1. 1 Networking Acronym Smorgasbord: 802.11, DVMRP, CBT, WFQ EE122 Fall 2011 Scott Shenker http://inst.eecs.berkeley.edu/~ee122/ Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxson and other colleagues at Princeton and UC Berkeley

2. Announcements Congratulations: You all got 100% on HW4 –Worksheet will provide practice This is last week of sections –See posting about additional office hours next week Next week will have office hours during class times –Will work through problems on work sheet –Be there or be square…. Wednesday’s Review: will figure something out…. 2

3. Today’s Lecture: Dim Sum of Design Wireless review Multicast Packet Scheduling Peer-to-peer 3

4. 4 Wireless Review

5. History MACA proposal: basis for RTS/CTS in lecture –Contention is at receiver, but CS detects sender! –Replace carrier sense with RTS/CTS MACAW paper: extended and altered approach –Implications of data ACKing –Introducing DS in exchange: RTS-CTS-DS-Data-ACK oShut up when hear DS or CTS –Other clever but unused extensions for fairness, etc. 802.11: uses carrier sense and RTS/CTS –RTS/CTS often turned off, just use carrier sense –When RTS/CTS turned on, shut up when hear either –RTS/CTS augments carrier sense 5

6. What Will Be on the Final? General awareness of wireless (lecture) Reasoning about a given protocol –If we used the following algorithm, what would happen? You are not expected to know which algorithm to use; we will tell you explicitly. 6

7. 7 Multicast

8. Motivating Example: Internet Radio Internet concert –More than 100,000 simultaneous online listeners –Could we do this with parallel unicast streams? Bandwidth usage –If each stream was 1Mbps, concert requires > 100Gbps Coordination –Hard to keep track of each listener as they come and go Multicast addresses both problems…. 8

9. 9 Unicast approach does not scale… Backbone ISP Broadcast Center

10. 10 Instead build data replication trees Backbone ISP Broadcast Center Copy data at routers At most one copy of a data packet per link LANs implement link layer multicast by broadcasting Routers keep track of groups in real-time Routers compute trees and forward packets along them

11. 11 Multicast Service Model Receivers join multicast group identified by a multicast address G Sender(s) send data to address G Network routes data to each of the receivers Note: multicast is both a delivery and a rendezvous mechanism –Senders don’t know list of receivers –For many purposes, the latter is more important than the former R 0 joins G R 1 joins G R n joins G S R0R0 R1R1...... [G, data] RnRn Net

12. 12 Multicast and Layering Multicast can be implemented at different layers –link layer oe.g. Ethernet multicast –network layer oe.g. IP multicast –application layer oe.g. End system multicast Each layer has advantages and disadvantages –Link: easy to implement, limited scope –IP: global scope, efficient, but hard to deploy –Application: less efficient, easier to deploy [not covered]

13. 13 Multicast Implementation Issues How is join implemented? How is send implemented? How much state is kept and who keeps it?

14. Link Layer Multicast Join group at multicast address G –NIC normally only listens for packets sent to unicast address A and broadcast address B –After being instructed to join group G, NIC also listens for packets sent to multicast address G Send to group G –Packet is flooded on all LAN segments, like broadcast Scalability: –State: Only host NICs keep state about who has joined –Bandwidth: Requires broadcast on all LAN segments Limitation: just over single LAN 14

15. 15 Network Layer (IP) Multicast Performs inter-network multicast routing –Relies on link layer multicast for intra-network routing Portion of IP address space reserved for multicast –2 28 addresses for entire Internet Open group membership –Anyone can join (sends IGMP message) oInternet Group Management Protocol –Privacy preserved at application layer (encryption) Anyone can send to group –Even nonmembers

16. How Would YOU Design this? 5 Minutes…. 16

17. 17 IP Multicast Routing Intra-domain (know the basics here) –Source Specific Tree: Distance Vector Multicast Routing Protocol (DVRMP) –Shared Tree: Core Based Tree (CBT) Inter-domain [not covered] –Protocol Independent Multicast –Single Source Multicast

18. 18 Distance Vector Multicast Routing Protocol Elegant extension to DV routing –Using reverse paths! Use shortest path DV routes to determine if link is on the source-rooted spanning tree –See whiteboard….. Three steps in developing DVRMP –Reverse Path Flooding –Reverse Path Broadcasting –Truncated Reverse Path Broadcasting (pruning)

19. 19 Reverse Path Flooding (RPF) If incoming link is shortest path to source Send on all links except incoming Otherwise, drop Issues: (fixed with RPB) Some links (LANs) may receive multiple copies Every link receives each multicast packet s:2 s s s:1 s:3 s:2 s:3 r r

20. 20 Other Problems Flooding can cause a given packet to be sent multiple times over the same link Solution: Reverse Path Broadcasting x x y y z z S S a b duplicate packet

