One document matched: draft-ietf-bfd-seamless-use-case-03.xml

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<rfc category="info" docName="draft-ietf-bfd-seamless-use-case-03" ipr="trust200902">
<front>
<title abbrev="S-BFD Use Case">Seamless Bidirectional Forwarding Detection (BFD) Use Case</title>

<author fullname="Sam Aldrin"  surname="Aldrin"
 initials="S.">
      <organization>Google, Inc</organization>
      <address>
        <postal>
          <street>1600 Amphitheatre Parkway </street>
          <!-- Reorder these if your country does things differently -->
          <city>Mountain View</city>
          <region>CA </region>
          <code></code>
          <country></country>
        </postal>
        <phone></phone>
        <email>aldrin.ietf@gmail.com</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
</author>

<author fullname="Manav Bhatia"  surname="Bhatia"
 initials="M.">
      <organization>Ionos Networks</organization>
      <address>
        <postal>
          <street></street>
          <!-- Reorder these if your country does things differently -->
          <city></city>
          <region></region>
          <code></code>
          <country></country>
        </postal>
        <phone></phone>
        <email>manav@ionosnetworks.com</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
</author>

<author fullname="Satoru Matsushima"  surname="Matsushima"
 initials="S.">
      <organization>Softbank</organization>
      <address>
        <postal>
          <street></street>
          <city></city>
          <region></region>
          <code></code>
          <country></country>
        </postal>
        <phone></phone>
        <email>satoru.matsushima@g.softbank.co.jp</email>
      </address>
</author>

<author fullname="Greg Mirsky"  surname="Mirsky"
 initials="G.">
      <organization>Ericsson</organization>
      <address>
        <postal>
          <street></street>
          <city></city>
          <region></region>
          <code></code>
          <country></country>
        </postal>
        <phone></phone>
        <email>gregory.mirsky@ericsson.com</email>
      </address>
</author>    

<author fullname="Nagendra Kumar"  surname="Kumar"
 initials="N.">
      <organization>Cisco</organization>
      <address>
        <postal>
          <street></street>
          <city></city>
          <region></region>
          <code></code>
          <country></country>
        </postal>
        <phone></phone>
        <email>naikumar@cisco.com</email>
      </address>
</author>

<date day="31" month="July" year="2015" />

<abstract>
	<t>
   This document provides various use cases for Bidirectional Forwarding
   Detection (BFD) such that extensions could be
   developed to allow for simplified detection of forwarding failures.
	 </t>
</abstract>

</front>

<middle>
<section title="Introduction"> <!-- 1, line 113-->
<t>
Bidirectional Forwarding Detection (BFD) is a lightweight protocol, as defined in <xref target="RFC5880"/>,
used to detect forwarding failures. Various protocols and applications rely on BFD for failure detection. Even
though the protocol is simple and lightweight, there are certain use cases, where faster setting up
of sessions and continuity check of the data forwarding paths is necessary. This document identifies 
use cases such that necessary enhancements could be made to BFD protocol to meet those requirements. 
</t>
<t>
BFD was designed to be a lightweight "Hello" protocol 
to detect data plane failures. With dynamic provisioning of forwarding paths on a large scale, establishing BFD
sessions for each of those paths creates complexity, not only from an operations point of view, but also in terms of 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.
</t>
<t>
This document specifically identifies those cases where certain requirements could be derived to be used as
reference, so that, protocol enhancements could be developed to address them. While the use cases could
be used as reference for certain requirements, it is outside the scope of this document to identify all of the 
requirements for all possible enhancements. Specific solutions and enhancement proposals are outside
the scope of this document as well.
</t>
<section title="Terminology"> <!-- 1.1, line 122-->
<t>
The reader is expected to be familiar with the BFD, IP, MPLS and Segment Routing (SR) terminology and protocol constructs. 
This section identifies only the new terminology introduced.
</t>
</section> <!-- ends: "1.1 from line 122-->
</section> <!-- ends: "1 from line 113-->

<section title="Introduction to Seamless BFD"> <!-- 2, line 128-->
<t>
BFD, as defined in <xref target="RFC5880"/>, requires two network nodes, to 
exchange locally allocated discriminators.  The discriminator enables identification of the sender and receiver of BFD
packets of the particular session and proactive continuity monitoring of the forwarding path between the two.
<xref target="RFC5881"/> defines single hop BFD whereas 
<xref target="RFC5883"/>  defines multi-hop BFD, <xref target="RFC5884"/> BFD for MPLS LSPs, and
<xref target="RFC5885"/> - BFD for PWs.
</t>
<t>
Currently, BFD is best suited to verify that two end points are reachable 
   or that an existing connection continues to be valid.  In order for BFD 
   to be able to initially verify that a connection is valid and that it 
   connects the expected set of end points, it is necessary to provide the
   node information associated with the connection at each end point 
   prior to initiating BFD sessions, such that this information can be used 
   to verify that the connection is valid.
</t>
<t>
   If this information is already known to the end-points of a potential BFD
   session, the initial handshake including an exchange of this node-specific
   information is unnecessary and it is possible for the end points to begin
   BFD messaging seamlessly.  In fact, the initial exchange of discriminator
   information is an unnecessary extra step that may be avoided for these
   cases.
</t>
<!--
<t>
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. 
</t>
-->
<t>
As an example of how Seamless BFD (S-BFD) might work, an entity (such as an operator, or centralized controller) determines 
a set of network entities to which BFD 
sessions might need to be established. Each of those network entities  is assigned a BFD discriminator, 
to establish a BFD session. These network entities will create a BFD session instance that listens for incoming 
BFD control packets. Mappings between selected network entities and corresponding BFD discriminators 
are known to other network nodes belonging in the same network by some means. A network entity in this network is then able to 
send a BFD control packet to a particular target with the corresponding BFD discriminator. Target network node, 
upon reception of such BFD control packet, will transmit a response BFD control packet back to the sender.
</t>
</section> <!-- ends: "2 from line 128-->

