Re: [Int-area] Proxy function for PTB messages on the tunnel end

[Joseph Touch <[e-mail]>]

Two points:

> On Mar 24, 2021, at 7:59 AM, Vasilenko Eduard <[e-mail]> wrote:
> 
> It would invalidate all tunneling implementations. It is not compatible with any one of them. PMTUD is killed. Revolution.

PMTUD is effectively dead, so if you’re worried about it, you’re 20+ years too late - as per the RFCs I’ve already cited.

> All complaints against RFC 2473 are minor (if right),
> Except this one that is definitely wrong:
>        o Tunnel ingress issues ICMPs
> This is a violation of RFC792 and 8200; the ICMPs issued are that of routers, not links. If the ingress is at the source host, these ICMPs would come from a device that is not a router.
> ICMP PTB is very important to deliver to the traffic source.

I’m saying something very specific:
	- tunnels are links
	- links NEVER *genenerate* ICMPs
	- routers and hosts *generate* ICMPs
		based on what happens inside them, e.g,, to their processes and links

So the question is “under what conditions does a link cause a router/host to generate an ICMP?”

There should be no unsolicited ICMPs, i.e., routers/hosts NEVER generate ICMPs unless in reaction to a packet being sent or received.

PTB means “I cannot send the packet over this link”. Not path - link. There is no PTB for a path; the assumption is that one link of a path that fails will send the ICMPs back to the source.

For a tunnel, when can it NOT send a packet?
	- only when that packet is larger than the tunnel EMTU_R (i.e., egress received max, reassembled if reassembly is supported)

A packet that can be fragmented and traverse a tunnel is not too big. It’s “bigger than you might like” or “bigger than desired”, but there is no ICMP to indicate that sort of ‘soft’ (non failure) error.

So what should happen:
	- tunnels ingress should know and update (if changing) the tunnel EMTU_R value
	- routers/hosts should use EMTU_R as the tunnel MTU
		again, because the tunnel path MTU is a preference; the tunnel EMTU_R is the actual strict limit
	- routers/hosts sending packets over a tunnel generate ICMP PTBs as needed
		again, the router/host generates the message, not the tunnel ingress
		this happens when the router/host tries to send a packet over out that tunnel interface that is larger than the tunnel MTU

So this all works, as long as ICMPs are relayed.

Draft-tunnels does not deprecate this behavior. It describes it and explains why this is the correct behavior.

Tunnel ingresses that relay PTBs inside are broken; they fail in ways they do not need to. That is the true error.

Joe