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CSCD 330Network Programming Spring 2019
Lecture 13Network Layer
Reading: Chapter 4
Some slides provided courtesy of J.F Kurose and K.W. Ross, All Rights Reserved, copyright 1996-2007
Network Superhighway
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Introduction• So far, have looked at
• Application Layer
• Transport Layer
• Now, move down protocol stack to
• Network Layer• What services are offered?
• How does this layer fit with other layers?
• Hardware is used at this layer?
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Network Layer
Goals• Understand principles behind network layer
• Network layer service models
• Forwarding versus routing, difference
• How a router works - Internally
• Routing itself
• Addresses, paths, algorithms
TCP/IP Model
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We areHere
Network
Data Link
Transport
Appliction
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Routing in the Internet
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Hierarchical Routing - Solution
• Internet is huge, distributed system managed by mostly private, possibly competing corporations
• Each corporate entity is responsible for their own routing within their IP space
• So, they aggregate routers into regions ...
Autonomous Systems (AS) • Routers in same AS run same routing protocol
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Autonomous System (AS)
• What is an Autonomous System?• Within Internet, an AS is
• Unit of router policy, either single network or group of networks controlled by common network administrator
• On behalf of single administrative entity• An autonomous system is assigned globally unique
number, sometimes called an Autonomous System Number (ASN)
• Report of current numbers http://www.cidr-report.org/as2.0/autnums.html
Routing Within and Between AS's
AS 123
AS 45
AS 678
Autonomous Systems
Example AS’s
AS # Provider 701 UUnet (U.S. ) (AS 701-705)
1239 Sprintlink U.S. Domestic
3356 Level 3
7018 AT&T WorldNet
209 Qwest
3935 Eastern Washington University
. . . .9
Autonomous Systems• Have Routing policy for each AS
• Can decide what routing algorithms to use• Typical to have different routing algorithms
• Interior to the AS• Interior Gateway Protocols (IGP's)
• Exterior networks between AS's• Exterior Gateway Protocols (EGP's)
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Network Layer
• Diagram shows at end points entire protocol stack implemented
• Need transport and applications layers for processes
• Routers only need layers up through network layer
• Data Link• Network
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
networkdata linkphysical network
data linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
Diagram of Differences Network vs. Transport
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Two Key Network-Layer Functions
• Forwarding• Move packets from router’s input queues to
router's output queues• Movement contained within one router
• Routing• Determine routing for packets from source to destination
•Routing algorithms important in efficiently routing packets•Movement of packets is distributed
Routing vs. Forwarding
Analogy
You take a road trip across country, go through many highway interchanges
• Routing: Process of planning trip from source to destination
• Forwarding: Process of getting through single interchange
How Routing Works …
• Every router has Forwarding Table
• Also called a Routing Table
• Router forwards packets by examining value in arriving packet header
– Destination IP Address
• Uses value to index into router’s forwarding table
• See following slide …14
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1
23
0111
Destination value in packet’s header
routing algorithm
local forwarding tableheader value output link
0100010101111001
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Interplay between Routing and Forwarding
All routers have routing tables
Specify next hop in route
Create Routing Table
Built by routing algorithms
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IP Datagram Delivery
Ethernet
TokenRingLANEthernet
H1
R1 R2
R3 R4
H2
Network ofEthernetswitches
Point-to-point link Point-to-point link
IP
• View at Data link Layer • Internetwork collection of LANs or point-to-point links or
switched networks that are connected by routers
Ted
Ed
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H1
R1 R2
R3 R4
H2
10.2.1.0/24
20.1.0.0/1610.1.2.0/24
10.1.0.0/24 10.3.0.0/16
20.2.1.0/28
IP Datagram Delivery
IP
• View at IP Layer• An IP network is a logical entity with a network number
• We represent an IP network as a “cloud”
• The IP delivery service takes the view of clouds, and ignores
data link layer view
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Routing tables• Each router keeps a routing table tells the router how to process an
outgoing packet• Main columns
– Destination address: Where is IP datagram going to?– Next hop: How to send the IP datagram?– Interface: What is the output port?
