ng_one2many v.s. AFT (NIC Fault Tolerance/Fail Over/Redundancy Revisited)

Sat Oct 15 16:25:16 PDT 2005

Re: 
http://lists.freebsd.org/pipermail/freebsd-questions/2005-October/100623.html

First: This is all very preliminary from some testing over the weekend.

Dell's reponse was that Intel's AFT/ALB was entirely software based.

That left me with few options:
1) Try userland layer 3 failover (ugly)
2) Use ng_one2many

However, ng_one2many only permits for two algorithms: 
NG_ONE2MANY_XMIT_ROUNDROBIN and NG_ONE2MANY_XMIT_ALL.

However, none of these meet the need:
- Round-Robin results in 50% packet loss if a hook/interface is lost (not 
acceptable in any mission critical environment).
- Xmit-All causes twice as much load on to be placed on the switch /fabric 
and switch CPU.

What ng_one2many needs is a "Active-Standy" XMIT algorithm (STP BOFH's 
will think BLOCKING/FORWARDING).  It could even be used on top of other 
NetGraph nodes like ng_fec or possibly (hopefully) ng_802.3ad >:}

Essentially, a single layer 3 IP address needs to be visible in a "switch 
fault tolerant" or "adapter fault tolerant" configuration.  A 
userland-level daemon could be scripted, and it has been done before:

http://lists.freebsd.org/pipermail/freebsd-isp/2003-November/001314.html

So when a fail-over occurs, the layer IP 3 address moves from one layer 2 
MAC address to another layer 2 MAC address on the same machine (and same 
subnet, same ethernet segment, just a different interface).  TCP sockets 
should not be affected due to layer abstraction.

This got me thinking about HSRP/VRRP.  That protocol is designed strictly 
to move a layer 3 address between two different hosts.   Excellent 
applications are Router/Firewall and VPN concentrator, as OpenBSD's 
carp(4) has implemented with the help of pfsync.  I was experimenting with 
the OpenBSD variant and I realized that client hosts weren't seeing the 
usual warnings about MAC address changes.

As of 3.7, OpenBSD's CARP shares a virtual MAC address between the hosts, 
Cisco's HSRP does not.

Then I was thinking about the OpenBSD/NetBSD bridge(4) interface.  If the 
host acting as the bridge wishes too, it can participate in the bridged 
networks by assigning a layer 3 address.  The address isn't ifconfig(8)'d 
do the "bridge0" interface.  Instead, it's assigned to the first interface 
included in the "bridge[0-9]", say fxp0.

Further more, regardless of what network segment/port a host participating 
in a bridge(4)'d network resides, the ARP'd IP address of the 
OpenBSD/NetBSD host is persistently the MAC first physical interface 
ifconfig(8)'d with the IP.

Plus OpenBSD/NetBSD bridge(4) supports 802.1d spanning tree >:}

This is important.  Spanning Tree as an alogirth could provide Intel AFT 
"Fault Tolerance" intelligence if the persistent layer2 address of a host 
was unchanged with the NIC interface change.  The function of STP is to 
provide a loop free path to every layer2 MAC in a segment.  But a STP 
enabled bridge(4) with an IP address assigned has a persistent MAC address 
associated with a layer 3 address!

Therefore, the solution has been there all along.  The attached diagram 
explains in greater detail.

http://digitalfreaks.org/~lavalamp/OpenBSD_Bridge_AFT.png

In this diagram, switch 0 is configured manually as the spanning tree root 
and switch 1 is the backup spanning tree root.  By default, rl0 will be in 
BLOCKING and rl1 will being FORWARDING.  However, as tcpdump(8) 
illustrates, regardless of which interface is the root port, ARP replys 
will always return the MAC if the bridge(4) member interface ifconfig(8)'d 
with the IP.

rl0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
 	address: 00:50:fc:9d:24:d6
 	media: Ethernet autoselect (100baseTX full-duplex)
 	status: active
 	inet 192.168.100.1 netmask 0xffffff00 broadcast 192.168.100.255

rl1: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
 	address: 00:50:fc:9d:08:cd
 	media: Ethernet autoselect (100baseTX full-duplex)
 	status: active

