atm interworking

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Copyright 1999 by Marko VuskovicC

M. Vuskovic ATM Networking CS596

Chapter 3

ATM INTERNETWORKING

Table of contents:

3.1 OVERVIEW3.1.1 IP over ATM

3.1.2 Logical IP Subnetworks (LIS)

3.1.3 Subnet Masks

3.1.4 IP Address Resolution

3.1.5 Example of LIS Connected to ATM

3.2 CLASSICAL IP OVER ATM (CLIP)

3.2.1 RFC 1577

3.2.2 ATMARP Server

3.2.3 Registration

3.2.4 Inverse ARP (InARP)

3.2.5 ARP Requests

3.2.6 Connection and Data Transfer

3.2.7 Interconnecting LIS

3.3 LAN EMULATION (LANE)

3.3.1 LANE Protocol Architecture

3.3.2 Components of LANE

3.3.3 Connections Between LANE Components

3.3.4 LANE Configuration

3.3.5 LEC Initialization and Configuration

3.3.6 LEC Registration

3.3.7 Data Transfer3.3.8

3.4 MULTI PROTOCOL OVER ATM (MPOA)

3.4.1 Next Hop Resolution Protocol (NHRP)

3.4.2 MPOA Components

3.4.3 MPOA Operation

3.4.4 Examples of MPOA Scenarios


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IP Over ATM

TCP and IP are prevalent protocols, hence there are large number of "legacy" appli-

cations which run on top of TCP/IP. Therefore it is important to support IP over

ATM. ATM Forum has specified several protocols which enable IP over ATM, andhas helped ATM to be successful.

The IP/ATM networking is relatively complex due to two reasons:

ATM is connection-oriented, IP is connection-less

ATM supports QoS, IP does not

There are two fundamental models for IP over ATM:

peer model

overlay model

In peer model both IP and ATM layers are considered as peer networking layers

which operate in the same address space (i.e. use the same addressing scheme

used in IP). This would simplify addressing but would significantly complicate the

ATM switch design because the ATM switches would have to extend all the func-

tionality of IP routers. This would be even more complex if ATM were to support

other protocols such as IPX and Appletalk.

In overlay model (which prevailed) IP runs on top of ATM and both operate in their

own unrelated address spaces which does not allow simple mapping. Consequently,

the end systems will have two addresses: an IP address and an ATM address.

With overlay model, there are three approaches for IP over ATM:

Classical IP over ATM (CLIP)

LAN emulation (LANE)

Multiprotocol over ATM (MPOA)

CLIP is simpler, it treats ATM as a LAN: it resolves IP addresses into ATM ad-dresses by using address resolution protocol which is essentially similar to IP ARP,

only the MAC addresses are replaced by ATM addresses. The end stations which

belong to the same logical subnet of IP communicate over VCCs thus treating the

entire ATM network as a LAN. The end stations which belong to different IP sub-

nets have to communicate through IP routers, even though they are attached to the

same ATM network.

OVERVIEW


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LANE uses ATM network to simulate a LAN (specifically Ethernet or token ring): all commu-

nications which are involved in LAN like IP address to MAC address resolution and broad-

casting of ARP requests, ARP replies and data are carried over ATM VCCs. The endstations which belong to different IP subnets still have to communicate through IP routers.

MPOA eliminates the inefficiency of CLIP and LANE in the case of interconnecting different

IP subnets. This is done by combining LANE with the Next Hop Resolution Protocol (NHRP)

in which a direct VCC is established between two end stations even though they belong to

different IP subnets.

Before moving to the discussion of these approaches, the rest of this section reviews some

basic facts about IP subnetting.

OVERVIEW (Cont.)


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OVERVIEW (Cont.)

Logical IP Subnetworks (LIS)

The concept of IP subnetting is very important part of the internetworking protocols dis-

cussed in this chapter.

Subnetting was introduced in IP to handle the complexity of large networks. An alternative

name for IP subnetting is "summarization of host addresses". Without subnetting, routers

and hosts would have to perform address resolution over large address space, which

would require an enormous amount of memory in each router and host, and an enormous

network management effort.

