INTERNET-DRAFT Erik Nordmark, Sun Microsystems
July 30, 1997
Site prefixes in Neighbor Discovery
<draft-ietf-ipngwg-site-prefixes-00.txt>
Status of this Memo
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Distribution of this memo is unlimited.
This Internet Draft expires January 30, 1998.
Abstract
This document specifies extensions to IPv6 Neighbor Discovery to
carry site-prefixes. The site prefixes are used to reduce the effect
of site renumbering by ensuring that the communication inside a site
uses site-local addresses.
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Contents
Status of this Memo.......................................... 1
1. INTRODUCTION AND MOTIVATION.............................. 2
2. TERMINOLOGY.............................................. 4
2.1. Requirements........................................ 4
3. OVERVIEW................................................. 4
3.1. Assumptions......................................... 5
4. UPDATED PREFIX OPTION FORMAT............................. 5
4.1. CONCEPTUAL VARIABLES................................ 7
5. SENDING RULES............................................ 8
6. RECEIVING RULES.......................................... 8
7. USING THE SITE PREFIXES.................................. 9
7.1. Host Name Lookups................................... 9
7.2. IPv6 Address Lookups................................ 10
8. SECURITY CONSIDERATIONS.................................. 11
9. OPEN ISSUES.............................................. 12
REFERENCES................................................... 13
AUTHOR'S ADDRESS............................................. 14
1. INTRODUCTION AND MOTIVATION
In order to maintain the aggregation of the global Internet routing
tables it might be necessary for whole sites to renumber to use
different prefixes for their global IPv6 addresses. Such renumbering
would not directly benefit the renumbered sites but instead be
necessary for the scaling of the Internet as a whole.
In order to increase the probability that such renumbering is viewed
favorably by the sites themselves, which see little or no direct
benefit, it is critical that both the effort of renumbering is kept
at a minimum and also that the risk associated with renumbering is as
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small as possible.
The Stateless address autoconfiguration [ADDRCONF] and support for
router renumbering [ROUTER-RENUM] make it easier to renumber a site.
Also, the IPv6 routing protocols use link-local or site-local
addresses to maintain their router adjacencies (as specified in
[RIPNG], [OSPF] etc) so reduce the effect of site renumbering on the
internal communication. However, these protocols do not by
themselves address long-running TCP connections or cases where IP
addresses have been stored in some configuration file. Thus
additional measures are needed to reduce the risk of renumbering.
For many sites it is much more critical to maintain the internal
communication than the intra-site communication over the Internet.
Based on that observation this proposal tries to limit the effect of
a site renumbering one or more of its global prefixes by ensuring
that intra-site communication can use site-local addresses that are
not effected by the site renumbering. With this proposal it is
possible to maintain internal long-running TCP connections or
otherwise store IPv6 addresses for longer time than would have been
possible without it.
As specified in [ADDR-TODAY] IP addresses are no longer temporarily
unique. This implies, among other things, that applications should
not store IPv6 addresses without a mechanisms for honoring the DNS
time-to-live and refreshing the IPv6 address. This protocol is not
intended to deter from that recommendation but is merely based on the
observation that the applications today might assume that IPv4
addresses are temporarily unique and it is likely that some
applications might not be corrected in their behavior as they are
moved to IPv6. It would be unfortunate if such application
"brokenness" would lead sites to view site renumbering as a too risky
or a too costly operation.
This document does not address the general issues of renumbering such
as renumbering a single host or a subnet. It is targeted at site
renumbering. The proposal does not attempt to address how long-
running TCP connections going outside a site will survive the site
renumbering.
The author would like to acknowledge the contributions the IPNGWG
working group and in particular Mike O'Dell who pointed out the
importance of the problem, and Robert Elz who suggested this approach
to solving the problem.
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2. TERMINOLOGY
This documents uses the terminology defined in [IPV6] and
[DISCOVERY].
2.1. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [KEYWORDS].