---

bridge0: flags=41<UP,RUNNING>
 	Configuration:
 		priority 32768 hellotime 2 fwddelay 15 maxage 20
 	Interfaces:
 		rl1 flags=b<LEARNING,DISCOVER,STP>
 			port 2 ifpriority 128 ifcost 55 forwarding
 		rl0 flags=b<LEARNING,DISCOVER,STP>
 			port 1 ifpriority 128 ifcost 55 blocking
 	Addresses (max cache: 100, timeout: 240):
 		00:01:63:bb:f7:c9 rl1 1 flags=0<>
 		00:0f:1f:c1:f2:b7 rl1 1 flags=0<>
-----
# tcpdump -i rl1 -n arp
12:38:17.806885 arp who-has 192.168.100.1 tell 192.168.100.254
12:38:17.806951 arp reply 192.168.100.1 is-at 0:50:fc:9d:24:d6
12:38:17.806966 arp reply 192.168.100.1 is-at 0:50:fc:9d:24:d6

bs0#sh spanning-tree vlan 11 interface fa0/9

Spanning tree 11 is executing the IEEE compatible Spanning Tree protocol
   Bridge Identifier has priority 100, address 0001.63bb.f7c2
   Configured hello time 2, max age 20, forward delay 15
   We are the root of the spanning tree
   Topology change flag not set, detected flag not set, changes 54
   Times:  hold 1, topology change 35, notification 2
           hello 2, max age 20, forward delay 15
   Timers: hello 0, topology change 0, notification 0

Interface Fa0/9 (port 22) in Spanning tree 11 is FORWARDING
    Port path cost 19, Port priority 128
    Designated root has priority 100, address 0001.63bb.f7c2
    Designated bridge has priority 100, address 0001.63bb.f7c2
    Designated port is 22, path cost 0
    Timers: message age 0, forward delay 0, hold 0
    BPDU: sent 10592, received 30

bs1#sh spanning-tree vlan 11 interface fa0/9

Spanning tree 11 is executing the IEEE compatible Spanning Tree protocol
   Bridge Identifier has priority 32768, address 0002.fd0e.f382
   Configured hello time 2, max age 20, forward delay 15
   Current root has priority 100, address 0001.63bb.f7c2
   Root port is 38, cost of root path is 19
   Topology change flag not set, detected flag not set, changes 54
   Times:  hold 1, topology change 35, notification 2
           hello 2, max age 20, forward delay 15
   Timers: hello 0, topology change 0, notification 0

Interface Fa0/9 (port 22) in Spanning tree 11 is FORWARDING
    Port path cost 19, Port priority 128
    Designated root has priority 100, address 0001.63bb.f7c2
    Designated bridge has priority 32768, address 0002.fd0e.f382
    Designated port is 22, path cost 19
    Timers: message age 0, forward delay 0, hold 0
    BPDU: sent 45454, received 1196

bs0#sh mac-address-table | include 24d6
0050.fc9d.24d6       Dynamic         11  FastEthernet0/9

bs1#sh mac-address-table | include 24d6
0050.fc9d.24d6       Dynamic         11  FastEthernet0/24

The behavior is similar in FreeBSD using ng_bridge(4) (I haven't tried 
FreeBSD bridge(4)).  However, both of these claim "a privative loop 
prevention algorithm"); ... 'debug stp events' shows no STP traffic from a 
FreeBSD host, though.

Also, FreeBSD differs in behavior in that the MAC address ARP'd is that of 
which ever NG node bridge member is assigned the IP.

The disadvantage is that without FreeBSD speaking 802.1d, it can't know to 
fail an interface on any event other than a media state change. i.e., the 
currently active port could be connected a switch that looses it's uplink. 
Of course, neither the FreeBSD or NetBSD/OpenBSD implementation features a 
"heartbeat" algorithm to add intelligence, as Intel AFT/ALB might, but 
that wasn't the design principal goal.

Also, my initial tests are with managed switches using PVST.  Behavior may 
differ with unmanaged switches where no STP debugging is possible or 
possibly a uni-stp is used.