                  message;
            A   All interfaces --
                0 indicates that the Command MUST NOT be applied to
                  interfaces which are administratively shut down;
                1 indicates that the Command MUST be applied to all
                  interfaces regardless of administrative shutdown
                  status.
            S   Site-specific -- This flag MUST be ignored unless
                the router treats interfaces as belonging to
                different "sites".
                0 indicates that the Command MUST be applied to
                  interfaces regardless of which site they belong
                  to;
                1 indicates that the Command MUST be applied only to
                  interfaces which belong to the same site as the
                  interface to which the Command is addressed.  If
                  the destination address is appropriate for
                  interfaces belonging to more than one site, then
                  the Command MUST be applied only to interfaces
                  belonging to the same site as the interface on
                  which the Command was received.
            P   Processed previously --
                0 indicates that the Result message contains the
                  complete report of processing the Command;

Crawford Standards Track [Page 8] RFC 2894 Router Renumbering for IPv6 August 2000

                1 indicates that the Command message was previously
                  processed (and is not a Test) and the responding
                  router is not processing it again.  This Result
                  message MAY have an empty body.
 MaxDelay   An unsigned 16-bit field specifying the maximum time, in
            milliseconds, by which a router MUST delay sending any
            reply to this Command.  Implementations MAY generate the
            random delay between 0 and MaxDelay milliseconds with a
            finer granularity than 1ms.

3.2. Message Body – Command Message

 The body of an RR Command message is a sequence of zero or more
 Prefix Control Operations, each of variable length.  The end of the
 sequence MAY be inferred from the IPv6 length and the lengths of
 extension headers which precede the ICMPv6 header.

3.2.1. Prefix Control Operation

 A Prefix Control Operation has one Match-Prefix Part of 24 octets,
 followed by zero or more Use-Prefix Parts of 32 octets each.

3.2.1.1. Match-Prefix Part

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    OpCode     |   OpLength    |    Ordinal    |   MatchLen    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    MinLen     |    MaxLen     |           reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-                                                             -+
 |                                                               |
 +-                         MatchPrefix                         -+
 |                                                               |
 +-                                                             -+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 OpCode      An unsigned 8-bit field specifying the operation to be
             performed when the associated MatchPrefix matches an
             interface's prefix or address.  Values are:
             1    the ADD operation

Crawford Standards Track [Page 9] RFC 2894 Router Renumbering for IPv6 August 2000

             2    the CHANGE operation
             3    the SET-GLOBAL operation
 OpLength    The total length of this Prefix Control Operation, in
             units of 8 octets.  A valid OpLength will always be of
             the form 4N+3, with N equal to the number of UsePrefix
             parts (possibly zero).
 Ordinal     An 8-bit field which MUST have a different value in each
             Prefix Control Operation contained in a given RR Command
             message.  The value is otherwise unconstrained.
 MatchLen    An 8-bit unsigned integer between 0 and 128 inclusive
             specifying the number of initial bits of MatchPrefix
             which are significant in matching.
 MinLen      An 8-bit unsigned integer specifying the minimum length
             which any configured prefix must have in order to be
             eligible for testing against the MatchPrefix.
 MaxLen      An 8-bit unsigned integer specifying the maximum length
             which any configured prefix may have in order to be
             eligible for testing against the MatchPrefix.
 MatchPrefix The 128-bit prefix to be compared with each interface's
             prefix or address.

Crawford Standards Track [Page 10] RFC 2894 Router Renumbering for IPv6 August 2000

3.2.1.2. Use-Prefix Part

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    UseLen     |    KeepLen    |   FlagMask    |    RAFlags    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Valid Lifetime                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Preferred Lifetime                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |V|P|                         reserved                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-                                                             -+
 |                                                               |
 +-                          UsePrefix                          -+
 |                                                               |
 +-                                                             -+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 UseLen      An 8-bit unsigned integer less than or equal to 128
             specifying the number of initial bits of UsePrefix to
             use in creating a new prefix for an interface.
 KeepLen     An 8-bit unsigned integer less than or equal to (128-
             UseLen) specifying the number of bits of the prefix or
             address which matched the associated Match-Prefix which
             should be retained in the new prefix.  The retained bits
             are those at positions UseLen through (UseLen+KeepLen-1)
             in the matched address or prefix, and they are copied to
             the same positions in the New Prefix.
 FlagMask    An 8-bit mask.  A 1 bit in any position means that the
             corresponding flag bit in a Router Advertisement (RA)
             Prefix Information Option for the New Prefix should be
             set from the RAFlags field in this Use-Prefix Part.  A 0
             bit in the FlagMask means that the RA flag bit for the
             New Prefix should be copied from the corresponding RA
             flag bit of the Matched Prefix.
 RAFlags     An 8 bit field which, under control of the FlagMask
             field, may be used to initialize the flags in Router
             Advertisement Prefix Information Options [ND] which
             advertise the New Prefix.  Note that only two flags have