130.191.0.0

R0

Router and hosts need to resolve

216

-2 = 65,534 possible addresses

which would require emormous size

of ARP tables (cache).

Route table of router R0

must

have only 28

= 256 entries for

subnet routers R1

,R2.... and x

entries for external routers,

130.191.1.0

130.191.2.0

130.191.0.0

R1

R2

Subnet 1

Subnet 2

Route table of routers Ri

and

hostsin each subnet need maximal

size of ARP tables of only28-2 =

254 entries.

Class B network

Local subnetworks

Reasons for subnetting:

- Smaller ARP caches and easier network management

- Less traffic (traffic is localized to subnets)

- Different subnets can use different (incompatible) network technologies

- Security (it is convenient to have different security attributes for different subnets)

R0


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OVERVIEW (Cont.)

Subnet Masks

Splitting networks into subnetworks depends on the network class. There are three

major network classes which IP uses (other classes, D and E, also exist but they are irrele-

vant for this discussion). The difference between the three classes is in the size of the net-work part versus the host part ("n" are bits of the network part, "h" are bits of the host

address part):

Class A:

0nnnnnnn.hhhhhhhh.hhhhhhhh.hhhhhhhh (1.0.0.0 - 127.255.255.255)

Class B:

10nnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh (128.0.0.0 - 191.255.255.255)

Class C:

110nnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh (192.0.0.0 - 223.255.255.255)

NOTICE: In definitions above is used term "host". More precise term would be "host's net-

work interface". A single host can have several network interface cards (NICs).

Each of the NICs can have an IP address. For the simplicity we will continue to

use term host.

Some IP addresses are not the addresses of the particular hosts. Examples:

68.0.0.0 - address of an A class network

130.191.0.0 - address of a B class network

200.123.224.0 - address of a C class network

68.255.255 - broadcast address of an A class network

130.191.255 - broadcast address of a B class network

200.123.224.255 - broadcast address of a C class network

Some IP addresses are used for private networks (which are not connected to the Internet).

The private addresses are in the following ranges:

10.0.0.0 - 10.255.355.255 (Class A)

172.16.0.0 - 172.31.255.255 (Class B)

192.168.0.0 - 192.168.255.255 (Class C)

A network mask is a 32-bit pattern which shows which bits of the IP address belong to the

network part (1s) and which bits belong to the host part (0s). For example, network masks

for the three classes above are: 255.0.0.0, 255.255.0.0, and 255.255.255.0 respectively.


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OVERVIEW (Cont.)

The corresponding mask is 255.255.252.0

10000010.10111111.sssssshh.hhhhhhh

191 subnets hosts130

11111111.11111111.11111100.00000000

255 252 0

subnet hosts

255

11111111.11111111.00011101.00000010

191 29 2

subnet 7 host 258

130

This would allow 26-2 = 62 subnets and 210-2 = 1022 hosts in each subnet.

For example, the IP address of the host number 258 in subnet 7 would be:

130.191.29.2. Proof:

A network can be split into subnetworks by using a portion of the host bits as a subnet-

work number. One way of forming subnets in class B networks is to use only last 8 bits

for hosts, while the next byte to the left is used for subnet number. For example, the net-

work 130.191.0.0 can be split into the following subnetworks 130.191.1.0, 130.191.2.0,......,130.191.254, which gives 28-2 subnets. (addresses 130.191.x.0 and 130.191.x.255 are

reserved for the subnet address and the subnet broadcast address respectively.) Subnet

masks would be 255.255.255.0.

Subnets are transparent to other networks, i.e. they have only local significance. For ex-

ample, network 130.192.0.0 is not aware of the subnetworks 130.191.xxx.0

It is not mandatory to create subnets at the byte boundary. For example, a subnet can be

defined by using only 6 leftmost bits of the last two bytes:


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OVERVIEW (Cont.)

Each subnet must have a router, which will forward trafic to other subnets. If a source

wants to send a frame to a destination which doesn't belong to the same subnet, it must be

addressed to the router for that subnet. If the destination belongs to the same subnet assource, it can go directly to that address.