<section title="Use Cases"> <!-- 3, line 138-->
<t>
As per the BFD protocol <xref target="RFC5880"/>, BFD sessions are established using handshake mechanism 
prior to validating the forwarding path. This section outlines some use cases where the existing mechanism 
may not be able to satisfy the requirements. In addition, some of the use cases also be identify the need for 
expedited BFD session establishment while preserving benefits of forwarding failure detection using existing BFD 
specifications.
</t>
<section title="Unidirectional Forwarding Path Validation"> <!-- 3.1, line 143-->
<t>
Even though bidirectional verification of forwarding path is useful, there are scenarios when verification is only required in 
one direction between a pair of nodes. 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, and validation by the target entity to the originating entity is not required.  Many LSPs have the same unidirectional 
characteristics and unidirectional validation requirements.  Such LSPs are common in Segment Routing and LDP based 
networks.  Another example is when a unidirectional tunnel uses BFD to validate reachability of an egress node.  
</t>
<t>
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 knows the target 
network entity's discriminator, it starts the session right away. When the targeted network entity receives the packet, it 
knows that BFD packet, based on the discriminator and processes it. That does not require establishment of a bi-directional 
session, hence the two way handshake to exchange discriminators is not needed as well. 
</t>
<t>
The primary requirement in this use case is to enable session establishment from source network entity to target network 
entity. This translates to a need for the target network entity (for the BFD session), should start processing 
for the discriminator received in the BFD packet. This will enable the source network entity to establish a unidirectional BFD session without 
the bidirectional handshake of discriminators for session establishment. 
</t>
</section> <!-- ends: "3.1 from line 143-->

<section title="Validation of forwarding path prior to traffic switching"> <!-- 3.2, line 152-->
<t>
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 <xref target="RFC5880"/>, LSP ping bootstrapping 
<xref target="RFC5884"/>, etc.  This use case does not require to have sequences for session negotiation and discriminator 
exchanges in order to establish the BFD session. 
</t>
<t>
When these sequences for handshake are eliminated, the network 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. However, this information is required at each entity to verify that
BFD messages are being received from the expected end-points, hence the handshake mechanism serves no purpose.
Elimination of the unnecessary handshake mechanism allows for faster reachability validation of BFD provisioned paths/LSPs. 
</t>
<t>
In addition, it is expected that some MPLS technologies will require traffic engineered LSPs to
be created dynamically, perhaps driven by external applications, e.g. in Software Defined
Networks (SDN).  It will be desirable to perform BFD validation very quickly to allow 
applications to utilize dynamically created LSPs in a timely manner.
</t>
</section> <!-- ends: "3.2 from line 152-->

<section title="Centralized Traffic Engineering"> <!-- 3.3, line 159-->
<t>
Various technologies in the SDN domain that involve controller based networks have evolved where intelligence, 
traditionally placed in a distributed and dynamic control plane, is separated from the data plane and resides in a logically 
centralized place. There are various controllers that  perform this exact function in establishing forwarding paths for the 
data flow. Traffic engineering is one important function, where the traffic flow is engineered depending upon various attributes 
of the traffic as well as the network state.
</t>
<t>
When the intelligence of the network resides in a centralized entity, ability to manage and maintain 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 engineered tunnels are created, it is operationally critical to ensure that the forwarding paths are 
working prior to switching the traffic onto the engineered tunnels. In the absence of control plane protocols, it may be  
desirable to verify the forwarding path but also of any arbitrary path in the network. With tunnels being engineered by a centralized 
entity, when the network state changes, traffic has to be switched with minimum latency and black holing of the data. 
</t>
<t>
Traditional BFD session establishment and validation of the forwarding path must not become a bottleneck in the case of 
centralized traffic engineering. If the controller or other centralized entity is able to instantly verify a forwarding path of the 
TE tunnel , it could steer the traffic onto the traffic engineered tunnel very quickly thus minimizing adverse effect on a service. 
This is especially useful and needed when the scale of the network and number of TE tunnels is very high. 
</t>
<t>
The cost associated with BFD session negotiation and establishment of BFD sessions to identify valid paths is very high 
and providing network redundancy becomes a critical issue.
</t>
</section> <!-- ends: "3.3 from line 159-->