Crawford Standards Track [Page 11] RFC 2894 Router Renumbering for IPv6 August 2000

             defined meanings to date: the L (on-link) and A
             (autonomous configuration) flags.  These flags occupy
             the two leftmost bit positions in the RAFlags field,
             corresponding to their position in the Prefix
             Information Option.
 Valid Lifetime
             A 32-bit unsigned integer which is the number of seconds
             for which the New Prefix will be valid [ND, SAA].
 Preferred Lifetime
             A 32-bit unsigned integer which is the number of seconds
             for which the New Prefix will be preferred [ND, SAA].
 V           A 1-bit flag indicating that the valid lifetime of the
             New Prefix MUST be effectively decremented in real time.
 P           A 1-bit flag indicating that the preferred lifetime of
             the New Prefix MUST be effectively decremented in real
             time.
 UsePrefix   The 128-bit Use-prefix which either becomes or is used
             in forming (if KeepLen is nonzero) the New Prefix.  It
             MUST NOT have the form of a multicast or link-local
             address [AARCH].

3.3. Message Body – Result Message

 The body of an RR Result message is a sequence of zero or more Match
 Reports of 24 octets.  An RR Command message with the "R" flag set
 will elicit an RR Result message containing one Match Report for each
 Prefix Control Operation, for each different prefix it matches on
 each interface.  The Match Report has the following format.

Crawford Standards Track [Page 12] RFC 2894 Router Renumbering for IPv6 August 2000

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         reserved          |B|F|    Ordinal    |  MatchedLen   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         InterfaceIndex                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +-                                                             -+
 |                                                               |
 +-                        MatchedPrefix                        -+
 |                                                               |
 +-                                                             -+
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 B           A one-bit flag which, when set, indicates that one or
             more fields in the associated PCO were out of bounds.
             The bounds check is described in section 4.2.
 F           A one-bit flag which, when set, indicates that one or
             more Use-Prefix parts from the associated PCO were not
             honored by the router because of attempted formation of
             a forbidden prefix format, such as a multicast or
             loopback address.
 Ordinal     Copied from the Prefix Control Operation whose
             MatchPrefix matched the MatchedPrefix on the interface
             indicated by InterfaceIndex.
 MatchedLen  The length of the Matched Prefix.
 InterfaceIndex
             The router's numeric designation of the interface on
             which the MatchedPrefix was configured.  This MUST be
             the same as the value of ipv6IfIndex which designates
             that index in the SNMP IPv6 MIB General Group [IPV6MIB].
 It is possible for a Result message to be larger than the Command
 message which elicited it.  Such a Result message may have to be
 fragmented for transmission.  If so, it SHOULD be fragmented to the
 IPv6 minimum required MTU [IPV6].

Crawford Standards Track [Page 13] RFC 2894 Router Renumbering for IPv6 August 2000

4. Message Processing

 Processing of received Router Renumbering Result messages is entirely
 implementation-defined.  Implementation of Command message processing
 may vary in detail from the procedure set forth below, so long as the
 result is not affected.
 Processing of received Router Renumbering Command messages consists
 of three conceptual parts: header check, bounds check, and execution.

4.1. Header Check

 The ICMPv6 checksum and type are presumed to have been checked before
 a Router Renumbering module receives a Command to process.  In an
 implementation environment where this may not be the case, those
 checks MUST be made at this point in the processing.
 If the ICMPv6 length derived from the IPv6 length is less than 16
 octets, the message MUST be discarded and SHOULD be logged to network
 management.
 If the ICMPv6 Code field indicates a Result message, a router which
 is not a source of RR Command messages MUST discard the message and
 SHOULD NOT log it to network management.
 If the IPv6 destination address is neither an All Routers multicast
 address [AARCH] nor one of the receiving router's unicast addresses,
 the message MUST be discarded and SHOULD be logged to network
 management.
 Next, the SequenceNumber is compared to the Recorded Sequence Number.
 (If no RR messages have been received and accepted since system
 initialization, the Recorded Sequence Number is zero.)  This
 comparison is done with the two numbers considered as unsigned
 integers, not as DNS-style serial numbers.  If the SequenceNumber is
 less than the Recorded Sequence Number, the message MUST be discarded
 and SHOULD be logged to network management.
 Finally, if the SequenceNumber in the message is greater than the
 Recorded Sequence Number or the T flag is set, skip to the bounds
 check.  Otherwise the SegmentNumber MUST now be checked.  If a
 correctly authenticated message with the same SequenceNumber and
 SegmentNumber has not already been processed, skip to the bounds
 check.  Otherwise, this Command is a duplicate and not a Test
 Command.  If the R flag is not set, the duplicate message MUST be
 discarded and SHOULD NOT be logged to network management.  If R is
 set, an RR Result message with the P flag set MUST be scheduled for
 transmission to the source address of the Command after a random time

Crawford Standards Track [Page 14] RFC 2894 Router Renumbering for IPv6 August 2000

 uniformly distributed between 0 and MaxDelay milliseconds.  The body
 of that Result message MUST either be empty or be a saved copy of the
 Result message body generated by processing of the previous message
 with the same SequenceNumber and SegmentNumber.  After scheduling the
 Result message, the Command MUST be discarded without further
 processing.

