Internet DRAFT - draft-akiya-bfd-seamless-sr
draft-akiya-bfd-seamless-sr
Internet Engineering Task Force N. Akiya
Internet-Draft C. Pignataro
Intended status: Standards Track N. Kumar
Expires: August 27, 2015 Cisco Systems
February 23, 2015
Seamless Bidirectional Forwarding Detection (S-BFD) for Segment Routing
draft-akiya-bfd-seamless-sr-04
Abstract
This document defines procedures to use Seamless Bidirectional
Forwarding Detection (S-BFD) for the Segment Routing environment.
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].
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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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on August 27, 2015.
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Inheritance of Code Points and Procedures . . . . . . . . . . 2
3. SBFDInitiator Models . . . . . . . . . . . . . . . . . . . . 3
3.1. Uncontrolled Return Path . . . . . . . . . . . . . . . . 3
3.2. Controlled Return Path . . . . . . . . . . . . . . . . . 3
4. S-BFD Echo Recommendations . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. Contributing Authors . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Seamless Bidirectional Forwarding Detection (S-BFD),
[I-D.ietf-bfd-seamless-base], defines a generalized mechanism to
allow network nodes to seamlessly perform continuity checks to remote
entities. This document defines necessary procedures to use S-BFD on
the Segment Routing environment described by
[I-D.ietf-spring-segment-routing].
The reader is expected to be familiar with the IP, MPLS, Segment
Routing [I-D.ietf-spring-segment-routing], BFD [RFC5880] and S-BFD
[I-D.ietf-bfd-seamless-base] terminologies and protocol constructs.
2. Inheritance of Code Points and Procedures
S-BFD on the Segment Routing MUST use the code points and procedures
defined in [I-D.ietf-bfd-seamless-ip] regarding following aspects:
o S-BFD Control UDP Port
o S-BFD Echo UDP Port
o S-BFD Control Packet Demultiplexing
o Initiator Procedures
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o Responder Procedures
The Segment Routing on the MPLS data plane is to use MPLS based
procedures, and the Segment Routing on the IPv6 data plane is to use
IP based procedures.
3. SBFDInitiator Models
The S-BFD technology defines an SBFDReflector and how SBFDInitiators
speak to SBFDReflectors. Outside of these definitions,
implementations are free to be flexible in terms of how
SBFDInitiators behave. The packet steering capability of the Segment
Routing allows for, at very high level, two distinct SBFDInitiator
models. This section describes the two SBFDInitiator models as an
implementation reference.
3.1. Uncontrolled Return Path
A network node sending S-BFD control packets to a remote target with
particular segment stack will allow the network node to determine
whether or not such packets reach the intended remote target. The
network node can conclude the reachability when valid response S-BFD
control packets are received back. In opposite, the network node can
conclude the lack of reachability when valid response S-BFD control
packet are not received back. Because S-BFD control packets back
from the responder to the initiator will be IP routed, how S-BFD
control packets traverse the network back to the initiator is
uncontrolled. If the network employs good set of local protection
mechanisms, this may not be concerning and the model of only sending
S-BFD control packets may be sufficient.
In this model, SBFDInitiator is to send only S-BFD control packets.
3.2. Controlled Return Path
In addition to SBFDInitiator sending S-BFD control packets, described
in Section 3.1, S-BFD echo packets can also be sent.
+-----B-------C-----+
/ \
A-----------E-----------D
\ /
+-----F-------G-----+
Forward Paths: A-B-C-D
IP Return Paths: D-E-A
Figure 1: S-BFD Echo Example
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Node A sending S-BFD control packets with segment stack {B, C, D}
will cause S-BFD control packets to traverse the paths A-B-C-D in the
forward direction. The response S-BFD control packets from node D
back to node A will be IP routed and will traverse the paths D-E-A.
