Turtles on the Wire: Understanding How the OS Uses the Modern NIC (2016)

http://dtrace.org/blogs/rm/2016/09/15/turtles-on-the-wire-understanding-how-the-os-uses-the-modern-nic/

by nz • 9 years ago

300 72 9 years ago

tedunangst• 9 years ago

Buggy firmware with edge cases is putting it mildly. I suppose a checksum of 0000 or ffff is technically an edge case, but not all that uncommon, and a pretty popular thing to get wrong.

gonzo• 9 years ago

Even the standards got it wrong. https://tools.ietf.org/html/rfc1624

pthreads• 9 years ago

This is a very useful write-up. I thoroughly enjoyed it i.e. found it informative. Thank you.

bluetech• 9 years ago

Interesting article.

Another related article I found interesting: https://www.coverfire.com/articles/queueing-in-the-linux-net... Discusses some of the queues in the Linux network stack.

drewg1232• 9 years ago

The biggest innovation that I've seen in quite some time in terms of making creative use of hardware offloads is Hans Petter Selasky's RSS assisted LRO in FreeBSD.

On our workloads (~100K connections, 16 core / 32 HTT FreeBSD based 100GbE CDN server) LRO was rather ineffective because there were roughly 3K connections / rx queue. Even with large interrupt coalescing parameters and large ring sizes, the odds of encountering 2 packets from the same connection within a few packets of each other, even in a group of 1000 or more, are rather small.

The first idea we had was to use a hash table to aggregate flows. This helped, but had the draw back of a much higher cache footprint.

Hps had the idea that we could sort packets by RSS hash ID before passing them to LRO. This would put packets from the same connection adjacent to each other, thereby allowing the LRO without a hash table to work. Our LRO aggregation rate went from ~1.1:1 to well over 2:1, and we reduce CPU use by roughly 10%.

This code is in FreeBSD-current right now (see tcp_lro_queue_mbuf())

pebblexe• 9 years ago

FreeBSD also has netmap https://www.freebsd.org/cgi/man.cgi?query=netmap&sektion=4

ams61102• 9 years ago

As an aside, is anyone using the Joyent cloud stuff in production? Any good comparisons to Openstack? Looking for something easier to manage.

policedemil3• 9 years ago

Great article! A lot of this is way beyond me, but I'm generally interested in the process of how a NIC filters based on MAC addresses.

I'm in the humanities and certain scholars working with culture and technology love to make a huge deal about data leakage and how intertwined we all are precisely because you can put a NIC in promiscuous mode and cap packets that weren't meant for you. The whole point is that because your NIC is constantly receiving data meant for others (i.e. because it's filtering the MAC addresses), something like privacy on networks is always problematic. I've always found the whole point somewhat overstated.

So, could anyone explain real quick the process of how a NIC decides whether a packet/frame is actually bound for it or link some good resources? For example, does the NIC automatically store the frame/packet in a buffer, then read the header, and then decide to discard? Or can it read the header before storing the rest of the frame? How much has been read at the point the NIC decides to drop it or move it up the stack? Reading all of every packet seems improbable to me because if it were the case, laptop 1 (awake but not downloading anything) would experience significant battery drain due to constantly filtering network traffic that was meant for laptop 2. I'm not sure that really maps to my experience. Also, I assume there are also differences for LAN vs WiFi?

Any help on the matter would be greatly appreciated! I've tried google diving on this question many times before and it's really hard to find much on it.

eru• 9 years ago

To answer the first point you brought up: privacy does not depend on who reads your network packages---it depend on encryption.

javajosh3• 9 years ago

The serial nature of the physical connection cannot be overstated. Bits flow to your NIC one at a time.

A 1Gb/s NIC is detecting a billion wiggles in voltage per second. Structure is imposed on these wiggles in stages: first, the A/D conversion happens, making the voltage wiggles 1's and 0's, then ethernet framing, then IP packet parsing, then TCP packet/ordering, then the application handles IO (and can, and often does, define even more structure, such as HTTP). You might look up the OSI network layering model (or the OSI 7-layer burrito, as I call it).

My understanding is that MAC filtering happens after ethernet framing, and before putting into the ring via DMA, and packets failing that test do not generate interrupts. Your NIC hardware is choosing to ignore packets not addressed to it because, generally, it's pretty useless to listen in on other people's packets. Especially these days when your most likely to capture HTTPS encrypted data.

policedemil3• 9 years ago

Thank you! This is starting to make a lot more sense! I guess what's still confusing to me has to do with the seriality (continuity?) of the process vs the OSI model, which seems like there are more discrete stages.

