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<rfc category="std" docName="draft-ietf-ospf-link-overload-09" ipr="trust200902">
<front>
<title abbrev="OSPF link overload"> OSPF Link Overload</title>
<author initials="S." surname="Hegde" fullname="Shraddha Hegde">
<organization>Juniper Networks, Inc.</organization>
<address>
<postal>
<street>Embassy Business Park</street>
<city>Bangalore</city>
<region>KA</region>
<code>560093</code>
<country>India</country>
</postal>
<email>shraddha@juniper.net</email>
</address>
</author>
<author initials="P." surname="Sarkar" fullname="Pushpasis Sarkar">
<organization>Individual</organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<email>pushpasis.ietf@gmail.com</email>
</address>
</author>
<author fullname="Hannes Gredler" initials="H." surname="Gredler">
<organization>Individual</organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<email>hannes@gredler.at</email>
</address>
</author>
<author fullname="Mohan Nanduri" initials="M." surname="Nanduri">
<organization>ebay Corporation</organization>
<address>
<postal>
<street>2025 Hamilton Avenue</street>
<city>San Jose</city>
<region>CA</region>
<code>98052</code>
<country>US</country>
</postal>
<email>mnanduri@ebay.com</email>
</address>
</author>
<author fullname="Luay Jalil" initials="L." surname="Jalil">
<organization>Verizon</organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<email>luay.jalil@verizon.com</email>
</address>
</author>
<date year="2017"/>
<area>Routing</area>
<workgroup>Open Shortest Path First IGP</workgroup>
<keyword>MPLS</keyword>
<keyword>IGP</keyword>
<keyword>OSPF</keyword>
<abstract>
<t> When a link is being prepared to be taken out of service, the traffic needs to be diverted from both ends of the link.
 Increasing the metric to the highest metric on one side of the link
is not sufficient to divert the traffic flowing in the other direction.</t> 

<t>It is useful for routers in an OSPFv2 or OSPFv3 routing domain to be able to
advertise a  link being in an overload state to indicate impending maintenance activity on the link.
This information can be used by the network devices to re-route the traffic effectively.
 </t>
<t>This document describes the protocol extensions to disseminate
link-overload information in OSPFv2 and OSPFv3.</t>
</abstract>
<note title="Requirements Language">
<t>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 <xref target="RFC2119">RFC 2119</xref>.</t>
</note>
</front>
<middle>

<section title="Introduction" anchor='intro'>

<t>When a node is being prepared for a planned maintenance or upgrade, 
<xref target="RFC6987"/> provides mechanisms to advertise the 
node being in an overload state by setting all outgoing link costs  to MAX-METRIC (0xffff). These procedures
are specific to the maintenance activity on a node and cannot be used when a single  link on the node
requires maintenance. </t>

<t>  In traffic-engineering deployments, 
LSPs need to be diverted from the link without disrupting the services.
<xref target="RFC5817"/> describes requirements and procedures for graceful shutdown of MPLS links.
It is useful to be able to advertise the impending maintenance
activity on the link and to have LSP re-routing policies at the ingress to route the LSPs away from the link. </t>

<t>Many OSPFv2 or OSPFv3 deployments run on overlay networks provisioned by means of pseudo-wires or L2-circuits.
Prior to devices in the underlying network going offline for maintenance, it is useful to
divert the traffic away from the node before the maintenance is actually scheduled. Since the nodes in the underlying network are not visible to OSPF,
the existing stub router mechanism described in <xref target="RFC6987"/> cannot be used.
 An application specific to this use case is described in <xref target="Pseudowire_services"/>.</t>

 <t>
This document provides mechanisms to advertise link-overload state in the flexible encodings provided by
OSPFv2 Prefix/Link Attribute Advertisement(<xref target="RFC7684"/>).
Throughout this document, OSPF is used when the text applies to both
OSPFv2 and OSPFv3.  OSPFv2 or OSPFv3 is used when the text is
specific to one version of the OSPF protocol.
</t>
</section>

