1

CMPE 252A: Computer Networks

Set 10:

IP Internetworking

2

Internetworking Arbitrary collection of physical networks

interconnected to provide an end-to-end (host-to-host) packet delivery service.

Networks differ in many ways: Service offered: datagrams vs connections Protocols and mechanisms used Address space Topology and physical media

An internetwork should make all these differences transparent to end nodes.

3

IP Internetworking Based on Cerf’s catenet model

V.G. Cerf, “The Catenet Model for Internetworking,” IEN 48, July 1978.

Basic premises: Heterogeneous transmission media Heterogeneous hardware and OS in hosts and gateways Common protocol for network interconnection runs in all

gateways and hosts! Common protocol used for data transfer and signaling Common address space used to identify where a host or

router is in the internetwork An address states at which network a node attaches to

the internetwork

4

Catenet Model

A network is the address of a host in the internet A single address space, with addresses that are globally unique A single protocol for delivering all user and control data Common protocol runs in all gateways and hosts A common definition of services

G

GG G

NET

NET NET

NET

5

Catenet Model

Routing within each network is transparent to internet protocol

Internet protocol is transparent to intra-network routing

G

GG

G

RR

R R

R

6

Service Model: Theory and Practice

The Internet Protocol (IP) evolved from the catenet model.

Theory: Datagram Delivery is assumed, so that packets can get lost, out of order, and multiple copies can be delivered.

Practice: TCP needs in-order delivery of packets to

work efficiently, and (as we will see) Internet routing protocols provide a single path for each destination and do not adapt very rapidly.

Too many destinations!

7

IP Internet today

A single path to each destination, link costs are static. Starting with NSFNET, routers run IP and the Internet

is based on routers running IP interconnecting autonomous systems.

R

RR

R

RR

R R

R

8

Internet Control Protocols In addition to packet forwarding and keeping routing

tables correct, sending IP packets requires a number of control protocols:

Host must be given an IP address (DHCP) Application in host has the name of an intended destination,

and an IP address has to be found for that name. The application calls a resolver in the Domain Name System

(DNS) or uses a static hosts file (e.g., /etc/hosts) Host determines if destination IP address is the same or

different. If different, packet is sent to an attached (default) IP router. If same, the IP address must be converted to a MAC address

(ARP). Destination router must also map IP address to MAC address

(ARP). Errors may have to be reported to the source of an IP packet

using a protocol (ICMP).

9

R

RR

R

R

R

R R

R

IP Internet Today

“Simple” store-and-forward networking

“Rich” end-to-end services:Processing and storage of content

Internet Protocol Internet Protocol (IP) is the glue(IP) is the glue

A Success tale of A Success tale of “two worlds with a “two worlds with a little glue”little glue”

““Networking” is Networking” is independent of independent of processing and processing and storage of content.storage of content.

10

IP Internet ApproachDatagram switching Datagram switching (using IP)(using IP) that that

- Optimizes link utilization by - Optimizes link utilization by means of short-term storage of means of short-term storage of packetspackets- Applies the same limited - Applies the same limited instruction set on all packetsinstruction set on all packets- Assumes that destinations are - Assumes that destinations are points of attachment to Internet (points of attachment to Internet (host@net))- Assumes that the context for - Assumes that the context for switching a packet is stated in its switching a packet is stated in its encapsulated headersencapsulated headers- Assumes that physical - Assumes that physical connectivity between switch and connectivity between switch and destination existsdestination exists

R

R

R

R

R

R R

R

R

To net b go to r2 nextTo net d go to r3 nextTo net e go to r2 next….

Great if link bandwidth is scarce Great if link bandwidth is scarce andand storage and storage and processing are very expensive. processing are very expensive.

11

IP Internet ApproachMapping of application-friendly names Mapping of application-friendly names to globally-unique identifiers linked to to globally-unique identifiers linked to the topology (IP addresses)the topology (IP addresses)

- - Early binding of end points Early binding of end points is inefficientis inefficient- DNS is prone to attacks- DNS is prone to attacks- Globally-unique addressing - Globally-unique addressing is inefficient is inefficient - Mapping is inefficient if - Mapping is inefficient if hosts move or many hosts hosts move or many hosts can map to the same can map to the same application-friendly nameapplication-friendly name- Mapping introduces long - Mapping introduces long latencies and is prone to latencies and is prone to configuration errors configuration errors (35% of DNS queries fail; many of (35% of DNS queries fail; many of them due to configuration errors them due to configuration errors [Jung et al, sigcomm IMW01])[Jung et al, sigcomm IMW01])

R

R

R

R

R

R R

R

R

12

R

R

R

R

R

R R

R

R

IP Internet Approach

Connection requires connectivity and a Connection requires connectivity and a bandwidth-delay product that permits feedback.bandwidth-delay product that permits feedback.

