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Where we are in the big picture. IP. IP. IP. Ethernet interface. Ethernet interface. Ethernet interface. host. host. HTTP message. HTTP. HTTP. TCP segment. TCP. TCP. router. router. IP packet. IP packet. IP packet. IP. Ethernet interface. SONET interface. SONET

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Where we are in the big picture
Where we are in the big picture

IP

IP

IP

Ethernet

interface

Ethernet

interface

Ethernet

interface

host

host

HTTP message

HTTP

HTTP

TCP segment

TCP

TCP

router

router

IP packet

IP packet

IP packet

IP

Ethernet

interface

SONET

interface

SONET

interface

CS118/Spring05


Broadcast routing
Broadcast Routing

R2

R3

R4

R2

R3

R4

R1

R1

  • Goal: Deliver packets from a source to all other nodes

  • Why letting source send to everyone not a good solution

    • the source may not know all the destination addresses

    • Redundant transmission over links near the source

replicate

replicate

in-network

replication

sourcereplication

CS118/Spring05


In network replication
In-network replication

  • Flooding: when a node receives a broadcast packet, sends a copy to all neighbors

    • Problems: packet looping

  • Controlled flooding: node broadcast a packet if it hasn’t seen the same packet before

    • Node must keep track of packet ids already seen

  • Reverse Path Forwarding (RPF): a node N forwards packet if it arrived on shortest path between N and source

    • Make use of forwarding table

      of unicast routing

source

R1

R4

R2

R5

R6

R3

R7

CS118/Spring05


Spanning tree
Spanning Tree

A

D

A

D

G

G

B

B

F

E

E

F

c

c

  • First construct a spanning tree

  • Nodes forward copies only along spanning tree

    • No redundant packets received by any node

  • All sources send data along the same tree

(b) Broadcast initiated at D

(a) Broadcast initiated at A

CS118/Spring05


Creating a center based spanning tree
Creating a center-based Spanning Tree

A

A

D

D

G

G

B

B

E

F

F

c

c

E

  • First pick a center node (E in this example)

  • Each node sends unicast join message towards the center node

    • Message forwarded until it arrives at a node already belonging to spanning tree

  • Stepwise construction of spanning tree

(b) Constructed spanning tree

CS118/Spring05


Multicast routing
Multicast Routing

Goal: build a tree to reach allmembers in a mcast group

  • shared-tree: same tree used by all group members

    • minimal spanning tree (Steiner)

    • center-based tree

  • source-based tree: one tree from each sender to receivers

    • Each tree made of shortest paths to all mcast members

    • 2 ways to build: link-state (MOSPF), RPF (DVMRP)

Shared tree

Source-based trees

CS118/Spring05


Shared tree steiner tree
Shared-Tree: Steiner Tree

  • Steiner Tree: minimum cost tree connecting all routers with attached group members

  • problem is NP-complete

  • excellent heuristics exists

  • not used in practice:

    • computational complexity

    • Infeasible to get/keep information about entire network

    • monolithic: rerun whenever a router needs to join/leave

CS118/Spring05


Center based shared routing tree
Center-based shared routing tree

  • one router identified as “center” of tree

  • routerwith mcast member attached sends unicast join-msg addressed to the center router

    • join-msg processed by intermediate routers before being forwarded towards center

    • join-msg either hits existing tree branch for this center, or arrives at center

    • path taken by join-msg becomes new branch of tree

R1

R4

R2

R5

R6

R7

R3

CS118/Spring05


Building shortest path tree mospf
Building Shortest Path Tree: MOSPF

  • OSPF: a link-state routing protocol

    • Each node maintains a complete network graph

    • Use Dijkstra algorithm to compute the shortest path

  • MOSPF (Multicast OSPF)

    • Carry multicast membership in link-state packet

    • Router computes shortest path tree for each source

S: source

R1

R4

LEGEND

router with attached

group member

R2

R5

router with no attached

group member

R3

R7

R6

CS118/Spring05


Reverse path forwarding example
Reverse Path Forwarding: example

S: source

LEGEND

R1

R4

router with attached

group member

R2

router with no attached

group member

R5

datagram will be forwarded

R3

R7

R6

datagram will not be

forwarded

  • result is a source-specific reverse shortest path broadcast tree

    • may not be a good tree if link cost is asymmetric

    • It's a broadcast tree, reaching every node

CS118/Spring05


Trim broadcast tree to mcast tree by pruning
Trim Broadcast Tree to Mcast Tree by Pruning

