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Added implementation note on IP routing.

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Added reason for min value of 512 on s_PXENV_TFTP_OPEN::PacketSize.
This commit is contained in:
Michael Brown 2005-05-27 11:36:36 +00:00
parent aeb984a1a9
commit 6e74dc3460
1 changed files with 33 additions and 2 deletions
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35
src/include/pxe_api.h
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@ -565,8 +565,8 @@ struct s_PXENV_TFTP_OPEN {
/** Requested size of TFTP packets
*
* This is the TFTP "blksize" option. This must be at least
* 512, according to the PXE specification, though no reason
* is offered.
* 512, since servers that do not support TFTP options cannot
* negotiate blocksizes smaller than this.
*/
UINT16_t PacketSize;
} PACKED;
@ -1633,6 +1633,37 @@ extern PXENV_EXIT_t undi_loader ( struct s_UNDI_LOADER *undi_loader );
/** @page pxe_notes Etherboot PXE implementation notes
@section pxe_routing IP routing
Several PXE API calls (e.g. pxenv_tftp_open() and pxenv_udp_write())
allow for the caller to specify a "relay agent IP address", often in a
field called "gateway" or similar. The PXE specification states that
"The IP layer should provide space for a minimum of four routing
entries obtained from the default router and static route DHCP option
tags in the DHCPACK message, plus any non-zero giaddr field from the
DHCPOFFER message(s) accepted by the client".
The DHCP static route option ("option static-routes" in dhcpd.conf)
works only for classed IP routing (i.e. it provides no way to specify
a subnet mask). Since virtually everything now uses classless IP
routing, the DHCP static route option is almost totally useless, and
is (according to the dhcp-options man page) not implemented by any of
the popular DHCP clients.
This leaves the caller-specified "relay agent IP address", the giaddr
field from the DHCPOFFER message(s) and the default gateway(s)
provided via the routers option ("option routers" in dhcpd.conf) in
the DHCPACK message. Each of these is a default gateway address.
It's a fair bet that the routers option should take priority over the
giaddr field, since the routers option has to be explicitly specified
by the DHCP server operator. Similarly, it's fair to assume that the
caller-specified "relay agent IP address", if present, should take
priority over any other routing table entries.
@bug Etherboot currently ignores all potential sources of routing
information other than the first router provided to it by a DHCP
routers option.
@section pxe_x86_modes x86 processor mode restrictions
On the x86 platform, different PXE API calls have different