Source Address Selection for IPv6 FUJIKAWA Kenji
Internet-Draft ROOT Inc.
Expires: August 25, 2008 February 2007
Source Address Selection Using Just Routing Information for IPv6
draft-fujikawa-ipv6-src-addr-selection-03.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This document describes a problem of source address selection Rule 8.
stated in RFC3484[RFC3484], and shows one solution, which is based
just on the destination based address routing and does not require
policy routing such as source address based routing.
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1. A Problem of at Source Address Selection Rule 8. in RFC3484
In RFC3484[RFC3404], the source address selection guidelines are
shown in the case of multihoming. However, according to them, a
problem occurs that a host cannot select the best path.
Rule 8 in 5. Source Address Selection in RFC3484 says:
Rule 8: Use longest matching prefix. If CommonPrefixLen(SA, D) >
CommonPrefixLen(SB, D), then prefer SA. Similarly, if
CommonPrefixLen(SB, D) > CommonPrefixLen(SA,D), then prefer SB.
Here, consider a network shown in Fig. 1.
+---+
|CN |
+-+-+
| 2001:db8:2001::CN
|
+---+---+2001:db8:2000:/36
| |
+---------+ ISP2 |
| | |
| +-------+
|
+---+---+2001:db8:1000:/36 +-------+2001:db8:3000::/36
| | | |
| ISP1 +-------------------+ ISP3 |
| | | |
+---+---+ +---+---+
| |
| |
+----------+ +---------+
2001:db8:1000:R | |2001:db8:3000:R
+-+-+ +-+-+
2001:db8:1001::/48|R1 | |R3 |2001:db8:3001::/48
+-+-+ +-+-+
fe80::R1| |fe80:R3
| |
--+---+---+--
2001:db8:1001:1:EN | 2001:db8:3001:1:EN
+-+-+
|EN |
+---+
Fig. 1
In Fig. 1,
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- Each of ISP1, ISP2, and ISP3 is assigned an address space,
2001:db8:1000::/36, 2001:db8:2000::/36, and 2001:db8:3000::/36,
respectively.
- Correspondent node CN belongs to ISP2, and is assigned an
address 2001:db8:2001::CN.
- The site is multihomed to ISP1 and ISP3, and the routers R1 and
R2 distributes address spaces to downstream nodes such as
2001:db8:1001::/48 and 2001:db8:3001::/48.
- End node EN is assigned two addresses, 2001:db8:1001:1:EN and
2001:db8:3001:1:EN.
Here, in the above IPv6 address notation, CN, R1, R2, and EN
indicates 64bit Interface ID's.
According to Rule 8, by means of the longest match method,
2001:db8:3001:EN is selected as the source address of a packet
directed from CN to EN. Therefore,
- For the purpose of avoiding the source address filtering,
policy routing or etc. is required in order to direct a packet to
ISP3.
- The route becomes roundabout passing through ISP3.
2. A Solution Using Just the Information of the Destination Address of
Packets
Here, one of the solutions of the above problem is shown, which is
based on the traditional destination address based routing, that is,
does not require policy routing such as source address based routing.
This approach is separated into two issues, an management issue and
implementation issue.
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2.1 Management Issue
+---+
|CN |
+-+-+
| 2001:db8:2001::CN
|
+---+---+2001:db8:2000:/36
| |
+---------+ ISP2 |
| | |
| +-------+
|
+---+---+2001:db8:1000:/36 +-------+2001:db8:3000::/36
| | | |
| ISP1 +-------------------+ ISP3 |
| | | |
+---+---+ +---+---+
| |
| |
+----------+ +---------+
2001:db8:1000:R | |2001:db8:3000:R
+-+-+ +-+-+
2001:db8:1001::/48|R1 | |R3 |2001:db8:3001::/48
+-+-+ +-+-+
2001:db8:1001:1:R1| |2001:db8:3001:1:R3 <- Change from Fig.1
| |
--+---+---+--
2001:db8:1001:1:EN | 2001:db8:3001:1:EN
+-+-+
|EN |
+---+
Routing Tables:
R1:
Destination Next Hop
2001:db8:1000::/36 address_of_ISP1's_router
2001:db8:2000::/36 address_of_ISP1's_router
R3:
2001:db8:3000::/36 address_of_ISP3's_router
EN:
Destination Next Hop
2001:db8:1000::/36 2001:db8:1001:1:R1
2001:db8:2000::/36 2001:db8:1001:1:R1
2001:db8:3000::/36 2001:db8:3001:1:R3
Fig.2
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First, mange a network such as shown in Fig. 2. Here:
- The global addresses 2001:db8:1001:R1 and 2001:db8:3001:R3 is
respectively assigned to the downstream interfaces of R1 and R3,
in addition to the link local addresses.
