router: enforce configured MTU

[matzf]

The border router should respect and enforce the configured inter and intra-AS MTU.

Assuming the MTU is configured correctly, IP fragmentation should not happen. So perhaps, the router should also set IP_DONTFRAG to ensure that packets are not fragmented.

Note: this used to be blocked by exceedingly large control plane messages, but this is no longer an issue as this now all uses QUIC.

[matzf]

Compare also

scion/go/border/rpkt/rpkt.go

Lines 43 to 44 in 1965468

	// FIXME(kormat): this should be reduced as soon as we respect the actual link MTU.
	const pktBufSize = 9 * 1024

[matzf]

Related: #1328

[matzf]

Discussion in the proposal review meeting; consensus to accept this . However, we need to be careful as doing so may unearth issues with MTUs, e.g. on long SCION paths, the effective MTU may fall below the minimum MTU required for QUIC.

This proposal will be accepted unless objections are raised in the 10 day final comment period starting now.