The SBFDInitiator sending such packets can also send S-BFD echo
packets with segment stack {B, C, D, C, A}. S-BFD echo packets will
u-turn on node D and traverse the paths D-C-B-A. If required, the
SBFDInitiator can possess multiple types of S-BFD echo packets, with
each having varying return paths. In this particular example, the
SBFDInitiator can be sending two types of S-BFD echo packets in
addition to S-BFD control packets.
o S-BFD control packets
* Segment stack: {B, C, D}
* Return path: D-E-A
o S-BFD echo packets #1
* Segment stack: {B, C, D, C, A}
* Return path: D-C-B-A
o S-BFD echo packets #2
* Segment stack: {B, C, D, G, A}
* Return path: D-G-F-A
The SBFDInitiator can correlate the result of each packet type to
determine the nature of the failure. One such example of failure
correlation is described in the figure below.
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+---+-----------------------------------------------------------+
| | S-BFD Echo Pkt |
| +------------------------------------+----------------------+
| | Success | Failure |
+-+-+------------------------------------+----------------------+
| |S| | |
|S|u| | |
|||c| |Forward SID stack good|
|B|c| All is well |Return SID stack bad |
|F|e| |Return IP path good |
|D|s| | |
| |s| | |
|C+-+----------------------+-------------+----------------------+
|t|F|Forward SID stack good| | |
|r|a|Return SID stack good |Send Alert | |
|l|i|Return IP path bad |Discrim S-BFD| |
| |l+--------- OR ---------+w/ Forward |Forward SID stack bad |
|P|u|Forward SID stack is |SID stack to | |
|k|r|terminating on wrong |differentiate| |
|t|e|node | | |
+-+-+----------------------+-------------+----------------------+
Figure 2: SBFDInitiator Failure Correlation Example
4. S-BFD Echo Recommendations
o It is RECOMMENDED to compute and use smallest number of segment
stack to describe the return path of S-BFD echo packets to prevent
the segment stack being too large. How SBFDInitiator determines
when to use S-BFD echo packets and how to identify corresponding
segment stack for the return paths are outside the scope of this
document.
o It is RECOMMENDED that SBFDInitiator does not send only S-BFD echo
packets. S-BFD echo packets are crafted to traverse the network
and to come back to self, thus there is no guarantee that S-BFD
echo are u-turning on the intended remote target. On the other
hand, S-BFD control packets can verify that segment stack of the
forward direction reaches the intended remote target. Therefore,
an SBFDInitiator SHOULD send S-BFD control packets when sending
S-BFD echo packets.
o It is RECOMMENDED that, for Segment Routing on the MPLS data
plane, destination IP address of S-BFD echo packets is chosen from
the 127/8 range for IPv4 and from the 0:0:0:0:0:FFFF:7F00/104
range for IPv6.
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5. Security Considerations
Security considerations for S-BFD are discussed in
[I-D.ietf-bfd-seamless-base] and [I-D.ietf-bfd-seamless-ip].
6. IANA Considerations
This document does not request any new code points from IANA.
7. Acknowledgements
Authors would like to thank Marc Binderberger from Cisco Systems for
providing valuable comments.
8. Contributing Authors
Dave Ward
Cisco Systems
Email: wardd@cisco.com
Tarek Saad
Cisco Systems
Email: tsaad@cisco.com
Siva Sivabalan
Cisco Systems
Email: msiva@cisco.com
9. References
9.1. Normative References
[I-D.ietf-bfd-seamless-base]
Akiya, N., Pignataro, C., Ward, D., Bhatia, M., and J.
Networks, "Seamless Bidirectional Forwarding Detection
(S-BFD)", draft-ietf-bfd-seamless-base-04 (work in
progress), January 2015.
[I-D.ietf-bfd-seamless-ip]
Akiya, N., Pignataro, C., and D. Ward, "Seamless
Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6
and MPLS", draft-ietf-bfd-seamless-ip-01 (work in
progress), January 2015.
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[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J.,
and E. Crabbe, "Segment Routing Architecture", draft-ietf-
spring-segment-routing-01 (work in progress), February
2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
Authors' Addresses
Nobo Akiya
Cisco Systems
Email: nobo@cisco.com
Carlos Pignataro
Cisco Systems
Email: cpignata@cisco.com
Nagendra Kumar
Cisco Systems
Email: naikumar@cisco.com
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