Network Layer4-1 Chapter 4 Network Layer Part 1: network layer overview datagram networks routers...

Network Layer 4-1

Chapter 4Network Layer

Part 1:network layer overviewdatagram networksrouters

Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith RossAddison-WesleyMarch 2012

Network Layer 4-2

Chapter 4: Network Layer

Chapter goals: understand principles behind network

layer services: network layer service models forwarding versus routing how a router works routing (path selection) dealing with scale advanced topics: IPv6, mobility

instantiation, implementation in the Internet

Network Layer 4-3


4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol

Datagram format IPv4 addressing ICMP IPv6

4.5 Routing algorithms Link state Distance Vector Hierarchical routing

4.6 Routing in the Internet RIP OSPF BGP

4.7 Broadcast and multicast routing

Network Layer 4-4

Network layer transport segment

from sending to receiving host

on sending side encapsulates segments into datagrams

on rcving side, delivers segments to transport layer

network layer protocols in every host, router

router examines header fields in all IP datagrams passing through it

application

transportnetworkdata linkphysical

application


networkdata linkphysical network

data linkphysical

networkdata linkphysical







networkdata linkphysicalnetwork

data linkphysical

Network Layer 4-5

Network layer network vs transport layer

connection service: network: between two

hosts (may also involve intervening routers in case of VCs)

transport: between two processes

application


application


networkdata linkphysical network

data linkphysical








networkdata linkphysicalnetwork

data linkphysical

Network Layer 4-6

Two Key Network-Layer Functions

forwarding: move packets from router’s input to appropriate router output

routing: determine route taken by packets from source to dest.

routing algorithms

analogy:

routing: process of planning trip from source to dest

forwarding: process of getting through single interchange

Network Layer 4-7

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Use a value in a packet’s header to index into the table

Each router has a forwarding table

Corresponding value is the output link on which to place the packet

Network Layer

Forwarding table

A routing algorithm is used to configure the table Algorithm may be centralized and thus each

router must download table Algorithm my be decentralized and run in

each router In either case, router receives routing

protocol messages that are used to configure the table

4-8

Network Layer

routers

Network level (layer 3): packet switch transfers a packet from the input link interface to the

output link interface according to a value in a field in the header of the

network-layer packet Link-layer (layer 2): switches

base their decision on values in the fields in the link-layer frame.

Routers are network level but must also act as link-layer switch since they are physically connected to another device

4-9

Network Layer 4-10

Connection setup

3rd important function in some network architectures: ATM (asynchronous transfer mode), frame

relay, X.25

before datagrams flow, two end hosts and intervening routers establish virtual connection routers get involved

Network Layer 4-11

Network service model

Q: What service model for “channel” transporting datagrams from sender to receiver?

Example services for individual datagrams:

guaranteed delivery guaranteed delivery with

bounded delay (say less than 40 msec delay)

Example services for a flow of datagrams:

in-order datagram delivery guaranteed minimum

bandwidth to flow (acts like transport layer):

if bit rate is below specified rate, guarantee delivery within specified delay

Network Layer 4-12

Network service model

Q: What service model for “channel” transporting datagrams from sender to receiver?

Example services for individual datagrams:

guaranteed delivery guaranteed delivery with

bounded delay (say less than 40 msec delay)

Example services for a flow of datagrams:

guaranteed max jitter. i.e., restrictions on changes in inter-packet spacing

security services. Network layer could encrypt the payloads of all datagrams sent to destination host.

Source and destination know a secret session key

…and other services!

Network Layer 4-13

Network layer service models:

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort*

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

CBR: constant bit rate ATM network serviceVBR: variable bit rate ATM network serviceABR: available bit rate ATM network serviceUBR: unspecified bit rate ATM network service

* i.e., no service at all! A network that delivered no packets would meet this requirement!

Network Layer

ATM networks

Constant bit rate (CBR) ATM network service Goal: provide a flow of packets (known as cells) with

a virtual pipe whose properties are the same as if a dedicated

fixed-bandwidth transmissions link existed between source and destination hosts.

