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CSR 1000V Software Architecture

Chapter Description

In this sample chapter from Virtual Routing in the Cloud, explore the software and data plane design of the CSR 1000V, a virtualized software router that runs the IOS XE operating system.

This chapter describes the software design of the CSR 1000V and details the data plane design. It also illustrates the software implementation and packet flow within the CSR 1000V, as well as how to bring up the CSR 1000V.

System Design

CSR 1000V is a virtualized software router that runs the IOS XE operating system.

IOS XE uses Linux as the kernel, whereas the IOS daemon (IOSd) runs as a Linux process providing the core set of IOS features and functionality. IOS XE provides a native Linux infrastructure for distributing the control plane forwarding state into an accelerated data path. The control and data planes in IOS XE are separated into different processes, and the infrastructure to communicate between these processes supports distribution and concurrent processing. In addition, IOS XE offers inherent multicore capabilities, allowing you to increase performance by scaling the number of processors. It also provides infrastructure services for hosting applications outside IOSd.

Originally, IOS XE was designed to run on a system with redundant hardware, which supports physical separation of the control and data plane units. This design is implemented in the ASR 1006 and ASR 1004 series routers. The original ASR 1000 family hardware architecture consisted of the following main elements:

  • Chassis

  • Route processor (RP)

  • Embedded service processor (ESP)

  • SPA interface processors (SIP)

The RP is the control plane, whereas the ESP is the data plane. In an ASR 1006 and ASR 1004, the RP and ESP processes have separate kernels and run on different sets of hardware. ASR 1000 was designed for high availability (HA). The ASR 1006 is a fully hardware redundant version of the ASR, and its RP and ESP are physically backed up by a standby unit. IOSd runs on the RP (as do the majority of the XE processes), and the RP is backed up by another physical card with its own IOSd process. The ASR 1004 and fixed ASR 1000s (ASR 1001-X and ASR 1002-X) do not have physical redundancy of the RP and ESP.

In the hardware-based routing platform for IOS XE, the data plane processing runs outside the IOSd process in a separate data plane engine via custom ASIC: QuantumFlow Processor (QFP). This architecture creates an important framework for the software design. Because these cards each have independent processors, the system disperses many elements of software and runs them independently on the different processors.

As the need for smaller form factor ASRs arose, a one rack unit (RU) ASR 1000 was conceptualized and developed: ASR 1001. The ASR 1001 is a 64-bit architecture in which all processes (RP, SIP, and ESP) are controlled by a single CPU. The SPA interface complex, forwarding engine complex, and IOS XE middleware all access the same Linux kernel. This is achieved by mapping the RP, ESP, and SIP domains into logical process groups. The RP’s process domain includes IOSd, a chassis manager process and forwarding manager. The ESP process domain includes the chassis manager process, QFP client/driver process, and forwarding manager.

The architecture diagram in Figure 4-1 provides a high-level overview of the major components.

Figure 4-1

Figure 4-1 ASR 1001 Platform Logical Architecture

The details on grouping of the components are as follows:

  • RP—RP mainly contains the IOS daemon (IOSd), the forwarding manager for RP (FMAN-RP), the chassis manager for RP (CMAN-RP), the kernel, and bootstrap utilities.

  • ESP (forwarding plane)—ESP contains FMAN-FP and CMAN-FP, as well as QFP microcode and data plane drivers and crypto offload ASIC for handling hardware assist encryption.

  • SIP/SPA—SIP/SPA houses the I/O interface for the chassis. It has its own CMAN and kernel process to handle the discovery, bootstrapping, and initialization of the physical interfaces.

Virtualizing the ASR 1001 into the CSR 1000V

There are a lot of commonalities between the system architectures of the CSR 1000V and the ASR 1001, and there are some differences as well. The CSR 1000V is essentially an ASR 1001 without the hardware. The following measures brought the ASR 1001 into the software-based design of the CSR 1000V:

  • All the inter-unit communication with the SIP/CC was removed.

  • The entire SIP/SPA interface complex was eliminated.