Flow and congestion control assumes a Flow and congestion control assumes a sender-receiver session against all otherssender-receiver session against all others..

Reliable connections (using Reliable connections (using TCP) for reliable byte TCP) for reliable byte delivery between two hostsdelivery between two hosts

Reliable content delivery via Reliable content delivery via connections between specific connections between specific hosts is wasteful hosts is wasteful ((>99% use of today’s networks is for entities to acquire named chunks of data (like web pages or email messages)

– Popular sites are hotspots and Popular sites are hotspots and prone to congestionprone to congestion

– Poor reliability from dependence Poor reliability from dependence on a channel to the data sourceon a channel to the data source

– Poor utilization of computing and Poor utilization of computing and storage resources in the networkstorage resources in the network

– End-to-end connectivity may not End-to-end connectivity may not be therebe there

13

IP Internet Approach

Good: Good:

- The “glue” provided by IP.The “glue” provided by IP.

- “ “Interoperability attained with Interoperability attained with common packet format model”common packet format model”

Bad: Bad:

- The network behaves as if storage The network behaves as if storage and processing were still very and processing were still very expensive!expensive!

- End-to-end, we still emulate End-to-end, we still emulate “pipes” similar to those of the “pipes” similar to those of the telephone network!telephone network!

R

R

R

RR

R R

R

R

IP

transmissionmedia

applications

14

The Opportunity: A New Kind of Network

A richer “instruction set” A richer “instruction set” for packet switching that for packet switching that takes advantage of takes advantage of contextcontext

New routers store and New routers store and process process contentcontent

Names of content, not Names of content, not host addresses, used as host addresses, used as the entities for routingthe entities for routing

Consumers and providers Consumers and providers of content collaborate of content collaborate based on their contextbased on their context

n

nn

n

““Store-process-forward” networking;Store-process-forward” networking;

Process and storage of content Process and storage of content inside the networkinside the network

15

Designing Communication Protocols

ARCHITECTURES

LIMITS

ANALYTICAL MODELS & SIMULATIONS

LOGIC: Self-Organizing, scalable

16

Research Areas

n

nn

n

transmissionmedia

applications

Opportunistic Opportunistic storage, processing, comm. storage, processing, comm.

Naming:Naming: No DNS, no IPv6, tell the network what you

want, not where it should be What’s a name, what’s an object?

Content dissemination & Content dissemination & search:search:

No need for e-t-e connectivity, routing tables become distributed search engine for content

R.T. conversations as objects, rather than reserved pipes

No need for large pkt headers stating context in each pkt; context establishment is part of distributed storage & processing

Handling billions of objects

Devices:Devices: No host/router divide; exploit devic context

Security:Security: Protect information objects and expressed

interests for content wherever they may be

New apps, no servers!New apps, no servers! New packet switching modelsNew packet switching models Others… New team-oriented Others… New team-oriented

retransmission & congestion retransmission & congestion control schemescontrol schemes

17

Hierarchical Routing

Scale: 100’s millions of destinations: Routing table cannot store an entry for each destination! Routing table exchange would swamp links.

Administrative autonomy Internet is a network of networks Each network administrator may want to control routing in its own

network.

18

Hierarchical Routing

Aggregate routers into regions, “autonomous systems” (AS)

Routers in same AS run same routing protocol

“intra-AS” routing protocol

Routers in different AS can run different intra-AS routing protocol

Special routers in AS Run intra-AS routing

protocol with all other routers in AS

Responsible for routing to destinations outside AS

run inter-AS routing protocol with other gateway routers

Gateway Routers

19

Intra-AS and Inter-AS Routing

Gateways:• Perform inter-AS routing amongst themselves• Perform intra-AS routers with other routers in their AS

inter-AS, intra-AS routing in

gateway A.c

network layer

link layer

physical layer

a

b

b

aaC

A

Bd

A.a

A.c

C.bB.a

cb

c

20

Intra-AS and Inter-AS Routing

a

b

b

aaC

A

Bd

A.a

A.c

C.bB.a

cb

c

Host S

Host D

Intra-AS routingwithin AS B

Inter-AS routingbetween A and B

Intra-AS routingwithin AS A

21

Layers in Internet Routing

PHYSICAL

LINK

NETWORK

TRANSPORT (TCP or UDP)

