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Chapter 4: Internetworking(Internet Protocol)

Dr. Rocky K. C. Chang16 March 2004

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1. The IP technology (except routing)• IP service model• IP protocol family• IP datagram structure• IP datagram fragmentation and reassembly• IP subnets• IP forwarding mechanisms• IP tunnels• Other IP layer protocols

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2. The IP service model• The IP service model consists of

– an addressing scheme to identify an IP host, and– a datagram (connectionless) model of data delivery.

• IP provides a best-effort service.– IP makes its best effort to send a datagram to its

destination.– The best-effort service does not guarantee reliable

datagram delivery, i.e., an unreliable service.

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3. Internet protocol suite (incomplete)

…

FTP HTTP NV TFTP

TCP UDP

IP

NET1 NET2 NETn

Application

Transport

Network

Data-link

ICMP IGMP

ARP & RARP

Ping DNS

RTPSSL

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4. IP datagram

Version HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

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4. IP datagram• Version: 4 for the current IP.• Type of service (TOS) for specifying how a

router should handle this datagram.• Header length handles a variable-length header.

– 20-byte IP header without IP options• A 16-bit length limits the size of an IP datagram

to 65,535 bytes, including the IP header.• Identification, flags, and offset are used for

packet fragmentation and reassembly.

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4. IP datagram• Time to live (TTL) limits the the number of

times that a datagram processed by routers.• Protocol specifies the type of payload, e.g., 6

for TCP and 17 for UDP.• Checksum is a 16-bit word checksum.• IP options, e.g.,

– Source routing– Record route

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5. MTU and packet fragmentation• Each network chooses a maximum packet size

that can be sent on it, Maximum Transmission Unit (MTU). For example,– 1500 bytes for 10-Mbps Ethernet– 4352 bytes for FDDI– 17914 bytes for 16-Mbps token ring

• Note that all MTUs are smaller than IP datagram’s maximum size.

• One internetworking problem is to accommodate various MTU values.

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5. MTU and packet fragmentation• To send datagrams to a directly attached host,

use the network’s MTU.• To send datagrams to a nondirectly attached

host, use the path MTU.– Path MTU is the minimum of the networks’ MTUs

on the path from the source to destination.• If the actual MTU used is larger than the path

MTU, packet fragmentation occurs.– Fragmentation occurs when a router attempts to

forward it to a network with a smaller MTU.

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5. MTU and packet fragmentation

H1 R1 R2 R3 H8

ETH IP (1400) FDDI IP (1400) PPP IP (512)

PPP IP (376)

PPP IP (512)

ETH IP (512)

ETH IP (376)

ETH IP (512)

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Ident = x Offset = 0

Start of header

0

Rest of header

1400 data bytes

(a)

Ident = x Offset = 0

Start of header

1

Rest of header

512 data bytes

(b)

Ident = x Offset = 512

Start of header

1

Rest of header

512 data bytes

Ident = x Offset = 1024

Start of header

0

Rest of header

376 data bytes

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5. MTU and packet fragmentation• Each IP fragment contains enough information

for forwarding to the destination.• A fragmented IP datagram will be reassembled

only at the destination node.• If any fragments do not arrive within a certain

time, other received fragments in the datagram will be discarded.

• Fragmentation could occur multiple times to an IP datagram.

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6. IP subnets• IP subnets introduce additional levels within an

IP network:– A network address, a subnet ID, and a host ID.

• IP subnets offer flexibility in allocating addresses to different sizes of sub-networks.

• A subnet mask is used to indicate which bits are referred to the network and subnet ID.– Each network interface stores subnet mask and its

unicast IP address.

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6. IP subnets• Subnetting for a class B address:

Network number Host number

Class B address

Subnet mask (255.255.255.0)

Subnetted address

1111111111111111 11111111 00000000

Network number Host IDSubnet ID

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6. IP subnetsSubnet mask: 255.255.255.128Subnet number: 128.96.34.0

128.96.34.15128.96.34.1

H1R1

128.96.34.130 Subnet mask: 255.255.255.128Subnet number: 128.96.34.128

128.96.34.129128.96.34.139

R2H2

128.96.33.1128.96.33.14

Subnet mask: 255.255.255.0Subnet number: 128.96.33.0

H3

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7. IP forwarding mechanisms• Assume that both routers and hosts already

have appropriate routing tables in place.– Routing tables for routers are constructed from

routing protocols.– Routing tables for hosts are constructed from other

means.• Problem: Given a routing table, how do hosts

and routers forward datagrams?