  • The kernel utilities have been shared across the RP and ESP software complexes.

  • The kernel utilities use the virtualized resources presented to it by the hypervisor.

The CSR is basically the ASR 1000 design stripped of its hardware components. When you compare the two designs, you find that the data path implementation is very different. This is because the ASR 1001 has a physical processor (the QFP) for running data path forwarding. In a CSR, the IOS XE data path is implemented as a Linux process.

The CSR 1000V is meant to leverage as much of the ASR 1001 architecture as possible. There are places in the CSR 1000V system where software emulation for hardware-specific requirements is needed. In general, the software architecture is kept the same, using the same grouping approach as for the hardware components. One of the major engineering efforts has been focused on migrating the QFP custom ASIC network processor capabilities onto general-purpose x86 CPU architectures and providing the distributed data path implementation for IOS XE. This effort creates a unique opportunity for Cisco to package this high-performance and feature-rich technology into the CSR 1000V. Figure 4-2 shows the high-level architecture of the CSR 1000V.

Figure 4-2

Figure 4-2 CSR 1000V High-Level Architecture

CSR 1000V Initialization Process

This section examines the initialization of a CSR 1000V running on a type 1 hypervisor. Refer to Chapter 2, “Software Evolution of the CSR 1000,” for details on the IOSd process running on the control plane.

When a CSR boots up as a virtual machine, interfaces are discovered by parsing the contents of /proc/net/dev on the Linux kernel. The gethd (Guest Ethernet Management Daemon) process performs the port enumeration at startup and then passes the interface inventory to the guest Ethernet driver within the IOS complex. The IOSd gethd driver then instantiates the Ethernet interfaces. This is how the I/O interfaces provided by the virtual NIC are managed by IOS.

The gethd process manages the interfaces on the CSR VM. It takes care of addition, removal, configuration, states, and statistics of the Ethernet interfaces on the CSR VM.

Figure 4-3 illustrates the CSR 1000V initialization sequence.

Figure 4-3

Figure 4-3 CSR 1000V Initialization Sequence

gethd is an important process that handles a variety of interface management functions, including interface removal/addition. It is an important part of the virtualized I/O used in CSR.

CSR 1000V Data Plane Architecture

Originally, IOS XE QFP data plane design consisted of four components: client, driver, QFP microcode (uCode), and crypto assist ASIC. Different ASR 1000 platforms package these components differently, but in general the four components are the same across platforms. CSR 1000V leverages the same client, driver, and uCode to support a multithread-capable packet processing data plane, with the exception of the crypto assist ASIC.

Figure 4-4 illustrates the CSR 1000V data plane architecture. The HW threads mentioned in the figure are packet processing engine (PPE) threads. The terms HW and PPE can be used interchangeably.

Figure 4-4

Figure 4-4 CSR 1000V Data Plane Architecture

The following is an overview of the three main components that make up the packet-processing data plane for CSR:

  • Client—The Client is software that ties together the control plane and the data plane. It is a collection of software modules that transform control plane information into various data plane forwarding databases and data structure updates. It is also responsible for updating the control plane with statistics from the data plane. It allocates and manages the resources of the uCode, including data structures in resource memory. The QFP Client is also responsible for restarting the QFP process in the event of failure. The Client provides a platform API layer that logically sits between IOSd and the uCode implementing the corresponding features. The Client API is called from FMAN-FP and then communicates with the uCode via both Interprocess Communicator (IPC) and shared memory interfaces provided by the Driver. Within the Client, feature processing support can be broken down into functional blocks known as Execution Agents (EA) and Resource Managers (RM). RMs are responsible for managing physical and logical objects, which are shared resources. An example of a physical object manager is the TCAM-RM, which manages allocation of TCAM resources, and an example of a logical object manager is the UIDB-RM, which manages the micro Interface Descriptor Block (uIDB) objects used to represent various forms of interfaces. The data plane (uCode) uses uIDB objects to see the logical interfaces.