SESSION

PRESENTATION

APPLICATION

PHYSICAL

LINK

NETWORK

TRANSPORT

SESSION

PRESENTATION

APPLICATION

RoutingTable

IP IP

RoutingProtocol

RoutingProtocol

RoutingTable

RoutingTable

RoutingTable

22

IPv4 Packet Information

Typically no options and header is 20 bytes

version HLen TOS length

identifier flags offset

TTL protocol checksum

0 3 7 15 23 31

source address

destination address

Options (variable) pad (variable)

data

23

IPv4 Datagram Format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (words)

max numberremaining hops

(decremented at each router)

forfragmentationandreassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) e.g., timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

20 bytes of TCP 20 bytes of IP = 40 bytes + app

layer overhead

24

IPv4 Addresses IP addresses are global and, unlike MAC

addresses, they are hierarchical. IP address has a network part and a host part

and specifies host@network A host has an address for each network to

which it attaches. IP addresses are denoted using the dotted-

decimal notation: Each byte of the address is written in its decimal form and is separated by a dot from the other bytes, e.g.,

5.7.2.1 => 00000101 00000111 00000010 00000001

25

IPv4 Addresses

IP address: 32-bit identifier for host, router interface

Interface: connection between host or router and physical link

Router’s typically have multiple interfaces

Host may have multiple interfaces

IP addresses associated with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

26

IPv4 Addresses

Routing table entries referring to destinations in the same AS refer to networks only.

223.1.1.*

223.1.2.*

223.1.3.*

27

IPv4 Addressing Problems There were too few networks left due to the

class structure used in IP address assignments! There are many more IP devices and appliances

coming. Routing tables cannot have millions of entries. Solutions:

Aggregation of addresses without classes (CIDR) New and much bigger global address space (IPv6) Locally unique addresses (NAT and other

techniques)

28

Class-based IPv4 Addresses (past)

network host

host

host

multicast address

0

8 16 24 310

Class A

network

network10Class B

Class C

16 million

65,534

110

126

16,382

2 million 254

reserved address

1110

11110

Class D

Class E

29

IP Addressing: CIDR Classful addressing:

Inefficient use of address space, address space exhaustion. A class B address has enough addresses for 65K hosts, even if only a

few more than 256 hosts are located in that network CIDR: Classless InterDomain Routing

Eliminate the strict assignment of address portion in class-full addressing. Enable a network portion of address of arbitrary length.

CIDR Address Format: a.b.c.d/x, where x is # bits in network portion of address

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23

30

Assigning IP Addresses to Hosts

Hard-coded by system administrator in a file Wintel: control-panel->network->configuration-> tcp/ip->properties UNIX: /etc/rc.config

Obtain address from as server dynamically (“plug-and-play”)

This is the purpose of DHCP: Dynamic Host Configuration Protocol:

31

Assigning Network Portion of IP Address to a Network

An ISP obtains a block of the address space. Net is allocated portion of its provider ISP’s

address space.

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23

Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23

Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

32

IANA: Internet Assigned Numbers Authority Domain names: Manage the DNS root, .int, .arpa

domains. Number resources: Coordination of global pool of IP

and AS numbers via Regional Internet Registries Protocol assignments: Manage Internet protocol

numbering systems together with standards bodies. Operated by Internet Corporation for Assigned Names and Numbers

(ICANN) under a US Department of Commerce contract http://www.iana.org/ http://www.icann.org/ Take a look and remind yourselves how it works.

