TCP/IP over ATM
LESSON 18

Outline:
TCP/IP over ATM

What is ATM ??

• Asynchronous Transfer Mode (ATM):
  • Is a lower-level communications technology which can perform at very high speeds.
• ATM is integration of multiple media types:
  • Data
  • Voice
  • Video
  • Images

Circuit vs. Packet Switching:

• Circuit:
  • End-to-end connection.
  • Constant delay.
  • Information is delivered in order (sequenced delivery).
  • Network is transparent to the user's information.
  • May be inefficient for some types of traffic.
• Virtual Circuit:
• End-to-end connection in a packet switched network.
• Packets delivered in order.
• Network is not transparent to what the user is sending.
• More efficient than circuits for certain applications.

ATM Components:

• ATM components include:
  • Hubs
  • Switches
  • Network Interface Adapter Cards
  • Multiplexers
  • Routers / Gateways
  • Access Nodes
  • Concentrators

ATM Network Interfaces:

• ATM components can be interconnected to support larger networks and to provide different interfaces to the same network.
• Two interfaces exist for ATM network components.
  • UNI (User-to-Network Interface):
    • Spans from customer premises to the network switch (common carrier) or can be used to connect switches within a private network.
  • NNI (Network-to-Network Interface):
    • Connects ATM switches together using slightly modified protocols at higher speeds than the user-to-network interface (UNI).
  

ATM Connection Paradigms:

• Hosts must establish a connection over an ATM network before they can transfer any data.
• To do so, they need to use a virtual circuit connection.
• Two forms exist:
  • Permanent Virtual Circuits (PVC)
  • Switched Virtual Circuits (SVC)
• Permanent Virtual Circuits:
  • The virtual circuit is manually setup through provisioning of network resources by the network providers.
  • No connection setup is needed prior to any data transmission.
• Switched Virtual Circuits:
  • Functions like a voice telephone call in that it must setup a connection prior to user of the network.
  • Signaling is used throughout the ATM network to allow devices to request circuit setup, take-down, and management.
  • SVCs use Meta-signaling.

Paths, Circuits, and Connection Identifiers:

• A circuit identifier used with UNI is a 24-bit integer divided into two fields.
• It uniquely identifies an open virtual circuit on one end of a connection.
  

• Virtual Circuit Identifier (VCI):
  • 16-bits which identifies the virtual circuit used for this particular connection.
• Virtual Path Identifier (VPI):
  • 8-bits which identifies the particular path the virtual circuit identifier is traveling over.
• Many VCI are usually contained within a single VPI.
• If many circuits follow the same path, the system administrator can give those circuits the same VPI value.
• ATM switches use VPI to efficiently route virtual channels.

ATM Cell Transport:

• ATM uses fixed-size packets of 53-octets called cells to carry all types of data over the network.
• Five octets of each cell are used as a header to carry control information.
  
  
  

  ATM Cell Format

  Header 5 octets

  Data Area (Payload) 48 octets

ATM Adaptation Layer:

• ATM transfers cells. Applications that transfer data over ATM do not read or write cells.
• Higher-layer protocols interact with ATM through the ATM Adaptation Layer (AAL).
• AAL performs many functions including:
  • Detection and correction of errors.
  • Multiplexing and Demultiplexing of data units.
  • Lost or corrupted cells.
  

• Usually the AAL is located in firmware on the network interface card.
• When a virtual circuit is created over ATM, both end- points must agree on and use the same AAL protocol.
• Several combinations of AAL protocols, services, and sublayers exist: each providing support for different types of transport services.

ATM Adaptation Layer:
Service Classes

• Service Classes:
  • Class A - Circuit emulation; constant bit rate
  • Class B - Variable bit rate; audio and video
  • Class C - Connection-oriented data transfer; variable bit rate (Frame Relay, TCP/IP)
  • Class D - Connectionless data transfer (SMDS)
  • Class X - Connection-oriented; best-effort delivery; user regulated.

