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Thoughts on how to coerce the PXE TFTP API into something resembling
the TFTP protocol.
This commit is contained in:
Michael Brown
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commit
aeb984a1a9
@ -24,7 +24,52 @@
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#include "pxe.h"
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/* PXENV_TFTP_OPEN
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/**
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* TFTP OPEN
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*
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* @v tftp_open Pointer to a struct s_PXENV_TFTP_OPEN
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* @v s_PXENV_TFTP_OPEN::ServerIPAddress TFTP server IP address
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* @v s_PXENV_TFTP_OPEN::GatewayIPAddress Relay agent IP address, or 0.0.0.0
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* @v s_PXENV_TFTP_OPEN::Filename Name of file to open
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* @v s_PXENV_TFTP_OPEN::TFTPPort TFTP server UDP port
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* @v s_PXENV_TFTP_OPEN::PacketSize TFTP blksize option to request
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* @ret #PXENV_EXIT_SUCCESS File was opened
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* @ret #PXENV_EXIT_FAILURE File was not opened
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* @ret s_PXENV_TFTP_OPEN::Status PXE status code
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* @ret s_PXENV_TFTP_OPEN::PacketSize Negotiated
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* @err ....... ..........
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*
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* Opens a TFTP connection for downloading a file a block at a time
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* using pxenv_tftp_read().
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*
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* If s_PXENV_TFTP_OPEN::GatewayIPAddress is 0.0.0.0, normal IP
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* routing will take place. See the relevant
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* @ref pxe_routing "implementation note" for more details.
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*
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* s_PXENV_TFTP_OPEN::PacketSize must be at least 512.
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*
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* You can only have one TFTP connection open at a time, because the
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* PXE API requires the PXE stack to keep state about the open TFTP
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* connection (rather than letting the caller do so).
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*
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* It is unclear precisely what constitutes a "TFTP open" operation.
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* Clearly, we must send the TFTP open request to the server. Since
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* we must know whether or not the open succeeded, we must wait for
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* the first reply packet from the TFTP server. If the TFTP server
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* supports options, the first reply packet will be an OACK; otherwise
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* it will be a DATA packet. In other words, we may only get to
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* discover whether or not the open succeeded when we receive the
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* first block of data. However, the pxenv_tftp_open() API provides
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* no way for us to return this block of data at this time. See the
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* relevant @ref pxe_note_tftp "implementation note" for Etherboot's
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* solution to this problem.
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*
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*
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* @note According to the PXE specification version 2.1, this call
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* "opens a file for reading/writing", though how writing is to be
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* achieved without the existence of an API call %pxenv_tftp_write()
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* is not made clear.
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*
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* Status: working
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*/
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@ -197,3 +242,98 @@ PXENV_EXIT_t pxenv_tftp_get_fsize ( struct s_PXENV_TFTP_GET_FSIZE
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tftp_get_fsize->Status = PXENV_STATUS_SUCCESS;
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return PXENV_EXIT_SUCCESS;
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}
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/** @page pxe_notes Etherboot PXE implementation notes
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@section pxe_note_tftp Welding together the TFTP protocol and the PXE TFTP API
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The PXE TFTP API is fundamentally poorly designed; the TFTP protocol
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simply does not map well into "open file", "read file block", "close
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file" operations. The problem is the unreliable nature of UDP
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transmissions and the lock-step mechanism employed by TFTP to
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guarantee file transfer. The lock-step mechanism requires that if we
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time out waiting for a packet to arrive, we must trigger its
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retransmission by retransmitting our previously transmitted packet.
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For example, suppose that pxenv_tftp_read() is called to read the
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first data block of a file from a server that does not support TFTP
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options, and that no data block is received within the timeout period.
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In order to trigger the retransmission of this data block
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pxenv_tftp_read() must retransmit the TFTP open request. However, the
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information used to build the TFTP open request is not available at
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this time; it was provided only to the pxenv_tftp_open() call.
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The question of when to transmit the ACK packets is also awkward. At
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a first glance, it would seem to be fairly simple: acknowledge a
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packet immediately after receiving it. However, since the ACK packet
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may itself be lost, the next call to pxenv_tftp_read() must be
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prepared to re-acknowledge the packet.
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Another problem to consider is that the pxenv_tftp_open() API call
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must return an indication of whether or not the TFTP open request
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succeeded. In the case of a TFTP server that doesn't support TFTP
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options, the only indication of a successful open is the reception of
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the first data block. However, the pxenv_tftp_open() API provides no
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way to return this data block at this time. Pretending that we lost
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the data block and requesting retransmission is problematic, because
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the only way to request retransmission of the first data block in such
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a case is to reissue the TFTP open request, which has side effects
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such as requiring the allocation of a new local port number.
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At least some PXE stacks (e.g. NILO) solve this problem by violating
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the TFTP protocol and never bothering with retransmissions, relying on
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the TFTP server to retransmit when it times out waiting for an ACK.
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This approach is dubious at best.
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The only viable solution seems to be to allocate a buffer for the
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storage of the first data packet returned by the TFTP server, since we
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may receive this packet during the pxenv_tftp_open() call but have to
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return it from the subsequent pxenv_tftp_read() call. This buffer
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must be statically allocated and must be dedicated to providing a
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temporary home to TFTP packets. There is nothing in the PXE
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specification that prevents a caller from calling
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e.g. pxenv_undi_transmit() between calls to the TFTP API, so we cannot
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use the normal transmit/receive buffer for this purpose.
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Having paid the storage penalty for this buffer, we can then gain some
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simplicity by exploiting it in full. There is at least one
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circumstance (pxenv_tftp_open() called to open a file on a server that
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does not support TFTP options) in which we will have to enter
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pxenv_tftp_read() knowing that our previous transmission (the open
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request, in this situation) has already been acknowledged.
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Implementation of pxenv_tftp_read() can be made simpler by making this
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condition an invariant. Specifically, on each call to
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pxenv_tftp_read(), we shall ensure that the following are true:
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- Our previous transmission has already been acknowledged. We
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therefore do not need to keep state about our previous
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transmission.
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- The next packet to read is already in a buffer in memory.
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In order to maintain these two conditions, pxenv_tftp_read() must do
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the following:
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- Copy the data packet from our buffer to the caller's buffer.
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- Acknowledge the data packet that we have just copied. This will
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trigger transmission of the next packet from the server.
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- Retransmit this acknowledgement packet until the next packet
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arrives.
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- Copy the packet into our internal buffer, ready for the next call
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to pxenv_tftp_read().
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It can be verified that this preserves the invariant condition, and it
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is clear that the resulting implementation of pxenv_tftp_read() can be
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relatively simple. (For the special case of the last data packet,
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pxenv_tftp_read() should return immediately after sending a single
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acknowledgement packet.)
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In order to set up this invariant condition for the first call to
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pxenv_tftp_read(), pxenv_tftp_open() must do the following:
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-
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*/
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