<section title="Motivation" anchor='Motivation'>

<t>The motivation of this document is to reduce manual intervention during maintenance activities.
The following objectives help to accomplish this in a range of deployment scenarios.</t>
<t>
<list style="numbers">
<t> Advertise impending maintenance activity so that traffic from both directions 
can be diverted away from the link.</t>
<t> Allow the solution to be backward compatible so that nodes that do not understand the new advertisement do not
    cause routing loops.</t>
<t> Advertise the maintenance activity to other nodes in the network so that LSP ingress routers/controllers can learn of the impending
    maintenance activity and apply specific policies to re-route the LSPs for traffic-engineering based deployments.</t>
<t> Allow the link to be used as last resort link to prevent traffic disruption when alternate paths are not available.</t>

</list>
</t>
</section>

<section title='Flooding Scope'>
<t>The link-overload information is flooded in area scoped Extended Link Opaque LSA <xref target='RFC7684'/>.
   The Link-Overload sub-TLV MAY be processed by the head-end nodes or the controller as described in the
 <xref target="applications"/>. The procedures for processing the Link-Overload sub-TLV are described in <xref target="procedures"/>.
 </t>

</section>

<section title='Link-Overload sub-TLV' anchor='link-overload-sub-tlv'>

<section title='OSPFv2 Link-overload sub-TLV'>
<t> The Link-Overload sub-TLV  identifies the link being in overload state.It is carried in extended Link TLV  in the Extended Link Opaque LSA as defined in 
<xref target='RFC7684'/>.

<vspace blankLines="2" />
<figure anchor="OSPFv2-link-overload-sub-TLV" title="Link-Overload sub-TLV for OSPFv2">
<artwork>
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Type             |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     

</artwork>
</figure>
<vspace blankLines="1" />

Type : TBA (suggested value 5)
<vspace blankLines="1" />
Length: 0
<vspace blankLines="1" />

<vspace blankLines="1" />
</t>
</section>

<section title='Remote IPv4 address sub-TLV' anchor='remote-ipv4-addr'>


<t>This sub-TLV specifies the IPv4 address of remote endpoint on the link. It is advertised in extended Link TLV as defined in 
<xref target='RFC7684'/>.This sub-TLV is optional and MAY be advertised in area scoped Extended Link Opaque LSA
to identify the link when there are multiple parallel links between two nodes. 

<vspace blankLines="2" />
<figure anchor="remote-ipv4-addr-sub-tlv" title="Remote IPv4 address sub-TLV">
<artwork>
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Type             |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Remote IPv4 address                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

</artwork>
</figure>
<vspace blankLines="1" />

Type : TBA (suggested value 4)
<vspace blankLines="1" />
Length: 4
<vspace blankLines="1" />
Value: Remote IPv4 address. The remote IP4 address is used to identify the particular link  when there are
       multiple parallel links between two nodes.
<vspace blankLines="1" />
</t>
</section>

<section title='Local/Remote Interface ID sub-TLV' anchor='local-remote-interface-id'>


<t>This sub-TLV specifies local and remote interface identifiers. It is advertised in extended Link TLV as defined in 
<xref target='RFC7684'/>.This sub-TLV is optional and MAY be advertised in area scoped Extended Link Opaque LSA
to identify the link when there are multiple parallel unnumbered links between two nodes. 
The local interface-id is generally readily available.
One of the mechanisms to obtain remote interface-id is described in <xref target='RFC4203'/>.

<vspace blankLines="2" />
<figure anchor="local-remote-interface-id-sub-tlv" title="Local/Remote Interface ID sub-TLV">
<artwork>
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Type             |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Local Interface ID                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Remote Interface ID                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

</artwork>
</figure>
<vspace blankLines="1" />

Type : TBA (suggested value 11)
<vspace blankLines="1" />
Length: 8
<vspace blankLines="1" />
Value: 4 octets of Local Interface ID followed by 4 octets of Remote interface ID.
<vspace blankLines="1" />
</t>
</section>

<section title='OSPFv3 Link-Overload sub-TLV'>
<t> The definition of OSPFv3 Link-Overload sub-TLV is defined below.
The area scoped advertisement of Link-Overload sub-TLV for OSPFv3 will be described
in a separate document.

