Download the network layer 1

Network Layer
Chapter 5
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Design Issues
Routing Algorithms
Congestion Control
Quality of Service
Internetworking
Network Layer of the Internet
Revised: August 2011
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
The Network Layer
Responsible for delivering packets
between endpoints over multiple
links
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Application
Transport
Network
Link
Physical
Network Layer Functionality
Getting packets from the source all the way to the
destination
Lowest layer that deals with end-to-end transmission
Must know the topology of the physical network
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Design Issues
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Store-and-forward packet switching »
Services Provided to the Transport Layer
Connectionless service – datagrams »
Connection-oriented service – virtual circuits »
Comparison of virtual-circuits and datagrams »
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Store-and-Forward Packet Switching
Hosts send packets into the network; packets are
forwarded by routers
ISP’s equipment
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Services Provided to the Transport
Layer
Frequently, network layer is the interface between the
carrier and the customer
• Services should be independent of the router
technology
• Transport layer should be shielded from the
topology of the routers present
• Network Layer addressing should be uniform
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Unreliable and connectionless
Just two primitives
• SEND PACKET
• RECEIVE PACKET
The complexity is in the transport layer (hosts)
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Connectionless Service – Datagrams
Packet is forwarded using destination address inside it
• Different packets may take different paths
ISP’s equipment
A’s table (initially)
A’s table (later)
C’s Table
E’s Table
Dest. Line
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Connection-Oriented Service
Properties
• Set up a connection to receiver
• Negotiate parameters
• Packets are guaranteed to be delivered in sequence
• Flow control provided
Complexity is in the network layer
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Connection-Oriented – Virtual Circuits
Packet is forwarded along a virtual circuit using tag inside it
• Virtual circuit (VC) is set up ahead of time
ISP’s equipment
A’s table
C’s Table
E’s Table
In: Line Tag Line Tag: Out
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Internal Organization of Network Layer
Virtual circuits
• No need to choose a route for each packet
Datagram
• No routes are worked out in advance
Note that the type of services provided to transport layer
and internal organization of the network are different
concepts
• E.g. Network Layer may provide a connectionoriented service to transport layer even when it
internally uses datagrams to move the packets
around
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Virtual Circuits
Between the source and destination a path is established
All the packets from the source to destination follow the
same path
Each router must remember where to forward packets for
each of the currently open virtual circuits passing through it
Each packet must contain a virtual circuit number field in
the header for identification purposes
Routers assign this number locally and maintain tables to
determine which incoming packet with a particular number
will be forwarded to which outgoing line
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Datagrams
Each packet must contain full destination address
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Comparison of VC and Datagrams
Router memory space for vc tables vs wasted bandwidth
for extra long address in each packet header
Set up time versus address parsing time
Easier to control congestion with vc
Datagram may be more suitable for transaction processing
systems
VC is vulnerable to crash of the router
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Comparison of Virtual-Circuits & Datagrams
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Routing Algorithms (1)
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Optimality principle »
Shortest path algorithm »
Flooding »
Distance vector routing »
Link state routing »
Hierarchical routing »
Broadcast routing »
Multicast routing »
Anycast routing »
Routing for mobile hosts »
Routing in ad hoc networks »
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Routing Algorithms (2)
Routing is the process of discovering network paths
• Model the network as a graph of nodes and links
• Decide what to optimize (e.g., fairness vs efficiency)
• Update routes for changes in topology (e.g., failures)
Forwarding is the sending of packets along a path
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Routing Algorithm Properties
Correctness
Simplicity
Robustness
• What happens if the topology changes?
