INTERNET-DRAFT Sam Aldrin
Intended Status: Informational (Huawei)
Expires: August 7, 2014 Manav Bhatia
(Alcatel-Lucent)
Greg Mirsky
(Ericsson)
Nagendra Kumar
(Cisco)
Satoru Matsushima
(Softbank)
February 3, 2014
Seamless Bidirectional Forwarding Detection (BFD) Use Case
draft-aldrin-bfd-seamless-use-case-00
Abstract
This document provides various use cases for Bidirectional Forwarding
Detection (BFD) such that simplified solution and extensions could be
developed for detecting forwarding failures.
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Copyright and License Notice
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Copyright (c) 2014 IETF Trust and the persons identified as the
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Contributors . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction to Seamless BFD . . . . . . . . . . . . . . . . . 4
3. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Unidirectional forwarding path Validation . . . . . . . . . 5
3.2. Validation of forwarding path prior to traffic switching . 5
3.3. Centralized Traffic Engineering . . . . . . . . . . . . . . 6
3.4. BFD in Centralized Segment Routing . . . . . . . . . . . . 6
3.5. BFD to Efficiently Operate under Resource Constraints . . . 7
3.6. BFD for Anycast Address . . . . . . . . . . . . . . . . . . 7
3.7. BFD Fault Isolation . . . . . . . . . . . . . . . . . . . . 7
3.8. Multiple BFD Sessions to Same Target . . . . . . . . . . . 7
4 Security Considerations . . . . . . . . . . . . . . . . . . . . 9
5 IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6 References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1 Normative References . . . . . . . . . . . . . . . . . . . 9
6.2 Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1 Introduction
Bidirectional Forwarding Detection (BFD) is a light weight protocol,
as defined in [RFC5880], used to detect forwarding failures. Various
protocols and applications rely on BFD for failure detection. Even
though the protocol is simple and light weight, there are certain use
cases, where a much faster setting up of sessions and continuity
check of the data forwarding paths is necessary. This document
identifies those use cases such that if necessary enhancements could
be made to BFD protocol to meet those requirements.
There are various ways to detecting faults and BFD protocol was
designed to be a light weight "Hello" protocol to detect data plane
failures. With dynamic provisioning of forwarding paths at a large
scale, establishing BFD sessions for each of those paths creates
complexity, not only from operations point of view, but also the
speed at which these sessions could be established or deleted. The
existing session establishment mechanism of the BFD protocol need to
be enhanced in order to minimize the time for the session to come up
and validate the forwarding path.
This document specifically identify those cases where certain
requirements could be derived and could be used as reference, so
that, new protocol enhancements could be developed. Whilst the use
cases could be used as reference for certain requirements, it is
outside the scope of this documents to identify all of the
requirements. Specific solutions and enhancement proposals are
outside the scope of this document as well.
1.1 Terminology
The reader is expected to be familiar with the BFD, IP, MPLS and SR
terminology and protocol constructs. This section identifies only
the new terminology introduced.
1.2 Contributors
Carlos Pignataro
Cisco Systems
Glenn Hayden
ATT
Santosh P K
Juniper
Mach Chen
Huawei
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Nobo Akiya
Cisco Systems
2. Introduction to Seamless BFD
BFD as defined in standard [RFC5880] requires two network nodes, as
part of handshake, exchange discriminators. This will enable the
sender and receiver of BFD packets of a session to be identified and
check the liveness of the forwarding path. [RFC5881] defines single
hop BFD where as [RFC5883] and [RFC5884] defines multi-hop BFD.
In order to establish BFD sessions between network entities and
seamlessly be able to have the session up and running, BFD protocol
should be capable of doing that. These sessions have to be
established a priori to traffic flow and ensure the forwarding path
is available and connectivity is present. With handshake mechanism
within BFD protocol, establishing sessions at a rapid rate and
ensuring the validity or existence of working forwarding path, prior
to the session being up and running, becomes complex and time
consuming. In order to achieve seamless BFD sessions, it requires a
mechanism where the ability to specify the discriminators and the
ability to respond to the BFD control packets by the network node,
should already be negotiated ahead of the session becoming active.
Seamless BFD by definition will be able to provide those mechanisms
within the BFD protocol in order to meet the requirements and
establish BFD sessions seamlessly, with minimal overhead, in order to
detect forwarding failures.