• Routing tables help datagrams gets closer to their destinationRouting table unique for each router
directdirectR4direct R4R4
NextHop
eth0eth0serial0eth1eth0eth0
interface
10.1.0.0/2410.1.2.0/2410.2.1.0/2410.3.1.0/2420.1.0.0/1620.2.1.0/28
Destination
IP datagrams can be directly delivered (“direct”) to connected networks or sent to another router (“R4”)
Routing Table Example
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Delivery with routing tables
D e s t i n a t i o n N e x t H o p 1 0 . 1 . 0 . 0 / 2 4 1 0 . 1 . 2 . 0 / 2 4 1 0 . 2 . 1 . 0 / 2 4 1 0 . 3 . 1 . 0 / 2 4 2 0 . 1 . 0 . 0 / 1 6 2 0 . 2 . 1 . 0 / 2 8
d i r e c t R 3 R 3 R 3 R 3 R 3
H 1
R 1 R 2
R 3 R 4
H 2
1 0 . 2 . 1 . 0 / 2 4
2 0 . 1 . 0 . 0 / 1 61 0 . 1 . 2 . 0 / 2 4
1 0 . 1 . 0 . 0 / 2 4 1 0 . 3 . 0 . 0 / 1 6
2 0 . 2 . 1 . 0 / 2 8
2 0 . 2 . 1 . 2 / 2 8
D e s t i n a t i o n N e x t H o p 1 0 . 1 . 0 . 0 / 2 4 1 0 . 1 . 2 . 0 / 2 4 1 0 . 2 . 1 . 0 / 2 4 1 0 . 3 . 1 . 0 / 2 4 2 0 . 1 . 0 . 0 / 1 6 2 0 . 2 . 1 . 0 / 2 8
d i r e c t d i r e c t R 4 d i r e c t R 4 R 4
D e s t i n a t i o n N e x t H o p 1 0 . 1 . 0 . 0 / 2 4 1 0 . 1 . 2 . 0 / 2 4 1 0 . 2 . 1 . 0 / 2 4 1 0 . 3 . 1 . 0 / 2 4 2 0 . 1 . 0 . 0 / 1 6 2 0 . 2 . 1 . 0 / 2 8
R 3 R 3 R 2 d i r e c t d i r e c t R 2
D e s t i n a t i o n N e x t H o p 1 0 . 1 . 0 . 0 / 2 4 1 0 . 1 . 2 . 0 / 2 4 1 0 . 2 . 1 . 0 / 2 4 1 0 . 3 . 1 . 0 / 2 4 2 0 . 2 . 0 . 0 / 1 6 3 0 . 1 . 1 . 0 / 2 8
R 3 d i r e c t d i r e c t R 3 R 2 R 2
D e s t i n a t i o n N e x t H o p 1 0 . 1 . 0 . 0 / 2 4 1 0 . 1 . 2 . 0 / 2 4 1 0 . 2 . 1 . 0 / 2 4 1 0 . 3 . 1 . 0 / 2 4 2 0 . 1 . 0 . 0 / 1 6 2 0 . 2 . 1 . 0 / 2 8
R 1 R 1 d i r e c t R 4 d i r e c t d i r e c t
D e s t i n a t i o n N e x t H o p 1 0 . 1 . 0 . 0 / 2 4 1 0 . 1 . 2 . 0 / 2 4 1 0 . 2 . 1 . 0 / 2 4 1 0 . 3 . 1 . 0 / 2 4 2 0 . 1 . 0 . 0 / 1 6 2 0 . 2 . 1 . 0 / 2 8
R 2 R 2 R 2 R 2 R 2 d i r e c t
to:20.2.1.2
Two Service Models at Network Layer• What services could network layer provide?
• Guarantee delivery,
• Guarantee bandwidth,
• Order packets,
• Time delay guarantee …
• Given what you know of the Network layer, are any of these services implemented?
No!
• Decided that network only does best effort delivery!!
