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Merge branch 'vxlan-MDB-support'
Ido Schimmel says: ==================== vxlan: Add MDB support tl;dr ===== This patchset implements MDB support in the VXLAN driver, allowing it to selectively forward IP multicast traffic to VTEPs with interested receivers instead of flooding it to all the VTEPs as BUM. The motivating use case is intra and inter subnet multicast forwarding using EVPN [1][2], which means that MDB entries are only installed by the user space control plane and no snooping is implemented, thereby avoiding a lot of unnecessary complexity in the kernel. Background ========== Both the bridge and VXLAN drivers have an FDB that allows them to forward Ethernet frames based on their destination MAC addresses and VLAN/VNI. These FDBs are managed using the same PF_BRIDGE/RTM_*NEIGH netlink messages and bridge(8) utility. However, only the bridge driver has an MDB that allows it to selectively forward IP multicast packets to bridge ports with interested receivers behind them, based on (S, G) and (*, G) MDB entries. When these packets reach the VXLAN driver they are flooded using the "all-zeros" FDB entry (00:00:00:00:00:00). The entry either includes the list of all the VTEPs in the tenant domain (when ingress replication is used) or the multicast address of the BUM tunnel (when P2MP tunnels are used), to which all the VTEPs join. Networks that make heavy use of multicast in the overlay can benefit from a solution that allows them to selectively forward IP multicast traffic only to VTEPs with interested receivers. Such a solution is described in the next section. Motivation ========== RFC 7432 [3] defines a "MAC/IP Advertisement route" (type 2) [4] that allows VTEPs in the EVPN network to advertise and learn reachability information for unicast MAC addresses. Traffic destined to a unicast MAC address can therefore be selectively forwarded to a single VTEP behind which the MAC is located. The same is not true for IP multicast traffic. Such traffic is simply flooded as BUM to all VTEPs in the broadcast domain (BD) / subnet, regardless if a VTEP has interested receivers for the multicast stream or not. This is especially problematic for overlay networks that make heavy use of multicast. The issue is addressed by RFC 9251 [1] that defines a "Selective Multicast Ethernet Tag Route" (type 6) [5] which allows VTEPs in the EVPN network to advertise multicast streams that they are interested in. This is done by having each VTEP suppress IGMP/MLD packets from being transmitted to the NVE network and instead communicate the information over BGP to other VTEPs. The draft in [2] further extends RFC 9251 with procedures to allow efficient forwarding of IP multicast traffic not only in a given subnet, but also between different subnets in a tenant domain. The required changes in the bridge driver to support the above were already merged in merge commit 8150f0cf ("Merge branch 'bridge-mcast-extensions-for-evpn'"). However, full support entails MDB support in the VXLAN driver so that it will be able to selectively forward IP multicast traffic only to VTEPs with interested receivers. The implementation of this MDB is described in the next section. Implementation ============== The user interface is extended to allow user space to specify the destination VTEP(s) and related parameters. Example usage: # bridge mdb add dev vxlan0 port vxlan0 grp 239.1.1.1 permanent dst 198.51.100.1 # bridge mdb add dev vxlan0 port vxlan0 grp 239.1.1.1 permanent dst 192.0.2.1 $ bridge -d -s mdb show dev vxlan0 port vxlan0 grp 239.1.1.1 permanent filter_mode exclude proto static dst 192.0.2.1 0.00 dev vxlan0 port vxlan0 grp 239.1.1.1 permanent filter_mode exclude proto static dst 198.51.100.1 0.00 Since the MDB is fully managed by user space and since snooping is not implemented, only permanent entries can be installed and temporary entries are rejected by the kernel. The netlink interface is extended with a few new attributes in the RTM_NEWMDB / RTM_DELMDB request messages: [ struct nlmsghdr ] [ struct br_port_msg ] [ MDBA_SET_ENTRY ] struct br_mdb_entry [ MDBA_SET_ENTRY_ATTRS ] [ MDBE_ATTR_SOURCE ] struct in_addr / struct in6_addr [ MDBE_ATTR_SRC_LIST ] [ MDBE_SRC_LIST_ENTRY ] [ MDBE_SRCATTR_ADDRESS ] struct in_addr / struct in6_addr [ ...] [ MDBE_ATTR_GROUP_MODE ] u8 [ MDBE_ATTR_RTPORT ] u8 [ MDBE_ATTR_DST ] // new struct in_addr / struct in6_addr [ MDBE_ATTR_DST_PORT ] // new u16 [ MDBE_ATTR_VNI ] // new u32 [ MDBE_ATTR_IFINDEX ] // new s32 [ MDBE_ATTR_SRC_VNI ] // new u32 RTM_NEWMDB / RTM_DELMDB responses and notifications are