Stability
• Must converge to an equilibrium
Fairness
Optimality
• Minimize mean packet delay
• Maximize total network throughput
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Classes of Routing Algorithms
Nonadaptive algorithms
• Route is computed in advance and stored in the router
• This is called static routing
Adaptive algorithms
• Base their decision on current measurement of traffic
and topology
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The Optimality Principle
Each portion of a best path is also a best path; the
union of them to a router is a tree called the sink tree
• Best means fewest hops in the example
B
Network
Sink tree of best paths to router B
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Shortest Path Algorithm (1)
Dijkstra’s algorithm computes a sink tree on the graph:
• Each link is assigned a non-negative weight/distance
• Shortest path is the one with lowest total weight
• Using weights of 1 gives paths with fewest hops
Algorithm:
• Start with sink, set distance at other nodes to infinity
• Relax distance to other nodes
• Pick the lowest distance node, add it to sink tree
• Repeat until all nodes are in the sink tree
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Shortest Path Algorithm (2)
A network and first five steps in computing the shortest
paths from A to D. Pink arrows show the sink tree so far.
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Shortest Path Algorithm (3)
Initialize
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T = {s} set of nodes incorporated (s source)
L(n)=w(i,j) for ns initial path costs to neighbors
Get next node
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Add the neighbor x not in T that has least cost L(x) to s.
Add the edge from this neighbor to old T that
contributes to the path
Update least cost paths
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For all nT, update costs by L(n)=min[L(n),L(x)+w(x,n)]
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Shortest Path Algorithm (4)
...
Start with the sink,
all other nodes are
unreachable
Relaxation step.
Lower distance to
nodes linked to
newest member of
the sink tree
...
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Shortest Path Algorithm (5)
...
Find the lowest
distance, add it to
the sink tree, and
repeat until done
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Flooding
A simple method to send a packet to all network nodes
Each node floods a new packet received on an
incoming link by sending it out all of the other links
Nodes need to keep track of flooded packets to stop the
flood; even using a hop limit can blow up exponentially
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Flooding (2)
Every incoming packet is sent out on every
outgoing line except the one it arrived on
Generates vast number of duplicate packets
Solutions
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Put a hop counter to every packet
Avoid flooding the same packet second time
− Needs packet sequence numbers
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Use selective flooding
− Do not put the packet to every outgoing line, instead put the
packet to certain outgoing lines
Used in military applications and measurements
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Distance Vector Routing (1)
Distance vector is a distributed routing algorithm
• Shortest path computation is split across nodes
Algorithm:
• Each node knows distance of links to its neighbors
• Each node advertises vector of lowest known
distances to all neighbors
• Each node uses received vectors to update its own
• Repeat periodically
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Distance Vector Routing 2
Each router maintains a table that contains the following for
each destination
• Best known distance to the destination
− Number of hops or
− Time delay in milliseconds or
− Total number of packets queued along the path
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Which line to use to get there
Each router is assumed to know the distance to each of its
neighbors
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Distance Vector Routing 3
Once every T msec, each router sends to each neighbor a
list of its estimated delays to each destination
A router, knowing the distance to its neighbor, can
calculate distances to all destinations via its respective
neighbor
Then, it can choose the minimum distance and forward
packets via that neighbor
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Distance Vector Routing (4)
Network
New vector
for J
Vectors received at J from
Neighbors A, I, H and K
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
The Count-to-Infinity Problem 1
Failures can cause DV to “count to infinity” while
seeking a path to an unreachable node
X
Good news of a path
to A spreads quickly
Bad news of no path to A
is learned slowly
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
The Count-to-Infinity Problem 2
In distance vector routing
• Good news travel fast
− One hop per exchange
− Within N (number of nodes) exchanges, everyone will know
about new changes
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Bad news travel slow
− Count-to-Infinity problem
» Routers increase their costs by a value of one to reach infinity
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Link State Routing (1)
Link state is an alternative to distance vector
• More computation but simpler dynamics
• Widely used in the Internet (OSPF, ISIS)
Algorithm:
• Each node floods information about its neighbors in
LSPs (Link State Packets); all nodes learn the full
network graph
• Each node runs Dijkstra’s algorithm to compute the
path to take for each destination
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Link State Routing 2
Distance vector routing
• Did not take line bandwidth into account
• Took too long to converge
Link state routing: Each router
• Discover its neighbors, learn their network address.
• Measure the delay or cost to each of its neighbors.
• Construct a packet telling all it has just learned.
• Send this packet to all other routers.
• Compute the shortest path to every other router.