3. OVERVIEW
The goal of this extension to Neighbor Discovery is to make
communication that is local to a single site use the site-local
addresses instead of the global addresses. If all communication
internal to a site uses site-local addresses then the site's global
addresses can be renumbered without having any affect on the internal
communication. Thus the risk associated with site renumbering is
lowered - applications that store IPv6 addresses and long-running TCP
connections will, as long as the communication is local to the site,
continue to operate across the renumbering of the site.
A few alternative solutions have been explored. An early proposal
was to place the site-local addresses in the name service (e.g., the
DNS) and make sure they are returned first in the list of addresses
returned to an application (to make it likely that the application
will use that address). That proposal has the disadvantage that the
name service must return different addresses depending on who asks
the question; if a node inside the site asks for an address it should
return the site-local address(es) but if a node outside the site asks
it must not return a site-local address. This is referred to as the
two-faced DNS. While some sites use a two-faced DNS today as part of
their firewall solution it would be rather unfortunate if each and
every site had to deploy such a solution. See [GSE-EVAL] for more
discussion.
This proposal takes a different approach. The name service will only
contain global addresses. The routing infrastructure will be used to
distribute information about which prefixes belong to the local site.
This document only specifies how the site-prefixes are distributed
from the routers to the hosts on each link. However, other protocols
such as [ROUTER-RENUM] might be extended to carry the site-prefixes
to all routers in a site. The use of the routing infrastructure to
carry the site-prefixes avoids the "two-faced" issue above - the
routers know which part of the network is inside the site thus they
can naturally prevent this information from being distributed outside
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the site.
The protocol is based on each host maintaining a list of all the
currently active site prefixes. The site prefixes are periodically
advertised in Neighbor Discovery Router Advertisement messages and
each prefix has an associated lifetime.
Once a host has a list of the prefixes that apply to its site it uses
this information to determine if the global addresses returned by the
name service is part of its site. If this is the case the host
constructs the site-local address that corresponds to the global
address by replacing the site-prefix with the constant site-local
prefix (fec0::0/48). This will result in one or more site-local
addresses being generated. These addresses are then added to the set
of addresses that will be used when communicating with the peer in
such a way that the site-local addresses are tried before any of the
global addresses.
The host will perform the reverse operation when doing a reverse
lookup (from an IPv6 address to a host name). If the address being
looked up is a site-local address the host constructs the
corresponding global addresses by using the list of site prefixes and
performs a reverse lookup on those addresses until a match is found.
It is expected that both the forward and reverse lookup rules can be
hidden from the applications by implementing them as part of the
library that handles host name lookups.
3.1. Assumptions
The protocol assumes that the site uses a consistent subnet numbering
scheme across all its global addresses and its site-local addresses.
Thus, for every subnet in the site, the 16-bit subnet ID field
[ADDR-ARCH] for the site-local address must have the same value as
the Site-Local Aggregator(s) field in the global addresses.
4. UPDATED PREFIX OPTION FORMAT
The protocol adds two new fields using previously reserved parts of
the Prefix Information Option defined in [DISCOVERY].
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Prefix Length |L|A|S| Resvd1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preferred Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Site PLength | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Prefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
New fields:
S 1-bit site prefix flag. When set indicates that
this prefix, in addition to what might be specified
by the L and A flags, should be used to create
site-local addresses when an address matches the
first Site PLength part of the prefix.
Site PLength 8-bit unsigned integer. This Site Prefix Length is
only valid when the S flag is set. The number of
leading bits in the Prefix that are valid. The
value ranges from 0 to 128.
The defined format above allows a single Prefix Information option to
carry a subnet prefix used for on-link and/or stateless address
autoconfiguration [ADDRCONF] together with a site prefix since the
site prefix(es) are normally sub-prefixes of the subnet prefixes.
For example, if the subnet prefix is
2000:1:2:653a::0/64
and the site prefix is:
2000:1:2::0/48
this can be encoded in a single Prefix Information option with Prefix
Length being 64, Site PLength being 48 and the Prefix being
2000:1:2:653a::0.
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4.1. CONCEPTUAL VARIABLES
This document makes use of internal conceptual variables to describe
protocol behavior and external variables that an implementation must
allow system administrators to change. The specific variable names,
how their values change, and how their settings influence protocol
behavior are provided to demonstrate protocol behavior. An
implementation is not required to have them in the exact form
described here, so long as its external behavior is consistent with
that described in this document.
