Reliable Multicast IRMA

8/13/2019 Reliable Multicast IRMA

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IRMA = Illinois Reliable Multicast Architecture


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Recent years have witnessed a tremendous increase in the use of

commerce, web access, software distribution, multimedia,communication. These a lications re uire reliable data transmission from a

sender to multiple receivers reliable multicast transmission. While it is possible to establish multiple unicast TCP connections

and accomplish such data transmissions between each sender-rece ver pa r, t s approac as two st nct sa vantages: it involves duplicating packet transmissions for connections which may

potentially be able to share common links, and it re uires an a lication-level s nchronization between the sender and

the receiver set.For these reasons, reliable multicast is gaining popularity as ahighly desirable feature of the future Internet.


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The goal of IRMA is to provide an architecture thatuarantees reliable se uenced and loosel s nchronized

delivery of multicast streams with support for flow control andcongestion control.

us, oes no requ re en os s o ns a newsoftware, the reliable multicast infrastructure makes the

TCP/IP protocol stack at the end hosts believe that thecommunication is unicast, while in fact, multiple receivers canparticipate in the reliable reception of the data.

additional functionality in a subset of the multicast routers inthe multicast tree.


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Use TCP to provide reliability.

stack at the end hosts. -

routers to implement efficient routing for multicast

packets and ACKs.


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Authors adopted their approach for three main reasons: ,

congestion control and flow control have already beenextensively studied in the context of unicast TCPcommunication and can be effectivel extended to multicastcommunication.

For practical reasons, widespread deployment of a new protocoltakes time, and requires showing demonstrable robustness ande c ency over a arge set o comp ex scenar os n t e nternet.

Most contemporary approaches treat reliable multicast as aspecialized service which needs to be instantiated in participating

ubiquitous service for a large number of hosts in the Internetmerely by adding functionality to a relatively small subset ofmulticast routers.


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There are three key issues in supporting reliable multicast:

The semantics of reliable multicast NAK-based versus ACK-based architectures

Loss recovery mechanisms


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There are three classes of reliable multicast semantics.

tr ct re a ty w t oose sync ron zat on:Data recept on s

synchronized among receivers, the reliability semantics can be end-to-end & the transmission rate of a multicast session is constrained

time. This end-to-end semantics is desirable as it provides better

robustness since it allows for intermediate node failures without.

Partial reliability with loose synchronization: Some applicationssend heterogeneous data traffic with partial reliability requirements

- . ., .In this case, an ideal multicast protocol should ensure that reliable

packets are guaranteed in-sequence delivery, and slow receivers donot stall the entire multicast session.


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Strict/partial reliability with no synchronization:If theheterogeneity in a multicast group is high, then split-connection semantics

with no synchronization among receivers is suitable. By caching the datastream at some intermediate network nodes, we can also allow late joinrece vers o ca c up rom e s ar o e mu cas sess on as we assupporting split-connection semantics.

T e es gners o IRMA ave c osen t e rst re a ty semant cs(Strict reliability with loose synchronization) in order to supporta lications with stron reliabilit re uirements.


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Several contemporary approaches have argued for a NAK basedreliabilit scheme with NAK su ression because it avoids the

feedback implosion problem. NAK-based schemes work well if theNAK-channel is reliable, otherwise the NAK-based schemes will not

NAK-based schemes require an infinite buffer at the sender in order toguaran ee correc opera on w e - ase sc emes can guaran eereliable transmission with bounded buffers.

The designers of IRMA have adopted an ACK-based approach inIRMA since they concluded that it is more suitable for the Internetenvironment.


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How and where does recovery of lost packets take place?

There are three possible solutions depending on who isresponsible for the retransmission:

en er- n t ate sc eme

Local server-initiated scheme

-


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In sender initiated recovery, ACKs go all the way back

to the sender, which then retransmits the lost packet. In local server initiated recovery, some intermediate

no e n t e mu t cast tree ca e , a repa r server, cac esdata packets, and retransmits packets locally without

.

In receiver-initiated recovery, any receiver who hasreceived and cached a acket can issue a local

retransmission upon hearing a loss notification for thepacket. This option does not work with standard TCP.


