ECSE-4670: Computer Communication Networks (CCN)

Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 1

ECSE-4670: Computer Communication Networks (CCN)

ECSE-4670: Computer Communication Networks (CCN)

Network Layer

Shivkumar Kalyanaraman: [email protected]

Biplab Sikdar: [email protected]

http://www.ecse.rpi.edu/Homepages/shivkuma

Network Layer

Shivkumar Kalyanaraman: [email protected]

Biplab Sikdar: [email protected]

http://www.ecse.rpi.edu/Homepages/shivkuma


• understand principles behind network layer services:– Internetworking concepts– The network layer– IP – routing (path selection)– how a router works

ChapterGoals


• Internetworking• network layer services• IP• routing principle: path selection• hierarchical routing• Internet routing protocols reliable transfer

– intra-domain– inter-domain

• what’s inside a router?

Overview


The Internetworking Problem• Two nodes communicating across a

“network of networks”…

– How to transport packets through this heterogeneous mass ?

A B

Cloud

Cloud

Cloud


The Internetworking Problem

• Problems: heterogeneity and scaling• Heterogeneity:

– How to interconnect a large number of disparate networks ? (lower layers)

– How to support a wide variety of applications ? (upper layers)

• Scaling: – How to support a large number of end-

nodes and applications in this interconnected network ?


Heterogeneity: Solutions

• Translation (eg: bridges): specify a separate mapping between every pair of protocols(+) No software changes in networks required.

() Need to specify N mappings when a new lower layer protocol is added to the list

() When many networks, subset = 0() Mapping may be asymmetric



– Overlay model: Define a new protocol (IP) and map all networks to IP(+) Require only one mapping (IP -> new

protocol) when a new protocol is added(+) Global address space can be created

for universal addressibility and scaling

() Requires changes in lower networks (eg: protocol type field for IP)



() IP has to be necessarily simple else mapping will be hard.

– Even in its current form mapping IP to ATM has proven to be really hard.

– Basis for “best-effort” forwarding() Protocol mapping infrastructure

needed: address hierarchy, address resolution, fragmentation


The Internet Network layer

routingtable

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


IP Addressing: introduction• IP address: 32-bit

identifier for host, router interface

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

multiple interfaces– host may have multiple

interfaces– IP addresses associated

with interface, not host, 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

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11


IP Addressing - 1• IP address:

– network part (high order bits)

– host part (low order bits)

• What’s a network ? (from IP address perspective)– device interfaces with

same network 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 IP networks(for IP addresses starting with 223, first 24 bits are network address)

LAN


IP Addressing - 2How to find the

networks?• Detach each

interface from router, host

• create “islands of isolated networks

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.1

223.1.2.6

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

Interconnected system consisting

of six networks

223.1.3.2223.1.3.1

223.1.3.27

223.1.2.2


IP Addresses

0 network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

given notion of “network”, let’s re-examine IP addresses:

“class-full” addressing:


Some Special IP Addresses

• All-0s This computer• All-1s All hosts on this net (limited

broadcast: don’t forward out of this net)

• All-0 host suffix Network Address (‘0’ means ‘this’)

• All-1 host suffix All hosts on the destination net (directed broadcast).

• 127.*.*.* Loopback through IP layer


IP addressing: CIDR - 1

• classful addressing: – inefficient use of address space,

address space exhaustion

– e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network


IP addressing: CIDR - 2

• CIDR: Classless InterDomain Routing– network portion of address of arbitrary

length– address format: a.b.c.d/x, where x is #

bits in network portion of address

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23


Subnet Addressing

– External routers need to store entries only for the “network ID”

– Internal routers & hosts use subnet mask to identify “subnet ID” and route packets between “subnets” within the “network”.

– Eg: Mask: 255.255.255.0 => subnet ID = 8 bits with upto 62 hosts/subnet


Subnet Addressing (Continued)

– Route table lookup: • IF ((Mask[i] & Destination Addr) = =

Destination[i]) Forward to NextHop[i]

– Subnet mask can end on any bit.– Mask must have contiguous 1s followed

by contiguous zeros. Routers do not support other types of masks.


Route Table Lookup: Example

Destination Mask Next Hop30.0.0.0 255.0.0.0 40.0.0.740.0.0.0 255.0.0.0 Deliver direct128.1.0.0 255.255.0.0 Deliver direct192.4.10.0 255.255.255.0 128.1.0.9

30.0.0.030.0.0.0 40.0.0.040.0.0.0 128.1.0.0128.1.0.0 192.4.0.0192.4.0.0

30.0.0.7

40.0.0.7 128.1.0.8 192.4.10.9

40.0.0.8 128.1.0.9


IP addresses: how to get one?

Hosts (host portion):• hard-coded by system admin in a file• DHCP: Dynamic Host Configuration

Protocol: dynamically get address: “plug-and-play”– host broadcasts “DHCP discover” msg– DHCP server responds with “DHCP offer” msg– host requests IP address: “DHCP request”

msg– DHCP server sends address: “DHCP ack” msg


IP addresses: how to get one?

Network (network portion):• get allocated portion of 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


Hierarchical addressing: route aggregation

“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: 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


Getting a datagramfrom source to dest. - 1

IP datagram:

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

B

E

miscfields

sourceIP addr

destIP addr data

datagram remains unchanged, as it travels source to destinationaddr fields of interest here

routing table in ADest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2



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

B

E

Starting at A, given IP datagram addressed to B:look up net. address of Bfind B is on same net. as Alink layer will send datagram directly to B inside link-layer frame

B and A are directly connected

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

miscfields 223.1.1.1 223.1.1.3 data



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

B

E

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

Starting at A, dest. E:look up network address of EE on different network

A, E not directly attached

routing table: next hop router to E is 223.1.1.4 link layer sends datagram to router 223.1.1.4 inside link-layer framedatagram arrives at 223.1.1.4 continued…..

miscfields 223.1.1.1 223.1.2.2 data


Getting a datagramfrom source to dest. – 4

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

B

E

Arriving at 223.1.4, destined for 223.1.2.2look up network address of EE on same network as router’s

interface 223.1.2.9 router, E directly attached

link layer sends datagram to 223.1.2.2 inside link-layer frame

via interface 223.1.2.9 datagram arrives at 223.1.2.2!!! (hooray!)

miscfields 223.1.1.1 223.1.2.2 data network router Nhops interface

223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

Dest. next


IP Features

• Connectionless service• Addressing• Data forwarding• Fragmentation and reassembly • Supports variable size datagrams• Best-effort delivery• Provides only “Send” and “Delivery”

services. Error and control messages generated by Internet Control Message Protocol (ICMP)


What IP does NOT provide

• End-to-end data reliability & flow control (done by TCP or application layer protocols)

• Sequencing of packets (like TCP)

• Error detection in payload (TCP, UDP or other transport layers)

• Error reporting (ICMP)


What IP does NOT provide (Continued)

• Setting up route tables (RIP, OSPF, BGP etc)

• Connection setup (it is connectionless)

• Address/Name resolution (ARP, RARP, DNS)

• Configuration (BOOTP, DHCP)• Multicast (IGMP, MBONE)


IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet 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, pecifylist of routers to visit.


IP Fragmentation & Reassembly - 1

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

different MTUs • large IP datagram divided

(“fragmented”) within net– 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 and Reassembly - 2

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=1480

fragflag=1

length=1500

ID=x

offset=2960

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams


Summary

• Internet architectural principles• IP addressing and header• Fragmentation/Reassembly, Path

MTU discovery

Documents

ECSE-4670: Computer Communication Networks (CCN)