The internetworking solution of the Internet. Single networks. The Internet approach to internetworking. Protocol stacks in the Internet

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1 Polytechnic University

The internetworking solution

of the Internet

Prof. Malathi Veeraraghavan

Elec. & Comp. Engg. Dept/CATT

Polytechnic University

[email protected]

What is the internetworking problem:

• how to connect different types of

networks

Simplest network – one link

Endpoint Endpoint

Switch Switch

One network – same type of switches – link rates can be different

Single networks

Endpoint Endpoint Endpoint A shared link: often used to create a LAN 3 Polytechnic University Endpoint Endpoint Switch Switch Network 1 Endpoint Endpoint Switch Switch Network 2 IP router An internetwork

The Internet approach to

internetworking

• Have all endpoints speak the IP (Internet Protocol) in addition to their own network protocols

• For loss-sensitive applications: run TCP, an end-to-end transport protocol, irrespective of whether

– both ends are within the same network – the two ends are on different networks

• IP routers are connectionless packet switches

– they forward IP packets from one network to another based on the destination IP address carried in the IP header and information stored in their routing tables

Network 3 4 Polytechnic University Network 1 Network 2 T1 Inter-T:TCP Inter-N: IP A N1 L1 P1 N1 L3 P3 T1 N2 L4 P4 T2 Inter-N: IP IP router N1 L2 P2 P3 N1 L1 L2 P1 P2 N2 L5 L6 P5P6 N2 L4 L5 P4P5 T2 Inter-T:TCP Inter-N: IP A N2 L6 P6 Switch Switch Switch Switch

Endpoint Endpoint

L3

Protocol stacks in the Internet

Today’s most common networks

in the Internet

• Ethernet within enterprises using a combination of

– shared-medium Ethernet LANs with hubs, or

– with Ethernet switches – which are connectionless

packet switches

• PDH/SONET networks in the MAN and WAN

domains

– Routers are interconnected by T1, T3, OC3 connections

that are set up through a PDH/SONET circuit-switched

network

– PPP, Point-to-Point Protocol, is executed on these

circuits

Ethernet frame structure

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PPP frame structure

7E flag 1 FF addr 1 03 ctrl 1 2 protocol <= 1500 data 2 CRC 7E flag 1 0021 IP datagram C021 link control data 8021 network control data

296 if low delay

IP Datagram Format

version

(4 bits) headerlength Type of Service/TOS(8 bits) Total Length (in bytes)(16 bits) Identification (16 bits) _{(3 bits)}flags Fragment Offset (13 bits)

Source IP address (32 bits) Destination IP address (32 bits)

Options (if any, <=40 bytes) DATA >= fi ve 32-b it w o rd s 32-bit word 0 31 TTL Time-to-Live

(8 bits) Protocol(8 bits) Header Checksum (16 bits)

FOCUS

9 Polytechnic University Application TCP IP Ethernet Driver User data User data Application Header Application data TCP Header Application data TCP Header IP Header Application data TCP Header IP Header Ethernet Header Ethernet Trailer IP datagram TCP segment Ethernet frame

Userplane interworking

-Encapsulation

As data moves down the protocol stack, each protocol adds layer-specific control information. 10 Polytechnic University

Layers used in ftp

FTP program TCP IP mng.poly.edu Ethernet Driver Ethernet Driver Ethernet Driver IP FTP program TCP IP photon.poly.edu Ethernet Driver FTP protocol TCP protocol IP protocol IP protocol Ethernet protocol Ethernet protocol

IP router: dibner-gw

Need Internet address and

Network address

Host A Host B Switch 1 Switch 2 Ethernet 1 Host C Host D Switch 3 Switch 4 Ethernet 2 IP router Internetwork Host E Host F Switch Switch Ethernet 3

Host A sends a packet to Host C: - Places Host C’s IP address in IP header

- To get through Ethernet 1, it needs Ethernet address of IP router’s interface 1

- Switch 1 and Switch 2 forward packets based on destination Ethernet address of IP router’s interface 1

- IP router forwards packet to port 2 to reach Host C (based on IP level routing data using destination IP address of host C) - IP router needs Ethernet address of Host C to send the packet through

Ethernet 2

1 2

3

Addresses for interfaces

• Both IP addresses and Ethernet addresses are assigned per

interface, not per node (router or host).