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Sender

Root Router

router index i

Intermediate Router

Leaf Router

User host

host index j


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Sender

Packet Flow and ACK Aggregation

packetAck

u1 u2 u3 u4 u5


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Every router i has the following data structures

parent(i) single upstream parent router

ackseq_no(i)ACK seq_no of the slowest child

awnd(i) advertised windowdup_no(i) number of duplicate ACKS

cached_seq(i) used for local repair and is equal to the

cached by router i


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For every childj of router i, // j belongs to child(i) ackseq_no(j) / last ACK sequence number

dup_no(j) // number of duplicate ACKs

awnd(j) // advertised window

cached_seq(j) // cashed sequence number

ackseq_no(i) = Min{ackseq_no(j)}

awnd(i) = Min{ackseq_no(j)+awnd(j)} - ackseq_no(i)

where j belongs to child(i)

dup_no(i) = Max{dup_no(j) | ackseq_no(j) = ackseq_no(i)} cached_seq = Max{cached_seq(i) , min{cached_seq(j)} }


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Updating ackseq_no andawnd

Routeri

has four children with the following (ackseq_no , awnd) values

Child 1 (50, 20) ; Child 2(55, 20) ; Child 3 (53,20); Child 4 (54,15)

70

55

53

75

73

54 69

Router i 50 19

ackseq_no of router i is determined by the slowest child and awnd of router i

is determined by the child whose advertised window has the least reach.

ackseq_no(i) = Min{ 50, 55, 53, 54} = 50awnd(i) = Min{70, 75, 73, 69} - 50 = 69 50 = 19

ac seq_no = n ac seq_no c ,

awnd(i) = Min{ackseq_no(j)+awnd(j) | j

child (i)} - ackseq_no(i)


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Sender

dup_no(i) = Max{dup_no(j) | ackseq_no(j) = ackseq_no(i)}

R

_

is 41 and there are two

children U1 and U2

having the slowest ACK

R41

41

2

sequence num er o .

The dup_no of R5 is the

highest dup_no of the two

children i.e. du no ofackseq_no

ac seq_no

dup_no

R5 R6

2 43

1

_

U2.dup_no

u1 u3 u4 u5u2

41

1

41

2

43

0

44

0

43

1ackseq_nodup_no


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Multicast packet travels downstream

Sender

R

Source Destination

156.23.1.7 224.17.9.2

R

R5


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Aggregated ACKs travel upstream

Sender

R

Source Destination

156.23.1.7 224.17.9.2

R

R5

u3 u5u2

Ack AggregationTakes Place


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Sender

Packet Transmission and ACK Aggregation


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Sender

State of R5 is

ackseq_no = 41

du no = 1

R

_

Source Destination

156.23.1.7 224.17.9.2

R

41

1ackseq_no

dup_no

R51 43

1

_

dup_no

u1 u3 u4 u5u2

41

1

42

0

43

0

44

0

43

1

ackseq_nodup_no


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Sender

State of R5 is

ackseq_no = 41

R

_

User U1 sends new ACKackseq_no = 43Source Destination

156.23.1.7 224.17.9.2

R

41

1

ackseq_no

dup_no

R51 43

1

ac seq_no

dup_no

u1 u3 u4 u5u2

43

0

42

0

43

0

44

0

43

1

ackseq_nodup_no


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Sender

Router R5 updates its state to

ackseq_no = 42

=

R

_

This reflects the new slowest

child U2 Source Destination156.23.1.7 224.17.9.2

R

41

1

ackseq_nodup_no

R50

43

1

_dup_no

u1 u3 u4 u5u2

43

0

42

0

43

0

44

0

43

1

ackseq_nodup_no


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Sender

Router R3 updates i ts state to

ackseq_no = 42

=

R

_

This reflects the updated state

of the slowest child R5 Source Destination156.23.1.7 224.17.9.2

R

42

0

ackseq_no

dup_no

R50

43

1

_

dup_no

u1 u3 u4 u5u2

43

0

42

0

43

0

44

0

43

1

ackseq_nodup_no


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As in TCP fast retransmit, if a repair server receives an ACK notificationwith three duplicate ACKs, it will initiate fast retransmit.

However, it also propagates the duplicate ACK count unchanged upstream

because local recovery should not interfere with the fast retransmit/fastrecovery mechanism of the sender TCP (to preserve end-to-end semantics)

There is a problem with the above approach: if there are two repair serversalong a path on the multicast tree, the higher level repair server will always

retransmit packets to the lower level repair server even though the lattersever may a rea y ave e pac e s cac e .

