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The world in which IPv6 was a good design (2017)

The world in which IPv6 was a good design

Last November I went to an IETF meeting for the first time. The IETF is an
interesting place; it seems to be about 1/3 maintenance grunt work, 1/3
extending existing stuff, and 1/3 blue sky insanity. I attended mostly
because I wanted to see how people would react to TCP BBR, which was being
presented there for the first time
. (Answer: mostly positively, but
with suspicion. It kinda seemed too good to be true.)

Anyway, the IETF meetings contain lots and lots of presentations about IPv6,
the thing that was supposed to replace IPv4, which is what the Internet runs
on. (Some would say IPv4 is already being replaced; some would say it has
already happened.) Along with those presentations about IPv6, there were
lots of people who think it’s great, the greatest thing ever, and they’re
pretty sure it will finally catch on Any Day Now, and IPv4 is just a giant
pile of hacks that really needs to die so that the Internet can be elegant

I thought this would be a great chance to really try to figure out what was
going on. Why is IPv6 such a complicated mess compared to IPv4? Wouldn’t
it be better if it had just been IPv4 with more address bits? But it’s not,
oh goodness, is it ever not. So I started asking around. Here’s what I found.

Buses ruined everything

Once upon a time, there was the telephone network, which used physical
circuit switching. Essentially, that meant moving connectors around so that
your phone connection was literally just a very long wire (“OSI layer 1″). A
“leased line” was a very long wire that you leased from the phone
company. You would put bits in one end of the wire, and they’d come out the
other end, a fixed amount of time later. You didn’t need addresses because
there was exactly one machine at each end.

Eventually the phone company optimized that a bit. Time-division
multiplexing (TDM) and “virtual circuit switching” was born. The phone
company could transparently take the bits at a slower bit rate from multiple
lines, group them together with multiplexers and demultiplexers, and let
them pass through the middle of the phone system using fewer wires than
before. Making that work was a little complicated, but as far as we
modem users were concerned, you still put bits in one end and they
came out the other end. No addresses needed.

The Internet (not called the Internet at the time) was built on top of these
circuits. You had a bunch of wires that you could put bits
into and have them come out the other side. If one computer had two or
three interfaces, then it could, if given the right instructions, forward
bits from one line to another, and you could do something a lot more
efficient than a separate line between each pair of computers. And so IP
addresses (“layer 3”), subnets, and routing were born. Even then, with
these point-to-point links, you didn’t need MAC addresses, because once a
packet went into the wire, there was only one place it could come out. You
used IP addresses to decide where it should go after that.

Meanwhile, LANs got invented as an alternative. If you wanted to connect
computers (or terminals and a mainframe) together at your local site, it was
pretty inconvenient to need multiple interfaces, one for each wire to each
satellite computer, arranged in a star configuration. To save on
electronics, people wanted to have a “bus” network (also known as a
“broadcast domain,” a name that will be important later) where multiple
stations could just be plugged into a single wire, and talk to any other
station plugged into the same wire. These were not the same people as the
ones building the Internet, so they didn’t use IP addresses for this. They
all invented their own scheme (“layer 2”).

One of the early local bus networks was arcnet, which is dear to my heart (I
wrote the first Linux arcnet driver and arcnet poetry way
back in the 1990s, long after arcnet was obsolete). Arcnet layer 2
addresses were very simplistic: just 8 bits, set by jumpers or DIP switches
on the back of the network card. As the network owner, it was your job to
configure the addresses and make sure you didn’t have any duplicates, or all
heck would ensue. This was kind of a pain, but arcnet networks were usually
pretty small, so it was only kind of a pain.

A few years later, ethernet came along and solved that problem once and for
all, by using many more bits (48, in fact) in the layer 2 address. That’s
enough bits that you can assign a different (sharded-sequential) address to
every device that has ever been manufactured, and not have any overlaps.
And that’s exactly what they did! Thus the ethernet MAC address was born.

