[dpdk-dev] zero copy of received segmented IP packet

Yossi Barshishat yossi at imvisiontech.com
Sun Mar 30 17:13:42 CEST 2014

Thanks David for the detailed answer.
In fact I am talking about a proprietary UDP datagrams exchange
implementation between two machines.
It is not a standard protocol but a proprietary, and it will be the only
protocol to be exchanged between the both machines.
Performance is important in this application and I hoped there will be a way
to really make it zero-copy.
Following my very specific needs I believe it is more doable than the
general use-case you have described in your answer.
Anyway I am more convinced now that this is not doable using the means that
DPDK (or alternative tools) provides.


-----Original Message-----
From: David P. Reed [mailto:david.reed at tidalscale.com] 
Sent: Sunday, March 30, 2014 5:18 PM
To: Yossi Barshishat
Cc: dev at dpdk.org
Subject: Re: [dpdk-dev] zero copy of received segmented IP packet

Yossi -

You may already understand this, but fragments of IP datagrams ("IP packet"
is non-standard slang that confuses IP fragments - packets - with the
end-to-end data unit of IP) need to be checksummed together with items from
the "virtual header" before delivery to TCP and then userspace.  Also, TCP
datagrams can overlap each other's sequence space and also be partially
"old".  There is no rule that says that a later IP datagram cannot transmit
the part of the sequence-number range of earlier received IP datagrams.  The
bytes must be identical, of course.

So, for example, if a prior TCP datagram had been received covering sequence
numbers 504-508, a subsequent TCP segment might cover sequence number
500-535 (if the sender has not seen the ack up to 508, which can happen for
many reasons).   504-508 would be covered by the segment's TCP checksum
(along with that  segment's virtual header).  

Whatever you do to handle zero-copy implementation of TCP direct into TCP
receiver buffers must, for example, be able to deliver bytes 509-535
directly into the user buffer, if bytes 504-508 have already been delivered.
Otherwise it is a non-standard implementation.

A simpler approach might work with certain sender-stacks  (those that use
the same "datagram-boundaries" for retransmission), but hardly all, since
the standard does not require retransmission on such boundaries.  In the old
days, terminal concentrators that used telnet over TCP would retransmit
larger segments than the "single character" segments in order to reduce the
overhead of catching up with packets dropped.  It's dangerous to presume
that one's "sending stack" and one's "receiving stack" are in the same
version of the same OS - especially dangerous to promote a technique that
fails on certain standard cases as a performance improving win.

I suspect that a zero-copy TCP requires that at least sometimes, given
fragmentation and this "overlapping sequence number" issue, actual copying,
especially with fragmentation involved.

So if you are talking about "almost always zero-copy with certain senders"
that might make the complexity far less.  Zero-copy fragment assembly only
in the IP layer is much more doable, but it still requires a copy from the
reassembled IP datagram into TCP sequence number space.

David P. Reed, Ph.D.
TidalScale, Inc.

On Mar 30, 2014, at 2:52 AM, Yossi Barshishat <yossi at imvisiontech.com>

> Hi,
> Assuming I know ahead that all IP segments related to one single IP 
> packet ID arrive consequently and I need to forward the entire IP 
> payload toward the application layer.
> One way to handle this is using a hash table for reassembly of the 
> packet data (like the ipv4_reassembly example), another way would be 
> to assume one single bucket (following the above assumption).
> However any means the DPDK provides doesn't enable a zero copy 
> mechanism (it will be required to copy the segments payloads into one
larger buffer).
> Does anybody has any idea regarding a method to control the place 
> where each part of the packet will be written to?
> e.g. allocating the first segment regularly while the packet data 
> buffer is set to the maximum packet length (rather than to MTU size), 
> and then reading n bytes after the start of each following segment into
the data buffer.
> That way I can forward the app layer the buffer without copying it.
> Thanks,

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