How can one know if an IP address belongs to the same subnet? Answer: by AND-ing the

IP addresses by their subnet mask, then by comparing the result.

Example:

Given are IP addresses: 192.168.48.2 and 192.168.49.102. Suppose the subnet

mask 255.255.252.0. Do these IP addresses belong to the same subnet?

11111111.11111111.11111100.00000000

11000000.10101000.00110000.00000010

11000000.10101000.00110001.01100110

168

168

48

49

2

102

subnet 12

subnet 12

host 2

host 358

192

255 255 252

192

Since both subnet numbers (bit patterns after AND-ing with the subnet mask)

are the same, the IP numbers belong to the same subnet.


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3-8


OVERVIEW (Cont.)

IP Address Resolution

Although all NICs are assigned IP addresses, they communicate only through MAC num-

bers. IP addresses are needed for applications which are above the layer 3. The NICs

however communicate at layer 2 and use MAC addresses. In other words, a frame sent to

a destination must contain the MAC address of that destination. Therefore an address res-olution is needed. Address resolution is performed by hosts that are using the Address

Resolution Protocol (ARP), which is part of the layer 3 protocol. For example, if host A

wants to send a frame to host B (see figure below), then the ARP of A will perform map-

ping IP.B!MAC.B. The following events happen when A wants to send a frame to B:

(1) A determines if B belongs to the same subnet (ANDs bitwise the IP addresses with

the subnet mask of A, then compares them)

(2) If A has the MAC address of B in its cache, it performs the address resolution by

finding the entry [IP.B, MAC.B] in ARP table, then sends the frame by using the

MAC address MAC.B.

(3) If A can't find the entry [IP.B, MAC.B] in its ARP table, it broadcasts an

ARP_REQUEST packet (which contains: IP.A, IP.B, MAC.A) to all hosts in subnet.

All hosts cache the info [IP.A, MAC.A] for their own use, but only B answers with an

ARP_RESPONSE packet which has IP.B, MAC.B. By now, A has the MAC address

of B, and can send the frame.

If A wants to send a frame to E (which is on different subnet than A), the following hap-

pens:

(1) A finds that E doesn't belong to the same subnet.

(2) A sends the frame to the router R1

(the default router for the subnet 1). The router

for wards the frame to the network, which will eventually route the frame to the router

R2

, which in turn delivers the frame to the end station E, by using its own ARP to

obtain the MAC address of E.

.

A B C D E F

IP.A

MAC.A

IP.R1

MAC.R1

IP.R1

MAC.R1

IP.YIP.X

IP.D

MAC.D

IP.B

MAC.BIP.E

MAC.E

IP.C

MAC.CIP.F

MAC.F

IP R2

Subnet 1 Subnet 2

R1

1

1

2

2

3


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OVERVIEW (Cont.)

Example of LIS Connected to ATM

Subnets can be directly connected to an ATM network, or indirectly through bridges,

LAN switches, routers and/or gateways.

.

COMMENTS:

End stations A, B, C, D, E, F, G, H and J are directly connected to the ATM network

and they must have the ATM NICs (they are called "ATM hosts").

Bridge and router are also directly connected to the ATM network and have ATM NICs

(they are called "ATM edge devices).

End stations K, L, M, P, Q, R, S, T and U are connected indirectly to the ATM network

and are ATM unaware, i.e. they don't have ATM NICs, instead they have Ethernet or

Token Ring NICs (they are called "legacy devices").

LAN Hosts

Switch 1

A B C

LIS1

P Q R

LIS5

S T U

LIS6

LIS2

Switch 2

LIS3

K L M

Bridge

LIS4

ATM Hosts

ATM Edge

Devices

Router

D E F

J

G

H


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RFC 1577

Classical IP over ATM (CLIP) is the simplest ATM internetworking protocol. Its speci-fications are originally given in RFC1577 (1994), later superseded by RFC2225

(1998). The protocol is designed to enable interworking between legacy IP applica-

tions distributed across an ATM network. The applications that originally worked on

Ethernet or token ring are not aware of ATM and do not need any modification.