Various LAN technologies came and went, including one of my favourites, IPX
(Internetwork Packet Exchange, though it had nothing to do with the “real”
Internet) and Netware, which worked great as long as all the clients and
servers were on a single bus network. You never had to configure any
addresses, ever. It was beautiful, and reliable, and worked. The golden
age of networking, basically.

Of course, someone had to ruin it: big company/university networks. They
wanted to have so many computers that sharing 10 Mbps of a single bus
network between them all became a huge bottleneck, so they needed a way to
have multiple buses, and then interconnect – “internetwork,” if you will –
those buses together. You’re probably thinking, of course! Use the Internet
Protocol for that, right? Ha ha, no. The Internet protocol, still not
called that, wasn’t mature or popular back then, and nobody took it
seriously. Netware-over-IPX (and the many other LAN protocols at the time)
were serious business, so as serious businesses do, they invented their own
thing(s) to extend the already-popular thing, ethernet. Devices on
ethernet already had addresses, MAC addresses, which were about the only
thing the various LAN protocol people could agree on, so they decided to use
ethernet addresses as the keys for their routing mechanisms. (Actually they
called it bridging and switching instead of routing.)

The problem with ethernet addresses is they’re assigned sequentially at the
factory, so they can’t be hierarchical. That means the “bridging table” is
not as nice as a modern IP routing table, which can talk about the route for
a whole subnet at a time. In order to do efficient bridging, you had to
remember which network bus each MAC address could be found on. And humans
didn’t want to configure each of those by hand, so it needed to figure
itself out automatically. If you had a complex internetwork of bridges,
this could get a little complicated. As I understand it, that’s what led to
the spanning tree
, and I think I’ll just leave it at that. Poetry is very important
in networking.

Anyway, it mostly worked, but it was a bit of a mess, and you got broadcast floods
every now and then, and the routes weren’t always optimal, and it was pretty
much impossible to debug. (You definitely couldn’t write something like
traceroute for bridging, because none of the tools you need to make it work
– such as the ability for an intermediate bridge to even have an address –
exist in plain ethernet.)

On the other hand, all these bridges were hardware-optimized. The whole
system was invented by hardware people, basically, as a way of fooling the
software, which had no idea about multiple buses and bridging between them,
into working better on large networks. Hardware bridging means the bridging
could go really really fast – as fast as the ethernet could go. Nowadays
that doesn’t sound very special, but at the time, it was a big deal.
Ethernet was 10 Mbps, because you could maybe saturate it by putting a bunch
of computers on the network all at once, not because any one computer could
saturate 10 Mbps. That was crazy talk.

Anyway, the point is, bridging was a mess, and impossible to debug, but it
was fast.

Internet over buses

While all that was happening, those Internet people were getting busy, and
were of course not blind to the invention of cool cheap LAN technologies. I
think it might have been around this time that the ARPANET got actually
renamed to the Internet, but I’m not sure. Let’s say it was, because the
story is better if I sound confident.

At some point, things progressed from connecting individual Internet
computers over point-to-point long distance links, to the desire to connect
whole LANs together, over point-to-point links. Basically, you wanted a
long-distance bridge.

You might be thinking, hey, no big deal, why not just build a long distance
bridge and be done with it? Sounds good, doesn’t work. I won’t go into the
details right now, but basically the problem is congestion
. The deep dark secret of ethernet bridging is that it assumes
all your links are about the same speed, and/or completely uncongested,
because they have no way to slow down. You just blast data as fast as you
can, and expect it to arrive. But when your ethernet is 10 Mbps and your
point-to-point link is 0.128 Mbps, that’s completely hopeless. Separately,
the idea of figuring out your routes by flooding all the links to see which
one is right – this is the actual way bridging typically works – is hugely
wasteful for slow links. And sub-optimal routing, an annoyance on local
networks with low latency and high throughput, is nasty on slow, expensive
long-distance links. It just doesn’t scale.

