1. Network Layer Datagram vs. Virtual circuit Routing Congestion control
Internetworking
Internet
Example Systems
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2. Datagram vs. Virtual Circuit --Two alternative for network layer
Datagram (connectionless):
Each packet passes through the network as independent entity with full destination address; routing decision is based on the routing table.
Virtual circuit (connection-oriented):
Full duplex virtual channel is established by creating an entry in virtual circuit table on each node along the path; based on another routing table.
Virtual circuit ID defined packet destination address (abbreviated addressing); routing decision is based on the virtual circuit table.
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3. Datagram Approach B C A D F E D C 2 F D B 3 packet: dest src ... data
Dest Next Delay
(Routing Table)
packet:
dest
src
...
data
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4. Virtual Circuit ApproachC H C 1 H 1 A A 1 F B D H 2 F 1 B 1 D 1
(Routing Table) F H E H E 1 D 2 D B 3 B C C H C 1 H 1 A D F H 2 F 1 H 3
host H B C 2 F E F H 1 E
router B E 1 H 2 B 1 A F E D H 3 E 2 A 1 H B D 1 A 2 C B 1 H
incoming
outgoing A 3 C 1 D B
(Virtual Circuit Table)
V.C. ID
packet: 2
data 1
data
data 1
data
(at A)
(at B)
(at F)
(at D)
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5. Comparison of Datagram and Virtual Circuit
Robustness
to failures
more
less
Overhead
call setup
memory for VC table
reassemply
Example
Internet
ATM
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6. IP vs. ATM datagram subnet vs. virtual-circuit subnet
soft-state flow vs. hard-state connection
Applications
Application
Q.2931
IP-over-ATM
LAN Emulation
TCP
UDP
RSVP
AAL IP
ATM
Ethernet
Ether Switch
HDLC
...
Sonet
connection subnet
connection-oriented subnet
connection-oriented socket
connection-oriented APIs
connectionless socket
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7. Routing Routing policies Shortest path routing Flow-based routing
Flooding
Distance vector routing
Link state routing
Hierarchical routing
Mobile routing
Broadcast routing
Multicast routing
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8. Routing Policies Static vs. dynamic routing
Centralized vs. distributed routing
Building blocks
--network status monitoring (local, global, partial)
--network status reporting (exchange, broadcast)
--route computation (delay, bandwidth)
--route implementation (routing table, virtual circuit table, route cache)
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9. Shortest Path Routing Static and centralized Dijkstra Algorithm (1959)
1. To compute the distance (cost) from A to all nodes, label all other nodes as (,–) and tentative, while marking A as permanent and working node X.
2. Re-label the nodes adjacent to X as (distance to A,X) if X provides a shorter path to A.
3. From the tentative nodes, mark the one with the smallest label permanent and new working X. Go to step 2 until all nodes are permanent.
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10. Shortest Path Routing (Cont.)
B (2,A)
C (9,8) 7
distance to A
next hop 2 2 3 3
(4,8) A E
F (6,E)
D (10,H) 2 6 1 2 2
G (5,E) 4
H (8,F)
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11. Flow-based Routing Static and centralized
Consider topology (connectivity and capacity) and traffic matrix to allocate traffic flows over the topology to optimize performance (delay, loss, blocking)
Optimization techniques exist in routing the flows.
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12. Flooding
Flood the incoming packet on every outgoing link except the one it arrived on.
Two ways to limit the scope of flooding:
--hop count in packet header
--sequence number in packet header to avoid duplicate flooding
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13. Distance Vector Routing
Also named as distributed Bellman-Ford, original ARPANET, RIP
Distance vector exchange:
Periodically each node sends the distance vector to its neighbors.
Routing table update:
Di(k) = min [ Di(j) + Dj(k)], j is i’s neighbor
Ri(k) = j*, which is i’s neighbor that minimize Di(k)
Initialization:
Di(k) = 0 if k=i
 if kI
Ri(k) = 
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14. Distance Vector Routing (Cont.) (For destination F)
(,) (,) (11,C) (9,D) (9,D) 7 2
(,) (4,F) (4,F) (4,F) (4,F) B C 1 3 1 4 3 2 2 2 F
(0,F) A D
(,)
(12,E)
(11,B)
(DA(F),RA(F)) 6 1 5 3 3 E 7
(,) (7,F) (7,F) (7,F) (7,F)
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15. Problems with Distance Vector Routing
Not taking link bandwidth into account (only considering instantaneous queue length)
instability and oscillation
Taking a long time to converge
good news: travel at the rate of one hop per exchange
bad news: count-to-infinity
(because no router ever has a value a few more higher than the minimum of all its neighbors)
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16. Link State Routing Examples: IS-IS, OSPF,
Learn about neighbors and their network address ( HELLO packet )
Measure link state ( ECHO packet )
Build link state packets ( router id, sequence, age, ( neighbor, cost ),.... )
Distribute link state packets to all other routers
--check and update the table ( source router, sequence, age, send flags, ACK flags )
Compute new routes ( run Dijkstra’s algorithm locally )
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17. Hierarchical Routing
A router has one entry, in its routing table, for each router in the same region, and also one representation entry for each of other regions.
