75 lines
3.2 KiB
ReStructuredText
75 lines
3.2 KiB
ReStructuredText
====================
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CONFIG_NET_GUARDSIZE
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====================
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Global Option for All Drivers
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=============================
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``CONFIG_NET_GUARD_SIZE`` is global option. It is added to the allocated size of
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each driver packet buffer. Currently it is a very small value, defaulting to only
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two bytes. So it is not a memory hog and should be added to the packetsize for
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all drivers for commonality. But why?
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It should (eventually) be larger and common for all drivers. We need to look at
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how it is used today and how it might be used tomorrow. There is a probably a lot
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more involved than you might be initially considering.
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Packet Receipt
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==============
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For packet receipt, it is necessary for some hardware, but not for others. Often
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the hardware will DMA a 2 byte FCS at the end of the packet or possibly other
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hardware-specific info. But that is only part of the whole story.
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``CONFIG_NET_GUARDSIZE`` is not just for hardware packet receipt.
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Packet Transmission
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===================
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There are several issues for packet transmission. These are less well defined
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and need further study, but we need to keep all of the driver packet definitions
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in place until we understand how we are going to handle these things:
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* Memory Overrun Bugs
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There was in the past, a bug that caused write past the end of the buffer by
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a couple of bytes during TX message formatting. I don't know if that bug still
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exists, but the minimum, two-byte ``CONFIG_NET_GUARDSIZE`` was sufficient to
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eliminate the bug. That is why it has the name GUARD: Its primary purpose is
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to protect from overrunning the packet buffer and corrupting the following memory.
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I do no know if we have any such bugs today. Perhaps they still do?
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Perhaps they do not? Having such a guard is a good thing for reliability in
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any case.
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* Variable size IP/TCP headers
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There is a limitation in the way IP packets are formatted now. Basically they
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are formatted like this:
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#. When the packet is received a pointer to the location of the payload is
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set (d_appdata). This is an offset into the packet buffer For TCP, that
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accounts for the MAC/Ethernet header, the minimum IPv4/IPv6 header size,
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and the minimum TCP header size.
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#. The TCP payload is written at that location,
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#. The correctly sized IPv4/IPv6 headers and the correctly sized TCP header
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are added below the payload, and finally
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#. The MAC/Ethernet header as added.
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The start offset of the packet in the packet is no longer zero, but some
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variable offset into the packet buffer. That new start offset would have
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to be passed to driver in order to send the packet.
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The key to making this all work is:
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* Keep ``CONFIG_NET_GUARDSIZE`` in all driver buffers, and
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* Set the ``CONFIG_NET_GUARDSIZE`` to the maximum size of IPv4/IPv6 and TCP options
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(depending on which IP version is enabled and if TCP is enabled)
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* Extend the driver interface to accept data offset into the driver's packet buffer.
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* Variable MSS
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Closely related to this is the MSS which is the maximum size of the payload.
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Currently that is a constant because it assumes the minimum header lengths.
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It should be variable, depending on the actual header sizes.
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