Luckily, those Internet people (if it was called the Internet yet) had been
working on that exact set of problems. If we could just use Internet stuff
to connect ethernet buses together, we’d be in great shape.

And so they designed a “frame format” for Internet packets over ethernet
(and arcnet, for that matter, and every other kind of LAN).

And that’s when everything started to go wrong.

The first problem that needed solving was that now, when you put an Internet
packet onto a wire, it was no longer clear which machine was supposed to
“hear” it and maybe forward it along. If multiple Internet routers were on
the same ethernet segment, you couldn’t have them all picking it up and
trying to forward it; that way lies packet storms and routing loops. No,
you had to choose which router on the ethernet bus is supposed to
pick it up. We can’t just use the IP destination field for that, because
we’re already using that for the final destination, not the router
destination. Instead, we identify the desired router using its MAC address
in the ethernet frame.

So basically, to set up your local IP routing table, you want to be able to
say something like, “send packets to IP address via the router at
MAC address 11:22:33:44:55:66.” That’s the actual thing you want to
express. This is important! Your destination is an IP address, but your
router is a MAC address. But if you’ve ever configured a routing table, you
might have noticed that nobody writes it like that. Instead, because the
writers of your operating system’s TCP/IP stack are stubborn, you write
something like “send packets to IP address via the router at IP

In truth, that really is just complicating things. Now your operating
system has to first look up the ethernet address of, find out
it’s 11:22:33:44:55:66, and finally generate a packet with destination
ethernet address 11:22:33:44:55:66 and destination IP address shows up nowhere in the packet; it’s just an abstraction at the
human level.

To do that pointless intermediate step, you need to add ARP (address
resolution protocol), a simple non-IP protocol whose job it is to convert IP
addresses to ethernet addresses. It does this by broadcasting to everyone
on the local ethernet bus, asking them all to answer if they own that
particular IP address. If you have bridges, they all have to forward all
the ARP packets to all their interfaces, because they’re ethernet broadcast
packets, and that’s what broadcasting means. On a big, busy ethernet with
lots of interconnected LANs, excessive broadcasts start becoming one of your
biggest nightmares. It’s especially bad on wifi. As time went on, people
started making bridges/switches with special hacks to avoid forwarding ARP
as far as it’s technically supposed to go, to try to cut down on this
problem. Some devices (especially wifi access points) just make fake ARP
answers to try to help. But doing any of that is a hack, albeit sometimes a
necessary hack.

Death by legacy

Time passed. Eventually (and this actually took quite a while), people
pretty much stopped using non-IP protocols on ethernet at all. So basically
all networks became a physical wire (layer 1), with multiple stations on a
bus (layer 2), with multiple buses connected over bridges (gotcha! still
layer 2!), and those inter-buses connected over IP routers (layer 3).

After a while, people got tired of manually configuring IP addresses, arcnet
style, and wanted them to auto-configure, ethernet style, except it was too
late to literally do it ethernet style, because a) the devices had already
been manufactured with ethernet addresses, not IP addresses, and b) IP
addresses were only 32 bits, which is not enough to just manufacture them
forever with no overlaps, and c) just assigning IP addresses sequentially
instead of using subnets would bring us back to square one: it would just be
ethernet over again, and we already have ethernet.

So that’s where bootp
and DHCP came from. Those protocols, by the way, are special kinda like ARP
is special (except they pretend not to be special, by technically being IP
packets). They have to be special, because an IP node has to be able to
transmit them before it has an IP address, which is of course impossible, so
it just fills the IP headers with essentially nonsense (albeit nonsense
specified by an RFC), so the headers might as well have been left out. (You
know these “IP” headers are nonsense because the DHCP server has to open a raw
socket and fill them in by hand; the kernel IP layer can’t do it.) But
nobody would feel nice if they were inventing a whole new protocol that
wasn’t IP, so they pretended it was IP, and then they felt nice. Well, as
nice as one can feel when one is inventing DHCP.

