Network Working Group M. Bagnulo
Internet-Draft UC3M
Expires: November 25, 2006 May 24, 2006
Default Locator-pair selection algorithm for the SHIM6 protocol
draft-bagnulo-shim6-locator-pair-selection-00
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Copyright (C) The Internet Society (2006).
Abstract
In this note, we present a locator-pair selection mechanism for the
shim6 protocol. The presented mechanism provides an ordered list of
available locator-pairs that can be used for outgoing traffic.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Preliminary considerations . . . . . . . . . . . . . . . . . . 4
2.1. Candidate Locator-pair set . . . . . . . . . . . . . . . . 4
2.2. Locator-pair States . . . . . . . . . . . . . . . . . . . 4
2.3. Locator preferences . . . . . . . . . . . . . . . . . . . 5
2.3.1. Remote locator preferences . . . . . . . . . . . . . . 5
2.3.2. Source locator preferences . . . . . . . . . . . . . . 6
2.4. Locator-pair selection table . . . . . . . . . . . . . . . 6
2.5. About IPv4 addresses . . . . . . . . . . . . . . . . . . . 6
3. Default Locator-Pair Selection Algorithm . . . . . . . . . . . 6
4. Security considerations . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction
Once that a shim6 context is established between two peers, they are
free to select the best locator pair to continue the communication.
In particular, when an outage is detected, they will need to select a
new locator pair to rehome the communication. Besides, policy or
other considerations may lead to change the locator pair used in the
communication even if no outage has occurred.
In this note, we present a locator-pair selection mechanism for the
shim6 protocol. The presented mechanism provides an ordered list of
available locator-pairs that can be used for outgoing traffic (note
that since unidirectional locator pairs are supported by the shim6
protocol, the locator pair used in the outgoing direction may not
affect the locator pair used by the peer to send incoming traffic).
The motivation for having a locator selection mechanism different
than RFC 3484 [3] is that RFC 3484 was designed to select addresses
that were both identifiers and locators, so, in some cases the
selection criteria differs from the one used when selecting addresses
that will used only as locators. In particular, when addresses are
to be used as identifiers and as locators, stable addresses such as
Home Addresses are preferred over more temporary addresses as Care-
Off Addresses. If an address is to be used only as a locator,
probably the stability property is not as important as achieving a
more direct path, making a Care-off Address more attractive than a
Home Address. Similar considerations can be made with respect to
private and public addresses. In addition, the locator pair
selection mechanism described in this document incorporates into the
selection mechanism shim-specific information, such as available
reachability information and local and remote locator preferences
obtained through the shim6 protocol. Finally, the mechanism
presented in this note is a locator pair selection mechanism as
opposed to separate source address and destination address selection
mechanisms as described in RFC 3484. We think that such approach is
more appropriate for the shim6 protocol, since reachability seems to
be a property of an address pair rather than a property of a single
address.
The presented mechanism takes into account general properties of the
available addresses, in particular the address family (v4 or v6),
address scope [3], mobility consideration (Home-Addresses and Care-
off Addresses) [5], [4], status of the addresses (Preferred and
Deprecated addresses) [6], privacy considerations (Public and
Temporary addresses) [7]. In addition it also takes into account
shim6 specific information such as whether the addresses are known to
be locally operational (as defined in [2]), if locator pairs are know
to be unidirectionally operational [2], the local and remote
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preferences for the different locators available in the shim6
context.
Multicast addresses are out of the scope of the document.
2. Preliminary considerations
2.1. Candidate Locator-pair set
We define the local set of locally-operational locators (LOLs(local))
as the local locators that are included in the local locator set
(Ls(local) as defined in [1]) and that are locally operational as
defined in [2]. Locally operational addresses are discovered through
local means that are outside of the scope of this document.
We define the set of the locally-operational locators of the peer
(LOLs(peer)) as the remote locators that are included in the peer
locator set (Ls(peer) as defined in [1]) and that are locally
operational in the peer as defined in [2]. The peers' locally
operational locators are discovered through the Locator List Option
and the Locator Preferences Option (in particular the BROKEN flag)
defined in the shim protocol [1].
