1. ENGS 116 Lecture 201 Interconnection Networks Cont’d Vincent H. Berk November 16, 2005 Reading for Friday: 8.1-8.7 Reading for Monday: 8.8-8.13 Homework for Friday 18th: 5.4, 5.17, 6.4, 6.10, 7.3, 7.21, 8.9/8.10, 8.17

2. ENGS 116 Lecture 202 Connection-Based vs. Connectionless Telephone: operator sets up connection between the caller and the receiver –Once the connection is established, conversation can continue for hours Share transmission lines over long distances by using switches to multiplex several conversations on the same lines –“Time division multiplexing” divide B/W transmission line into a fixed number of slots, with each slot assigned to a conversation Problem: lines busy based on number of conversations, not amount of information sent Advantage: reserved bandwidth

3. ENGS 116 Lecture 203 Connection-Based vs. Connectionless Connectionless: every package of information must have an address  packets –Each package is routed to its destination by looking at its address –Analogy, the postal system (sending a letter) –Also called “Statistical multiplexing” Each packet requires a new/separate routing decision Depending on implementation the switching stations may also be called routers.

4. ENGS 116 Lecture 204 Routing Messages Within a network: –Shared media: Broadcast to everyone Internetwork routing. Options: –Source-based routing: message specifies path to the destination (changes of direction) –Virtual circuit: circuit established from source to destination, message picks the circuit to follow –Destination-based routing: message specifies destination, switch must pick the path: deterministic vs. non-deterministic deterministic: always follow same path adaptive: pick different paths to avoid congestion, failures randomized routing: pick between several good paths to balance network load

5. ENGS 116 Lecture 205 mesh: dimension-order routing –(x 1, y 1 )  (x 2, y 2 ) –first  x = x 2 – x 1, –then  y = y 2 – y 1, hypercube: edge-cube routing –X = x o x 1 x 2... x n  Y = y o y 1 y 2... y n –R = X xor Y –Traverse dimensions of differing address in order tree: common ancestor Deadlock free? Deterministic Routing Examples 001 000 101 100 010 110 111 011

6. ENGS 116 Lecture 206 Store and Forward vs. Cut-Through Store-and-forward policy: each switch waits for the full packet to arrive in switch before sending to the next switch (good for WAN) Cut-through routing or wormhole routing: switch examines the header, decides where to send the message, and then starts forwarding it immediately –In wormhole routing, when head of message is blocked, message stays strung out over the network, potentially blocking other messages (needs only buffer the piece of the packet that is sent between switches). CM-5 uses it, with each switch buffer being 4 bits per port. –Cut-through routing lets the tail continue when head is blocked, “accordioning” the whole message into a single switch. (Requires a buffer large enough to hold the largest packet).

7. ENGS 116 Lecture 207 Congestion Control Packet switched networks do not reserve bandwidth; this leads to contention (connection-based limits input) Solution: prevent packets from entering until contention is reduced (e.g., freeway on-ramp metering lights) Options: –Packet discarding: If packet arrives at switch and no room in buffer, packet is discarded (e.g., UDP) –Flow control: between pairs of receivers and senders; use feedback to tell sender when allowed to send next packet Back-pressure: separate wires to tell to stop Window: give original sender right to send N packets before getting permission to send more; overlaps latency of interconnection with overhead to send & receive packet (e.g., TCP), adjustable window –Choke packets: aka “rate-based”; each packet received by busy switch in warning state sent back to the source via choke packet. Source reduces traffic to that destination by a fixed % (e.g., ATM, ICMP source quench)

8. ENGS 116 Lecture 208 Practical Issues for Interconnection Networks Standardization advantages: –low cost (components used repeatedly) –stability (many suppliers to chose from) Standardization disadvantages: –Time for committees to agree –When to standardize? Before anything built?  Committee does design? Too early suppresses innovation Perfect interconnect vs. Fault Tolerant? –Will SW crash on single node prevent communication? (MPP typically assumes perfect) Reliability (vs. availability) of interconnect Most successful system is not always the best design.

9. ENGS 116 Lecture 209 Practical Issues InterconnectionMPPLANWAN ExampleCM-5EthernetATM StandardNoYesYes Fault Tolerance?NoYesYes Hot Insert?NoYesYes Standards: required for WAN, LAN! Fault Tolerance: Can nodes fail and still deliver messages to other nodes? Required for WAN, LAN! Hot Insert: If the interconnection can survive a failure, can it also continue operation while a new node is added to the interconnection? Required for WAN, LAN!