extended with corresponding attributes. One MDB entry that can be installed in the VXLAN MDB, but not in the bridge MDB is the catchall entry (0.0.0.0 / ::). It is used to transmit unregistered multicast traffic that is not link-local and is especially useful when inter-subnet multicast forwarding is required. See patch #12 for a detailed explanation and motivation. It is similar to the "all-zeros" FDB entry that can be installed in the VXLAN FDB, but not the bridge FDB. "added_by_star_ex" entries -------------------------- The bridge driver automatically installs (S, G) MDB port group entries marked as "added_by_star_ex" whenever it detects that an (S, G) entry can prevent traffic from being forwarded via a port associated with an EXCLUDE (*, G) entry. The bridge will add the port to the port group of the (S, G) entry, thereby creating a new port group entry. The complexity associated with these entries is not trivial, but it needs to reside in the bridge driver because it automatically installs MDB entries in response to snooped IGMP / MLD packets. The same in not true for the VXLAN MDB which is entirely managed by user space who is fully capable of forming the correct replication lists on its own. In addition, the complexity associated with the "added_by_star_ex" entries in the VXLAN driver is higher compared to the bridge: Whenever a remote VTEP is added to the catchall entry, it needs to be added to all the existing MDB entries, as such a remote requested all the multicast traffic to be forwarded to it. Similarly, whenever an (*, G) or (S, G) entry is added, all the remotes associated with the catchall entry need to be added to it. Given the above, this patchset does not implement support for such entries. One argument against this decision can be that in the future someone might want to populate the VXLAN MDB in response to decapsulated IGMP / MLD packets and not according to EVPN routes. Regardless of my doubts regarding this possibility, it can be implemented using a new VXLAN device knob that will also enable the "added_by_star_ex" functionality. Testing ======= Tested using existing VXLAN and MDB selftests under "net/" and "net/forwarding/". Added a dedicated selftest in the last patch. Patchset overview ================= Patches #1-#3 are small preparations in the bridge driver. I plan to submit them separately together with an MDB dump test case. Patches #4-#6 are additional preparations centered around the extraction of the MDB netlink handlers from the bridge driver to the common rtnetlink code. This allows reusing the existing MDB netlink messages for the configuration of the VXLAN MDB. Patches #7-#9 include more small preparations in the common rtnetlink code and the VXLAN driver. Patch #10 implements the MDB control path in the VXLAN driver, which will allow user space to create, delete, replace and dump MDB entries. Patches #11-#12 implement the MDB data path in the VXLAN driver, allowing it to selectively forward IP multicast traffic according to the matched MDB entry. Patch #13 finally enables MDB support in the VXLAN driver. iproute2 patches can be found here [6]. Note that in order to fully support the specifications in [1] and [2], additional functionality is required from the data path. However, it can be achieved using existing kernel interfaces which is why it is not described here. Changelog ========= Since v1 [7]: Patch #9: Use htons() in 'case' instead of ntohs() in 'switch'. Since RFC [8]: Patch #3: Use NL_ASSERT_DUMP_CTX_FITS(). Patch #3: memset the entire context when moving to the next device. Patch #3: Reset sequence counters when moving to the next device. Patch #3: Use NL_SET_ERR_MSG_ATTR() in rtnl_validate_mdb_entry(). Patch #7: Remove restrictions regarding mixing of multicast and unicast remote destination IPs in an MDB entry. While such configuration does not make sense to me, it is no forbidden by the VXLAN FDB code and does not crash the kernel. Patch #7: Fix check regarding all-zeros MDB entry and source. Patch #11: New patch. [1] https://datatracker.ietf.org/doc/html/rfc9251 [2] https://datatracker.ietf.org/doc/html/draft-ietf-bess-evpn-irb-mcast [3] https://datatracker.ietf.org/doc/html/rfc7432 [4] https://datatracker.ietf.org/doc/html/rfc7432#section-7.2 [5] https://datatracker.ietf.org/doc/html/rfc9251#section-9.1 [6] https://github.com/idosch/iproute2/commits/submit/mdb_vxlan_rfc_v1 [7] https://lore.kernel.org/netdev/20230313145349.3557231-1-idosch@nvidia.com/ [8] https://lore.kernel.org/netdev/20230204170801.3897900-1-idosch@nvidia.com/ ==================== Signed-off-by:David S. Miller <davem@davemloft.net>