Topic 3 – Network Layer and WAN Introduction Forwarding and Routing Network service models Virtual...

Topic 3 – Network Layer and WAN

•Introduction•Forwarding and Routing•Network service models•Virtual circuit networks•Datagram networks•Router architecture and performance•IPv4•IPv4 addressing•DHCP•NAT•ICMP

Learning Outcomes• Clear understanding of the role of the network layer

in achieving source to destination delivery.• Appreciate the differences between datagram and

virtual circuit packet switching.• The distinction between forwarding and routing.• The basic semantics and syntax of IPv4.• Able to fragment according to the IPv4 protocol.• Good working knowledge of the IPv4 address

formats and structure.• Understand the role of DHCP and its basic

operation • Understand the role of NAT and its basic operation.

Introduction• Delivers transport segments from sending to receiving host .

• On sending side encapsulates segments into datagrams.

• On receiving side, delivers segments to transport layer.

• Network layer protocols in every host, router.

• Router examines header fields in all IP datagrams passing through.

See next slide for schematic

application

transportnetworkdata linkphysical

application


networkdata linkphysical network

data linkphysical

networkdata linkphysical







networkdata linkphysicalnetwork

data linkphysical

Datagram sent by H1 to H2. The diagram show the networking layers in action.

Network Layers

Forwarding and Routing

• Forwarding: Transfer packets from router’s input to appropriate router output.

• Routing: Determine route taken by packets from source to destination.

• Routing algorithms/protocols: Methods enable efficient routing for a given set of conditions.

Interplay between routing and forwarding

Datagram arriving at router

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Network Service models

What service model for transporting datagrams from sender to receiver should be followed?

Possible requirements for individual datagrams:• Guaranteed delivery.• Guaranteed delivery with less than 40 ms. delay.For the flow of datagrams:• In-order datagram delivery• Guaranteed minimum bandwidth to flow• Restrictions on changes in inter-packet spacing

The next slide shows the service model taken by the two major forms of packet switching.

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)N/A - nocongestionN/A - nocongestionyes

no

Guarantees ?

Actual Network layer service models:

Virtual circuit and Datagram networks

Network layer connection and connection-less service.

• Datagram network provides a network-layer connectionless service

• VC network provides a network-layer connection service - analogous to the transport-layer services’ but:

Virtual circuit networks“Source-to-destination path behaves much like a

telephone circuit”

• Call setup, teardown for each call before data can flow

• Each packet carries VC identifier (not destination host address)

• Every switch on source-destination path maintains “state” for each passing connection

• Link and switch resources (bandwidth, buffers) may be allocated to VC (dedicated resources offer predictable services)

VC implementationA VC consists of:

1. Path from source to destination

2. VC identifiers, one identifier for each link along the path.

3. Entries in forwarding tables of switches along path

• A packet belonging to a VC carries a VC identifier (more like L2 operation than interpreting a L3 destination address)

• VC number can be changed on each link.– New VC number comes from 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 :

Switches maintain connection state information!

Forwarding table

Virtual circuits: signaling protocols

• Used to setup, maintain and teardown VC• Used in ATM, frame-relay, X.25• Not used in today’s Internet (datagram)

application


application


1. Initiate call 2. incoming call

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

Datagram Networks• 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-destination pair may take different paths

application


application


1. Send data 2. Receive data

Network

DestinationSource

Destination Address Range Link Interface

11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111

otherwise 3

> 4 billion (232)possible entries

Ex. Forwarding table for IPv4

(Used by the Router to determine the forwarding decision)

Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?

The initial part of some Net prefix’ (Net-ids)are the same with sub-netting so Longest prefix matching

Internet (datagram)• Independent data exchange

among computers– “elastic” service, no strict timing

req. • “Smart” end systems (computers)

– can adapt, perform control, error recovery

– Allows simple network processes - complexity at “edge”

• Many link types – different characteristics– uniform service difficult (QoS)

ATM (VC)• Evolved from telephony• QoS:

– strict timing, reliability requirements

– Offers guaranteed service

• “Dumb” end systems– telephones– complexity inside

network

Datagram or VC network: why?

