1 Networking Acronym Smorgasbord: 802.11, DVMRP, CBT, WFQ EE122 Fall 2011 Scott Shenker ee122/...
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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 Paxsonand other colleagues at Princeton and UC Berkeley
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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
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Today’s Lecture: Dim Sum of Design
• Wireless review
• Multicast
• Packet Scheduling
• Peer-to-peer
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Wireless Review
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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
o Shut 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
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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.
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Multicast
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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
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Unicast approach does not scale…
BackboneISP
BroadcastCenter
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Instead build data replication trees
BackboneISP
BroadcastCenter
• 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
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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
R0 joins GR1
joins G
Rn joins G
S
R0
R1
.
.
.
[G, data]
[G, data]
[G, data]
[G, data]
Rn
Net
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Multicast and Layering
• Multicast can be implemented at different layers– link layer
o e.g. Ethernet multicast
– network layero e.g. IP multicast
– application layero e.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]
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Multicast Implementation Issues
• How is join implemented?
• How is send implemented?
• How much state is kept and who keeps it?
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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 LAN14
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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– 228 addresses for entire Internet
• Open group membership– Anyone can join (sends IGMP message)
o Internet Group Management Protocol
– Privacy preserved at application layer (encryption)
• Anyone can send to group– Even nonmembers
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How Would YOU Design this?
• 5 Minutes….
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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
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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)
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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:1
s:3
s:2
s:3
r
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Other Problems• Flooding can cause a given packet to be sent
multiple times over the same link
• Solution: Reverse Path Broadcasting
x y
z
S
a
b
duplicate packet
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Reverse Path Broadcasting (RPB)
x y
z
S
a
b
5 6
child link of xfor S
forward onlyto 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
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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
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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
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Distance Vector Multicast Scaling
• State requirements: – O(Sources Groups) active state
• How to get better scaling?– Hierarchical Multicast– Core-based Trees
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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)
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Use Shared Tree for Delivery
• Group members: M1, M2, M3• M1 sends data root
M1
M2 M3
control (join) messagesdata
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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
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Packet Scheduling
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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
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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
o E.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
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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
o High-priority packet arrives during transmission of low-priorityo Router completes sending the low-priority traffic first
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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
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Max-Min Fairness
• Given a set of bandwidth demands ri and a total bandwidth C, the max-min bandwidth allocations are:
ai = min(f, ri)
• where f is the unique value such that Sum(ai) = C
• Property:– If you don’t get full demand, no one gets more than you
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Computing Max-Min Fairness
• Denote– C – link capacity– N – number of flows– ri – arrival rate
• Max-min fair rate computation:1. compute C/N (= the remaining fair share)2. if there are flows i such that ri ≤ C/N
then update C and N
and go to 1
3. if not, f = C/N; terminate
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Example
• C = 10; r1 = 8, r2 = 6, r3 = 2; N = 3
• C/3 = 3.33 – Can service all of r3
– Remove r3 from the accounting: C = C – r3 = 8; N = 2
• C/2 = 4 – Can’t service all of r1 or r2
– So hold them to the remaining fair share: f = 4
8
6
244
2
f = 4: min(8, 4) = 4 min(6, 4) = 4 min(2, 4) = 2
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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
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Example
1 2 3 4 5
1 2 3 4
1 23
1 24
3 45
5 6
1 2 1 3 2 3 4 4
5 6
55 6
Flow 1(arrival traffic)
Flow 2(arrival traffic)
Servicein fluid flow
system
Packetsystem
time
time
time
time
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Fair Queuing (FQ)
• Provides isolation: – Misbehaving flow can’t impair others– Could change congestion control paradigm
o But 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)
o Allocations to aggregates