Does each stage of the model need to complete for the whole packet before moving on to the next stage? For example, does A/D conversion take place until all of the packet information is converted, then the whole binary blob is enframed as a header and a packet... then we filter for MAC address and move on up the stack in discrete and consecutive stages? Or are the voltages read off the line and once there is enough information to construct the header, compare it, then choose to continue reading the rest from the line or stop the A/D conversion because it is just a waste of energy? The latter makes a lot more sense to me.

EDIT: words

dom06• 9 years ago

A 1Gb/s NIC is detecting a billion wiggles in voltage per second.

Well technically :) ... 1000BASE-T - regular gigabit ethernet - uses a five-level modulation and four pairs in parallel (which reduces high frequency components, making gigabit over old cables possible).

ams61102• 9 years ago

MAC is a hexadecimal number so testing a packet header is literally a bitwise AND and then drop the packet if the result is not equal to the card's MAC address, or an XOR and drop if the result is not zero. I am not speaking from knowledge but I'd be astonished if this doesn't happen on the card and never involves the CPU.

0x0• 9 years ago

It can't possibly an AND operation, can it? If your MAC address has a lot of zero bits you'd get a ton of false positives?

walrus012• 9 years ago

scholars working with culture and technology love to make a huge deal about data leakage

one of the fundamental considerations is that if things are very sensitive, they need to be on their own air gapped network. Or at least not on the same layer 2 fabric as a ton of other things that it can arp. Network engineers who understand all of the myriad possible ways that topology can be set up (both at OSI layer 1 and logically) are key.

Properly set up with a secure gateway/VLAN delivery for a critical workstation that has a special route outbound to the internet through a firewall, there will be only two MACs showing up on the fabric: The workstation itself and the device that is serving as its default route/gateway.

dom06• 9 years ago

one of the fundamental considerations is that if things are very sensitive, they need to be on their own air gapped network.

Or maybe consider not using an information-dispersal machine for "very sensitive things".

skissane• 9 years ago

I'm in the humanities and certain scholars working with culture and technology love to make a huge deal about data leakage and how intertwined we all are precisely because you can put a NIC in promiscuous mode and cap packets that weren't meant for you.

For wired networks, a lot of those concerns about machines receiving other machines traffic are somewhat outdated–they were very valid in the 1980s and 1990s, but now in the 2010s they are far less pressing (although not completely gone). Back when we used coax Ethernet or Ethernet hubs, the norm was every machine got every other machine's traffic, and the machine's NIC was responsible for filtering out the traffic destined for other machines, so spying on other people's traffic was easy, and could be done without being detected. Now, with Ethernet switches, the norm is that each machine only gets its own traffic (plus broadcast traffic destined for all machines.) It is possible to overload a switch into a hub by MAC flooding, but in a well-maintained corporate network you can't get away with that for long without being caught. (A home network or small business network you probably can do it for a long time without being detected, since those networks are usually poorly monitored.)

So, in a contemporary well-maintained Ethernet network, it is unlikely your traffic is being sent to other people's machines. Of course, you shouldn't rely on that if you care about your security and privacy. But, encryption is far more common (and far stronger) nowadays, so even if you see someone else's traffic, you are much less likely to understand it. That is the best answer to the concern – who cares if someone else gets your traffic if they can't read it? (Well, if they save it for a few decades, computers might become fast enough to be able to break it – but, it is very unlikely anyone could be bothered.)

For wireless networks, these concerns are still very valid. The best advice with wireless networks, even secure ones, is always use a VPN.

toast0• 9 years ago

For wired Ethernet, a (decoded) frame starts with a 7-octet preamble of alternating 1 and 0 (0x55), then a "start of frame delimiter" which continues the alternation, except the last bit is 1 instead of zero (0xD5 -- least significant bit is sent first). Then the next 6 octets are the destination mac address. If the NIC is in normal mode, it will wake up when it sees a preamble, use the alternating pattern to synchronize its clock, wait for the start of frame delimiter (which is really the two 1 bits together -- in case the wakeup was a little slow); and then compare the destination mac address to its own mac address one bit at a time. The least significant bit of the first octet of the mac address indicates if it's a broadcast/multicast or intended for only one station; this is the first bit sent on the wire, so that the adapter knows to not do the normal comparison right away. Once the adapter determines the destination address doesn't match, it can stop processing the line. Ethernet requires 96 bits of idle between frames (interframe gap), which it can detect to rearm the preamble trigger.

Different versions of ethernet encode the logical signals differently, so the adapter may decode multiple bits at the same time (100BaseTX transmits in 4-bit groups, 1000BaseT in 8-bit groups), so preamble detection and address comparison would likely be a little different, but the same idea applies.

The Ethernet frame page [1] might be a helpful place to look for more info.

