[e2e] Protocols breaking the end-to-end argument
perfgeek at mac.com
Fri Oct 23 18:47:52 PDT 2009
On Oct 23, 2009, at 8:38 AM, David P. Reed wrote:
> On 10/23/2009 08:39 AM, William Allen Simpson wrote:
>> You could add the Broadcom chip sets to your list. Not a protocol
>> per se,
>> but they inexplicably "handle" TCP segmentation. Usually used in a
>> (bad enough in my opinion), but could create utter havoc in a router.
>> So far, I've noticed:
>> NetXtreme II 1 Gigabit
>> Tigon 3
> This is an interesting observation, but I don't understand what you
> Explain "handling TCP segmentation" please? Exactly what chips do
> that? What exactly do they do in the chip?
> The chips might do IP fragmentation, but I find it hard to see how
> they could do TCP segmentation, unless of course they are acting as
> a host. Nothing wrong with a chipset being a host, too (perhaps to
> present a web, ssh or SNMP interface).
Perhaps he is referring to chips which provide TCP/Transport
Segmentation Offload - aka TSO - the functionality that allows the
stack to hand the chip a chunk of data > the MTU, along with the
initial TCP/IP headers and the connection's on the wire MSS, and then
have the chip otherwise statelessly segment that larger chunk of data
into MSS-sized segments for transmission on the wire/fibre/etc.
If that is the functionality of which he speaks, it is in virtually
every contemporary 1GbE card I can think of (but my thoughts cannot
span the entirety of the space I suspect). Also, virtually every 10G
NIC out there offers the same functionality.
And if that upsets him, we better not tell him about the 10G NICs also
doing receive offload... :)
BTW, I do not believe that any router actually has TSO happen to TCP
segments contained within the IP datagrams passing through it -
although there have been issues in Linux with LRO (Large Receive
Offload, distinct from General Receive Offload) when the system was
acting as either a router or a bridge - because TSO doesn't happen in
that path :)
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