<vspace blankLines="2" />
<figure anchor="OSPFv3-link-overload-TLV" title="Link-Overload sub-TLV for OSPFv3">
<artwork>
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Type             |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
</artwork>
</figure>
<vspace blankLines="1" />
Type : TBA (Suggested value 4)
<vspace blankLines="1" />
Length: 0
</t>
</section>
</section>

<section title='Elements of procedure' anchor='procedures'>
<t>The Link-Overload sub-TLV indicates that the link identified by the
 sub-TLV is overloaded. The node that has the link to be taken out of service 
SHOULD advertise the Link-Overload sub-TLV in the Extended Link TLV in the
Extended Link Opaque LSA as defined in  <xref target="RFC7684"/> for OSPFv2.
The Link-Overload information is advertised 
as a property of the link and is flooded across the area. This information can be used by ingress routers or controllers to take
special  actions. An application specific to this use case is described in  <xref target="controller_TE"/>.</t>


<t>The precise action taken by the remote node at the other end of the link identified as overloaded depends on the link type.</t>

<section title='Point-to-point links'>
<t>The node that has the link to be taken
   out of service MUST set metric of the link to MAX-METRIC (0xffff) and re-
   originate the Router-LSA.  The TE metric SHOULD be set to MAX-TE-METRIC -1
   (0xfffffffe) and the node SHOULD re-originate the TE Link Opaque LSAs.
When a Link-Overload sub-TLV is received for a point-to-point link, the remote node MUST identify the local link which corresponds to the
overloaded link and set the metric to MAX-METRIC (0xffff)and the remote node MUST re-originate  the router-LSA with the changed metric. 
The TE metric SHOULD be set to MAX-TE-METRIC -1 (0xfffffffe) and the TE opaque LSA for the link
 SHOULD be re-originated with new value.</t>
 
 <t> Extended link opaque LSAs and the Extended link TLV are not scoped for multi-topology <xref target="RFC4915"/>.
 In multi-topology deployments <xref target="RFC4915"/>, the Link-Overload sub-TLV advertised in an Extended Link opaque LSA corresponds to all the topologies which include the link.
 
 The receiver node SHOULD change the metric in the reverse direction for all the topologies which include the remote link  and
 re-originate the Router LSA as defined in <xref target="RFC4915"/>.</t>
 
 <t>When the originator of the Link-Overload sub-TLV purges the Extended Link Opaque LSA or re-originates it without the Link-Overload sub-TLV,
 the remote node must re-originate the appropriate LSAs with the metric and TE metric values set to their original values.</t>
 
</section>

<section title='Broadcast/NBMA links'>
<t>Broadcast or NBMA networks in OSPF are represented by a star topology where the Designated Router (DR) is the central point to
which all other routers on the broadcast or NBMA network connect logically.  As a result, routers on the broadcast or NBMA network advertise only 
their adjacency to the DR.  Routers that do not act as DR do not form or advertise adjacencies with each other. 
 For the Broadcast links, the MAX-METRIC on the remote link cannot be changed since all the neighbours are on same link. 
Setting the link cost to MAX-METRIC would impact paths going via all neighbours. 
<vspace blankLines="1" />
The node that has the link to be taken out of service MUST set metric of the link to MAX-METRIC(0xffff) and re-originate the Router-LSA.  
The TE metric SHOULD be set to MAX-TE-METRIC -1(0xfffffffe) and the node SHOULD re-originate the corresponding TE Link Opaque LSAs.
For a broadcast link, the two part metric as described in <xref target="RFC8042"/> is used. The node originating the Link-Overload
sub-TLV MUST set the metric in the Network-to-Router Metric sub-TLV to MAX-METRIC 0xffff for OSPFv2 and OSPFv3
and re-originate the corresponding LSAs.
The nodes that receive the two part metric should follow the procedures described in <xref target="RFC8042"/>.
The backward compatibility procedures described in <xref target="RFC8042"/> should be followed to ensure 
loop free routing. 
</t>
</section>

<section title='Point-to-multipoint links'>
   <t> Operation for the point-to-multipoint links is similar to the point-to-point links.
    When a Link-Overload sub-TLV is received for a point-to-multipoint link the remote node MUST identify the neighbour which corresponds to the
    overloaded link and set the metric to MAX-METRIC (0xffff). The remote node MUST re-originate the Router-LSA with the changed metric.</t>
</section>

<section title='Unnumbered interfaces'>
   <t> Unnumbered interface do not have a unique IP address and borrow their address from other interfaces.  
   <xref target="RFC2328"/> describes procedures to handle unnumbered interfaces in the context of the Router LSA. 
   We apply a similar procedure to the Extended Link TLV advertising the Link-Overload sub-TLV in to handle 
   unnumbered interfaces.
 