  • no need to forward packets down branches which has no mcast group members

  • router with no downstream group members sends “prune” message upstream

    • Routers keep state regarding prune msgs

LEGEND

S: source

R1

router with attached

group member

R4

router with no attached

group member

R2

P

P

R5

prune message

P

R3

links with multicast

forwarding

R7

R6

CS118/Spring05


Distance vector multicast routing protocol dvmrp specified in rfc1075
Distance-Vector Multicast Routing Protocol(DVMRP, specified in RFC1075)

  • DVMRP routers run RIP to build routing table

  • Use reverse path forwarding to build source-based tree

    • initial datagram to mcast group floods everywhere via RPF

    • Edge routers with no members send prune msg upstream

  • soft state: router periodically delete prune state

    • Sender may have finished by then

    • If not, downstream router prune again

  • routers can quickly graft to tree

    • By canceling the prune state

R1

R4

R2

P

R5

P

R3

R7

R6

CS118/Spring05


Pim protocol independent multicast
PIM: Protocol Independent Multicast

  • independent from underlying unicast routing algorithm

    • Either get "next hop" information for each node from the unicast forwarding table, or

    • Use unicast routing to forward mcast join message

  • two different multicast distribution scenarios:

    • Dense: group members densely packed, in “close” proximity

    • Sparse: # networks with group members small wrt to the total # of interconnected networks, group members widely dispersed

CS118/Spring05


Consequences of sparse dense dichotomy
Consequences of Sparse-Dense Dichotomy:

Dense

group membership by routers assumed until routers explicitly prune

data-driven construction on mcast tree (e.g., RPF)

Sparse

no membership until routers explicitly join

receiver- driven construction of mcast tree (e.g., center-based)

Implementation:

  • flood-and-prune RPF

  • similar to DVMRP but using info from underlying unicast protocol for RPF checking

CS118/Spring05


Pim sparse mode
PIM - Sparse Mode

Build center-based shared tree

router sends join msg to rendezvous point (RP)

intermediate routers update state and forward join msgs

Data sources:

unicast packets to RP, which forwards down RP-rooted tree

RP can send stop msg to source if no receivers joined the group

“no one is listening!”

R1

R4

join

R2

join

R5

join

R3

R7

R6

all data multicast

from rendezvous

point

rendezvous

point

CS118/Spring05


Components of the ip multicast architecture
Components of the IP Multicast Architecture

multicast routing protocols(MOSPF, DVMRP, PIM)

hosts

  • IGMP operates between Router and local Hosts on the same network (e.g., Ethernet)

    • Router queries local Hosts for mcast group membership info

    • Hosts respond with membership reports

host-to-router protocolInternet Group Management Protocol

routers

CS118/Spring05


1

1

1

0

group ID

IP Multicast Address

131.179.26.38

18.4.157.100

Class D IP addresses:

in “dotted decimal” notation: 224.0.0.0 — 239.255.255.255

Two administrative categories:

  • well-known multicast addresses, assigned by IANA

  • (the rest) transient addresses, assigned & reclaimed dynamically

CS118/Spring05


How igmp works i
How IGMP Works (I)

routers:

Q

hosts:

  • One router is elected the “querier” on each local/physical network

  • querierperiodically sends Membership Query message to “all-systems group” (224.0.0.1) with TTL=1

CS118/Spring05


How igmp works ii
How IGMP Works (II)

  • On receipt, a host starts a random timer [0–10 sec] for each multicast group it wants to join

  • when a host’s timer for group G expires, it sends a Membership Report to group G (TTL = 1)

    • other members of G hear the report, stop their timers

    • routers hear all reports

  • Normal case: only one report message per group is sent in response to a query

  • when a host first joins a group, it can send unsolicited reports immediately

Q

G

G

G

G

CS118/Spring05


How igmp works leaving a multicast group
How IGMP Works: Leaving a Multicast Group

  • host sends a Leave Group msg to group address Gif it was the most recent host to report membership in that group

  • Upon receiving Leave Group msg: query router sends a few queries to group G with a small max-response-time

