Internetworking with TCP/IP DCLAB-ghcho-internet03 2001 Fall 1 Chap. 7 Internet Protocol (IP) A...

Internetworking with TCP/IP DCLAB-ghcho-internet03 2001 Fall 1

Chap. 7 Internet Protocol (IP)

A user thinks of an internet as a single virtual network that interconnects all hosts, and through which communication is possible; its underlying hardware is both hidden and irrelevant

Internet architecture and philosophy := hierarchy => adaptability, robust

Connectionless delivery service := unreliable, best-effort, connectionless

Connectionless Delivery Service

Underlying Hardware

Application Services

Reliable Transport Service


The Basic of IP (I)

Host A

Application

TCP

IP

NetworkInterface

Hardware

Uses TCP/IP Services

Virtual Circuit

Routes Datagrams

Application

TCP

IP

Host B

NetworkInterface

Hardware

NetworkInterface

Hardware

IP

Gateway G


The Basic of IP (II)

IP provides three important definitions defines the basic unit of data transfer performs the routing function includes a set of rules that embody the idea of unreliable packet

delivery, such as packet processing, error control The unit of hardware transfer is a frame that contains a header

and data, where the header gives information, which includes the source and destination addresses

The unit of Internet transfer is a datagram, which has the same structure as the frame

Because datagram processing occurs in software, the contents and format are not constrained by any hardware


IP Format (I)

Format of an Internet datagram

Vers HLen Service Type Total Length

IP Identification Flags Fragment Offset

Time to Live Protocol Num. Header Checksum

Source IP Address

Destination IP Address

Options Padd.

Data (variable length)

15 310


IP Format (II)

Vers : IP version number, currently 4 HLen : IP header length in word (16) Total Length : IP datagram length in octets (65535) Service type field : a kind of transport specification this is only a hint to the routing algorithm, that is, it does not gua

rantee the type of transport requested

Data encapsulation : to support the different physical frame

0 4321 765

Precdence D T R unused

Datagram data area

Frame Data AreaFrame Header

Dataram Header


IP Format (III)

Now, the problem is the difference size between IP datagram (40 ~ 65535) and physical frame

The network’s maximum transfer unit (MTU) - Ethernet : 1500, FDDI : 4470, someone : 128, ATM : 54 … Total Length : IP datagram length in octets (65535) Again, Internet design basement is to hide underlying network t

echnologies and make communication convenient for the user A datagram does not always fit into a single network frame How the Internet has resolved this problem? permit to use any size of datagram, and arranges a way to divid

e large datagrams into smaller pieces when the datagram needs to traverse a network that has a small MTU

This process of dividing a datagram is called as fragmentation, and the small pieces into which a datagram is divided are called as fragment


IP Format (IV)

Fragmentation example (pp. 96, 97)

H1

Net 1 Net 3

Net 2

H2

R2R1

MTU = 1500MTU = 1500

MTU = 620

Data1600 octets

Dataram Header Data3300 octets

Data2600 octets

Data1Fragment1Header



Fragment 1 (offset 0)




IP Format (V)

Identification : a unique integer that identifies the datagram, the destination uses it along with the datagram source address to identify the datagram

Flag : - + do not fragment + more fragment Fragment Offset : the offset in the original datagram of the data

being carried in the fragment, in units of 8 octets TTL : how long, in seconds, the datagram is allowed to remain i

n the Internet, but usually handled with the number of hop Protocol : which high-level protocol was used to create the mes

sage being carried in the DATA area of a datagram Header Checksum : checksum the header as a sequence of 16

bit integers, adding them using 1’s complement arithmetic Source and Destination Addresses IP Option Padding : 32 bit alignment


Datagram Option (I)

Aims for network testing or debugging The length varies depending on which options are selected Option format

Copy : how routers treat options during fragmentation Option number : network control + - + debugging + -

Option code Option data (variable)Length

0 8 16

Copy Option class

0 1 3

Option number

7


Datagram Option (II)

