On 20.06.2018 09:49, R. Diez wrote:
My TCP/IP and lwIP expertise are limited. Please help me understand
the situation by correcting my assumptions:
- The Ethernet driver has a fixed, limited number of DMA slots, so it
already limits the amount of data at that level.
- An Ethernet frame is normally limited to 1500 bytes (or a little
more). However, an UDP packet can be up to roughly 64 KiB long.
- In the case of large UDP (or even TCP) packets, lwIP is merging
several Ethernet frames (IP fragmentation) into a large buffer (or a
pbuffer chain). Therefore, it is "allocating" a bigger logical chunk.
In the process of doing so, it could check whether some maximum
reassembly packet size has been reached, and then discard the whole
packet. Depending on the allocation strategy, that would significantly
drop the temporary memory requirements.
- If I turn off IP_REASSEMBLY, would it help here? I guess the maximum
size of UDP (or even TCP) packets would then be limited by the largest
possible Ethernet frame. Would turning off IP_REASSEMBLY break other
things, or severely impact performance, at least on IPv4?
Unless you expect to get fragmented packets, you should be able to
safely turn off reassembly. However, you *may* get fragmented packets
when connected to some larger network (like the internet) where there
maybe routes that require fragmentation because they have MTUs smaller
than 1500 bytes. This is the case e.g. for PPPoE or some IP4-in-IPv6
tunnels and others.
- Is there some constant to limit the size in bytes of reassembled
packets? I found IP_REASS_MAX_PBUFS, but that is not a byte limit. If
I set it too low, it may break, because the sender may choose to send
many small packets. If I set it too high, the sender could send few
but large frames and trigger big memory allocations.
No, such a config option does not exist. However, you never trigger 'big
memory allocations'. All you have is a linked list of all the small
packets. The stack does not copy them into one big memory block. And
from a DoS perspecive, those many small allocations are not worse than
getting many partly received fragments of different IP packets, or am I
wrong there?
Simon
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