[Xorp-users] question bad done

[Bruce M Simpson]

Hi there.

This is a bit of a detailed explanation because I'm currently reviewing
Layer 2 and Layer 3 QoS support across the board.

On Wed, Feb 22, 2006 at 09:14:18AM +0100, riccardo.sciaccaluga@tin.it wrote:
> Sorry my question was bad done!
> I don't wanna know about priority 
> within IP packet flows but about this:
> In a topology configuration like 
> this  ForwardingMachine---->Xorp machine---->>traffic destination 
> machine.
> Xorp pc has two interfaces running OPSF protocol towards the 
> Fw and destination machines.
> The Fw machine is forwarding an IP traffic 
> flows higher then the forwarding ability of xorp pc.
> So, xorp pc will 
> start to drop packets.
> I am asking my self if it drops just IP packets 
> saving OPSF one (protecting the opsf link) or it drops IP packets and 
> ospf packets without any difference?

This isn't really a XORP issue; XORP only provides a router control plane,
not a forwarding plane or a TCP/IP stack.

If the machine on which you are running XORP isn't configured to
prioritize delivery of the OSPF traffic in some way, then yes, it
is possible that the traffic could be dropped.

This is a function of the TCP/IP stack in the host operating system, and
the network card driver. If running Linux or BSD, you could configure some
form of priority queueing e.g. in IPTABLES, IPFW or PF. Windows has support
for RSVP and DiffServ since Windows 2000, but we don't currently do anything
with it -- you'd have to configure an inbound flow explicitly with TCMON.EXE
from the Resource Kit.

This doesn't offer any guarantees that the traffic will actually be delivered
to user-space, where XORP lives; if you saturate the buffers in your XORP PC's
Ethernet card (I'm assuming you're using an Ethernet link layer here (+)), then
traffic of any kind will be dropped, not just OSPF or indeed IPv4, and
this condition will most likely manifest itself in the form of high interrupt
load, creating bigger problems.

The place to look for the drop, if it's badly affecting performance, is
the network card and its driver. If the card supports 802.1p (*) Packet
Priority in hardware, this might improve things, but Ethernet as a rule
only offers best-effort delivery because of the nature of the MAC protocol.

Regards,
BMS

(+) If you're running XORP with a non-Ethernet link layer, please tell us!

(*) FreeBSD doesn't currently support 802.1p correctly -- you need to
configure a VLAN for it to work.