Option Class

OptionNumber Length Description

0 -0 End of option list. Used if optionsdo not end at end of header

0 -1 No operation (used to align octets ina list of operations)

0 112 Security and handling restrictions(for military applications)

0 var3 Loose source routing. Used to routea datagram along a specified path

0 var7 Record route. Used to trace a route

0 48 Stream identifier. Used to carry a SATNET stream identifier (Obsolete)

0 var9 Strict source routing. Used to routea datagram along a specified path

2 var4 Internet timestamp. Used to recordtimestamps along the route

Refer to pp. 102


Datagram Option (III)

Record route option : provide a way to monitor or control how internet routers route datagrams

create an empty list of IP addresses arrange for each router that handle the datagram to add its IP a

ddress to the list

Code (7) Not used

0 8 16 24 31

Length

First IP address

Second IP address

...

Pointer


Datagram Option (VI)


Chap. 8 Routing IP Datagrams (I)

In a packet switching network, routing refers to the process of choosing a path over which to send datagrams

In the Internet, the IP layer chooses the next hop for each datagram that it sends

single homed host vs. multi-homed host

Host

R1 R2

subnet 1

subnet 2 subnet 3 subnet 4


Direct delivery vs. Indirect delivery

Direct delivery : if the datagrams is destined for a host that is on a directly connected network, it is sent directly to the host

does not involve routers identify the destination using the ARP (mapping from IP addres

s to a corresponding physical address) encapsulates the datagram in a physical frame (if necessary, th

e datagram may fragmented) in order to passing down how can it find out the destination lies on a directly connected? Indirect delivery : for destinations that are not on a directly conn

ected network, the IP layer must decide to which next-hop gateway to send the datagram, based on the network ID portion of the destination IP address

how can a router know where to send each datagram?


Table-driven IP Routing(I)

The IP routing algorithm employs an Internet routing table on each machine (host and router), which contains information about the possible destinations and how to reach them

It consults the table to decide where to send the datagram Then what information should be kept in routing tables? minimal information principle : keep network prefix only - makes routing efficient and keeps routing table small information hiding principle : the details of specific hosts confine

d to the local environment : next- hop routing - the routing table in a router only specifies one step along the p

ath from the router to a destination default routing : if no route appears in the table, the routing routi

nes send the datagram to a default router

- it makes their routing decisions efficiently to possible distant destinations


Table-driven IP Routing(II)


Table-driven IP Routing (Example)

Refer to pp 114

Network10.0.0.0

Q Network20.0.0.0

R Network30.0.0.0

S Network40.0.0.0

40.0.0.720.0.0.5

20.0.0.6

30.0.0.6

30.0.0.710.0.0.5

To reach hostson network

30.0.0.0

Route tothis address

10.0.0.0

40.0.0.0

20.0.0.0 Deliver Directly

Deliver Directly

20.0.0.5

30.0.0.7


Routing Algorithm

Route_IP_Datagram(datagram, routing_table)

Extract destination IP address, ID, from datagram

Compute IP address of destination network, IN

if IN matches any directly connected network address

send datagram to destination over that network;

else if ID appears as a host-specific route

route datagram as specified in the table;

else if IN appears in routing table

route datagram as specified in the table;

else if a default route has been specified

route datagram to the default gateway;

else declare a routing error;

Refer to pp. 116


Routing Examples (I)


Routing Examples (II)


Routing Examples (III)


Routing Examples (IV)


IP Routing (I)

IP routing is based on the destination network ID alone, what? all IP traffic for a given network tales the same path regardless t

o the delay or throughput of physical network only the final router can determine if the destination exists or is

operational, the router only can report the delivery to the sender each router routes traffic independently - someone should find o

ut if two-way communication is always possible IP routing selects the next hop to be sent the datagram, what? where does IP store the next hop address? not IP itself! IP simply passes the datagram and the next hop address to the

network interface software (so-called network driver) the driver software responsible for the physical network over whi

ch the datagram must be sent - binds the next hop IP address to a physical address, forms a frame, and sends it


IP Routing (II)

Routing tables store the IP address of a next hop for each destination network

When those addresses must be translated into corresponding physical addresses before the datagram can be sent?

