From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ilya Matveychikov <matvejchikov@gmail.com>
Subject: A question about GRO neighbor packet matching
Date: Wed, 6 Dec 2017 18:02:21 +0400
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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)?

~~~
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.