The candidate locator-pair set is the set of locator pairs that can
be used to send packets in a shim context.
The candidate locator-pair set contains in all the possible locator
pairs formed with the first of them belonging to the local set of
locally-operational locators (LOLs(local)) and the second locator
belonging to the locally-operational locators of the peer
(LOLs(peer)).
Cand_Loc_Pair_Set ={(x,y)/[x in LOLs(local) and y in LOLs(peer)}
2.2. Locator-pair States
Locator pairs can be in the following state:
o Unidirectionally Operational state: As defined in [2], is when
packets send with the first locator as the source address and the
second locator as a destination address are known to reach the
destination. In the shim6 case, a locator pair is know to be
unidirectionally operational when there is fresh information about
packets reaching the peer, using the mechanisms defined in [2] or
thanks to recent ULP feedback. When the information about
reachability expires, the locator pair moves to Unknown state.
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o Non-Operational state: The locator pair is known to be non-
operational i.e. that packets containing the first locator as
source address and the second locator as destination address do
not reach the destination. In the shim6 case this can be known
because recent attempts to exchange packets have failed. When the
information about unreachability expires, the locator pair moves
to Unknown state.
o Unknown state: No recent reachability information is available for
this locator-pair.
2.3. Locator preferences
2.3.1. Remote locator preferences
Remote locator preferences can be obtained through the shim6 protocol
using the Locator Preference option. The preferences consist in a
Flag octet, a Priority octet and an optional Weight octet.
The weight field express the relative weight for locators with the
same priority, and as defined in [8] larger weights should be given a
proportionally higher probability of being selected. In order to
include this probability information in the locator-pair selection
algorithm, a new weight* information is generated from the weight
values as following:
We order each set of destination addresses with the same priority and
defined weight values using the following algorithm defined in [8]:
Arrange all addresses (that have not been ordered yet) in any order,
except that all those with weight 0 are placed at the beginning of
the list.
Compute the sum of the weights of those addresses, and with each
address associate the running sum in the selected order. Then choose
a uniform random number between 0 and the sum computed (inclusive),
and select the address whose running sum value is the first in the
selected order which is greater than or equal to the random number
selected. This address is the next one to be included in the ordered
list. Remove this address from the set of the unordered addresses
and apply the described algorithm to the unordered address set to
select the next target address. Continue the ordering process until
there are no unordered addresses.
The weight* (W*1, W*2,...,W*N) values for each of the addresses is
their final position in the resulting ordered list.
The procedure is repeated for each one of the sets containing
destination addresses with equal priority.
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The Weight information is not used in the locator-pair selection
mechanism, but the Weight* information is.
2.3.2. Source locator preferences
With respect to the local locator preferences, this document assumes
that the host will have a mechanism to express Priority and Weight
information for local locators similar to the one defined in [8].
The same procedure is used to assign Weight* values to the source
locators that have the same priority value.
Note that destination and source addresses are never included in the
same set, even if they have the same priority value.
The Weight information is not used in the locator-pair s election
mechanism, but the Weight* information is.
2.4. Locator-pair selection table
We define the Locator-pair selection table to express preferences
about which source address prefix to use when communicating with a
given destination address prefix. The table contains entries having
a source prefix and a destination prefix each. Given a locator pair,
it is then possible to find a match when both the source prefix is
contained in the source address and the destination prefix is
contained in the destination address.
2.5. About IPv4 addresses
IPv4 addresses are considered to be Public in the RFC3041 sense,
Preferred in the RFC2462 sense.
3. Default Locator-Pair Selection Algorithm
The goal of the defualt locator-pair selection algorithm is to
produce an ordered list of locator pairs to be tried for rehoming an
ongoing communication. The ordered list can be produced with any
sorting algorithm. The set of rules described next are the
comparison criteria to be used in the locator-pair sorting algorithm.
This rules act must be processed in order and if a given rule selects
a locator pair over the other one, then the following rules don't
need to be processed and the selected locator pair is prefered.
We are comparing two locator pairs (src1,dst1) and (src2,dst2). Note
that in some cases the source or the destination addresses of the two
pairs may be equal.
