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Week 2 Reliable transmission Building a network
| Date post: | 26-May-2015 |
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Engineering |
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Week 2Reliable transmissionBuilding a network
Agenda
•Reliable transmission
•Checksums and hash functions
•Framing
•Go-back-n
•Selective repeat
•Building a network
Agenda
•Reliable transmission
•Checksums and hash functions
•Framing
•Go-back-n
•Selective repeat
•Building a network
Checksums
•How to verify that a transmission has not been affected by random errors
•Checksum
•CRC
•Should the Checksum/CRC also cover the header ?
Hash functions
•What is the role of hash functions like MD5 or SHA-1 ?
•Do they play the same role as CRCs and checksums ?
Agenda
•Reliable transmission
•Checksums and hash functions
•Framing
•Go-back-n
•Selective repeat
•Building a network
Bit stuffing
•How to combine bit stuffing and checksums ?
•Stuffing first and then compute checksum or
•Compute checksum and then stuffing
Character stuffing
•What are the advantages/drawbacks of character stuffing compared to bit stuffing ?
Agenda
•Reliable transmission
•Checksums and hash functions
•Framing
•Go-back-n
•Selective repeat
•Building a network
Go-back-n•What are the key principles behind go-back-n ?
•How does it deal with losses/errors ?
•How does it deal with losses of acknowledgements ?
•How to set the retransmission timer ?
Go-back-n : example
A B
0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3
0 1 2 3
Data.req(e)
Data.req(a)
Data.ind(a)
D(0,a)
Data.req(c)
D(2,c)
C(OK,0)
C(OK,0)
Sending window
Data.req(b)
D(1,b)
Lost
Not expected seq num,discarded
Retransmissiontimer expires
Sending window is full
0 1 2 3 0 1 2 3
D(1,b)
Data.ind(b)
Data.req(d)
D(3,d)Data.ind(d)
D(2,c)
Data.ind(c)
Maximum window
•What is the maximum window size that can be used with go-back-n ?
•Sequence numbers are encoded in N bits
Agenda
•Reliable transmission
•Checksums and hash functions
•Framing
•Go-back-n
•Selective repeat
•Building a network
Selective repeat•What are the key principles behind selective repeat ?
•How does it deal with losses/errors ?
•How does it deal with losses of acknowledgements ?
•How to inform about losses ?
•How to set the retransmission timer ?
Selective repeatA B
0 1 2 3 0 1 2 3 0 1 2 3
0 1 2 3
Data.ind(b)
Data.req(a)
Data.ind(a)
D(0,a)
0 1 2 3 Data.req(c)
D(2,c)
C(OK,0)
C(OK,0)
Sending window
Data.req(b)
D(1,b)
Lost segment
Segment stored
D(1,b)
Retransmissiontimer expires0 1 2 3
0 1 2 3 0 1 2 3
0 1 2 3
0 1 2 3
Data.ind(c)
Data.req(d)
D(3,d)
Data.ind(d)
0 1 2 3
0 1 2 3 C(OK,2)
C(OK,3)
Rec. window0 1 2 3
0 1 2 3
Maximum window
•What is the maximum window size that can be used with selective repeat ?
•Sequence numbers are encoded in N bits
Performance with losses
•What is the minimum window size required to achieve high throughput
•When there are no losses ?
•When a few frames are errored/lost ?
•Go-back-n
•Selective repeat
Take back lessons
•Usage of Checksums/CRCs
•Cumulative acknowledgements
•Maximum window size with gbn/sr
•Bandwidth x delay product
Agenda
•Reliable transmission
•Building a network
•Datagram organisation
•Virtual circuit organisation
•Distance vector routing
•Link state routing
Datagram organisation
•Principles
•Nodes
•Hosts
•Routers
•Packet
Forwarding table
•Contents of the forwarding table
•Destination, nexthop/outgoing if
•How to compute the forwarding table ?
Port-station table
•When a node boots, its forwarding table is empty, is it possible to learn automatically the forwarding table from the packets ?
•Tree-shaped networks only !
A network
B
A R1
R2
R3
Source routing•Basic idea
•Each node has a unique address
•Each node knows its direct neighbours
•How ?
•Each packet contains the list of the addresses of intermediate nodes
A network
B
A R1
R2
R3
How can A discover the paths towards B ?
Agenda
•Reliable transmission
•Building a network
•Datagram organisation
•Virtual circuit organisation
•Distance vector routing
•Link state routing
Virtual circuit•Basic idea
•Create circuits through the network to transfer data divided in packets
•Packet contains one label
•Simpler forwarding table
•incoming label, nexthop, outgoing label
Virtual circuit
B
A R1
R2
R3
Create one virtual circuit from A to B
C
Agenda
•Reliable transmission
•Building a network
•Datagram organisation
•Virtual circuit organisation
•Distance vector routing
•Link state routing
Computing forwarding tables•How to compute the forwarding
tables for all nodes in a network ?
