And I am totally against this. I agree for human / interactive input. But for automated things, be as strict as you can be, and hard fail on anything else. Specs are there for a reason, and not adhering them creates compatibility issues.
> RFC 1122 (1989) expanded on Postel's principle by recommending that programmers "assume that the network is filled with malevolent entities that will send in packets designed to have the worst possible effect
Well, that's gonna be a tough one if you try to make sense of garbage data (or data which doesn't adhere to the specs)
> Protocols should allow for the addition of new codes for existing fields in future versions of protocols by accepting messages with unknown codes (possibly logging them).
I agree with that. That does not mean "be liberal in what you accept". It means the protocol is designed in a better way.
> Basically be liberal in what you accept and conservative in what you send
That's not the same as "be conservative in what you do, be liberal in what you accept from others". "Doing" is a lot more than "sending".
You know.. many programmers sometimes say that when things go wrong it's "a user error", or "a human error". But most of the time it is either a programming error, or a lazy programmer. It's so easy for a programmer to send the correct data. But we live in a world where usually the least correct/secure software is chosen, because it's slightly easier to get started, or allows for input errors (mysql data truncation, mongodb, etc, etc)
Totally agreed on this. When a human inputs a name with a trailing space, trim it for them. Or when they input a phone number with funny dashes, remove them.
But when an API does this, it means the API client is simply incorrect. Incorrect requests should be rejected. Don't leave it up to the server to try to interpret it, this just leads to incorrect behavior.
I experienced this first hand recently with a lower MTU limit and it's pretty confusing. The annoying thing is that when it's only wrong at one end, most of the internet and web pages will actually carry on working, i.e it's only some HTTP requests with large enough request header that push past the MTU fragmentation threshold and disappear. Which results in websites half loading, or sometimes loading fine, but not other times.
I have to use LTE to get decent internet in the city I live, and have been experimenting with different providers. Turns out most LTE networks use quite low MTUs, and so PMTUD (MTU discovery) _must_ work correctly otherwise you get the behaviour I described above... For the network with the best reception and backhaul, PMTUD just didn't seem to work, so it defaults to 1500, and all the wifi connections to the LTE router will be told to use 1500, so my wiregaurd sets itself to 1420 (less 80 bytes) and even that wont save itself when the UDP packets vanish.
Now when the internet seems fishy my first response after a simple ping is to try ping at the network interface supposed MTU, e.g `ping -M do -s 1472 1.1.1.1` (-28 bytes for the header).
Even more confusing is that for these LTE networks the effective MTU seems to change, possible as different combinations of cells are used for carrier aggregation... so if you are trying to use this for home internet you need to keep testing for the MTU over time to discover the lowest, then change your router wifi MTU (not even possible for most LTE routers) to the minimum for reliable internet.
The reason why you can browse the web at all with PMTU <1500 is due to the awful hack that is TCP MSS clamping, where the router will rewrite your TCP SYN packets to advertise a different MSS (which is roughly the same as MTU, only on layer 3) based on what it thinks the PMTU to the host is.
Seriously, try browsing the Internet with MTU 1500, PMTU <1500 and a router in place that does _not_ do TCP MSS clamping. It's an incredibly frustrating experience.
I don't think PMTUD ever really worked over the whole internet. Path MTU blackhole discovery can work, but it defaults to off on a lot of OSes (including Android!), and it's behavior isn't always great anyway. iOS actually does do a good job here (which doesn't change my overall opinion). I don't think Windows will pay attention to the MTU from DHCP, but I think it will try path mtu blackhole detection (but it may do it with rfc timers, which are way too slow)
Also, most TCP stacks send out their MSS on SYN+ACK instead of sending max(received MSS, local MSS), which means that those networks that tried to fix their MTU problem by tweaking outgoing SYNs but don't tweak incoming SYN+ACKs don't get fixed. FreeBSD used to do the right thing, but it changed recently. People will argue that the RFC says to do it the way I'm calling the wrong way, but how many connections do you run into that can send a 1500 byte packet, but only receive a 1492 packet?
Most/many large internet sites just send MSS indicating 1480 or less and call it a day.