4.2. Bounds Check

 If the SequenceNumber is greater than the Recorded Sequence Number,
 then the list of processed SegmentNumbers and the set of saved Result
 messages, if any, MUST be cleared and the Recorded Sequence Number
 MUST be updated to the value used in the current message, regardless
 of subsequent processing errors.
 Next, if the ICMPv6 Code field indicates a Sequence Number Reset,
 skip to section 5.
 At this point, if T is set in the RR header and R is not set, the
 message MAY be discarded without further processing.
 If the R flag is set, begin constructing an RR Result message.  The
 RR header of the Result message is completely determined at this time
 except for the Checksum.
 The values of the following fields of a PCO MUST be checked to ensure
 that they are within the appropriate bounds.
 OpCode      must be a defined value.
 OpLength    must be of the form 4N+3 and consistent the the length
             of the Command packet and the PCO's offset within the
             packet.
 MatchLen    must be between 0 and 128 inclusive
 UseLen, KeepLen
             in each Use-Prefix Part must be between 0 and 128
             inclusive, as must the sum of the two.
 If any of these fields are out of range in a PCO, the entire PCO MUST
 NOT be performed on any interface.  If the R flag is set in the RR
 header then add to the RR Result message a Match Report with the B
 flag set, the F flag clear, the Ordinal copied from the PCO, and all
 other fields zero.  This Match Report MUST be included only once, not
 once per interface.

Crawford Standards Track [Page 15] RFC 2894 Router Renumbering for IPv6 August 2000

 Note that MinLen and MaxLen need not be explicitly bounds checked,
 even though certain combinations of values will make any matches
 impossible.

4.3. Execution

 For each applicable router interface, as determined by the A and S
 flags, the Prefix Control Operations in an RR Command message must be
 carried out in order of appearance.  The relative order of PCO
 processing among different interfaces is not specified.
 If the T flag is set, create a copy of each interface's configuration
 on which to operate, because the results of processing a PCO may
 affect the processing of subsequent PCOs.  Note that if all
 operations are performed on one interface before proceeding to
 another interface, only one interface-configuration copy will be
 required at a time.
 For each interface and for each Prefix Control Operation, each prefix
 configured on that interface with a length between the MinLen and
 MaxLen values in the PCO is tested to determine whether it matches
 (as defined in section 2.1) the MatchPrefix of the PCO.  The
 configured prefixes are tested in an arbitrary order.  Any new prefix
 configured on an interface by the effect of a given PCO MUST NOT be
 tested against that PCO, but MUST be tested against all subsequent
 PCOs in the same RR Command message.
 Under a certain condition the addresses on an interface are also
 tested to see whether any of them matches the MatchPrefix.  If and
 only if a configured prefix "P" does have a length between MinLen and
 MaxLen inclusive, does not match the MatchPrefix "M", but M does
 match P (this can happen only if M is longer than P), then those
 addresses on that interface which match P MUST be tested to determine
 whether any of them matches M.  If any such address does match M,
 process the PCO as if P matched M, but when forming New Prefixes, if
 KeepLen is non-zero, bits are copied from the address.  This special
 case allows a PCO to be easily targeted to a single specific
 interface in a network.
 If P does not match M, processing is finished for this combination of
 PCO, interface and prefix.  Continue with another prefix on the same
 interface if there are any more prefixes which have not been tested
 against this PCO and were not created by the action of this PCO.  If
 no such prefixes remain on the current interface, continue processing
 with the next PCO on the same interface, or with another interface.

Crawford Standards Track [Page 16] RFC 2894 Router Renumbering for IPv6 August 2000

 If P does match M, either directly or because a configured address
 which matches P also matches M, then P is the Matched Prefix.
 Perform the following steps.
    If the Command has the R flag set, add a Match Report to the
    Result message being constructed.
    If the OpCode is CHANGE, mark P for deletion from the current
    interface.
    If the OpCode is SET-GLOBAL, mark all global-scope prefixes on the
    current interface for deletion.
    If there are any Use-Prefix parts in the current PCO, form the New
    Prefixes.  Discard any New Prefix which has a forbidden format,
    and if the R flag is set in the command, set the F flag in the
    Match Report for this PCO and interface.  Forbidden prefix formats
    include, at a minimum, multicast, unspecified and loopback
    addresses.  [AARCH]  Any implementation MAY forbid, or allow the
    network manager to forbid other formats as well.
    For each New Prefix which is already configured on the current
    interface, unmark that prefix for deletion and update the
    lifetimes and RA flags.  For each New Prefix which is not already
    configured, add the prefix and, if appropriate, configure an
    address with that prefix.
    Delete any prefixes which are still marked for deletion, together
    with any addresses which match those prefixes but do not match any
    prefix which is not marked for deletion.
    After processing all the Prefix Control Operations on all the
    interfaces, an implementation MUST record the SegmentNumber of the
    packet in a list associated with the SequenceNumber.
    If the Command has the R flag set, compute the Checksum and
    schedule the Result message for transmission after a random time
    interval uniformly distributed between 0 and MaxDelay
    milliseconds.  This interval SHOULD begin at the conclusion of
    processing, not the beginning.  A copy of the Result message MAY
    be saved to be retransmitted in response to a duplicate Command.