Since the application knows only the source's IP address, the RFC1577 needs to

provide an ATM address resolution, i.e. the mapping

IP " AESAso that UNI can make the call setup for the destination end station B. This address

resolution is done by an ATMARP server. (NOTE: the traditional ARP, which

mapped IP " MAC, did not require an ARP server. The protocol was invoked as a

passive system component, part of the layer 3.)

CLASSICAL IP OVER ATM

PHY

ATM

ALL5

Host A

UNI

RFC1577

TCP/IP

Applica-

tion

PHY

ATM

ATM Switch

PHY

ATM

ATM Switch

PHY

ATM

ALL5

Host B

UNI

RFC1577

TCP/IP

Applica-

tion

ATM

ATM

NIC

IP Host A IP Host B

IP.A

AESA.AMAC.A

IP.B

AESA.BMAC.B

PVC

SVC

ATM

NIC


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ATMARP Server

Why ATMARP needs a server? The traditional ARP could broadcast the ARP re-

quests to all (unknown) hosts at the Ethernet or token ring segment and wait for an

ARP response. ATM is however a point-to-point communication through VCCs and in

order to broadcast an ATMARP request a list of all hosts attached to the ATM net-

work must be known and maintained. These hosts therefore must register, so that

their AESA is known and can be used for broadcast. The registration of ad-hoc hosts

can only be done by an active system component, which is a server.

An ATMARP server can run on ATM switch, or on any host attached to ATM switch.

Each LIS of hosts attached to the ATM network must have its own ATMARP server.

The role of the ATMARP server involves two stages:

registration

address resolution

Once the AESA of the destination is known, the source station can start the call set-

up, followed by data transfer.

CLASSICAL IP OVER ATM (Cont.)


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Registration

In order to be able to send an ARP request, an end station attached to an ATM net-

work must register first with ATMARP server. After registration, the end station be-

comes an ATMARP client.

The AESA of the ATMARP server must be manually configured in the NIC of each IP

station so that the station can initiate an ATM call to the server.


AESA.A

IP.A

VCC

established

by setup

call from A

VCC

established

by setup

call from B

ATM

Once the connection is made with the ATMARP server, an end station can make ARP

requests. An ATMARP server builds and maintains its ATMARP table which has the fol-

lowing entries:

[IP.A, AESA.A, ]

There are two ways an ATMARP server gets information for its table:

through inverse ARP requests

through ARP requests from clients

Time stamp is used for entries aging. The entries older than certain time period (common-

ly 20 minutes) are considered stale and are replaced by the server.

AESA.S

IP End

Station A

ATMARP

Server S

IP End

Station B

AESA.B

IP.B


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Inverse ARP (InARP)

In classical IP over ATM, the inverse ARP performs the following mapping:

AESA.X " IP.X

(NOTE: In IP world the equivalent of inverse ARP is called "reverse ARP" and itperforms mapping: MAC.X " IP.X)

The purpose of InARP is to obtain the IP addresses of all ATM hosts that have regis-

tered with the server. Since the server knows the AESA of all registered end stations,

it can broadcast an InARP request to obtain their IP addresses and to build its ARP

table.


A S B

In order to refresh its ARP table, the ATMARP server sends InARPs periodically (every

15 minutes) to all IP stations for which a VCC is still in place. The VCC will clear auto-

matically due to inactivity. In this case the ATMARP server can't send the InARP and

must wait for the station which has lost the VCC to register again.


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ARP Requests

Once the VCC is established between the end station and the ATMARP server, thestation can ask for AESA for any given IP address, provided that the address belongs

to an ATM NIC attached to the ATM network and the IP address belongs to the LIS

which is served by the ATMARP server.


S

If ATMARP server cannot resolve

the requested address it will send

this message.

ATMARP server checks first the entry

[IP.A, AESA.A, time]. If entry is not there

it will add it to its ARP table. If entry is

there it will update it, including the time

stamp.

