Download The Network Layer

Chapter 5
The Network Layer
Network Layer Design Isues
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Store-and-Forward Packet Switching
Services Provided to the Transport Layer
Implementation of Connectionless Service
Implementation of Connection-Oriented Service
Comparison of Virtual-Circuit and Datagram Subnets
Store-and-Forward Packet Switching
fig 5-1
The environment of the network layer protocols.
Services Provided to the Transport
Layer
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3.
The services should be independent of the router technology
The transport layer should be shielded from the number, type and
topology of the routers present
The network addresses made available to the transport layer should
use a uniform numbering plan, even across LANs and WANs
Internet – Connection Less
ATM – Connection-oriented.
Implementation of Connectionless Service
Routing within a diagram subnet.
Implementation of Connection-Oriented Service
Routing within a virtual-circuit subnet.
Comparison of Virtual-Circuit and
Datagram Subnets
5-4
Routing Algorithms
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The Optimality Principle
Shortest Path Routing
Flooding
Distance Vector Routing
Link State Routing
Hierarchical Routing
Broadcast Routing
Multicast Routing
Routing for Mobile Hosts
Routing in Ad Hoc Networks
Routing Algorithms (2)
A – A’, B – B’, C – C’, can fill the channel, then X-X’ doesn’t get a chance
Conflict between fairness and optimality.
The Optimality Principle
Optimality Principle – If router J is on the optimal path from router I to router K
then the optimal path from J to K also falls along the same route.
(a) A subnet. (b) A sink tree for router B.
Shortest Path Routing
The first 5 steps used in computing the shortest path from A to D.
The arrows indicate the working node.
Dijkstra
5-8 top
Dijkstra's algorithm to compute the shortest path through a graph.
Dijkstra
5-8
bottom
Dijkstra's algorithm to compute the shortest path through a graph.
Flooding
Robust but costly.
a)
b)
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d)
Used military application
Wireless Networks
Distributed Database
Metrics against which other routing algorithms are compared.
Distance Vector Routing
(a) A subnet. (b) Input from A, I, H, K, and the new
routing table for J.
Distance Vector Routing (2)
The count-to-infinity problem.
Hierarchical Routing
Hierarchical routing.
Problems – optimal paths are sacrificed
Hierarchical Routing
How many levels of hierarchy
720 routers
720 routers in 24 regions
Three levels of hierarchy – 8 clusters each containing 9 regions of 10
clusters
Link State Routing
Each router must do the following:
1. Discover its neighbors, learn their network address.
2. Measure the delay or cost to each of its neighbors.
3. Construct a packet telling all it has just learned.
4. Send this packet to all other routers.
5. Compute the shortest path to every other router.
Learning about the Neighbors
(a) Nine routers and a LAN. (b) A graph model of (a).
Setting Link Cost
a)
b)
Bandwidth
Delay – measured by sending special ECHO
Measuring Line Cost
A subnet in which the East and West parts are connected by two lines.
Including queueing delay may lead to a lot of oscillation
Load spread over both lines
Building Link State Packets
(a) A subnet. (b) The link state packets for this subnet.
Few Problems
Algorithm – Sequence number less means obsolete
a)If sequence numbers wrap around, confusion will reign
b)Router crashes, sequence number is lost
c)Sequence number gets corrupted
Aging and then dropping the packet
When a link-state packet comes in to router it is not queued immediately
rather wait if any other packets arrive
Distributing the Link State Packets
The packet buffer for router B in the previous slide (Fig. 5-13).
E has arrived twice.
Cs copy arrive from F, then F will be ack not send
OSPF (Open Shortest Path First)
a)
Refreshed every 60 seconds.
b)
Hardware Problem router getting corrupt etc.
Broadcast Routing
Reverse path forwarding. (a) A subnet. (b) a Sink tree. (c) The
tree built by reverse path forwarding.
Broadcast Routing
Multidimensional Routing – each packet contains either a list of
destinations. When the packet comes, checks the set of
destinations needed
Spanning Tree –
Reverse Path Forwarding – Broadcast.
