One document matched: draft-tsou-bfd-ds-lite-00.txt
Internet Engineering Task Force T. Tsou, Ed.
Internet-Draft Huawei Technologies (USA)
Intended status: Informational March 5, 2012
Expires: September 5, 2012
BFD Support DS-Lite
draft-tsou-bfd-ds-lite-00
Abstract
In DS-Lite, the tunnel is not associated with any state information,
which makes it difficult to manage and diagnose. Bidirectional
Forwarding Detection (BFD) can be used in this case to detect the
state of the IPv4-in-IPv6 tunnel by creating a BFD session between
the DS-Lite CPE and the Address Family Transitional Router (AFTR).
Status of this Memo
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This Internet-Draft will expire on August 30, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. BFD for DS-Lite . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. DS-Lite Scenario . . . . . . . . . . . . . . . . . . . . . 4
3.2. Parameters for BFD . . . . . . . . . . . . . . . . . . . . 4
3.3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 5
3.4. Failover . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.5. Implementation Considerations . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
In DS-Lite [RFC6333], there is no status information about the IPv4-
in-IPv6 tunnel; no keep-alive mechanism is available. It is
difficult to know whether the tunnle is up or down, which creates a
problem for operation and maintenance. Although administor can use
ping to test the connectivity, that is not a commonly used keep-alive
mechanism.
BFD [RFC5880] is a mechanism intended to detect faults in the
bidirectional path. It is usually used in conjunction with
applications like OSPF, IS-IS, etc, for fast fault recovery/fast re-
route.
BFD [RFC5880] can be used in DS-Lite, by creating a BFD session
between the CPE and the AFTR to provide tunnel status information.
If a fault is detected the CPE can try to create a DS-Lite tunnel
with another AFTR and terminate the existing one, so as to continue
network service.
[I-D.vinokour-bfd-dhcp] proposes using a DHCP option to distribute
BFD parameters to the CPE. But in case of DS-Lite, some of the key
BFD parameters are already available (e.g., peer IP address is
already available), and other parameters can be negotiated by BFD
signaling or statically configured, so that no extra DHCP option(s)
need to be defined.
1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. Terminology
BFD: Bidirectional Forwarding Detection.
AFTR: Address Family Transition Router.
CPE: Customer Premise Equipment (i.e., the DS-Lite B4).
FQDN Fully Qualified Domain Name
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3. BFD for DS-Lite
3.1. DS-Lite Scenario
In DS-Lite [RFC6333], the BFD packet SHOULD be sent through an IPv4-
in-IPv6 tunnel, as shown in Figure 1. The IPv4 addresses of the CPE
and AFTR SHOULD be the endpoints of a BFD session.
+--------------+ +--------------+
+-----+ | | +------+ | |
| |-----+--------------+-----| | | |
| CPE | IPv6 Tunnel | AFTR |-----| IPv4 Network |
| |-----+--------------+-----| | | |
+-----+ | IPv6 Network | +------+ | |
192.0.0.2 +--------------+ 192.0.0.1 +--------------+
Figure 1: DS-Lite Scenario
3.2. Parameters for BFD
In order to set up a BFD session, the following parameters are
needed, as shown in Section 4.1 of [RFC5880]:
o Peer IP address
o My Discriminator
o Your Discriminator
o Desired Min TX Interval
o Required Min RX Interval
o Required Min Echo RX Interval
In DS-Lite [RFC6334], the CPE WAN-side IPv4 address is a well-known
address 192.0.0.2, and the AFTR's IPv4 address is 192.0.0.1, as
defined in section 5.7 of [RFC6333]. Because all the CPEs and AFTRs
use the same well-known IP addresses, IPv4 addresses are not
sufficient for setting up a BFD session. From the CPE's point of
view, the CPE needs to create an IPv6 tunnel to an AFTR so as to get
network connectivity to the AFTR, and send IPv4 BFD packets through
the tunnel to manage it. From the AFTR's point of view, a lot of
CPEs with the same IPv4 address will setup BFD sessions with it, so
in order to distinguish the CPEs, the AFTR needs to take account of
both the IPv4 address and the IPv6 address of the CPE that
establishes a BFD session. [Editor's note - I think this isn't quite
consistent with the following from Section 6 of RFC 5881:
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"On a point-to-point link, the source address of a BFD Control packet
MUST NOT be used to identify the session. This means that the
initial BFD packet MUST be accepted with any source address, and that
subsequent BFD packets MUST be demultiplexed solely by the Your
Discriminator field (as is always the case)."]
[PTT - Should be a reference to RFC 5881 Section 3 here.] When a CPE
goes online and sets up a tunnel with an AFTR, then it should
initiate a BFD session with the AFTR, generating a local
discriminator, and send the first BFD packet to AFTR with peer
discriminator set to zero; when receiving the first BFD packet from
CPE, AFTR should get a local discriminator and put it in the response
BFD packet to the CPE.