Available bit rate (ABR) ATM network service Slightly-better-than-best-effort-service Cells may be lost, but cannot be reordered and a

minimum transmission rate is guaranteed. Can provide congestion feedback to transport layer

4-14

Network Layer 4-15







Network Layer 4-16

Network layer connection and connection-less service

datagram network provides network-layer connectionless service Used for the internet

VC (Virtual-Circuit) network provides network-layer connection service Used for some DSL services Mostly deployed by telephone companies Mostly used to access internet today In 90’s was “next big thing”

Network Layer 4-17

Network layer connection and connection-less service

VC network is analogous to the transport-layer services, but: service: host-to-host instead of process-to-

process no choice: network provides connection-

oriented or connectionless but cannot provide both

implementation: • in network core (in the routers) • transport-layer connection service is only in the

edges (hosts).

Network Layer 4-18

Virtual circuitsnetwork-layer connections

call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host

address) every router on source-dest path maintains “state” for

each passing connection link, router resources (bandwidth, buffers) may be

allocated to VC (dedicated resources = predictable service)

“source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path

Network Layer 4-19

VC implementation

a VC consists of:1. path from source to destination (a series of links

and routers)2. VC numbers, one number for each link along path3. entries in forwarding tables in routers along path

packet belonging to VC carries VC number (rather than dest address)

VC number can be changed on each link. New VC number comes from forwarding table

Network Layer 4-20

Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …

Forwarding table innorthwest router:

Routers maintain connection state information!

Network decides on path for this VC: A-R1-R2-B and assigns VC numbers 12, 22, 32 to the links.

Network Layer

Virtual Circuits

Why not keep same VC number for all links? Replacing the number keeps the VC number

field small VC setup is simplified

• Each router can choose a different number• Otherwise would have to coordinate the number

across all routers on the path

4-21

Network Layer

Virtual Circuits

Three phases in VC VC setup. Network layer software does:

• Determines path (series of links and routers)• Determines VC number for each link along path• Adds entry in the forwarding table in each router• May reserve resources along path (e.g.,

bandwidth) Data transfer. Packets flow. VC teardown. Initiated by sender or

receiver.• Network informs other side of shutdown• Update forwarding tables of all routers on path

4-22

Network Layer 4-23

Virtual circuits: signaling protocols

used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet

application


application


1. Initiate call 2. incoming call

3. Accept call4. Call connected5. Data flow begins 6. Receive data

We’re not covering signaling protocols!

Network Layer 4-24

Datagram networks (internet) 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

application


application


1. Send data 2. Receive data

Network Layer 4-25

Datagram networks Each router still has forwarding table Table only matches destination address to link interface. When packet arrives at router, router uses dest addr to

index into the forwarding table & determine output interface

Then forwards packet to that interface Table is changed very slowly by a routing algorithm

(typically 1-to-5 minutes!)

application


application


1. Send data 2. Receive data

Network Layer 4-26

1

23

Datagram forwarding table

IP destination address in arriving packet’s header

routing algorithm

local forwarding tabledest address output

linkaddress-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses (in 32 bits), so rather than list individual destination addresslist range of addresses(aggregate table entries)

Network Layer 4-27

Destination Address Range

11001000 00010111 00010000 00000000through 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q: but what happens if ranges don’t divide up so nicely?

Datagram forwarding table

Network Layer 4-28

Longest prefix matching

Destination Address Range

11001000 00010111 00010*** *********

11001000 00010111 00011000 *********

11001000 00010111 00011*** *********

otherwise

DA: 11001000 00010111 00011000 10101010

examples:DA: 11001000 00010111 00010110 10100001 which interface?

which interface?

when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.

longest prefix matching

Link interface

0

1

2

3Problem: address may match two or more table entries:

the second DA matches both second and third entriesResolution: use the longest match

Network Layer 4-29

Datagram or VC network: why?

Internet (datagram) data exchange among

computers “elastic” service, no strict

timing req. “smart” end systems

(computers) can adapt, perform

control, error recovery simple inside network,

complexity at “edge” many link types

different characteristics uniform service difficult

Easy to implement new application level service: just define new protocol in host devices!