Assigning Blocks of Addresses to ISPs

33

Hierarchical Addressing for Route Aggregation

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

My-ISP

Organization 0

Organization 7Internet

Organization 1

Another-ISP“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

Allow efficient advertisement of routing information

34

Hierarchical Addressing and More Specific Routes

Another-ISP has a more specific route to Organization 1

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

My-ISP

Organization 0

Organization 7Internet

Organization 1

Another-ISP “Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

35

Internet Routing Protocols Itra-domain routing:

RIP, OSPF, EIGRP Single-path routing protocols, static link

costs Performance (shortest path)

Inter-domain routing: Border Gateway Protocol (BGP) Single path Policy based

36

RIP Based on DBF Used in small internets Problems: Counting to infinity and looping, single-

path routing, link cost should be 1 or infinity Update specifies only a destination network and a

distance to it; hence, no variable subnet masks are allowed in “local” internet and a static subnetting convention must be used for all routers

Router sends its routing table to its neighbors every 30 sec. or when it must update its routing table.

Runs on top of UDP.

37

RIPv2 Adds the next hop to a destination

and subnet mask in each update. Variable subnets are allowed. Performance does not improve much.

38

OSPF: Open Shortest Path First Dijkstra’s SPF used to compute shortest paths

locally based on topology map. Flooding is used to disseminate topology maps. Sequence numbers and age fields are used to

validate link-state updates. Runs on top of IP and implements its own reliable

transmission of link-state updates. Designated routers are used to reduce overhead

within a LAN, and areas connected by a backbone are used to reduce overhead across LANs.

A handshake is used to reduce overhead of sending large portions of the topology map between neighbors.

HELLOs used to identify neighbors.

39

R not allowed

R

R

R

R

OSPF

Areas must be connected by a connected backbone (area 0)

A2

A4A3

R

R

R R

R

A1

backbone

40

OSPF

R

R

R

RA2

A4A3

R

R

R R

R

A1

area border routerTo otherdomains

boundaryrouter,backbonerouter

internal router

EA1EA2…EAn

41

OSPF Areas need unique IDS, an IP address. Zero or more address ranges can be reached in an area. Different types of routers have different views of topology. End result is a hybrid of link-state and distance information.

42

Topology Information at Backbone Router

R

R

R

RA2

A4A3

A1

EA1

EA2…EAn

43

Topology Information at Area Border Router

R

R

R

RA2

A4A3

A1

EA1

EA2…EAn

R

R

R R

R

44

Topology Information atInternal Router

A2

A4A3

A1

EA1

EA2…EAn

R

R

R

R

R

45

OSPF

In a broadcast LAN, designated router eliminates too many link state updates.

LSUs, HELLOs and topology updates sent unicast to designated router, which keeps all routers in LAN updated.

R R RR

46

Differences between Intra- and Inter-AS Routing

Policy: Inter-AS: admin wants control over how its

traffic routed, who routes through its net. Intra-AS: single administration, so no policy

decisions needed (usually; changing with traffic engineering)

Scale: Hierarchical routing saves table size, reduced

update trafficPerformance: Intra-AS: can focus on performance Inter-AS: policy dominates over performance

47

Inter-AS routing in the Internet: BGP

Figure 4.5.2-new2: BGP use for inter-domain routing

AS2 (OSPF

intra-AS routing)

AS1 (RI P intra-AS

routing) BGP

AS3 (OSPF intra-AS

routing)

BGP

R1 R2

R3

R4

R5

48

BGP BGP (Border Gateway Protocol): the de facto

standard for Internet inter-AS routing. Path Vector protocol:

Similar to Distance Vector protocol Each Border Gateway broadcast to neighbors

(peers) entire path (i.e., sequence of AS’s) to destination

BGP routes to networks (ASs), not individual hosts

e.g., Gateway X may send its path to dest. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z

49

BGP Gateway X send its path to peer gateway W W may or may not select path offered by X

cost, policy (don’t route via competitors AS), loop prevention reasons.

If W selects path advertised by X, then:Path (W,Z) = w, Path (X,Z)

Note: X can control incoming traffic by controlling its route advertisements to peers: e.g., if X does not want to forward traffic to Z then do not advertise

any routes to Z

50

Controlling Who Routes to You

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

A,B,C are provider networks X,W,Y are customer (of provider networks

to which they attach directly) X is dual-homed: attached to two networks

X does not want to forward from B via X to C

.. so X will not advertise to B a route to C

Do not allow

51

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW?

“No”... B gets no “revenue” for routing CBAW since neither W nor C are B’s customers

B wants to force C to route to W via A B wants to route only to/from its customers!

Controlling Who Routes to You

52

BGP Operation

Receiving and filtering route advertisements from directly attached neighbor(s).