ATM Adaptation Layer:
Protocols

• AAL Protocols:

  AAL1 AAL2 AAL3/4, 5 AAL3/4, 5 AAL3/4, 5 AAL0

  = Class A = Class B = Class C = Class D = Class X = Null

ATM Adaptation Layer:
Sublayers

• AAL Sublayers (lowest to highest):
  • Segmentation and Reassembly Sublayer (SAR):
    • Required for all AAL protocols (except AAL 0), SAR handles segmentation and reassembly of data.
  • Common Part Convergence Sublayer (CPCS):
    • Used only in AAL1, 3/4, 5. Handles cell delay variation, sequencing, & error correction/detection.
  • Service Specific Convergence Sublayer (SSCS)
    • Optional in AAL3/4, 5. Used for specific features.

ATM Adaptation Layer:
AAL-1

• AAL1 accepts and sends data across an ATM network at a fixed bit rate.
• It's used for applications like video because it provides a fixed-rate delivery service.
• This is required to guarantee a constant video stream free of pauses or flickering.

ATM Adaptation Layer:
AAL-0, 2, 3/4

• AAL0 - is NULL defined so there is no protocol used. It is a raw interface.
• AAL2 - is not yet complete. The standard definition has not been finished by ITU-T or the ATM Forum.
• AAL3/4 - is a complex protocol which provides a similar service to AAL5. It is not as popular because of this complexity.

ATM Adaptation Layer:
AAL-5

• AAL5 - is used to send conventional data packets across an ATM network.
• It provides an interface that accepts and delivers large, variable-length packets (1..65,535 octets).
• AAL5 packets are divided into ATM cells in exact multiples of 48 octets (including 8-bit trailer).
• If all cells are not completely full, the last one can contain from 0..40 octets of data, padded with zeros to fill it.
  

• AAL5 places control information in the 8 octet trailer, which includes:
  • Length field (16-bits).
  • Cyclic Redundancy Check (32-bit CRC) - for frame checksum.
  • UU field (8-bits) -unused and can contain any value.
  • CPI field (8-bits) - unused, must be zero filled.

Datagram Encapsulation and IP-MTU Size:

• AAL5 uses a 16-bit length field, allowing up to 64K octets in a single packet.
• IP imposes an MTU of 9180 octets on ATM networks.
• If a datagram is larger than 9180 octets, IP fragments the datagram and passes each fragment to AAL5.
• AAL5 accepts, transfers, and delivers datagrams of 9180 octets or less.

Packet Types and Multiplexing:

• The AAL5 trailer does not have a type field, so each frame is not self-identifying.
• ATM has two approaches to sending and identifying information of a connection:
  • A virtual circuit is used specifically for a single high-level protocol (IP - no type needed).
  • Several high-level protocols can use the same virtual circuit (type field is required).

Packet Types and Multiplexing:
Single Protocol Transport:

• Advantage:
  • No additional information is required per packet to identify the high-level protocol.
• Disadvantages:
  • A separate virtual circuit is needed for each different high-level protocol (cannot share).
  • Most companies are charged per virtual circuit, so it is more costly.

Packet Types and Multiplexing:
Multiple Protocol Transport:

• Advantage:
  • Allows all traffic to travel over the same circuit, reducing costs.
• Disadvantages:
• Requires extra overhead (type field) in each packet to determine the high-level protocol.
• Packets from all protocols travel with the same delay and priority characteristics.

Packet Types and Multiplexing:

• The TCP/IP standard allows computers to access AAL5 using either transport method.
• If a Computer chooses to include the type field, it is required to use two specific link-layer headers:
  • IEEE 802.2 Logical Link Control (LLC) header, followed by the
  • SubNetwork Attachment Point (SNAP) header.

