Computer Networks. Main Functions

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Computer Networks

The Network Layer

Main Functions

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Design Issues

• Services provided to transport layer. • How to design network-layer protocols.

Store-and-Forward Packet Switching

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Services

• What kind of services provided to transport

layer?

• Connection-oriented versus connectionless

service?

Connectionless Service

• Datagram network.

• “Move all intelligence to the edges”.

– Routers just route.

– Everything else should be done end-to-end.

• No ordering, no flow/congestion control, no

reliable delivery.

• Best-effort service model.

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Connection-Oriented Service

• Virtual circuit networks. • `A la telephone network. • Reliable, ordered service.

• Virtual connection established from source to

destination.

• E.g., X-25, ATM.

Datagram Network Operation

• How does it work?

• Data from transport layer is broken into

packets, or datagrams.

• Network layer at host adds network-layer

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Datagram Network: Example

• Routing within a diagram subnet.

Virtual Circuit Network Operation

• Connection-establishment before sending

data.

– All traffic for that connection follows same

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Virtual Circuit Network: Example

• Routing within a virtual-circuit subnet.

Virtual-Circuit versus Datagram Subnets

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Routing

Routing

• One of the main functions of network layer. • Routing versus forwarding?

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Routing Algorithm

• Computes routing tables. • Properties:

– Correctness. – Robustness. – Stability. – Optimality.

• Try to optimize a certain metric.

Optimality Principle

• General statement about optimal routes

(topology, routing algorithm independent).

• If router J is on optimal path between I

and K, then the optimal path from J to K

also falls along the same route.

– Proof by contradiction.

• Corollary:

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Types of Routing Algorithms

• Non-adaptive versus adaptive.

Adaptive and Non-adaptive Routing

• Non-adaptive routing:

– Fixed routing, static routing.

– Do not take current state of the network (e.g.,

load, topology).

– Routes are computed in advance, off-line, and

downloaded to routers when booted.

• Adaptive routing:

– Routes change dynamically as function of current

state of network.

– Algorithms vary on how they get routing

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Static Algorithms



(Non-Adaptive)

1.Shortest-path routing.

2.Flooding.

Shortest-Path Routing

• Problem: Given a graph, where nodes

represent routers and edges, links, find

shortest path between a given pair of nodes.

• What is shortest in shortest path?

– Depends on the routing metric in use.

– Example: number of hops (static), geographic

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Dijkstra’s Shortest-Path Algorithm

• Initially, links are assigned costs.

• As the algorithm executes, nodes are labeled

with its distance to source along best known path.

• Initially, no routes known, so all nodes are

labeled with infinity.

• Labels change as the algorithm proceeds. • Labels can be temporary or permanent.

– Initially all labels are tentative.

– A label becomes permanent if it represents the

shortest path from the source to the node.

Shortest Path Routing

Start

Label each adjacent node with distance to

A.

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Flooding

• Every incoming packet forwarded on every

outgoing link except the one it arrived on.

• Problem: duplicates. • Constraining the flood:

– Hop count.

– Keep track of packets that have been flooded.

• Robust, shortest delay (picks shortest path as

one of the paths).

Flooding: Example

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Dynamic Routing Algorithms

•

(

Adaptive Routing)

– Distance vector routing.

– Link state routing.

Distance Vector Routing

• Aka, Bellman-Ford (1957), Ford-Fulkerson

(1962).

• Original ARPANET routing; also used by

Internet’s RIP.

• Each router keeps routing table (or routing

vector) with best known distance to each

destination and corresponding outgoing

interface.

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Distance Vector (Cont’d)

• Routing table at each router:

– One entry per participating router.

– Each entry contains outgoing interface and distance to

corresponding destination.

– Metric: number of hops, delay, queue length. – Each router knows distance to its neighbors.

• Old ARPANET algorithm: DV where cost metric

is outgoing link queue length.

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Routing Updates

•

Every T interval, routers exchange

routing updates.

•

Routing update from router X consists of

a vector with all destinations and the

corresponding distance from X to them.

•

When router Y receives an update from

X, it can estimate its distance to router Z

through X as D

= D

+ D

.