Checks if the packet has arrived following the correct hop or not.
If correct hop then rebroadcast.
Multicast Routing
(a) A network. (b) A spanning tree for the leftmost router.
(c) A multicast tree for group 1. (d) A multicast tree for group 2.
Routing for Mobile Hosts
A WAN to which LANs, MANs, and wireless cells are attached.
Routing for Mobile Hosts (2)
Packet routing for mobile users.
Routing in Ad Hoc Networks
Possibilities when the routers are mobile:
1. Military vehicles on battlefield.
– No infrastructure.
2. A fleet of ships at sea.
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All moving all the time
3. Emergency works at earthquake .
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The infrastructure destroyed.
4. A gathering of people with notebook computers.
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In an area lacking 802.11.
Route Discovery
a) (a) Range of A's broadcast.
b) (b) After B and D have received A's broadcast.
c) (c) After C, F, and G have received A's broadcast.
d) (d) After E, H, and I have received A's broadcast.
Shaded nodes are new recipients. Arrows show possible reverse routes.
Route Discovery (2)
Format of a ROUTE REQUEST packet.
Route Discovery (2)
The (Source Address, Request ID) pair is looked up in a local history table
Receiver looks up the destination in its route table. If a fresh route is known, then a
ROUTE REPLY is sent.
Destination sequence number is higher than the Destination sequence in the Route
Discovery Packet
Increments Hop count and rebroadcasts ROUTE REPLY
Stores the data in a new entry in its reverse route table.
Route Discovery (3)
Format of a ROUTE REPLY packet.
Route Discovery (3)
IN response – Source addr, destination addr and Hop Count
copied but Dest seq number taken from its counter
Hopcount is set to 0, Lifetime field controls how long the route is
valid.
At each intermediate node
1.No route to I is known,
2.Sequence number of I in the ROUTE REPLY packet is greater
than the value in the routing table
3.The sequence numbers are equal but the new route is shorter
4.Hop Count incremented
5.In large network Discovery increases with Time to Live
incrementally being increased from 1, 2, 3
Route Maintenance
Active Neigbors that have fed in those destinations to A in last t
seconds
(a) D's routing table before G goes down.
(b) The graph after G has gone down.
Node Lookup in Peer-to-Peer Networks
(a) A set of 32 node identifiers arranged in a circle. The shaded ones
correspond to actual machines. The arcs show the fingers from
nodes 1, 4, and 12. The labels on the arcs are the table indices.
(b) Examples of the finger tables.
a)
CASE STUDY – Key = 3, Key = 14, Key = 16
Congestion Control Algorithms
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General Principles of Congestion Control
Congestion Prevention Policies
Congestion Control in Virtual-Circuit Subnets
Congestion Control in Datagram Subnets
Load Shedding
Jitter Control
Congestion
When too much traffic is offered, congestion sets in and
performance degrades sharply.
General Principles of Congestion Control
1. Monitor the system .
– detect when and where congestion occurs.
2. Pass information to where action can be taken.
3. Adjust system operation to correct the problem.
Congestion Prevention Policies
5-26
Policies that affect congestion.
Congestion Control in Virtual-Circuit
Subnets
(a) A congested subnet. (b) A redrawn subnet, eliminates
congestion and a virtual circuit from A to B.
(b) Problem is in virtual circuits there may be
provisioning but not real usage.
Congestion Control in Datagram
Subnets
u_new = au_old + (1 – a)f – instant line utilization
where the constant a determines how fast the router forgets the history.
If u_new is above a threshold action is taken.
Warning Bit
Went to destination and then source
Every router along the path set the warning bit, traffic increased only
when no router is in trouble.
Congestion Control in Datagram
Subnets
Choke Packets
In this approach the router sends a choke packet back to the source
The source on receiving the choke packets, reduce the traffic by X%
Wait for the next choke packet
Hop-by-Hop
Choke Packets
(a) A choke packet that affects
only the source.
(b) A choke packet that affects
each hop it passes through.
Sending choke packets to the
source may be costly.