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Two Service Models at Network Layers• Recall, we discussed Circuit Switched Network and
Datagram Network
• However, you can try to emulate these services in network layer virtually
• Datagram service is what we typically have1. Datagram service – best effort, no previous connection setup
• Virtual service model lets you obtain services other than best effort
2. Virtual Service – tries to compensate for lack of guaranteed service, sets up connections ahead of time
Examples: ATM, Asynchronous Transfer Mode 21
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Datagram Service
• Connectionless Unacknowledged Network Service• Attitude to packets Characterized by
• “Send and forget!”• Does not guarantee actual delivery• Does not guarantee data is undamaged• Does not guarantee data delivered in
order• Does not guarantee that only one copy of
the data will be delivered
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Virtual Service
• Call setup, teardown for each call before data can flow
• Each packet carries Virtual Circuit (VC) identifier, VC25, VC601– Not same as destination host address
• Every router on source-destination path maintains “state” for each passing connection
• Sets Up ... Source-to-destination path ahead of time
• Uses that route to send all packets• Behaves much like telephone circuit
Virtual Service
• Book discusses mechanics of a generic virtual service , Section 4.2
• Have virtual identifiers instead of IP addresses
• Can run a virtual service over the Internet
• Routers must recognize the virtual identifiers
Two Examples: ATM and MPLS
Asynchronous Transfer Mode http://www2.rad.com/networks/infrastructure/atm/main.htm
Multiprotocol Label Switching http://www2.rad.com/networks/infrastructure/ipmpls/main.htm
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Datagram Networks Basics
• No call setup at network layer
• Routers: No state about end-to-end connections• No network-level concept of “connection”
• Packets forwarded using destination host address• Packets between same source-dest pair may take
different paths
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
1. Send data 2. Receive data
Datagram Networks
• No advance setup of paths, link to link connections
• Need a global number recognized by all network components, hosts and routers
• How would you set up routing tables in order to accommodate … currently, over 4 billion entries?
• Question: Whats the best way to do this?
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Routing Table Organization
• One dumb idea, global table 4 billion entries
• 232
• But … didn't do that
• IP addresses assigned contiguous address blocks by region
• So, can use a matching algorithm and match the network prefix to route packets
• Have distributed address space
• More on addressing later ...27
Routers
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Routers Internals
• Routers are the “glue” that holds the Internet together
• Router speeds greatly affect how well traffic gets moved around the network
• Performance of routers turns out to be critical
• Today, look at evolution of routers and the speedups that have occurred
Routers Commercial Realities
• A router is sold as one big box• Cisco, Juniper, Redback, Avici, …• No standard interfaces between components• Cisco switch, Juniper cards, and Avici software
• Vendors vs. Service providers• Vendors: Build the routers and obey standards• Providers: Buy the routers and configure them
EWU is a provider
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Routing Architectures
Routing Processor
Switching Fabric
A Router Consists of Ports Connections to wires to other
network entities
Switching fabric A “network” inside the router that
transfers packets between ports
Routing processor Brain of the router ...Maintains
lookup tables
ports
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Router Architecture Overview
Routers do two important things• Build and Maintain Routing Tables
• Performs Packet Switching and Updating
Several input ports
Several output ports
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Generic Router Architecture
LookupIP Address
UpdateHeader
Header ProcessingData Hdr Data Hdr
1M prefixesOff-chip DRAM
AddressTable
AddressTable
IP Address Next Hop
QueuePacket
BufferMemory
BufferMemory
1M packetsOff-chip DRAM
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Input Port Functions
Input Port Function: • Given datagram destination lookup output
port using forwarding table in input port memory
Physical layer:bit-level reception
Data link layer:EthernetProcess packetup to network layer
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Input Port Functions
• Needs to perform lookup at line speed• Gbps in most networks
Example:• OC-48 link – runs at 2.5 Gbps
• (OC) Optical Carrier – SONET fiber optic network, different sizes
• If packets are 256 bytes – small packet
• Must lookup speeds of 1 Million lookups/sec
• Binary search typically done to speed things up
• Uses special tree structures
• And other methods ... more later
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Switching Fabrics
• Switching fabric is heart of a router• Through switching that datagrams are actually moved
from an input port to an output port
• Switching can be accomplished in a number of ways ...
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Three types of switching fabrics
1. 2.
3.
Relate to Router Evolution
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First Generation, Switching Via Memory
First Generation Routers Traditional computers with switching under direct control of CPU, act as router Packet copied to system’s memory Speed limited by Memory Bandwidth 2 bus crossings per datagram, looks like shared memory multi-processors
InputPort
OutputPort
Memory
System Bus
CISCO Catalyst 8500's
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RouteTableCPU Buffer
Memory
LineInterface
MAC
LineInterface
MAC
LineInterface
MAC
Typically <0.5Gb/s aggregate capacity
Shared Bus
Line Interface
CPU
Memory
First Generation Routers
Off-chip BufferOne route table
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First GenerationSwitching via Memory
• Comment• Modern routers also switch via memory
• Difference from early routers
• Address lookup and packet switching in memory
• Performed by processors on input line cards
• Greatly speeds things up !!!!