Anyway, I digress.
The salient detail here is that unlike real IP services, bootp and DHCP need to
know about ethernet addresses, because after all, it’s their job to hear
your ethernet address and assign you an IP address to go with it. They’re
basically the reverse of ARP, except we can’t say that, because there’s a
protocol called RARP that is literally the reverse of ARP. Actually, RARP
worked quite fine and did the same thing as bootp and DHCP while being
much simpler, but we don’t talk about that.

The point of all this is that ethernet and IP were getting further and
further intertwined. They’re nowadays almost inseparable. It’s hard to
imagine a network interface (except ppp0) without a 48-bit MAC address, and it’s hard to imagine
that network interface working without an IP address. You write your IP
routing table using IP addresses, but of course you know you’re lying when
you name the router by IP address; you’re just indirectly saying that you
want to route via a MAC address. And you have ARP, which gets bridged but
not really, and DHCP, which is an IP packet but is really an ethernet
protocol, and so on.

Moreover, we still have both bridging and routing, and they both get more
and more complicated as the LANs and the Internet get more and more
complicated, respectively. Bridging is still, mostly, hardware based and
defined by IEEE, the people who control the ethernet standards. Routing is
still, mostly, software based and defined by the IETF, the people who
control the Internet standards. Both groups still try to pretend the other
group doesn’t exist. Network operators basically choose bridging vs routing
based on how fast they want it to go and how much they hate configuring DHCP
servers, which they really hate very much, which means they use bridging as
much as possible and routing when they have to.

In fact, bridging has gotten so completely out of control that people
decided to extract the layer 2 bridging decisions out completely to a higher
level (with configuration exchanged between bridges using a protocol layered
over IP, of course!) so it can be centrally managed. That’s called
software-defined networking (SDN). It helps a lot, compared to letting your
switches and bridges just do whatever they want, but it’s also fundamentally
silly, because you know what’s software defined networking? IP. It is
literally and has always been the software-defined network you use for
interconnecting networks that have gotten too big. But the problem is, IPv4
was initially too hard to hardware accelerate, and anyway, it didn’t get
hardware accelerated, and configuring DHCP really is a huge pain, so network
operators just learned how to bridge bigger and bigger things. And nowadays
big data centers are basically just SDNed, and you might as well not be
using IP in the data center at all, because nobody’s routing the packets.
It’s all just one big virtual bus network.

It is, in short, a mess.

Now forget I said all that…

Great story, right? Right. Now pretend none of that happened, and we’re
back in the early 1990s, when most of that had in fact already happened,
but people at the IETF were anyway pretending that it hadn’t happened and
that the “upcoming” disaster could all be avoided. This is the good part!

There’s one thing I forgot to mention in that big long story above:
somewhere in that whole chain of events, we completely stopped using bus
. Ethernet is not actually a bus anymore. It just
pretends to be a bus. Basically, we couldn’t get ethernet’s famous
to keep working as speeds increased, so we went back to the good old star
topology. We run bundles of cables from the switch, so that we can run one
cable from each station all the way back to the center point. Walls and
ceilings and floors are filled with big, thick, expensive bundles of
ethernet, because we couldn’t figure out how to make buses work well… at
layer 1. It’s kinda funny actually when you think about it. If you find
sad things funny.

In fact, in a bonus fit of insanity, even wifi – the ultimate bus network,
right, where literally everybody is sharing the same open-air “bus” – we
almost universally use wifi in a mode, called “infrastructure mode,” which
simulates a giant star topology. If you have two wifi stations
connected to the same access point, they don’t talk to each other directly,
even when they can hear each other just fine.
They send a packet to the access point, but addressed to the MAC address of
the other node. The access point then bounces it back out to the
destination node.

there. When node X wants to send to Internet node Z, via IP router Y, via
wifi access point A, what does the packet look like? Just to draw a
picture, here’s what we want to happen:

X -> [wifi] -> A -> [wifi] -> Y -> [internet] -> Z

Z is the IP destination, so obviously the IP destination field has to be Z.
Y is the router, which we learned above that we specify by using its
ethernet MAC address in the ethernet destination field. But in wifi, X
can’t just send out a packet to Y, for various reasons (including that they
don’t know each other’s WPA2 encryption keys). We have to send to A. Where do
we put A’s address, you might ask?

