Re: A question about GRO neighbor packet matching

[Ilya Matveychikov <matvejchikov@gmail.com>]

> On Dec 6, 2017, at 10:12 PM, Stephen Hemminger <stephen@networkplumber.org> wrote:
> 
> On Wed, 6 Dec 2017 18:02:21 +0400
> Ilya Matveychikov <matvejchikov@gmail.com> wrote:
> 
>> Hello all,
>> 
>> 
>> My question is about neighbor packet matching algorithm for TCP. Is it
>> correct to expect that IP packets should have continuous ID enumeration
>> (i.e. iph-next.id = iph-prev.id + 1)?
> 
> 
> No.
> 
>> ~~~
>> lib/librte_gro/gro_tcp4.c:check_seq_option()
>> 	...
>> 	/* check if the two packets are neighbors */
>> 	tcp_dl0 = pkt0->pkt_len - pkt0->l2_len - pkt0->l3_len - tcp_hl0;
>> 	if ((sent_seq == (item->sent_seq + tcp_dl0)) &&
>> 			(ip_id == (item->ip_id + 1)))
>> 		/* append the new packet */
>> 		return 1;
>> 	else if (((sent_seq + tcp_dl) == item->sent_seq) &&
>> 			((ip_id + item->nb_merged) == item->ip_id))
>> 		/* pre-pend the new packet */
>> 		return -1;
>> 	else
>> 		return 0;
>> ~~~
>> 
>> As per RFC791:
>> 
>>  Identification:  16 bits
>> 
>>    An identifying value assigned by the sender to aid in assembling the
>>    fragments of a datagram.
> 
> The IP header id is meaningless in most TCP sessions.
> Good TCP implementations use PMTU discovery which sets the Don't Fragment bit.
> With DF, the IP id is unused (since no fragmentation).
> Many implementations just send 0 since generating unique IP id requires an
> atomic operation which is potential bottleneck.

So, is my question correct and the code is wrong?