IP

2

Orientation

IP (Internet Protocol) is a Network Layer Protocol.

IP’s current version is Version 4 (IPv4). It is specified in RFC 891.

NetworkLayer

Link Layer

IP

ARPNetworkAccess

Media

ICMP IGMP

TransportLayer

TCP UDP

IPv4 Address Model

Class Network ID Host ID # of Addresses

# of Networks

A 0 + 7 bit 24 bit 224-2 126

B 10 + 14 bit 16 bit 65,536 - 2 214

C 110 + 21 bit 8 bit 256 - 2 221

D 1110 + Multicast Address IP Multicast

E Future Use

0Network (7 bits)

Network (14 bits)

1 1 0

1 0

Network (21 bits)

Host (24 bits)

Host (16 bits)

Host (8 bits)

Class A:

Class B:

Class C:

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IP: The waist of the hourglass

IP is the waist of the hourglass of the Internet protocol architecture

Multiple higher-layer protocols

Multiple lower-layer protocols

Only one protocol at the network layer.

Applications

HTTP FTP SMTP

TCP UDP

IP

Data link layer protocols

Physical layer protocols

5

Application protocol

IP is the highest layer protocol which is implemented at both routers and hosts

Application

TCP

IP

Data Link

Application

TCP

IP

NetworkAccess

Application protocol

TCP protocol

IP protocol IP protocol

DataLink

DataLink

IP

DataLink

DataLink

IP

DataLink

DataLink

DataLink

IP protocol

RouterRouter HostHost

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IP Service

Delivery service of IP is minimal

IP provide provides an unreliable connectionless best effort service (also called: “datagram service”).

Unreliable: IP does not make an attempt to recover lost packets

Connectionless: Each packet (“datagram”) is handled independently. IP is not aware that packets between hosts may be sent in a logical sequence

Best effort: IP does not make guarantees on the service (no throughput guarantee, no delay guarantee,…)

IP

IP is connectionless in the end-to-end delivery Data delivered in datagrams (packets / frames),

each with a header Combines collection of physical networks

into single, virtual network

Transport protocols use this connectionless service to provide connectionless data delivery (UDP) and connection-oriented data delivery (TCP)

IP is Best Effort Delivery

IP provides service equivalent to LAN Does not guarantee to prevent

Duplicate datagrams Delayed or out-of-order delivery Corruption of data Datagram loss

Reliable delivery provided by transport layer Network layer - IP - can detect and report

errors without actually fixing them

IP Problems

• Higher layer protocols have to deal with losses or with duplicate packets

• Packets may be delivered out-of-sequence

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IP Service

IP supports the following services: one-to-one (unicast) one-to-all (broadcast) one-to-several (multicast)

IP multicast also supports a many-to-many service. IP multicast requires support of other protocols (IGMP, multicast

routing)

unicastbroadcast multicast

The IP Datagram

Formally, the unit of IP data delivery is called a datagram

Includes header area and data area Datagrams can have different sizes

Header area usually fixed (20 octets) but can have options Data area can contain between 1 octet and 65,535 octets

(216- 1) Usually, data area much larger than header

Forwarding Datagrams

The header contains all the information needed to deliver a datagram to a destination computer Destination address Source address Identifier Other delivery information

Routers examine the header of each datagram and forwards the datagram along a path to the destination Use routing table to compute next hop Update routing tables using algorithms previously

discussed Link state, distance vector, manually

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IP Datagram Format

20 bytes ≤ Header Size < 24 x 4 bytes = 60 bytes 20 bytes ≤ Total Length < 216 bytes = 65536 bytes

ECNversionheaderlength

DS total length (in bytes)

Identification Fragment offset

source IP address

destination IP address

options (0 to 40 bytes)

payload

4 bytes

time-to-live (TTL) protocol header checksum

bit # 0 15 23 248 317 16

0MF

DF

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IP Datagram Format

Transmission is row by row For each row:

1. First transmit bits 0-72. Then transmit bits 8-153. Then transmit bits 16-234. Then transmit bits 24-31

Note: Many computers (incl. Intel processors) store 32-bit words in little endian format. Others (incl. Motorola processors) use big endian.

This is called network byte order (LE or BE)"Big Endian" means that the high-order byte of the

number is stored in memory at the lowest address, and the low-order byte at the highest address.

0x1234: BE: 12 34 LE: 34 12

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Maximum Transmission Unit

Maximum size of IP datagram is 65535, but the data link layer protocol generally imposes a limit that is much smaller

Example: Ethernet frames have a maximum payload of 1500 bytes

IP datagrams encapsulated in Ethernet frame cannot be longer than 1500 bytes

The limit on the maximum IP datagram size, imposed by the data link protocol is called maximum transmission unit (MTU)

• MTUs for various data link protocols: Ethernet: 1500 FDDI: 4352802.3: 1492 ATM AAL5: 9180802.5: 4464 PPP: negotiated

Datagram Lifetime

Datagrams could loop indefinitely Consumes resources Transport protocol may need upper bound on

datagram lifeDatagram marked with lifetime

Time To Live field in IP Once lifetime expires, datagram discarded (not

forwarded) Hop count

Decrement time to live on passing through a each router Time count

Need to know how long since last router

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IP Fragmentation

FDDIRing

RouterHost A Host B

Ethernet

MTUs: FDDI: 4352 Ethernet: 1500

• Fragmentation: • IP router splits the datagram into several datagram• Fragments are reassembled at receiver

• What if the route contains networks with different MTUs? Or exceeds?

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Where is Fragmentation done?

Fragmentation can be done at the sender or at intermediate routers

The same datagram can be fragmented several times. Reassembly of original datagram is only done at

destination hosts

Router

IP datagram H Fragment 1 H1Fragment 2 H2

IP Fragmentation and Reassembly

ID=x

offset=0

moreflag=0

length=4000

ID=x

offset=0

moreflag=1

length=1500

ID=x

offset=1480

moreflag=1

length=1500

ID=x

offset=2960

moreflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Fragmenting Fragments

A fragment may encounter a subsequent network with even smaller MTU Router fragments the fragment to fit Resulting (sub)fragments look just like original

fragments (except for size) No need to reassemble hierarchically; (sub)fragments

include position in original datagram

Dealing with Failure

Re-assembly may fail if some fragments get lost

Need to detect failureRe-assembly time out

Assigned to first fragment to arrive If timeout expires before all fragments arrive, discard

partial data

Error Control

Not guaranteed deliveryRouter should attempt to inform source if

packet discarded e.g. for time to live expiring

Source may modify transmission strategyMay inform high layer protocolDatagram identification needed

Destination doesn’t ACK or NAK if checksum fails, no retries, best-effort like Ethernet