Then, the server looks for the entry

[IP.B, AESA.B, time]. If it is there, it will

send back the ARP_REPLY. If the entry

is not there, it will broadcast the InARP

request to get the AESA from IP.B.

If this fails, it will send to A the negative

acknowledgement ATMARO_NAK.

A

or


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Connection and Data Transfer

Once an end station knows the AESA of the destination, it can place a setup call byinvoking the UNI 3.1/4.0 protocol (the call is placed through VCC VPI=0, VCI = 5, see

section "UNI Signaling").


AESA.S

The ARP VCCs can

be automatically cleared

due to inactivity (20 min)

Data VCC

ATM

PeriodicInARP

requests

AESA.A

IP.A

IP End

Station A

IP End

Station B

AESA.B

IP.BData

Data VCCs can age much faster than the ARP VCCs. This is to emulate the connection-

less paradigm. The data VCC will release if there is no packet flow within a few min-

utes.

ATMARP

Server S


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Interconnecting LIS

An ATMARP server has a scope of a single LIS. The RFC 1577 (and newer RFC

2225) require that the hosts at different subnets must communicate through routers,

even though they are connected to the same ATM network. This is one of the major

limitations of the CLIP.


If A wants to communicate with C (which belongs to different LIS), the following steps

will occur:

- A sends an ATMARP_REQUEST to S

1

to get AESA.C

- S1 returns AESA.R to A, which establishes VCC1 with R, then sends IP packet

- R routes the packet to IP.C: it sends first an ATMARP_REQUEST to S2- S2 resolves the address IP.C and returns AESA.C to R

- R establishes VCC2

with C

- R sends the IP packet to C

LIS1

A

R LIS2

C

D

IP Router

B

ATM

NIC

ATM Network

S1 S2

ATMARP

server

for LIS2

ATMARP

server

for LIS1

A

B

C

Since A and B belong to

the same LIS, a direct data

VCC can be established

between them

Since A and C belong to

different LIS, there must be two

data VCCs which are connected

through router

VCC1

VCC2

R


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With two different LISs and one router, the data packets have to go three times through

the AAL5 and IP layers (i.e. one time more than it would be normally necessary). This

is because the communication is performed hop-by-hop through a router. In case of

more LIS and more routers this overhead would be even greater, because there would

be hops between routers. This is a limitation of CLIP which is unnatural because there

is a physical connection between the hosts from different LIS, provided they are all con-

nected to the ATM network. This limitation is removed by NHRP and MPOA, discussed

in the following sections.


A C

VCC1 VCC2

VCC1

VCC2

PHY

ATM

ALL5

Router R

UNI

RFC1577

IP

PHY

ATM

ATM Switch

PHY

ATM

ATM Switch

PHY

ATM

ALL5

Host A

UNI

RFC1577

IP

Applica-tion

TCP

PHY

ATM

ALL5

Host C

UNI

RFC1577

IP

Applica-tion

TCP

Overhead due

to inter LIS

communication

R


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LAN EMULATION


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LAN EMULATION (Cont.)


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Establishment of VCCs that connect LANE components, as well as usage of the VCCs to

transfer data involves the following five phases:

1. LANE initialization

2. LEC initialization and configuration

3. LEC registration

4. Data transfer


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LANE Configuration

In order to be able to register a LEC with a LES and establish the VCCs by which the

LEC is connected to other LANE components, these components (LECS, LES and BUS)

must exist in the first place. Bringing these components into existence involvesthe following steps:

(1) Design the LANE (decide how many ELANs are going to be used, what are

their names, decide where to place LECS, LESs, and BUSs).

(2) Determine the LANE default addresses on every device which will participate

in LANE and which will be running the LANE components (ATM switches and

edge devices. For example, for CISCO products that support IOS use command

"show lane default"This command will display the default ATM addresses of

LECS, LES, BUS and LEC for each ATM interface (port). The displayed address-

es should be put down on a configuration worksheet.

(3) Enter (manually) the address of the chosen LECS into all switches and edge

devices. Save the addresses permanently.