As an example of how Seamless BFD (S-BFD) works, a set of network
entities are first identified, to which BFD sessions have to be
established. Each of those network nodes, will be assigned a special
BFD discriminator, to establish a BFD session. These network nodes
will also create a BFD session instance that listens for incoming BFD
control packets. Mappings between selected network entities and
corresponding special BFD discriminators are known to other network
nodes belonging in the same network. A network node in such network
is then able to send a BFD control packet to a particular target with
corresponding special BFD discriminator. Target network node, upon
reception of such BFD control packet, will transmit a response BFD
control packet back to the sender.
3. Use cases
As per the BFD protocol RFC[5880], BFD sessions are established using
handshake mechanism prior to validating the forwarding path. This
section outlines some of the use cases where the existing mechanism
may not be able to satisfy the requirements. In addition, some of the
use cases will also be identifying the need for seamless BFD session
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establishment and detection of forwarding failures using BFD.
3.1. Unidirectional forwarding path Validation
Even though bidirectional verification of forwarding path is useful,
there are scenarios when only one side of the BFD, not both, are
interested in verifying connectivity between a pair of systems. One
such case is, when a static route uses BFD to validate reachability
to the next-hop IP router. In this case, the static route is
established from one network entity to another. The requirement in
this case is only to validate the forwarding path for that statically
established path. Validating the reverse direction is not required in
this case. Many of these network scenarios are being proposed as part
of segment routing [TBD]. Another example is when a unidirectional
tunnel uses BFD to validate reachability to the egress node.
If the traditional BFD is to be used, the target network entity has
to be provisioned as well, even though the reverse path validation
with BFD session is not required. But with unidirectional BFD, the
need to provision on the target network entity is not needed. Once
the mechanism within the BFD protocol is in place, where the source
network entity know the target network entity's discriminator, it
starts the session right away. When the target network entity
receives the packet, it knows that BFD packet, based on the
discriminator and process it. This do not require to have a bi-
directional session establishment, hence the two way handshake to
exchange discriminators is not needed as well.
The primary requirement in this use case is to enable session
establishment from source network entity to target network entity.
This translates to, the target network entity for the BFD session,
upon receiving the BFD packet, should start processing for the
discriminator received. This will enable the source network entity to
establish a unidirectional BFD session without bidirectional
handshake of discriminators for session establishment.
3.2. Validation of forwarding path prior to traffic switching
BFD provides data delivery confidence when reachability validation is
performed prior to traffic utilizing specific paths/LSPs. However
this comes with a cost where traffic is prevented to use such
paths/LSPs until BFD is able to validate the reachability, which
could take seconds due to BFD session bring-up sequences [RFC5880],
LSP ping bootstrapping [RFC5884], etc. This use case does not
require to have sequences for session negotiation and discriminator
exchanges in order to establish the BFD session.
When these sequences for handshake are eliminated, the network
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entities need to know what the discriminator values to be used for
the session. The same is the case for S-BFD i.e., when the three-way
handshake mechanism is eliminated during bootstrap of BFD sessions.
Due to this faster reachability validation of BFD provisioned
paths/LSPs could be achieved. In addition, it is expected that some
MPLS technologies will require traffic engineered LSPs to get created
dynamically, driven by external applications (ex: Software Defined
Networks a.k.a. SDN). It would be desirable to perform BFD
validation very quickly to allow applications to utilize dynamically
created LSPs in timely manner.
3.3. Centralized Traffic Engineering
Various technologies in the SDN domain have evolved which involves
controller based networks, where the intelligence, traditionally
within the control plane, is separated and resides in a logically
centralized place. There are various controllers which performs this
exact function in establishing forwarding paths for the data flow.
Traffic engineering is one important function, where the traffic is
engineered depending upon various attributes of the traffic as well
as the network state.
When the intelligence of the network resides in the centralized
entity, ability to manage and maintaining the dynamic network becomes
a challenge. One way to ensure the forwarding paths are valid and
working is to establish BFD sessions within the network. When traffic
engineering tunnels are created, it is very much necessary to ensure
the forwarding paths are working prior to switching the traffic onto
the traffic engineering tunnels. In the absence of control plane
protocols, it is not only the desire to verify the forwarding path
but also the the desired path in the network. With tunnels being
engineered from the centralized entity, when network state changes,
traffic has to be switched without much latency and black holing of
the data.