C
E
A B C
D E
Computing forwarding tables
•
•
C
E
A B C
D E
Routing tableA : LocalD : SouthB : East
C : East [via B]
E: East [via B]
Routing tableA : WestB : LocalC : EastD : South [via E]
E : South
Routing tableA : West [via
B]B : WestC : Local
D : West [via B]
E : South West
Routing tableA : NorthB : North [via A]
C : East [via E]
D : LocalE : East
Routing tableA : North [via B]B : NorthC : North-
EastD : WestE : Local
Cost=1
Cost=1 Cost=1
Cost=1
Cost=1Cost=1
Distance vector routing
•Objectives of the routing protocol
•Compute forwarding tables so that all nodes can reach all nodes in the network
•Principle
•Each node regularly sends a summary of its forwarding table to its neighbours
Distance vector
C
D E
Routing tableA : 0 [ Local
]
A B C
D E
Routing tableB : 0 [Local] Routing table
C : 0 [Local]
Routing table
E : 0 [Local]
Routing tableD : 0 [Local]
Distance vector
example (2)
C
D E
Routing tableA : 0 [ Local
]
A B C
D E
Routing table
B : 0 [Local]A : 1 [West]
Routing table
C : 0 [Local]
Routing table
E : 0 [Local]
Routing table
D : 0 [Local]A : 1 [North]
D=0 ; A=1
D=0 ; A=1
Distance vector
example (3)
C
D E
Routing tableA : 0 [ Local
]D : 1 [South]
A B C
D E
Routing table
B : 0 [Local]A : 1 [West]
Routing table
C : 0 [Local]
Routing table
E : 0 [Local]D : 1 [West]A : 2 [West]
Routing table
D : 0 [Local]A : 1 [North]
C=0
C=0
Distance vector
example (4)
C
D E
Routing tableA : 0 [ Local
]D : 1 [South]
A B C
D E
Routing table
B : 0 [Local]A : 1 [West]C : 1 [East]
Routing table
C : 0 [Local]
Routing tableE : 0 [Local]D : 1 [West]A : 2 [West]C : 1 [North-
East]
Routing table
D : 0 [Local]A : 1 [North]
E=0;D=1;A=2;C=1
E=0;D=1;A=2;C=1
E=0;D=1;A=2;C=1
Distance vector
example (5)
C
D E
Routing tableA : 0 [ Local
]D : 1 [South]
A B C
D E
Routing tableB : 0 [Local]A : 1 [West]C : 1 [East]E : 1 [South]D : 2 [South]
Routing tableC : 0 [Local]E : 1 [South-
West]D : 2 [South-
West]A : 3 [South-
West]
Routing table
E : 0 [Local]D : 1 [West]A : 2 [West]C : 1 [North-
East]
Routing table
D : 0 [Local]A : 1 [North]E : 1 [East]C : 2 [East]
B=0;A=1;C=1;D=2;E=1
B=0;A=1;C=1;D=2;E=1
Distance vector
example (6)
C
D E
Routing tableA : 0 [ Local
]D : 1 [South]B : 1 [East]C : 2 [East]E : 2 [East]
A B C
D E
Routing tableB : 0 [Local]A : 1 [West]C : 1 [East]E : 1 [South]D : 2 [South]
Routing tableC : 0 [Local]E : 1 [South-
West]D : 2 [South-
West]A : 2 [West]B : 1 [West]
Routing tableE : 0 [Local]D : 1 [West]A : 2 [West]C : 1 [North-
East]B : 1 [North]
Routing table
D : 0 [Local]A : 1 [North]E : 1 [East]C : 2 [East]
A=0;B=1;C=2;D=1;E=2
Distance vector
example (7)
C
D E
Routing tableA : 0 [ Local
]D : 1 [South]B : 1 [East]C : 2 [East]E : 2 [East]
A B C
D E
Routing tableB : 0 [Local]A : 1 [West]C : 1 [East]E : 1 [South]D : 2 [South]
Routing tableC : 0 [Local]E : 1 [South-
West]D : 2 [South-
West]A : 2 [West]B : 1 [West]
Routing tableE : 0 [Local]D : 1 [West]A : 2 [West]C : 1 [North-
East]B : 1 [North]
Routing tableD : 0 [Local]A : 1 [North]E : 1 [East]C : 2 [East]B : 2 [North]
Agenda
•Reliable transmission
•Building a network
•Datagram organisation
•Virtual circuit organisation
•Distance vector routing
•Link state routing
Link state routing
•Objectives of the routing protocol
•Compute all forwarding tables
•Principle
•Each node detects its neighbours
•Each node regularly sends list of its neighbours to all nodes
Discovering neighbours
•Manually
•Automatically
E
A B
E
B:HELLO
A:HELLO
E:HELLO
Link state packets
•How to reliably send LSPs to all nodes ?