   The link-data field in the Extended Link TLV includes the Local interface-id instead of the IP address. 
   The Local/Remote Interface ID sub-TLV MUST be advertised when there are multiple parallel unnumbered interfaces between two nodes.
   One of the mechanisms to obtain the interface-id of the remote side are defined in  <xref target="RFC4203"/>. </t>
</section>

<section title='Hybrid Broadcast and P2MP interfaces'>
   <t> Hybrid Broadcast and P2MP interfaces  represent a broadcast network modeled as P2MP interfaces.
   <xref target="RFC6845"/> describes procedures to handle these interfaces. Operation for the Hybrid
   interfaces is similar to the P2MP interfaces.
    When a Link-Overload sub-TLV is received for a hybrid link the remote node MUST identify the neighbour which corresponds to the
    overloaded link and set the metric to MAX-METRIC (0xffff). All the remote nodes connected to originator
    MUST re-originate the Router-LSA with the changed metric.</t>
</section>
    
</section>


<section title='Backward compatibility'>
   <t> The mechanisms described in the document are fully backward compatible.
    It is required that the node adverting the Link-Overload sub-TLV as
    well as the node at the remote end of the overloaded link support the
    extensions described herein for the traffic to diverted from the 
    overloaded link. If the remote node doesn't support the capability, 
    it will still use the overloaded link but there are no other adverse 
    effects.  In the case of broadcast links using two-part metrics, the 
    backward compatibility procedures as described in <xref target="RFC8042"/> are 
    applicable.</t>

</section>

<section title='Applications' anchor='applications'>
<section anchor="Pseudowire_services" title='Pseudowire Services'> 


 <t>Many service providers offer pseudo-wire services to customers using L2 circuits. The IGP protocol that
runs in the customer network would also run over the pseudo-wire to create a seamless private network for the customer. 
Service providers want to offer overload functionality when the PE device is taken-out for maintenance.
The provider should guarantee that the PE is taken out for maintenance only after the service is successfully diverted
on an alternate path. There can be large number of customers attached to a PE node and the remote end-points for these pseudo-wires 
are spread across the service provider's network. It is a tedious and error-prone process to change the metric for all
pseudo-wires in both directions. The link-overload feature 
simplifies the process by increasing the metric on the link in the reverse direction as well so that traffic
in both directions is diverted away from the PE undergoing maintenance.
 The Link-Overload feature allows the link to be used as a last resort link so that traffic is not disrupted when alternative
paths are not available.</t>

<figure anchor="Pseudo_wire_services" title="Pseudowire Services">
<artwork>
                Private VLAN
        =======================================
       |                                       |
       |                                       |
       |     ------PE3---------------PE4------CE3
       |   /                             \          
       | /                                 \       
     CE1---------PE1----------PE2---------CE2
       |                       \                 
       |                        \                
       |                         ------CE4
       |                                 |
       |                                 |
       |                                 |
        =================================   
                Private VLAN

</artwork>
</figure>

<t> In the example shown in <xref target="Pseudo_wire_services"/>, when the PE1 node is going out of service for maintenance, service providers set the PE1 to overload state. 
The PE1 going in to overload state
triggers all the CEs (In this example CE1)connected to the PE to set their pseudowire links passing via PE1 to link-overload state. The mechanisms used
to communicate between PE1 and CE1 is outside the scope of this document. CE1 sets the link-overload state on its private VLAN connecting CE3, CE2 and CE4 and changes the metric to
MAX_METRIC and re-originates the corresponding LSA.
The remote end of the link at CE3, CE2, and CE4 also set the metric on the link to MAX-METRIC and the traffic from
 both directions gets diverted away from the pseudowires.</t>
</section>

<section anchor="controller_TE" title='Controller based Traffic Engineering Deployments'>  


 <t>In controller-based deployments where the controller participates in the IGP protocol, 
 the controller can also receive the link-overload information as a warning that link maintenance is imminent.
 Using this information, the controller can find alternate paths for traffic which uses the affected link.  
 The controller can apply various policies and re-route the LSPs away from the link undergoing maintenance. 
 If there are no alternate paths satisfying the traffic engineering constraints, the controller might
 temporarily relax those constraints and put the service on a different path. Increasing the link metric alone does not specify
 the maintenance activity as the metric could increase in events such as LDP-IGP synchronisation. An explicit indication
from the router using the link-overload sub-TLV is needed to inform the Controller or head-end routers. </t>
 <figure anchor="TE_deployments" title="Controller based Traffic Engineering">
<artwork>
                      _____________
                     |             |
        -------------| Controller  |--------------
       |             |____________ |             |
       |                                         |
       |--------- Primary Path ------------------|
       PE1---------P1----------------P2---------PE2
                   |                  |
                   |                  |
                   |________P3________|      
                   
