internetworking The solution of the Internet
Prof. Malathi Veeraraghavan Elec. & Comp. Engg. Dept/CATT Polytechnic University [e-mail]
What is the internetworking problem: how to connect different types of networks
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Single networks
Simplest network one link Endpoint Endpoint A shared link: often used to create a LAN Endpoint Endpoint
Endpoint
One network same type of switches link rates can be different Endpoint Switch Endpoint
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Endpoint Switch Endpoint
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The Internet approach to internetworking
An internetwork
Endpoint Switch Endpoint Switch Switch Switch Endpoint Endpoint
Protocol stacks in the Internet
Endpoint A
Inter-T:TCP
Endpoint A IP router
Inter-N: IP
Inter-T:TCP
Inter-N: IP
Inter-N: IP
Network 1
IP router Network 3
T1 N1 L1 P1
Switch N1 L1 P1 L2 P2
Switch N1 L2 P2
L3
T1 N1 L3 P3
T2 N2 L4 P4
Switch N2 L4 P4 L5 P5
Switch N2 L5 P5 L6 P6
T2 N2 L6 P6
Network 2
P3
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
Network 1
Network 2
IP routers are connectionless packet switches
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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
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Todays 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
Ethernet frame structure (RFC 894 and 893)
FOCUS: Ethernet addresses (also called MAC addresses) are 6 bytes long
Dest. Src. Addr Addr. Type . 6 6 2 Type 0800 2 Type 0806 2 Type 8035 2 Data 46-1500 IP datagram 46-1500 CRC 4
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
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ARP req./reply 28
PAD 18
RARP req./reply 28
PAD 18
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PPP frame structure
flag 7E 1 addr ctrl protocol FF 1 03 1 2 0021 C021 8021 <= 1500 IP datagram 2 data CRC flag 7E 1
>= five 32-bit words version (4 bits)
IP Datagram Format
header length Type of Service/TOS (8 bits) flags (3 bits) Total Length (in bytes) (16 bits) Fragment Offset (13 bits) Header Checksum (16 bits) Identification (16 bits) TTL Time-to-Live (8 bits) Protocol (8 bits)
Source IP address (32 bits) Destination IP address (32 bits) Options (if any, <=40 bytes) DATA
296 if low delay
link control data network control data
FOCUS
0 32-bit word
31
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User-plane interworking Encapsulation
Application
Layers used in ftp
mng.poly.edu FTP program TCP FTP protocol photon.poly.edu FTP program TCP
As data moves down the protocol stack, each protocol adds layer-specific control information.
User data
Application Header
User data
TCP
TCP Header
Application data
TCP segment
TCP protocol
IP
IP Header TCP Header
Application data
IP Ethernet Driver
IP protocol Ethernet Driver
IP
IP protocol Ethernet Driver
IP Ethernet Driver
Ethernet Driver
Ethernet Header IP Header
IP datagram
TCP Header
Application data
Ethernet Trailer
Ethernet protocol
Ethernet protocol
Ethernet frame
IP router: dibner-gw
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Need Internet address and Network address
Internetwork IP router
Host A Switch 1 Switch 2
Addresses for interfaces
Host C Host D
1 3
2
Switch 3
Switch 4
Host B
Ethernet 1
Host A sends a packet to Host C: - Places Host Cs IP address in IP header - To get through Ethernet 1, it needs Ethernet address of IP routers interface 1 - Switch 1 and Switch 2 forward packets based on destination Ethernet address of IP routers 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 11 - Switch 3 and 4 forward packets based on destination Ethernet address of Host C
Ethernet 2
Host E Switch Switch Host F
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 A host typically has only one interface; hence it is assigned one IP address and one Ethernet address if its interface is an Ethernet link
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Ethernet 3
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FTP session from host mng to photon
mng.poly.edu photon.poly.edu
mng.poly.edu
Packet sent from mng to IP router dibner-gw
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 0:0:c:1:a2:e 5:6:7:1:a1:f
128.238.42.105
128.238.42.1
128.238.32.1
128.238.32.22
dibner-gw.poly.edu
router
dibner-gw.poly.edu
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 mngs 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 14 Polytechnic University destination MAC address
Note that IP router dibner-gw has more than one IP address
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At the IP router, dibner-gw
When the packet arrives at the IP router, dibner-gw, it looks its up routing table
For destination IP address 128.238.32.22, the routing table shows which output port to use.
dibner-gw 128.238.32.1 0:0:c:1:a2:d
Packet sent from mng to IP router dibner-gw
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 photon.poly.edu
photon.poly.edu (128.238.32.22)
router
0:0:5e:3f:4d:2c 0:0:c:1:a2:d
dibner-gw.poly.edu
dibner-gw 128.238.42.1 0:0:c:1:a2:e
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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.
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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)
ARP (Address Resolution Protocol)
128.238.42.1 ARP mng.poly.edu 0:0:c:1:a2:e ARP
all stations on the same Ethernet
128.238.32.22 ARP 0:0:5e:3f:4d:2c ARP
Same thing when dibner-gw needs to send packet to photon.
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dibner-gw .poly.edu
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all stations on the same Ethernet
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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.
Finding MAC address of an interface whose IP address is available
(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 HOSTB? (3) HOST-B responds with an ARP Reply which contains its hardware address. (4) HOST-A transmits the IP datagram to HOST-B.
HOST-A HOST-B IP IP ARP ARP
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Ethernet Driver
3 2 4
Ethernet Driver
IP address (32 bit)
ARP RARP
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Ethernet MAC address (48 bit)
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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)
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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)
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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
it does this by sending a DNS (Domain Name Service) query to the DNS Server whose IP address is stored on host (as we just saw)
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For the file transfer example from mng to photon
p...