21. 21 Reverse Path Broadcasting (RPB) x x y y z z S S a b 5 6 child link of x for S forward only to child link Parent of z on reverse path Choose single parent for each link along reverse shortest path to source Only parent forwards to child link Identifying parent links –Distance –Lower address as tie- breaker

22. Even after fixing this, not done This is still a broadcast algorithm – the traffic goes everywhere Need to “Prune” the tree when there are subtrees with no group members Networks know they have members based on IGMP messages Add the notion of “leaf” nodes in tree –They start the pruning process 22

23. 23 Pruning Details Prune (Source,Group) at leaf if no members –Send Non-Membership Report (NMR) up tree If all children of router R send NMR, prune (S,G) –Propagate prune for (S,G) to parent R On timeout: –Prune dropped –Flow is reinstated –Down stream routers re-prune Note: a soft-state approach

24. 24 Distance Vector Multicast Scaling State requirements: –O(Sources  Groups) active state How to get better scaling? –Hierarchical Multicast –Core-based Trees

25. Core-Based Trees (CBT) Pick “rendevouz point” for the group (called core) Build tree from all members to that core –Shared tree More scalable: –Reduces routing table state from O(S x G) to O(G) 25

26. 26 Use Shared Tree for Delivery Group members: M1, M2, M3 M1 sends data root M1 M2 M3 control (join) messages data

27. 27 Barriers to Multicast Hard to change IP –Multicast means changes to IP –Details of multicast were very hard to get right Not always consistent with ISP economic model –Charging done at edge, but single packet from edge can explode into millions of packets within network

28. 28 Packet Scheduling

29. 29 Scheduling Decide when and what packet to send on output link Classifier partitions incoming traffic into flows In some designs, each flow has their own FIFO queue 1 2 Scheduler flow 1 flow 2 flow n Classifier Buffer management

30. 30 Packet Scheduling: FIFO What if scheduler uses one first-in first-out queue? –Simple to implement –But everyone gets the same service Example: two kinds of traffic –Video conferencing needs low bandwidth and low delay oE.g., 1 Mbps and 100 msec delay –E-mail not sensitive to delay, but need bandwidth Cannot admit much e-mail traffic –Since it will interfere with the video conference traffic

31. 31 Packet Scheduling: Strict Priority Strict priority –Multiple levels of priority –Always transmit high-priority traffic, when present –.. and force the lower priority traffic to wait Isolation for the high-priority traffic –Almost like it has a dedicated link –Except for the (small) delay for packet transmission oHigh-priority packet arrives during transmission of low-priority oRouter completes sending the low-priority traffic first

32. 32 Scheduling: Weighted Fairness Limitations of strict priority –Lower priority queues may starve for long periods –… even if the high-priority traffic can afford to wait –Traffic still competes inside each priority queue Weighted fair scheduling –Assign each queue a fraction of the link bandwidth –Rotate across the queues on a small time scale –Send extra traffic from one queue if others are idle 50% red, 25% blue, 25% green

33. Max-Min Fairness Given a set of bandwidth demands r i and a total bandwidth C, the max-min bandwidth allocations are: a i = min(f, r i ) where f is the unique value such that Sum(a i ) = C Property: –If you don’t get full demand, no one gets more than you 33

34. 34 Computing Max-Min Fairness Denote –C – link capacity –N – number of flows –r i – arrival rate Max-min fair rate computation: 1.compute C/N (= the remaining fair share) 2.if there are flows i such that r i ≤ C/N then update C and N and go to 1 3.if not, f = C/N; terminate

35. 35 Example C = 10; r 1 = 8, r 2 = 6, r 3 = 2; N = 3 C/3 = 3.33  –Can service all of r 3 –Remove r 3 from the accounting: C = C – r 3 = 8; N = 2 C/2 = 4  –Can’t service all of r 1 or r 2 –So hold them to the remaining fair share: f = 4 8 6 2 4 4 2 f = 4 : min(8, 4) = 4 min(6, 4) = 4 min(2, 4) = 2 10

36. 36 Fair Queuing (FQ) Conceptually, computes when each bit in the queue should be transmitted to attain max-min fairness (a “fluid flow system” approach) Then serve packets in the order of the transmission time of their last bits Allocates bandwidth in a max-min fairly

37. 37 Example 12345 1234 12 3 12 4 34 5 56 12132344 56 556 Flow 1 (arrival traffic) Flow 2 (arrival traffic) Service in fluid flow system Packet system time

38. 38 Fair Queuing (FQ) Provides isolation: –Misbehaving flow can’t impair others –Could change congestion control paradigm oBut not used…. Doesn’t “solve” congestion by itself: –Still need to deal with individual queues filling up Generalized to Weighted Fair Queuing (WFQ) –Can give preferences to classes of flows –Used for quality of service (QoS) oAllocations to aggregates