| LSR Working Group | A. Wang |
| Internet-Draft | China Telecom |
| Intended status: Standards Track | June 28, 2018 |
| Expires: December 30, 2018 |
OSPF Extend for Inter-Area Topology Retrieval
draft-wang-lsr-ospf-inter-area-topology-ext-00
This document describes method to transfer the source router id of inter-area prefixes for OSPFv2 [RFC2328] and OSPFv3 [RFC5340], which is needed in topology retrieval processing for inter-area scenario.
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BGP-LS [RFC7752] describes the methodology that using BGP protocol to transfer the Link-State information. Such method can enable SDN controller to collect the underlay network topology automatically.
But if the underlay network is divided into multi area and running OSPF protocol, it is not easy for the SDN controller to rebuild the multi-area topology, because normally the ABR that locates on the boundary of different area will hide the detail topology information in non-backbone area, and the router in backbone area that runs BGP-LS protocol can only get and report the summary network information in non-backbone area.
[RFC7794] introduces “IPv4/IPv6 Source Router IDs” TLV to label the source of the prefixes redistributed from different Level, this TLV can be used to reconstruct the detail overall topology within level 1 and level 2. Such solution can also be applied into network that run OSPF protocol, but the related LSP message must be redefined.
This draft gives such solution for the OSPF v2 and OSPF v3 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 [RFC2119] .
Fig.1 illustrates the topology retrieval scenario when OSPF is running in multi-area. R0-R4 are routers in backbone area, S1-S4,T1-T4 are interal router in area 1 and area 2 respectively. R1 and R3 are border routers between area 0 and area 1; R2 and R4 are border routers between area 0 and area 2. N1 is the network between router S1 and S2, N2 is the network between router T1 and T2.
Normally, ABR router R1 or R3 will send the summary LSA(for OSPFv2) or Inter-Area-Prefix-LSAs(for OSPFv3) for network N1. When R0 receives such LSA, it can only know network N1 locates behind R1, and does not know where it is originated. When R0 reports the summary LSA information via BGP-LS protocol, the IP SDN controller can’t certainly deduce the detail network topology within area 1. The situation is same as that in Area 2.
+-----------------+
|IP SDN Controller|
+--------+--------+
|
|BGP-LS
|
+---------------------+------+--------+-----+--------------+
| +--+ +--+ ++-+ ++-+ +-++ + -+ +--+|
| |S1+--------+S2+---+R1+---|R0+----+R2+---+T1+--------+T2||
| +-++ N1 +-++ ++-+ +--+ +-++ ++++ N2 +-++|
| | | | | || | |
| | | | | || | |
| +-++ +-++ ++-+ +-++ ++++ +-++|
| |S4+--------+S3+---+R3+-----------+R4+---+T3+--------+T4||
| +--+ +--+ ++-+ +-++ ++-+ +--+|
| | | |
| | | |
| Area 1 | Area 0 | Area 2 |
+---------------------+---------------+--------------------+
Fig.1 OSPF Inter-Area Topology Retrieval Scenario
Because traditional OSPFv2/v3 packet is not in the TLV format, we need to find some solutions to reuse or redefine the existing fields in summary LSA (OSPFv2) and Inter-Area-Prefix-LSAs(for OSPFv3)to transfer the additional information. The extend methods should not conflict with the usage of existing semantics.
Section 3.1 and section 3.2 give the proposed solutions for OSPFv2 and OSPFv3 respectively.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | TOS metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Fig.2 Summary LSA Format
Fig.2 illustrates the format of summary LSA. There is one byte that originately defined for the number of TOS types but in actually this feature does not applied in real network or implemented in the main stream router.
To transfer the additional information, this draft proposes to reuse/redefine this field. In order to prevent possible conflict, even it is in very rare event, we can start the usage of this field from the upper limit, for example, 0xFE. Then the proposed extend summary LSA format is the followings:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xFE | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Source Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig.3 Extended Summary LSA Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Age |0|0|1| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig.4 Inter-Area-Prefix-LSA Format
For OSPFv3, this draft proposes the similar method, because the semantic of the Inter-Area-Prefix-LSA format is almost same as the summary LSA format.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Age |0|0|1| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS Checksum | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xFE | Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Source Router ID |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fig.5 Extended Inter-Area-Prefix-LSA Format
If the value of “Numbers of TOS” equal “0xFE”, then the “IPv6 source router ID” (16 bytes) and its corresponding area ID (4 bytes) information are inserted in the “Inter-Area-Prefix-LSA” after the field “Metric”. After this, the normal Prefix information is followed as shown in Fig.5
[RFC7684] and [RFC8362] define the TLV format extension for OSPFv2 and OSPFv3 respectively. These documents give the flexibility to add new attributes for the prefixes and links. Based on these formats, we can define new sub TLV to transfer the “Prefix Source Router ID”, as that defined in [RFC7794].
The proposed “Prefix Source Router ID” format is the following:
For IPv4 network, it is the following:
This sub TLV should be included in the “OSPFv2 Extended Prefix Opaque LSA” that defined in
For IPv6 network, it is the following:
This sub TLV should be included in “E-Inter-Area-Prefix-LSA” that defined in [RFC8362]
When ABR(for example R1 in Fig.1)receives the “Router LSA” announcement in area 1, it should generate the corresponding extend “Summary LSA” or “Inter-Area-Prefix-LSA” that includes the “Source Router ID” of the network prefixes, which labels the corresponding link and the “area ID” that the source router belongs to.
When R0 receives such extend LSA, it then strips this additional information, put it into the corresponding part that in BGP-LS protocol as described in[I-D.wang-idr-bgpls-inter-as-topology-ext] and reports them to the IP SDN Controller.
When IP SDN Controller receives this information, it should compare the prefix NLRI that included in the BGP-LS packet. When it encounters the same prefix but with different source router ID, it should extract the corresponding area ID, rebuild the link between these two different source router in non-backbone area.
Iterating the above process continuously, the IP SDN controller can then retrieve the detail topology that span multi-area.
TBD.
TBD.
| [I-D.wang-idr-bgpls-inter-as-topology-ext] | Wang, A., "BGP-LS extend for inter-AS topology retrieval", Internet-Draft draft-wang-idr-bgpls-inter-as-topology-ext-00, March 2018. |