Route selection. To route to destination X, which path )of

several advertised) will be taken? Sending route advertisements to

neighbors.

53

BGP Messages BGP messages exchanged using TCP. BGP messages:

OPEN: opens TCP connection to peer and authenticates sender

UPDATE: advertises new path (or withdraws old)

KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request

NOTIFICATION: reports errors in previous msg; also used to close connection

54

Additional Control More than just route computation and

forwarding!

55

Sending Datagrams from Source to Destination

IP datagram: remains unchanged, as it travels from source to destination

miscfields

sourceIP addr

destIP addr data

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

Forwarding table at A:

223.1.1.1

223.1.1.2

223.1.1.3

223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

223.1.1.4

223.1.1

223.1.3

223.1.2

56

Sending Datagrams from Source to Destination

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

Forwarding table at A:

223.1.1.1

223.1.1.2

223.1.1.3

223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

223.1.1.4

223.1.1

223.1.3

223.1.2

miscfields 223.1.1.1223.1.1.3 data

Starting at A, send IP datagram addressed to B:

Look up net. address of B in forwarding table

Find B is on same net as A Link layer sends datagram

directly to B inside link-layer frame

B and A are directly connected

57

Sending Datagrams from Source to Destination

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

Forwarding table at A:

223.1.1.1

223.1.1.2

223.1.1.3

223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

223.1.1.4

223.1.1

223.1.3

223.1.2

Starting at A, dest. E: Look up network address of

E in forwarding table E is on a different network

A, E not directly attached

Routing table: next hop router to E is 223.1.1.4

Link layer sends datagram to router 223.1.1.4 inside link-layer frame

Datagram arrives at 223.1.1.4

miscfields 223.1.1.1223.1.2.2 data

58

Sending Datagrams from Source to Destination

223.1.1.1

223.1.1.2

223.1.1.3

223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

223.1.1.4

223.1.1

223.1.3

223.1.2

Dest. Net router Nhops interface

223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

Forwarding table at router

Arriving at 223.1.4, destined for 223.1.2.2

Look up network address of E in router’s forwarding table

E on same network as router’s interface 223.1.2.9

Router and E are directly attached

Link layer sends datagram to 223.1.2.2 inside link-layer frame via interface 223.1.2.9

Datagram arrives at 223.1.2.2

miscfields 223.1.1.1223.1.2.3 data

59

Fragmentation

Packet length is in bytes and includes header; maximum length is then 65,535 bytes

MAC protocol my not support such long packets, and an IP packet may have to be fragmented.

Ethernet accepts frames of up to 1500 bytes and FDDI of up to 4500 bytes

Each fragment is a self-contained datagram. Fragmentation is handled with:

The packet ID, which is the same for all fragment The offset, which states the byte (position) of the

fragment A flag indicating that there a more fragments for the same

ID coming.

60

Fragmentation Example

Each datagram has a 20-byte header. Re-assembly done at IP module of destination D. Fragmentation is avoided in general by limiting the size of TCP

segments to 536 B, because all data-link protocols must support MTUs of 576 B. (536 + 20 of IP header + 20 of TCP header).

R

R

R

R R

R

Length = 3980 BID = 50

Max MTU size = 1500 B

ID = 50; length = 1480B; flag = 1 (more)

ID = 50; length = 1480B; flag = 1 (more)

ID = 50; length = 1020B; flag =0 (last)

D

S

61

IPv4 Header TTL (time to live indicates how long the packet can stay in

the network; it is specified in hops and is decremented each time the packet is forwarded.

Default is 64 hops; nodes can play with the field to limit the scope Protocol specifies the type of payload Checksum is computed considering the entire header as a

sequence of 16-bit words, adding them up with 1’s complement arithmetic and taking the 1’s complement of the result.

This checksum is NOT as powerful as a CRC but is simple to do in software.

Why this way? Because it is done at each hop (software) What if we process headers in hardware?

62

Error Reporting

In general, errors can be reported to the origin of a packet or to intermediate relays or both.

In the IP Internet, errors are reported to the source using ICMP (internet control message protocol).

The choice stems from using IP for all signaling and user data transfer in the Internet.

