Announcements Pending Questions Homework 3 out due Nov 6 Note due date a week later than originally announced Homework 4 to be out Nov 6 tentatively due Nov 27 Additional office hours Midterm Review Sukun this week Friday 4 5PM Me next week Monday 1 30 3 30PM EE 122 Intro to Communication Networks Physical layer design issues EE 121 Spring 07 Fall 2006 MW 4 5 30 in Donner 155 Identifier for electing a Spanning Tree root Vern Paxson Switch s MAC address configurable priority knobs thanks to Gene Zhang Wikipedia Mike Bennett TAs Dilip Antony Joseph and Sukun Kim http inst eecs berkeley edu ee122 How does switch know when spanning tree done Materials with thanks to Jennifer Rexford Ion Stoica and colleagues at Princeton and UC Berkeley It doesn t it just uses what it currently has 1 Robust Spanning Tree Algorithm 2 Moving From Switches to Routers Algorithm must react to failures Failure of the root node Advantages of switches over routers Need to elect a new root with the next lowest identifier Failure of other switches and links Plug and play Fast filtering and forwarding of frames Need to recompute the spanning tree Root switch continues sending messages Periodically reannouncing itself as the root 1 0 1 Other switches continue forwarding messages Disadvantages of switches over routers Topology restricted to a spanning tree Large networks require large ARP tables Broadcast storms can cause the network to collapse Can t accommodate non Ethernet segments why not Detecting failures through timeout soft state Switch waits to hear from others Eventually times out and claims to be the root See Section 3 2 2 in the textbook for details and another example 3 Midterm Review Comparing Hubs Switches Routers hubs traffic isolation plug play optimized routing cut through switches 4 In class next Monday routers no yes yes yes yes no no no yes yes yes no Closed book You can have one regular sized 8 5 x11 sheet of paper with notes on both sides No PDAs calculators electronic Internet gadgets smart cell phones jeweler s loupes etc No Blue Books all answers on exam sheets Ensure legibility pencil eraser 5 6 1 Taxonomy of Communication Networks Fundamental Challenges for Networking Speed of light Communication networks can be classified based on the way in which the nodes exchange information Desiring a pervasive global network Co m m u n ic a t io n Ne t wo rk Need for it to work efficiently cheaply Failure of components Swit c h e d Co m m u n ic a t io n Ne t wo rk Enormous dynamic range Bro a d c a s t Co m m u n ic a t io n Ne t wo rk no such thing as typical Disparate parties must work together Circ u it Swit c h e d Co m m u n ic a t io n Ne t wo rk Rapid growth evolution Pa c ke t Swit c h e d Co m m u n ic a t io n Ne t wo rk Virt u a l Circ u it Ne t wo rk Da t a g ra m Ne t wo rk Crooks other bad guys 7 8 Timing in Circuit Switching Circuit Switching e g Phone Network Establish source creates circuit to destination Host 1 Node along the path store connection info Nodes generally reserve resources for the connection If circuit not available Busy signal Switch 1 Host 2 Switch 2 Transmission delay Transfer source sends data over the circuit propagation delay between Host 1 and Switch1 Circuit Establishment No destination address since nodes know path propagation delay between Host 1 and Host 2 Teardown source tears down circuit when done incoming links Node outgoing links Transfer Information time Circuit Teardown 9 10 Timing of Datagram Packet Switching Time Division Multiplexing Demultiplexing Host 1 Node 1 Host 2 Node 2 Frames Slots 0 1 2 3 4 5 0 1 2 3 4 5 Time divided into frames frames into slots Relative slot position inside a frame determines to which conversation data belongs transmission time of Packet 1 at Host 1 E g slot 0 belongs to orange conversation Requires synchronization between sender and receiver surprisingly difficult In case of non permanent conversations Packet 2 Packet 3 Packet 1 processing delay of Packet 1 at Node 2 Packet 2 Packet 3 Packet 1 Packet 2 Need to dynamically bind a slot to a conversation How to do this Packet 1 propagation delay between Host 1 and Node 1 Packet 3 If a conversation does not use its circuit the capacity is lost 11 12 2 Protocol Standardization Packet Switching vs Circuit Switching Ensure communicating hosts speak the same protocol Critical advantage of packet switching over circuit switching Exploitation of statistical multiplexing Standardization to enable multiple implementations Or the same folks have to write all the software Another since routers don t know about individual conversations when a router or link fails it s easy to fail over to a different path Standardization Internet Engineering Task Force Based on working groups that focus on specific issues Produces Request For Comments RFCs A third easier for different parties to link their networks together because they re not promising to reserve resources for one another Promoted to standards via rough consensus and running code IETF Web site is http www ietf org RFCs archived at http www rfc editor org per Homework 1 However packet switching must handle congestion More complex routers Harder to provide good network services e g delay and bandwidth guarantees In practice sometimes combined e g IP over SONET De facto standards same folks writing the code 13 Layering A Modular Approach 14 Drawbacks of Layering Layer N may duplicate lower level functionality Paritition the system E g error recovery to retransmit lost data Each layer solely relies on services from layer below Each layer solely exports services to layer above Layers may need same information E g timestamps maximum transmission unit size Interface between layers defines interaction Strict adherence to layering may hurt performance Hides implementation details Layers can change without disturbing other layers E g hiding details about what is really going on Some layers are not always cleanly separated Application Inter layer dependencies for performance reasons Some dependencies in standards header checksums Application to application channels Host to host connectivity Link hardware P2P file sharing Skype your protocol here Headers start to get really big Sometimes header bytes actual content 15 Layer Violations 16 Layer Encapsulation Sometimes the gains from not respecting layer boundaries too great to resist Can occur with higher layer entity inspecting lowerlayer information E g TCP over wireless system that
View Full Document
Unlocking...