Asynchronous Transfer Mode (ATM) NETE0521 Presented by Dr.Apichan Kanjanavapastit.

Asynchronous Transfer Mode (ATM)

NETE0521

Presented by

Dr.Apichan Kanjanavapastit

Definition

• Asynchronous transfer mode (ATM) is a high-performance, cell-oriented switching and multiplexing technology that utilizes fixed-length packets to carry different types of traffic

• ATM was designed by the ATM Forum and adopted by the ITU-T

Packet Networks

• Data communications are based on packet switching and packet networks

• A packet is a combination of data and overhead bits that can be passed through the network as a self-contained unit

• The overhead provides identification and addressing information as well as the data required for routing, flow control, error control, and so on

• Different protocols use packets of varying size and intricacy

• As networks become more complex, the information carried in the header becomes more extensive

Packet Networks (con’t)

• The result is larger overhead relative to the size of data unit

• Some protocols have enlarged the size of data unit to make header use more efficient

• Thus, packets can be as long as 60,000 bytes sharing long-haul links with packets of fewer than 200 bytes

Mixed Network Traffic

• Since packet networks have unpredictable packet sizes, switches, multiplexers, and router must incorporate elaborate software systems to manage the various sizes of packets

• A grate deal of header information must be read and each bit counted and evaluated to ensure the integrity of every packet

• Another problem is that of providing consistent data-rate delivery when packet sizes are unpredictable and can vary so dramatically

• To get the most out of broadband technology, traffic must be time-division multiplexed onto shared paths

Mixed Network Traffic (con’t)

• Because audio and video packets ordinarily are small, mixing them with conventional data traffic often creates unacceptable delays of this type and makes shared packet links unusable for audio and video information

router

Cell Networks

• Many of the problems associated with the packet internetworking are solved by adoption a concept called cell networking

• A cell is a small data unit of fixed size; thus all data loaded into identical cells can be transmitted with complete predictability and uniformity

• As packets of different sizes and formats reach the cell network, they are split into multiple small data units of equal length and loaded into cells

• The cells are then multiplexed with other cells and routed through the cell network

• Since each cell is the same size and all are small, the problem associated with multiplexing different-sized packets are avoided

Cell Networks (con’t)

• In this way, a cell network can handle real-time transmission, such as phone call, without the parties aware of the segmentation or multiplexing at all

MU

X

Asynchronous TDM

• ATM uses asynchronous time-division-multiplexing to multiplex cells coming from different channels. It uses fixed-size slots the size of a cell

• ATM multiplexers fill a slot with a cell from any input channel that has a cell; the slot is empty if none of the channels has a cell to send

• ATM uses fixed-size slots (total 53 bytes: 48 bytes for payload and 5 bytes for overhead)

Asynchronous TDM (con’t)

A1

C1

A2

B1

C2

B2

C3

A3

A1C1A2B2 B1C3 C2A3

WHY USE A 48-BYTE PAYLOAD?48 bytes corresponds to approximately 6 milliseconds of voice

• Losing one 48-byte payload wouldn’t be disruptive to a listener (a speech phoneme is about 32 milliseconds long)

The U.S. preferred a 64-byte payload• Studies indicated that data communication efficiency would be improved with somewhat

larger cells (i.e., less overhead per PDU)Europe preferred a 32-byte payload

• Echo cancellers for audio wouldn’t be needed in smaller countries if PDU sizes were kept small enough

Everyone wanted the payload size to be a power of two• Memory transfer and switching would all be simplified

The Solution?

ATM Architecture

End points are user access devices

ATM Architecture (cont.)

• Virtual Connection– Connection between two end points is accomplished through

transmission paths (TPs), virtual paths (VPs), and virtual circuits (VCs)

• A transmission path (TP) is the physical connection (wire, cable, satellite, and so on) between an end point and a switch or between two switches

• A transmission path is divided into several virtual paths. A virtual path provides a connection or a set of connections between two switches

• Cell networks are based on virtual circuits (VCs). All cells belonging to a single message follow the same virtual channel and remain in their original order until they reach their destination

ATM Architecture (cont.)This virtual connection is uniquelydefined using the (VPI, VCI) pair:

(14 , 21)

Since the virtual connections need to be identified, there are two levels of identifier: a virtual path identifier (VPI) and a virtual circuit identifier (VCI).