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Learning about the Neighbors
When a rooter is booted, it sends HELLO packet to all of its
neighbors
Neighbors reply by sending their globally unique names
Modeling LANs need extra mechanisms
• In the next slide an artificial node N is added to
represent the LAN
− From A to C, the path is represented as ANC
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Learning about the Neighbors 2
(a) Nine routers and a LAN. (b) A graph model
of (a).
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Measuring Line Cost
Each router should know an estimate of delay to each of its
neighbors
To estimate the delay, a router can send an echo packet to
its neighbor and measure the time when it receives it back
While measuring time, it is possible to include or exclude
the time the echo packet spends on the queues which
corresponds to consider load on the line
Including load may cause oscillations in the algorithm
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Building Link State Packets
Packets contain
• Identity of the sender
• Sequence number
• Age
• List of neighbors and delays
Packets can be issued
• Periodically or
• When a significant event occurs
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Link State Routing (3) – LSPs
LSP (Link State Packet) for a node lists neighbors and
weights of links to reach them
Network
LSP for each node
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Distributing the Link State Packets
Flooding is used
Packet sequence numbers are used to discard
duplicates or old packets by holding packets for a
short while before flooding them
32-bit sequence numbers are used to prevent
wrap-arounds
For each packet, in addition to sequence number,
age is used and decremented every second to
solve
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Router crash problem
Corrupted sequence number problem
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Link State Routing (4) – Reliable Flooding
Seq. number and age are used for reliable flooding
• New LSPs are acknowledged on the lines they are
received and sent on all other lines
• Example shows the LSP database at router B
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Computing the New Routes
When a router accumulates a full set of link state
packets, it can construct the entire network graph
It then runs Dijkstra’s algorithm to construct
shortest paths to all possible destinations
Even for a large subnet, the memory requirements
are reasonable
Malfunctioning routers may cause problems
Protocols using link state routing
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OSPF and IS-IS
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Hierarchical Routing
As networks grow
• Routing table memory requirements increase
• CPU time to execute routing algorithm increases
Solution: Use hierarchical routing
• Routers are divided into regions
• A router knows every detail about its region but knows
nothing about other regions
• Each region is represented by a “monitor” router
• Monitor routers form a higher level in hierarchy
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Hierarchical Routing 2
Hierarchical routing reduces the work of route computation
but may result in slightly longer paths than flat routing
Best choice to
reach nodes in 5
except for 5C
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Hierarchical routing (3)
Path length increases
How many levels should the hierarchy have?
• If there are N nodes, the optimal number of levels is ln
N
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Broadcast Routing
Broadcast
• Sending a packet to all destinations simultaneously
Broadcast method 1:
• Send a distinct packet to each destination
• Needs addresses of destinations and wastes bandwidth
Broadcast method 2:
• Flooding
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Broadcast Routing 2
Broadcast method 3:
• Multidestination routing
− Each packet contains a list of destinations
− A router receiving a packet checks the destinations and
determines the output lines needed
− The router generates a new packet for each output line and
modifies the destination list by including only those destinations
that are on that particular line
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Broadcast Routing (3)
Broadcast method 4:
• Spanning tree
− A subset of network that includes all routers but contains no
loops
− Each router knows which of its lines belong to the spanning tree
(this is an important restriction)
− Any incoming packet on a spanning tree line is forwarded to all
other spanning tree lines
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Broadcast Routing (4)
Broadcast method 5:
• Reverse Path Forwarding
− No need to know the spanning tree
− When a broadcast packet arrives, the router checks to see if it
arrived on the line that it uses forwarding packets to the source
node of the broadcast
− If yes, it forwards the packets to all other lines except the one it
arrived on.
− If no, it discards the packet, considering that the packet did not
arrive through optimal route.
