EE 122: Switching and Forwarding

Transcription

1 Direct Lk Network Review EE : Switchg and Forwardg Kev Lai September, Data lk layer presents a sgle media (eg, sgle wire) network model Problem and solutions - Framg character stuffg, byte countg, bit stuffg, clocked framg - Error detection parity, checksum, CRC - Reliability stop and go, slidg wdow - solutions also apply to similar problems higher layers problems can not be completely solved at data lk layer only implemented data lk layer as optimization laik@csberkeleyedu Limitations of Direct Lk Networks Direct Lk Networks vs Switchg distance - distance creases propagation delay - large propagation delay causes large coordation delay - eg, collision detection requires *prop_delay number of hosts - More hosts creases the probability of collisions - collisions decrease efficiency of lk bandwidth - bandwidth of lk is shared among all connected nodes sgle media type - different media (eg, fiber,, wireless) have different tradeoffs for performance, cost, etc Direct Lk Network Sgle lk Switch n lks Switched Network Emulates clique laik@csberkeleyedu laik@csberkeleyedu Defitions Properties switch (aka bridge) - does switchg - operates at data lk layer - rer also does switchg, but at network layer switchg consists of - forwardg read data from put lks, decide which put lk to forward on, and exame packet header or comg circuit, and look up forwardg table transmit it on one of the put lks (unicast) - rg how the switch/rer builds up its forwardg table spans larger physical area than direct lk network (DLN) - can connect multiple switches together supports more hosts than DLN - hosts on separate lks can transmit at same time higher aggregate bandwidth than DLN - approaches (n/)*b stead of b, n = number of switched lks, b = bandwidth of one lk supports more than one media type - more expensive for bridge than rer laik@csberkeleyedu laik@csberkeleyedu 6

2 Bridge/Rer Comparison Forwardg Techniques 8b Rer terconnects different lk layer protocols more easily Switch Rer E-to-E E-to-8 E-to-A E-to-S 8b O(n ) converters n = different lk types 8b E-to-IP 8-to-IP A-to-IP S-to-IP IP-to-E IP-to-8 IP-to-A IP-to-S O(n) converters 8b packet switchg - aka [packet datagram connectionless] [switchg forwardg] source rg virtual circuit switchg - aka virtual circuit forwardg circuit switchg despite names, all ways for switch to decide which put port to forward data laik@csberkeleyedu laik@csberkeleyedu 8 Packet Switchg Statistical Multiplexg vs Resource Reservations Data is separated to packets Each packet is forwarded dependently of previous packets - packets between two hosts can fol different paths On lk failure, adjog switches select new re and contue forwardg packets Statistical multiplexg - any one host may use % of a lk s bandwidth Statistical Multiplexg Resource Reservations Advantage Mb/s Mb/s S H9 Mb/s Mb/s Mb/s S Mb/s H9 Mb/s / Mb/s Mb/s / Mb/s Problem Mb/s Mb/s Mb/s S Mb/s H9 Mb/s Mb/s Mb/s S Mb/s H9 Reserve explicit amount of resources (eg, bandwidth) - get exactly that amount Statistical multiplexg: get whatever is available Mb/s / Mb/s congestion, packet loss utilization Mb/s / Mb/s laik@csberkeleyedu 9 laik@csberkeleyedu Packet Switchg Operation Packet Switchg Properties Each switch matas a forwardg table - forwardg entry: (address, put port) Upon packet arrival - put port forwards the packet to the put port whose address matches packet s destation address 8 exact match 86 longest prefix match - forwardg entry: (address prefix, put port) - forward packet to the put port whose address matches packet s destation address the longest number of bits 86 xxxxxxxxx 8xxxxxx Expensive forwardg - forwardg table size depends on number of different destations - must lookup forwardg table for every packet Robust - lk and rer failure may be transparent for end-hosts High bandwidth utilization - statistical multiplexg No service guarantees - Network als hosts to send more packets than available bandwidth congestion dropped packets laik@csberkeleyedu laik@csberkeleyedu

3 Source Rg Source Rg (cont d) source Each packet specifies the sequence of rers, or alternatively the sequence of put ports, from source to destation Gives the source control of the path Not scalable - Packet overhead proportional to the number of rers - Typically, require variable header length which is harder to implement Hard for source to have complete formation Loose source rg sender specifies only a subset of rers along the path laik@csberkeleyedu laik@csberkeleyedu Virtual Circuit (VC) Switchg VC Forwardg: Example Packets not switched dependently - establish virtual circuit before sendg data Forwardg table entry - (put port, put VCI, put port, put VCI) - VCI Virtual Circuit Identifier Each packet carries a VCI its header Upon a packet arrival at terface i - Input port uses i and the packet s VCI v to fd the rg entry (i, v, i, v ) - Replaces v with v the packet header - Forwards packet to put port i source -VCI -VCI -VCI -VCI -VCI -VCI destation laik@csberkeleyedu laik@csberkeleyedu 6 VC Forwardg (cont d) Virtual Circuit Switchg Properties A signalg protocol is required to set up the state for each VC the rg table - A source needs to wait for one RTT (round trip time) before sendg the first data packet Can provide per-vc QoS - When we set the VC, we can also reserve bandwidth and buffer resources along the path Less expensive forwardg - forwardg table size depends on number of different circuits - must lookup forwardg table for every packet Much higher delay for short fs - RTT delay for connection setup Less Robust - end host must spend RTT to establish new connection after lk and rer failure Flexible service guarantees - either statistical multiplexg or resource reservations laik@csberkeleyedu laik@csberkeleyedu 8

4 Circuit Switchg Circuit Switchg Properties Packets not switched dependently - establish circuit before sendg data Circuit is a dedicated path from source to destation - eg, old style telephone switchboard, where establishg circuit means connectg wires all the switches along path - eg, modern dense wave division multiplexg (DWDM) form of optical networkg, where establishg circuit means reservg an optical wavelength all switches along path No forwardg table Cheap forwardg - no table lookup Much higher delay for short fs - RTT delay for connection setup Less robust - end host must spend RTT to establish new connection after lk and rer failure Must use resource reservations laik@csberkeleyedu 9 laik@csberkeleyedu Forwardg Comparison Rg forwardg cost bandwidth utilization pure packet switchg high high resource none reservations robustness high virtual circuit switchg flexible flexible circuit switchg none yes Update forwardg/rg tables Manual configuration - simple, error prone, work for admistrator Learng bridges - all that is needed for sgle bridge Spanng Tree - necessary for multiple bridges Described ternetworkg section - ance Vector - Lk State laik@csberkeleyedu laik@csberkeleyedu Learng Bridges Learng Bridge Problem A H H H AC BA C A A B A B H B H H H B H B H H H B H laik@csberkeleyedu laik@csberkeleyedu

5 Spanng Tree H Summary B - H H B B - H B As if I am B Get I am I m root Uh, no B/ is B/ is root I m root B/ is root B H B/ is root H Cha B/ is B B B B B B B/ is root laik@csberkeleyedu Switchg - overcome limitations of direct lk networks Forwardg techniques - packet switchg - source rg - virtual circuit switchg - circuit switchg Rg techniques laik@csberkeleyedu 6