•

Router Y receives update from all its

neighbors and builds a new RT.

Distance Vector: Example

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2 ₃

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1 Node Distance Next 2 3 3 2 1 9 9 5 1 2 1 0 -2 -2 -2 3 5 3 4 1 4 5 6 3 6 8 3 T=T₀ T=T1 3 7 5 2 3 4 0 _{4 2} 3 0 2 2 2 0 3 1 1 5 3 3

Node Distance Next

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Problems

1.Routing loops.

2.Slow convergence.

3.Counting to infinity.

Count-to-Infinity

• Good news propagate faster.

A B C D E

Initially, A down:

A comes up: infinity₁

1 2 infinity infinity (after 2 exchanges) 1 2 3 infinity (after 3 exchanges) 1 2 3 4 (after 4 exchanges) infinity infinity infinity infinity infinity

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Count-to-Infinity (Cont’d)

• But, bad news propagate slower!

A B C D E

Initially, all up:

A goes down: 1 2 3 4_{3 2 3 4 (after 1 exchange)} 3 4 3 4 (after 2 exchanges) 5 4 5 4 (after 3 exchanges) 5 6 5 6 (after 4 exchanges) 7 6 7 6 (after 5 exchanges) 7 8 7 8 (after 6 exchanges) …. infinity

Count-to-Infinity (Cont’d)

• Gradually routers work their way up to infinity.

• Number of exchanges depends on how large is

infinity.

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Solution

•

Routing loops:

– Path vector: record actual path used in the DV. – Previous hop tracing: records preceding router.

•

Count-to-infinity:

– Split horizon: router reports to neighbor cost “infinity”

for destination if route to that destination is through that neighbor.

Split Horizon

• Tries to make bad news spread faster.

• A node reports infinity as distance to node

X on link packets to X are sent.

• Example, in the first exchange, C tells D

its distance to A but tells B its distance to

A is infinity.

– So B discovers its link to A is down and C’s

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Link State Routing

• DV routing used in the ARPANET until 1979,

when it was replaced by link state routing.

• Used by the Internet’s OSPF.

• Based on Dijkstra’s “all pairs shortest path”

algorithm.

• Plus link state updates.

Link State Routing (Cont’d)

• Link state routing is based on:

– Discover your neighbors and measure the

communication cost to them.

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Finding Neighbors

• When router is booted, its first task is to find

who its neighbors are.

• Special single-hop “hello” packets.

• Cost metric:

– Number of hops: in this case, always 1. – Delay: “echo” packets and measure RTT/2. – Load?

Generating Link State Updates

• Link state packets (LSP).

– Sender identity. – Sequence number. – TTL.

– List of (neighbor, cost).

• When to send updates?

– Proactive: periodic updates; how often?

– Reactive: whenever some significant event is detected,

e.g., link goes down.

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Processing Updates

• When LSP is received:

– Check sequence number.

– If higher than current sequence number, keep it and

flood it; otherwise, discard it.

– Periodically decrement TTL.

• When TTL=0, purge LSP.

Computing Routes

• Routers have global view of network.

– They receive updates from all other routers with their

cost to their neighbors.

– Build network graph.

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Measuring Line Cost

• A subnet in which the East and West parts

are connected by two lines.

Building Link State Packets

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Distributing the Link State

Packets

B’s LSP buffer: each row corresponds to a recently LSP that hasn’t been processed yet.

Link State Routing: Problems

• Scalability:

– Storage: kn, where n is number of routers and

k is number of neighbors.

– Computation time.

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DV versus LS

• DV:

– Node tells its neighbors what it knows about everybody. – Based on other’s knowledge, node chooses best route. – Distributed computation.

• LS:

– Node tells everyone what it knows about its neighbors. – Every node has global view.

– Compute their own routes.

Hierarchical Routing

•

For scalability:

– As network grows, so does RT size, routing update

generation, processing, and propagation overhead, and route computation time and resources.

•

Divide network into routing regions.

– Routers within region know how to route packets to

all destinations within region.

