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

[Vasilenko Eduard <[e-mail]>]

Hi Joseph,
You have presented below (and in many other messages) a long list of policies (extensive usage of “SHOULD”, “NEVER”, “MUST”)
That are new – would change how current tunnels operate
And are not justified by any reasoning.
It is religion, not technology.

Why virtual link could not send ICMP PTB (like on a physical link)? Just because… it is “unsolicited”. But one moment – any other PTB is unsolicited too - It is an event.

You have not answered any of my questions – you continue to promote the solution from the draft-ietf-intarea-tunnels putting some excerpts in a different order.

PS: I am especially sorry that draft-ietf-intarea-tunnels would scrape the best tunneling RFC that we have for IPv6. RFC 2473 was really good.
Eduard
From: Joseph Touch [mailto:[e-mail]]
Sent: Wednesday, March 24, 2021 10:01 PM
To: Vasilenko Eduard <[e-mail]>
Cc: [e-mail]; int-area <[e-mail]>
Subject: Re: Proxy function for PTB messages on the tunnel end

Two points:


On Mar 24, 2021, at 7:59 AM, Vasilenko Eduard <[e-mail]<mailto:[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