VP-only Switching

ATM Layers

• The ATM standard defines three layers, from the top to bottom, the application layer, the ATM layer, and the physical layer. The physical and ATM layer are used in both switches and end points. The AAL is used only by the end points.

ATM Reference Model Relates to the OSI Reference Model

Application Adaptation Layer (AAL)

• The AAL allows existing networks (such as packet networks) to connect to ATM facilities

• AAL protocols accept transmissions from upper-layer services (e.g., packet data) and map them into fixed-sized ATM cells

• These transmissions can be of any type (voice, data, audio, and video) and can be of variable or fixed rates

• At the receiver, this process is reversed– segments are reassembled into their original formats and passed to the receiving service

AAL (con’t)

Physical Layer

ATM Layer

Segmentation and Reassembly Sublayer• Pack Convergence Sublayer information into 48-byte blocks for transfer down to the ATM Layer.• Unpack ATM Layer cells for transfer up to the Convergence Sublayer.

Convergence Sublayer• Provide application-specific interface• Handle lost and delayed cells• Error detection and handling

Upper Layers

TRAVERSING THE AAL

Segmentation and Reassembly Sublayer (continued)Segmentation and Reassembly Sublayer (continued)

Segmentation and Reassembly SublayerSegmentation and Reassembly Sublayer

Convergence Sublayer

Application Layer

MessageMessage

CS CS HeadeHeade

rr

CSCSTrailerTrailer

PaPadd

SARSARHdrHdr

SARSARTrlrTrlr

SARSARHdrHdr

SARSARTrlrTrlr

SARSARHdrHdr

SARSARTrlrTrlr

SARSARHdrHdr

SARSARTrlrTrlr

SARSARHdrHdr

SARSARTrlrTrlr

ATM LayerATM Layer

ATMATMHdrHdr

ATMATMHdrHdr

ATMATMHdrHdr

ATMATMHdrHdr

ATMATMHdrHdr

AAL (cont.)

• AAL Type 1 supports constant bit rate (CBR), synchronous, connection oriented traffic. Examples include T1 (DS1), E1, and x64 kbit/s emulation.

• AAL Type 2 supports time-dependent Variable Bit Rate (VBR-RT) of connection-oriented, synchronous traffic. Examples include Voice over ATM. AAL2 is also widely used in wireless applications due to the capability of multiplexing voice packets from different users on a single ATM connection.

• AAL Type 3/4 supports VBR, data traffic, connection-oriented, asynchronous traffic (e.g. X.25 data) or connectionless packet data (e.g. SMDS traffic) with an additional 4-byte header in the information payload of the cell. Examples include Frame Relay and X.25.

AAL (cont.)

• AAL Type 5 is similar to AAL 3/4 with a simplified information header scheme. This AAL assumes that the data is sequential from the end user and uses the Payload Type Indicator (PTI) bit to indicate the last cell in a transmission. Examples of services that use AAL 5 are IP over ATM, Ethernet Over ATM

AAL5

• AAL 5 is sometimes called the simple and efficient adaptation layer (SEAL), assumes that all cells belonging to a single message travel sequentially and that control functions are included in the upper layers of the sending application (addressing, sequencing, or other header information)

• AAL5 accepts an IP packet of no more than 65,535 bytes and adds an 8-byte trailer as well as any padding required to ensure that the position of the trailer falls where the receiving equipment expects it (at the last 8 bytes of the last cell)

AAL5 (cont.)

ATM Layer

• The ATM layer provides routing, traffic management, switching, and multiplexing services

• It processes outgoing traffic by accepting 48-byte segments from the AAL and transforming them into 53-byte cells by the addition of a 5-byte header

• Most of the header is occupied by the VPI and VCI. The combination of VPI and VCI can be thought of as a label that defines a particular virtual connections

Physical Layer

• The physical layer defines the transmission medium, bit transmission, encoding, and electrical to optical transformation

• It provides convergence with physical transport protocol such as SONET/SDH as well as the mechanisms for transforming the flow of cells into a flow of bits

• The ATM Forum has left most of the specifications for this level to the implementer

QoS, PVC, and SVC

• Quality of Service (QoS) requirements are handled at connection time and viewed as part of signaling.

• ATM provides permanent virtual connections and switched virtual connections.– Permanent Virtual Connections (PVC)

permanent connections set up manually by network provider. The VPIs and VCIs are defined for the permanent connections and the values are entered in a table for each switch– Switched Virtual Connections (SVC) set up and released on demand by the end user via signaling

procedures.