4.4. Summary of Effects

 The only Neighbor Discovery [ND] parameters which can be affected by
 Router Renumbering are the following.

Crawford Standards Track [Page 17] RFC 2894 Router Renumbering for IPv6 August 2000

    A router's addresses and advertised prefixes, including the prefix
    lengths.
    The flag bits (L and A, and any which may be defined in the
    future) and the valid and preferred lifetimes which appear in a
    Router Advertisement Prefix Information Option.
    That unnamed property of the lifetimes which specifies whether
    they are fixed values or decrementing in real time.
 Other internal router information, such as the time until the next
 unsolicited Router Advertisement or MIB variables MAY be affected as
 needed.
 All configuration changes resulting from Router Renumbering SHOULD be
 saved to non-volatile storage where this facility exists.  The
 problem of properly restoring prefix lifetimes from non-volatile
 storage exists independently of Router Renumbering and deserves
 careful attention, but is outside the scope of this document.

5. Sequence Number Reset

 It may prove necessary in practice to reset a router's Recorded
 Sequence Number.  This is a safe operation only when all
 cryptographic keys previously used to authenticate RR Commands have
 expired or been revoked.  For this reason, the Sequence Number Reset
 message is defined to accomplish both functions.
 When a Sequence Number Reset (SNR) has been authenticated and has
 passed the header check, the router MUST invalidate all keys which
 have been used to authenticate previous RR Commands, including the
 key which authenticated the SNR itself.  Then it MUST discard any
 saved RR Result messages, clear the list of recorded SegmentNumbers
 and reset the Recorded Sequence Number to zero.
 If the router has no other, unused authentication keys already
 available for Router Renumbering use it SHOULD establish one or more
 new valid keys.  The details of this process will depend on whether
 manual keying or a key management protocol is used.  In either case,
 if no keys are available, no new Commands can be processed.
 A SNR message SHOULD contain no PCOs, since they will be ignored.  If
 and only if the R flag is set in the SNR message, a router MUST
 respond with a Result Message containing no Match Reports.  The
 header and transmission of the Result are as described in section 3.
 The invalidation of authentication keys caused by a valid SNR message
 will cause retransmitted copies of that message to be ignored.

Crawford Standards Track [Page 18] RFC 2894 Router Renumbering for IPv6 August 2000

6. IANA Considerations

 Following the policies outlined in [IANACON], new values of the Code
 field in the Router Renumbering Header (section 3.1) and the OpCode
 field of the Match-Prefix Part (section 3.2.1.1) are to be allocated
 by IETF consensus only.

7. Security Considerations

 The Router Renumbering mechanism proposed here is very powerful and
 prevention of spoofing it is important.  Replay of old messages must,
 in general, be prevented (even though a narrow class of messages
 exists for which replay would be harmless).  What constitutes a
 sufficiently strong authentication algorithm may change from time to
 time, but algorithms should be chosen which are strong against
 current key-recovery and forgery attacks.
 Authentication keys must be as well protected as any other access
 method that allows reconfiguration of a site's routers.  Distribution
 of keys must not expose them or permit alteration, and key validity
 must be limited in terms of time and number of messages
 authenticated.
 Note that although a reset of the Recorded Sequence Number requires
 the cancellation of previously-used authentication keys, introduction
 of new keys and expiration of old keys does not require resetting the
 Recorded Sequence Number.

7.1. Security Policy and Association Database Entries

 The Security Policy Database (SPD) [IPSEC] of a router implementing
 this specification MUST cause incoming Router Renumbering Command
 packets to either be discarded or have IPsec applied.  (The
 determination of "discard" or "apply" MAY be based on the source
 address.)  The resulting Security Association Database (SAD) entries
 MUST ensure authentication and integrity of the destination address
 and the RR Header and Message Body, and the body length implied by
 the IPv6 length and intervening extension headers.  These
 requirements are met by the use of the Authentication Header [AH] in
 transport or tunnel mode, or the Encapsulating Security Payload [ESP]
 in tunnel mode with non-NULL authentication.  The mandatory-to-
 implement IPsec authentication algorithms (other than NULL) seem
 strong enough for Router Renumbering at the time of this writing.
 Note that for the SPD to distinguish Router Renumbering from other
 ICMP packets requires the use of the ICMP Type field as a selector.
 This is consistent with, although not mentioned by, the Security
 Architecture specification [IPSEC].

