RE: [Xen-devel] Xen Networking query

["Pradeep Singh, TLS-Chennai" <pradeep_s@xxxxxx>]

-----Original Message-----
From: Petersson, Mats [mailto:Mats.Petersson@xxxxxxx]
Sent: Fri 13-Apr-07 3:40 PM
To: Pradeep Singh, TLS-Chennai; xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] Xen Networking query

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> Pradeep Singh, TLS-Chennai
> Sent: 13 April 2007 09:33
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] Xen Networking query
>
> Hi All,
>
> Assume ,I create a connection from remote host to a Xen
> machine using sockets.
>
> AFAIK xen hypervisor doesnot have a network stack but uses
> dom0's tcp/ip stack for networking.Right?

Correct, Dom0 has all the networking protocols, Xen's Hypervisor doesn't
even know what a network card is, nor has any idea of what happens on
the network in general [of course, the Hypervisor receives the interrupt
from the hardware, but the interrupt is just forwarded to the guest
owning the hardware, and xen doesn't actually know (or care) what the
reason/purpose of the interrupt is].
>
> This mean, the very first time a conncetion is made the
> packet is recieved by the NIC which is handled by the driver
> and the TCP/IP stack in the dom0. Am i right?

This is a good question. From what I understand, and this page
http://linux-net.osdl.org/index.php/Bridge agrees, the networking to
forward packets from Dom0 to DomU is done at "ethernet level", e.g. the
TCP/IP stack (or any other protocol stack above Ethernet) is not used in
Dom0 to determine the destination or purpose of the DomU packet.

>
> Subsequent data transfer to this connection(to the domU)
> doesnot goes through dom0 TCP/IP stack but goes through the
> domU's tcp/ip stack. Is my understanding of the Xen
> networking correct?

All packets received by DomU goes through the DomU TCP/IP stack,
regardless of whether it was necessary for Dom0 to parse them too, or
not. [But if the above is correct, Dom0 doesn't even look at the packet
aside from the Ethernet address].

Thanks Mats.
That was really helpful.

But then how does Xen hypervisor makes sure that first time the data is recieved is for which DomU?
Because the IP the remote machine sees is that of the dom0 right? This means if i send data to the Xen server it is for dom0 and not for the domU.Am i correct in my understanding?

Or is it that Xen hypervisor just sends it to the dom0's tcp/ip stack and then dom0's stack chooses the domU's who should handle the packet? If so are there any parameters governing this selective switching?

Thank you

~psr
--
Mats
>
> Thank you
>
> ~psr
>
>
> -----------------------
> Pradeep Singh
> Member Technical Staff
> Server and Storage Group
> 51, Greams Road.
> HCL Technologies, Chennai
>
>
>
>

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