    • if no Membership Report heard, stop data forwarding

Q

G

G

G

G

CS118/Spring05


Igmp message types
IGMP message types

IGMP Msg type Sent by Purpose

membership query: general router

membership query: specific router

membership report host

leave group host

query for current active multicast groups

query for specific m-cast group

host wants to join group

host leaves the group

IP header

CS118/Spring05


Chapter 5 the data link layer overview
Chapter 5: the Data Link Layer: overview

M

H

H

H

H

H

H

H

H

H

l

n

l

n

t

n

t

t

t

M

M

M

  • data delivery between two physically connected devices

  • implementation of various link layer technologies:

    • Ethernet, hubs, bridges, IEEE 802.11 LANs, PPP

  • Address mapping between LAN and IP: ARP

application

transport

network

link

physical

data link

protocol

network

link

physical

M

phys. link

frame

adapter card

CS118/Spring05


Link layer services
Link Layer Services

  • sending data over a physical link

    • bit encoding: transmitting sequence of 1’s and 0’s by signals

    • Framing: defining the beginning & end of a data chunk

    • bit error detection

    • reliable transmission

  • MAC (Medium Access Control) addresses to identify source, destination nodes

    • Different address, not IP address!

  • Channel access if shared medium

    • e.g, Ethernet, wireless, etc.

  • Flow Control: pacing between sender and receivers

  • implemented in “adapter” (e.g., PCMCIA card, Ethernet card)

  • Link type: Half-duplex vs. full-duplex

CS118/Spring05


Data framing
Data Framing

  • Terminology: for a block of data

    • at link layer: normally called a data frame

    • at network layer: a packet

    • at transport level: TCP segment; UDP  datagram

  • A frame/packet has a header field

    • optionally there may be a trailer field too

  • Byte-Oriented Framing Protocol: delineate frame with a special bit sequence: 01111110

    Q: What if the bit sequence 01111110 occurs in data stream?

data

CS118/Spring05


Byte stuffing
Byte stuffing

  • Sender: adds (“stuffs”) extra < 01111110> byte after each apperance of < 01111110>

  • Receiver:

    • single 01111110: flag byte

    • two back-to-back 01111110 bytes: discard first byte, continue data reception

  • stuffing changes the total length of data to be sent

CS118/Spring05


Error detection
Error Detection

  • EDC= Error Detection and Correction bits (redundancy)

  • D = Data protected by error checking

  • Error detection not 100% reliable!

    • protocol may miss some errors, though rarely

    • larger EDC field gives better detection and correction

CS118/Spring05


Parity checking
Parity Checking

Two Dimensional Bit Parity:

Detect and correct single bit errors

Single Bit Parity:

Detect single bit errors

  • consider a data frame as

  • a m  n matrix

    • A parity bit for each row

    •  n-bit checksum,

    • and

    • A parity bit for each column

    •  m-bit checksum

CS118/Spring05


C yclic r edundancy c heck crc
Cyclic Redundancy Check (CRC)

  • consider a data frame as a bit sequence M(x)

    • e.g. 10011010  M(x) = x7 + x4 + x3 + x1

    • k-term polynomial has a degree of (k-1)

      • e.g. 10011010 represents a 8-term polynomial

  • use a (r+1)-bit generator polynomial G(x) to compute the checksum for M(x)

    • sender makes the transmitted bit sequence dividable by G(x) by appending r-bit remainder to M(x)

    • receiver divides the received sequence by G(x)

M

M

CS118/Spring05


How to compute crc
How to compute CRC

1. append r zero bits to M(x) to get xrM(x)

M(x) = 10011010 = x7 + x4 + x3 + x1, G(x) = 1101=x3 + x2 + 1, r = 3

then xrM(x) = x10 + x7 + x6 + x4 10011010000

2. divide xrM(x) by G(x)

  • 10011010000 / 1101 = 11111001, remainder 101

    3. subtract the remainder from xrM(x)  T(x), check-summed frame to be transmitted

    xrM(x) / G(x) = Q(x), remainder R(x),T(x) = xrM(x) - R(x)

  • In reality: just append R(x) to the end of data frame M(x) : 10011010101

    Because: xrM(x) = shifting M(x) to the left by r bits, and

    for XOR: r bits of 0's – R(x) = R(x)

