Internet DRAFT - draft-tsou-softwire-bfd-ds-lite
draft-tsou-softwire-bfd-ds-lite
Internet Engineering Task Force T. Tsou
Internet-Draft Huawei Technologies (USA)
Intended status: Informational B. Li
Expires: August 17, 2014 C. Zhou
Huawei Technologies
J. Schoenwaelder
Jacobs University Bremen
R. Penno
Cisco Systems, Inc.
M. Boucadair
France Telecom
February 13, 2014
DS-Lite Failure Detection and Failover
draft-tsou-softwire-bfd-ds-lite-06
Abstract
In DS-Lite, the tunnel is stateless, not associated with any state
information, and the CGN function at the AFTR is stateful.
Currently, there is no failure detection and failover mechanism for
both stateless tunnel and stateful CGN function, which makes it
difficult to manage and diagnose if there is a problem. This draft
analyzes the applicability of some of the possible solutions.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 17, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Failover Mechanisms . . . . . . . . . . . . . . . . . . . . . 3
3.1. Anycast Approach . . . . . . . . . . . . . . . . . . . . . 4
3.2. VRRP Approach . . . . . . . . . . . . . . . . . . . . . . 4
4. Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Bidirectional Forwarding Detection (BFD) . . . . . . . . . 4
4.1.1. DS-Lite Scenario . . . . . . . . . . . . . . . . . . . 5
4.1.2. Parameters for BFD . . . . . . . . . . . . . . . . . . 5
4.1.3. Elements of Procedure . . . . . . . . . . . . . . . . 6
4.1.4. BFD for NAT failure detection . . . . . . . . . . . . 6
4.1.5. Implementation Considerations . . . . . . . . . . . . 6
4.2. Port Control Protocol (PCP) . . . . . . . . . . . . . . . 7
4.3. ICMP Echo Request / Echo Reply (PING) . . . . . . . . . . 7
4.4. Comparison of Different Solutions . . . . . . . . . . . . 8
5. State Synchronization and Session Re-establishment . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
In DS-Lite [RFC6333], the IPv4-in-IPv6 DS-Lite tunnel is stateless,
no status information about the tunnel is available, and no keep-
alive mechanism is available. It is difficult to know whether the
tunnel is up or down; and if there is a link problem, the Basic
Bridging BroadBand (B4) element can not automatically switch to
another Address Family Transition Router (AFTR) so as to continue the
network service automatically, without the involvement of operators.
Besides, In DS-Lite [RFC6333], the CGN function at the AFTR is
stateful and there is no mechanism to detect whether the NAT44 CGN is
functioning in the AFTR. These will create problems for network
operation and maintenance.
Possible solutions for failure detection include the usage of
Bidirectional Forwarding Detection (BFD), the Port Control Protocol
(PCP), and ICMP Echo Request / Echo Reply (PING). The properties of
these solutions are discussed in this document and guidelines are
provided how to implement failure detection and automatic failover.
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
AFTR: Address Family Transition Router.
B4: Basic Bridging BroadBand.
BBF: BroadBand Forum.
BFD: Bidirectional Forwarding Detection.
CPE: Customer Premise Equipment (i.e., the DS-Lite B4).
FQDN Fully Qualified Domain Name.
PCP Port Control Protocol.
3. Failover Mechanisms
The FQDN of the AFTR is sent to the B4 element via a DHCP option, as
defined in [RFC6334]. Multiple IP addresses can be configured for
the FQDN of an AFTR on the DNS server. If a B4 element detects a
failure on the link to the AFTR, the B4 element MUST terminate the
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current DS-Lite tunnel, choose another AFTR address in the list, and
create a tunnel to the new AFTR. If necessary, the B4 element SHOULD
re-configure the connectivity test tool accordingly and restart the
test procedures.
3.1. Anycast Approach
Anycasts may also be used for failover. But there is an ICMP-error-
message problem with anycast, that is, when a packet is sent from the
AFTR to a B4 element, if one of the routers along the path generates
an ICMP error message, e.g., Packet Too Big (PTB), then the error
message may not be sent back to the source AFTR but to another AFTR.
There's also a problem with anycast for stateful CGN/AFTR. If there
is an asymmetric path though the CGNs, then return path traffic will
be dropped as there is no corresponding state table entry in the
AFTR.
3.2. VRRP Approach
For active/passive HA in NAT gateways, it's quite common to have a
single virtual address offered by VRRP (or a proprietary equivalent)
that the upstream routers will use as their next hop. In the event
that the master CGN fails, the standby takes over the virtual L3
address. If a VRRP based virtual address is used as the tunnel
endpoint, then the clients wouldn't need to be aware of the failover.
