IP over ATM

Integrated Network Services

Almerindo Graziano

Introduction

• Characteristics of IP and ATM• IP over ATM - Overlay Model• Data encapsulation• CLIP• NHRP• LANE• MPOA• Conclusions

ATM

• ATM is connection oriented

• IP is connectionless

• ATM has built-in QoS support

• IP uses best-effort

• Two approaches– Connections established on demand– IP traffic is carried over pre-configured circuits

• Both approaches have pros and cons

IP over ATM

• Overlay Model

• ATM is treated as a Data Link layer on which IP runs– Classical IP over ATM (CLIP)– Next Hop Resolution Protocol (NHRP)– LAN Emulation (LANE)– Multiprotocol Over ATM (MPOA)

CLIP

• All members of a LIS (host or router) are under the same administrative control

• A LIS is given ONE IP subnet address

• CLIP

Classical IP over ATM

• The ATM network is divided into a number of Logical IP Subnet (LIS)

• Systems within a LIS communicate through direct ATM connections– Each LIS has an ARP server called ATMARP

– ARP queries are sent to the ATMARP server

• Systems in different LIS communicate through a router– A router is a member of multiple LIS

ATMARP server

• There is one for each LIS– It holds a table of <IP address,ATM address>

• A wants to communicate with B and it knows B’s IP address– A send an ARP_REQUEST to the ATMARP

server– If the IP address is found an ARP_REPLY is sent– Otherwise an ARP_NACK packet is sent

Classical IP Over ATM

H3

H2

H5H4

H6

ATM ARP server

ATM ARP server

ATM ARP server

H1

R4

R3

R1

R5

R2

LIS 1

LIS 3

LIS 2

ATM Network

Data Encapsulation

• How do we carry different protocols

• How do we identify different protocols

– VC multiplexing– LLC/SNAP– TULIP– TUNIC

VC multiplexing

• VC multiplexing or null encapsulation

• A different VC is required to carry each layer 3 protocol– One VC carries the IP protocol– One VC carries the IPX protocol etc..

• Not very suitable in a multiprotocol environment

LLC/SNAP encapsulation

• Similar to the IEEE 802

• Multiple protocols can be carried in the same VC

• Less expensive

• Can we do better than this?– Once established a VC, we don’t actually need

the IP header

TULIP/TUNIC

• TULIP (TCP an UDP over Lightweight IP)– Only the layer 4 protocol identifier is kept

• TUNIC (TCP and UDP over Nonexistent IP) Connection– The IP header is eliminated– A different VC is created for each layer 4

protocol

CLIP Limitations

• Inter-LIS communication has to go through a router– Both parties are attached to same ATM network

• Can be a problem in an ATM WAN– NHRP– MPOA

NHRP

• NHRP (Next Hop Resolution Protocol)

• NHRP servers and NHRP client

• Each LIS has at least 1 NHRP server– A server can serve more than 1 LIS– A server has a table of <IP address,ATM

address>

• Every ES is a NHRP client

NHRP• For intra-LIS communication, an NHRP server works

as an ATMARP server– It resolves IP addresses into ATM addresses

• For inter-LIS communication, NHSs are interconnected to exchange NHRP queries– When an NHS cannot solve an IP address, it forwards the

query to another NHS– If the destination host is not part of the same ATM network,

the NHS provides the address of the egress router– Intermediate NHSs store NHRP replies into their cache

NHRP

H3

H2

H5H4

H6

ATM ARP server

ATM ARP server

ATM ARP server

H1

R4

R3

R1

R5

R2

LIS 1

LIS 3

LIS 2

ATM Network

NHS NHS

LANE• LAN technologies such as Ethernet are widely used• However, new applications require higher

bandwidth and QoS support• LANE (LAN Emulation)

– Allows ATM to coexist with legacy LANs– Allows ATM to be gradually introduced into existing

legacy LANs– Emulates IEEE 802 LANs without any change to upper

layer protocols

Legacy LANs

• Connectionless MAC– ATM is connection oriented

• Broadcast transmission– Difficult to achieve in ATM

• No guaranteed QoS– In-built support in ATM

LANE entities

• LEC (LAN Emulation Client)

• LES (LAN Emulation Server)

• LECS (LAN Emulation Configuration Server)

• BUS (Broadcast and Unknown Server)

LEC

• Runs on an ATM station and simulates an Ethernet or Token Ring network

• Encapsulates upper layer protocols into ELAN frames

• Decapsulate incoming ELAN frames into upper layer protocol data unit (e.g. IP packets)

LES and BUS• Every ELAN has a LES

– It acts as a coordinator and resolves MAC addresses into ATM addresses

– LECs register with the LES giving <MAC address,ATM address>

• BUS– Used to emulate broadcast feature of a legacy LAN– Packet sent to the BUS are sent to all LECs in the ELAN– It is used before a direct connection is established between

two LECs

LECS

• More than one ELAN can run on an ATM network

• A LECS assigns LECs to their ELAN– When a station starts up it queries the LECS to

find out its LES– The station then registers with the LES– The BUS address is determined through an

LE_ARP with a MAC address of all 1s

Advantages and Disadvantages

• Advantages– Higher speed (not completely true)– Allows the creation of multiple VLANs

• Disadvantages– It hides QoS features of ATM– With LANEv2 a LEC can provide 8 levels of

QoS to higher layers, each with a different VC

MPOA

• Communication over multiple sub-networks requires routers– Bottleneck– A connection is set up with router and then

from the router to the destination

• Combination of LANE and NHRP– Aims at optimal, direct communication between

end systems without crossing any router

MPOA

• MPOA (Multi-Protocol Over ATM)– ATM hosts– Edge devices such as switches, routers etc..

• MPOA is built on top of LANE– Intra-LANE traffic follows LANE specification– Inter-LANE traffic is optimized by integrating

NHRP functionalities

MPOA

• MPOA Client (MPC)

• MPOA Server (MPS)

ATM Network

H1 H2

H6H5

H4H3H2

ELAN 1

ELAN 2

R

MPOA

Conclusions

• Problems with IP over ATM

• Overlay Model

• Integration with legacy systems– LANE

• CLIP, NHRP, MPOA