(4) Set up manually the LECS database (configuration table). The configuration table

contains entries [, ].

(5) Enable LECS on the selected machine (where the LECS is "placed")

(6) Set up the LES/BUS. Usually the LES and BUS are collocated on the same

machine. They have to be set up for each ELAN. An ELAN can have several

LES/BUS pairs, the primary LES/BUS and the backup LES/BUS (the latter are

usually placed on different machines).

(7) Set up LECs on edge devices (routers, Ethernet switches, bridges).

1

2

3

4

5

7

6


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LEC Initialization and Configuration

At the LEC start-up or new installation the following happens:

(1) LEC obtains its own AESA through ILMI automatic address registration

(gets from the switch the prefix part and sends to the switch its ESI)

(2) LEC determines the LECS AESA, by one of the following ways:

- By preconfigured LECS AESA

- By using ILMI to get LECS AESA from the switch

- By using well known LECS address (see below)

- By using well known label (VPI = 0, VCI = 17)

(3) LEC sets up the configuration direct VCC with the LECS

(4) LEC sends to LECS the CONFIGURE_REQUEST message which includes

the following data:

- my AESA

- my maximal frame size (MTU - max. transfer unit)

- my LAN type (802.3 or 802.5)

- name of ELAN I wish to join

- my layer 3 address (IP address)

(5) LECS sends back to LEC the CONFIGURE_RESPONSE message which

includes the following data:

- maximum frame size (MTU)

- LAN type

- ELAN ID

- LES AESA (for the requested ELAN)

Well known AESA of the LECS is defined by the ATM Forum:

47.0079.00000000000000000000.00A03E000001

1

2

3

4

5


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LEC Registration

After a LEC learns the AESA of the LES, it can register with the LES and join the ELAN.This is done through the following steps:

(1) LEC clears the configuration direct VCC, which is no longer needed

(2) LEC sets up the control direct VCC with the LES (by using UNI/PNNI)

(3) LEC sends LE_JOIN_REQUEST which includes the following data:

- my LAN type (802.2 or 802.5)

- my ELAN name

- my AESA

- my MAC address

- my MTU

(4) LES adds the LEC to the control distribute VCC and sends LE_JOIN_RESPONSE

with the following data:

- updated information from LE_JOIN_REQUEST

- LEC ID (which is unique to ELAN and is used in LAN frame)

(5) Now LEC wants to connect to the BUS. Therefore it must first find its AESA.

LEC sends to LES the ARP_REQUEST for the MAC number "FFFFFFFFFFFF"

(the broadcast MAC address).

(6) LES forwards the LEC's ARP_REQUEST to everybody on the control distribute

VCC (BUS included)

(7) BUS sends ARP_REPLY to LES (the reply contains its AESA)

(8) LES forwards ARP_REPLY to LEC

(9) LEC sets up the multicast send VCC wit the BUS

(0) BUS adds the LEC to the multicast forward VCC

1

2

3

4

5

6

7

8

9

10


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Data Transfer

When a LEC becomes officially a member of ELAN, it can communicate with other LECs

which are either direct members of ELAN (i.e. registered with the LES) or are behind a

LAN bridge or a LAN router. Of course, the latter have to have an ATM interface end

must be registered with the LES.

There are four phases of this communication:

1. Resolving the MAC address of destination (IP ARP)

2. Initial data transmission through BUS

3. Resolving the ATM address of destination (LE ARP)

4. Data transmission through data direct VCC

The second and third phases are overlapped.


