Topology-Aware Overlay Networks for Group Communication

1
Topology-Aware Overlay Networksfor Group
Communication
Minseok Kwon and Sonia Fahmy Department of
Computer Sciences Purdue University kwonm,
fahmy_at_cs.purdue.edu http//www.cs.purdue.edu/fah
my/
2
Overlay Multicast
Switzerland
Russia
Austria
Germany
USA
Japan
Korea
3
End System Multicast

• Proposed by Y. Chu, S. Rao, S. Seshan and H.
  Zhang (CMU) in 2000.
• One of the early application-level multicast
  protocols
• Easy to deploy
• Self-organizing
• Target application small sparse groups
  (audio/video conferencing groups)

4
Challenges

• Efficiency
• How to reduce delay penalty (relative to unicast)
  and reduce number of duplicate packets (link
  stress)?
• Scalability
• How to reduce the amount of data maintained by
  each group member, and reduce routing information
  exchange?

5
Our Primary Focus

• Can we exploit underlying network topology data
  which routing protocols anyway establish
  (together with measurements) for constructing
  efficient low-overhead overlays for application
  level multicast?

6
Benefiting from overlapping shortest paths
Source
Shortest path for the relay node
Shortest path for the new member
Other group member (now serving as relay Node)

• Minimize the delay penalty and the number of
  duplicate packets.

New member
7
TAG (Topology Aware Grouping)

• Mostly single source or core-based multicast
• Overlay tree is organized based on path overlap
  information
• Delay (primary) and bandwidth (secondary) are
  considered as metrics
• Target application latency-based applications
  (limited bandwidth streaming applications,
  multi-player online game)

8
TAG Features

• Reduces delay penalty
• Reduces the number of duplicate packets
• Low space complexity a small amount of
  information (IP addresses and paths of only
  parent and children) is maintained at each group
  member
• Low average time complexity member join and
  leave require O((log n)2) and O(log n),
  respectively, where n is the total number of
  group members

9
Difficulties with TAG

• When to re-organize?
• Overhead at (or near) the sender/core if many
  members join/re-join at the same time
• Bandwidth is only considered as a secondary
  metric (as a loose constraint, and to break ties
  among equal delay paths)

10
TAG Definitions

• Path from A to B
• A sequence of routers comprising the shortest
  path from A to B
• Spath of A
• Path from the sender/core to A
• Length of path P
• The number of routers in the path P
• A ? B
• if spath of A is a prefix of spath of B

11
TAG Member Join
Root
Path Matching
Member1
New Member
Member2

• A new member finds the parent and children by
  recursively applying the path matching algorithm

12
TAG Complete Path Matching
New member
D8 (R1,R2,R4,R5)
D8 (R1,R2)
D8 (R1,R5)
Family Table
D8 ? D4, D8 ? D7
New member subscribes here
D4 and D7 are the children of new member
D8. New member subscribes here
D4 ? D8
D4 is the best candidate to Proceed with
13
TAG Member Join Example
Source
S
R0
R1
R2
R3
R4
14
TAG Member Leave Example
Source
S
R0
R5
R1
R2
R3
D4 is leaving.
R4
15
TAG Partial Path Matching

• Complete path matching does not consider
  available bandwidth
• Minus-k (or partial) path matching
• Node A can be the parent of node B if A has a
  common spath prefix of length (spath of A) k
  with B
• Example

S
R0
D1 (R0,R1,R3)
R1
R3
D1
R2
D4 (R0,R1,R2,R4)
R4
k1
D4
16
TAG Partial Path Matching

• Mitigates possibly high link stress and limited
  bandwidth near a constrained node
• When is partial path matching activated?
• Partial path matching is activated when the
  available bandwidth lt bwthresh (loose constraint)
• With partial matching, a new member examines
  several delay-based paths and selects the path
  which maximizes bandwidth (tie breaker)
• k may be dynamically increased depending on the
  available bottleneck bandwidth and other
  constraints
• Last hop(s) delay bounds, etc. can also be used
  as (loose) constraints, in addition to available
  bandwidth

17
How do we obtain topology and bandwidth data?

• Topology
• Traceroute (experiments show that 10 of the
  routers do not respond)
• Topology server (e.g., OSPF topology server,
  AS-level maps)
• Comparing common subsequences can be used instead
  of matching paths when complete information is
  not easily available
• Bandwidth/delay
• Bandwidth estimation tools (e.g., pathchar,
  nettimer)
• In-band measurements

18
Ongoing work

• More intelligent path matching with multiple
  tight or loose constraints and incomplete
  topology data
• Fault resilience
• Periodic probing of parent and children
• Adaptivity to changes
• An intermediate node probes paths to its children
• Path-based aggregation of destinations
• A change in spath affects members which overlap
  with the spath

19
Economies of Scale Factor

• Two important questions to answer about an
  overlay multicast tree
• How much bandwidth does TAG save compared to
  unicast (1) ?
• How much additional bandwidth does TAG consume
  compared to IP multicast (0.8) ?

