| Open Shortest Path First IGP | S. Hegde |
| Internet-Draft | P. Sarkar |
| Intended status: Standards Track | H. Gredler |
| Expires: January 17, 2016 | Juniper Networks, Inc. |
| M. Nanduri | |
| Microsoft Corporation | |
| L. Jalil | |
| Verizon | |
| July 16, 2015 |
OSPF Link Overload
draft-hegde-ospf-link-overload-01
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, existing stub router mechanism described in [RFC3137] cannot be used. It is useful for routers in OSPFv2 or OSPFv3 routing domain to be able to advertise a link being in overload state to indicate impending maintenance activity in the underlying network devices.
This document describes the protocol extensions to disseminate link overload information in OSPFv2 and OSPFv3 protocol.
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].
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It is useful for routers in OSPFv2 or OSPFv3 routing domain to be able to advertise a link being in overload state to indicate impending maintenance activity on the link. 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.
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Link overload sub TLV for OSPFv2
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
Length: 4
Value: Remote IPv4 address. The remote IP4 address is used to identify the particular link that is in 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
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 neighbor interface-id and can uniquely identify the link on the remote node.
Type : TBA
Length: 0
The Link Overload sub TLV indicates that the Link which carries the sub TLV is overloaded. The node that has the link going for maintenance, sets metric of the link to MAX-METRIC and re-originates the router LSA. The metric in the reverse direction also need to change to divert the traffic from reverse direction. The node SHOULD originate Link overload sub TLV and include it in Extended link TLV and originate the Extended Link Opaque LSA as defined in [I-D.ietf-ospf-prefix-link-attr] for OSPFv2 and E-Router-LSA as defined in [I-D.ietf-ospf-ospfv3-lsa-extend] for OSPFv3 and flood in the OSPF area.
when the originator of the Link Overload sub TLV, purges the extended link opaque LSA or re-originates without the Link Overload sub TLV, the metric on the remote node SHOULD be changed back to the original value.
Based on the link type of the overloaded link below actions MAY be taken by the receiver.
When a link overload TLV is received for a point-to-point link the receiver SHOULD identify the local link which corresponds to the overloaded link and set the metric to MAX-METRIC (0xffff). Receiver node MUST re-originate the router-LSA with the changed metric and flood into the OSPF area.
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. 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 outgoing link cannot be changed since all the neighbors are on same link. Setting the link cost to MAX-METRIC would impact paths going via all neighbors.
When a link-overload TLV is received by the remote end for a broadcast/NBMA link
- If it’s DROther or BDR for that link, SHOULD not take any action. - If receiving node is DR for the link, it MUST remove the originator of the link overload TLV from the list of connected neighbors and MUST re-originate the network LSA and flood into the OSPF area.
Operation for the point-to-multipoint links is similar to the point-to-point links. When a link overload TLV is received for a point-to-multipoint link the receiver SHOULD identify the neighbor which corresponds to the overloaded link and set the metric to MAX-METRIC (0xffff). Receiver node MUST re-originate the router-LSA with the changed metric and flood into the OSPF area.
The mechanism described in the document is fully backward compatible.It is required that the originator and receiver of link-overload sub TLV understand the extensions defined in this document and in case of broadcast links the originator and the DR need to understand the extensions. Other nodes in the network compute based on increased metric and hence the feature is backward compatible.
---------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 to run over the pseudo-wire to get 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 the alternate path. Link overload feature provides facilities to achieve this service by increasing the metric on the link but still allowing the traffic to use the link when there is no alternate path available.
_____________
| |
-------------| Controller |--------------
| |____________ | |
| |
| ------- Primary Path --------------- |
PE1---------P1----------------P2---------PE2
| |
| |
|________P3________|
Alternate Path
Figure 4: Controller based Traffic Engineering
Controller based deployments where the controller participates in the IGP protocol gets the link-overload information when the link maintenance is impending. Using this information controller finds an alternate path. If there are no alternate paths satisfying the traffic engineering constraints, controller might temporarily relax the constraints and put the service on different path. In the above example when P1->P2 link goes for maintenance, controller gets the link-overload information and sets up an alternate path via P1->P3->P2. 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 TLV OSPFV3 Router Link TLV Registry
i) TBD – Link Overload TLV
| [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. |
| [RFC3137] | Retana, A., Nguyen, L., White, R., Zinin, A. and D. McPherson, "OSPF Stub Router Advertisement", RFC 3137, DOI 10.17487/RFC3137, June 2001. |
| [RFC5340] | Coltun, R., Ferguson, D., Moy, J. and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008. |
| [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. |