Re: [PATCH v2 net-next 08/14] ipv6: Add hop-by-hop header to jumbograms in ip6_output

On Fri, Mar 4, 2022 at 7:48 AM Alexander H Duyck
[off-list ref] wrote:

On Thu, 2022-03-03 at 21:33 -0700, David Ahern wrote:

quoted

On 3/3/22 11:16 AM, Eric Dumazet wrote:

quoted

From: Coco Li <redacted>

Instead of simply forcing a 0 payload_len in IPv6 header,
implement RFC 2675 and insert a custom extension header.

Note that only TCP stack is currently potentially generating
jumbograms, and that this extension header is purely local,
it wont be sent on a physical link.

This is needed so that packet capture (tcpdump and friends)
can properly dissect these large packets.

I am fairly certain I know how you are going to respond, but I will ask
this anyways :-) :

The networking stack as it stands today does not care that skb->len >
64kB and nothing stops a driver from setting max gso size to be > 64kB.
Sure, packet socket apps (tcpdump) get confused but if the h/w supports
the larger packet size it just works.

The jumbogram header is getting adding at the L3/IPv6 layer and then
removed by the drivers before pushing to hardware. So, the only benefit
of the push and pop of the jumbogram header is for packet sockets and
tc/ebpf programs - assuming those programs understand the header
(tcpdump (libpcap?) yes, random packet socket program maybe not). Yes,
it is a standard header so apps have a chance to understand the larger
packet size, but what is the likelihood that random apps or even ebpf
programs will understand it?

Alternative solutions to the packet socket (ebpf programs have access to
skb->len) problem would allow IPv4 to join the Big TCP party. I am
wondering how feasible an alternative solution is to get large packet
sizes across the board with less overhead and changes.

I agree that the header insertion and removal seems like a lot of extra
overhead for the sake of correctness. In the Microsoft case I am pretty
sure their LSOv2 supported both v4 and v6. I think we could do
something similar, we would just need to make certain the device
supports it and as such maybe it would make sense to implement it as a
gso type flag?

Could we handle the length field like we handle the checksum and place
a value in there that we know is wrong, but could be used to provide
additional data? Perhaps we could even use it to store the MSS in the
form of the length of the first packet so if examined, the packet would
look like the first frame of the flow with a set of trailing data.

I am a bit sad you did not give all this feedback back in August when
I presented BIG TCP.

We did a lot of work in the last 6 months to implement, test all this,
making sure this worked.

I am not sure I want to spend another 6 months implementing what you suggest.

For instance, input path will not like packets larger than 64KB.

There is this thing trimming padding bytes, you probably do not want
to mess with this.