ATM Signaling Protocol

• Signaling protocol consists of two parts• User-Network Interface (UNI)

– defines how end points talk to switches• Network-Network Interface (NNI)

– defines how switches talk to other switches

• Cell formats of the two protocols are slightly different

UNI Signaling

• UNI signaling is performed between an end station and a private ATM switch, or between a private ATM switch and the public ATM network

• The UNI signaling is simpler because it does not involve routing. The standards are produced by the ATM Forum and are called UNI 3.1 (1994) and UNI 4.0 (1996)

• UNI 4.0 is an addition to UNI 3.1, UNI 3.1 is derived from the Public Network Signaling protocol Q.2931 brought by the ITU-T which is further derived from Q.931 used in ISDN and Frame Relay

UNI Header Format

• GFC---4 bits of generic flow control that are used to provide local functions, such as identifying multiple stations that share a single ATM interface. The GFC field is typically not used and is set to a default value.

• VPI---8 bits of virtual path identifier that is used, in conjunction with the VCI, to identify the next destination of a cell as it passes through a series of ATM switch routers on its way to its destination.

• VCI---16 bits of virtual channel identifier that is used, in conjunction with the VPI, to identify the next destination of a cell as it passes through a series of ATM switch routers on its way to its destination.

UNI Header Format (cont.)

• PT---3 bits of payload type. The first bit indicates whether the cell contains user data or control data. If the cell contains user data, the second bit indicates congestion, and the third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame.

• CLP---1 bit of congestion loss priority that indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network.

• HEC---8 bits of header error control that are a checksum calculated only on the header itself.

UNI Header Format (cont.)

NNI Signaling

• NNI signaling is performed between the switches of a public ATM network. Since a public network generally involves several (or many) switches the routing becomes very important component of the NNI signaling

• NNI signaling has two major standards: IISP (Interim Inter-switch Signaling Protocol) and PNNI (Private Network-to-Network Interface)

• IISP is a simple signaling protocol which uses static routing which have to be manually created and maintained and is designed for small private ATM networks

• PNNI is a signaling protocol that uses very elaborate dynamic routing algorithms which can easily handle small to large ATM networks which can have hundreds, thousands and even tens of thousands of ATM switches

NNI Header Format

• The GFC field is not present in the format of the NNI header. Instead, the VPI field occupies the first 12 bits, which allows ATM switch routers to assign larger VPI values. With that exception, the format of the NNI header is identical to the format of the UNI header.

ATM End System Addressing (AESA)

• All ATM switches and end stations in an ATM network must have a unique ATM address

• The address is a crucial part of ATM signaling. This address must be long enough to accommodate a potentially huge number of ATM devices.

ATM End System Addressing (AESA) (cont.)

Automatic Address Registration in UNI

• The ATM addresses (prefix only) of switches must be entered manually by the network manager

• Once the address is in place, each work station (edge device) attached to that switch can now be configured automatically

• The configuration is dynamic, it happens each time a device is attached to the switch, or when the device is moved from one switch to another

• Automatic address registration is performed through the Integrated Local Management Interface (ILMI)

Integrated Local Management Interface (ILMI)

• ILMI is based on IP's SNMP and uses a similar MIB and access procedures like Get, Set and Trap requests and responses

• All ILMI communications go over a dedicated (default) VC (VPI = 0, VCI = 16)

• Each ATM device (edge device or switch) that implements UNI (private or public) has ILMI and a component called Interface Management Entity (IME)

• This entity acts as a symmetric component that can both send requests and respond to a peer IME

• IME is responsible to maintain MIB and interpret/respond to SNMP messages. There are four types of SNMP messages used in automatic address registration: trap, get, getnext, set

Integrated Local Management Interface (ILMI) (cont.)

Automatic Address Registration in UNI (cont.)

ATM SwitchEdge Device

UNI Signaling

• Once an AESA address is established the user can place a call across an ATM network

• The calls are accomplished by a set of signaling frames– connection setup frames– maintenance frames– connection teardown frames

• All frames use dedicated VC, VPI = 0, VCI = 5

UNI Call Set-Up

NNI Signaling: IISP

• IISP (Interim Interswitch Signalling Protocol) is an extension of UNI 3.1/4.0 (approved in 1994) which includes simple hop-to-hop routing based on AESA addresses

• Usually, the routing table has two additional fields for output ports: the second and the third routing choice in case the link for the first choice fails.

