5G and the End-to-End Argument
One of the most surprising things I learned while writing the 5G Book with Oguz Sunay is how the cellular network’s history, starting 40+ years ago, parallels that of the Internet. And while from an Internet-centric perspective the cellular network is just one of many possible access network technologies, the cellular network in fact shares many of the “global connectivity” design goals of the Internet. That is, the cellular network makes it possible for a cell phone user in New York to call a cell phone in Tokyo, then fly to Paris and do the same thing again. In short, the 3GPP standard federates independently operated and locally deployed Radio Access Networks (RAN) into a single logical RAN with global reach, much as the Internet federated existing packet-switched networks into a single global network.
For many years, the dominant use case for the cellular network has been access to cloud services. With 5G expected to connect everything from home appliances to industrial robots to self-driving cars, the cellular network will be less-and-less about humans making voice calls and increasingly about interconnecting swarms of autonomous devices working on behalf of those humans to the cloud. This raises the question: Are there artifacts or design decisions in the 3GPP-defined 5G architecture working at cross-purposes with the Internet architecture?
Another way to frame this question is: How might we use the end-to-end argument—which is foundational to the Internet’s architecture—to drive the evolution of the cellular network? In answering this question, two issues jump out at me, identity management and session management, both of which are related to how devices connect to (and move throughout) the RAN.
The 5G architecture leverages the fact that each device has an operator-provided SIM card, which uniquely identifies the subscriber with a 15-digit International Mobile Subscriber Identity (IMSI). The SIM card also specifies the radio parameters (e.g., frequency band) needed to communicate with that operator’s Base Stations, and includes a secret key that the device uses to authenticate itself to the network. The IMSI is a globally unique id and plays a central role in devices being mobile across the RAN, so in that sense it plays the same role as an IP address in the Internet architecture. But if you instead equate the 5G network with a layer 2 network technology, then the IMSI is effectively the device’s “ethernet address.”
Ethernet addresses are also globally unique, but the Internet architecture makes no attempt to track them with a global registry or treat them as a globally routable address. The 5G architecture, on the other hand, does, and it is a major source of complexity in the 3GPP Mobile Core. Doing so is necessary for making a voice call between two cell phones anywhere in the world, but is of limited value for cloud-connected devices deployed on a manufacturing floor, with no aspiration for global travel. Setting aside (for the moment) the question of how to also support traditional voice calls without tracking IMSI locations, the end-to-end argument suggests we leave global connectivity to IP, and not try to also provide it at the link layer.
Let’s turn from identity management to session management. Whenever a mobile device becomes active, the nearest Base Station initiates the establishment of a sequence of secure tunnels connecting the device back to the Mobile Core, which in turn bridges the RAN to the Internet. (You can find more details on this process here.) Support for mobility can then be understood as the process of re-establishing the tunnel(s) as the device moves throughout the RAN, where the Mobile Core’s user plane buffers in-flight data during the handover transition. This avoids dropped packets and subsequent end-to-end retransmissions, which may make sense for a voice call, but not necessarily for a TCP connection to a cloud service. As before, it may be time to apply the end-to-end argument to the cellular network’s architecture in light of today’s (and tomorrow’s) dominant use cases.
To complicate matters, sessions are of limited value. The 5G network maintains the session only when the same Mobile Core serves the device and only the Base Station changes. This is often the case for a device moving within some limited geographic region, but moving between regions—and hence, between Mobile Cores—is indistinguishable from power cycling the device. The device is assigned a new IP address and no attempt is made to buffer and subsequently deliver in-flight data. This is important because any time a device becomes inactive for a period of time, it also loses its session. A new session is established and a new IP address assigned when the device becomes active. Again, this makes sense for a voice call, but not necessarily for a typical broadband connection, or worse yet, for an IoT device that powers down as a normal course of events. It is also worth noting that cloud services are really good at accommodating clients who’s IP addresses change periodically (which is to say, when the relevant identity is at the application layer).
This is all to say that the cellular network’s approach, which can be traced to its roots as a connection-oriented voice network, is probably not how one would design the system today. Instead, we can use IP addresses as the globally routable identifier, lease IP addresses to often-sleeping and seldom-moving IoT devices, and depend on end-to-end protocols like TCP to retransmit packets dropped during handovers. Standardization and interoperability will still be needed to support global phone calls, but with the ability to implement voice calls entirely on top of IP, it’s not clear the Mobile Core is the right place to solve that problem. And even if it is, this could potentially be implemented as little more than legacy APIs supported for backward compatibility. In the long term, it will be interesting to see if 3GPP-defined sessions hold up well as the foundation for an architecture that fully incorporates cellular radio technology into the cloud.