Crawford Standards Track [Page 19] RFC 2894 Router Renumbering for IPv6 August 2000

 At the time of this writing, there exists no multicast key management
 protocol for IPsec and none is on the horizon.  Manually configured
 Security Associations will therefore be common.  The occurrence of
 "from traffic" in the table below would therefore more realistically
 be a wildcard or a fixed range.  Use of a small set of shared keys
 per management station suffices, so long as key distribution and
 storage are sufficiently safeguarded.
 A sufficient set of SPD entries for incoming traffic could select
    Field         SPD Entry           SAD Entry
    -------       ---------           ---------
    Source        wildcard            from traffic
    Destination   wildcard            from SPD
    Transport     ICMPv6              from SPD
    ICMP Type     Rtr. Renum.         from SPD
    Action        Apply IPsec
    SA Spec       AH/Transport Mode
 or there might be an entry for each management station and/or for
 each of the router's unicast addresses and for each of the defined
 All-Routers multicast addresses, and a final wildcard entry to
 discard all other incoming RR messages.
 The SPD and SAD are conceptually per-interface databases.  This fact
 may be exploited to permit shared management of a border router, for
 example, or to discard all Router Renumbering traffic arriving over
 tunnels.

8. Implementation and Usage Advice for Reliability

 Users of Router Renumbering will want to be sure that every non-
 trivial message reaches every intended router.  Well-considered
 exploitation of Router Renumbering's retransmission and response-
 directing features should make that goal achievable with high
 confidence even in a minimally reliable network.
 In one set of cases, probably the majority, the network management
 station will know the complete set of routers under its control.
 Commands can be retransmitted, with the "R" (Reply-requested) flag
 set in the RR header, until Results have been collected from all
 routers.  If unicast Security Associations (or the means for creating
 them) are available, the management station may switch from multicast
 to unicast transmission when the number of routers still unheard-from
 is suitably small.

Crawford Standards Track [Page 20] RFC 2894 Router Renumbering for IPv6 August 2000

 To maintain a list of managed routers, the management station can
 employ any of several automatic methods which may be more convenient
 than manual entry in a large network.  Multicast RR "Test" commands
 can be sent periodically and the results archived, or the management
 station can use SNMP to "peek" into a link-state routing protocol
 such as OSPF [OSPFMIB].  (In the case of OSPF, roughly one router per
 area would need to be examined to build a complete list of routers.)
 In a large dynamic network where the set of managed routers is not
 known but reliable execution is desired, a scalable method for
 achieving confidence in delivery is described here.  Nothing in this
 section affects the format or content of Router Renumbering messages,
 nor their processing by routers.
 A management station implementing these reliability mechanisms MUST
 alert an operator who attempts to commence a set of Router
 Renumbering Commands when retransmission of a previous set is not yet
 completed, but SHOULD allow the operator to override the warning.

8.1. Outline and Definitions

 The set of routers being managed with Router Renumbering is
 considered as a set of populations, each population having a
 characteristic probability of successful round-trip delivery of a
 Command/Result pair.  The goal is to estimate a lower bound, P, on
 the round-trip probability for the whole set.  With this estimate and
 other data about the responses to retransmissions of the Command, a
 confidence level can be computed for hypothesis that all routers have
 been heard from.
 If the true probability of successful round-trip communication with a
 managed router were a constant, p, for all managed routers then an
 estimate P of p could be derived from either of these statistics:
    The expected ratio of the number of routers first heard from after
    transmission (N + 1) to the number first heard from after N is
    (1 - p).
    When N different routers have been heard from after M
    transmissions of a Command, the expected total number of Result
    messages received is pNM.  If R is the number of Results actually
    received, then P = R/MN.
 The two methods are not equivalent.  The first suffers numerical
 problems when the number of routers still to be heard from gets
 small, so the P = R/MN estimate should be used.

Crawford Standards Track [Page 21] RFC 2894 Router Renumbering for IPv6 August 2000