4. Solutions
4.1. Bidirectional Forwarding Detection (BFD)
Bidirectional Forwarding Detection [RFC5880] (BFD) is a mechanism
intended to detect faults in a bidirectional path. It is usually
used in conjunction with applications like OSPF, IS-IS, for fast
fault recovery and fast re-route [RFC5882]. BFD is being made
mandatory for keep-alive for subscriber sessions, including DS-Lite,
by the BroadBand Forum (BBF) [WT-146].
BFD can be used in DS-Lite, by creating a BFD session between the B4
element and the AFTR to provide tunnel status information. If a
fault is detected, the B4 element can try to create a DS-Lite tunnel
with another AFTR and terminate the existing one, so as to continue
network service. BFD could also be used to detect the CGN state at
the AFTR, but the detection should be based on per-user.
[I-D.vinokour-bfd-dhcp] proposes using a DHCP option to distribute
BFD parameters to B4 elements. But in case of DS-Lite, some of the
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key BFD parameters are already available (e.g., peer IP address), and
other parameters can be negotiated by BFD signaling or statically
configured, so that no extra DHCP option(s) need to be defined.
4.1.1. 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 B4
element and the AFTR SHOULD be the endpoints of a BFD session.
+--------------+ +--------------+
+------+ | | +------+ | |
| |-----+--------------+-----| | | |
| CPE | IPv6 Tunnel | AFTR |-----| IPv4 Network |
| (B4) |-----+--------------+-----| | | |
+------+ | IPv6 Network | +------+ | |
192.0.0.2 +--------------+ 192.0.0.1 +--------------+
Figure 1: DS-Lite Scenario
4.1.2. 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
B4's WAN-side IPv4 address is the well-known address 192.0.0.2, and
the AFTR's well-known IPv4 address is 192.0.0.1, as defined in
section 5.7 of [RFC6333]. The B4 element 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.
The other parameters listed above can be negotiated by BFD signaling,
and initial values can be configured on B4 elements and AFTRs.
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4.1.3. Elements of Procedure
When a B4 element gets online, it will be assigned an IPv6 prefix or
address, and also the FQDN of the AFTR, as defined in [RFC6334]. The
B4 element will create an IPv6 tunnel to the AFTR with which the B4
element can initiate a BFD session to the AFTR. BFD packets will be
sent through the DS-Lite tunnel. As defined in section 4 of
[RFC5881], BFD control packets MUST be sent in UDP packets with
destination port 3784, and BFD echo packets MUST be sent in UDP
packets with destination port 3785.
When sending out the first BFD packet, the B4 element can generate a
unique local discriminator, and set the remote discriminator to zero.
When the AFTR receives the first BFD packet from a B4 element, the
AFTR will also generate a corresponding local discriminator, and put
it in the response packet to the B4 element. This will finish the
discriminator negotiation in the B4 to AFTR direction, without any
manual configuration.
When an AFTR receives the first packet from a B4 element, the AFTR
will get the IPv6 address and discriminator of the B4 element, so
that the AFTR can initiate the BFD session in the other direction and
a similar discriminator negotiation can be carried out.
4.1.4. BFD for NAT failure detection
B4 creates PCP mapping. BFD at AFTR uses an external public
interface (or another external mapping) to send a BFD packet to the
public PCP mapping created by B4. In this case, the AFTR BFD packet
will have a public source IP of interface, which will go through the
NAT, therefore exercising the NAT function. B4 will reply to the
AFTR external interface.
4.1.5. 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 B4 elements, which means an AFTR
will need to support the same number of BFD sessions. In order to
meet performance requirements on an AFTR, it may be necessary to
extend the time period between BFD packet transmissions to a longer
time, e.g., 10s or 30s.
Compared to other solutions, BFD has a simple and fixed packet
format, which is easy to implement by logic devices (e.g., ASIC,
FPGA). Complicated protocols are usually processed by software which
is relatively slow. An AFTR may need to support 10000-20000 users,
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and if the protocol is handled by software, it will bring extra load
to the AFTR.
4.2. Port Control Protocol (PCP)
[RFC6887]PCP is a NAT traversal tool. It can also be used for
network connectivity test if PCP is supported in the network. A
common use case of PCP is to create a pinhole so that external users
can visit the servers located behind a NAT. The lifetime of the
pinhole mapping is usually long, e.g., hours, and the lifetime will
be refreshed periodically by the client before it is expired. For
the purpose of network connectivity tests, a B4 element can create a
mapping in the CGN via PCP, with a short life time, e.g., 10s of
seconds, and keep on refreshing the mapping before it expires. If
any refresh requests fail, the B4 element knows that something is
wrong with the link or the PCP server or the CGN.
In order to detect the network connectivity of the DS-Lite tunnel,
the encapsulation mode MUST be used for PCP: PCP packets are sent
through the DS-Lite tunnel.
PCP can detect the failure of more components of the DS-Lite system.
Besides failures of the link and the routing, it also covers NAT
functions.