Two key router functions: • Run routing algorithms/protocol (RIP, OSPF, BGP)• Forwarding datagrams from incoming to outgoing

link

Router Architecture Overview

Decentralized switching: • Given datagram destination, lookup output

port using forwarding table in input port memory (local – detached from central 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., Ethernet

Input Port Functions

Three types of switching fabrics

Interconnection network

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

Switching via memory

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

• bus contention: switching speed limited by bus bandwidth

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

Switching Via a Bus

Switching via An Interconnection Network

• overcome bus bandwidth limitations• Banyan networks, other interconnection

nets initially developed to connect processors in multiprocessor

• advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.

• Cisco 12000: switches 60 Gbps through the interconnection network

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

• Scheduling discipline chooses among queued datagrams for transmission

Output Ports

• buffering when arrival rate via switch exceeds output line speed

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

Output port queuing

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

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

• queuing delay and loss due to input buffer overflow!

Input Port Queuing

forwardingtable

Host, router network layer functions:

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

The Internet Network layer

IPv4 introduction• The IP network address is a data structure understood by a network

which uniquely identifies the source and destination within the network. • A unicast/broadcast/multicast IPv4 address is a 32 bit value (4 bytes)

which is allocated to each system in the Internet. • The 32-bit value uniquely identifies this system, and therefore no two

systems may have the same IP address. Some systems have more than one IP address, in which case they may be reached by any of their IP addresses.

• Each IP address consists of two parts, the network part (identifying the network/subnet or LAN broadcast domain, to which the computer is attached) and the host part (which identifies the host within the local network).

• This is a flat allocation technique. Administrators of a specific IP network may freely allocate host addresses within their network, without co-ordination with any other administrators in the Internet. However, they are not allowed to allocate host addresses belonging to a network number which has not been assigned to them.

• IPv4 addresses are normally written in a format known as "dotted decimal notation".

• An IP address may be unicast (for a specific end system), network broadcast (for all systems on a LAN) or multicast (for a group of end systems).

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

header checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

• 20 bytes of TCP• 20 bytes of IP• = 40 bytes + app

layer overhead

IPv4 datagram format

IPv4 Semantics and syntax I• Version, 4bits (always set to the value 4 in the current version

of IP). • HLEN, IP Header Length, 4bits (The number of 32 -bit words

or 4 Byte or rows forming the header - usually five).• Type of Service, 8bits , now known as Differentiated Service

Code Point (usually set to 0, but may indicate particular Quality of Service needs from the network, the DSCP defines the way routers should queue packets while they are waiting to be forwarded).

• Total Length, 16 bits, size of Datagram (in bytes, this is the combined length of the header and the data)

• Identification (16-bit number which together with the source address uniquely identifies this packet - used during reassembly of fragmented data-grams)

• Flags, 3bits, (a sequence of three flags (one of the 4 bits is unused) used to control whether routers are allowed to fragment a packet (i.e. the Don’t fragment flag, DF flag), and to indicate the parts of a packet to the receiver)

IPv4 Semantics and syntax II

• Fragmentation Offset, 13bits (a byte count from the start of the original sent packet, set by any router which performs IP fragmentation router)

• Time To Live (Number of hops /links which the packet may be routed over, decremented by most routers- used to prevent accidental routing loops)

• Protocol, 8bits (SAP) which indicates the type of transport packet being carried (e.g. 1 = ICMP; 2= IGMP; 6 = TCP; 17= UDP).