Hosts will need to maintain the following pieces of information.
Unlike the information specific in [DISCOVERY] this information is
not per interface but for the host as a whole.
Site Prefix List - A list of the site prefixes that have been
received in Router Advertisement messages that have
not yet timed out. Each entry has an associated
invalidation timer value (extracted from the
advertisement) used to expire site prefixes when
they become invalid. A special "infinity" timer
value specifies that a prefix remains valid
forever, unless a new (finite) value is received in
a subsequent advertisement.
Note that the Site Prefix List is separate from the
list of on-link prefixes called Prefix List in
[DISCOVERY].
The conceptual Router variable called AdvPrefixList in [DISCOVERY] is
extended to also contain site prefixes. Conceptually this can be
done by having each prefix both contain a AdvSubnetPrefixLength and a
AdvSitePrefixLength field. If one of the length fields is zero the
prefix is not used as a on-link and/or addrconf prefix or a site
prefix, respectively. The same lifetime values will apply to both
the subnet and site prefix aspects of a prefix in the AdvPrefixList.
The above are conceptual variables; Implementations are free to
implement the router variables as a separate list for the site
prefixes and the existing Neighbor Discovery AdvPrefixList for subnet
prefixes. However, it is desirable that such implementations still
use a single Prefix Information option to encode both a site and a
subnet prefix when the site prefix is just a sub-prefix of the subnet
prefix.
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5. SENDING RULES
When a router is sending Prefix options as part of sending Router
Advertisement messages in addition to the rules in [DISCOVERY] is
performs the following operations:
o If the AdvSitePrefixLength field in the AdvPrefixList entry is
non-zero set the S flag in the Prefix option to one and set the
Site PLength to the AdvSitePrefixLength.
o Only if the AdvSubnetPrefixLength field is non-zero should the L-
bit and the A-bit be set from the AdvOnLinkFlag and the
AdvAutonomousFlag fields, respectively.
o The Prefix field and the lifetime fields are set is specified in
[DISCOVERY].
6. RECEIVING RULES
The host receiving a valid Router Advertisement follows the rules as
specified in [DISCOVERY] with the following additions when parsing
each received Prefix Information option. For each prefix that has
the S-flag set:
o If the Site PLength is zero ignore the prefix.
o If the prefix is the link-local or the site-local prefix ignore
the prefix.
o If the prefix is a multicast address ignore the prefix.
o If the prefix is not already present in the Site Prefix List and
the Valid Lifetime is zero, ignore the prefix.
o If the prefix is not already present in the Site Prefix List and
the Valid Lifetime is non-zero, create a new entry for the prefix
in the Site Prefix List and initialize its invalidation timer to
the Valid Lifetime value in the Prefix Information option.
o If the prefix is already present in the host's Site Prefix List as
the result of a previously-received advertisement, reset its
invalidation timer to the Valid Lifetime value in the Prefix
Information option. If the new Lifetime value is zero,
immediately remove the prefix from the Site Prefix List.
The bits in the Prefix after the first Site PLength bits MUST be
ignored when the prefix is entered in the Site Prefix List and/or
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when it is compared against other site prefixes. These bits might be
non-zero when the Prefix option carries a subnet prefix in addition
to a site prefix.
Timing out a site prefix from the Site Prefix List SHOULD NOT affect
any existing communication. New communication will use the updated
Site Prefix List after performing a host name lookup.
7. USING THE SITE PREFIXES
The following rules apply when a node looks up host names and
addresses in a name service such as DNS.
7.1. Host Name Lookups
The goal is that when an address or multiple addresses are returned
by the name server to the host that the host should, if one or more
of those addresses match one of the site prefixes, prepend the
corresponding site local address to the list of addresses that the
application will attempt to use.
It is important to prepend them to the list so that the applications
try the site-local addresses before the corresponding global
addresses in order to be able to prevent the applications from using
global addresses for communication that is local to the site.
A possible algorithm for doing these comparisons is as follows:
1) Assume the name service returns the addresses A1, A2, A3, ...