• For routing are used only the first n octets of the address (n is the column indicated by "Octets to use"). An IISP routing table must be configured by the network administrator.

NNI Signaling: IISP (cont.)

Routing Loop Problem in IISP

ATM Classes of Services

• Constant Bit Rate (CBR)– This class is used for emulating circuit switching. The cell

rate is constant with time. CBR applications are quite sensitive to cell-delay variation. Examples of applications that can use CBR are telephone traffic (i.e., nx64 kbps), videoconferencing, and television

• Variable Bit Rate–Non-Real Time (VBR–NRT) – This class allows users to send traffic at a rate that varies

with time depending on the availability of user information. Statistical multiplexing is provided to make optimum use of network resources. Multimedia e-mail is an example of VBR–NRT

ATM Classes of Services (con’t)

• Variable Bit Rate–Real Time (VBR–RT) – This class is similar to VBR–NRT but is designed for applications

that are sensitive to cell-delay variation. Examples for real-time VBR are voice with speech activity detection (SAD) and interactive compressed video

• Available Bit Rate (ABR)– This class provides rate-based flow control and is aimed at data

traffic such as file transfer and e-mail. Although the standard does not require the cell transfer delay and cell-loss ratio to be guaranteed or minimized, it is desirable for switches to minimize delay and loss as much as possible. Depending upon the state of congestion in the network, the source is required to control its rate. The users are allowed to declare a minimum cell rate, which is guaranteed to the connection by the network

ATM Classes of Services (con’t)• Unspecified Bit Rate (UBR)

– The bandwidth allocation service of this class does not guarantee any throughput levels and uses only available bandwidth. UBR is often used when transmitting data that can tolerate delays. The most widely use today is the TCP/IP data

ATM Technical Parameters

• Cell Loss Ratio (CLR)– CLR is the percentage of cells not delivered at their destination

because they were lost in the network due to congestion and buffer overflow

• Cell Transfer Delay (CTD)– The delay experienced by a cell between network entry and exit

points is called the CTD. It includes propagation delays, queuing delays at various intermediate switches, and service times at queuing points

• Cell Delay Variation (CDV)– CDV is a measure of the variance of the cell transfer delay. High

variation implies larger buffering for delay-sensitive traffic such as voice and video

ATM Technical Parameters (con’t)

• Peak Cell Rate (PCR) – The maximum cell rate at which the user will transmit.

PCR is the inverse of the minimum cell inter-arrival time

• Sustained Cell Rate (SCR) – This is the average rate, as measured over a long

interval, in the order of the connection lifetime • Burst Tolerance (BT)

– This parameter determines the maximum burst that can be sent at the peak rate. This is the bucket-size parameter for the enforcement algorithm that is used to control the traffic entering the network

IP-over-ATM: why?

• because it’s there- use ATM network as a link-layer to connect IP routers

• can manage traffic more carefully in ATM network (e.g., rate-limit source/dest pairs, provide CBR service)

• leave IP untouched – leverage the fact that many users have IP addresses already

IP-Over-ATMClassic IP only

• 3 “networks” (e.g., LAN segments)

• MAC (802.3) and IP addresses

IP over ATM • replace “network” (e.g.,

LAN segment) with ATM network

• ATM addresses, IP addresses

ATMnetwork

EthernetLANs

EthernetLANs

IP-Over-ATM

AALATMphyphy

Eth

IP

ATMphy

ATMphy

apptransport

IPAALATMphy

apptransport

IPEthphy

IP Over ATM

•Set of IP hosts within a same IP domain (subnet) communicate with each other directly over ATM network.

•The IP hosts outside their subnet (domain)

communicate with other IP hosts in another subnet via an IP router.

ATM Network

IP Host 1

IP Host 2

IP Host 3

IP Host 4

CASE 1:

CASE 2:

ATM Network ATMNETWORK

IP Router

IP Host 1

IP Host 2

IP Host 3

IP Host 4

Classical IP-over ATM [RFC 1577]

AB C D

E

R1 R2

LIS: logical IP subnet• end systems in same LIS

have same IP network addr• LIS looks like a LAN• ATM net divided into multiple

LIS• Intra-LIS communication via

direct ATM connections– How to go from IP addr to

ATM addr: ATMARP resolves IP addr to ATM addr (similar to ARP)

LIS1 LIS2 LIS3

Classical IP-over ATM [RFC 1577]