• An IP router has many interfaces; each interface has an IP

address; interfaces that connect the IP router to an Ethernet

network also have Ethernet addresses, one per interface

• An Ethernet switch has many interfaces; each has an

Ethernet address

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FTP session from host mng to photon

128.238.42.105 dibner-gw.poly.edu photon.poly.edu mng.poly.edu 128.238.32.1 128.238.42.1 128.238.32.22

Note that IP router dibner-gw has more than one IP address

Packet sent from mng

to IP router dibner-gw

mng.poly.edu router

dibner-gw.poly.edu

src IP address: 128.238.42.105 dst IP address: 128.238.32.22 src MAC address: 5:6:7:1:a1:f dst MAC address: 0:0:c:1:a2:e 5:6:7:1:a1:f

0:0:c:1:a2:e

• Host mng consults its IP routing table. This says that to reach destination IP address 128.238.32.22, it needs to send the packet to the IP router because this destination is on a different network

• Hence it sends the packet within its Ethernet network to destination Ethernet (MAC) address 0:0:c:1:a2:e because this is the Ethernet address of the router interface that is connected to mng’s Ethernet network. This destination MAC address allows the Ethernet packet (called frame) to be routed through the first Ethernet network – Ethernet switches determine how to route based on destination MAC address

At the IP router, dibner-gw

• When the packet arrives at the IP router, dibner-gw, it looks up its routing table

– For destination IP address 128.238.32.22, the routing table shows which output port to use.

photon.poly.edu (128.238.32.22) dibner-gw 128.238.42.1 0:0:c:1:a2:e dibner-gw 128.238.32.1 0:0:c:1:a2:d 16 Polytechnic University

Packet sent from mng

to IP router dibner-gw

photon.poly.edu router

dibner-gw.poly.edu

src IP address: 128.238.42.105 dst IP address: 128.238.32.22 src MAC address: 0:0:c:1:a2:d dst MAC address: 0:0:5e:3f:4d:2c 0:0:c:1:a2:d

0:0:5e:3f:4d:2c

• IP router, dibner-gw, finds MAC address of photon and adds the IP header and Ethernet header to the packet with the four addresses as shown and sends it. • The destination MAC address allows for routing through the second Ethernet

network; each Ethernet switch that the frame encounters will forward packets based on destination MAC address and its routing table.

Intra-network addresses and

inter-network addresses

• Consider example:

– mng knows that to reach photon it has to route the

packet to the IP router dibner-gw from its IP-level

routing data

– it needs to find the MAC address of the router to get

through the first Ethernet

– it does this using ARP (Address Resolution Protocol)

• Same thing when dibner-gw needs to send packet

to photon.

ARP

(Address Resolution Protocol)

ARP

dibner-gw .poly.edu all stations on the

same Ethernet 128.238.32.22

0:0:5e:3f:4d:2c ARP

ARP

mng.poly.edu _{all stations on the}

same Ethernet 128.238.42.1

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ARP and RARP

• The IP protocol uses 32-bit addresses.

• Ethernet networks use 48-bit Ethernet (MAC) addresses

• The ARP and RARP protocols perform the translation

between IP addresses and MAC layer addresses.

• We will discuss ARP for broadcast LANs, particularly

Ethernet LANs.

RARP

Ethernet MAC address (48 bit)

ARP

IP address (32 bit) 20 Polytechnic University

Finding MAC address of an interface

whose IP address is available

HOST-B HOST-A ARP IP Ethernet Driver 1 Ethernet Driver ARP 2 3 IP 4

(1) HOST-A wants to send an IP datagram to HOST-B. (2) HOST-A broadcasts an ARP

request to all stations on the network: “What is the hardware address of HOST-B?”

(3) HOST-B responds with an ARP Reply which contains its hardware address. (4) HOST-A transmits the IP

datagram to HOST-B.

ARP reply

• The ARP reply is sent by the node whose IP

address matches the address sent in the ARP

request

• All other nodes receiving the broadcast

ARP ignore the request (since their IP

addresses do not match the address that is

being resolved)

ARP cache

• Clearly, sending an ARP request/reply for

each IP datagram is inefficient.

• Each station maintains a cache (ARP

Cache) of current entries. The entries expire

after 20 minutes.

• Everytime the ARP cache is consulted for a

MAC address, the expiry timer is reset in

common implementations.

) at (incomplete)

Whose addresses does a host

store on initialization?

• Go to Control Panel – Network on a

Windows PC

– Point out that a host needs to have initialized

• host IP address

• gateway IP address (default router interface)

• DNS server IP address

For Internet applications, what

type of “address” do you need?

• To begin with, a user obtains “domain

name” of a host to which the user wants to

connect for a web file download, to send

email etc.

• Host needs to find IP address corresponding

to domain name

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For the file transfer example from

mng to photon

ftp program Domain Name Server mng.poly.edu gatekeeper.poly.edu photon.poly.edu 128.238.32.22