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Rule 1: Prefer the same address: If src1 = dst1 and src2 <> dst2,
then prefer (src1,dst1).
Rule 2: Prefer appropriate scope: If scope(src1) >= scope(dst1) and
scope(src2) < scope(dst2), then prefer (src1,dst1).
Rule 3: Avoid Non-Operational pairs: If (src1,dst1) is in Non-
Operational state and (src2,dst2) is in Unidirectionally
Operational or in Unknown state, then prefer (src2,dst2).
Rule 4: Prefer Unidirectionally Operational state: If (src1,dst1) is
in Unknown state and (src2,dst2) is in Unidirectionally
Operational, then prefer (src2,dst2).
Rule 5: Prefer fresher reachability information: If (src1,dst1) and
(src2,dst2) are both in Unidirectionally Operational state,
then prefer the one with smallest age information i.e. the
one for which newer reachability information is available.
Rule 6: Prefer same address family: If (src1,dst1) are both of the
same address family (v4/v6) and (src2,dst2) are of different
address family, then prefer (src1,dst1) (This could also be
done with the Locator-pair selection table)
Rule 7: Prefer matching scope: If scope(src1) = scope(dst1) and
scope(src2) < scope(dst2), then prefer (src1,dst1)
Rule 8: Prefer Locator-pair table match: If (dst1,src1) has a match
in the Locator-pair selection table and (src2,dst2) does not
have a match in the locator-pair selection table, then
prefer (dst1,src1).
Rule 9: Prefer Preferred addresses: If src1 address is a Preferred
address in the RFC2462 sense and src2 is a deprecated
address in the RFC2462 sense, then prefer (src1,dst1)
Rule 10: Prefer Local Priority: If src1 of (src1,dst1) has a lowest
Priority than src2 of (src2,dst2) then prefer (src1,dst1)
Rule 11: Prefer Local Weight*: If src1 of (src1,dst1) has a lowest
Weight* than src2 of (src2,dst2) then prefer (src1,dst1)
Rule 12: Prefer Temporary addresses: If src1 is a temporary address
[7] and src2 is a public address, the prefer (src1,dst1)
over (src2,dst2)
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Rule 13: Prefer Local Care-off Addresses: If src1 is a Care-off
address [4] [5] and src2 is a Home Address, the prefer
(src1,dst1)
Rule 14: Prefer Remote Priority: If dst1 of (src1,dst1) has a lowest
Priority than dst2 of (src2,dst2) then prefer (src1,dst1)
Rule 15: Prefer Remote Weight*: If dst1 of (src1,dst1) has a lowest
Weight* than dst2 of (src2,dst2) then prefer (src1,dst1)
Rule 16: Prefer Remote Care-off Addresses: If dst1 is a Care-off
address (Temporary flag set in the Locator preferences
options defined in [1]) and dst2 is not a Care-off address,
the prefer (src1,dst1)
Rule 17: Prefer ULID-Pair: If (src1,dst1) is the ULID-pair of the
context, the prefer (src1,dst1)
Other rules that may be worth taking into account are:
o Prefer native transport
o Prefer smaller scope
o Prefer most dissimilar locator pair to the currently used
o Prefer locator pair contained in incoming packet
o Longest prefix match
4. Security considerations
TBD
5. Acknowledgements
The idea of pre-assigning Weight* values for introducing the Weight
probability in the locator-pair selection process was suggested by
Albert Banchs.
Marcelo Bagnulo worked on this document while visiting Ericsson
Research laboratory Nomadiclab.
6. References
[1] Nordmark, E. and M. Bagnulo, "Level 3 multihoming shim
protocol", draft-ietf-shim6-proto-04 (work in progress),
March 2006.
[2] Arkko, J. and I. Beijnum, "Failure Detection and Locator Pair
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Exploration Protocol for IPv6 Multihoming",
draft-ietf-shim6-failure-detection-03 (work in progress),
December 2005.
[3] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[4] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[5] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.
[6] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[7] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[8] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[9] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
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Author's Address
Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: 34 91 6248814
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es
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