We conclude by noting that while we have framed this discussion as a thought experiment, it illustrates the potential power of the software-defined architecture being embraced by 5G. With the Mobile Core in particular implemented as a set of micro-services, an incremental evolution that addresses the issues outlined here is not only feasible, but actually quite likely. This is because history teaches us that once a system is open and programmable, the dominant use cases will eventually correct for redundant mechanisms and sub-optimal design decisions.
Why 5G Matters
Over the last month I undertook a detailed review of a new book in the Systems Approach series, 5G Mobile Networks: A Systems Approach by Larry Peterson and Oguz Sunay. Talking to people outside the technology world about my work, I soon found myself trying to explain "why does 5G matter" to all sorts of folks without a technical background. At this point in 2020, we can generally assume people know two things about 5G: the Telcos are marketing it as the greatest innovation ever (here's a sample); and conspiracy theorists are having a field day telling us all the things that 5G causing or covering up (which has in turn led to more telco ads like this one). By the end of reviewing the new book from Larry and Oguz, I felt I had finally grasped why 5G matters. Spoiler alert: I'm not going to bother debunking conspiracy theories, but I do think there is something quite important going on with 5G. And frankly, there is plenty of hype around 5G, but behind that hype are some significant innovations.
What is clear about 5G, technically, is that there will be a whole lot of new radio technologies and new spectrum allocation, which will enable yet another upgrade in speeds and feeds. If you are a radio person that's quite interesting–there is plenty of innovation in squeezing more bandwidth out of wireless channels. It's a bit harder to explain why more bandwidth will make a big difference to users, simply because 4G generally works pretty well. Once you can stream video at decent resolution to your phone or tablet, it's a bit hard to make a case for the value of more bandwidth alone. A more subtle issue is bandwidth density–the aggregate bandwidth that can be delivered to many devices in a certain area. Think of a sporting event as a good example (leaving aside the question of whether people need to watch videos on their phones at sporting events).
Lowering the latency of communication starts to make the discussion more interesting–although not so much to human users, but as an enabler of machine-to-machine or Internet-of-things applications. If we imagine a world where cars might communicate with each other, for example, to better manage road congestion, you can see a need for very low latency coupled with very high reliability–which is another dimension that 5G aims to address. And once we start to get to these scenarios, we begin to see why 5G isn't just about new radio technology, but actually entails a whole new mobile network architecture. Lowering latency and improving availability aren't just radio issues, they are system architecture issues. For example, low latency requires that a certain set of functions move closer to the edge–an approach sometimes called edge computing or edge clouds.
The Importance of Architecture
The high points of the new cellular architecture for 5G are all about leveraging trends from the broader networking and computing ecosystems. Three trends stand out in particular:
If you want to know more about the architecture of 5G, the application requirements that are driving it, and how it will enable innovation, you should go read the book as I did!
Photo by Sander Weeteling on Unsplash
It's Been a Fun Ride
We are officially shutting down PlanetLab at the end of May, with our last major user community (MeasurementLab) having now migrated to new infrastructure. It was 18 years ago this month (March 2002) that 30 systems researchers got together at the Intel Lab in Berkeley to talk about how we could cooperate to build out the distributed testbed to support our research. There were no funding agencies in the room, no study group, and no platinum sponsors. Just a group of systems people that wanted to get their research done. We left the meeting with an offer from David Tennenhouse, then Director of Research at Intel, to buy 100 servers to bootstrap the effort. In August, the day before SIGCOMM, a second underground meeting happened in Pittsburgh, this time drawing 80 people. The first machines came online at Princeton and Berkeley in July, and by October, we had the 100 seed machine up and running at 42 sites. The rest, as they say, is history.
In retrospect, it was a unique moment in time. The distributed systems community, having spent the previous 15 years focused on the LAN, was moving on to wide-area networking challenges. The networking community, having architected the Internet, was ruminating about how it had become ossified. Both lacked a realistic platform to work on. My own epiphany came during an Internet End-to-End Research Group meeting in 2001, when I found myself in a room full of the Internet’s best-and-brightest, trying to figure out how we could possibly convince Cisco to interpret one bit in the IP header differently. I realized we needed to try a different approach.