An and the prefixes in the Site Prefix List are SP1, SP2, ...
SPm. The Site PLength of each of the prefixes is Length(SPj).
2) For each Ai compare it against all the SPj. If the first
Length(SPj) bits of Ai are equal to the first Length(SPj) bits
of SPj then construct the site-local address corresponding to
Ai by concatenating the first Length(SPj) bits out of the
site-local address (prefix) FEC0::0 and the last (128 -
Length(SPj)) of Ai to form a new address. Add this address to
the front of the list (i.e. before A1).
3) In some cases the above algorithm will add the same site-local
address more than once to the list thus it is desirable to
remove the duplicates from the resulting list.
For example, if the name service returns these addresses for a
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multihomed node:
2837:a:b:987:X:Y:W:Z1
2000:1:2:987:X:Y:W:Z1
2837:a:b:34:X:Y:W:Z2
2000:1:2:34:X:Y:W:Z2
2abc:77:66:23:X:Y:W:Z3
and the prefixes in the Site Prefix List are:
2837:a:b::0/48
2000:1:2::0/48
The resulting list that the application should use should be (in
order):
fec0::987:X:Y:W:Z1
fec0::987:X:Y:W:Z1 (duplicate - can be removed)
fec0::34:X:Y:W:Z2
fec0::34:X:Y:W:Z2 (duplicate - can be removed)
2837:a:b:987:X:Y:W:Z1
2000:1:2:987:X:Y:W:Z1
2837:a:b:34:X:Y:W:Z2
2000:1:2:34:X:Y:W:Z2
2abc:77:66:23:X:Y:W:Z3
7.2. IPv6 Address Lookups
It is not sufficient to handle the forward lookup. For instance, the
node that receives packets and/or connections from a site-local
address might have the desire to perform a reverse lookup to get a
host name. Thus these rules allow such a reverse lookup to succeed
as long as the Site Prefix List contains all the prefixes that apply
to the site.
A possible algorithm for doing this is as follows:
1) Assume the site-local address is A and the prefixes in the Site
Prefix List are SP1, SP2, ... SPm. The Site PLength of each of
the prefixes is Length(SPj).
2) First perform a regular reverse lookup of the IPv6 address. If
the lookup succeeds or the lookup fails and the IPv6 address is
not a site-local address there is nothing more to do.
3) When the reverse lookup of a site-local address fails use the
Site Prefix List to construct global addresses corresponding to
the site-local address. This is done by taking each entry in
the Site Prefix List and using it to construct a global
address. For each of the SPj concatenate the first Length(SPj)
bits from SPj and the last (128 - Length(SPj)) bits from A to
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form a new address. Look up each of the resulting addresses
until a match is found.
For example, if the site-local address is:
fec0::987:X:Y:W:Z1
and the prefixes in the Site Prefix List are:
2837:a:b::0/48
2000:1:2::0/48
The address that should be tried in the reverse lookup are:
2837:a:b:987:X:Y:W:Z1
2000:1:2:987:X:Y:W:Z1
8. SECURITY CONSIDERATIONS
Router Advertisements are not required to be authenticated and even
if they are authenticated it is unclear whether or not there would be
a mechanisms to verify the authority of a particular node to send
Router Advertisements.
Neighbor Discovery uses the rule of HopCount 255 (set to 255 on
transmit and verified to be 255 on reception) to drop any Neighbor
Discovery packets that are sent non-neighboring nodes. This limits
any attack using ND to the neighbors.
Without authentication and authorization this new mechanisms
introduces a new type of denial of service attack. A node on the
link can send a router advertisement listing site-prefixes that are
in fact not part of the site. For instance, it could advertise all
addresses (prefix 0::0/0) as a site-prefix). Such an attack would
result in all nodes on the link to fail initiate any new
communication with any node outside the site.
Furthermore this mechanism can also be used by an attacker on the
link to "redirect" an arbitrary global prefix to a node inside the
site that has the same low-order part of the address as the intended
recipient. Thus such an attack consists of one attacker on the link
plus one cohort that has the same low order part of the address as
the intended destination.