10. ENGS 116 Lecture 2010 Inter-Network-Routing Connecting >2 networks together. Requires: –Addressing Hierarchy –Common Protocols –Courtesy and Security Each step in a route (hop) decides: –What first? –Where next? Transparent or explicit

11. ENGS 116 Lecture 2011 Bridging (transparent routing)

12. ENGS 116 Lecture 2012 OSI model This one has to be in every network presentation 7. ApplicationWeb browser 6. PresentationNetwork library interface 5. SessionTCP 4. TransportIP 3. NetworkPacket 2. Data LinkEthernet Frame 1. PhysicalElectrical signals

13. ENGS 116 Lecture 2013 Networking Protocols: HW/SW Interface Internetworking: allows computers on independent and incompatible networks to communicate reliably and efficiently; –Enabling technologies: SW standards that allow reliable communications without reliable networks –Hierarchy of SW layers, giving each layer responsibility for portion of overall communications task, called protocol families or protocol suites Transmission Control Protocol/Internet Protocol (TCP/IP) –This protocol family is the basis of the Internet –IP makes best effort to deliver; TCP “guarantees” delivery –TCP/IP used even when communicating locally: NFS uses IP even though communicating across homogeneous LAN

14. ENGS 116 Lecture 2014 Protocol Key to protocol families is that communication occurs logically at the same level of the protocol, called peer-to-peer, but is implemented via services at the lower level Danger is each level increases latency if implemented as hierarchy (e.g., multiple check sums) Logical T H T H T H T H T H T H Actual T H THT H THT H THT H TH T T T T HH Message

15. ENGS 116 Lecture 2015 IP, TCP, and UDP IP = internet protocol, used at network layer –IP routes datagrams to destination machine, makes best effort to deliver packets but does not guarantee delivery or order of datagrams –For IP, every host and router must have unique IP address IPv4 uses 32-bit addresses IPv6 uses 16-byte addresses (not that straight forward, though!!!) TCP = transmission control protocol, used at transport layer –TCP is connection-oriented, makes guarantee of reliable, in-order delivery –Up to 4 retries on failure to deliver (or acknowledge!) UDP = user data protocol, used at transport layer –Connectionless protocol, makes no guarantees of delivery

16. ENGS 116 Lecture 2016 Packet Formats Fields: Destination, Checksum (C), Length (L), Type (T) Data/Header Sizes in bytes: (4 to 20)/4, (0 to 1500)/26, 48/5 L T Route Data (4 - 20) C 32 bits CM-5ATM Destination 32 bits C Data (48) T 32 bits Preamble Source Destination Data (0 - 1500) Pad (0 -46) Checksum Length Destination Source Ethernet

17. ENGS 116 Lecture 2017 Networking Summary Protocols allow heterogeneous networking Protocols allow operation in the presence of failures Routing issues: store and forward vs. cut-through, congestion,... Standardization key for LAN, WAN Internetworking protocols used as LAN protocols  large overhead for LAN Integrated circuit revolutionizing networks as well as processors Switch is a specialized computer

18. ENGS 116 Lecture 2018 Cluster (Multicomputer) A collection of low-cost nodes connected by a fast network. Applications: –Less synchronization required than for MP applications –Less need for communication –No need for one large homogeneous memory –Many copies of one application run in parallel Each node: –cheap –redundant Easily expandable –Scales if the software application scales

19. ENGS 116 Lecture 2019 Possible Applications Distributed Database –Each node works as the query engine for data on local disk(s) –All nodes together implement redundancy: Failure of 1 or more nodes doesn’t damage the database Scientific applications: –Nuclear or Oceanographic simulations –Diskless nodes. Each node uses NFS (of similar SAN-based system) to access central data repository. –Applications are started over the network. –Think SETI@home

20. ENGS 116 Lecture 2020 Book Example: google cluster 1000’s of cheap PC’s with a very fast network connection. Most of bandwidth is used in keeping database updated. How are queries handled? Distributed? How is their database constructed? What is the algorithm used? Paper: “The Anatomy of a Large-Scale Hypertextual Web Search Engine” by Sergey Brin and Lawrence Page