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Example: A wants to communicate with B

ATM host

(LEC) LAN host

LAN host

ATM switch

EthernetATM

BUS LES LECS

ATM host(LEC)

B

LAN Bridge/Switch

(LEC)

S

C

A

LEC on S encapsulates the

IP ARP Request into a

LANE frame and sends it to

BUS via multicast send VCC

LEC on S decapsulates the

IP ARP replyh and forwards

it to A

BUS broadcasts the IPARP request to all LECs

via multicast forward VCC

B recognizes IP.B andsends its MAC address to

BUS, which in turn forwards

the frame to the LEC on Svia multicast send VCC

As soon as A gets the MAC

address of B it starts sending

data to LAN

LEC on S forwards data to BUS

via multicast send VCC

BUS broadcasts data to all LECs via

multicast forward VCC. This getsdata moving immediately without

waiting for data direct VCC

between A and B


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ATM host

(LEC) LAN host

LAN host

ATM switch

EthernetATM

BUS LES LECS

ATM host

(LEC)

B

LAN Bridge/Switch

(LEC)

S

C

A

In order to communicate more

efficiently, LEC on S needs

to know AESA of B andsends

an LE_ARP request to B

LEC on B recognizes MAC.B

and replies to LES

LES broadcasts the LE_ARP request

to all LECs over control distribute VCC

LES broadcasts the LE_ARP

reply to all LECs over

control distribute VCC

After it gets the AESA.B, LEC

on S sets up a data direct

VCC with B

LEC on S decapsulates

LE framesand sends LANframes to A

LEC on S encapsulates

LAN frames and sends LE

frames to B

After the data direct VCC is

established between S and B

they can communicate faster.

Data braoadcast via BUS is nolonger needed


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MULTI PROTOCOL OVER ATM

LANE has a similar limitation as CLIP: if end stations belong to different subnets, the

data transfer over the ATM network has to go over several VCCs that connect the edge

devices with intermediate routers. This introduces an unnecessary overhead, since thedata frames have to traverse the AALs several times. Another limitation is that the LANE

can work only with two types of LAN protocols: Ethernet and token ring.

Therefore Multi Protocol Over ATM (MPOA) was brought by ATM Forum in 1997 under

the industry concensus to achieve two goals:

- to extend LANE to other protocols such as IPX, ApleTalk and DECNET

- to create a more efficient internetworking (specifically to eliminate routers in

inter-ELAN data transfer)

MPOA is an extension of LANE which uses Next Hop Routing Protocol (NHRP) to getthe AESA of the destination edge device and to create a direct VCC for data transfer,

called shortcut or cut through. Consequently MPOA retains the LANE components and

connections between them. If the communication involves two end stations which belong

to the same subnet, the VCC for data transfer will be established according to LANE

specification. If the end stations belong to different subnets, the data will start to flow

over several VCC through router(s) until the ingress edge device discovers such flow

and organizes the shortcut.

E2

Edge

device

Edge

device

Router

A B C E FD

LIS1 LIS2

LANE

LAN LAN

LANE

Shortcut

(MPOA)

RATM

Network

E1


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MULTI PROTOCOL OVER ATM (Cont.)

Next Hop Resolution Protocol (NHRP)

NHRP is a protocol that eliminates routers in data transfer between different LISs. Since

the ATM hosts or edge devices are directly connected to the ATM network, there is a

possibility to establish a single direct VCC for data transfer. Such VCC is called "short-

cut" or "cut-through" data VCC. In order to establish a VCC, the source station must

know the AESA of the destination station.

NHRP specifies one NHRP server (NHS) for each LIS. Each potential end station is an

NHRP client (NHC).

The main purpose of NHS is to resolve IP addresses into ATM addresses even if IP be-

longs to a different LIS than the NHC. If an NHS gets an ARP_REQUEST for LIS it

doesn't serve, it will forward the request to the next NHS. Eventually the ARP request

will get to the NHS which serves the target LIS. The target NHS will then send

ARP_REPLY (orARP_NAK) back to the end station, following the same route in reversedirection. The intermediate NHSs will cache the information into their ARP tables, so that

next time they can generate the ARP_REPLY instead of forwarding the ARP_REQUEST

to the next NHS.

NHRP ARP_REQUEST ("I am IP.A, what is AESA of IP.B?")