PlanetLab enabled a lot of good research, much of which has been documented in the website’s bibliography. Those research results are certainly important, but from my point of view, PlanetLab has had impact in other, more lasting ways. One was a model for how computer scientists can share research infrastructure. Many of the early difficulties we faced deploying PlanetLab had to do with convincing University CIOs that hosting PlanetLab servers had an acceptable risk/reward tradeoff. A happy mistake we made early on was asking the VP for Research (not the University CIO) for permission to install servers on their campus. By the time the security-minded folks figured out what was going on, it was too late. They had no choice but to invent Network DMZs as a workaround.
A second was to expose computer scientists to real-world operational issues that are inevitable when you’re running Internet services. Researchers that had been safely working in their labs were suddenly exposed to all sorts of unexpected user behavior, both benign and malicious, not to mention the challenges of keeping a service running under varied network conditions. There were a lot of lessons learned under fire, with unexpected traffic bursts (immediately followed by email from upset University system admins), a common right-of-passage for both grad students and their advisors. I’m not surprised when I visit Google and catch up with former faculty colleagues to hear that they now spend all their time worrying about operational challenges. Suddenly, network management is cool.
Then there were the non-technical, policy-related issues, forcing us to deal with everything from DMCA take-down notices to FBI subpoenas to irate web-surfers threatening to call the local Sheriff on us. These and similar episodes were among the most eye-opening aspects of the entire experience. They were certainly the best source of war stories, and an opportunity to get to know Princeton’s General Counsel quite well. Setting policy and making judgements about content is really hard… who knew.
Last, but certainly not least, is the people. In addition to the fantastic and dedicated group of people that helped build and operate PlanetLab, the most gratifying thing that happens to me (even still today) is running into people--usually working for an Internet company of one sort or another--who tell me that PlanetLab was an important part of their graduate student experience. If you are one of those people and I haven’t run into you recently (or even if I have) please leave a comment and let me know what you’re up to. It will be good to hear from you.
The transition to 5G is happening, and unless you’ve been actively trying to ignore it, you’ve undoubtedly heard the hype. But if you are like 99% of the CS-trained, systems-oriented, cloud-savvy people in the world, the cellular network is largely a mystery. You know it’s an important technology used in the last mile to connect people to the Internet, but you’ve otherwise abstracted it out of your scope-of-concerns.
The important thing to understand about 5G is that it implies much more than a generational upgrade in bandwidth. It involves transformative changes that blur the line between the access network and the cloud. And it will encompass enough value that it has the potential to turn the “Access-as-frontent-to-Internet” perspective on its head. We will just as likely be talking about “Internet-as-backend-to-Access” ten years from now. (Remember, you read it here first.)
The challenge for someone that understands the Internet is penetrating the myriad of acronyms that dominate cellular networking. In fairness, the Internet has its share acronyms, but it also comes with a sufficient set of abstractions to help manage the complexity. It’s hard to say the same for the cellular network, where pulling on one thread seemingly unravels the entire space. It has also been the case that the cellular network had been largely hidden inside proprietary devices, which has made it impossible to figure it out for yourself.
In retrospect, it's strange that we find ourselves in this situation, considering that mobile networks have a 40-year history that parallels the Internet’s. But unlike the Internet, which has evolved around some relatively stable "fixed points," the cellular network has reinvented itself multiple times over, transitioning from from voice-only to data-centric, and from circuit-oriented to IP-based. 5G brings another such transformation, this time heavily influenced the cloud. In the same way 3G defined the transition from voice to broadband, 5G’s promise is mostly about the transition from a single access service (broadband connectivity) to a richer collection of edge services and devices, including support for immersive user interfaces (e.g., AR/VR), mission-critical applications (e.g., public safety, autonomous vehicles), and the Internet-of-Things (IoT). Because these use cases will include everything from home appliances to industrial robots to self-driving cars, 5G won’t just support humans accessing the Internet from their smartphones, but also swarms of autonomous devices working together on their behalf. All of this requires a fundamentally different architecture that will both borrow from and impact the Internet and Cloud.
