Outline: Xinu architecture

• Xinu overview
• TCP/IP layers
• overall data flow
• network interface
• IP thread
• TCP threads:
  • input
  • output
  • timer
• application thread
• Xinu source code at http://www.ics.hawaii.edu/~esb/1999spring.ics651/xinu

Xinu Overview

• simple operating system
• Xinu Is Not Unix:
  • publicly available source code (in the '80s)
  • threads, not processes
  • multi-threaded OS, including IPC primitives
  • different system calls
  • simpler, less powerful
  • optimized for speed, small size
• educational focus: everything well documented, explained, clear
• simplest method to get the job done
• Comer at Purdue

Xinu Networking

• Full TCP/IP stack, including:
  • Ethernet
  • ARP, RARP
  • IP, ICMP, IP multicast and IGMP
  • RIP, OSPF
  • TCP, UDP
  • SNMP
• multithreaded implementation
• layered implementation

TCP/IP layers

• device layer (ethernet driver)
• IP layer (including ICMP and most UDP processing)
• TCP layer
• application layer

Data Flow

• Comer+Stevens book, Figure 2.6 (page 23)
• Comer+Stevens book, Figure 2.7 (page 24)

Network Interface

• struct netif
• name
• state: up, down, testing
• IP, network number, subnetwork number, mask, broadcast address
• MTU
• ARP hardware type, hardware address length, hardware address, hardware broadcast address
• device descriptor
• IP input queue
• device output queue
• statistics

IP thread

• basic algorithm: loop forever
  1. read a packet
  2. do input processing (checksum checking if needed)
  3. route (or discard) the packet
  4. forward the packet on the correct interface.
• why this works:
  • local interface to upper levels
  • queues are used on each interface
  • if there are no packets, IP sleeps
  • any function that adds a packet to a queue wakes up IP (no-op if IP is awake)
  • fairness: IP reads the queues in round-robin order

ICMP, UDP

• ICMP is part of IP
  • icmpin called by netwrite, handles incoming ICMP packets
  • ipproc calls icmp to send ICMP messages such as destination unreachable, time exceeded, or redirect
• udpin is also called directly by netwrite, runs as part of the IP thread
• udpsend calls ipsend which enqueues the packet on the local network interface and wakes up the IP thread

TCP input thread

• Input thread, tcpinp, loops forever:
  1. read packets from the TCP input queue
  2. checksum the TCP packet
  3. find a TCB
  4. process TCP options, if any
  5. call a TCB-state-dependent procedure: tcpswitch[ptcb->tcbstate] (ptcb, pep);

TCP output thread

• output thread
• output state machine has 4 states:
  • idle
  • transmit
  • retransmit
  • persist (wait for window to become non-zero)
• output state machine handles 4 messages:
  • send this packet
  • persist (send probe packet)
  • retransmit queued packet
  • delete a TCB
  tcpout loops forever

TCP timer thread

• delta list:
  • events are stored in a list in order of increasing deadline
  • delay for each event is from expiration of previous event
  • only the first element of the list ever needs to be checked -- O(1)
  • insertion of a new event may traverse entire list -- O(n)
• timer thread tcptimer loops forever:
  1. sleep 0.1s
  2. if delta list is empty, suspend
  3. for each expired event (since last time we executed), send a message to a Xinu port (usually to TCP output thread)