Asynchronous Transfer Mode (ATM) Protocol Stack

1. EE 122: Lecture 19(Asynchronous Transfer Mode - ATM) Ion Stoica Nov 13, 2001 (* based on some on-line slides of J. Kurose & K. Rose)

2. Goals • Provide better services than best-effort • Able to carry both data and voice (telephony) traffic • Can be implemented at very high speeds istoica@cs.berkeley.edu

3. ATM Protocol Stack • Physical layer • ATM layer: performs routing (similar in function to IP + data link layer) • ATM Adaptation Layer (AAL) – performs segmentation and reassembly, multiplexing (similar in function to the transport layer) Source Destination AAL AAL ATM ATM ATM Phys Phys Phys istoica@cs.berkeley.edu

4. Design Decisions • Fixed size packets – cells • Based on virtual circuit istoica@cs.berkeley.edu

5. Why Fixed Size? • Easier to implement high speed switches • Easier to do processing when the cell length is known in advance • Easier to implement parallel and pipeline solutions when the processing of all cells take the same time • A cell is in general much smaller than the maximum packet size • A high priority cell needs to wait less before being transmitted (a low priority cell will take less time to be transmitted than a packet of maximum size) • When the is empty, the first data bits are in general transmitted faster • Cell size: 48 byte payload + 5 byte header • A compromise between US (64 byte payload) and Europe (32 byte payload) istoica@cs.berkeley.edu

6. ATM Cell Header • VCI: virtual channel ID • will change from link to link through network • PT:Payload type (e.g. control cell versus data cell) • CLP: Cell Loss Priority bit • CLP = 1 implies low priority cell, can be discarded if congestion • HEC: Header Error Checksum • cyclic redundancy check istoica@cs.berkeley.edu

7. ATM Cell Header • First 4 bits in VCI – GFC (Generic Flow Control) • Local significance at the end-host • Arbitrate the access to the link if a shared medium is used to connect to ATM • GFC bits can be overwritten by the newtork istoica@cs.berkeley.edu

8. ATM VCs • Advantages of ATM VC approach: • QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) • Drawbacks of ATM VC approach: • Inefficient support of datagram traffic • One VC between each source/dest pair) does not scale (N*2 connections needed) • VC introduces call setup latency, processing overhead for short lived connections istoica@cs.berkeley.edu

9. Virtual Circuit (VC) Forwarding • Each router maintains a routing table • A routing entry: (input port, input VCI, output port, output VCI); • VCI – Virtual Circuit Identifier • Upon a cell arrival at interface i • Input port uses i and the packet’s VCI v to find the routing entry (i, v, i’, v’) • Replaces v with v’ in the packet header • Forwards packet to output port I’ istoica@cs.berkeley.edu

10. 1 11 5 7 VC Forwarding: Example in-VCI out-VCI out in … … … … in-VCI out-VCI out in 1 7 4 1 … … … … … … … … destination source 3 5 4 11 … … … … 1 1 2 2 1 1 3 3 2 2 4 4 3 3 4 4 1 1 2 2 3 3 4 4 in-VCI out-VCI out in … … … … 2 11 3 7 … … … … istoica@cs.berkeley.edu

11. Virtual Path Identifier (VPI) • ATM splits the VCI in two • 16 bits Virtual Path Identifier (VPI) • 8-12 bits VCI • Use to implement hierarchical routing • All VCI between two subnets share the same VPI • Backbone switches switch based on VPI • Switches in edge networks switch based on the entire VCI Backbone(Public) Network Virtual path Network A Network B istoica@cs.berkeley.edu

12. Virtual Path Identifier (VPI) • ATM splits the VCI in two • 16 bits Virtual Path Identifier (VPI) • 8-12 bits VCI • Use to implement hierarchical routing • All VCI between two subnets share the same VPI • Backbone switches switch based on VPI • Switches in edge networks switch based on the entire VPI Backbone(Public) Network Virtual path Network A Network B istoica@cs.berkeley.edu

13. ATM Adaptation Layer • Goal: allow existing protocols and applications to run on top of ATM • AAL is implemented only at endpoints • AAL has two sub-layers • Convergence Sub-layer (CS) • Segmentation and Reassembly (SAR) sub-layer • Usually CS adds • Common Part Convergence Sub-layer (CPCS) header and trailer • Checksum istoica@cs.berkeley.edu

14. AAL Structure Protocol Data Unit (PDU) • Example of AAL header information • Type (e.g., first cell, last cell in PDU) • Sequence # of the cell within PDU CPCS header PDU CPCS trailer ATM cell header AAL header Payload data <= 48 ATM cell trailer istoica@cs.berkeley.edu

15. ATM Quality of Service (QoS) • Constant bit rate (CBR) • Variable bit rate – real-time (VBR-rt) • Variable bit rate –non-real-time (VBE-nrt) • Available bit rate (ABR) • Unspecified bit rate (UBR) istoica@cs.berkeley.edu

16. ATM QoS (cont’d) • VBR-rt – similar to Guaranteed Service in Intserv • Traffic is specified by a token bucket, and the end-to-end delay is specified • CBR – very similar to VBR-rt, but the source is expected to send at a specified rate (e.g., telephony traffic) • Specified by a token-bucket with very small bucket, e.g., 1 cell • VBR-nrt – similar to the Controlled –load Service in Intserv • Traffic specified by token-bucket, but no hard delay guarantees istoica@cs.berkeley.edu

17. ATM QoS (cont’d) • UBR - similar to best-effort service, but • There can still be an admission control: • However, UBR allows the source to specify a maximum rate, which can be used in the admission control • ABR – implements congestion control: use explicit notification • Source sends periodic Resource Management (RM) cells • Each switch put in the RM cell the available bit rate of the output link – after the RM cell traverses all switches it will have the minimum available rate amongst all routers along the path • Routers can use fair queueing to compute the available bit rate • Receiver sends back the RM cell to the sender • Sender adjust its rate accordingly istoica@cs.berkeley.edu

18. AAL Types • AAL 1: support CBR • AAL 2: support VBR • AAL 3/4: support variable-length packets • AAL 5: more efficient support for data packets (used to implement to support data packets and UBR service) istoica@cs.berkeley.edu

19. IP-Over-ATM IP over ATM • Replace “network” (e.g., LAN segment) with ATM network • ATM addresses, IP addresses Classic IP only • 3 “networks” (e.g., LAN segments) • MAC (802.3) and IP addresses ATM network Ethernet LANs Ethernet LANs istoica@cs.berkeley.edu

20. IP-Over-ATM Issues: • IP datagrams into ATM AAL5 PDUs • From IP addresses to ATM addresses ATM network Ethernet LANs istoica@cs.berkeley.edu

21. Datagram Transmission in IP-over-ATM Network • At source: • IP layer finds mapping between IP, ATM dest address (using ARP) • passes datagram to AAL5 • AAL5 encapsulates data, segments to cells, passes to ATM layer • ATM network:moves cell along VC to destination • At destination: • AAL5 reassembles cells into original datagram • if CRC OK, datgram is passed to IP istoica@cs.berkeley.edu

22. Summary • ATM was an effort to build a new network to support data and voice applications, and to provide QoS • Two main design decisions: • Use fixed-size, packets (cells) • Use virtual circuit switching • ATM did not replace IP • To many legacy applications • Hard to change these applications to take advantage of ATM QoS • Not appropriate to Web traffic • Today ATM is used largely by • Voice carrier • In the Internet backbones (see IP-over-ATM) istoica@cs.berkeley.edu