NHRP ARP_REPLY ("IP.B has AESA.B")

NHCA

ATM host or edge

device which

IP address

belongs to LIS1

ATM host or edge

device which

IP address

belongs to LIS3

This server serves

LIS3

and resolves

IP.B " AESA.B

These servers cache

[IP.B, AESA.B]

1

2 3

45

6

LIS2

LIS3

LIS1

Direct VCC ("shortcut", "cut-through")

7

NHS1 NHS3NHS2

NHC

B


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MPOA Components

MPOA uses similar client/server architecture as LANE. Consequently MPOA uses all

components of LANE defined in the previous section. It also defines additional compo-

nents, which can be classified into two types:

MPOA Client (MPC)

Every ATM host, edge device or router with ATM interface run a copy of MPC. When

MPC is in ingress role it monitors traffic and detects the frames sent to a router that

contains an MPS. If it realizes that the flow would benefit from a shortcut, it starts a

NHRC-based query-response protocol to obtain necessary information to setup a

shortcut. If such shortcut is available it will establish it between the source MPC and

the destination MPC, and will continue frame forwarding over the shortcut.

In egress role MPC receives internetwork data frames and forwards them to its local

users.

MPOA Server (MPS)

Runs on a router. It includes NHRC server (NHS) and answers MPOA queries from in-

gress MPCs.


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MPOA Operation

The operation of MPOA has the following steps:

1. Configuration

MPC and MPS obtain the address of LECS. There are four methods to do this (same as

in LEC configuration):

- By preconfigured LECS AESA

- By using ILMI to get LECS AESA from the switch

- By using well known LECS address

- By using well known label (VPI = 0, VCI = 17)

Once MPC and MPS obtain the necessary addresses they establish control direct VCC

with LES.

2. Discovery

MPC and MPS discover each other. This is done over LANE (by now the LANE compo-

nents are already connected, and the communication over LUNI is in place). An MPC

sends an LE_ARP_REQUEST for MPS to LES (this message has an additional field

identifying that the request is in connection to MPS). LES returns LE_ARP_REPLY with

the AESA of MPS. Once the MPC has MPS's address it will establish the connection.

3. Target Resolution

As soon as the data direct VCCs are established between the LECs, the data will start to

flow from the source LEC to the destination LEC via the router(s). At the same time the

source MPC will start counting the forwarded frames. When this count reaches a "signifi-

cant flow" called: Shortcut Setup Frame Count (default value is 10), the MPC will send

an MPOA_ARP_REQUEST to its MPS, to get the AESA of the destination MPC. The re-

quest goes from one MPS to another (hop-by-hop) until it reaches the egress MPS which

serves the target ELAN. The egress MPS will then send an MPOA_ARP_REPLY back

to the requesting MPC. The reply follows the same route in reversed direction. This ad-

dress resolution is performed by multiple MPSs in accordance with the NHRP which is

part of MPS.

4. Data Transfer

When the source MPC gets the AESA of the destination MPC, it will establish a data di-

rect VCC (the shortcut) with the destination MPC.


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Examples of MPOA Scenarios

Suppose the following (simplified) network:

S

ATM host LAN host

LAN host

EthernetATM

Network

ATM host

B

C

D

ARouter

LAN

Switch

The MPOA environment can be pictured as follows:

Two types of scenarios exist for the environment:

Intra-ELAN scenarios: A$ B, C$ D

Inter-ELAN scenarios: A$ C, A$ D, B $ C, B $ D

RF

LEC

ATM host

ATM host

B

C

D

ARouter

MPC

SLAN

Switch

MPS

MPC

MPC

ELAN1LEC

LECLEC

ELAN2

LAN

host

LAN

host

FWD

RF - Routing Function

FWD - Layer 3 Forwarding Function

MFC - MPOA Client

MPS - MPOA ServerLEC - LANE Client

MPOA VCCs

LANE VCCsLAN

LIS1

LIS2


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Intra-ELAN scenarios:

Inter-ELAN scenarios (default path):

Inter-ELAN scenarios (shortcut path):

A C S DB

ELAN LAN

LAN

A,B R S C,D

ELAN

ELAN

LAN

A,B R S C,DMPOA ARP

request MPOA ARP

request

MPOA ARP

reply

MPOA ARP

reply

LAN

Shortcut

atm interworking

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