We have attempted to document this emerging architecture in a book that is accessible to people with a general understanding of the Internet and Cloud. The book (5G Mobile Networks: A Systems Approach) is the result of a mobile networking expert teaching a systems person about 5G as we’ve collaborated on an open source 5G implementation. The material has been used to train other software developers, and we are hopeful it will be useful to anyone that wants a deeper understanding of 5G and the opportunity for innovation it provides. Readers that want hands-on experience can also access the open source software introduced in the book.
Democratizing the Network Edge
Two industry trends with significant momentum are on a collision course. One is the cloud, which in pursuit of low-latency/high-bandwidth applications is moving out of the datacenter and towards the edge. The promise and potential of applications ranging from Internet-of-Things (IoT) to Immersive UIs, Public Safety, Autonomous Vehicles, and Automated Factories, has triggered a gold rush to build edge platforms and services. The other is the access network that connects homes, businesses, and mobile devices to the Internet. Network operators (Telcos and CableCos) are transitioning from a reliance on closed and proprietary hardware to open architectures leveraging disaggregated and virtualized software running on white-box servers, switches, and access devices.
The confluence of cloud and access technologies raises the possibility of convergence. For the cloud, access networks provide low-latency connectivity to end users and their devices, with 5G in particular providing native support for the mobility of those devices. For the access network, cloud technology enables network operators to enjoy the CAPEX & OPEX savings that come from replacing purpose-built appliances with commodity hardware, as well as accelerating the pace of innovation through the softwartization of the access network.
It is clear that the confluence of cloud and access technologies at the access-edge is rich with opportunities to innovate, and this is what motivates the CORD-related platforms we are building at ONF. But it is impossible to say how this will all play out over time, with different perspectives on whether the edge is on-premise, on-vehicle, in the cell tower, in the Central Office, distributed across a metro area, or all of the above. With multiple incumbent players—e.g., network operators, cloud providers, cell tower providers—and countless startups jockeying for position, it’s impossible to predict how the dust will settle.
On the one hand, cloud providers believe that by saturating metro areas with edge clusters and abstracting away the access network, they can build an edge presence with low enough latency and high enough bandwidth to serve the next generation of edge applications. In this scenario, the access network remains a dumb bit-pipe, allowing cloud providers to excel at what they do best: run scalable cloud services on commodity hardware. On the other hand, network operators believe that by building the next generation access network using cloud technology, they will be able to co-locate edge applications in the access network. This scenario comes with built-in advantages: an existing and widely distributed physical footprint, existing operational support, and native support for both mobility and guaranteed service.
While acknowledging both of these possibilities, there is a third outcome that not only merits consideration, but is also worth actively working towards: the democratization of the network edge. The idea is to make the access-edge accessible to anyone, and not strictly the domain of incumbent cloud providers or network operators. There are three reasons to be optimistic about this possibility:
HTTP is the New Narrow Waist
The Internet has been described as having a narrow waist architecture, with one universal protocol in the middle (IP), widening to support many transport and application protocols above it (e.g., TCP, UDP, RTP, SunRPC, DCE-RPC, gRPC, SMTP, HTTP, SNMP) and able to run on top of many network technologies below (e.g., Ethernet, PPP, WiFi, SONET, ATM). This general structure has been a key to the Internet becoming ubiquitous: by keeping the IP layer that everyone has to agree to minimal, a thousand flowers were allowed to bloom both above and below. This is now a widely understood strategy for any platform trying to achieve universal adoption.
But something else has happened over the last 30 years. By not addressing all the issues the Internet would eventually face as it grew (e.g., security, congestion, mobility, real-time responsiveness, and so on) it became necessary to introduce a series of additional features into the Internet architecture. Having IP’s universal addresses and best-effort service model was a necessary condition for adoption, but not a sufficient foundation for all the applications people wanted to build.
It is informative to reconcile the value of a universal narrow waist with the evolution that inevitably happens in any long-lived system: the “fixed point” around which the rest of the architecture evolves has moved to a new spot in the software stack. In short, HTTP has become the new narrow waist; the one shared/assumed piece of the global infrastructure that makes everything else possible. This didn’t happen overnight or by proclamation, although some did anticipate it would happen. The narrow waist drifted slowly up the protocol stack as a consequence of a evolution (to mix geoscience and biological metaphors).
