Linux Networking Overview COMS W Spring 2010

Outline Layering in Networks ISO Network Model IP Network Model Linux Kernel Outline Network Subsystem Outline

Layer-Based Communication Models Instance (N) Instance (N-1) Application Instance (N+1) Application (N+1)-Protocols (N)-protocol (N-1)-protocol Instance (N+1) Instance (N+1) Instance (N) Instance (N-1) Instance (N+1) Transmission medium Intermediate system End systems Layer (N-1) Layer (N) Layer (N+1) Application

Concepts in Layered Model Protocols Rules that two parties talk and understand each other Horizontal interface Services Functions provided by a lower layer to the neighboring upper layer Vertical interface Service interface (function calls)

ISO/OSI Reference Model ISO/OSI Reference model Application Presentation Session Transport Network Data link Physical Application: Protocols for different applications, HTTP, SMTP, FTP, etc Presentation layer: Regulating data presentation (formatting, ASN/1) Session : Handling structured message exchange, multiplexing sessions Transport: End-to-end functions between applications. Flow control, packet ordering, etc. Network: Connecting networks. Packet routing/forwarding Data link layer: Moving data between two directly connected stations. Flow control, error detection, etc. Shared medium: access control. LLC/MAC Physical: Media types, coding methods, bit sequences vs. physical signals

ISO vs. The Internet ISO/OSI reference model Internet reference model Application Presentation Application (HTTP, SMTP, SSH) Transport (TCP/UDP) Internet (IPv4/v6) Data link (802.x, PPP, SLIP) Session Transport Network Data link Physical

Design Principles Optimize for the common case E.g., TCP header prediction Never touch/copy data E.g., checksum offload Mistakes can be made at each layer Use common facilities at each layer Buffer management, hash tables, timers Use best-of-breed practices in these facilities

Kernel Structure vimapachesshd Shared C Library System Call Interface User Kernel Process Mgmt Memory Mgmt File System Device Control Network Subsys CPU Support Code Hardware MMU Support Code CPURAM Character Devices Network Devices Network device drivers Character device drivers Block device drivers Block Devices

Process management Creating, destroying, putting to sleep, waking up, and scheduling processes. Memory management Allocates memory to processes; maps virtual memory to physical memory; enforces protection File system In UNIX, almost everything is handled over the file system interface. Device drivers can be addressed as files /proc file system allows us to access data and parameters in the kernel Kernel Structure

Device drivers Abstract away the underlying hardware and allow us to access the hardware with well-defined APIs The use of kernel modules allow device drivers to be dynamically loaded/unloaded Networking Provides communication between end hosts Incoming packets are asynchronous events and have to be collected and identified, before a process can handle them. Many network operations occur asynchronously and cannot be associated to a specific process. Instead, interrupts and timers are used extensively. Kernel Structure (2)

Kernel Structure vimapachesshd Shared C Library System Call Interface User Kernel Process Mgmt Memory Mgmt File System Device Control Network Subsys CPU Support Code Hardware MMU Support Code CPURAM Block Devices Character Devices Network Devices Network device drivers Character device drivers Block device drivers COMS W6998

Network Subsystem System Call Interface User Kernel Interrupts Soft IRQs Lists UDP Wait Queues Hardware Timers Intel E1000 E1000 driver Application Intel E1000 Hash Tables Synch & Atomic Ops E1000 driver Sockets ip_proto TCPSCTP data link layer ARPIPV4IPV6bridging ICMP sk_buff net_device U/K copy DMAPCI Mem Alloc Notifiers VFS sock socket

Network-specific facilities sk_buff : Core networking data structure for managing data (i.e., packets) net_device: Core data structure that represents a network interface (e.g., an Intel E1000 Ethernet NIC). proto_ops: Data structure for different IP protocol families SOCK_STREAM, SOCK_DGRAM, SOCK_RAW Virtual functions for bind(), accept(), connect(), etc. struct sock/ struct socket : Core data structures for representing sockets

Kernel facilities (1) Timers Facility for scheduling work in the future (e.g., retransmitting a lost TCP segment) Hash tables Facility for creating associations (e.g., 4-tuple  TCP connection block), looking them up, deleting them User/kernel copying Library for safely transferring data across the user/kernel boundary Memory allocation Mechanism for the network subsystem to obtain memory (e.g., pinned pages for arriving packets to land in) Linked lists What you think

Kernel facilities (2) Atomic operations and synchronization Mechanisms for managing concurrency correctly Interrupts Hardware interface for notifying OS of an event (e.g., a packet arrival) Soft IRQs “Software interrupts” that are asynchronously executed in response to a hardware interrupt Wait Queues Mechanism for processes/threads/tasks to wait for an event, put themselves to sleep, or wake another process up Notifiers Publish/subscribe system for notifying other systems about an event (e.g., interface goes down)

That’s the big picture… Later lectures will delve into the details Not enough time to cover everything Suggestions about what to cover welcome Or how to cover it Bottom-up or top-down? Use your project to cover something interesting and/or important to you

For next week Install a distribution inside a VM Download/build/install the appropriate kernel source in the VM Enable kgdb, kprobes, oprofile, magic sysreq, debugfs If this is difficult for you, you probably are in the wrong class..