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    <p>There are some pictures <a href="http://vger.kernel.org/~davem/tcp_output.html" rel="nofollow noreferrer">http://vger.kernel.org/~davem/tcp_output.html</a> Googled with <code>tcp_transmit_skb()</code> which is a key part of tcp datapath. There are some more interesting thing on his site <a href="http://vger.kernel.org/~davem/" rel="nofollow noreferrer">http://vger.kernel.org/~davem/</a></p> <p>In <code>user - tcp</code> <strong>transmit</strong> part of datapath there is <strong>1 copy</strong> from user to skb with <code>skb_copy_to_page</code> (when sending by <em><code>tcp_sendmsg()</code></em>) and <strong>0 copy</strong> with <code>do_tcp_sendpages</code> (called by <em><code>tcp_sendpage()</code></em>). Copy is needed to keep a backup of data for case of undelivered segment. skb buffers in kernel can be cloned, but their data will stay in first (original) skb. Sendpage can take a page from other kernel part and keep it for backup (i think there is smth like COW)</p> <p>Call paths (manually from lxr). Sending <code>tcp_push_one</code>/<code>__tcp_push_pending_frames</code></p> <pre><code>tcp_sendmsg() &lt;- sock_sendmsg &lt;- sock_readv_writev &lt;- sock_writev &lt;- do_readv_writev tcp_sendpage() &lt;- file_send_actor &lt;- do_sendfile </code></pre> <p>Receive <code>tcp_recv_skb()</code></p> <pre><code>tcp_recvmsg() &lt;- sock_recvmsg &lt;- sock_readv_writev &lt;- sock_readv &lt;- do_readv_writev tcp_read_sock() &lt;- ... spliceread for new kernels.. smth sendfile for older </code></pre> <p>In <strong>receive</strong> there can be <strong>1 copy</strong> from kernel to user <code>skb_copy_datagram_iovec</code> (called from <code>tcp_recvmsg</code>). And for tcp_read_sock() there can be copy. It will call <code>sk_read_actor</code> callback function. If it correspond to file or memory, it may (or may not) copy data from DMA zone. If it is a other network, it has an skb of received packet and can reuse its data inplace.</p> <p>For udp - receive = <strong>1 copy</strong> -- skb_copy_datagram_iovec called from udp_recvmsg. transmit = <strong>1 copy</strong> -- udp_sendmsg -> ip_append_data -> getfrag (seems to be ip_generic_getfrag with 1 copy from user, but may be a smth sendpage/splicelike without page copiing.)</p> <p>Generically speaking there is must be at least 1 copy when sending from/receiving to userspace and 0 copy when using zero-copy (surprise!) with kernel-space source/target buffers for data. All headers are added without moving a packet, DMA-enabled (all modern) network card will take data from any place in DMA-enabled address space. For ancient cards PIO is needed, so there will be one more copy, from kernel space to PCI/ISA/smthelse I/O registers/memory.</p> <p>UPD: In path from NIC (but this is nic-dependent, i checked 8139too) to tcp stack there is <strong>one more copy</strong>: from rx_ring to skb and the same for receive: from skb to tx buffer <strong>+1copy</strong>. You must to fill in ip and tcp header, but does skb contain them or place for them?</p>
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    1. POCount of memory copies in *nix systems between packet at NIC and user application?
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    1. CO"The Performance Analysis of Linux Networking – Packet Receiving" (thnx to http://hackingnasdaq.blogspot.com/2010/01/myth-of-procsysnetipv4tcplowlatency.html - myth of tcp_low_latency sysctl)
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    2. COhackingnasdaq.blogspot.com - this blog is very interesting. There are a lot of posts about low-latency linux networking
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    3. CO**"Potential performance bottleneck in Linux TCP"** is another **very good** (and longer) paper by WWu & MCrawford about linux network packet path
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