    At receiving end: if the received frame P(x) divisible by G(x), P(x)/G(x) = 0, P(X) considered error free

CS118/Spring05


An example crc computation
An example CRC computation

11111001

11111001

1101 10011010000

1101 10011010101

1101

1001

1101

1000

1101

1011

1101

1100

1101

1000

1101

101

1101

1001

1101

1000

1101

1011

1101

1100

1101

1101

1101

0

remainder

computation by sender computation by receiver

CS118/Spring05


Multiple access protocols
Multiple Access protocols

  • The problem: single shared communication channel, only one node can send successfully at a time

  • 3 broad classes:

    • Channel Partitioning

      • divide channel into smaller “pieces” (time slots, frequency band, code modulation)

      • allocate piece to node for exclusive use

    • “Taking turns”

      • coordinate shared access to avoid collision

    • Random Access

      • Detect and resolve collisions

CS118/Spring05


Channel partitioning tdma and fdma
Channel Partitioning: TDMA and FDMA

time

frequency bands

  • TDMA: Time Division Multiple Access

    • channel divided into N fixed length time slots, one per station

    • each station takes turns to access channel

    • unused slots go idle

    • example: 6-station LAN, 1,3,4 send data, slots 2,5,6 idle

  • FDMA

    • channel spectrum divided into

      frequency bands

    • each station assigned fixed

      frequency band

    • unused frequency bands go idle

    • example: 6-station LAN, 1, 3, 4

      send data, frequency bands 2,5,6 idle

CS118/Spring05


Taking turns mac protocols
“Taking Turns” MAC protocols

  • channel partitioning: commonalities

    • Share channel efficiently with constant, uniform load

    • But inefficient with random, non-uniform load

      • delay in channel access

      • 1/N bandwidth allocated even if only 1 active node!

  • “taking turns” protocols: on-demand channel allocation

    • Polling: master node asks slave nodes to transmit in turn

      • Concerns: polling latency, single point of failure

    • Token passing

CS118/Spring05


Taking turns by token passing
“Taking Turns” by Token Passing

  • One token message passed from one node to next sequentially

  • whoever gets the token can send one data frame, then passes token to next node

  • A master station generates the token and monitors its circulation

  • concerns:

    • token overhead

    • latency

    • single point of failure (token)

CS118/Spring05


Random access protocols
Random Access protocols

  • When node has packet to send

    • transmit at full channel data rate R.

    • no a priori coordination among nodes

  • If two or more nodes transmitting at the same time “collision”

  • random access MAC protocol specifies:

    • how to detect collisions

    • how to recover from collisions

  • Examples of random access MAC protocols:

    • ALOHA

    • slotted ALOHA

    • CSMA and CSMA/CD

CS118/Spring05


Aloha
ALOHA

  • If a station has data to send: just send

  • collision probability:

    • frame sent at t0 collide with other packets sent in [t0-1, t0+1]

P(success by given node) = P(node transmits) .

P(no other node transmits in [t0-1,t0] .

P(no other node transmits in [t0,t0+1]

= p . (1-p)N-1 . (1-p)N-1

P(success by any node) = N p . (1-p) 2(N-1)choosing optimum p as n ∞ ...

= 1/(2e) = 0.18

CS118/Spring05


Slotted aloha
Slotted Aloha

Assumptions:

  • All frames the same size; clocks in all nodes are synchronized

  • Divide time into equal size slots (= pkt trans. time)

  • If 2 or more nodes transmit in the same slot, all nodes detect collision

CS118/Spring05


Slotted aloha1
Slotted Aloha

Operations:

  • When a node gets data to send, it transmits at beginning of next slot

  • If no collision, node can send new frame in next slot

  • If collision: node retransmits frame in each subsequent slots with probability p, until successful.

Success (S), Collision (C), Empty (E) slots

CS118/Spring05


Slotted aloha efficiency
Slotted Aloha efficiency

At best: channel

use for useful

transmissions 37%

of time!

Q: what is the max fraction of slots successful?

  • each node transmits in a slot with probability p

  • prob. successful transmission S is

    by a given node: S= p (1-p)(N-1)

    by any of N nodes

    S = Prob (only one transmits) = N p (1-p)(N-1)

    … choosing optimum p as n ∞ ...