No problem! 802.11 has a thing called 3-address mode. They add a third
ethernet MAC address to every frame, so they can talk about the real
ethernet destination, and the intermediate ethernet destination. On top of
that, there are bit fields called “to-AP” and “from-AP,” which tell you if
the packet is going from a station to an AP, or from an AP to a station,
respectively. But actually they can both be true at the same time, because
that’s how you make wifi repeaters (APs send packets to APs).

Speaking of wifi repeaters! If A is a repeater, it has to send back to the
base station, B, along the way, which looks like this:

X -> [wifi] -> A -> [wifi-repeater] -> B -> [wifi] -> Y -> [internet] -> Z

X->A uses three-address mode, but A->B has a problem: the ethernet source
address is X, and the ethernet destination address is Y, but the packet on
the air is actually being sent from A to B; X and Y aren’t involved at all.
Suffice it to say that there’s a thing called 4-address mode, and it works
pretty much like you think.

(In 802.11s mesh networks, there’s a 6-address mode, and that’s about where
I gave up trying to understand.)

Avery, I was promised IPv6, and you haven’t even mentioned IPv6

Oh, oops. This post went a bit off the rails, didn’t it?

Here’s the point of the whole thing. The IETF people, when they were
thinking about IPv6, saw this mess getting made – and maybe predicted some
of the additional mess that would happen, though I doubt they could have
predicted SDN and wifi repeater modes – and they said, hey wait a minute,
stop right there. We don’t need any of this crap! What if instead the
world worked like this?

  • No more physical bus networks (already done!)
  • No more layer 2 internetworks (that’s what layer 3 is for)
  • No more broadcasts (layer 2 is always point-to-point, so where would you
    send the broadcast to? replace it with multicast instead)
  • No more MAC addresses (on a point-to-point network, it’s obvious who the
    sender and receiver are, and you can do multicast using IP addresses)
  • No more ARP and DHCP (no MAC addresses, no so mapping IP addresses to MAC
  • No more complexity in IP headers (so you can hardware accelerate IP
  • No more IP address shortages (so we can go back to routing big subnets
  • No more manual IP address configuration except at the core (and there are
    so many IP addresses that we can recursively hand out subnets down the
    tree from there)

Imagine that we lived in such a world: wifi repeaters would just be IPv6
routers. So would wifi access points. So would ethernet switches. So
would SDN. ARP storms would be gone. “IGMP snooping bridges” would be
gone. Bridging loops would be gone. Every routing problem would be
traceroute-able. And best of all, we could drop 12 bytes (source/dest ethernet
addresses) from every ethernet packet, and 18 bytes (source/dest/AP
addresses) from every wifi packet. Sure, IPv6 adds an extra 24 bytes of
address (vs IPv4), but you’re dropping 12 bytes of ethernet, so the added
overhead is only 12 bytes – pretty comparable to using two 64-bit IP addresses
but having to keep the ethernet header. The idea that we could someday drop
ethernet addresses helped to justify the oversized IPv6 addresses.

It would have been beautiful. Except for one problem: it never happened.

Requiem for a dream

One person at work put it best: “layers are only ever added, never removed.”

All this wonderfulness depended on the ability to start over and throw away
the legacy cruft we had built up. And that is, unfortunately, pretty much
impossible. Even if IPv6 hits 99% penetration, that doesn’t mean we’ll be
rid of IPv4. And if we’re not rid of IPv4, we won’t be rid of ethernet
addresses, or wifi addresses. And if we have to keep the IEEE 802.3 and
802.11 framing standards, we’re never going to save those bytes. So we will
always need the “IPv6 neighbour discovery” protocol, which is just a
more complicated ARP. Even though we no longer have bus networks, we’ll
always need some kind of simulator for broadcasts, because that’s how ARP
works. We’ll need to keep running a local DHCP server at home so that our
obsolete IPv4 light bulbs keep working. We’ll keep needing NAT so that our
obsolete IPv4 light bulbs can keep reaching the Internet.

