CS 268: IP Multicast Routing

Transcription

1 Motivtion CS 268: IP Multicst Routing Ion Stoic April 5, 2004 Mny pplictions requires one-to-mny communiction - E.g., video/udio conferencing, news dissemintion, file updtes, etc. Using unicst to replicte pckets not efficient thus, IP multicst needed - Wht bout the e2e rguments? istoic@cs.berkeley.edu 2 Open group semntic Semntic - A group is identified by loction-independent ddress - Any source (not necessry in the group) cn multicst to ll members in group Advntges: - Query n object/service when its loction is not known Disdvntge - Difficult to protect ginst unuthorized listeners Multicst delivery widely vilble on individul LANs - Exmple: Ethernet multicst But not cross interconnection of LANs istoic@cs.berkeley.edu 3 istoic@cs.berkeley.edu 4 Three Approches [Deering & Cheriton 89] Single spnning-tree (T) Distnce-vector multicst (DVM) Link-stte multicst (LSM) Also: Sketches hierrchicl multicst Multicst Service Model Built round the notion of group of hosts: - Senders nd receivers need not know bout ech other Sender simply sends pckets to logicl group ddress No restriction on number or loction of receivers - Applictions my impose limits Norml, best-effort delivery semntics of IP istoic@cs.berkeley.edu 5 istoic@cs.berkeley.edu 6 1

2 Multicst Service Model (cont d) Dynmic membership - Hosts cn join/leve t will No synchroniztion or negotition - Cn be implemented higher lyer if desired Key Design Gols 1. Delivery efficiency s good s unicst 2. Low join ltency 3. Low leve ltency istoic@cs.berkeley.edu 7 istoic@cs.berkeley.edu 8 Network Model Distnce Vector Multicst Routing Interconnected LANs LANs support link-level multicst Mp globlly unique multicst ddress to LAN-bsed multicst ddress (LAN-specific lgorithm) An elegnt extension to DV routing Use shortest pth DV routes to determine if link is on the source-rooted spnning tree istoic@cs.berkeley.edu 9 istoic@cs.berkeley.edu 10 Reverse Pth Flooding (RPF) A router forwrds brodcst pcket from source (S) iff it rrives vi the shortest pth from the router bck to S Pcket is replicted out ll but the incoming interfce Reverse shortest pths esy to compute just use info in DV routing tbles - DV gives shortest reverse pths - Works if costs re symmetric Forwrd pckets tht rrives tt on shortest pth from t to S (ssume symmetric routes) istoic@cs.berkeley.edu 11 Flooding cn cuse given pcket to be sent multiple times over the sme link duplicte pcket b Solution: Reverse Pth Brodcsting istoic@cs.berkeley.edu 12 2

3 Reverse Pth Brodcsting (RPB) Bsic ide: forwrd pcket from S only on child links for S Child link of router R for source S: link tht hs R s prent on the shortest pth from the link to S Identify Child Links Routing updtes identify prent Since distnces re known, ech router cn esily figure out if it's the prent for given link In cse of tie, lower ddress wins 5 6 forwrd only to child link child link of x for S b istoic@cs.berkeley.edu 13 istoic@cs.berkeley.edu 14 Truncted RBP This is still brodcst lgorithm the trffic goes everywhere First order solution: Truncted RPB Don't forwrd trffic onto network with no receivers 1. Identify leves 2. Detect group membership in lef istoic@cs.berkeley.edu 15 istoic@cs.berkeley.edu 16 Reverse Pth Multicst (RPM) Bsic RPM Ide Prune bck trnsmission so tht only bsolutely necessry links crry trffic Use on-demnd pruning so tht router group stte scles with number of ctive groups (not ll groups) Prune (Source,Group) t lef if no members - Send Non-Membership Report (NMR) up tree If ll children of router R prune (S,G) - Propgte prune for (S,G) to prent R On timeout: - Prune dropped - Flow is reinstted - Down strem routers re-prune Note: gin soft-stte pproch istoic@cs.berkeley.edu 17 istoic@cs.berkeley.edu 18 3

4 Detils RMP Scling How to pick prune timers? - Too long lrge join time - Too short high control overhed Wht do you do when member of group (re)joins? - Issue prune-cncelltion messge (grfts) Both NRM nd grft messges re positively cknowledged (why?) Stte requirements: - O(Sources Groups) ctive stte How to get better scling? - Hierrchicl Multicst - Core-bsed Trees istoic@cs.berkeley.edu 19 istoic@cs.berkeley.edu 20 Core Bsed Trees (CBT) Exmple Bllrdie, Frncis, nd Crowcroft, - Core Bsed Trees (CBT): An Architecture for Sclble Inter- Domin Multicst Routing, SIGCOMM 93 Similr to Deering s Single-Spnning Tree Unicst pcket to core nd bounce it bck to multicst group Tree construction is receiver-bsed - One tree per group - Only nodes on tree involved Reduce routing tble stte from O(S x G) to O(G) Group members: M1, M2, M3 M1 sends dt M1 control (join) messges dt M2 root M3 istoic@cs.berkeley.edu 21 istoic@cs.berkeley.edu 22 Disdvntges IP Multicst Revisited Sub-optiml dely Smll, locl groups with non-locl core - Need good core selection - Optiml choice (computing topologicl center) is NP complete Despite mny yers of reserch nd mny compelling pplictions, nd despite the fct tht the mny of tody routers implement IP multicst, this is still not widely deployed Why? istoic@cs.berkeley.edu 23 istoic@cs.berkeley.edu 24 4

5 Possible Explntions [Holbrook & Cheriton 99] Solution: EXPRE Violtes ISP input-rte-bsed billing model - No incentive for ISPs to enble multicst! No indiction of group size (gin needed for billing) Hrd to implement sender control ny node cn send to the group (remember open group semntic?) Multicst ddress scrcity Limit to single source group Use both source nd destintion IP fields to define group - Ech source cn llocte 16 millions chnnels (i.e., multicst groups) Use RPM lgorithm Add counting mechnism - Use recursive CountQuery messge Use session rely pproch to implement multiple source multicst trees istoic@cs.berkeley.edu 25 istoic@cs.berkeley.edu 26 Summry Deering s DV-RMP n elegnt extension of DV routing CBT ddresses some of the DV-RMP sclbility concerns but is sub-optiml nd less robust Protocol Independent Multicst (PIM) - Sprse mode similr to CBT - Dense mode similr to DV-RPM Lesson: economic incentives plys mjor role in deploying technicl solution - See EXPRE work istoic@cs.berkeley.edu 27 5