    = 1/e = 0.37

0.4

S = throughput = “goodput”

(success rate)

0.3

Slotted Aloha

0.2

0.1

Pure Aloha

G = offered load = N*p

1.5

2.0

0.5

1.0

CS118/Spring05


Csma carrier sense multiple access
CSMA: Carrier Sense Multiple Access

listen before transmit

If channel sensed idle: transmit

If channel sensed busy, wait

p-persistent CSMA: when channel becomes idle, retry immediately with probability p

Non-persistent CSMA: retry

after a random interval

collisions still possible:

propagation delay two or more nodes may send simultaneously

Chance of collision goes up with distance between nodes

To cut the loss early: CSMA/CD

CS118/Spring05


Csma cd c ollision d etection
CSMA/CD (Collision Detection)

  • Collision Detection:compare transmitted with received signals

  • Abort collided transmissions

CS118/Spring05


Classification of different multiaccess protocols
Classification ofdifferent multiaccess protocols

controlled access

random access

ALOHA,

slotted

ALOHA

Static

allocation

adaptive

to demand

CSMA/CD

polling

token

passing

TDMA

FDMA

CDMA

Multiple data sources sharing

the same communication link

(Ethernet, FDDI, Appletalk, etc)

CS118/Spring05


Lan addresses and arp
LAN Addresses and ARP

32-bit IP address:network-layer address

  • used to get IP packet to destination host

    LAN (or MAC or physical) address:

  • used to get frame from one interface to another physically connected interface (same network)

  • 48 bit MAC address (for most LANs)

    burned in the adapter ROM

    • Each adapter on LAN has

      a unique LAN address

    • Broadcast address:

      FF-FF-FF-FF-FF-FF

CS118/Spring05


Lan address more
LAN Address (more)

  • MAC address allocation administered by IEEE

  • manufacturer buys portion of MAC address space (to assure uniqueness)

  • Analogy:

    (a) MAC address: like Social Security Number

    (b) IP address: like postal address

  • MAC flat address => portability

    • can move LAN card from one LAN to another

  • IP hierarchical address NOT portable

    • depends on network to which one attaches

CS118/Spring05


Recall earlier routing discussion
Recall earlier routing discussion

223.1.1.1

223.1.2.1

E

B

A

223.1.1.2

223.1.2.9

223.1.1.4

223.1.2.2

223.1.3.27

223.1.1.3

223.1.3.2

223.1.3.1

  • Node A sends IP packet to B:

  • look up network address of B, find B is on the same net as A

  • link layer send datagram to B inside link-layer frame

frame source,

dest address

datagram source,

dest address

A’s IP

addr

B’s IP

addr

B’s MAC

addr

A’s MAC

addr

IP payload

datagram

frame

CS118/Spring05


Arp address resolution protocol
ARP: Address Resolution Protocol

Question: how to determine

MAC address of B

given B’s IP address?

  • Each IP node (Host, Router) on LAN has ARP table

  • ARP Table: IP/MAC address mappings for some LAN nodes

    < IP address; MAC address; TTL>

    • TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

237.196.7.78

1A-2F-BB-76-09-AD

237.196.7.23

237.196.7.14

LAN

71-65-F7-2B-08-53

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

237.196.7.88

CS118/Spring05


Arp protocol
ARP protocol

  • A knows B's IP address, wants to learn B's MAC address

  • A broadcasts ARP query pkt, containing B's IP address

    • Dest MAC address = FF-FF-FF-FF-FF-FF

    • all machines on LAN receive ARP query

  • B receives ARP packet, replies to A with its (B's) physical layer address

    • reply sent to A’s MAC address (unicast)

  • A caches (saves) IP-to-physical address pairs until information becomes old (times out)

    • soft state: information that times out (goes away) unless refreshed

  • ARP is “plug-and-play”:

    • nodes create their ARP tables without intervention from net administrator

CS118/Spring05


Routing to another lan
Routing to another LAN

  • A creates IP packet with source A, destination B

  • A uses ARP to get R’s physical layer address for 111.111.111.110

  • A creates Ethernet frame with R's MAC address as destination

  • A’s data link layer sends the Ethernet frame

  • R’s data link layer receives Ethernet frame

  • R extracts IP datagram from Ethernet frame, sees it’s destined to B

  • R uses ARP to get B’s physical layer address

  • R creates frame containing A-to-B IP packet, sends to B

A

R

B

CS118/Spring05


Ethernet
Ethernet

  • first widely used LAN technology, dominant LAN today

  • Kept up with speed race: 10 Mbps – 10 Gbps

  • Bus topology popular through mid 90s

  • Now star topology prevails

  • Connection choices: hub or switch (more later)

hub or

switch

CS118/Spring05


Schedule for next week
Schedule for next week

  • Makeup lecture: Monday 6/6

    • 8:00-9:50AM, 5422BH

    • 6:00-7:50PM, 5419BH

  • Tuesday 6/7: regular class hours

  • Wednesday 6/8: office hour cancelled

  • Thursday 6/9: no class

  • Saturday 6/11:

    • Office hour: 10:00AM - 1:00PM

    • Final exam: 3:00-6:00PM, 2444BH

CS118/Spring05


Ethernet frame structure
Ethernet Frame Structure

  • Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

  • Preamble:

    • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011

    • used to synchronize receiver, sender clock rates

  • Addresses: 6 bytes

  • Type: indicates the higher layer protocol

    • IEEE802.3 changed the “type” field to “length”, defined a separate type field carried in the data part

  • CRC: checked at receiver, if error, drop the frame

46 – 1500 bytes

CS118/Spring05


Ethernet frame length limit
Ethernet Frame length limit

  • All packets/frames have an upper bound on length

  • Ethernet has a lower bound for reliable collision detection:

    • cable max length 2500m 

  • transceiver can send & listen at the same time

    • If the received data stream differs from the one transmitted  collision

    • to detect collision: the sender must still be transmitting when garbled signal propagates back

    • minimum frame = 64bytes = 512bits, take 51 to transmit at 10Mbps

A

B

CS118/Spring05


Ethernet mac protocol csma cd
Ethernet MAC protocol: CSMA/CD

A: sense channel, wait if necessary until it is idle

transmit and monitor the channel;

If detect another transmission

then {

abort and send jam signal;

update # collisions (n++);

delay for K x 512bits trans time

goto A

}

else {done with the frame; set #collisions to zero (n = 0)}

Jam Signal: make sure all other transmitters are aware of collision; 48 bits

Exponential Backoff algorithm:

  • first collision (n=1): choose K from {0, 1}

  • after second collision (n =2): choose K from {0, 1, 2, 3}…

  • after 10 collisions (n=10), choose K from {0,1,2,3,4,…,1023}

CS118/Spring05


Csma cd efficiency
CSMA/CD efficiency

  • Tprop = max. propagation delay between 2 nodes in LAN

  • Ttrans = time to transmit a max-size frame

  • Efficiency goes to 1

    • as Tprop goes to 0

    • as Ttrans goes to infinity

  • Much better than ALOHA, but still decentralized, simple, and cheap

    • Best effort data delivery

CS118/Spring05


Dhcp client server scenario
DHCP client-server scenario

arriving new host

DHCP discover

src : 0.0.0.0, 68

dest.: 255.255.255.255,67

yiaddr: 0.0.0.0

transaction ID: 654

DHCP offer

src: 223.1.2.5, 67

dest: 255.255.255.255, 68

yiaddrr: 223.1.2.4

transaction ID: 654

Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68

dest:: 255.255.255.255, 67

yiaddrr: 223.1.2.4

transaction ID: 655

Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67

dest: 255.255.255.255, 68

yiaddrr: 223.1.2.4

transaction ID: 655

Lifetime: 3600 secs

223.1.2.1

DHCP server 223.1.2.5

223.1.1.1

223.1.1.2

223.1.2.9

223.1.1.4

223.1.2.2

arriving

client

223.1.1.3

223.1.3.27

DHCP server: 223.1.2.5

223.1.3.2

223.1.3.1

time

CS118/Spring05


Dhcp relay ip tunneling

Either have a DHCP server on each network, or a DHCP-relay to forward request to server

DHCP Relay & IP Tunneling

Unicast to server

broadcast

Source=A1

Dest. =A2

  • IP tunnel:

broadcast

Put another IP header in

front of the original packets,

with Source=tunnel entry, Destination=tunnel exit

host

Other

networks

DHCP

server

DHCP

relay

A1

A2

Broadcast

CS118/Spring05


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