BGP Extension for ECMP Group ID

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Table of Contents

1. Introduction

Currently, there are two granularities for network load balancing: per-flow ECMP and per-packet forwarding.¶

The per-flow ECMP method employs flow characteristic-based hashing (typically using the five-tuple) to distribute traffic across multiple ECMP paths. Its primary advantage is the elimination of packet reordering issues. However, this method presents limitations when dealing with either a limited number of flows, or the presence of elephant flows. In such cases, five-tuple-based hashing may lead to hash collisions, causing disproportionate mapping of oversized flows to the same path. This results in suboptimal load balancing performance.¶

The per-packet forwarding approach applies hashing to each individual packet. Theoretically, it achieves optimal load-balancing granularity. However, it introduces severe packet reordering within the same flow, necessitating additional mechanisms (e.g., reordering buffers or sequence tracking) to handle out-of-order delivery.¶

In AI training scenarios, the current mainstream load balancing technology is per-flow ECMP. However, hash collision issues lead to imbalanced traffic distribution, adversely affecting application performance.¶

To address this problem, an enhanced ECMP method that resolves load imbalance caused by hash collisions has been proposed in [I-D.cheng-rtgwg-enhanced-ecmp]. In one of the proposed methods, the ECMP egress interfaces on the ingress router are divided into subgroups, and flows are mapped to subgroups based on the remote egress group index carried in the route. This requires the route to carry such an egress group index.¶

This document defines a new BGP Extended Community to carry the ECMP Group ID associated with a route, as required by the enhanced ECMP mechanism.¶

2. Protocol Extension

This document defines a new extended community attribute type [RFC4360] to carry the ECMP ID associated with a BGP route [RFC4271]. The ID comprises a 4-byte Router ID and a 2-byte Group-ID. The format is as follows:¶

   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |    0x83       | Sub-Type(TBD) |           Router ID           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Router ID                   |        Group-ID               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Sub-Type: BGP_EXT-COMM-ECMP-GROUPID (TBD)¶

Value Structure:¶

• Local BGP RouterID (4 bytes)¶
• ECMP-Group-ID Value (2 bytes)¶

When a route is advertised with the enhanced ECMP mechanism described in [I-D.cheng-rtgwg-enhanced-ecmp], the route MUST carry this extended community. The Router ID field MUST be set to the local BGP RouterID of the advertising device, and the ECMP-Group-ID field MUST be set to the egress group index assigned on the advertising device. A router that receives this extended community but does not support the enhanced ECMP mechanism MAY ignore it and perform normal ECMP forwarding.¶

3. Security Considerations

TBD.¶

4. IANA Considerations

Registry Name: Transitive BGP_EXT-COMM-ECMP-GROUPID Community Sub-Types¶

TBD: BGP_EXT-COMM-ECMP-GROUPID¶

Authors' Addresses