<section title="BFD in Centralized Segment Routing"> <!-- 3.4, line 168-->
<t>
A centralized controller based Segment Routing network monitoring technique is described in <xref target="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.
</t>
</section> <!-- ends: "3.4 from line 168-->

<section title="BFD Efficient Operation Under Resource Constraints"> <!-- 3.5, line 173-->
<t>
When BFD sessions are being setup, torn down or modified (i.e. parameters ? such as  interval, multiplier, etc are being modified), BFD  
requires additional packets other than 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 witho minimal additional BFD packets
exchanged to establish a new or modified session.
</t>
</section> <!-- ends: "3.5 from line 173-->

<section title="BFD for Anycast Address"> <!-- 3.6, line 178-->
<t> 
BFD protocol requires two endpoints to host BFD sessions, both sending packets to each other. This BFD model does not 
fit well with 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.
</t>
</section> <!-- ends: "3.6 from line 178-->

<section title="BFD Fault Isolation"> <!-- 3.7, line 183-->
<t> 
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 a fault anywhere along the path. Fast failure detection allows
for rapid path recovery procedures. However, operators often have to follow up, manually or 
automatically, to attempt to identify and localize the fault that caused BFD sessions to fail. Usage of other tools to 
isolate the fault may cause the packets to traverse a different path through the network (e.g. if ECMP is used). In addition, the longer it 
takes from BFD session failure to fault isolation attempt, more likely that the fault cannot be isolated, e.g. a 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 that went missing.

</t>
</section> <!-- ends: "3.7 from line 183-->

<section title="Multiple BFD Sessions to Same Target"> <!-- 3.8, line 188-->
<t> 
BFD is capable of providing very fast failure detection, as relevant network nodes continuously transmitting BFD packets 
at negotiated rate. If BFD packet transmission is interrupted, even for a 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 interruption of BFD packet transmissions. With distributed architectures of BFD 
implementations, 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.
</t>
</section> <!-- ends: "3.8 from line 188-->

<section title="MPLS BFD Session Per ECMP Path"> <!-- 3.9, line 193-->
<t>
BFD for MPLS, defined in <xref target="RFC5884"/>, describes procedures to run BFD as LSP in-band continuity check 
mechanism, through usage of MPLS echo request <xref target="RFC4379"/> to bootstrap the BFD session on the egress 
node. Section 4 of <xref target="RFC5884"/> also describes a possibility of running multiple BFD sessions per alternative 
paths of LSP.  However, details on how to bootstrap and maintain correct set of BFD sessions on the egress node is absent.
</t>
<t>
When an LSP has ECMP segment, it may be desirable to run in-band monitoring that exercises every path of ECMP.  
Otherwise there will be scenarios where in-band BFD session remains up through one path but traffic is black-holing 
over another path.  One way to achieve BFD session per ECMP path of LSP is to define procedures that update 
<xref target="RFC5884"/> in terms of how to bootstrap and maintain correct set of BFD sessions on the egress node.  
However, that may require constant use of MPLS Echo Request messages to create and delete BFD sessions on the 
egress node, when ECMP paths and/or corresponding load balance hash keys change.  If a BFD session over any 
paths of the LSP can be instantiated, stopped and resumed without requiring additional procedures of bootstrapping 
via MPLS echo request, it would simplify implementations and operations, and benefits network devices as less 
processing are required by them.
</t>
</section> <!-- ends: "3.9 from line 193-->
</section> <!-- ends: "3 from line 138-->

<section title="Security Considerations"> <!-- 4, line 201-->
<t>
There are no new security considerations associated with this draft.
</t>
</section> <!-- ends: "4 from line 201-->

<section title="IANA Considerations"> <!-- 5, line 206-->
<t>
There are no IANA considerations introduced by this draft
</t>
</section> <!-- ends: "5 from line 206-->

<section title="Contributors"> <!-- 8, line 276-->
<t>Carlos Pignataro
</t>
<t>Cisco Systems
</t>
<t>Email: cpignata@cisco.com
</t>
<t>Glenn Hayden
</t>
<t>ATT
</t>
<t>Email: gh1691@att.com
</t>
<t>Santosh P K
</t>
<t>Juniper
</t>
<t>Email: santoshpk@juniper.net
</t>
<t>Mach Chen
</t>
<t>Huawei
</t>
<t>Email: mach.chen@huawei.com
</t>
<t>Nobo Akiya
</t>
<t>Cisco Systems
</t>
<t>Email: nobo@cisco.com
</t>
</section> <!-- ends: "8 from line 276-->

    <section title="Acknowledgements">
      <t>The authors would like to thank Eric Gray for
      his useful comments.</t>
    </section>

</middle>

<back>
<references title="Normative References">
    &RFC5881;

    &RFC4379;

    &RFC5883;

    &RFC5880;

    &RFC5884;

    &RFC5885;

</references>

<references title="Informative References">

   &I-D.geib-spring-oam-usecase;
   
</references>
</back>
</rfc>
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