 Since the round-trip probability is not expected to be uniform in the
 real world, and the less-reliable units are more important to a
 lower-bound estimate but more likely to be missed in sampling, the
 sample from which P is computed is biased toward the less-reliable
 routers.  After the Nth transmission interval, N > 2, neglect all
 routers heard from in intervals 1 through F from the reliability
 estimate, where F is the greatest integer less than one-half of N.
 For example, after five intervals, only routers first heard from in
 the third through fifth intervals will be counted.
 A management station implementing the methods of this section should
 allow the user to specify the following parameters, and default them
 to the indicated values.
 Ct      The target delivery confidence, default 0.999.
 Pp      A presumptive, pessimistic initial estimate of the lower
         bound of the round-trip probability, P, to prevent early
         termination.  (See below.)  Default 0.75.
 Ti      The initial time between Command retransmissions.  Default 4
         seconds.  MaxDelay milliseconds (see section 3.1) must be
         added to the retransmission timer.  Knowledge of the
         routers' processing time for RR Commands may influence the
         setting of Ti.  Ti+MaxDelay is also the minimum time the
         management station must wait for Results after each
         transmission before computing a new confidence level.  The
         phrase "end of the Nth interval" means a time Ti+MaxDelay
         after the Nth transmission of a Command.
 Tu      The upper bound on the period between Command
         retransmissions.  Default 512 seconds.
 The following variables, some a function of the retransmission
 counter N, are used in the next section.
 T(N)    The time between Command transmissions N and N+1 is V*T(N) +
         MaxDelay, where V is random and roughly uniform in the range
         [0.75, 1.0].  T(1) = Ti and for N > 1, T(N) = min(2*T(N-1),
         Tu).
 M(N)    The cumulative number of distinct routers from which replies
         have been received to any of the first N transmissions of
         the Command.

Crawford Standards Track [Page 22] RFC 2894 Router Renumbering for IPv6 August 2000

 F=F(N)  FLOOR((N-1)/2).  All routers from which responses were
         received in the first F intervals will be effectively
         omitted from the estimate of the round-trip probability
         computed at the Nth interval.
 R(N,F)  The total number of RR Result messages, including
         duplicates, received by the end of the Nth interval from
         those routers which were NOT heard from in any of the first
         F intervals.
 p(N)    The estimate of the worst-case round-trip delivery
         probability.
 c(N)    The computed confidence level.
 An asterisk (*) is used to denote multiplication and a caret (^)
 denotes exponentiation.
 If the difference in reliability between the "good" and "bad" parts
 of a managed network is very great, early c(N) values will be too
 high.  Retransmissions should continue for at least Nmin = log(1-
 Ct)/log(1-Pp) intervals, regardless of the current confidence
 estimate.  (In fact, there's no need to compute p(N) and c(N) until
 after Nmin intervals.)

8.2. Computations

 Letting A = N*(M(N)-M(F))/R(N,F) for brevity, the estimate of the
 round-trip delivery probability is p(N) = 1-Q, where Q is that root
 of the equation
      Q^N - A*Q + (A-1) = 0
 which lies between 0 and 1.  (Q = 1 is always a root.  If N is odd
 there is also a negative root.)  This may be solved numerically, for
 example with Newton's method (see any standard text, for example
 [ANM]).  The first-order approximation
      Q1 = 1 - 1/A
 may be used as a starting point for iteration.  But Q1 should NOT be
 used as an approximate solution as it always underestimates Q, and
 hence overestimates p(N), which would cause an overestimate of the
 confidence level.
 If necessary, the spurious root Q = 1 can be divided out, leaving
      Q^(N-1) + Q^(N-2) + ... + Q - (A-1) = 0

Crawford Standards Track [Page 23] RFC 2894 Router Renumbering for IPv6 August 2000

 as the equation to solve.  Depending on the numerical method used,
 this could be desirable as it's just possible (but very unlikely)
 that A=N and so Q=1 was a double root of the earlier equation.
 After N > 2 (or N >= Nmin) intervals have been completed, Compute the
 lower-bound reliability estimate
      p(N) = R(N,F)/((N-F)*(M(N) - M(F))).
 Compute the confidence estimate
      c(N) = (1 - (1-p(N))^N)^(M(N) - M(F) + 1).
 which is the Bayesian probability that M(N) is the number of routers
 present given the number of responses which were collected, as
 opposed to M(N)+1 or any greater number.  It is assumed that the a
 priori probability of there being K routers was no greater than that
 of K-1 routers, for all K > M(N).
 When c(N) >= Ct and N >= Nmin, retransmissions of the Command may
 cease.  Otherwise another transmission should be scheduled at a time
 V*T(N) + MaxDelay after the previous (Nth) transmission, or V*T(N)
 after the conclusion of processing responses to the Nth transmission,
 whichever is later.
 One corner case needs consideration.  Divide-by-zero may occur when
 computing p.  This can happen only when no new routers have been
 heard from in the last N-F intervals.  Generally, the confidence
 estimate c(N) will be close to unity by then, but in a pathological
 case such as a large number of routers with reliable communication
 and a much smaller number with very poor communication, the
 confidence estimate may still be less than Ct when p's denominator
 vanishes.  The implementation may continue, and should continue if
 the minimum number of transmissions given in the previous paragraph
 have not yet been made.  If new routers are heard from, p(N) will
 again be non-singular.
 Of course no limited retransmission scheme can fully address the
 possibility of long-term problems, such as a partitioned network.
 The network manager is expected to be aware of such conditions when
 they exist.