Traditional BFD session establishment and validation of the
forwarding path does become bottleneck in the case of centralized
traffic engineering. When the controller or other centralized entity
could quickly verify a forwarding path of the TE tunnel , it could
steer the traffic on to the traffic engineered tunnel very quickly.
This is especially useful and needed when the scale of the network
and number of TE tunnels is too high. Session negotiation and
establishment of BFD sessions to identify valid paths is way to high
in terms of time and providing network redundancy becomes a critical
issue.
3.4. BFD in Centralized Segment Routing
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Centralized controller based segment routing network monitoring
techniques, is described in [I-D.geib-spring-oam-usecase]. In
validating this use case, one of the requirements is to ensure the
BFD packet's behavior is according to the requirement and monitoring
of the segment, where the packet is U-turned at the expected node.
One of the criterion is to ensure the continuity check to the
adjacent segment-id.
3.5. BFD to Efficiently Operate under Resource Constraints
When BFD sessions are being setup, torn down or parameters (i.e.
interval, multiplier, etc) are being modified, BFD protocol requires
additional packets outside of scheduled packet transmissions to
complete the negotiation procedures (i.e. P/F bits). There are
scenarios where network resources are constrained: a node may require
BFD to monitor very large number of paths, or BFD may need to operate
in low powered and traffic sensitive networks (i.e microwave, low
powered nano-cells, etc). In these scenarios, it is desirable for BFD
to slow down, speed up, stop or resume at will without requiring
additional BFD packets to be exchanged.
3.6. BFD for Anycast Address
BFD protocol requires the two endpoints to host BFD sessions, both
sending packets to each other. This BFD model does not fit anycast
address monitoring, as BFD packets transmitted from a network node to
an anycast address will reach only one of potentially many network
nodes hosting the anycast address.
3.7. BFD Fault Isolation
BFD multi-hop and BFD MPLS traverse multiple network nodes. BFD has
been designed to declare failure upon lack of consecutive packet
reception, which can be caused by any fault anywhere along the path.
Fast failure detection provides great benefits, as it can trigger
recovery procedures rapidly. However, operators often have to follow
up, manually or automatically, to attempt to identify the failure
which caused the BFD sessions to fail. Usage of other tools to
isolate the fault may cause the packets to traverse differently
throughout the network (i.e. ECMP). In addition, longer it takes from
BFD session failure to fault isolation attempt, more likely that
fault cannot be isolated, i.e. fault can get corrected or routed
around. If BFD had built-in fault isolation capability, fault
isolation can get triggered at the earliest sign of fault and such
packets will get load balanced in very similar way, if not the same,
as BFD packets which went missing.
3.8. Multiple BFD Sessions to Same Target
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BFD is capable of providing very fast failure detection, achieve
through relevant network nodes continuously transmitting BFD packets
at negotiated rate. If BFD packet transmission is interrupted, even
for very short period of time, that can result in BFD to declare
failure irrespective of path liveliness. It is possible, on a system
where BFD is running, for certain events, intentionally or
unintentionally, to cause a short interrupt to BFD packet
transmissions. With distributed BFD architectures, this can be
protected, if a node was to run multiple BFD sessions to targets,
hosted on different parts of the system (ex: different CPU
instances). This can reduce BFD false failures, resulting in more
stable network.
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4 Security Considerations
There are no new security considerations introduced by this draft.
5 IANA Considerations
There are no new IANA considerations introduced by this draft
6 References
6.1 Normative References
[KEYWORDS] 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)", RFC5880, June 2010.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC5881, June 2010.
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC5883, June 2010.
[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC5884, June 2010.
6.2 Informative References
[EVILBIT] Bellovin, S., "The Security Flag in the IPv4 Header",
RFC 3514, April 1 2003.
[RFC5513] Farrel, A., "IANA Considerations for Three Letter
Acronyms", RFC 5513, April 1 2009.
[RFC5514] Vyncke, E., "IPv6 over Social Networks", RFC 5514, April 1
2009.
Authors' Addresses
Sam Aldrin
Huawei Technologies
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2330 Central Expressway
Santa Clara, CA 95051
EMail: aldrin.ietf@gmail.com
Manav Bhatia
Alcatel-Lucent
EMail: manav.bhatia@alcatel-lucent.com
Satoru Matsushima
Softbank
EMail: satoru.matsushima@g.softbank.co.jp
Greg Mirsky
Ericsson
EMail: gregory.mirsky@ericsson.com
Nagendra Kumar
Cisco
EMail: naikumar@cisco.com
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