Also, as mschuster91 pointed out, an ethernet switch that can do cut-through switching [2], will be able to avoid buffering the whole packet to decrease latency; although it will need to have some capacity for buffering whole packets anyway -- if it's already sending a packet on the destination port, cut through switching isn't appropriate. An IP router can also do cut-through switching, once the IP header with destination comes in (or as much addressing as required for hashing among multiple links, if the destination is link aggregated)

[1] https://en.wikipedia.org/wiki/Ethernet_frame [2] https://en.wikipedia.org/wiki/Cut-through_switching

en4bz8• 9 years ago

This is why I'm really hoping RDMA [1] will catch on soon. It would be great if there was a cloud provider that would enable this feature on some of their offerings. Amazon has done something similar by allowing kernel bypass via DPDK [2] with their ENA offering but Kernel bypass is inferior to RDMA in so many ways IMO.

At this point we have 200Gbit/s NICs being provided by Mellanox [3]. CPUs aren't getting any faster and the scale out approach is extremely difficult to get right without going across NUMA domains [4]. Based on the progression of CPUs lately there just isn't going to be enough time to process all these packets AND have time left over to actually run your application. There's a lot of work focusing on data locality at the moment but at this point it's still not fool proof and the work that has been done is woefully under documented.

As the article mentioned we've already added a bunch of hardware offloads. RDMA is just a continuation of these offloads but unfortunately it requires some minor changes on the application side to take advantage of which is why it's probably been slow to be adopted.

RDMA has so many great applications for data-transfer for backend services. Whether it's queries between a web-server and a DB, replication/clustering of DBs, or micro-service fabric with micro seconds latency. Overall there's a lot of low hanging fruit that could be optimized with RDMA.

[1] https://en.wikipedia.org/wiki/Remote_direct_memory_access

[2] http://dpdk.org/doc/nics

[3] http://www.mellanox.com/page/products_dyn?product_family=266...

[4] http://rhelblog.redhat.com/2015/09/29/pushing-the-limits-of-...

kev0099• 9 years ago

This has been ongoing debate since the beginning. RDMA means running a large amount of protocol in silicon and firmware. Can you imagine what might go wrong?

In the FreeBSD community it is somewhat common knowledge that Chelsio is the best Ethernet vendor, mainly because their driver isn't a trainwreck like the dominant Intel (which share BSD HAL code on BSD/Linux/Illumos and are quite similar)

If you are alright with the trade offs of a firmware stack, Chelsio do have some pretty slick TCP full offloads that require no substantial application changes: http://www.chelsio.com/wp-content/uploads/resources/T6-100G-...

en4bz8• 9 years ago

At the end of the day I think we will end up with RDMA being the norm. Additional offloads can only go so far and are going to require firmware anyway so why not just cut to the chase.

Last time I checked Chelsio backed iWarp which has pretty much been defeated in the RDMA wars by RoCE since iWarp had terrible performance because it has to run over TCP. Intel has even removed from it their new cards and have pushed DPDK as a replacement. I'm also not surprised Chelsio has added additional offloads to compensate for their lose. That being said RoCE does have that Converged Ethernet part (CE) which is still not mainstream in many datacenters.

I get people will be disappointed that their previously Open Source drivers will be replaced with proprietary firmware but as we see FPGAs take off I think that you may see VHDL code going open source on new fully programmable cards.

I for one welcome our new hardware based overlords but perhaps I'm being far to optimistic.

_kp6z• 9 years ago

One other point, these cloud companies like Amazon and Google do a great deal of self congratulations for some pretty embarrassing architectures. Necessitating million server datacenters is as WTF to me as being proud of millions of lines of code when someone else is doing it with magnitudes less. 40G networking has been commercially viable since the top of the decade and 100G (or partial variants) for a couple years now while ENA tops out at 20G. They are basically asleep at the wheel because their architectures are predicated on poor assumptions, shirking understanding of computer architecture for 10s of thousands of SREs and "full stack developers".

Dovetailing a bit, back in the commercial UNIX and mainframe markets it is pretty common to have 80%-90% percent system utilization. In the Linux world, it's usually single digits. For some reasons (I guess it's more inviting to think holistically due to the base OS model), we are getting those 90% figures in the BSD community. See drewg's comment, WhatsApp, Isilon, LLNW:

I led the creation of an OS team, to look from first principles, where we were spending CPU/bus/IO bandwidth, and focusing on north-south delivery instead of horizontal scale. A team of 5, we are able to deliver significant shareholder value http://investors.limelight.com/file/Index?KeyFile=38751761.

en4bz8• 9 years ago

Yeah I suppose what I really want is physical procurement of hardware with the ease of cloud.

bogomipz4• 9 years ago

"One other point, these cloud companies like Amazon and Google do a great deal of self congratulations for some pretty embarrassing architectures. Necessitating million server datacenters is as WTF to me as being proud of millions of lines of code when someone else is doing it with magnitudes less"

Can you explain what these "embarrassing architectures"?

