Closing the IPv6 first-packet gap with GRAND

mm_montazeri about 3 hours ago 10 commentsRead Article on labs.ripe.net

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mort96•about 1 hour ago

Wait I don't get this.

If I understand correctly, traditional IPv6 flow is:

    * A host configures its own IP address via SLAAC
    * The host sends a packet to its gateway with some destination address
    * The gateway forwards the packet to the Internet
    * Eventually, a response packet arrives to the gateway
    * At this point, the gateway does neighbour discovery to try to figure out how to send the packet to the host
    * The gateway might drop the packet or delay forwarding it until neighbour discovery completes

Why couldn't we change the flow to:

    * A host configures its own IP address via SLAAC
    * The host sends a packet to its gateway with some destination address
    * The gateway forward the packet, and at the same time starts neighbour discovery because almost all computers which send outgoing packets will eventually receive some incoming packet
    * When the response packet arrives, neighbour discovery is likely already done, or if not it got a good head start

Isn't this the obvious solution which wouldn't require changes to hosts or new protocols, just a small tweak to the router? Usually, when there's a seemingly obvious simple solution to a real problem and that solution hasn't been implemented by any of the clever people working in networking standards, there's a good reason and the solution isn't as simple as it seems. So what am I missing?

gsliepen•29 minutes ago

Even weirder: when the router forwards the packet from the host to the Internet, it already sees both the source IPv6 and MAC address, so it could store them.

Maybe there are some weird situations where a host that just got its own IP address starts proxying for a third node that wants return packets to asymmetrically bypass the host?

convolvatron•40 minutes ago

Im mildly confused as well, there is an even more immediate shortcut that I've certainly implemented before. in arp its not unusual to to just create a ip->mac binding from the source information in the ethernet header. where this potentially breaks down if we start looking at issues of trust. but its already the case in ND that we trust the endpoint to have executed the state machine to search for duplicates. so what's preventing us from doing the same thing here? maybe just layering concerns?

mayhemducks•24 minutes ago

This was my immediate thought as well. Though I have not thought through any of the details, it did occur to me that the information needed would be in the "source" section of the header. I wonder if it is too much work to validate it somehow before using it?

I haven't ever done any programming at this layer of the stack, so I'm purely spitballing.

amluto•26 minutes ago

I’ve always thought that IPv6 has dramatically worse layering than IPv4. In IPv4 over Ethernet, there’s ARP, which layers over plain Ethernet, and IPv4 sits on top of the combination of ARP+Ethernet.

In the IPv6 world, neighbor discovery is IPv6, but only sort of, because the participants don’t necessarily have real addresses. So it’s a mess.

zamadatix•12 minutes ago

IPv6 link local layers over Ethernet the same way Arp does. Both contain a source/dest MAC which is used for forwarding, both contain the relevant neighbor info. If anything, keeping the protocol's self-discovery messages wrapped in the protocol itself is actually cleaner layering at the cost of complexity (the extra link local signalling addresses).

cyberax•11 minutes ago

Ah, one of the recursive WTFs in the IPv6 design.

Technically, there's no broadcast in IPv6, so the host is supposed to join the local multicast group and do the neighbor discovery flow to find the "on link" address. And it's not guaranteed that the network is "symmetric".

Technically, this is also true for IPv4. You can have a proxy-ARP host impersonating the sender, but since it had never been fully specced, nobody cares about this scenario.

JackSlateur•32 minutes ago

Wild guess: packet forward is implemented in hardware while arp/nd is software, with probably some things (think "hardware interrupt" or something alike) that allows hardware to "call" the software stack (for instance, when the link-layer addr is unknown)

So, to implement what you said, we need more than a simple router upgrade: we'd need to change the hardware, so that when a packet is forwarded from a source that's not in the mac table, the software can (asynchronously) perform an arp/nd lookup

There is probably a world of issue behind that behavior, but I do not know

basilikum•18 minutes ago

Interesting topic.

Started reading, the text is dreadfully lengthy, hard to stay concentrate on due to all the filler and mindlessly arranged words that obscure the actual content, notice AI tell signs.

Stop reading, paste into Pangram, mostly AI,

scroll down on the text, find this:

> Note: The author used AI tools to assist with drafting and language. The technical work, analysis and conclusions are the author’s own. For more information about our policy on AI-generated content, see our Contributing to RIPE Labs page.

Do better RIPE. I assumed RIPE had some standard. If you decide to not have any standards, at the very least put the AI disclaimer at the top so I don't waste my time on slop. It's disrespectful.

Does anyone have an article about NDP and GRAND that is not slop and worth sharing and reading?