ICMP messages are encapsulated in IP. An IP packet specifies the source and

destination and not the relays (options are not supported in general)

63

ICMP: Internet Control Message Protocol

Used by hosts and routers for

Error reporting: unreachable host, network, port, protocol

Echo request/reply (used by ping)

Network-layer “above” IP: ICMP messages are

carried in IP datagrams ICMP message: type, code

plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

64

Address Resolution Protocol Goal: Enable a host to build a table of

mappings between IP addresses and MAC addresses in a dynamic manner.

Mappings are called ARP cache or ARP table. Approach:

ARP is designed assuming a fully connected, broadcast link layer (LAN) and the requestor is responsible for persisting.

Hosts and routers broadcast requests and responses and listen to requests and responses from any other node in the LAN.

Different approach would be needed in a multihop LAN.

65

protocol type

ARP

Hardware type = hardware interface type (e.g., Ethernet is 1) HLEN and PLEN = length of MAC address and high-level protocol address Protocol type = type of high-level protocol address (e.g., IP) OPERATION = ARP request (1), ARP response (2), RARP request (3), RARP response (4).

hardware type

8 16 24 310

operation PLENHLEN

sender MAC address

sender IP addresssender MAC address

target MAC addresssender IP address

target MAC address

target IP address

66

ARP Implementation considerations:

Minimize the number of ARP requests sent Queue requests for the same IP address Learn form requests from others Only one node is the target of an ARP A node can act as a proxy for others

Reverse ARP (RARP) provides the requestor with its own IP address Nodes responding should play response

avoidance using randomized timers

67

Dynamic Host Configuration Host must be assigned an IP address, because it

is not committed to hardware as a MAC address. Configuring hosts with proper IP addresses is

involved. DHCP (dynamic host configuration protocol) is a

solution to this configuration and management problem.

DHCP is intended to support manual, automatic and dynamic configurations

DHCP is designed to work with no pre-configured addresses of servers and across networks.

68

DHCP

my IP???HOST

discover to broadcast IP address (all hosts and routers in LAN)

DHCPrelay

internet

unicast besteffort to server

SERVER

Derived from BOOTP

Host knows nothing, relay is configured with IP address of server

Sever assigns IP addresses statically or dynamically

Relies on end-to-end level datagrams (UDP)

UDP specifies “DHCP”

69

DHCP: Dynamic Host Configuration Protocol

Goal: Allow host to dynamically obtain its IP address from network server when it joins network.Can renew its lease on address in use

Allows reuse of addresses (only hold address while connected an “on”

Support for mobile users who want to join network (more shortly)

DHCP overview: host broadcasts “DHCP discover” msg DHCP server responds with “DHCP offer” msg host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg

70

Special IP Addresses

All 0’s IP address means: “This host” Address with net.host = 0.host means:

“host in this network” All 1’s IP address means:

“Broadcast on this network” Address with net.host = net.111…1

means: “broadcast on net” 127.anything means “loopback”

71

DHCP Client-Server Scenario

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

72

DHCP Client-Server ScenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offer

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

73

NAT: Network Address Translation

NAT router

Translates between local and globaladdresses available

locally-unique IP addresses

used

host

Local IP address

Many locally unique addresses.A few globally unique addresses available.

INTERNET

Globally unique addresses must be used

74

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this network

have 10.0.0/24 address for source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

75

NAT Motivation

Local network uses just one IP address as far as outside world is concerned

No need to be allocated range of addresses from ISP: - just one IP address is used for all devices

Can change addresses of devices in local network without notifying outside world

Can change ISP without changing addresses of devices in local network

Devices inside local net not explicitly addressable, visible by outside world (a security plus).

76

Functions of NAT Router Outgoing datagrams: Replace (source IP address, port #) of every

outgoing datagram to (NAT IP address, new port #). . . remote clients/servers will respond using (NAT

IP address, new port #) as destination addr. Remember (in NAT translation table) every (source IP address, port #) to (NAT IP

address, new port #) translation pair Incoming datagrams: Replace (NAT IP address, new port #) in dest fields

of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

77

NAT Example

10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4

S: 138.76.29.7, 5001D: 128.119.40.186, 802

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3

3: Reply arrives dest. address: 138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345

78

NAT Issues 16-bit port-number field:

60,000 simultaneous connections with a single LAN-side address!

NAT is “controversial”: “Routers should only process up to layer 3” “Violates end-to-end argument”

NAT possibility must be taken into account by app designers, e.g., P2P applications

Should address shortage be solved using IPv6 instead? (a new address space of globally unique IDs)