This problem is solved by using two variables in router i: cached_seq(i) is the highest sequence number cached by router i

c _cac e _seqs e owes sequence num er cac e a e su ree o rou er

Router i reports to its parent the maximum of cached_seq(i) andchild_cached_seq.

cac e _seq = ax cac e _seq , m n cac e _seq


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SenderExample 1:

R

When router R3 sends an ACK

to router R1, it reports thevalue of its cached_seq as 50

R

42

47

ackseq_nocached_seq

R553 50

_cached_seq

u1 u3 u4 u5u2

43

0

42

0

43

0

44

0

43

0

ackseq_nocached_seq


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SenderExample 2:

R

When router R3 sends an ACK

to router R1, it reports thevalue of its cached_seq as 55

R

42

55

ackseq_nocached_seq

R553 50

_cached_seq

u1 u3 u4 u5u2

43

0

42

0

43

0

44

0

43

0

ackseq_nocached_seq


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Receiving ACK (src, dest, ackseq_no, awnd, dup_no, cached_seq)

// Update state of child jackseq_no(j) = ACK.ackseq_no; awnd(j) = ACK.awnd

= =_ . _ _ . _

// Update state of router i

Update ackseq_no(i), dup_no(i), awnd(i) // Use equations described earlier

If state of i has chan ed send ACK to arent i

// Perform local repair by fast retransmission of lost packet to child j

If (Repair router & ACK.dup_no == 3 & ACK.cached_seq < ACK.ack_seqno &

cached_seq(i) >= ACK.ackseq_no)

{Retransmit the packet whose sequence no == ACK.ackseq_no}If (Repair router & child j is the slowest child)

{ update cache as appropriate }


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Reception of ACK (src, dest, ackseq_no, awnd)

// src is and dest is i ACK from host has two arameters

// Update state of child j

If (ACK.ackseq_no == ackseq_no(j)) { dup_no(j)++};elseif (ACK.ackseq_no > ackseq_no(j))

ac seq_no = .ac seq_no, up_no = e se ex nva ;awnd(j) = ACK.awnd;cached_seq(j) = 0; //

// Update state of router iUpdate ackseq_no(i), dup_no(i), awnd(i) // Use equations described earlierIf state of i has changed, send ACK to parent(i)

Note: If the leaf router is a repair router, additional code should be

added to do fast retransmission and cache maintenance if needed.


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Receiving ACK (src, dest, ackseq_no, awnd, dup_no, cached_seq)

/ src is j and dest is i ; ACK from router j has four parameters

// Update state of child j

ackseq_no(j) = ACK. ackseq_no

awnd(j) = ACK.awnd

dup_no(j) = ACK.dup_no

=_ . _

// Update state of root router i

Update ackseq_no(i), dup_no(i), awnd(i)

If (state of i has changed){send a normal TCP ACK or multiple ACKs to the sender host}

Perform local recover if needed

Note: normal TCP ACK has two parameters: ackseq_no and awnd


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The senderA with unicast addressa initiates the connection request bysendin a SYN acket with the multicast destination addressm.

The root multicast router in the subnet of A picks up this packet andforwards it to the nodes in the downstream multicast tree.

downstream.

When a leaf multicast router receives the multicast SYN packet, it picks alocall uni ue class E address e then creates a d namic association for

and replaces the source address of the SYN packet,a , with theclass E address e, (i.e., a e ).

Source Destination

=156.23.1.7 m 224.17.9.2

This slide is for information


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This is required because the leaf router must trap the SYN+ACK responsesrom e rece vers n or er o aggrega e em an orwar e aggrega e

SYN+ACK to the source. The modified SYN packet is locally multicast to all receivers in the

su ne . ac rece ver respon s w a pac e w es na onaddress e.

The SYN+ACK packet gets picked up by the local multicast router, which. .,back up the upstream tree to the root roouter.

The root router then unicasts the packet back to the sender A.

This slide is for information


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Dynamic Joins: details are given in the paper.

Dynamic Leave: the leaf router removes the receiver from itslocal receiver set. If the local receiver set is now empty, then

.details are given in the paper.

Reliable Multicast IRMA

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