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Hierarchical Routing: Example

1B 1A 1C 2A _2B 2C 2D 3A _3B 4A 4B 4C 5E 5D 5C 5B 5A

Dest. Next Hops

1A - -1B -1B 1 1C 1C 1 2A 1B 2 2B 1B 3 2C 1B 3 2D 1B 4 3A 1C 3 3B 1C 2 4A 1C 3 4B 1C 4 4C 1C 4 5A 1C 4 5B 1C 5 5C 1B 5 5D 1C 6 5E 1C 5 1A Flat routing:

Hierarchical Routing: Example

1B 1A 1C 2A _2B 2C 2D 3A _3B 4A 4B 4C 5E 5D 5C 5B 5A

Dest. Next Hops

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Hierarchical Routing

• Optimal paths are not guaranteed.

– Example: 1A->5C should be via 2 and not 3.

• How many hierarchical levels?

– Example: 720 routers.

• 1 level: each router needs 720 RT entries.

• 2 levels: 24 regions of 30 routers: each router’s

RT has 30+23 entries.

• 3 levels: 8 clusters of 9 regions with 10 routers:

each router’s RT 10+8+7.

Many-to-Many Routing

• Support many-to-many communication.

• Example applications: multi-point data

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Broadcasting

• Send to ALL destinations.

• Several possible routing mechanisms to broadcasting.

• Simplistic approach: send separate packet to

each destination.

– Simple but expensive.

– Source needs to know about all destinations.

• Flooding:

– May generate too many duplicates (depending on node connectivity).

Multidestination Routing

•

Packet contains list of destinations.

•

Router checks destinations and determines on

which interfaces it will forward packet.

– Router generates new copy of packet for each output

line and includes in packet only the appropriate set of destinations.

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Spanning Tree Routing

• Use spanning tree (sink tree) rooted at

broadcast initiator.

• No need for destination list.

• Each on spanning tree forwards packets on all

lines on the spanning tree (except the one the packet arrived on).

• Efficient but needs to generate the spanning

tree and routers must have that information.

Reverse Path Forwarding

•

Routers don’t have to know spanning tree.

•

Router checks whether broadcast packet

arrived on interface used to send packets to

source of broadcast.

– If so, it’s likely that it followed best route and thus not

a duplicate; router forwards packet on all lines.

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Broadcast Routing

Reverse path forwarding. (a) A subnet. (b) a Sink tree. (c)The tree built by reverse path forwarding.

Multicasting

• Special form of broadcasting:

– Instead of sending messages to all nodes, send

messages to a group of nodes.

• Multicast group management:

– Creating, deleting, joining, leaving group.

– Group management protocols communicate group

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Multicast Routing

• Each router computes spanning tree covering all

other participating routers.

– Tree is pruned by removing that do not contain

any group members.

1,2 1 1,2 2 2 1 1 2 1,2 1 1,2 2 2 1 1 2 1 1 1 1 1 2 2 2 2 2

Shared Tree Multicasting

• Source-rooted tree approaches don’t scale well!

– 1 tree per source, per group!

– Routers must keep state for m*n trees, where m is number

of sources in a group and n is number of groups.

• Core-based trees: single tree per group.

– Host unicast message to core, where message is

multicast along shared tree.

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Internetworking

Internetworking

• What is it?

– Connecting networks together forming a single

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Connecting Networks

• A collection of interconnected networks.

How Networks Differ

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How Networks Can Be Connected

• (a) Two Ethernets connected by a switch.

• (b) Two Ethernets connected by routers.

How to Internet?

• Connection-oriented versus connectionless

internetworking.

• Connection oriented internetworking:

– Based on VC concatenation.

• Connectionless internetworking follows the

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Concatenated Virtual Circuits

. Builds VC crossing the different networks.

. Use of gateways to perform necessary conversions. Gateway

Connectionless Internetworking

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Translating versus “Gluing”

• Translation: converting between different

protocols.

• Hard!

• Alternative: “gluing”.

– I.e., using the same network layer protocol

everywhere.

– That’s what IP does!

Tunneling

• Interconnecting source and destination on

separate networks but of the same type.

S

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Tunneling Analogy

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Internetworking

Internetwork Routing

• Inherently hierarchical.