The other parameters listed above can be negotiated by BFD signaling,
and initial values can be configured on the CPE and AFTR.
3.3. Procedures
In DS-Lite [RFC6333], when a CPE gets online, it will be assigned an
IPv6 prefix/address, and also the FQDN of the AFTR, as defined in
[RFC6334]. The CPE will create an IPv6 tunnel to the AFTR with
which, along with the well known CPE IPv4 address 192.0.0.2 and AFTR
IPv4 address 192.0.0.1, the CPE can initiate a BFD session to the
AFTR. BFD packets will be sent through DS-Lite tunnel. [PTT- should
refer to RFC 5881 Section 4 for source and destination port
settings.]
When sending out the first BFD packet, the CPE can generate a unique
local discriminator, and set the remote discriminator to zero. When
the AFTR receive the first BFD packet from a CPE, the AFTR will also
generate a corresponding local discriminator, and put it in the
response packet to the CPE. This will finish the discriminator
negotiation in the CPE to AFTR direction, without any manual
configuration.
When the AFTR receives the first packet from a CPE, AFTR will get the
IPv6 address and discriminator of the CPE, so that the AFTR can
initiate the BFD session in the other direction and a similar
discriminator negotiation can be carried out.
The procedure to set up a BFD session is illustrated below:
[Is this really necessary given that it is covered by RFC
5880?]
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CPE AFTR
| (CPE get online) |
| BFD DOWN |
| -------------------------------------------------------> |
| local discriminator = 1234 |
| remote discriminator = 0 |
| |
| BFD INIT |
| <------------------------------------------------------- |
| local discriminator = 5678 |
| remote discirminator = 1234 |
| |
| BFD UP |
| -------------------------------------------------------> |
| local discriminator = 1234 |
| remote discriminator = 5678 |
| |
| (BFD session in one direction is finished) |
| (Start BFD session in the other direction) |
| |
| BFD DOWN |
| <------------------------------------------------------- |
| local discriminator = 5678 |
| remote discriminator = 1234 |
| |
| BFD INIT |
| -------------------------------------------------------> |
| local discriminator = 1234 |
| remote discriminator = 5678 |
| |
| BFD UP |
| <------------------------------------------------------- |
| local discriminator = 5678 |
| remote discriminator = 1234 |
| |
| (Bidirectional session created!) |
| |
Figure 2: BFD session setup procedures
3.4. Failover
The FQDN of the AFTR is sent to CPE via a DHCP option, as defined in
[RFC6334]. Multiple IP addresses can be configured for an FQDN on
the DNS server. If BFD detects a fault on the link to an AFTR, the
CPE can choose another AFTR address and use a different AFTR to
provide network sevice.
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If anycast is used for load balancing and failover, there might be an
ICMP error message problem, that is, when a packet is sent from AFTR
to CPE, one of the routers along the path may generate a error ICMP
message, e.g., packet too big, and the error message is not sent back
to the source AFTR, but sent to another AFTR. [PTT - this has to do
with DS-Lite, but not with BFD. Off topic!]
3.5. Implementation Considerations
BFD is usually used for quick fault detection, at a very small time
scale, e.g. milliseconds. But in DS-Lite, it may not be necessary to
detect faults in such a short time. On the other hand, an AFTR may
need to support tens of thousands of CPEs, which means the AFTR will
need to support the same number of BFD sessions. In order to meet
performance requirements on the AFTR, it may be necessary to extend
the time period between BFD packet transmissions to a longer time,
e.g., 10s or 30s.
4. IANA Considerations
This memo includes no request to IANA.
5. Security Considerations
In DS-Lite [RFC6333], the CPE may not be directly connected to the
AFTR; there may be other routers between them. Then there are
potential spoofing problems, as described in [RFC5883]. Hence
cryptographic authentication should be used as described in
[RFC5880].
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
June 2010.
[RFC5882] Katz, D. and D. Ward, "Generic Application of
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Bidirectional Forwarding Detection (BFD)", RFC 5882,
June 2010.
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC 5883, June 2010.
[RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", RFC 6333, August 2011.
[RFC6334] Hankins, D. and T. Mrugalski, "Dynamic Host Configuration
Protocol for IPv6 (DHCPv6) Option for Dual-Stack Lite",
RFC 6334, August 2011.
6.2. Informative References
[I-D.vinokour-bfd-dhcp]
Vinokour, V., "Configuring BFD with DHCP and Other
Musings", May 2008.
Author's Address
Tina Tsou (editor)
Huawei Technologies (USA)
2330 Central Expressway
Santa Clara CA 95050
USA
Phone: +1 408 330 4424
Email: tina.tsou.zouting@huawei.com
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