CS162 Operating Systems and Systems Programming Lecture 23 Networking III November 19 2008 Prof John Kubiatowicz http inst eecs berkeley edu cs162 Review Network Protocols Protocol Agreement between two parties as to how information is to be transmitted Example system calls are the protocol between the operating system and application Networking examples many levels Physical level mechanical and electrical network e g how are 0 and 1 represented Link level packet formats error control for instance the CSMA CD protocol Network level network routing addressing Transport Level reliable message delivery Protocols on today s Internet NFS Transport RPC UDP Network Physical Link 11 17 08 WWW e mail ssh TCP IP Ethernet ATM Packet radio Kubiatowicz CS162 UCB Fall 2008 Lec 22 2 Goals for Today Networking Continue discussion of reliable messaging Sequence numbers for ordering Acknowledgments for reliability TCP windowing Sockets Messages Send receive One vs two way communication Note Some slides and or pictures in the following are adapted from slides 2005 Silberschatz Galvin and 11 17 08 Kubiatowicz CS162 UCB Fall 2008 Lec 22 3 Gagne Many slides Gagne generated from my lecture notes Performance Considerations Before we continue need some performance metrics Overhead CPU time to put packet on wire Throughput Maximum number of bytes per second Depends on wire speed but also limited by slowest router routing delay or by congestion at routers Latency time until first bit of packet arrives at receiver Raw transfer time overhead at each routing hop Router LW1 LR1 Router LW2 LR2 Lw3 Contributions to Latency Wire latency depends on speed of light on wire about 1 1 5 ns foot Router latency depends on internals of router Could be 1 ms for a good router Question can router handle full wire throughput 11 17 08 Kubiatowicz CS162 UCB Fall 2008 Lec 22 4 Sample Computations E g Ethernet within Soda Latency speed of light in wire is 1 5ns foot which implies latency in building 1 s if no routers in path Throughput 10 1000Mb s Throughput delay packet doesn t arrive until all bits So 4KB 100Mb s 0 3 milliseconds same order as disk E g ATM within Soda Latency same as above assuming no routing Throughput 155Mb s Throughput delay 4KB 155Mb s 200 E g ATM cross country Latency assuming no routing 3000miles 5000ft mile 15 milliseconds How many bits could be in transit at same time 15ms 155Mb s 290KB In fact Berkeley MIT Latency 45ms 872KB in flight if routers have wire speed throughput Requirements for good performance Local area minimize overhead improve bandwidth Wide area keep pipeline full 11 17 08 Kubiatowicz CS162 UCB Fall 2008 Lec 22 5 Sequence Numbers Ordered Messages Several network services are best constructed by ordered messaging Ask remote machine to first do x then do y etc Unfortunately underlying network is packet based Packets are routed one at a time through the network Can take different paths or be delayed individually IP can reorder packets P0 P1 might arrive as P1 P0 Solution requires queuing at destination Need to hold onto packets to undo misordering Total degree of reordering impacts queue size Ordered messages on top of unordered ones Assign sequence numbers to packets 0 1 2 3 4 If packets arrive out of order reorder before delivering to user application For instance hold onto 3 until 2 arrives etc Sequence numbers are specific to particular connection Reordering among connections normally doesn t matter If restart connection need to make sure use different range of sequence numbers than previously 11 17 08 Kubiatowicz CS162 UCB Fall 2008 Lec 22 6 Reliable Message Delivery the Problem All physical networks can garble and or drop packets Physical media packet not transmitted received If transmit close to maximum rate get more throughput even if some packets get lost If transmit at lowest voltage such that error correction just starts correcting errors get best power bit Congestion no place to put incoming packet Point to point network insufficient queue at switch router Broadcast link two host try to use same link In any network insufficient buffer space at destination Rate mismatch what if sender send faster than receiver can process Reliable Message Delivery on top of Unreliable Packets Need some way to make sure that packets actually make it to receiver Every packet received at least once Every packet received at most once Can combine with ordering every packet received by process at destination exactly once and in order 11 17 08 Kubiatowicz CS162 UCB Fall 2008 Lec 22 7 Using Acknowledgements A Pack et ack B A Timeout Pack et B Pack et ack How to ensure transmission of packets Detect garbling at receiver via checksum discard if bad Receiver acknowledges by sending ack when packet received properly at destination Timeout at sender if no ack retransmit Some questions If the sender doesn t get an ack does that mean the receiver didn t get the original message No What if ack gets dropped Or if message gets delayed Sender doesn t get ack retransmits Receiver gets message twice acks each 11 17 08 Kubiatowicz CS162 UCB Fall 2008 Lec 22 8 How to deal with message duplication Solution put sequence number in message to identify re transmitted packets Receiver checks for duplicate s Discard if detected Requirements Sender keeps copy of unack ed messages Easy only need to buffer messages Receiver tracks possible duplicate messages Hard when ok to forget about received message Alternating bit protocol A Pkt Send one message at a time don t send 0 next message until ack received 0 k c Sender keeps last message receiver A tracks sequence of last message receivedPkt 1 Pros simple small overhead 1 k c Con Poor performance A Wire can hold multiple messages want to Pkt 0 fill up at wire latency throughput 0 k Ac Con doesn t work if network can delay or duplicate messages arbitrarily Kubiatowicz CS162 UCB Fall 2008 11 17 08 Lec 22 9 B Better messaging Window based acknowledgements Window based protocol TCP A p Send up to N packets without ack Each packet has sequence number Receiver acknowledges each packet Ack says received all packets up to sequence number X send more Acks serve dual purpose kt 0 pk t 4 0 k ac Reliability Confirming packet received Flow Control Receiver ready for packet Queue Allows pipelining of packets N 5 Window size N queue at destination B 4 k ac Remaining space in queue at receiver can be returned with ACK What if packet gets garbled dropped Sender will timeout waiting for ack
View Full Document
Unlocking...