Group Communication

1
Group Communication

• Group oriented activities are steadily
  increasing.
• There are many types of groups
• ? Open and Closed groups
• ? Peer-to-peer and hierarchical groups

We focus on how the members of a group communicate
2
Major issues

• Different types of multicast
• Atomic multicast
• Ordered multicast
• Dynamic groups
• How to communicate when the membership constantly
  changes
• Failure handling
• Keeping track of membership and membership
  changes

3
Atomic multicast

• A multicast is called atomic, when the message
  is delivered to every correct member, or to no
  member at all.

How can we implement atomic multicast?
4
Basic vs. reliable multicast

• Basic multicast does not consider crash failures.
• Reliable multicast does.
• Three criteria for basic multicast
• Liveness. Each process must receive every
  message
• Integrity. No spurious message received
• No duplicate. Accepts exactly one copy of a
  message

5
Reliable atomic multicast

• Senders program Receivers program
• i0 if m is new ?
• do i ? n ? accept it
• send message to i multicast m
• i i1 ? m is duplicate ? discard m
• od fi

Tolerates process crashes.
6
Multicast support in networks

• Sometimes, certain features available in the
  infrastructure of a distributed system simplify
  the implementation of multicast. Examples are
• Multicast on an ethernet LAN
• IP multicast

7
IP Multicast

• Although multicasts can be implemented using
  point-to-point communications, there are some
  practical forms of multicasts that make use of
  the inherent multicasting ability of the
  underlying medium. IP multicast is a
  bandwidth-conserving technology that reduces
  traffic by simultaneously delivering a single
  stream of information to multiple clients.
• Applications that take advantage of multicast
  include distance learning, videoconferencing, and
  distribution of software, stock quotes, and news.
  The source sends only one copy, which is
  replicated by the routers.

8
Distribution trees

• Class D addresses are assigned to multicast
  groups.
• Routers maintain and update distribution trees
  when members join / leave a group.
• Concern Too much load on routers. Application
  layer multicast overcomes this.

9
Ordered multicasts

• Total order
• Causal order
• Local order
• (a.k.a. Single source FIFO)
• Definitions?
• Applications?

• Total order multicast is useful in the consistent
  update of replicated servers
• Causal order multicast is relevant in
  implementing bulletin boards
• Local order multicast is useful in updating cache
  memories in multi-computers

10
Implementing total order multicast

• Basic multicast using a sequencer
• The sequencer S
• define seq integer (initially 0
• do receive m ?
• multicast (m, seq)
• seq seq1
• deliver m
• od

sequencer
11
Implementing basic total order multicast
Distributed implementation without a sequencer
Uses the idea of 2PC
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Implementing basic total order multicast

• Step 1. Sender i sends (m, ts) to all
• Step 2. Receiver j saves it in a holdback queue,
  and sends an ack (a, ts)
• Step 3. Receive all acks, and pick the largest
  ts. Then send (m, ts, commit) to all.
• Step 4. Receiver removes it from the holdback
  queue and delivers m in the ascending order of
  timestamps.
• Why does it work?

13
Implementing basic causal order multicast

• Use vector clocks.The
• recipient i delivers a
• message from j iff
• 1. VCj(j) LCj(i) 1
• LC local vector clock
• 2. ?k k?j VCk(j) LCk(i)
• VC incoming vector clock
• LC Local vector clock

Note the slight difference in the implementation
of the vector clocks
14
Reliable multicast

• Tolerates process crashes. The additional
  requirements are as follows
• Only correct processes are required to receive
  the messages from all correct processes in the
  group. Multicasts by faulty processes will either
  be received by every correct process, or by none
  at all.

15
A theorem on reliable multicast

• In an asynchronous distributed system, total
  order reliable multicasts cannot be implemented
  when even a single process undergoes a crash
  failure.
• Why? Since it will violate the FLP impossibility
  result.

16
Scalable Reliable Multicast

• IP multicast or application layer multicast has
  to detect the loss of messages and use
  retransmission for achieving reliability. For
  large groups (like distance learning
  applications) scalability is a major problem.

17
Scalable Reliable Multicast

• Difficult to scale
• Sender state explosion
• Message implosion

Statereceiver 1, receiver 2, receiver n
18
Scalable Reliable Multicast

• If omission failures are rare, then receivers
  will only report the non-receipt of messages
  using NACK, This has the potential to trigger
  selective point-to-point retransmission The
  reduction of acknowledgements is the underlying
  principle of Scalable Reliable Multicasts (SRM).
• If several members of a group fail to receive a
  message, then each such member waits for a random
  period of time before sending its NACK. This
  helps to suppress redundant NACKs. Sender
  multicasts the missing copy only once.