IPng Working Group Matt Crawford
Internet Draft Fermilab
January 27, 1999
Router Renumbering for IPv6
<draft-ietf-ipngwg-router-renum-07.txt>
Status of this Memo
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Distribution of this memo is unlimited.
1. 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.
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Table of Contents
Status of this Memo ............................................... 1
1. Abstract ...................................................... 1
2. Functional Overview ........................................... 3
3. Definitions ................................................... 4
3.1. Terminology ............................................. 4
3.2. Requirements ............................................ 5
4. Message Format ................................................ 5
4.1. Router Renumbering Header ............................... 7
4.2. Message Body -- Command Message ......................... 9
4.2.1. Prefix Control Operation .......................... 9
4.2.1.1. Match-Prefix Part ........................... 9
4.2.1.2. Use-Prefix Part ............................. 11
4.3. Message Body -- Result Message .......................... 12
5. Message Processing ............................................ 14
5.1. Header Check ............................................ 14
5.2. Bounds Check ............................................ 15
5.3. Execution ............................................... 16
5.4. Summary of Effects ...................................... 18
6. Sequence Number Reset ......................................... 18
7. IANA Considerations ........................................... 19
8. Security Considerations ....................................... 19
8.1. Security Policy and Association Database Entries ........ 19
9. Implementation and Usage Advice ............................... 20
9.1. Reliability Control Mechanism ........................... 22
10. Usage Examples ............................................... 23
10.1. Maintaining Global-Scope Prefixes ...................... 23
10.2. Renumbering a Subnet ................................... 24
11. Acknowledgments .............................................. 25
12. References ................................................... 25
13. Author's Address ............................................. 26
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2. 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, 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.
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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 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.
3. Definitions
3.1. 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.
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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.
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.
3.2. 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].
4. Message Format
There are two types of Router Renumbering messages: Commands, which
are sent to routers, and Results, which are sent by routers. A
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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.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ 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
8, "Security Considerations".
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The use of authentication for Router Renumbering Result messages is
RECOMMENDED.
4.1. 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 6.
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 |
+-+-+-+-+-+-+-+-+
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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;
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
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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.
4.2. 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.
4.2.1. 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.
4.2.1.1. 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:
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1 the ADD operation
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.
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4.2.1.2. 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
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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].
4.3. 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.
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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 5.3.
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].
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5. 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.
5.1. 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
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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 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.
5.2. 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 6.
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
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once, not once per interface.
Note that MinLen and MaxLen need not be explicitly bounds checked,
even though certain combinations of values will make any matches
impossible.
5.3. 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 3.1) 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
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interface.
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.
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5.4. Summary of Effects
The only Neighbor Discovery [ND] parameters which can be affected by
Router Renumbering are the following.
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.
6. 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.
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The invalidation of authentication keys caused by a valid SNR
message will cause retransmitted copies of that message to be
ignored.
7. IANA Considerations
Following the policies outlined in [IANACON], new values of the Code
field in the Router Renumbering Header (section 4.1) and the OpCode
field of the Match-Prefix Part (section 4.2.1.1) are to be allocated
by IETF consensus only.
8. 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.
8.1. 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-
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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].
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.
9. Implementation and Usage Advice
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 in a modestly reliable network.
In one set of cases, the network management station will know the
complete set of routers under its control. Commands can be
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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. Multicast Test commands may be useful in
maintaining a list of managed routers.
When the set of managed routers is not known but reliable execution
is desired, a Command should be transmitted several times, governed
by the scheme in the following subsection. The interval between
transmissions must be greater than the sum of the estimated
propagation time on both directions, the maximum processing time,
and the chosen random delay interval for replies. The
retransmission interval may be much greater than this sum, but it
should not be so large that long-term congestion conditions are
likely to change markedly during the course of several intervals.