- Each of R1, R3 and EN keeps the traditional routing table
shown in Fig.2, respectively.
2.2 Implementation Issue
When an entry of a routing table is hit, a source address is selected
which longest-matches the next hop in the entry.
In the above example, on end node EN, when the entry
"2001:db8:2000::/36 2001:db8:1001:1:R" is hit for the destination
"2001:db8:2001::CN", the next hop becomes "2001:db8:1001::R", as a
result, the address "2001:db8:1001:1:EN" is selected, because it
longest-matches the next hop.
3. Modification to RFC3484
Before Rule 8 (Use longest matching prefix) in section 5. (Source
Address Selection) in RFC3484, the rule using longest-matching prefix
to the next hop is to be added.
4. How to distribute the routing information
Any intra-domain routing protocol can be adaptable. In order to
deliver the routing information to be used with this rule, employing
ND may not be suitable. Delivering destination and next-hop pairs
with routing protocols such as RIPng is the way to go.
5. Relations to [I-D.ietf-6man-addr-select-sol]
This method is categorized into the most proactive approach in [I-
D.ietf-6man-addr-select-sol].
In order to select a precise source address for any destination at
any time, the full routing table is required to the host. This is an
unavoidable issue for all the most proactive approaches, if one of
those approaches is solely adopted. It is also the same to the issue
that in order to select the best path for any destination at any
time, the full routing table is required.
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6. Adaptation to a Single Router Multihomed Site.
+---+
|CN |
+-+-+
| 2001:db8:2001::CN
|
+---+---+2001:db8:2000:/36
| |
+---------+ ISP2 |
| | |
| +-------+
|
+---+---+2001:db8:1000:/36 +-------+2001:db8:3000::/36
| | | |
| ISP1 +-------------------+ ISP3 |
| | | |
+---+---+ +---+---+
| |
| |
+------------+ +------------+
2001:db8:1000:R| |2001:db8:3000:R
++-++
2001:db8:1001::/48| R |2001:db8:3001::/48
+-+-+
2001:db8:1001:1:R | 2001:db8:3001:1:R
|
2001:db8:1001:1:EN | 2001:db8:3001:1:EN
+-+-+
|EN |
+---+
Routing Tables:
R:
Destination Next Hop
2001:db8:1000::/36 address_of_ISP1's_router
2001:db8:2000::/36 address_of_ISP1's_router
2001:db8:3000::/36 address_of_ISP3's_router
EN:
Destination Next Hop
2001:db8:1000::/36 2001:db8:1001:1:R
2001:db8:2000::/36 2001:db8:1001:1:R
2001:db8:3000::/36 2001:db8:3001:1:R
Fig.3
This method is also adaptable to a single router multihomed site. In
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the site, the single router should be assigned multiple address
spaces, and it assigns multiple addresses to the downstream
interfaces. This is required when it has only a single downstream
interface. (see Fig. 3)
Author's Address
FUJIKAWA Kenji
ROOT Inc., Kyoto Information Laboratory
59 Minami-yonnotsubo-cho, Iwakura, Sakyo-ku, Kyoto 606-0033, Japan
Phone: +81-3-5436-8380 (Ext. 1593)
Email: fujikawa@root-hq.com
References
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[I-D.ietf-6man-addr-select-sol]
Matsumoto, A., at el., Solution approaches for
address-selection problems,
draft-ietf-6man-addr-select-sol-00.txt (work in progress),
January 2007.
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