C
D E
LinksA-B : 1A-D : 1
A B C
D E
LinksB-C : 1C-E : 1
LinksE-D : 1E-B : 1E-C : 1
LinksA-D : 1D-E : 1
LinksA-B : 1B-E : 1B-C : 1
LinksA-D : 1D-E : 1
LSP : E [D:1];[B:1];[C:1]
LSP : E [D:1];[B:1];[C:1]
LSP : E [D:1];[B:1];[C:1]
Flooding•Naive approach
•Send received LSP to all neighbours
C
D E
LinksA-B : 1A-D : 1
A B C
D E
LinksB-C : 1C-E : 1B-E : 1D-E : 1
LinksE-D : 1E-B : 1E-C : 1
LinksA-B : 1B-E : 1B-C : 1E-D : 1E-C : 1
LinksA-D : 1D-E : 1B-E : 1E-C : 1
LSP : E [D:1];[B:1];[C:1]
LSP : E [D:1];[B:1];[C:1]
LSP : E [D:1];[B:1];[C:1]
Flooding
C
D E
LinksA-B : 1A-D : 1
A B C
D E
LinksB-C : 1C-E : 1
LinksE-D : 1E-B : 1E-C : 1
LinksA-D : 1D-E : 1
LinksA-B : 1B-E : 1B-C : 1
LinksA-D : 1D-E : 1
LSP : E-0 [D:1];[B:1];[C:1]
LSP : E-0 [D:1];[B:1];[C:1]
LSP : E-0 [D:1];[B:1];[C:1]
LSPs
LSPs
LSPsE-0 [D:1];[B:1];[C:1]
LSPsLSPs
Flooding
C
D E
LinksA-B : 1A-D : 1
A B C
D E
LinksB-C : 1C-E : 1B-E : 1D-E : 1
LinksE-D : 1E-B : 1E-C : 1
LinksA-B : 1B-E : 1B-C : 1E-D : 1E-C : 1
LinksA-D : 1D-E : 1B-E : 1E-C : 1
LSP : E-0 [D:1];[B:1];[C:1]
LSP : E-0 [D:1];[B:1];[C:1]
LSPsE-0 [D:1];[B:1];[C:1]
LSPsE-0 [D:1];[B:1];[C:1]
LSPsE-0 [D:1];[B:1];[C:1]
LSPsE-0 [D:1];[B:1];[C:1]
LSPs
LSP : E-0 [D:1];[B:1];[C:1]
After flooding•Each node knows full network topology
C
D E
A B C
D E
LinksA-B, B-A :
1B-E, E-B :
1B-C, C-B :
1E-D, D-E :
1E-C, C-E :
1A-D, D-A :
1
LSPsE-0 [D:1];[B:1];[C:1]
A-0 [D:1];[B:1]B-0 [A:1] [C:1]
[E:1]C-0 [B:1] [E:1]D-0 [A:1] [E:1]
LinksA-B, B-A :
1B-E, E-B :
1B-C, C-B :
1E-D, D-E :
1E-C, C-E :
1A-D, D-A :
1
LSPsE-0 [D:1];[B:1];[C:1]
A-0 [D:1];[B:1]B-0 [A:1] [C:1]
[E:1]C-0 [B:1] [E:1]D-0 [A:1] [E:1]
LinksA-B, B-A :
1B-E, E-B :
1B-C, C-B :
1E-D, D-E :
1E-C, C-E :
1A-D, D-A :
1
LSPsE-0 [D:1];[B:1];[C:1]
A-0 [D:1];[B:1]B-0 [A:1] [C:1]
[E:1]C-0 [B:1] [E:1]D-0 [A:1] [E:1]
LinksA-B, B-A :
1B-E, E-B :
1B-C, C-B :
1E-D, D-E :
1E-C, C-E :
1A-D, D-A :
1
LSPsE-0 [D:1];[B:1];[C:1]
A-0 [D:1];[B:1]B-0 [A:1] [C:1]
[E:1]C-0 [B:1] [E:1]D-0 [A:1] [E:1]
LinksA-B, B-A :
1B-E, E-B :
1B-C, C-B :
1E-D, D-E :
1E-C, C-E :
1A-D, D-A :
1
LSPsE-0 [D:1];[B:1];[C:1]
A-0 [D:1];[B:1]B-0 [A:1] [C:1]
[E:1]C-0 [B:1] [E:1]D-0 [A:1] [E:1]
