netdev - Re: SKB Reference Question

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Message-Id: <20171102.143557.1541082339282323563.davem@davemloft.net>
Date:   Thu, 02 Nov 2017 14:35:57 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     codesoldier1@...il.com
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: SKB Reference Question

From: Joe Smith <codesoldier1@...il.com>
Date: Wed, 1 Nov 2017 10:27:49 -0700

> How strictly are references on the SKB enforced. For example,
> tcp_transmit_skb() clones the SKB and adds a TCP header. Can I assume
> that in case of re-transmission the header added will be there and can
> be reused instead of creating a new one from scratch. Some fields like
> time stamp would need to be updated but they should be unmodified.

Every time a data packet goes out, whether it is the initial transmission
or a retransmission, it goes through tcp_transmit_skb() which pushes the
TCP headers and fills in all of the TCP header options as well.