This could be viewed as allowing some form of indirect spoofing of
the addresses returned by the DNS independent whether or not the DNS
itself is secure. Thus introducing a secure DNS [DNSsec] would not
remove this form of "address spoofing". However, it seems like this
threat is no worse than the other threats in [DISCOVERY] where any
node on the link can intercept all packets sent on the link.
The packets used to discover site prefixes, just like all other
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Neighbor Discovery protocol packet exchanges, can be authenticated
using the IP Authentication Header [IPv6-AUTH]. A node SHOULD
include an Authentication Header when sending Neighbor Discovery
packets if a security association for use with the IP Authentication
Header exists for the destination address. The security associations
may have been created through manual configuration or through the
operation of some key management protocol.
Received Authentication Headers in these packets, just like all
Neighbor Discovery packets, MUST be verified for correctness and
packets with incorrect authentication MUST be ignored.
Confidentiality issues are addressed by the IP Security Architecture
and the IP Encapsulating Security Payload documents [IPv6-SA, IPv6-
ESP].
9. OPEN ISSUES
o This scheme has the assumption that the same subnet number (called
Site-Local Aggregator(s) field for the global addresses) is
assigned to a link and used for both site-local addresses and all
global addresses that are advertised as site prefixes. Is this a
reasonable assumption?
o This proposal can be viewed as creating a security hole in a
secure name service. The proposal tries to limit the security
hole by only allowing the mapping to/from the site-local prefix
i.e., it does not allow arbitrary remapping from one IPv6 address
to another. When communication actually takes place after
resolving a host name this hole is not any worse than the fact
that any node on the link can intercept all packets sent on the
link as described in [DISCOVERY]. But do we need to be concerned
about the security of host name lookups and IP address lookups in
the absence of any communication with the peer node?
o Should the formed site-local addresses replace the global
addresses in the list returned to the application? As currently
specified a node might end up using a global address if it tries
all of the returned addresses and for whatever reason it could not
reach the node when it tried the site-local address(es).p
o Do we need to specify a common API that e.g. the BIND DNS resolver
can use to access the Site Prefix List?
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REFERENCES
[KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol, Version
6 (IPv6) Specification", RFC 1883, January 1996.
[ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6
Addressing Architecture", RFC 1884, January 1996.
[DISCOVERY] T. Narten, E. Nordmark, and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 1970, August 1996.
[RIPNG] G. Malkin, R. Minnear, "RIPng for IPv6", RFC 2080, January
1997.
[OSPF] R. Coltun, D. Ferguson, J. Moy, "OSPF for IPv6", Internet
Draft, draft-ietf-ospf-ospfv6-04.txt.
[ADDR-TODAY] B. Carpenter, J. Crowcroft, Y. Rekhter, "IPv4 Address
Behavior Today", RFC 2101, February 1997.
[GSE-EVAL] M. Crawford, A. Mankin, T. Narten, J. Stewart, L. Zhang,
"IPng Analysis of the GSE Proposal", Internet Draft,
draft-ietf-ipngwg-esd-analysis-00.txt.
[ROUTER-RENUM] M. Crawford, and R. Hinden, "Router Renumbering for
IPv6", Internet Draft, draft-ietf-ipngwg-router-renum-
00.txt.
[ADDRCONF] S. Thomson, T. Narten, "IPv6 Address Autoconfiguration",
RFC 1971, August 1996.
[IPv6-SA] R. Atkinson. "Security Architecture for the Internet
Protocol". RFC 1825, August 1995.
[IPv6-AUTH] R. Atkinson. "IP Authentication Header", RFC 1826,
August 1995.
[IPv6-ESP] R. Atkinson. "IP Encapsulating Security Payload (ESP)",
RFC 1827, August 1995.
[DNSsec] D. Eastlake, C. Kaufman, "Domain Name System Security
Extensions", RFC 2065, January 1997.
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AUTHOR'S ADDRESS
Erik Nordmark
Sun Microsystems, Inc.
901 San Antonio Road
Palo Alto, CA 94303
USA
phone: +1 415 786 5166
fax: +1 415 786 5896
email: nordmark@sun.com
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