IGP Extensions for Sub-interface Relationship Information

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Table of Contents

1. Introduction

The SRv6 BE bandwidth pooling technology enables network devices to detect link bandwidth and bandwidth usage in real time, thereby quickly identifying faults and congestion, and automatically calculating available alternative paths to load balance traffic. This function relies on network devices advertising their interface bandwidth and bandwidth usage information to external systems. There are several RFCs defining how to advertise the maximum bandwidth and utilized bandwidth of a link by IGP.¶

[RFC5305] describes extensions to Intermediate System to Intermediate System (IS-IS) protocol to support Traffic Engineering(TE). It defines the Maximum Link Bandwidth sub-TLV used to describe the maximum bandwidth of a link between two directly connected IS-IS neighbors.¶

[RFC8750] further extends [RFC5305], and defines the Unidirectional Utilized Bandwidth Sub-TLV to describe the bandwidth utilization between two directly connected IS-IS neighbors.¶

[RFC5305] describes extensions to OSPF protocol to support Traffic Engineering(TE). It defines the Maximum Link Bandwidth sub-TLV in the Link TLV used to describe the maximum bandwidth of a link between two directly connected OSPF neighbors¶

[RFC7471] further extends [RFC5305], and defines the Unidirectional Utilized Bandwidth Sub-TLV to describe the bandwidth utilization between two directly connected OSPF neighbors.¶

However, these layer 3 protocols, such as IS-IS and OSPF, often establish neighbor relationships through sub-interfaces. When two directly connected IS-IS neighbors are established based on the sub-interface, then the Maximum Link Bandwidth and Unidirectional Utilized Bandwidth of the sub-interfaces are the same as their parent physical interface, since the sub-interfaces don't have independent bandwidth and bandwidth utilization information.¶

The remote devices don't know the relationship between the sub-interfaces and their parent physical interface. Therefore, the remote devices don't know the maximum link bandwidth and unidirectional utilized bandwidth information is shared among all the sub-interfaces of a specific physical interface. This makes a remote link based on sub-interfaces difficult to participate in traffic load balancing as an alternative forwarding path.¶

This document extends ISIS and OSPF, allowing a network device to advertise the relationship between a physical interface and its sub-interfaces. This extension enables the links based on sub-interfaces to participate in the alternative paths for load balancing in SRv6 BE bandwidth polling.¶

2. ISIS extensions

There are two extension options to advertise the relationship between a physical interface and its sub-interfaces.¶

    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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             L3 Neighbor System ID + pseudonode ID             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     L3 Neighbor System ID + pseudonode ID     |     Flags     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /              Link Local/Remote Identifiers sub-TLV            /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Inter-Length  | Inter-Number  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /    Sub-Interface Member Link Local Identifiers (variable)     /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                      Sub-TLVs (variable)                      /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    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    |             Flags             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Physical Local Link Identifier                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                      Sub-TLVs (variable)                      /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3. OSPF extensions

TBD¶

4. Use cases

As shown in the following figure, there are four nodes A, B, C, and D on the network. There are two links between node D and node C. IS-IS neighbor relationships are established between sub-interfaces d1 and d2 of node D and sub-interfaces c1 and c2 of node C. Sub-interfaces d1 and d2 correspond to the same physical interface d, and sub-interfaces c1 and c2 correspond to the same physical interface c. The physical bandwidth of the link between A and B is 100 Gbit/s, and 70 Gbit/s is used. The physical bandwidth of the link between C and D is 50 Gbit/s, and 20 Gbit/s is used.¶

+-----+     70/100G     +-----+                   +-----+
|  A  |-----------------|  B  |-------------------|  C  |
+-----+                 +-----+                   +-----+
   |                                              c1 || c2
   |                    +-----+ d1      20/50G       ||
   +--------------------|  D  |======================+
                        +-----+ d2

After the link state synchronization, node A receives and parses the related TLVs, and establishes the following mapping between the physical interface and sub-interfaces:¶

• Node D¶

  • Physical interface d¶

    • Bandwidth attributes of interface d（physical bandwidth and bandwidth utilization ratio）¶

    • sub-interface d1¶

    • sub-interface d2¶

• Node C¶

  • Physical interface c¶

    • Bandwidth attributes of interface c（physical bandwidth and bandwidth utilization ratio）¶

    • sub-interface c1¶

    • sub-interface c2¶

Assume that node A is the source node and node C is the destination node, the traffic rate is 60Gbit/s. In normal cases, the primary path is A->B->C. Node A calculates an alternative path A->D->C for node C, where two links are established between node D and node C through two sub-interfaces.¶

Node A perceives that there is a congestion on the path A->B->C, it decides to use the alternative paths for load balancing.¶

Based on the above established mapping relationship, node A knows that the two links corresponds to one physical interface, then uses them as a single load balancing link. Then node A will allocate 30Gbit/s on each path.¶

However, if node A does not know the mapping relationship, it will consider A->D->C as two independent paths, allocating 20Gbit/s on each path. This result in the traffic on the physical interfaces d exceeds its maximum bandwidth, therefore a large number of packets will be lost.¶

5. IANA Considerations

TBD¶

6. Security Considerations

TBD¶

7. References

Acknowledgements

TBD¶

Authors' Addresses