Load Shedding/ Random Early
Detection
a)
Which packets to discard may depend on application
Multimedia – old packets
Text – Recent Packets
Packets importance can be marked in the beginning (application layer),
then which packets to discard can be taken.
RED ---- Discard selected packets and not report, source knows when
not getting acknowledgement and reduces the
Jitter Control
Schedule Delay according to Deadline Miss
(a) High jitter.
(b) Low jitter.
Quality of Service
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Requirements
Techniques for Achieving Good Quality of Service
Integrated Services
Differentiated Services
Label Switching and MPLS
Requirements
5-30
How stringent the quality-of-service requirements are.
Buffering
Smoothing the output stream by buffering packets.
The Leaky Bucket Algorithm
(a) A leaky bucket with water. (b) a leaky bucket with packets.
The Leaky
Bucket
Algorithm
(a) Input to a leaky bucket.
(b) Output from a leaky
bucket. Output from a token
bucket with capacities of (c)
250 KB, (d) 500 KB, (e)
750 KB, (f) Output from a
500KB token bucket feeding
a 10-MB/sec leaky bucket.
The Token Bucket Algorithm
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(a) Before.
(b) After.
If we call the burst length S sec, the token bucket capacity C bytes, the
token arrival rate \rho bytes/sec and the maximum output rate M
bytes/sec, we see an output burst contains a maximum of C + \rho . S
bytes. We also know that the number of bytes in a maximum –speed
burst of length S seconds is MS.
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Resource reservation – 1. Bandwidth, 2. Buffer Space, 3. CPU cycles
T = 1/\mu x 1/(1- \lambda/\mu)
\lambda/\mu – CPU utilization
\mu = 1,000,000 packets/sec, \lambda = 950,000 packets/sec
Admission Control (RFC 2210, 2211)
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Token Bucket Rate is 1 Mbps and Token Bucket Size is 500 KB,
the bucket can fill continuously for 4 sec
Peak data rate – Maximum tolerable rate
Smallest Packet which can be handled
An example of flow specification.
Packet Scheduling
(a) A router with five packets queued for line O.
(b) Finishing times for the five packets.
The Network Layer in the Internet
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The IP Protocol
IP Addresses
Internet Control Protocols
OSPF – The Interior Gateway Routing Protocol
BGP – The Exterior Gateway Routing Protocol
Internet Multicasting
Mobile IP
IPv6
Design Principles for Internet
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Make sure it works.
Keep it simple.
Make clear choices.
Exploit modularity.
Expect heterogeneity.
Avoid static options and parameters.
Look for a good design; it need not be perfect.
Be strict when sending and tolerant when receiving.
Think about scalability.
Consider performance and cost.
Collection of Subnetworks
The Internet is an interconnected collection of many networks.
The IP Protocol
The IPv4 (Internet Protocol) header.
Version – IPV4/IPV6
IHL – how long the header is – in 32 bit word, minimum is 5, maximum
is 15 (60 bytes)
Type of Service – (6 bits) – four queuing priority, three discard
probabilities and the historical class
Total Length – header + data, maximum is 65,535
Identification Field – fragments of a datagram have same Identification
value
DF – Don’t Fragment (may mean moving through sub-optimal route,
MF- More Fragments. (all except the last fragment set )
Fragment Offset – 8192 fragments per datagram (multiple of 8 bytes)
Time to Live –
Protocol – TCP, UDP
Header Checksum - verified
The IP Protocol (2)
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Some of the IP options.
Addressing Scheme for IPV4
a)
Network Number --- Host number
IP Addresses
IP address formats.
IP Addresses (2)
Special IP addresses.
First Octet Rule for Class
Determination
a)
A (1-126), B – (126 – 191), C – (192-223) D -- (224 – 239)
Subnets
A campus network consisting of LANs for various departments.
Subnets (2)
A class B network subnetted into 64 subnets.
<net-id> <all-is> Directed broadcast
<all – 1s><all 1s> Local broadcast within the network
<all 0s> <all 0s> ‘This network’ , ‘This host’
<all 0s> <host-id> A host on this network
<127> <any> -- loopback address
CDR – Classless InterDomain Routing
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A set of IP address assignments.