The management station implementation should estimate the
reliability of round-trip communication from the responses received
after each transmission. There are at least two different ways to
form this estimate. If the true probability of successful round-
trip communication with a managed router is P, then an estimate p of
P may be derived from:
The expected ratio of the number of routers first heard
from after transmission (M + 1) to the number first heard
from after M 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.
After two or more transmission intervals, a reliability estimate
should be used to compute the a posteriori probability that all
managed routers have been heard from. When this probability is
greater than or equal to the network manager's desired confidence
level, retransmissions may cease.
As stated so far, this algorithm would not deliver the confidence
level one might expect. The reason for the shortfall is that packet
loss is not, in general, uniform to and from all managed routers.
And the routers in the "lossier" parts of the network are less
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likely to contribute to the reliability estimate, p. This effect
can be compensated in the following way. After the Mth transmission
interval, M > 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 M. For example, after five intervals,
only routers first heard from in the third through fifth intervals
will be counted. This strongly biases the reliability estimate
toward the subset of managed routers with the worst reliability.
9.1. Reliability Control Mechanism
With this "moving estimate" estimator, the full algorithm for
computing the confidence that all routers have been heard from is as
follows. Let Ct < 1 represent the target confidence level.
After M > 2 transmission intervals, including the waiting
period for processing for delayed replies, let
F = FLOOR((M-1)/2), N(M) be the number of different
routers from which replies have been received, and R(M,F)
be the total number of replies received from routers which
were not heard from in intervals 1 through F.
Compute the reliability estimate
p(M) = R(M,F)/((M-F)*(N(M) - N(F))).
(The asterisk represents multiplication.)
Compute the confidence estimate
c(M) = (1 - (1-p(M))^M)^(N(M) - N(F)).
(The caret represents exponentiation.)
When c(M) >= Ct, retransmissions of the Command may cease.
A few corner cases need consideration. First, if the difference in
reliability between the "good" and "bad" parts of a managed network
is very great, early estimates c(M) of the confidence level will be
too high. If the network manager can specify a worst-case
reliability q, 0 < q < 1, then retransmissions should continue for
at least log(1-Ct)/log(1-q) intervals, regardless of the current
confidence estimate. If no worst-case guess is provided, the
implementation should perform at least five transmissions in all
cases. This corresponds to Ct = 0.999, q = 0.75.
Divide-by-zero may occur when computing p. This can happen only
when no new routers have been heard from in the last M-F intervals.
Generally, the confidence estimate c(M) 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
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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(M) 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.
10. 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.
10.1. 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)
First (and only) PCO:
Match-Prefix Part
OpCode = 3 (SET-GLOBAL)
OpLength = 4 N + 3 (assuming N global prefixes)
Ordinal = 0 (arbitrary)
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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.
10.2. 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)
First (and only) PCO:
Match-Prefix Part
OpCode = 2 (CHANGE)
OpLength = 11 (reflects 2 Use-Prefix Parts)
Ordinal = 0 (arbitrary)
MatchLen = 64
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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.
11. 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 was very helpful with
the IPsec details.
12. References
[AARCH] R. Hinden, S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373.
[AH] S. Kent, R. Atkinson, "IP Authentication Header", RFC 2402.
[ESP] S. Kent, R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406.
[IANACON] T. Narten, H. T. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434.
[ICMPV6] A. Conta, S. Deering, "Internet Control Message Protocol
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(ICMPv6) for the Internet Protocol Version 6 (IPv6)", RFC 2460.
[IPSEC] S. Kent, R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401.
[IPV6] S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460.
[IPV6MIB] D. Haskin, S. Onishi, "Management Information Base for IP
Version 6: Textual Conventions and General Group", RFC 2466.
[KWORD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119.
[ND] T. Narten, E. Nordmark, W. Simpson, "Neighbor Discovery for IP
Version 6 (IPv6)", RFC 2461.
13. Author's Address
Matt Crawford
Fermilab MS 368
PO Box 500
Batavia, IL 60510
USA
Phone: +1 630 840 3461
Email: crawdad@fnal.gov
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