Also two of the largest tech companies on the planet using millions of servers is a "WTF" to you? Care to explain? Also I don't think either Amazon or Google has ever released actual numbers for server counts. Please provide a citation for this.

"as being proud of millions of lines of code when someone else is doing it with magnitudes less"

This is also a pretty vague statement. Lines of code for what? And who is doing that same "what" with less code?

"They are basically asleep at the wheel because their architectures are predicated on poor assumptions, shirking understanding of computer architecture for 10s of thousands of SREs and "full stack developers".

Do you really believe that Google and Amazon don't understand computer architecture?

Please provide citations for how you arrived at the at the number of SREs and "Full Stack Developers" at Google and Amazon. I don't think Google/Amazon even have "Full Stack Developer" as a job title.

"I led the creation of an OS team, to look from first principles, where we were spending CPU/bus/IO bandwidth, and focusing on north-south delivery instead of horizontal scale"

You did this at Limelight Networks then I presume according to the link you provided? You realize that a CDN is predominantly North-South traffic right? CDNs are a very different business than being general purpose cloud providers which in addition to north-south traffic have substantial east-west traffic.

You fault Google and Amazon for "congratulating" themselves yet the link you provide presumably to substantiate your own grand achievements is fluffy press release. A press release with zero detail around the claims being made and no link to a white paper or any technical specifics are given.

Limelight is not exactly what I would call a market leader or a name associated with being a great technical innovator. Limelight lost 73 million dollars last years and has a 270 million market cap. They were also sued by CDN provider Akami and agreed to pay Akami licensing fees as part of the settlement for that infringement.[1]

"A team of 5, we are able to deliver significant shareholder value"

Your company's stock trades at $2.56 a share while Amazon and Alphabets trade at well over $800 a share. I will gladly take the share holder value that Google's and Amazon's "10s of thousands of SREs and "full stack developers" provide over your team of 5.

[1] http://www.bizjournals.com/phoenix/news/2017/02/08/limelight...

drewg1232• 9 years ago

RDMA can be rather challenging for a cloud provider, as it typically requires a lossless fabric. Cloud providers don't want to enable flow control on switches (to prevent packet drops) as that has the potential to deadlock the entire network if a buggy or malicious VM stops consuming traffic.

Heck, I took our corp. net down a decade back by causing a test machine to drop into a kernel debugger with flow control enabled on the switches and the test machine. All of a sudden, my remote access to the serial console kernel debugger froze, and I was cursing the flaky VPN. Then a second later, I had the 'Oh Crap, that was me!' realization.

ra1n85• 9 years ago

Flow-control as in ECN or Ethernet/pause?

Loss is inevitable in most cloud network fabrics due to the use of shallow buffer commodity ASICs, the amount of fan-in, and the reliance on tenants to use the network in a typical manner (don't fragment, don't encap, preserve entropy).

RDMA is going to be tough - microseconds count, you can't wait for TCP to retrans, and UDP doesn't provide the reliability needed. A new transport protocol is likely the answer.

jsolson6• 9 years ago

Note that DPDK works just fine with virtio-net on GCE as well, although we have not declared official support for it.

In terms of RDMA and bypass messaging, I agree there can be substantial advantages to offloading transport layer message processing from VMs. Regrettably, I can't say anything more on the subject today.

(I lead the team that owns the virtual NICs exposed to guests in GCE)

en4bz8• 9 years ago

Do you not use SR-IOV for GCE?

i336_2• 9 years ago

I can't say anything more on the subject today.

Cool. When should people start poking around GCE's network blog for Interesting Updates™ then?

(Keeping in mind that I acknowledge/note that the "6 months from now" or "next year" or whatever that you say is the date you think we should start keeping an eye out, not necessarily the point in time anything will happen.)

shaklee38• 9 years ago

All good points, but keep in mind the 200gbps is somewhat of a gimmick since pcie3 will top out at 128gbps. It's useful for a redundant port, but you won't be pushing 200gbps for a while.

en4bz8• 9 years ago

Apparently they have PCIe4 x16 and PCIe3 x32 models. Not sure if either of these exist at the moment but they so satisfy the PCIe bandwidth constraints.

bogomipz4• 9 years ago

RDMA has been around for years though. It's big in the HPC world Oracle Rac had used it for some time. In Solaris 11, the default transport for NFS uses RDMA:

https://docs.oracle.com/cd/E23824_01/html/821-1454/rfsrefer-...

en4bz8• 9 years ago

Infiniband RDMA is quite old yes but the Ethernet protocols are still rather new and they (iWarp & RoCE) are still competing for supremacy which doesn't really help adoption. At this point it looks like RoCE will win because Intel dropped iWarp but at this point there is basically only one major manufacturer backing each protocol.

Turtles on the Wire: Understanding How the OS Uses the Modern NIC (2016)

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