                      Alternate Path

</artwork>
</figure>
 <t>In the above example, PE1->PE2 LSP
 is set-up to satisfy a constraint of 10 Gbps bandwidth on each link. The links P1->P3 and P3->P2 have only 1 Gbps capacity 
 and there is no alternate path satisfying the bandwidth constraint of 10GB.
 When P1->P2 link is being prepared for maintenance, 
 the controller receives the link-overload information, as there is no alternate path available which satisfies the constraints,
 controller chooses a path that is less optimal and temporarily sets up an alternate path via P1->P3->P2.
 Once the traffic is diverted, the P1->P2 link can be taken out of service for maintenance/upgrade.</t>
</section>
<section anchor="L3VPN" title='L3VPN Services and sham-links'> 


 <t>Many service providers offer L3VPN services to customers and CE-PE links run OSPF <xref target="RFC4577"/>.
  When PE goes out of service for maintenance, all the links on the PE can be set to link-overlaod state which will gurantee that the traffic to/from dual-homed CEs gets diverted.
 The interaction between OSPF and BGP is outside the scope of this document.</t>

<t> Another useful usecase is when ISPs provide sham-link services to customers <xref target="RFC4577"/>.When PE goes out of service for maintenance, all sham-links 
 on the PE can be set to link-overload state and traffic can be divered from both ends without having to touch the configurations on the remote end of the sham-links.</t>
</section>

<section anchor="Hub_and_spoke" title='Hub and spoke deployment'> 


 <t>OSPF is largely deployed in Hub and Spoke deployments with a number of spokes connecting to the Hub. It is a general practice to deploy
 multiple Hubs with all spokes connecting to these  Hubs to achieve redundancy. When a Hub node goes down for maintenance,
 all links on the Hub can be set to link-overload state and traffic 
 gets divered from the spoke sites as well without having to make configuration changes on the spokes.</t>

</section>
</section>
<!-- HG: FIXME: add traffic-engineering reference -->
<section title='Security Considerations' anchor='sec-con'>
<t>
This document does not introduce any further security issues other
than those discussed in <xref target="RFC2328"/> and <xref target="RFC5340"/>.
</t>
</section>
<section anchor="IANA" title="IANA Considerations">

<t>This specification updates one OSPF registry:</t>
<t><vspace blankLines="1"/>
OSPF Extended Link TLVs Registry</t>
<t><vspace blankLines="1"/>
  i)   Link-Overload sub-TLV  - Suggested value 5</t>
  <t><vspace blankLines="1"/>
  ii)  Remote IPv4 address sub-TLV - Suggested value 4 </t>
  <t><vspace blankLines="1"/>
  iii) Local/Remote Interface ID sub-TLV - Suggested Value 11</t>
 

  <t>OSPFV3 Router Link TLV Registry</t>
 <t><vspace blankLines="1"/>
   i) Link-Overload sub-TLV - suggested value 4</t>
  
     
   <t>BGP-LS Link NLRI Registry <xref target="RFC7752"/></t>
   <t><vspace blankLines="1"/>
   i)Link-Overload TLV - Suggested 1101 </t>

</section>
<section title='Acknowledgements'>
<t>Thanks to Chris Bowers for valuable inputs and edits to the document.
 Thanks to Jeffrey Zhang, Acee Lindem and Ketan Talaulikar for inputs.
 Thanks to Karsten Thomann for careful review and inputs on the applications where link-overload
 is useful.</t>
</section>


</middle>
<back>

<references title='Normative References'>

&RFC7684;
&RFC8042;
&RFC7752;
&RFC6845;
</references>
<references title='Informative References'>
&RFC2328;
&RFC5340;
&RFC2119;
&RFC6987;
&RFC4203;
&RFC4915;
&RFC4577;
&RFC5817;
</references>

</back>
</rfc>