8.3. Additional Assurance Methods

 As a final means to detect routers which become reachable after
 missing renumbering commands during an extended network split, a
 management station MAY adopt the following strategy.  When performing
 each new operation, increment the SequenceNumber by more than one.

Crawford Standards Track [Page 24] RFC 2894 Router Renumbering for IPv6 August 2000

 After the operation is believed complete, periodically send some
 "no-op" RR Command with the R (Result Requested) flag set and a
 SequenceNumber one less than the highest used.  Any responses to such
 a command can only come from router that missed the last operation.
 An example of a suitable "no-op" command would be an ADD operation
 with MatchLen = 0, MinLen = 0, MaxLen = 128, and no Use-Prefix Parts.
 If old authentication keys are saved by the management station, even
 the reappearance of routers which missed a Sequence Number Reset can
 be detected by the transmission of no-op commands with the invalid
 key and a SequenceNumber higher than any used before the key was
 invalidated.  Since there is no other way for a management station to
 distinguish a router's failure to receive an entire sequence of
 repeated SNR messages from the loss of that router's single SNR
 Result Message, this is the RECOMMENDED way to test for universal
 reception of a SNR Command.

9. Usage Examples

 This section sketches some sample applications of Router Renumbering.
 Extension headers, including required IPsec headers, between the IPv6
 header and the ICMPv6 header are not shown in the examples.

9.1. Maintaining Global-Scope Prefixes

 A simple use of the Router Renumbering mechanism, and one which is
 expected to to be common, is the maintenance of a set of global
 prefixes with a subnet structure that matches that of the site's
 site-local address assignments.  In the steady state this would serve
 to keep the Preferred and Valid lifetimes set to their desired
 values.  During a renumbering transition, similar Command messages
 can add new prefixes and/or delete old ones.  An outline of a
 suitable Command message follows.  Fields not listed are presumed set
 to suitable values.  This Command assumes all router interfaces to be
 maintained already have site-local [AARCH] addresses.
 IPv6 Header
    Next Header = 58 (ICMPv6)
    Source Address = (Management Station)
    Destination Address = FF05::2 (All Routers, site-local scope)
 ICMPv6/RR Header
    Type = 138 (Router Renumbering), Code = 0 (Command)
    Flags = 60 hex (R, A)

Crawford Standards Track [Page 25] RFC 2894 Router Renumbering for IPv6 August 2000

 First (and only) PCO:
    Match-Prefix Part
        OpCode = 3 (SET-GLOBAL)
        OpLength = 4 N + 3 (assuming N global prefixes)
        Ordinal = 0 (arbitrary)
        MatchLen = 10
        MatchPrefix = FEC0::0
    First Use-Prefix Part
        UseLen = 48 (Length of TLA ID + RES + NLA ID [AARCH])
        KeepLen = 16 (Length of SLA (subnet) ID [AARCH])
        FlagMask, RAFlags, Lifetimes, V & P flags -- as desired
        UsePrefix = First global /48 prefix
    . . .
    Nth Use-Prefix Part
        UseLen = 48
        KeepLen = 16
        FlagMask, RAFlags, Lifetimes, V & P flags -- as desired
        UsePrefix = Last global /48 prefix
 This will cause N global prefixes to be set (or updated) on each
 applicable interface.  On each interface, the SLA ID (subnet) field
 of each global prefix will be copied from the existing site-local
 prefix.

9.2. Renumbering a Subnet

 A subnet can be gracefully renumbered by setting the valid and
 preferred timers on the old prefix to a short value and having them
 run down, while concurrently adding adding the new prefix.  Later,
 the expired prefix is deleted.  The first step is described by the
 following RR Command.
 IPv6 Header
    Next Header = 58 (ICMPv6)
    Source Address = (Management Station)
    Destination Address = FF05::2 (All Routers, site-local scope)
 ICMPv6/RR Header
    Type = 138 (Router Renumbering), Code = 0 (Command)
    Flags = 60 hex (R, A)

Crawford Standards Track [Page 26] RFC 2894 Router Renumbering for IPv6 August 2000

 First (and only) PCO:
    Match-Prefix Part
        OpCode = 2 (CHANGE)
        OpLength = 11 (reflects 2 Use-Prefix Parts)
        Ordinal = 0 (arbitrary)
        MatchLen = 64
        MatchPrefix = Old /64 prefix
    First Use-Prefix Part
        UseLen = 0
        KeepLen = 64 (this retains the old prefix value intact)
        FlagMask = 0, RAFlags = 0
        Valid Lifetime = 28800 seconds (8 hours)
        Preferred Lifetime = 7200 seconds (2 hours)
        V flag = 1, P flag = 1
        UsePrefix = 0::0
    Second Use-Prefix Part
        UseLen = 64
        KeepLen = 0
        FlagMask = 0, RAFlags = 0
        Lifetimes, V & P flags -- as desired
        UsePrefix = New /64 prefix
 The second step, deletion of the old prefix, can be done by an RR
 Command with the same Match-Prefix Part (except for an OpLength
 reduced from 11 to 3) and no Use-Prefix Parts.  Any temptation to set
 KeepLen = 64 in the second Use-Prefix Part above should be resisted,
 as it would instruct the router to sidestep address configuration.