– Routing within each network: interior gateway

protocol (IGP).

– Routing between networks: exterior gateway

protocol (EGP).

• Within each network, different routing

algorithms can be used.

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Internetwork Routing: Example

• (a) An internetwork. (b) A graph of the internetwork.

Internetwork Routing (Cont’d)

• Typically, packet starts in its LAN. Gateway

receives it (broadcast on LAN to “unknown”

destination).

• Gateway sends packet to gateway on the

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Fragmentation

• Happens when internetworking.

• Network-specific maximum packet size.

– Width of TDM slot. – OS buffer limitations.

– Protocol (number of bits in packet length field).

• Maximum payloads range from 48 bytes (ATM

cells) to 64Kbytes (IP packets).

Problem

• What happens when large packet wants to

travel through network with smaller maximum packet size? Fragmentation.

• Gateways break packets into fragments; each

sent as separate packet.

• Gateway on the other side have to reassemble

fragments into original packet.

• 2 kinds of fragmentation: transparent and

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Types of Fragmentation

• (a) Transparent fragmentation. (b)

Nontransparent fragmentation.

Transparent Fragmentation

• Small-packet network transparent to other

subsequent networks.

• Fragments of a packet addressed to the same

exit gateway, where packet is reassembled.

– OK for concatenated VC internetworking.

• Subsequent networks are not aware

fragmentation occurred.

• ATM networks (through special hardware)

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Problems with Transparent

Fragmentation

• Exit gateway must know when it received all the

pieces.

– Fragment counter or “end of packet” bit.

• Some performance penalty but requiring all

fragments to go through same gateway.

• May have to repeatedly fragment and

reassemble through series of small-packet networks.

Non-Transparent

Fragmentation

• Only reassemble at destination host.

– Each fragment becomes a separate packet. – Thus routed independently.

• Problems:

– Hosts must reassemble.

– Every fragment must carry header until it reaches

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Keeping Track of Fragments

• Fragments must be numbered so that original data stream can be reconstructed.

• Tree-structured numbering scheme:

– Packet 0 generates fragments 0.0, 0.1, 0.2, …

– If these fragments need to be fragmented later on, then 0.0.0, 0.0.1, …, 0.1.0, 0.1.1, …

– But, too much overhead in terms of number of fields needed.

– Also, if fragments are lost, retransmissions can take alternate routes and get fragmented differently.

Keeping Track of Fragments

(Cont’d)

• Another way is to define elementary

fragment size that can pass through every

network.

• When packet fragmented, all pieces equal

to elementary fragment size, except last

one (may be smaller).

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Fragmentation: Example

• Fragmentation when the elementary data size is 1 byte.

• (a) Original packet, containing 10 data bytes.

• (b) Fragments after passing through a network with maximum packet size of 8 payload bytes plus header.

• (c) Fragments after passing through a size 5 gateway.

Keeping Track of Fragments

• Header contains packet number, number of first

fragment in the packet, and last-fragment bit.

27 0 1 A B C D E F G H I J

27 0 0 A B C D E F G H _{27 8 1 I J} Packet number Number of_{first fragment}

Last-fragment bit

(a) Original packet with 10 data bytes.

(b) Fragments after passing through network with maximum packet size = 8 bytes.

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The Internet

Design Principles for Internet

• Keep it simple.

• Exploit modularity.

• Expect heterogeneity.

• Think robustness.

• Avoid static options and parameters.

• Think about scalability.

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Internet as Collection of

Subnetworks

IP (Internet Protocol)

• Glues Internet together.

• Common network-layer protocol spoken

by all Internet participating networks.

• Best effort datagram service:

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IP

• Transport layer breaks data streams into

datagrams; fragments transmitted over Internet,

possibly being fragmented.

• When all packet fragments arrive at destination,

reassembled by network layer and delivered to

transport layer at destination host.

IP Versions

• IPv4: IP version 4.

– Current, predominant version. – 32-bit long addresses.

• IPv6: IP version 6 (aka, IPng).

– Evolution of IPv4.

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IP Datagram Format

• IP datagram consists of header and data (or

payload).