Example: For a subnet with 720 routers partitioned into 24 regions of 30 routers each, each router needs 53 entries ( 30 local + 23 remote ).
For a subnet with n routers, optimal number of hierarchical levels is ln(n) and number of entries per router is eln(n)
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18. Mobile Routing
Mobile ( wired or wireless ) hosts travel without changing their network address. Need home agent at home site and foreign agent at remote site.
Mobile hosts registers to foreign agent when travelling to a foreign LAN or cell. Foreign agent informs home agent of the mobile hosts.
Packets sent to the mobile host are intercepted by home agent and tunneled to foreign agent. Home agent informs the packet sender foreign agent’s address.
Subsequent packets to mobile host are re-directed to foreign agent.
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19. Broadcast Routing
Possible methods: flooding, multi-destination routing, optimal sink tree, reverse path forwarding
Reverse path forwarding: approximate the optimal sink tree ( router checks to see if the packet arrived on the line that is normally used to send packets to the source of the broadcast ) I B • C • B • C • F H J N A • D • A • D • F • E • E • F • A D E K G O M O I • G • I • G • E C G D N K
H •
H • • L • N
• J • L • N
• J H B L • K • K • M
• O • M
• O L B
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20. Multicast Routing
To do multicast routing, two approach exist:
--pruned spanning tree per source per group
--core-base tree per group
Pruned spanning tree per source router per group
1.with link state routing, pruned locally at source router
2.with distance vector routing, PRUNE message stop multicasting
Core-base tree per group
--send multicast packets to the core ( root ) of the group
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21. Congestion Control Congestion phenomena
Policies that affect congestion
Classification of congestion control: best-effort and guaranteed
Choke packet scheme
Sliding window-based flow control
Rate-based flow control
Weighted fair queueing
Admission control
Traffic shaping and policing
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22. Congestion Phenomena Throughput Offered Load 22 Ying-Dar Lin@CIS.NCTU
capacity of subnet
ideal
Throughput
controlled
uncontrolled
deadlock
Offered Load
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23. Policies That Affect Congestion
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24. Classification of Congestion Control
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25. Choke Packet Scheme
When queue length or utilization exceeds a threshold, the router sends a choke packet back to (1) the source host or (2) the immediate upstream router, giving it the destination found in the affected packet stream.
Case (1): source choke packet
slower response
Case (2): hop-by-hop choke packet
higher buffer requirement
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26. Sliding Window-based Flow Control
To limit the number of outstanding ( un-acknowledged ) packets, a fixed or variable window is allocated for a packet stream ( flow ) between a pair of source and destination.
Only when the left-most packet is acknowledged can the window be slided forward.
Window size depends on bandwidth, propagation delay, traffic load.
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27. Rate-based Flow Control
When queue length or utilization reaches a certain value, router/switch (1) computes the allowed rate for the source of each packet stream, or (2) informs the source the congestion status.
Case (1): explicit rate flow control
Source sets its rate to the assigned allowed rate.
Less oscillation
Complex router/switch computation
Case (2): congestion notification flow control
Source adjusts its rate according to some rule, e.g. linear increase, exponential decrease
larger oscillation
simpler router/switch
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28. Admission Control
For a flow or connection that requires guaranteed service, admission control is exercised to determine the grant/reject, depending on the traffic descriptor performance parameters of the flow or connection.
Traffic descriptor ( Tspec ): peak rate, mean rate, burst length
Performance parameters ( Rspec ): delay, loss, jitter
Request for the flow or connection is granted if network resource, bandwidth and buffer, is available along a path to accomodate the flow or connection.
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29. Traffic Shaping and Policing
Leaky Bucket scheme can be used to (1) smooth/enforce a flow or connection, at host side, to conform to the declared traffic descriptor, or (2) police a flow or connection, at network-side, to regulate the input traffic.
token
: token arrival rate (mean rate)
: token bucket size (burst length)
packet/cell
packet/cell
c: link capacity (allocated bandwidth) b
:buffer size
(allocated buffer)
Leaky (Token) Bucket Queue
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30. Packet Scheduling
At routers/switches, certain queueing discipline is enforced to schedule (or arbitrate), for each output line, which input stream to serve next.