RoutingTable

RoutingAlgorithm

Datagram to be sent + physical address

Network Board

Network Driver

Datagram to be sent

Data stream to be sent

reference

update

initialize

IPaddress

physicaladdress


IP Routing (III) Why does IP S/W avoid using physical addresses when storing

and computing routes? provides a clean interface between IP and high-level provides an easy method to maintain the routing table provides an abstraction hides the details of underlying networks When a host (a router) received a datagram if the datagram’s destination address matches the host’s IP addr

ess, IP accepts the datagram and passes it to high-level else, simply discard the datagram (in the case of a host) else, forward the datagram using the standard routing algorithm (in the case of a router) why a host should not route datagrams? 1. bad effect propagation 2. unnecessary network traffic 3. simple errors can cause chaos 4. a host does not has any function to correct the route


Broadcast and Multicast

Broadcast: no filtering done at net interface limited: to local net => 255.255.255.255 net: to all on specified net => netid.x, where x is the all ones ho

st portion of the address subnet: to all on specified subnet => netid.subnetid.x all subnets: to all on subnets of one net => the host portion of th

e address is all ones Multicasting: like broadcasting, but: multicast messages are sent to multicast group addresses individual interfaces can select group addresses of interest Distribution handled by collection of multicast routers IGMP (internet group management protocol) used to manage gr

oup membership DVMRP, PIM ...


Multicast Example


Chap. 9 ICMP (Internet Control Message Protocol) (I)

ICMP allows routers to send error or control message to other routers or hosts; it provides communication between the IP software on one machine and on another

Usually used to provide information about problems : Not intended to make IP reliable, but to improve the operation of the internet

failures of communication lines and processors a temporarily or permanently disconnection from the network the time-to-live counter expiration network congestion ICMP messages are grouped into two classes error message : destination unreachable, source quench,

redirect, time exceeded, parameter problem query message : echo request/ reply, timestamp request/reply,

information request/reply, address mask request/reply


ICMP (II)

ICMP is built on top of IP, but is considered an integral part of IP ICMP message are transmitted as the data portion of an IP data

gram

ICMP header

IP header ICMP header

Optional data

ICMP message

IP datagram

type

0 8 16 24 31

code

identifier

optional data

checksum

sequence number


ICMP Examples


ICMP Examples : ping Use ICMP echo request/reply Source can calculate round trip time (RTT) of packets


ICMP Examples : traceroute Records the route that packets take To determine the route, progressively increase TTL


The Internet Routing Architecture (I) Internet = a core system + a set of autonomous systems The core system is the glue, as which is controlled by the INOC(Internet Network Operations Center) provides reliable and consistent routers for all possible dest. does not use the default route has complete infor. about optimal routes to all possible dest. The autonomous system is an ever-growing component of core

system, as which is a collection of networks and gateways managed by one admi

nistrative authority are hierarchically grouped into an autonomous system (nesting) allows gateways to advertise only the reachability of those netw

orks within the gateway’s autonomous system restricts the Internet’s topology to a tree structure in which a cor

e system forms the root - only one path from the core system


The Internet Routing Architecture (II)

Core system : GGP (Gateway-to-Gateway Protocol) Core and autonomous system(s) : EGP (Exterior Gateway Proto

col) Autonomous system : IGP (Interior Gateway Protocol)

Core System

Gateway 1 Gateway 3Gateway 2

Autonomous System 1

Autonomous System 3

Autonomous System 2

Internetworking with TCP/IP DCLAB-ghcho-internet03 2001 Fall 1 Chap. 7 Internet Protocol (IP) A...

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Transcript of Internetworking with TCP/IP DCLAB-ghcho-internet03 2001 Fall 1 Chap. 7 Internet Protocol (IP) A...