baldur.norddahl at gmail.com
Sat Jul 14 12:13:32 UTC 2018
I am investigating Linux as a BNG. The BNG (Broadband Network Gateway)
being the thing that acts as default gateway for our customers.
The setup is one VLAN per customer. Because 4095 VLANs is not enough, we
have QinQ with double VLAN tagging on the customers. The customers can
use DHCP or static configuration. DHCP packets need to be option82
tagged and forwarded to a DHCP server. Every customer has one or more
static IP addresses.
IPv4 subnets need to be shared among multiple customers to conserve
address space. We are currently using /26 IPv4 subnets with 60 customers
sharing the same default gateway and netmask. In Linux terms this means
60 VLAN interfaces per bridge interface.
However Linux is not quite ready for the task. The primary problem being
that the system does not scale to thousands of VLAN interfaces.
We do not want customers to be able to send non routed packets directly
to each other (needs proxy arp). Also customers should not be able to
steal another customers IP address. We want to hard code the relation
between IP address and VLAN tagging. This can be implemented using
ebtables, but we are unsure that it could scale to thousands of customers.
I am considering writing a small program or kernel module. This would
create two TAP devices (tap0 and tap1). Traffic received on tap0 with
VLAN tagging, will be stripped of VLAN tagging and delivered on tap1.
Traffic received on tap1 without VLAN tagging, will be tagged according
to a lookup table using the destination IP address and then delivered on
tap0. ARP and DHCP would need some special handling.
This would be completely stateless for the IPv4 implementation. The IPv6
implementation would be harder, because Link Local addressing needs to
be supported and that can not be stateless. The customer CPE will make
up its own Link Local address based on its MAC address and we do not
know what that is in advance.
The goal is to support traffic of minimum of 10 Gbit/s per server.
Ideally I would have a server with 4x 10 Gbit/s interfaces combined into
two 20 Gbit/s channels using bonding (LACP). One channel each for
upstream and downstream (customer facing). The upstream would be layer 3
untagged and routed traffic to our transit routers.
I am looking for comments, ideas or alternatives. Right now I am
considering what kind of CPU would be best for this. Unless I take steps
to mitigate, the workload would probably go to one CPU core only and be
limited to things like CPU cache and PCI bus bandwidth.
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