atm interworking
Post on 03-Apr-2018
228 Views
Preview:
TRANSCRIPT
-
7/28/2019 ATM Interworking
1/33
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
-
7/28/2019 ATM Interworking
2/33
3-2
Copyright 1999 by Marko VuskovicC
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
-
7/28/2019 ATM Interworking
3/33
3-3
Copyright 1999 by Marko VuskovicC
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.)
-
7/28/2019 ATM Interworking
4/33
3-4
Copyright 1999 by Marko VuskovicC
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
-
7/28/2019 ATM Interworking
5/33
3-5
Copyright 1999 by Marko VuskovicC
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.
-
7/28/2019 ATM Interworking
6/33
3-6
Copyright 1999 by Marko VuskovicC
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:
-
7/28/2019 ATM Interworking
7/33
3-7
Copyright 1999 by Marko VuskovicC
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.
-
7/28/2019 ATM Interworking
8/33
3-8
Copyright 1999 by Marko VuskovicC
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
-
7/28/2019 ATM Interworking
9/33
3-9
Copyright 1999 by Marko VuskovicC
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
-
7/28/2019 ATM Interworking
10/33
3-10
Copyright 1999 by Marko VuskovicC
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
-
7/28/2019 ATM Interworking
11/33
3-11
Copyright 1999 by Marko VuskovicC
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.)
-
7/28/2019 ATM Interworking
12/33
3-12
Copyright 1999 by Marko VuskovicC
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.
CLASSICAL IP OVER ATM (Cont.)
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
-
7/28/2019 ATM Interworking
13/33
3-13
Copyright 1999 by Marko VuskovicC
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.
CLASSICAL IP OVER ATM (Cont.)
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.
-
7/28/2019 ATM Interworking
14/33
3-14
Copyright 1999 by Marko VuskovicC
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.
CLASSICAL IP OVER ATM (Cont.)
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
-
7/28/2019 ATM Interworking
15/33
3-15
Copyright 1999 by Marko VuskovicC
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").
CLASSICAL IP OVER ATM (Cont.)
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
-
7/28/2019 ATM Interworking
16/33
3-16
Copyright 1999 by Marko VuskovicC
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.
CLASSICAL IP OVER ATM (Cont.)
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
-
7/28/2019 ATM Interworking
17/33
3-17
Copyright 1999 by Marko VuskovicC
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.
CLASSICAL IP OVER ATM (Cont.)
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
-
7/28/2019 ATM Interworking
18/33
3-18
Copyright 1999 by Marko VuskovicC
LAN EMULATION
-
7/28/2019 ATM Interworking
19/33
3-19
Copyright 1999 by Marko VuskovicC
LAN EMULATION (Cont.)
-
7/28/2019 ATM Interworking
20/33
3-20
Copyright 1999 by Marko VuskovicC
LAN EMULATION (Cont.)
-
7/28/2019 ATM Interworking
21/33
3-21
Copyright 1999 by Marko VuskovicC
LAN EMULATION (Cont.)
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
-
7/28/2019 ATM Interworking
22/33
3-22
Copyright 1999 by Marko VuskovicC
LAN EMULATION (Cont.)
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
-
7/28/2019 ATM Interworking
23/33
3-23
Copyright 1999 by Marko VuskovicC
LAN EMULATION (Cont.)
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
-
7/28/2019 ATM Interworking
24/33
3-24
Copyright 1999 by Marko VuskovicC
LAN EMULATION (Cont.)
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
-
7/28/2019 ATM Interworking
25/33
3-25
Copyright 1999 by Marko VuskovicC
LAN EMULATION (Cont.)
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.
-
7/28/2019 ATM Interworking
26/33
3-26
Copyright 1999 by Marko VuskovicC
LAN EMULATION (Cont.)
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
-
7/28/2019 ATM Interworking
27/33
3-27
Copyright 1999 by Marko VuskovicC
LAN EMULATION (Cont.)
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
-
7/28/2019 ATM Interworking
28/33
3-28
Copyright 1999 by Marko VuskovicC
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
-
7/28/2019 ATM Interworking
29/33
3-29
Copyright 1999 by Marko VuskovicC
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
-
7/28/2019 ATM Interworking
30/33
3-30
Copyright 1999 by Marko VuskovicC
MULTI PROTOCOL OVER ATM (Cont.)
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.
-
7/28/2019 ATM Interworking
31/33
3-31
Copyright 1999 by Marko VuskovicC
MULTI PROTOCOL OVER ATM (Cont.)
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.
-
7/28/2019 ATM Interworking
32/33
3-32
Copyright 1999 by Marko VuskovicC
MULTI PROTOCOL OVER ATM (Cont.)
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
-
7/28/2019 ATM Interworking
33/33
3-33
MULTI PROTOCOL OVER ATM (Cont.)
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
top related