net: bridge: mcast: add multicast exponential field encoding [LWN.net]

From:		Ujjal Roy <royujjal-AT-gmail.com>
To:		"David S . Miller" <davem-AT-davemloft.net>, Eric Dumazet <edumazet-AT-google.com>, Jakub Kicinski <kuba-AT-kernel.org>, Paolo Abeni <pabeni-AT-redhat.com>, Simon Horman <horms-AT-kernel.org>, Nikolay Aleksandrov <razor-AT-blackwall.org>, Ido Schimmel <idosch-AT-nvidia.com>, David Ahern <dsahern-AT-kernel.org>
Subject:		[PATCH 0/4] net: bridge: mcast: add multicast exponential field encoding
Date:		Thu, 26 Mar 2026 15:07:38 +0000
Message-ID:		<20260326150742.50289-1-royujjal@gmail.com>
Cc:		Ujjal Roy <ujjal-AT-alumnux.com>, bridge-AT-lists.linux.dev, netdev-AT-vger.kernel.org, linux-kernel-AT-vger.kernel.org
Archive-link:		Article

Description:
This series addresses a mismatch in how multicast query
intervals and response codes are handled across IPv4 (IGMPv3)
and IPv6 (MLDv2). While decoding logic currently exists,
the corresponding encoding logic is missing during query
packet generation. This leads to incorrect intervals being
transmitted when values exceed their linear thresholds.

The patches introduce a unified floating-point encoding
approach based on RFC3376 and RFC3810, ensuring that large
intervals are correctly represented in QQIC and MRC fields
using the exponent-mantissa format.

Key Changes:
* ipv4: igmp: get rid of IGMPV3_{QQIC,MRC} and simplify calculation
  Removes legacy macros in favor of a cleaner, unified
  calculation for retrieving intervals from encoded fields,
  improving code maintainability.

* ipv6: mld: rename mldv2_mrc() and add mldv2_qqi()
  Standardizes MLDv2 terminology by renaming mldv2_mrc()
  to mldv2_mrd() (Maximum Response Delay) and introducing
  a new API mldv2_qqi for QQI calculation, improving code
  readability.

* ipv4: igmp: encode multicast exponential fields
  Introduces the logic to dynamically calculate the exponent
  and mantissa using bit-scan (fls). This ensures QQIC and
  MRC fields (8-bit) are properly encoded when transmitting
  query packets with intervals that exceed their respective
  linear threshold value of 128 (for QQI/MRT).

* ipv6: mld: encode multicast exponential fields
  Applies similar encoding logic for MLDv2. This ensures
  QQIC (8-bit) and MRC (16-bit) fields are properly encoded
  when transmitting query packets with intervals that exceed
  their respective linear thresholds (128 for QQI; 32768
  for MRD).

Impact:
These changes ensure that multicast queriers and listeners
stay synchronized on timing intervals, preventing protocol
timeouts or premature group membership expiration caused
by incorrectly formatted packet headers.

Ujjal Roy (4):
  ipv4: igmp: get rid of IGMPV3_{QQIC,MRC} and simplify calculation
  ipv6: mld: rename mldv2_mrc() and add mldv2_qqi()
  ipv4: igmp: encode multicast exponential fields
  ipv6: mld: encode multicast exponential fields

 include/linux/igmp.h      | 158 +++++++++++++++++++++++++++++++--
 include/net/mld.h         | 179 ++++++++++++++++++++++++++++++++++++--
 net/bridge/br_multicast.c |  22 +++--
 net/ipv4/igmp.c           |   6 +-
 net/ipv6/mcast.c          |  19 +---
 5 files changed, 336 insertions(+), 48 deletions(-)

-- 
2.43.0