AB C D

E

R1 R2

Inter-LIS communication:• source, dest. in different LIS• each LIS looks like a LAN• hop-by hop forwarding:

– A-R1-R2-ELIS1 LIS2 LIS3

59

Architecture HostATM_ARP

Server

Host

Host

Host

Host

Host

ATM_ARPServer

ATM_ARPServer

IP RouterIP Router

LogicalIP

Subnet 1

LogicalIP

Subnet 2

LogicalIP

Subnet 3

LISLIS LIS

Configuration Requirements (Intra-subnet)

(ATM Network)

LIS 1

ATM ARP Server

LIS 2ATM Network

IP Router ATM ARP Server

IP Host 1

IP Host 2

IP Host 3

IP Host 4

Routing the IP over ATM Cells

• The ATM network creates a route between 2 routers: entering point and exiting-point routers

ATM Network

ATM cell

Entering-pointrouter

Exiting-pointrouter

I II III

IP Packet

IPP

acke

t

Routing the IP over ATM Cells (cont.)

• Routing the cells requires 3 types of addressing: IP addresses, physical addresses, and virtual circuit identifiers

• Each router connected to the ATM network has also a physical address associated with the ATM network. It plays the same role as the MAC address in a LAN.

• The ATM Forum defines 20-byte addresses for ATM networks. Each address must be unique in a network and is defined by the network administrator.

Address Binding

• An ATM network needs virtual circuit identifiers to route the cells. The IP datagram contains only source and destination IP addresses. The virtual circuit identifiers must then be determined from the destination IP address by the following steps:

1. The entering-point router receives an IP datagram. It uses the destination address and its routing table to find the IP address of the exiting-point router

2. The entering-point router uses the services of a protocol called ATMARP to find the physical address of the exiting-point router

3. The virtual circuit identifier is bounded to the physical address

ATMARP SERVER• Primary purpose is to maintain a table or cache of IP address mappings.• At least one ATMARP server must be configured for each LIS, along with a

specific IP and ATM address.• A single ATMARP server may service more than one LIS as long as it is IP and

ATM addressable within each LIS. • An ATMARP server learns about the IP and ATM addresses of specific

members (IP clients) of the LIS through the use of ATMARP and InATMARP messages exchanged between the ATMARP server and LIS members.

• Finally, an ATMARP server can run on an IP host or router. Figure shows an LIS with 2 IP clients and a stand-alone ATMARP server.

ATMSwitchATM

SwitchIP Client# 1IP address=176.13.11.1ATM address=AAA

IP Client# 2IP address=176.13.11.2ATM address=BBB

ATMARP serverIP address=176.13.11.99ATM address=ZZZ

ADDRESS RESOLUTION• If the ATMARP server contains an IP/ATM address entry for IP Client #2, it will return that information in an ATMARP reply message.

• IP Client #1 then knows the ATM address of IP Client #2 and can set up an SVC.

• If not, then the ATMARP server will return an ARP NAK message.

ATMSwitchATM

Switch

IP Client# 1IP address=176.13.11.1ATM address=AAA

IP Client# 2IP address=176.13.11.2ATM address=BBB

ATMARP serverIP address=176.13.11.99ATM address=ZZZ

ATMARP_Req (IP addr of Client #2, ATM addr ???)

ATMARP_Reply (ATM addr = BBB)

Setup VC and Send Data

How does the ATMARP server build its mapping table?

• This is done through the use of ATMARP and the two inverse messages

• When a router is connected to an ATM network for the first time and a PVC is established between the router and the server, the server sends an inverse request message to the router

• The router sends back an inverse reply message• Using these two addresses, the server creates

an entry in its routing table to be used if the router becomes an exiting-point router in the future

Registration• The registration process flow for IP Client #1 is shown in Figure.

• Of course, IP Client #2 will register its own address with the ATMARP server once it is initialized.

ATM Switch

Setup VC

InATMARP_Req (IP addr of client #1???)

InATMARP_Reply (176.13.11.1)

IP address=176.13.11.99 ATM address=ZZZ

ATMARP Server

IP Client #1 IP address=176.13.11.1 ATM address=AAA

IP Client #2 IPaddress=176.13.11.2 ATM address=BBB

Figure 7.28 Address binding in IP over ATM

Packet Format of ATMARP

Packet Format of ATMARP (cont.)

• Operation (OPER). This 16-bit field defines the type of the packet. Five packet types are defined as shown in the table.