• Header Checksum, 16bits, is inserted by the sender and updated whenever the packet header is modified by a router - Used to detect processing errors. Packets with an invalid checksum are discarded by all nodes in an IP network)

• Source Address, 32bits (theIPv4 address of the original sender of the packet)

• Destination Address, 32bits (the IPv4 address of the final destination of the packet)

• Options (not normally used, but, when used, the IP header length will be greater than five 32-bit words to indicate the size of the options field)

• network links have MTU (max.transfer size) - largest possible link-level frame.– different link types,

different MTUs • large IP datagram divided

(“fragmented”) within network– one datagram becomes

several datagrams– “reassembled” only at

final destination– IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

IP Fragmentation & ReassemblyAn example of the operation of one of the IPv4 header fields

ID=x

offset=0

fragflag=0

length=4000

ID777

offset=0

fragflag=1

length=1500

ID777

offset=185

fragflag=1

length=1500

ID777

offset=370

fragflag=0

length=1040

One large datagram becomesthree smaller datagrams

Example• 4000 byte datagram

is sent• MTU = 1500 bytes

is the largest packet that can be forwarded.

1480 bytes in data field

offset = 1480/8

IP Fragmentation and Reassembly example

Original datagram

Example detail

IPv4 address: 32-bit identifier for host and router interface

interface: connection between host/router and physical link– router’s typically have

multiple interfaces– host typically has one

interface– IP addresses

associated with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

IP Addressing: introduction

IPv4 address - dotted decimal

Useful stuff about IPv4

Diagram shows internetworking running within and between autonomous systems (networks under a single management regime)

Autonomous System

Larger AS having greater internal networking structure

IPv4 Net prefix and Netmasks• The IPv4 network address is identified as the bit-wise logical

AND of the 32-bit IPv4 address with another 32-bit quantity, the netmask. All systems with the same network number share the same netmask (sometimes called a "subnet mask").

• The netmask has a bit with a logical '1' for each bit that is a part of the network number, and a logical '0' for each bit which is a part of the host number. The netmask may be written in dotted decimal notation.

• e.g. a 24-bit network prefix has a netmask which may be written as 255.255.255.0.

• Hence, the IP address 129.7.1.10 with a netmask of 255.255.255.0 has the network prefix of 129.7.1.0.

• A 24-bit network prefix leaves a host part of 8 bits. That is a network with 254 unique hosts addresses. (Special addresses - 0 is reserved for the network itself, and the all one's host address is reserved for use as the network broadcast address).

• The netmask is often represented by writing the IP address followed by a slash ('/') with the number of bits of the net prefix. The above net prefix can also be represented as /24

Some examples of masking

IP address subnet mask Net

prefix host part

7.7.7.7/8 255.0.0.0 7.0.0.0 7.7.7

139.133.7.10/24 255.255.255.0 139.133.7.0 10

129.5.255.2/16 255.255.0.0 129.5.0.0 255.2

131.108.2.1/30 255.255.255.252 131.108.2.0 1

• IP address: – subnet part (high

order bits)– host part (low order

bits) • What is a subnet ?

– device interfaces with same subnet part of IP address

– can physically reach each other without intervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet

Subnets – distinction from network

223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Recipe• To determine the

subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.

Subnet mask: /24

Subnets

How many?

Hopefully you decided on 6

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

Subnets

CIDR: Classless InterDomain Routing– subnet portion of address of arbitrary length– address format: a.b.c.d/x, where x is # bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

IP addressing: CIDR - in contrast to the class based approach

Q: How does a host get an IP address?

• hard-coded by system admin. in a file, eg.– Windows: control-panel->network-

>configuration->tcp/ip->properties– UNIX: /etc/rc.config. or

• DHCP: Dynamic Host Configuration Protocol: dynamically get address from ISP server– “plug-and-play”

IP addresses: how to get one?

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network server when it joins a networkCan renew its lease on address in use

Allows reuse of addresses (only hold on to address while connected and “on”)

Support for mobile users who want to join network (more intermittently)

DHCP overview:– Host broadcasts “DHCP discover” message.– DHCP server responds with “DHCP offer” msg.– Host requests IP address: “DHCP request” msg.– DHCP server sends address: “DHCP ack” msg.

Arriving DHCP client needsaddress in thisnetwork

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

DHCP server

DHCP client-server scenario

DHCP server: 223.1.2.5 arriving client

time

DHCP discover

src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offer

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secsDHCP request

src: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP client-server scenario

Q: How does network get subnet part of IP address?

A: Gets allocated portion of its provider ISP’s address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

IP addresses: how to get one?