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7.1 Examples of routing tables• For example, R1’s routing table:

– Network/Subnet Subnet Mask Next Hop– 128.96.34.0 255.255.255.128 upper int.– 128.96.34.128 255.255.255.128 lower int.– 128.96.33.0 255.255.255.0 128.96.34.129

• For example, H1’s routing table:– Network/Subnet Subnet Mask Next Hop– 128.96.34.0 255.255.255.128 upper int.– 0.0.0.0 0.0.0.0

128.96.34.1

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7.2 Host’s forwarding mechanisms• A host sends a datagram to another host on the

same LAN or not.– In the former, it sends the datagram to the

destination directly.– In the latter, it sends the datagram to a default

router.– In both cases, the host uses ARP cache or ARP to

find out the corresponding MAC addresses.

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7.3 A general forwarding mechanism

D = Destination IP address

for each entry (Network/Subnet ID, Subnet Mask, Next Hop)

D1 = Subnet mask & D

if D1 = Network/Subnet ID

if Next Hop is an interface

deliver datagram directly to destination

else

deliver datagram to Next Hop (a router)

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7.4 Characteristics of IP forwarding• Both hosts and routers are involved in

forwarding.– Compared with routers, a host makes a much

simpler binary decision. • IP forwarding is done on a hop-by-hop basis.• It is assumed that the next-hop router is really

closer to the destination.• IP forwarding is able to specify a route to a

network, and not have to specify a route to every host.

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8. IP tunnels• Two network nodes (hosts or routers) may

“tunnel” IP datagrams between them.– Other nodes on the path are not aware of the other

datagram encapsulated by the outer datagram.– A tunnel configured from R1 to R2, which is

assigned with a virtual number of 0:• Network Next Hop• 1 Interface 0• 2 Virtual interface 0• Default Interface 1

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8. IP tunnels• Network interfaces configured as tunnel

endpoints perform IP-in-IP encapsulation.– When sending datagrams to each other, the sender

uses its IP address as the source address and the other’s IP address as the destination address.

– Each performs IP-in-IP encapsulation/decapsulation and then IP routing.

– A datagram may traverse several IP tunnels before arriving at the destination.

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8.1 An example of IP tunnels

IP header ,Destination = 2.x

IP payload

IP header ,Destination = 10.0.0.1

IP header ,Destination = 2.x

IP payload

IP header ,Destination = 2.x

IP payload

Network 1 R1 Internetwork Network 2R2

10.0.0.1

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8.2 Uses of IP tunnels• Mobile IP: IP tunnel between a foreign agent

(or a mobile host) and a home agent.• Mbone (Multicast backbone): IP tunnels

connect islands of multicast-enabled IP networks.

• IPv6: IP tunnels will be used for IPv4-IPv6 transition.

• IPSec: IP tunnels with security is used in establishing Virtual Private Networks (VPNs).

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8.3 Virtual private networksC

A

Corporation X private network

B

K L

M

Corporation Y private network

C

A B

K L

M

Physical links

Physical links

Virtual circuits

(a)

(b)

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9. Dynamic host configuration protocol• DHCP provides a framework for passing

configuration information to hosts.– IP addresses, address of a default router, etc.

• DHCP is a client-server system, including a relay agent.

• DHCP operation:– A DHCP client initially broadcasts a DISCOVER

message to find a DHCP server.• If the server is not directly connected to the client, a relay

agent on the LAN will forward this message to the server.

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9. Dynamic host configuration protocol– The server sends an OFFER message back to the

relay agent, which then forwards it in either unicast or broadcast back to the client.

– Upon accepting an OFFER from a DHCP server, the client sends a REQUEST message to that server.

– The final step is for the server to send a REQUEST ACK back to the client.

• DHCP provides IP addresses to clients for a finite lease duration.– The client either renews the lease or rebinds to

another new address.

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9. Dynamic host configuration protocol

DHCPrelay

DHCPserver

Other networks

Unicast to server

Broadcast

Host

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10. Internet control message protocol• The main functions associated with the ICMP

are error reporting, reachability test, and route-change notification.

• ICMP reports errors to the source for host unreachable, lost of fragments, etc.

• Ping program uses ICMP echo request and reply to test a host’s aliveness.

• ICMP sends a re-direct message for a better route back to the source.

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10. Internet control message protocol

Host

R1 R2

(1) IP datagram

(2) IP datagram

(3) ICMP redirect

to the destination