• Header:

– 20-byte fixed (mandatory) part. – Variable length optional part.

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IP Options

5-54

IP Addresses

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IP Addresses (Cont’d)

• Class A: 128 networks with 16M hosts each. • Class B: 16,384 networks with 64K hosts

each.

• Class C: 2M networks with 256 hosts each. • More than 500K networks connected to the

Internet.

• Network numbers centrally administered by

ICANN.

IP Addresses (Cont’d)

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Scalability of IP Addresses

• Problem: a single A, B, or C address refers to

a single network.

• As organizations grow, what happens?

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Solution

• Subnetting: divide the organization’s address

space into multiple “subnets”.

• How? Use part of the host number bits as the

“subnet number”.

• Example: Consider a university with 35

departments.

– With a class B IP address, use 6-bit subnet

number and 10-bit host number.

– This allows for up to 64 subnets each with

1024 hosts.

Subnets

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Subnet Mask

• Indicates the split between network and subnet

number + host number.

Subnet Mask: 255.255.252.0 or

/22 (network + subnet part)

Subnetting: Observations

• Subnets are not visible to the outside world. • Thus, subnetting (and how) is a decision

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Subnet: Example

Problem with IPv4

• IPv4 is running out of addresses.

• Problem: class-based addressing scheme.

– Example: Class B addresses allow 64K hosts.

• More than half of Class B networks have fewer

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Solution: CIDR

• CIDR: Classless Inter-Domain Routing.

– RFC 1519.

• Allocate remaining addresses in

variable-sized blocks without considering classes.

• Example: if an organization needs 2000

addresses, it gets 2048-address block.

• Forwarding had to be modified.

– Routing tables need an extra entry, a 32-bit

mask, which is ANDed with the destination IP address.

– If there is a match, the packet is forwarded on

that interface.

Network Address Translation

• Each organization gets a single (or small number of)

IP addresses.

– This is used for Internet traffic only.

– For internal traffic, each host gets its own “internal” IP

address.

• Three IP ranges have been declared as “private”. – 10.0.0.0 – 10.255.255.255/8

– 172.16.0.0 – 172.31.255.255/12 – 192.168.0.0 – 192.168.255.255/16

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NAT – Network Address

Translation

Internet Control Protocols

• “Companion” protocols to IP.

• Control protocols used mainly for signaling

and exchange of control information.

• Examples: ICMP, ARP, RARP, BOOTP, and

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ICMP

• Internet Control Message Protocol.

• A way to “debug” the Internet and find out

what is happening at routers.

• Defines a dozen different messages that are

generated typically by routers upon some unexpected event.

ICMP Message Types

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Address Resolution Protocol

• ARP.

• RFC 826.

• Protocol for machines to map IP addresses to

Ethernet addresses.

– This is needed when packet needs to be

delivered to a local host on a LAN (Ethernet).

ARP: Example

. Host 1 wants to send packet to host 2.

. Assume that host 1 knows host 2’s IP address. . Host 1 builds packet with host 2’s IP address.

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ARP Operation

• Host 1 broadcasts an ARP request on the

Ethernet asking who owns host 2’s IP address.

• Host 2 replies with its Ethernet address. • Some optimizations:

– ARP caches.

– Piggybacking host’s own Ethernet address on

ARP requests.

– Proxy ARP: services ARP requests for hosts

on separate LANs.

Beyond ARP

• ARP solves the problem of mapping IP

address to Ethernet address.

• How do we solve the inverse problem?

– I.e., how to map an Ethernet address to an IP

address?

• Older protocols: RARP (RFC 903) and

BOOTP (RFC 951).

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DHCP

• Dynamic Host Configuration Protocol. • RFCs 2131 and 2132.

• Assigns IP addresses to hosts dynamically. • DHCP server may not be on the same LAN

as requesting host.

• DHCP relay agent.

DHCP Operation

• Newly booted host broadcasts a DHCP

DISCOVER message.

• DHCP relay agent intercepts DHCP

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DHCP Operation

DHCP: Address Reuse

• How long should an IP address be allocated? • Issue: hosts come and go.

• IP addresses may be assigned on a “Lease”

basis.