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

Hierarchical addressing allows efficient advertisement of routing information:

Hierarchical addressing: route aggregation

Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned

Names and Numbers– allocates addresses– manages DNS– assigns domain names, resolves disputes

Class From To Decimal Mask

Class "A 10.0.0.0 10.255.255.255 255.0.0.0

Class "B 172.16.0.0 172.31.255.255 255.240.0.0 (or 255.255.0.0)

Class "C192.168.0.0

192.168.255.255

255.255.0.0 (or 255.255.255.0)

Private address spaceAn IP (Internet Protocol) Address is a 32-bit number broken up into "quads" of 1 byte each, separated by dots. 1 byte is 8 bits which in decimal is a number in the range 0 to 255. For example, 10.234.56.71 is an IP Address. There are only so many "real" IP addresses, and they are (and have been) perpetually very close to being used up and thus are very difficult to get.One of the solutions to this problem is so-called "private" IP Addresses. These are ranges of IP Addresses set aside expressly for use by a company or other entity internally. Private IP Addresses cannot be used to connect directly to the Internet--that is they are non-routable. These are also often called RFC1918 addresses.

http://www.faqs.org/rfcs/rfc1918.html

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this network

have 10.0.0/24 address for source, destination (as

usual)

All datagrams leaving localnetwork have same single source NAT IP address:

138.76.29.7,different source port

numbers


NAT: Network Address Translation• Motivation: local network uses just one IP

address as far as outside world is concerned:– range of addresses not needed from ISP:

just one IP address for all devices– can change addresses of devices in local

network without notifying outside world– can change ISP without changing

addresses of devices in local network– devices inside local net not explicitly

addressable, visible by outside world (a security plus).

Implementation: NAT router must:

– outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)

. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr.

– remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

– incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 10.0.0.1, 3345

4

S: 138.76.29.7, 5001D: 128.119.40.186, 80

2

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001

3

3: Reply arrives dest. address: 138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345


• 16-bit port-number field: – 60,000 simultaneous connections with a single

LAN-side address!

• NAT is controversial:– routers should only process up to layer 3– violates end-to-end argument

• NAT possibility must be taken into account by app designers, eg, P2P applications

– address shortage should instead be solved by IPv6


• client wants to connect to server with address 10.0.0.1– server address 10.0.0.1 local

to LAN (client can’t use it as destination addr)

– only one externally visible NATted address: 138.76.29.7

• solution 1: statically configure NAT to forward incoming connection requests at given port to server– e.g., (123.76.29.7, port 2500)

always forwarded to 10.0.0.1 port 25000

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

Client?

NAT traversal problem

• solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to:learn public IP address

(138.76.29.7)add/remove port mappings

(with lease times)

i.e., automate static NAT port map configuration

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

IGD


• solution 3: relaying (used in Skype)– NATed client establishes connection to relay– External client connects to relay– relay bridges packets between to connections

138.76.29.7

Client

10.0.0.1

NAT router

1. connection torelay initiatedby NATted host

2. connection torelay initiatedby client

3. relaying established


• used by hosts & routers to communicate network-level information– error reporting:

unreachable host, network, port, protocol

– echo request/reply (used by ping)

• network-layer “above” IP:– ICMP msgs carried in IP

datagrams• ICMP message: type, code

plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

ICMP: Internet Control Message Protocol

• Source sends series of UDP segments to dest– First has TTL =1– Second has TTL=2, etc.– Unlikely port number

• When nth datagram arrives to nth router:– Router discards

datagram– And sends to source an

ICMP message (type 11, code 0)

– Message includes name of router& IP address

• When ICMP message arrives, source calculates RTT

• Traceroute does this 3 times

Stopping criterion• UDP segment eventually

arrives at destination host• Destination returns ICMP

“host unreachable” packet (type 3, code 3)

• When source gets this ICMP, stops.

Traceroute and ICMP

The End

Keep reviewing the tutorial sheets.

Topic 3 – Network Layer and WAN Introduction Forwarding and Routing Network service models Virtual...

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