And that’s not the worst of it. The worst of it is we still need the
infinite abomination that is layer 2 bridging, because of one more mistake
the IPv6 team forgot to fix. Unfortunately, while they were
blue-skying IPv6 back in the 1990s, they neglected to solve the “mobile IP”
problem. As I understand it, the idea was to get IPv6 deployed first – it
should only take a few years – and then work on it after IPv4 and MAC
addresses had been eliminated, at which time it should be much easier to
solve, and meanwhile, nobody really has a “mobile IP” device yet anyway. I
mean, what would that even mean, like carrying your laptop around and
plugging into a series of one ethernet port after another while you ftp a
file? Sounds dumb.

The killer app: mobile IP

Of course, with a couple more decades of history behind us, now we know a
few use cases for carrying around a computer – your phone – and letting it
plug into one ethernet port wireless access point
after another. We do it all the time. And with LTE, it even mostly works!
With wifi, it works sometimes. Good, right?

Not really, because of the Internet’s secret shame: all that stuff only
works because of layer 2 bridging. Internet routing can’t handle mobility –
at all. If you move around on an IP network, your IP address changes, and
that breaks any connections you have open.

Corporate wifi networks fake it for you, bridging their whole LAN together
at layer 2, so that the giant central DHCP server always hands you the same
IP address no matter which corporate wifi access point you join, and then
gets your packets to you, with at most a few seconds of confusion while the
bridge reconfigures. Those newfangled home wifi systems with multiple
extenders/repeaters do the same trick. But if you switch from one wifi network to
another as you walk down the street – like if there’s a “Public Wifi”
service in a series of stores – well, too bad. Each of those gives you a
new IP address, and each time your IP address changes, you kill all your

LTE tries even harder. You keep your IP address (usually an IPv6 address in
the case of mobile networks), even if you travel miles and miles and hop
between numerous cell towers. How? Well… they typically just tunnel all
your traffic back to a central location, where it all gets bridged together
(albeit with lots of firewalling) into one super-gigantic virtual layer 2
LAN. And your connections keep going. At the expense of a ton of
complexity, and a truly embarrassing amount of extra latency, which they
would really like to fix, but it’s almost impossible.

Making mobile IP actually work1

So okay, this has been a long story, but I managed to extract it from those
IETF people eventually. When we got to this point – the problem of mobile
IP – I couldn’t help but ask. What went wrong? Why can’t we make it work?

The answer, it turns out, is surprisingly simple. The great design flaw was
in how the famous “4-tuple” (source ip, source port, destination ip,
destination port) was defined. We use the 4-tuple to identify a given TCP
or UDP session; if a packet has those four fields the same, then it belongs
to a given session, and we can deliver it to whatever socket is handling that
session. But the 4-tuple crosses two layers: internetwork (layer 3) and
transport (layer 4). If, instead, we had identified sessions using
only layer 4 data, then mobile IP would have worked perfectly.

Let’s do a quick example. X port 1111 is talking to Y port 80, so it sends
a packet with 4-tuple (X,1111,Y,80). The response comes back with
(Y,80,X,1111), and the kernel delivers it to the socket that generated the
original packet. When X sends more packets tagged (X,1111,Y,80), then Y
delivers them all to the same server socket, and so on.

Then, if X hops IP addresses, it gets a new name, say Q. Now it’ll start
sending packets with (Q,1111,Y,80). Y has no idea what that means, and
throws it away. Meanwhile, if Y sends packets tagged (Y,80,X,1111), they
get lost, because there is no longer an X to receive them.