  • Driver—The Driver is a software layer that enables software components to communicate with the hardware. It glues the software components to the hardware. The Driver is made up of libraries, processes, and infrastructure that are responsible for initialization, access, error detection, and error recovery. The Driver has hardware abstraction layering known as the Device Object (devobj) Model that allows it to support different QFP ASICs. Below the devobj API are implementations of various emulation and adaptation layers. In addition to the emulation and adaptation layers required to support the RMs listed in the Client section, the Driver is also responsible for coordinating memory access and IPC messaging between various QFP control plane software components and the QFP data plane packet processing uCode. The driver is completely segregated from the IOS code in an XE architecture, and this makes XE a very robust and flexible software architecture that offers complete separation of the control and data planes.

  • QFP uCode (packet processing)—The uCode is where all the feature packet processing occurs. The uCode runs as a single process in the same VM/container as the Client and the Driver processes. IOSd initiates a packet process request through FMAN-FP. This request is then driven by the Client and the Driver interacting with the uCode to control the PPE behavior. The QFP uCode is broken up into four main components: Feature Code, Infrastructure, Platform Abstraction Layer (PAL), and Hardware Abstraction Layer (HAL). The PAL and HAL are essentially glue for the portability of software features to different hardware platforms. Originally, the PAL and HAL were designed for Cisco forwarding ASICs, such as QFP. In order for uCode software to run on top of x86 in a Linux environment, a new PAL layer is needed to support the specifics of the CSR 1000V platform. In addition, a new HAL is introduced for running QFP software on top of x86 in a Linux environment.

The intention is for the CSR 1000V data plane to leverage as much of the existing QFP code base as possible to produce a full-featured software data plane capable of leveraging the processing capacity and virtualization capabilities of modern multicore CPUs. One way to minimize changes to the existing QFP software code base is to emulate QFP hardware ASIC in such a way that the existing Client, Driver, and QFP uCode are not aware that they are running on a non-QFP platform. However, due to the complexity of QFP hardware and the differences in platform requirements, a pure emulation is impractical. There are some cases where we choose to emulate hardware because doing so is the straightforward approach for code leverage. In other cases, it is best to replace the corresponding functionality with an implementation that is compatible at an API level but may be a completely different algorithmic implementation.

CSR 1000V Software Crypto Engine

Cisco router platforms are designed to run IOS with hardware acceleration for crypto operations. Like other ASR 1000 platforms, the ASR 1001 includes a crypto acceleration engine on board to deliver crypto offload and to increase encryption performance. In this environment, the main processor performing the data path processing is offloaded from the computing-intensive crypto operations. Once the crypto offload engine completes the encrypt/decrypt operation, it generates an interrupt to indicate that the packet should be reinserted back into the forwarding path.

The CSR 1000V runs completely on general-purpose CPUs without an offload engine; therefore, the software implementation of the IPsec/crypto feature path is needed to support the encryption function. To that end, the CSR 1000V includes a software crypto engine that uses low-level cryptographic operations for encrypting and decrypting traffic. The software crypto engine is presented to the IOS as a slower crypto engine. One thing to note is the software crypto engine runs as an independent process within the CSR 1000V, and it therefore may run as a parallel process in a multicore environment. To improve the crypto performance of the CSR 1000V software router, the crypto data path is implemented to take advantage of the latest Advanced Encryption Standard (AES) crypto instruction set from Intel (AES-NI) for encryption/decryption operations.

The newer Intel processors, such as the Xeon Westmere-EP family and mobile Sandy Bridge family, provide instruction sets for enhancing Advanced Encryption Standard (AES-NI) cryptographic operations performance. These instructions are included in the CSR 1000V crypto library, along with other cryptographic and hash algorithms for low-level crypto operations. The crypto library is used by the software crypto engine as well as by other subsystems within IOS that require cryptographic operations. The inclusion of Intel’s crypto instruction set allows the CSR 1000V to take advantage of the latest Intel CPUs for encryption and decryption operations in the data path.

2. Life of a Packet on a CSR 1000V: The Data Plane | Next Section

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