Method (1): mixed single queue for each output line
--unfairness, does not take connection or flow bandwidth into account
Method (2): fair queueing (multiple queue for each output line, one for each source)
packet-by-packet round robin
byte-by-byte round robin
weighted byte-by-byte round robin
(weight can the number or total bandwidth of the flows/connections from the source)
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31. Internetworking Scenario, Devices, Approaches
Scenario: LAN--LAN
LAN--WAN--LAN
WAN--WAN
Devices: Repeater, Bridge, Router, Gateway
( Higher-level internetworking devices are more capable to handle and convert the difference in interconnected networks )
Approaches: Connection-oriented: concatenated virtual circuit
Connectionless: through different networks, different routes
Tunneling: special approach for LAN-WAN-LAN
where two LANs are homogeneous and WAN can be connection-oriented or connectionless
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32. Internetworking Local and Wide Area LAN Interconnection
Local area LAN interconnection: bridge, router, switch
Wide area LAN interconnection:
modem, leased lines, ISDN
X-25, Frame Relay, SMDS
B-ISDN, Internet
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33. Internet Internet protocol hierarchy IP header
IP address and subnetting
Control protocols: ICMP, ARP, RARP, BOOTP
Routing protocols: OSPF and BGP
IGMP, Mobile IP, CIDR, IPv6, RSVP
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34. Internet Protocol Hierarchy
OSPF
BGP
SMTP
NNTP
HTTP
Telnet
FTP
NFS YP
Mount
DNS
BOOTP
RPC
TCP
UDP
ICMP
ARP
RARP IP
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35. Options (0 or more words)
IP Header
32 Bits
Version
IHL
Type of service
Total length
Identification D F M F
Fragment offset
Time to live
Protocol
Header checksum
Source address
Destination address 
Options (0 or more words) 
Option
Description
Security
Specifies how secret the data gram is
Strict source routing
Gives the complete path to be followed
Loose source routing
Gives a list of routers not to be missed
Record route
Makes each router append its IP address
Timestamp
Makes each router append its address and timestamp
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36. IP Address and Subnetting
32 Bits
class
Range of host address A
Network
Host to B 10
Network
Host C
110
Network
Host to D
1110
Multicast address to E
11110
Reserved for future use to to
This host …
Host
A host on this network
Broadcast on the local network
Network …
Broadcast on a distant network
127
(Anything)
Loop back
class B
add entries of (this-network, subnet, o)
and (this-network, this-subnet,host)
to routing table 10
network
subnet
host
subnet mask (e.g.)
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37. Internet Control Protocols
ICMP ( Internet Control Message Protocol )
Destination Unreachable, Time Exceeded, Parameter Problem, Source Quench, Redirect, Echo Request, Echo Reply, Timestamp Request, Timestamp Reply, etc.
ARP ( Address Resolution Protocol)
Symbolic name Domain Name System IP address ARP MAC address
RARP (Reverse Address Resolution Protocol) server --through local broadcast IP
BOOTP server ---- through UDP
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38. OSPF (Open Shortest Path First) Interior Gateway Routing Protocol for AS (Autonomous System)
Requirements: open, variety of distance metrics, dynamic, based on type of service, load balancing, hierarchical security, tunnel allowed.
Each AS, divided into areas, has a backbone area connecting all other areas, by point-to-point links, broadcast links, or tunnels in WAN.
Three kinds of routes:
intra-area: link state shorting path routing
inter-area: (1)from source to backbone, (2)across backbone to dest area, (3)to dest
inter-AS: BGP (Border Gateway Protocol) - exterior gateway routing protocol
OSPF messages: Hello, Link state update, Link state ack, Database description, Link state request
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39. BGP (Border Gateway Protocol) Exterior Gateway Routing Protocol for Inter-AS
Designed to allow many kinds of routing policies to be enforced in the inter AS traffic, by route scoring function.
A distance vector protocol where BGP router keeps track of the exact path used in routing table and tells its neighbor the exact path, instead of cost, it is using.
Pairs of BGP routers communicate with each other by establishing TCP connections.
No count-to-infinity problem
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40. IGMP (Internet Group Management Protocol)
Permanent and temporary group addresses
Permanent groups: (e.g.)
all systems on a LAN
all routers on a LAN
all OSPF routers on a LAN
all designated OSPF routers on a LAN
Temporary groups:
IGMP query: each multicast router multicasts to hosts on its LAN to ask them the groups their processes belong to
IGMP response: each host responds class D address it is interested in
Each multicast router constructs a pruned spanning tree per group, using a modified distance vector protocol. Tunneling is used in the pruned spanning tree to bypass nodes not in a spanning tree.
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41. CIDR (Classless InterDomain Routing)
More than half of all class B networks have fewer than 50 hosts. Too many small class C networks would enlarge routing table dramatically.
CIDR: allocate the remaining class C networks (about 2 million) in variable-sized blocks
~ : for Europe (194... and 195... entries) ~ : for North America ~ : for Central and South America
~ : for Asia and the Pacific
~ : reversed for future use
Example: NCTU asks for 4096 addresses and is assigned through along with mask If a packet is addressed to :
mask
base address
next . . .
...
AND . . .
match
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42. IPv6 To address three major problems:
address space, encryption and authentication, flow label for QoS treatment
32 Bits
Version
Priority
Flow label
Payload length
Next header
Hop limit
Source address
(16 bytes)
Destination address
(16 bytes)
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