Imagine now that we tagged sockets without reference to their IP address.
For that to work, we’d need much bigger port numbers (which are currently 16
bits). Let’s make them, say, 128 or 256 bits, some kind of unique hash.

Now X sends out packets to Y with tag (uuid,80). Note, the packets
themselves still contain the (X,Y) addressing information, down at layer 3
– that’s how they get routed to the right machine in the first place.
But the kernel doesn’t use the layer 3 information to decide which
socket to deliver to; it just uses the uuid. The destination port (80 in
this case) is only needed to initiate a new session, to identify what
service you want to connect to, and can be ignored or left out after that.

For the return direction, Y’s kernel caches the fact that packets for (uuid)
go to IP address X, which is the address it most recently received (uuid)
packets from.

Now imagine that X changes addresses to Q. It still sends out packets
tagged with (uuid,80), to IP address Y, but now those packets come
from address Q. On machine
Y, it receives the packet and matches it to the socket associated with
(uuid), notes that the packets for that socket are now coming from address
Q, and updates its cache. Its return packets can now be sent, tagged as
(uuid), back to Q instead of X. Everything works! (Modulo some care
to prevent connection hijacking by impostors.2)

There’s only one catch: that’s not how UDP and TCP work, and it’s too late
to update them. Updating UDP and TCP would be like updating IPv4 to IPv6; a
project that sounded simple, back in the 1990s, but decades later, is less
than half accomplished (and the first half was the easy part; the long tail
is much harder).

The positive news is we may be able to hack around it with yet another
layering violation. If we throw away TCP – it’s getting rather old anyway –
and instead use QUIC over UDP, then we can just stop using the UDP 4-tuple
as a connection identifier at all. Instead, if the UDP port number is
the “special mobility layer” port, we unwrap the content, which can be
another packet with a proper uuid tag, match it to the right session, and
deliver those packets to the right socket.

There’s even more good news: the experimental QUIC protocol already, at
least in theory, has the right packet structure to work like this. It turns
out you need unique session identifiers (keys) anyhow if you want to use stateless
packet encryption and authentication, which QUIC does. So, perhaps with not
much work, QUIC could support transparent roaming. What a world that would

At that point, all we’d have to do is eliminate all remaining UDP and TCP
from the Internet, and then we would definitely not need layer 2
bridging anymore, for real this time, and then we could get rid of
broadcasts and MAC addresses and SDN and DHCP and all that stuff.

And then the Internet would be elegant again.

1 Edit 2017-08-16: It turns out that nothing in this
section requires IPv6. It would work fine with IPv4 and NAT, even roaming
across multiple NATs.

2 Edit 2017-08-15: Some people asked what “some care to
prevent connection hijacking” might look like. There are various ways to do
it, but the simplest would be to do something like the SYN-ACK-SYNACK
exchange TCP does at connection startup. If Y just trusts the first packet
from the new host Q, then it’s too easy for any attacker to take over the
X->Y connection by simply sending a packet to Y from anywhere on the
Internet. (Although it’s a bit hard to guess which 256-bit uuid to fill
in.) But if Y sends back a cookie that Q must receive and process and send
back to Y, that ensures that Q is at least a man-in-the-middle and not just
an outside attacker (which is all TCP would guarantee anyway). If you’re
using an encrypted protocol (like QUIC3), the handshake can also be protected
by your session key.

3 Edit 2017-10-24: Besides QUIC, there are several other
candidates for such a protocol, including MinimaLT. I didn’t mention
MinimaLT originally because it wasn’t part of my original conversation with
the IETF people, but I don’t mean to imply that QUIC is the only possible
option as a roaming-capable TCP replacement. In fact, MinimaLT is the first
protocol I heard of that elegantly solved the roaming problem. Future
solutions that might get adopted, including by QUIC, will likely be modeled
after MinimaLT’s solution.

Update 2020-07-09: I’ve posted more thoughts on IPv4/IPv6 migration
and interoperability
on the Tailscale blog.

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