Bird does not include next hop information in babel retractions, while babeld requires them

[Fabian Bläse]

Hi,

when using both bird and babeld in our babel network, babeld regularly complains about malformed packets. This happens when bird sends route retractions without including router-id and next-hop TLVs first.
According to rfc6126 the router-id is not used for retractions. The resulting error message was an issue with babeld and has been fixed already [1].

However, babeld also does not like if retractions are not preceded by next-hop TLVs, which are indeed evaluated for retracting a route.
Bird does not send next-hop TLVs for route retractions though [3] and ignores next-hop information, when processing received retractions itself [4][5].

I could not find any information which of these behaviours is correct.
But other than for the router-id, the encoding description of Update TLVs (rfc6126, section 4.4.9) does not say that the next-hop address is ignored for retractions.

Regards,
Fabian

[1] https://github.com/jech/babeld/issues/66
[3] https://gitlab.nic.cz/labs/bird/-/blob/master/proto/babel/packets.c#L720-731
[4] https://gitlab.nic.cz/labs/bird/-/blob/master/proto/babel/packets.c#L617-619
[5] https://gitlab.nic.cz/labs/bird/-/blob/master/proto/babel/babel.c#L1242-1243

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