ATMARP Operation on PVC Connection

• If a permanent virtual circuit is established between 2 routers, there is no need for an ATMARP server

• However, the router must be able to bind a physical address to an IP address. The inverse request message and inverse reply message can be used for the binding.

• When a PVC is established for a router, the router sends an inverse request message. The router at the other end receives the message (which contains the physical and IP address of the sender) and sends back an inverse reply message (which contains its own physical and IP address)

ATMARP Operation on PVC Connection (cont.)

• After the exchange, both routers add a table entry that maps the physical addresses to the PVC

• Now, when a router receives an IP datagram, the table provides information so that the router can encapsulates the datagram using the virtual circuit identifier

time time

Two routers connected through PVC

I II IIIATM

1

2

ATMARP Operation on SVC Connection

• In a SVC, each time a router wants to make a connection with another router, a new virtual circuit must be established

• However, the virtual circuit can be created only if the entering-ping router knows the physical address of the exiting-point router

• To map the IP addresses to physical addresses, each router runs a client ATMARP program, but only one computer runs an ATMARP server program

• The process of establishing a virtual connection requires 3 steps: connecting to the server, receiving the physical address, and establishing the connection

Connecting to the Server

• Normally, there is a permanent virtual circuit established between each router and the server

• If there is no PVC connection between the router and the server, the router must at least know the physical address of the server to crate an SVC connection just for exchanging ATMARP request and reply messages

Receiving the Physical Address

• When there is a connection between the entering-point router and the server, the router sends an ATMARP request to the server

• The server sends back an ATMARP reply if the physical address can be found or an ATMARP NACK otherwise

• If the entering-point router receives a NACK, the datagram is dropped

Establishing Virtual Circuits

• After the entering-point router receives the physical address of the exiting-pint router, it can request an SVC between itself and the exiting-point router

• The ATM network uses the two physical addresses to set up a virtual circuit which lasts until the entering-point router asks for disconnection

• In this step, each switch inside the network adds an entry to its tables to enable them to route the cells carrying the IP datagram

ATM_ARPServer

DestinationSwitch

SourceSwitch Host 2Host 1

Set Up

Set Up

ConnectConnect

Connection EstablishedInARP request

InARP RP

ARP Request

ARP Response

Set Up

Set Up

Set Up

Connect

Connect

Connect

Connection Established

Reg

istr

atio

nA

ddre

ssR

esol

utio

nC

onne

ctio

nE

stab

lish

men

tOperation of Classical IP over ATM

Next Hop Resolution Protocol (for Inter-Subnets)(NHRP: pronounced nerp)

LIS(ATM

Network)

LIS(ATM

Network)

LIS(ATM

Network)

LIS(ATM

Network)

Host

Host

RouterGo through a router that is a member of multiple logical IP subnets. This router may become a bottleneck. Solution NHRP

NHRP (next hop resolution protocol)

[RFC 2332]

• source/dest. not in same LIS: ATMARP can not provide ATM dest. address

• NHRP: resolve IP-to-ATM address of remote dest.– client queries local NHRP server– NHRP server routes NHRP request

to next NHRP server – destination NHRP returns dest ATM

address back through NHRP server chain (like routed DNS)

• source can send directly to dest. using provided ATM address

AB C D

E

NHRP server, S1

LIS1 LIS2 LIS3

NHRP server, S2

NHRP server, S3

“ARP over multiple hops”

1. NON-BROADCAST MULTI-ACCESS NETWORK (NBMA)

An NBMA network is defined as:

* Does not support an inherent broadcast or multicast capability.

* Enables any host (or router) attached to the NBMA network to communicate directly with another host on the same NBMA network.

ATM, Frame Relay, SMDS, and X.25 are all examples of NBMA networks. An NBMA ATM network may contain one or more LISs.

* The NBMA is partitioned into administrative domains. Logical NBMA Subnets (LNS)

* Each LNS is served by an NHS (Next Hop Server)

NHRP Terminology

2. NEXT HOP SERVER (NHS)

(These are responsible for answering NHRP resolution requests by means of NHRP replies.)

• NHS serves a set of hosts (or NHRP stations) in the NBMA network and answers NHRP resolution requests from these stations called NHC (Next Hop Clients).

• Both NHS and NHC contain a CACHE or table of IP & ATM addresses for devices attached to the ATM network (Address Resolution Cache).

