[lwip-users] RFC: destination unreachable propagation to user

[leon . woestenberg]

Hello

I have been thinking about propagating incoming ICMP "destination unreachable"
messages to the user. A cheap solution is given here:

---
My proposal is to add a new flag that can be set in both the UDP pcb->flags and TCP pcb->flags
field.

Whenever we receive a destination unreachable in ICMP, it searches for a corresponding
connection in the TCP and UDP pcb list and sets the flag.

Whenever an application sends to the destination again, using its pcb, it tries to send out that
packet as usual, but returns a ERR_DST_UNR to the user indicating this connection has
received (asynchronous) ICMP destination unreachable errors.

The application can then handle this case if desired.

The flags have to be expired in a timely fashion. This can be done within udp_send() just after
remembering the current state of the flag, and in the tcp timer function.
---

The disadvantage is that the ICMP function becomes reasonably "heavy". It parses the list
of PCBs for each incoming destination unreachable. Internet host requirements indicate that
these ICMP messages should not be sent out often, but there may be mis-behaved hosts
to would hog our stack.

In this sense, it seems we are misbehaving as well, see next RFC topic.

Regards,

Leon Woestenberg