ATM (VC) evolved from telephony human conversation:

strict timing, reliability requirements

need for guaranteed service

“dumb” end systems telephones complexity inside

network Hard to implement new

service: must update every router!

Network Layer 4-30







Network Layer 4-31

Router Architecture Overview

Two key router functions:

run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing

link

Forwarding is the same as switching

Routing management Control plane (software)

forwardingData plane (hardware)

PCI bus from processor to input ports

Network Layer

Cisco 1200 router

4-32

Network Layer

Cisco

4-33

Network Layer

Cisco 2505 router

4-34

Network Layer

Routing processor

Executes routing protocols Maintains routing tables and attached

link state information Computes the forwarding table for the

router Performs network management

functions. These functions operate at the

millisecond or second timescale, so can be implemented in software

4-35

Network Layer

Router forwarding plane

The input ports, output ports, and switching fabric together are implemented in hardware and are called the router forwarding plane

why implement these functions in hardware? 10Gbps input link, 64-byte IP datagram Input link has 51.2ns to process the datagram before

another one arrives If N input ports are on a line card (common), must

operate N times faster

4-36

Network Layer 4-37

Input Port Functions

Decentralized switching: given datagram dest., lookup output port

using forwarding table in input port memory

Network control packets sent to the processor

goal: complete input port processing at ‘line speed’

queuing: if datagrams arrive faster than forwarding rate into switch fabric

Physical layer:bit-level reception

Data link layer:e.g., Ethernetsee chapter 5

Note: port here is a physical port. Different from the software ports used with sockets.

Network Layer

Input Port Functions

Input ports have small memory to contain forwarding table (uses cache)

Output port lookup must be very fast; linear search is too slow!

Packet may have to be queued if switching fabric is busy

Other actions Physical and link-layer processing Packet’s version number, checksum and time-to-live

fields must be checked and latter two rewritten Counters used for network management (e.g.,

number of datagrams received) must be updated.

4-38

Network Layer 4-39

Three types of switching fabrics

Network Layer 4-40

Switching Via Memory

First generation routers: traditional computers with switching under direct control of CPU

packet copied to system’s memory speed limited by memory bandwidth (2 bus crossings per datagram)

InputPort

OutputPort

Memory

System Bus

Single bus/input/output port limits throughput

Network Layer 4-41

Switching Via a Bus

datagram from input port memory to output port memory via a shared bus

(processor not used) Input port pre-pends an internal header to indicate

output port Label removed at output port before sending

bus contention: switching speed limited by bus bandwidth; only one packet can be on bus at a time

32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers

Network Layer 4-42

Switching Via An Interconnection Network

overcome bus bandwidth limitations Banyan networks, other interconnection nets initially

developed to connect processors in multiprocessor All input ports have lines to all output ports Switching fabric can selectively open connections Still possible to have contention: two input ports trying to

connect to the same output port advanced design: fragmenting datagram into fixed

length cells, switch cells through the fabric. Cisco 12000: switches 60 Gbps through the

interconnection network

Network Layer 4-43

Output Ports

Buffering required when datagrams arrive from fabric faster than the transmission rate

Scheduling discipline chooses among queued datagrams for transmission

Network Layer 4-44

Output port queueing

buffering when arrival rate via switch exceeds output line speed

queueing (delay) and loss due to output port buffer overflow!

Network Layer 4-45

How much buffering?

RFC 3439 rule of thumb*: average buffering equal to “typical” RTT (say 250 msec) times link capacity C e.g., C = 10 Gps link: 2.5 Gbit buffer

Recent recommendation**: with N flows, buffering equal to RTT C.

N*based on an analysis of queueing dynamics of a relatively small number of TCP flows (Villamizar 1994)

**based on theoretical and experiments on large flows (Appenzeller 2004)

Network Layer 4-46

Input Port Queuing

Fabric slower than input ports combined -> queueing may occur at input queues

Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward

queueing delay and loss due to input buffer overflow!

Can occur when packet arrival rate reaches 58% of capacity! There are solutions…beyond the scope of this class!

Network Layer4-1 Chapter 4 Network Layer Part 1: network layer overview datagram networks routers...

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