10. Acknowledgments

 This protocol was designed by Matt Crawford based on an idea of
 Robert Hinden and Geert Jan de Groot.  Many members of the IPNG
 Working Group contributed useful comments, in particular members of
 the DIGITAL UNIX IPv6 team.  Bill Sommerfeld provided helpful IPsec
 expertise.  Relentless browbeating by various IESG members may have
 improved the final quality of this specification.

Crawford Standards Track [Page 27] RFC 2894 Router Renumbering for IPv6 August 2000

11. References

 [AARCH]   Hinden, R. and S. Deering, "IP Version 6 Addressing
           Architecture", RFC 2373, July 1998.
 [AH]      Kent, S. and R. Atkinson, "IP Authentication Header", RFC
           2402, November 1998.
 [ANM]     Isaacson, E. and H. B. Keller, "Analysis of Numerical
           Methods", John Wiley & Sons, New York, 1966.
 [ESP]     Kent, S. and R. Atkinson, "IP Encapsulating Security
           Payload (ESP)", RFC 2406, November 1998.
 [IANACON] Narten, T. and H. Alvestrand, "Guidelines for Writing an
           IANA Considerations Section in RFCs", BCP 26, RFC 2434,
           October 1998.
 [ICMPV6]  Conta, A. and S. Deering, "Internet Control Message
           Protocol (ICMPv6) for the Internet Protocol Version 6
           (IPv6)", RFC 2463, December 1998.
 [IPSEC]   Kent, S. and R. Atkinson, "Security Architecture for the
           Internet Protocol", RFC 2401, November 1998.
 [IPV6]    Deering, S. and R. Hinden, "Internet Protocol, Version 6
           (IPv6) Specification", RFC 2460, December 1998.
 [IPV6MIB] Haskin, D. and S. Onishi, "Management Information Base for
           IP Version 6: Textual Conventions and General Group", RFC
           2466, December 1998.
 [KWORD]   Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [ND]      Narten, T., Nordmark, E. and W. Simpson, "Neighbor
           Discovery for IP Version 6 (IPv6)", RFC 2461, December
           1998.
 [OSPFMIB] Baker, F. and R. Coltun, "OSPF Version 2 Management
           Information Base", RFC 1850, November 1995.

Crawford Standards Track [Page 28] RFC 2894 Router Renumbering for IPv6 August 2000

12. Author's Address

 Matt Crawford
 Fermilab MS 368
 PO Box 500
 Batavia, IL 60510
 USA
 Phone: +1 630 840 3461
 EMail: crawdad@fnal.gov

Crawford Standards Track [Page 29] RFC 2894 Router Renumbering for IPv6 August 2000

Appendix – Derivation of Reliability Estimates

 If a population S of size k is repeatedly sampled with an efficiency
 p, the expected number of members of S first discovered on the nth
 sampling is
      m = [1 - (1-p)^n] * k
 The expected total number of members of S found in samples, including
 duplicates, is
      r = n * p * k
 Taking the ratio of m to r cancels the unknown factor k and yields an
 equation
      [1 - (1-p)^n] / p = nm/r
 which may be solved for p, which is then an estimator of the sampling
 efficiency.  (The statistical properties of the estimator will not be
 examined here.)  Under the substitution p = 1-q, this becomes the
 first equation of Section 8.2.
 With the estimator p in hand, and a count m of members of S
 discovered after n samplings, we can compute the a posteriori
 probability that the true size of S is m+j, for j >= 0.  Let Hj
 denote the hypothesis that the true size of S is m+j, and let R
 denote the result that m members have been found in n samplings.
 Then
      P{R | Hj} = [(m+j)!/m!j!] * [1-(1-p)^n]^m * [(1-p)^n]^j
 We are interested in P{H0 | R}, but to find it we need to assign a
 priori values to P{Hj}.  Let the size of S be exponentially
 distributed
      P{Hj} / P{H0} = h^(-j)
 for arbitrary h in (0, 1).  The value of h will be eliminated from
 the result.
 The Bayesian method yields
      P{Hj | R} / P{H0 | R} = [(m+j)!/m!j!] * [h*(1-p)^n]^j
 The reciprocal of the sum over j >= 0 of these ratios is
      P{H0 | R} = [1-h*(1-p)^n] ^ (m+1)

Crawford Standards Track [Page 30] RFC 2894 Router Renumbering for IPv6 August 2000

 and the confidence estimate of Section 8.2 is the h -> 1 limit of
 this expression.

Crawford Standards Track [Page 31] RFC 2894 Router Renumbering for IPv6 August 2000

Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Crawford Standards Track [Page 32]

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