Address Moving in CIDR
a)
A packet comes in .addressed 194.24.17.4 –compare it with each
mask….
Largest Suffix match
Aggregated entry of the three –
194.24.0.0/19
Omaha  Minneapolis, New York, Dallas, Denver
194.24.12.0/22
NAT – Network Address Translation
Placement and operation of a NAT box.
NAT
a)
While leaving – ip, port are put in table, checksum is calculated and
inserted into the packets.
b)
While coming back – ip, port, checksum of ip and port checksum
c)
Three ranges of ip addresses as private
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10.0.0.0
172.16 ---- 172.31
192.168
NAT – criticism
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Architectural model of IP where each IP is identified by a single
machine
Internet from connection-less to connection-oriented
NAT violates the most fundamental rule of protocol layering
Processes on the Internet are not required to use TCP or UDP
Some application insert IP address in their body – (FTP) – NAT
doesn’t know about it.
Internet Control Message Protocol
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The principal ICMP message types. (encapsulated in an IP packet)
ARP– The Address Resolution Protocol
Three interconnected /24 networks: two Ethernets and an FDDI ring.
Optimization on ARP
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Caching
Braodcast the mapping when booting
Send it to router if not local
Dynamic Host Configuration Protocol
Operation of DHCP.
DHCP Relay, Broadcasts a DISCOVER,
Leasing of IP, if not renewed it is taken back.
OSPF – (Open Shortest Path First)
The Interior Gateway Routing Protocol
(a) An autonomous system. (b) A graph representation of (a).
Salient Features
a)O – open source
b)Support variety of distant metrics
c)Dynamic algorithm – have to adapt
d)Load balancing and not the best path
e)Hierarchical systems
f)Security is needed
OSPF supports connection – point-to-point/multiaccess with broadcast
OSPF (2)
The relation between ASes, backbones, and areas in OSPF, every router
has .
OSPF (3)
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The five types of OSPF messeges.
BGP – The Exterior Gateway Routing
Protocol
No transit through particular AS etc..
(a) A set of BGP routers.
(b) Information sent to F.
END HERE
The Main IPv6 Header
The IPv6 fixed header (required).
Extension Headers
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IPv6 extension headers.
Extension Headers (2)
The hop-by-hop extension header for large datagrams (jumbograms).
Extension Headers (3)
The extension header for routing.
RSVP-The ReSerVation Protocol
(a) A network, (b) The multicast spanning tree for host 1.
(c) The multicast spanning tree for host 2.
RSVP-The ReSerVation Protocol (2)
(a) Host 3 requests a channel to host 1. (b) Host 3 then requests a
second channel, to host 2. (c) Host 5 requests a channel to host 1.
Expedited Forwarding
Expedited packets experience a traffic-free network.
Assured Forwarding
A possible implementation of the data flow for assured forwarding.
Label Switching and MPLS
Transmitting a TCP segment using IP, MPLS, and PPP.
Internetworking
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How Networks Differ
How Networks Can Be Connected
Concatenated Virtual Circuits
Connectionless Internetworking
Tunneling
Internetwork Routing
Fragmentation
Connecting Networks
A collection of interconnected networks.
How Networks Differ
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Some of the many ways networks can differ.
How Networks Can Be Connected
(a) Two Ethernets connected by a switch.
(b) Two Ethernets connected by routers.
Concatenated Virtual Circuits
Internetworking using concatenated virtual circuits.
Connectionless Internetworking
A connectionless internet.
Tunneling
Tunneling a packet from Paris to London.
Tunneling (2)
Tunneling a car from France to England.
Internetwork Routing
(a) An internetwork. (b) A graph of the internetwork.
Fragmentation
(a) Transparent fragmentation. (b) Nontransparent fragmentation.
Fragmentation (2)
Fragmentation when the elementary data size is 1 byte.
(a) Original packet, containing 10 data bytes.
(b) Fragments after passing through a network with maximum
packet size of 8 payload bytes plus header.
(c) Fragments after passing through a size 5 gateway.