1. J. Chuang and M. Sirbu, Pricing Multicast
Communications A Cost-based Approach. Proc.
of Internet Society INET, 1998. 2. G. Phillips,
S. Shenker, and H. Tangmunarunkit, Scaling of
multicast trees Comments on the Chuang-Sirbu
scaling law., ACM SIGCOMM, 1999.
20
A Simple Model
Primary Source
Router
End host
k

• An end-host can be attached to any router
• A router can have more than one end host attached
  to it

k-ary tree
21
TAG Model

• Case 1
• At least one host connected to A

• Case 2
• No host connected to A

B
B
A
C(k)
A
C(1)
C(2)
A single packet hop over the link B
22
Economies of Scale Factor

• Modeling results

• Simulation results

• Can we develop a more realistic model? (e.g.,
  unary nodes representing transit routers added to
  the tree)

23
Performance Evaluation

• Simulations
• Session-level simulations for TAG and ESM
• TAG
• Minus-k partial matching fixed
  k1, loose bwthresh200 KB
• ESM
• Degree bounds of a member in mesh lower bound
  3, upper bound 6

24
Performance Evaluation

• Topologies
• Transit-Stub model GT-ITM
• TS1 (492 nodes), TS2 (984 nodes), TS3 (1640
  nodes)
• Random symmetric link delays from 1 to 55 ms in
  transits and 1 3 ms in stubs
• 100 MB to 500 MB backbone bandwidth and 500 KB to
  1 MB for the bandwidth of edge links
• AS-level AS maps from NLANR, Inet
• AS97, Inet97 (3015 nodes)
• AS98, Inet98 (3878 nodes)
• AS99, Inet99 (4872 nodes)

25
Performance Evaluation

• Performance Metrics
• Relative Delay Penalty (RDP) The relative delay
  increase between two nodes in TAG against unicast
  delay between the same two nodes
• Link Stress (Total or maximum) Number of
  duplicate copies of a packet over a physical link
• Mean Available Bandwidth The mean available
  end-to-end bandwidth between every two nodes

26
Results Mean RDP
ESM performance significantly improves when
upper degree bound is increased to 12
27
Results Total Stress
28
Results Maximum Stress
Partial path matching helps reduce the stress
near highly constrained nodes.
29
Results Mean Bandwidth
30
Results ASMap and Inet
Config-uration Mean RDP Mean RDP Total Stress Total Stress Max Stress Max Stress Mean Bandwidth Mean Bandwidth
Config-uration TAG ESM TAG ESM TAG ESM TAG ESM
AS97 4.69 3.47 12162 13665 291 411 172 408
Inet97 4.87 6.24 12893 11103 404 310 167 371
AS98 2.67 3.03 16074 17607 347 352 166 448
Inet98 5.34 9.55 15436 15580 187 258 188 468
AS99 4.12 4.93 23774 24666 460 710 113 396
Inet99 4.40 9.56 20745 19590 379 313 161 468
31
Related Work

• End System Multicast
• ScatterCast, Yoid, ALMI, Overcast, Bayeux,
  SCRIBE, CAN-based multicast
• Overlay networks
• RON, Detour
• Unicast-based multicast protocol
• REUNITE
• Theoretical studies
• Node degree constraints and diameter bounds in
  overlay multicast networks

32
Conclusions

• Network topology information is used to construct
  an overlay multicast network low delay penalty
  and a small number of duplicate packets
• Delay (primary metric) and bandwidth (secondary)
  are considered as metrics
• Economies of scale factor is 0.94 for TAG
• Simulation results indicate the effectiveness of
  TAG in building efficient trees for a large
  number of group members, with appropriate
  parameter values

33
Ongoing and Future Work

• Two-tiered TAG
• Core receivers should meet given requirements
  (latency or bandwidth)
• More scalable
• More adaptive to dynamic changes
• Implementation and experiments