| Open Shortest Path First IGP | S. Hegde |
| Internet-Draft | P. Sarkar |
| Intended status: Standards Track | Juniper Networks, Inc. |
| Expires: July 9, 2016 | H. Gredler |
| Individual | |
| M. Nanduri | |
| Microsoft Corporation | |
| L. Jalil | |
| Verizon | |
| January 6, 2016 |
OSPF Link Overload
draft-ietf-ospf-link-overload-01
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.
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.
This document describes the protocol extensions to disseminate link overload information in OSPFv2 and OSPFv3.
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].
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on July 9, 2016.
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When a node is being prepared for a planned maintenance or upgrade, [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 attached to the node requires maintenance.
In traffic-engineering deployments, LSPs need to be moved away from the link without disrupting the services. 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.
Many OSPFv2 or OSPFv3 deployments run on overlay networks provisioned by means of pseudo-wires or L2-circuits. When the devices in the underlying network go 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 [RFC6987] cannot be used. Application specific to this use case is described in Section 6.1
This document provides mechanisms to advertise link overload state in the flexible encodings provided by OSPFv2 Prefix/Link Attribute Advertisement( [I-D.ietf-ospf-prefix-link-attr]) and OSPFv3 Extended LSA ([I-D.ietf-ospf-ospfv3-lsa-extend]). 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.
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.
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 IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Link Overload sub-TLV for OSPFv2
The Link Overload sub-TLV is carried as part of the Extended Link TLV as defined in [I-D.ietf-ospf-prefix-link-attr] for OSPFv2.
Type : TBA (suggested value 4)
Length: 4
Value: Remote IPv4 address. The remote IP4 address is used to identify the particular link that is in the overload state when there are multiple parallel links between two nodes.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Link Overload sub-TLV for OSPFv3
The Link Overload sub-TLV is carried in the Router-Link TLV as defined in the [I-D.ietf-ospf-ospfv3-lsa-extend] for OSPFv3. The Router-Link TLV contains the neighbour interface-id and can uniquely identify the link on the remote node.
Type : TBA (Suggested value 4)
Length: 0
The Link Overload sub-TLV indicates that the link identified in by the sub-TLV is overloaded. The node that has the link to be taken out of service SHOULD originate the Link Overload sub-TLV in the Extended Link TLV in the Extended Link Opaque LSA as defined in [I-D.ietf-ospf-prefix-link-attr] for OSPFv2 or in the E-Router-LSA as defined in [I-D.ietf-ospf-ospfv3-lsa-extend] for OSPFv3. The link-overload information is carried 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. Application specific to this use case is described in Section 6.2.
The precise action taken by the remote node at the other end of the link identified as overloaded depends on the link type.
The node that has the link to be taken out of service SHOULD 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 SHOULD identify the local link 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 and flood into the OSPF area. The TE metric SHOULD be set to MAX-TE-METRIC-1 (0xfffffffe) and the TE opaque LSA for the link MUST be re-originated with new value.
In multi-topology deployments [RFC4915], the Link overload Sub-TLV carried in an Extended Link opaque LSA corresponds to all the topologies the link belongs to. The receiver node SHOULD change the metric in the reverse direction corresponding to all the topologies to which the reverse link belongs.
When the originator of the Link Overload sub-TLV purges the Extended Link Opaque LSA/E-Router-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.
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.
The node that has the link to be taken out of service SHOULD 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. For a broadcast link, the two part metric as described in [I-D.ietf-ospf-two-part-metric] 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 LSAs the TLV is carried-in. The nodes that receive the two part metric should follow the procedures described in [I-D.ietf-ospf-two-part-metric]. The backward compatibility procedures described in [I-D.ietf-ospf-two-part-metric] should be followed to ensure loop free routing.
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 SHOULD 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 and flood into the OSPF area.
Unnumbered interface do not have a unique IP addresses and borrow address from other interfaces. [RFC2328] describes procedures to handle unnumbered interfaces. The link-data field in the Extended Link TLV carries the interface-id instead of the IP address. The Link Overload sub-TLV carries the remote interface-id in the Remote-ip-address field if the interface is unnumbered. Procedures to obtain interface-id of the remote side is defined in [RFC4203].
The mechanism described in the document is fully backward compatible.It is required that the originator of the Link Overload sub-TLV as well as the node at the remote end of the link identified as overloaded understand the extensions defined in this document. In the case of broadcast links, the backward compatibility procedures as described in [I-D.ietf-ospf-two-part-metric] are applicable. .
---------PE3----------------PE4----------
| |
| |
CE1---------PE1----------------PE2---------CE2
| |
| |
-----------------------------------------
Private VLAN
Figure 3: Pseudowire Services
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 seamless private network for the customer. Service providers want to offer overload kind of 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.
_____________
| |
-------------| Controller |--------------
| |____________ | |
| |
|--------- Primary Path ------------------|
PE1---------P1----------------P2---------PE2
| |
| |
|________P3________|
Alternate Path
Figure 4: Controller based Traffic Engineering
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 using 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.
In the above example, PE1->PE2 LSP is set-up which satisfies a constraint of 10 GB bandwidth on each link.The links P1->P3 and P3->P2 have only 1 GB 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 sets up an alternate path via P1->P3->P2 temporarily. Once the traffic is diverted, P1->P2 link can be taken out for maintenance/upgrade.
This document does not introduce any further security issues other than those discussed in [RFC2328] and [RFC5340].
This specification updates one OSPF registry:
OSPF Extended Link TLVs Registry
i) TBD – Link Overload sub TLV
OSPFV3 Router Link TLV Registry
i) TBD – Link Overload sub TLV
Thanks to Chris Bowers for valuable inputs and edits to the document. Thanks to Jeffrey Zhang and Acee Lindem for inputs.
| [I-D.ietf-ospf-ospfv3-lsa-extend] | Lindem, A., Mirtorabi, S., Roy, A. and F. Baker, "OSPFv3 LSA Extendibility", Internet-Draft draft-ietf-ospf-ospfv3-lsa-extend-06, February 2015. |
| [I-D.ietf-ospf-prefix-link-attr] | Psenak, P., Gredler, H., Shakir, R., Henderickx, W., Tantsura, J. and A. Lindem, "OSPFv2 Prefix/Link Attribute Advertisement", Internet-Draft draft-ietf-ospf-prefix-link-attr-03, February 2015. |
| [I-D.ietf-ospf-two-part-metric] | Wang, L., Lindem, A., DuBois, D., Julka, V. and T. McMillan, "OSPF Two-part Metric", Internet-Draft draft-ietf-ospf-two-part-metric-01, July 2015. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC2328] | Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, April 1998. |
| [RFC4203] | Kompella, K. and Y. Rekhter, "OSPF Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005. |
| [RFC4915] | Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L. and P. Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", RFC 4915, DOI 10.17487/RFC4915, June 2007. |
| [RFC5340] | Coltun, R., Ferguson, D., Moy, J. and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008. |
| [RFC6987] | Retana, A., Nguyen, L., Zinin, A., White, R. and D. McPherson, "OSPF Stub Router Advertisement", RFC 6987, DOI 10.17487/RFC6987, September 2013. |