Putting the narrow waist label purely on HTTP is an over simplification. It’s actually a team effort, with the HTTP/TLS/TCP/IP combination now serving as the Internet’s common platform.
Somewhat less obviously, HTTP also provides a good foundation for dealing with mobility. If the resource you want to access has moved, you can have HTTP return a redirect response that points the client to a new location. Similarly, HTTP enables injecting caching proxies between the client and server, making it possible to replicate popular content in multiple locations and save clients the delay of going all the way across the Internet to retrieve some piece of information. (See how in Section 9.4.) Finally, HTTP has been used to deliver real-time multi-media, in an approach known as adaptive streaming. (See how in Section 7.2.)
Feature Velocity vs Stability
It is important to recognize the various perspectives on computer networks (e.g., that of network architects, application developers, end users, and network operators) to understand the technical requirements that shape how networks are designed and built. But this presumes all design decisions are purely technical, which is certainly not the case. Many other factors, from economic forces, to government policy, to societal influences, to ethical considerations, influence how networks are designed and built.
Of these, the marketplace is often the most influential, and corresponds to the interplay between network operators that sell access and connectivity (e.g., AT&T, Comcast, Verizon, DT, NTT, China Mobile), network equipment venders that sell hardware to network operators (e.g., Cisco, Juniper, Ericsson, Nokia, Huawei, NEC), cloud providers that host content and scalable applications in their datacenters (e.g., Google, Amazon, Microsoft), service providers that deliver content and cloud apps to end-users (e.g., Facebook, Apple, Netflix, Spotify), and of course, subscribers and customers that download content and run cloud applications (i.e., individuals, but also enterprises and businesses). Not surprisingly, the lines between all these players are not crisp, with many companies playing multiple roles. For example, service providers like Facebook run their own clouds and network operators like Comcast and AT&T own their own content.
The most notable example of this cross-over are the large cloud providers, who (a) build their own networking equipment, (b) deploy and operate their own networks, and (c) provide end-user services and applications on top of their networks. It's notable because it challenges the implicit assumptions of the simple "textbook" version of the technical design process. One such assumption is that designing a network is a one-time activity. Build it once and use it forever (modulo hardware upgrades so users can enjoy the benefits of the latest performance improvements). A second is that the job of designing and implementing the network is completely divorce from the job of operating the network. Neither of these assumptions is quite right.
On the first point, the network’s design is clearly evolving. The only question is how fast. Historically, the feature upgrade cycle involved an interaction between network operators and their vender partners (often collaborating through the standardization process), with timelines measured in years. But anyone that has downloaded and used the latest cloud app knows how glacially slow anything measured in years is by today's standards.
On the second point, the companies that build networks are almost always the same ones that operate them. The only question is whether they develop their own features or outsource that process to their venders. If we once again look to the cloud for inspiration, we see that develop-and-operate isn’t just true at the corporate level, but it is also how the fastest moving cloud companies organize their engineering teams: around the DevOps model. (If you are unfamiliar with DevOps, we recommend you read "Site Reliability Engineering: How Google Runs Production Systems" to see how Google practices it.)
What this all means is that computer networks are now in the midst of a major transformation, due largely to market pressure being applied by agile cloud providers. Network operators are trying to simultaneously accelerate the pace of innovation (sometimes known as feature velocity) and yet continue to offer a reliable service (preserve stability). And they are increasingly doing this by adopting the best practices of cloud providers, which can be summarized as having two major themes: (1) take advantage of commodity hardware and move all intelligence into software, and (2) adopt agile engineering processes that break down barriers between development and operations.
This transformation is sometimes called the “cloudification” or “softwarization” of the network, but by another name, it’s known as Software Defined Networks (SDN). Whatever you call it, this new perspective will (eventually) be a game changer, not so much in terms of how we address the fundamental technical challenges of framing, routing, fragmentation/reassembly, packet scheduling, congestion control, security, and so on, but in terms of how rapidly the network evolves to support new features and to accommodate the latest advances in technology.
This general theme is important and we plan to return to it in future posts. Understanding networks is partly about understanding the technical underpinnings, but also partly about how market forces (and other factors) drive change. That you are able to make informed design decisions about technical approach A versus technical approach B is a necessary first step, but that you are able to deploy that solution and bring it to market more rapidly and for less cost than the competition is just as important, if not more so.