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2nd GenerationSwitching Via a Bus
• Use shared bus• No intervention by routing processor
Since bus is shared• Only one packet at a time can be transferred over bus• And since every packet must cross bus, switching
bandwidth of router limited to bus speed
Bus contention: Switching speed limited by bus bandwidth, one packet at a time
Another speedup have buffer memory on input cards!
• Example: 1 Gbps bus, Cisco 1900:
OK … for access and enterprise routers• Not regional or backbone
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RouteTableCPU
LineCard
BufferMemory
LineCard
MAC
BufferMemory
LineCard
MAC
BufferMemory
FwdingCache
FwdingCache
FwdingCache
MAC
BufferMemory
Typically <5Gb/s aggregate capacity
Second Generation Routers
Shared bus
Buffer Memory
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Third GenerationCrossbar Switch
• A crossbar switch is a matrix of switches between inputs and outputs
• Overcomes bandwidth limitation of single, shared bus
• More sophisticated interconnection network
• Used in past to interconnect processors in multiprocessor computer architectures
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Crossbar Switch Improvements Crossbar switch enables high performance for two reasons:
First, connections from line cards to central switch are now simple point-to-point links
Operate at very high speed. Semiconductor companies have developed chip-to-chip serial links operating at over 1 Gbps
Second, can support multiple bus transactions simultaneously – Parallelizing the process
This greatly increases aggregate bandwidth of the system
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Crossbar Switch• How it works
• Consists of 2N buses• Connect - N Input ports to N Output ports
• Packet arrives at input port travels along horizontal bus attached to input port until it intersects with vertical bus leading to desired output port
• Key idea – parallel processing of packets• If vertical bus leading to output port is free
Packet is transferred to output port• Else If vertical bus being used to transfer packet from another input
port to same output port, Arriving packet is blocked and must be queued at the input port …
More on queuing later …Paper on routershttp://www.cs.cmu.edu/~srini/15-744/F02/readings/McK97.html
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Third Generation Routers
LineCard
MAC
LocalBuffer
Memory
CPUCard
LineCard
MAC
LocalBuffer
Memory
“Crossbar”: Switched Backplane
Line Interface
CPU
Memory
FwdingTable
RoutingTable
FwdingTable
Typically <50Gb/s aggregate capacity
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Output Ports
• Output port processing• Datagrams then stored in output port's memory,
transmits them over outgoing link
• Queuing and buffer management needed when switch fabric delivers packets to output port at a rate that exceeds output link rate
• Cover output port queuing below
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Output Ports
• Buffering required when datagrams arrive from fabric faster than transmission rate
• Scheduling algorithm chooses among queued datagrams for transmission• Can be simple FIFO or some fairness algorithm
based on destination packet distribution
encapsulation
Here is where many packets get dropped
Where Does Packet Queue Occur?
• Consider That ... • Packet queues can form at both input ports and
output ports
• As queues grow large, router's buffer space will eventually be exhausted
• Packet loss will occur!!!
Input and Output Queues
• Question of Speeds
• Suppose that input line speeds and output line speeds are all identical,
• With n input ports and n output ports
• If switching fabric speed is at least n times as fast as the input line speed,
• Will queues form at input ports?
• NO.
Input and Output Queues• What about output ports?
• Worst case, packets arrive at each n input ports will be destined to same output port
• In time it takes to receive (or send) a packet, n packets will arrive at this output port
• Since output port can only transmit a single packet in a unit of time (the packet transmission time), n arriving packets will have to queue (wait) for transmission over the outgoing link
Input and Output Queues• If switch fabric is not fast enough to transfer all arriving
packets through fabric without delay, then packet queuing will also occur at input ports
• If two packets at front of two input queues are destined to same output queue, then one packet will be blocked and must wait at input queue -
Input and OutputQueues
• This phenomenon is known as
• Head-of-the-line (HOL) blocking • On an input-queued switch
• Queued packet in an input queue must wait for transfer through fabric due to blocking of another packet at head-of-the-line
• What would be one solution?
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Solution: Virtual Output Queues
• Maintain N virtual queues at each input– one per output
Output 1
Output 2
Output 3
Input 1
Input 2
Input 3
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Summary
• Multiple things can be done to enhance router performance• Input cards
• Have own CPU processors• Use caches for address lookup• Have copies of routing tables
• Switch fabric
Cross Switches faster - u2013-22013-2018-2013-2018-2013-2018-2013-2018-se parallelism to switch packets
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Reading: Chapter 4 - Network Layer