4.3. ICMP Echo Request / Echo Reply (PING)
PING is commonly implemented using the Echo Request and Echo Response
messages of the Internet Control Message Protocol (ICMP) [RFC0792]
[RFC4443]. In case of DS-Lite, a B4 element can send Echo Request
packets to the AFTR periodically. If the B4 element does not receive
Echo Response packets for a certain number (e.g., 3) of Echo Request
packets, then the B4 element decides that a fault has been detected.
In order to test the connectivity of DS-Lite tunnel, Echo Request
packets MUST be sent using ICMPv4, rather than ICMPv6.
Since ICMP is an integral part of any IP implementation, the usage of
PING to detect tunnel failures does not require any special
implementation efforts on the B4 elements. However, on AFTRs that
process ICMP messages in software rather than in hardware, the usage
of PING might lead to scalability issues.
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4.4. Comparison of Different Solutions
+--------+-------------+------+-------------------+-------------+
| | |Packet|Additional |Configuration|
| |Availablility|format|functionality |/provisioning|
| | | |ontop of keepalives| overheads |
+--------+-------------+------+-------------------+-------------+
| BFD |Widely used/ | | | |
| |network side,|Simple|Bidirectional | |
| |less used/ |fixed |status | |
| |terminal side| |synchronization | |
+--------+-------------+------+-------------------+ Similar |
| PCP |Less than | |No bidirectional | |
| |BFD/ICMP |Vari- | detection | |
+--------+-------------+able +-------------------+ |
| ICMP |Ubiquitous | |Network/CGN | |
| | | |initiated detection| |
+--------+-------------+------+-------------------+-------------+
Figure 2: Comparison of different solutions
Figure 2 gives a direct comparison among different solutions.
Compared to other solutions, BFD has a simple and fixed packet
format, which is easy to implement by logic devices (e.g., ASIC,
FPGA). Complicated protocols are usually processed by software which
is relatively slow. ICMP is widely used than PCP/BFD, while BFD is
more widely used in the router and CGN side than in the terminal
side. However, from the aspect of failure detection, BFD has
explicit capability of bidirectional status synchronization to
guarantee the consistency of the failure status of both sides. ICMP
could actively initiate status detection from the network side or CGN
side, while PCP could not. PCP has no capability of bidirectional
detection. Considering the configuration/provisioning overheads,
since there is normally TR-069 server at the network management side.
So it is similar for each approach.
From the above comparison, BFD is selected as the failure detection
approach in this document.
5. State Synchronization and Session Re-establishment
There should be a state sync mechanism between active AFTR and backup
AFTR, to synchronize the state of each user between the two AFTRs.
This mechanism is to guarantee that the traffic returning to the B4
is from the backup AFTR, if the service is shifted to that AFTR. The
BFD link for both active AFTR and backup AFTR should be set up in the
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initial state. When the active AFTR is detected in failure, the
service will be shifted to the backup AFTR. If the backup AFTR is
detected in failure, it will notify the network management server to
fix the failure.
In the hot-standby case, the master AFTR and the backup AFTR will
synchronize and backup the session. So there is no need to re-
establish the TCP session in the event of an AFTR failure. But in
the cold-standby case, if there is an active TCP session through the
CGN function of an AFTR, and this AFTR fails, then the TCP session
will need to be re-established by the client because only the
capability is reserved but the session is not backup.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
In the DS-Lite [RFC6333] application, the B4 element may not be
directly connected to the AFTR; there may be other routers between
them. In such a deployment, there are potential spoofing problems,
as described in [RFC5883]. Hence cryptographic authentication SHOULD
be used with BFD as described in [RFC5880] if security is concerned.
8. Acknowledgements
The authors would like to thank Ian Farrer for his valuable comments.
9. References
9.1. Normative References
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
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(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
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.
[RFC6887] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", RFC 6887,
April 2013.
[WT-146] Kavanagh, A., Klamm, F., Boucadair, W., and R. Dec, "WT-
146 Subscriber Sessions (work in progress)", Apr 2012.
9.2. Informative References
[I-D.vinokour-bfd-dhcp]
Vinokour, V., "Configuring BFD with DHCP and Other
Musings", May 2008.
Authors' Addresses
Tina Tsou
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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Brandon Li
Huawei Technologies
M6, No. 156, Beiqing Road, Haidian District
Beijing 100094
China
Phone:
Email: brandon.lijian@huawei.com
Cathy Zhou
Huawei Technologies
China
Phone:
Email: cathy.zhou@huawei.com
Juergen Schoenwaelder
Jacobs University Bremen
Campus Ring 1
Bremen 28759
Germany
Phone:
Email: j.schoenwaelder@jacobs-university.de
Reinaldo Penno
Cisco Systems, Inc.
170 West Tasman Drivee
San Jose, California 95134
USA
Phone:
Email: repenno@cisco.com
Mohamed Boucadair
France Telecom
Rennes,35000
France
Phone:
Email: mohamed.boucadair@orange.com
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