IMHO, something in-band would have been a lot more workable; let the router that can't forward the whole packet mark it, truncate, and forward it, because we can assume the router has a working forward path (otherwise there's no connection), but we can't assume the router has a working reverse path. For TCP, this would be relatively straight forward to manage; UDP wouldn't be great though.
Interesting, I didn't know this existed, it's built into linux too but unfortunately it's only for TCP `man tcp`:
tcp_mtu_probing (integer; default: 0; since Linux 2.6.17)
This parameter controls TCP Packetization-Layer Path MTU Discovery.
The following values may be assigned to the file:
0 Disabled
1 Disabled by default, enabled when an ICMP black hole detected
2 Always enabled, use initial MSS of tcp_base_mss.
Historically, ICMP blackholes tended to be on long distance international transit, though. So maybe your ISP was 1500 MTU throughout and most of its transit and peering was too (or sent ICMPs anyway), but routes to Mordor were few and far between, so your ISPs transit provider used one that was IPIP tunneled (so effective MTU was 1480) and either didn't send ICMPs or was rate limited on sending ICMPs so it was as if they didn't send them. Everything would work OK to most of the internet, but if you made a connection to Mordor (or someone from Mordor made a connection to you), small packets make it through and large packets get dropped, and you get that kind of works but doesn't really work behavior.
In that situation, if you're in Mordor and most of the things you connect to aren't, it makes sense to set a smaller MTU. If you're not in Mordor, but sometimes connect to it, you want probing to happen for those connections, but setting a smaller MTU probably isn't the right thing (but, popular sites do it). If you're in Mordor and mostly connect to Mordor services, but sometimes the outside world, you again want probing sometimes.
But keep in mind, the probing really only works if both sides are doing it. It's hard to induce your TCP peer to send smaller packets if you sent a too big MSS.
All that said, my experience is that in recent times, it seems like problems are usually some ISP PPPoE CPE that isn't clamping MSS. International transit has gotten pretty good over the past 30 years.
MTU discovery is "robust". Most nodes already forward any packet they can handle. The problem is that _some_ nodes filter out ICMP packets (hence breaking MTU discovery) but the link still "kind-of-works" (tm). Nodes that break MTU discovery should be considered 100% broken, but instead of fixing them we "accommodate" them by statically (manually) lowering our outgoing MTU, hence proliferating the problem.
Robustness works against us. If half-broken firewalls were considered fully-broken, they would be replaced instead of accommodated.
Aside: the `Fragmentation Needed` packet should have been part of the IP header, not a separate protocol.
Although the low default was probably established because of the aforementioned ICMP filtering issue.
I'm not sure if the LWN author just doesn't understand the distinction, or if they were trying to simplify things for their readers..
> Modern network interfaces perform segmentation offloading, meaning that much of the work of creating individual packets is done within the interface itself. Making segmentation offloading work with jumbo packets tends to involve a small number of tweaks; a few drivers are updated in the patch set.
The Internet will work with large L3/L4 packets assuming the path is able to create fragments. The article talked about the kernel's ability to hold fragments but it wasn't clear to me how Internet fragmentation comes into play or whether the performance gain is lost when using this on the Internet versus a LAN.
Side tangent, this is one issue I have with the major cloud providers. Azure, AWS, et al all advertise high-bandwidth interfaces upwards of 100Gbps, but you can't saturate it anywhere near those rates with 1500 Byte packets from the Internet.
I honestly don’t think an implementation of TCP that would try and follow this principle would bring anything but misery. I lived on the internet since IE5… imagine including “hacks for Cisco” in your packets
The problem with TCP is that the actual Layer 2 and 3 infrastructure doesn't obey this principle. It's taken the opposite stance to simply reject anything that's weird or unexpected. There's a reason and history for why this is. The most defensible is security. Less defensible is things like deep packet inspection.
This is a well-knwon problem and falls under the term (which I love) "ossification" [2].
As an example, much of the Internet stops working with MTUs above ~1500 (which this article mentions). Large packets (eg MTU ~9000) are almost a necessity for 10+ GbE but that often won't work outside of the LAN.
So I guess my point is changes like this will take a long time (if ever) to get widespread support. This could still impact, say, data center and other "local" deployments but I guess you have to start somewhere.
[1]: https://en.wikipedia.org/wiki/Robustness_principle
[2]: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7738442