− Simple and efficient
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Broadcast Routing 5
Broadcast sends a packet to all nodes
• RPF (Reverse Path Forwarding): send broadcast
received on the link to the source out all remaining links
• Alternatively, can build and use sink trees at all nodes
Network
Sink tree for I is
efficient broadcast
RPF from I is larger than
sink tree
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Multicast Routing
Process Groups
• A set of processes that work in an interactive manner
Multicasting
• Sending message to a group
Multicast Routing
• Routing of multicast messages
Group Management
• Creating and destroying groups
• Processes join and leave groups
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Multicast Routing (2)
Routers need to know which host belongs to which group
Each router needs to compute a spanning tree covering all other
routers in the group subnet
Multicast messages are forwarded only along the appropriate spanning
tree
Link state or distance vector approaches are used to
disseminate topology information
Pruning
• Forming the spanning tree for a group
Pruning by link state routing
• Each router knows the complete topology
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Multicast Routing (3) – Dense Case
Multicast sends to a subset of the nodes called a group
• Uses a different tree for each group and source
S
Network with groups 1 & 2
S
Spanning tree from source S
S
Multicast tree from S to group 1
Multicast tree from S to group 2
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Multicast Routing (4) – Sparse Case
CBT (Core-Based Tree) uses a single tree to multicast
• Tree is the sink tree from core node to group members
• Multicast heads to the core until it reaches the CBT
p 1.
Sink tree from core to group 1
Multicast is send to the core then
down when it reaches the sink tree
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Multicast Routing (5)
Pruning by distance vector routing
• Use reverse path forwarding to eliminate routers that
reject that particular group communication
• Scales poorly
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Anycast Routing
Anycast sends a packet to one (nearest) group member
• Falls out of regular routing with a node in many places
Anycast routes to group 1
Apparent topology of
sink tree to “node” 1
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Routing for Mobile Hosts
To route a packet to a mobile host, the network
first has to find it
Classification of users
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Stationary: Never move
Mobile: They move
− Migratory: They move but physically connect to network
somewhere
− Roaming: They maintain connection as they move around
Locations
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Home
Foreign
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Routing for Mobile Hosts (2)
Each area has
• Home agent
− Keeps track of users whose home is in the area
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Foreign agent
− Keeps track of all mobile users who are visiting the area
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Routing for Mobile Hosts (3)
Each mobile host entering a foreign area must
register with the foreign agent of that area as
follows
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Mobile host provides foreign agent
− Its home address
− Current data link address
− Security information
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Foreign agent contacts mobile host’s home agent and
report the presence of the mobile host in its region
The home agent confirms security information and
permits proceed
Foreign agent registers the mobile host
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Encapsulation and Tunneling
After registration,
• Any packet sent to the mobile host is intercepted by the
home agent
• The packet is encapsulated by the home agent and sent
to the foreign agent
• The home agent also informs the sender of the packet
about the new address of the mobile host
• Subsequent packets are directly sent to the mobile host
(tunneling)
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Routing for Mobile Hosts 4
Mobile hosts can be reached via a home agent
• Fixed home agent tunnels packets to reach the mobile
host; reply can optimize path for subsequent packets
• No changes to routers or fixed hosts
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Routing in Ad Hoc Networks
Possibilities when the routers are mobile:
Military vehicles on battlefield.
• No infrastructure.
A fleet of ships at sea.
• All moving all the time
Emergency works at earthquake .
• The infrastructure destroyed.
A gathering of people with notebook computers.
• In an area lacking 802.11.
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Routing in Ad Hoc Networks 2
The network topology changes as wireless nodes move
• Routes are often made on demand, e.g., AODV (below)
A’s starts to
find route to I
A’s broadcast
reaches B & D
B’s and D’s
broadcast
reach C, F & G
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
C’s, F’s and G’s
broadcast
reach H & I
Route Discovery
Request ID
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Incremented each time a ROUTE REQUEST is
broadcast
Other nodes check source address and request id and
discard duplicates
Source sequence #
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Each node maintains a second counter that is increased
whenever a Route Request is sent or someone else’s
Route Request is replied
Dest. Sequence #
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Most recent value of I’s sequence number that A has
seen
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AODV Algorithm
A node receiving ROUTE REQUEST examines source
address and request ID and discards it if it is a duplicate
Looks up for destination in its route table. If fresh route is
known, a ROUTE REPLY packet is sent back to source
Else, increases hop count and broadcasts the packet It
also stores the data in its reverse route table
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Reading
5-1
5-2
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011
Homework
5 (6)
9 (11)
CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011