• LLCM (24-bits) = AA.AA.03 (hexadecimal)
• SNAP (40-bits) - Two parts:
  • OUI (Organizationally Unique Identifier) - 3 octets. Identifies the org. that administers the values in the TYPE field. TYPE (2-octets) - identifies the packet type.
• For IP datagrams encapsulated over Ethernet:
  • OUI = 00.00.00 TYPE = 08.00

IP Address Binding in ATM:

• Encapsulating a datagram over ATM is straight forward, however address binding is difficult.
• ATM assigns addresses to all machines wishing to use a virtual circuit.
• ATM physical addresses are larger than IP addresses, so they cannot be encoded within them as static bindings.
• ATM does not support broadcasting (only multicasting via meta-signaling), therefore IP cannot use ARP for address resolution.
• Since PVCs are setup prior to use of the ATM network (by provisioning), each node does not know it's ATM physical address.
• They only know the circuit's VPI/VCI pair. This further complicates things.
• Switched virtual circuits (SVC) further complicate address binding because they require two levels of binding:

  1) IP address of the destination must be mapped to an ATM end-point address (hardware).

  2) Destination IP address must be mapped to the VPI/VCI pair for the SVC.

• Binding 2 is used each time a datagram is sent over ATM, binding 1 is done only once.

Logical IP Subnets:

• When a group of computers uses a large ATM network in place of a single local physical network, it forms a logical IP subnet (LIS) or Local area network ATM (LATM).
• This is used as a restricted form of address binding for larger neworks.
• Multiple logical IP subnets can be defined from all computers attached to the same ATM network.
  

• Each logical IP subnet functions like a separate LAN.
• All machines configured for a specific subnet exchange packets.
• Each LIS is treated by IP standard rules as a completely separate network.
• All LIS members share a single IP network ID, but it differs from all other LIS groups on the same physical network.
• All computers in a LIS can choose non-standard MTU sizes, but all must use the same size MTU on all LIS virtual circuits.
• Hosts communicating between separate LIS groups must use a router that participates among multiple logical subnets.
• No direct connections are allowed outside of LIS groups.

Connection Management:

• Creating a V.C., sending a single datagram, and then closing the V.C. is too expensive.
• Hosts must maintain a record of open virtual circuits so that they can be reused.
• Hosts use conventional IP routing when sending a datagram.
• The datagram and next-hop address are passed to the network interface which then examines it’s table of open virtual circuits to see if it can send it.
• If not, the network interface must:
  • Locate the next-hop computer's address.
  • Establish a switched virtual circuit.
  • Send the datagram using AAL5.
  • Add the circuit to it’s open circuit table.
• Logical subnets are justified by hardware and software constraints. Storing large tables of circuits, however can get costly and inefficient.

Address Binding within LIS
(LATM):

• Classic ARP relies on an efficient broadcast serviceM from the physical layer.
• ATM does not offer a hardware broadcast mechanism, only a proposed multicast delivery scheme.
• ATM's point-to-multipoint connections offer bi-directional communication only between the originator and all destinations, but not among those destinations.
• To simulate broadcast, many point-to-multipoint connections would be needed, which is inefficient and not always possible.
• TCP/IP over ATM approach:
  • Use a dedicated service on each LIS (LATM) that maintains a cache of mappings.
• Mappings - are established through registration at boot time and periodically updated.

ATMARP and InATMARP:

• Address Resolution Protocol for ATM (ATMARP):
  • The same protocol as ARP, but with extensions to support functionality in a unicast server environment.
  • Given a network protocol address, it determines the associated ATM address.
• Inverse ATMARP:
  • Similar to the original RARP protocol.
  • Given a virtual circuit identifier (VCI) between two end-points, it determines the end-point's network layer and ATM address.
• Each LIS (LATM) must have at least one ATMARP server.
• Both protocols use messages similar to ARP and RARP, to provide registration and address resolution.

• The messages used include:
  • (In)ATMARP_REQUEST - request a service.
  • (In)ATMARP_REPLY - reply to a service request.
  • ATMARP_NAK - avoids ambiguity when a resolution cannot be made.
• Backup ATMARP service is desirable for an LIS since only one machine is responsible for all address resolution.

Last Modification: (Sunday, August 25, 1996)

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1996 Illusion Industries Inc.