• If the desired destination IP address is not on the ATM network, then the NHS will provide the ATM address of the router nearest to the destination.

• The NHS should run on a router so as to facilitate forwarding of NHRP requests, replies, and other messages over the default-routed path.

• The NHS responds to queries from NHRP clients.

• The NHS serves a specific set or domain of NHRP clients for whom it is responsible.

NHRP Terminology (Cont.)

3. NEXT HOP CLIENTS (NHC)

• NHRP cloud contains entities called NHCs.

• These are responsible for initiating NHRP resolution

request packets.

REMARK:

• Both NHC and NHS maintain an ADDRESS RESOLUTION

CACHE.

• An NHC in NHRP replaces an ATMARP client in CLIP

(Classical IP over ATM Case)

• NHS replaces an ATMARP server.

NHRP Terminology (Cont.)

•NHRP clients can be serviced by more than one NHS.

• NHRP Servers are configured with their own IP and ATM addresses, a set of IP address prefixes that correspond to the domain of NHRP clients it is serving, and an NBMA (ATM) network identifier.

• If the NHRP server is located on an egress router attached to a non-ATM network, then the NHRP server must exchange routing information between the ATM and non-ATM network.

NHRP Configuration

• NHRP clients must be attached to an ATM network and must be configured with the ATM address of the NHS that is serving the client. Alternatively, it should have a means of locating its NHS.

• NHRP clients register with their NHRP server in one of the two ways:

1- Manual Configuration

2- NHRP Registration Packets

• The NHRP registration packet contains the following information along with additional values:

{NHC’s ATMaddress, NHC’s IPaddress, NHS’s IPaddress}

• With this information, the NHRP server can begin to build its table of IP and ATM addresses.

NHRP Client Registration

NHRP Client Registration

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

ATMSwitch

SubnetX

SubnetY

SubnetZ

NHS X NHS Z

X.1 Z.3

NHRP Registration Request

NHRP Registration Reply

NHRP Registration Request

NHRP Registration Reply

NHRP ADDRESS RESOLUTIONNHS X NHS Z

Subnet XATM Switch

ATM Switch

ATM Switch

ATM Switch

ATM Switch

ATM Switch

ATM Switch

ATM Switch

ATM Switch

ATM Switch

ATM Switch

ATM Switch

Subnet Y Subnet Z

X.1Z.3

IP address = X.1ATM address = AAA

IP address = Z.3ATM address = BBB

First PacketFirst Packet First PacketNHRP Resolution Request

NHRP Resolution RequestNHRP Resolution ReplyNHRP Resolution Reply

Setup SVCData

A single NBMA ATM network that contains 2 LISs: X and Z. Actually 3 if we count the LIS connecting the two routers omitted.

NHRP ADDRESS RESOLUTION

• Station X.1 builds a packet and addresses it to Z.3. • If Z.3 ATM address known, then X.1 uses an existing VCC to send its data.• If not, I.e., X.1 does not know the ATM address of Z.3, then it sends NHRP.• This packet is forwarded over an existing ATM VC to the default router. • This causes X.1 to send a NHRP Next Hop Resolution Request message to

NHS X with the following information: [AAA, X.1, Z.3]. • Station X.1 may also opt to hold onto the packet until a NHRP reply is

received or drop it. • The first option, the default, is the better choice because that allows data to

flow over the default-routed path.

• The LISs are connected by two routers that serve as NHRP servers for subnets X and Z, respectively.• The routers are running a normal intra-AS routing protocol, OSPF, and are connected by an ATM PVC so they are exchanging routing information.• The station attached to subnet X with the IP address of X.1 wishes to communicate with station Z.3.

NHRP ADDRESS RESOLUTION

• NHS X checks to see if it serves station Z.3.• It also checks to see if it has an entry in its cache for Z.3.• SUPPOSE Neither is true so the NHRP (Next Hop Resolution

Request) is forwarded to the adjacent NHRP server, NHS Z.• NHS Z receives the NHRP Next Hop Resolution Request from NHS X. • NHS Z determines that it serves the destination IP address contained

in the request message. • An entry is contained in the cache or table of NHS Z which contains

an IP to ATM address mapping for the destination IP address of Z.3. • NHS Z resolves the destination IP address, Z.3, with its matching

ATM address, BBB.• It places this information in a NHRP Next Hop Resolution Reply and

returns it to station X.1 over a default-routed path that the request came from.