Packet Switch Architectures

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1 Packet Switch Architectures The following are (sometimes modified and rearranged slides) from an ACM Sigcomm 99 Tutorial by Nick McKeown and Balaji Prabhakar, Stanford University Slides used with permission from authors All rights reserved by authors. Outline Introduction: What is a Packet Switch? Packet Lookup and Classification: Where does a packet go next? Switching Fabrics: How does the packet get there? Copyright 999. All Rights Reserved

2 Introduction What is a Packet Switch? Basic Architectural Components Some Example Packet Switches Copyright 999. All Rights Reserved Basic Architectural Components Datapath: per-packet processing. Table Decision. Interconnect. Output Scheduling Table Decision Table Decision Copyright 999. All Rights Reserved 4

3 Where high performance packet switches are used - Carrier Class Core Router - ATM Switch - Frame Relay Switch The Internet Core Edge Router Enterprise WAN access & Enterprise Campus Switch Copyright 999. All Rights Reserved 5 Some Example Packet Switches Packet switches exist for different networking technologies Internet: IP protocol suite Ethernet: Ethernet switches ATM (Asynchronous Transfer Mode): ATM switch MPLS (Multiprotocol label switching): MPLS switch There are many similarities in the architecture of the switches Copyright 999. All Rights Reserved 6

4 Packet Lookup Where does a packet go next? ATM and MPLS switches Direct Lookup Bridges and Ethernet switches Associative Lookup Hashing IP Routers Patricia trees/tries Copyright 999. All Rights Reserved 7 Lookup in an ATM Switch Lookup cell VCI/VPI in VC table. Replace old VCI/VPI with new. Forward cell to outgoing interface. Transmit cell onto link. Copyright 999. All Rights Reserved 8 4

5 Lookup in an Ethernet Switch Lookup frame DA in forwarding table. If known, forward to correct port. If unknown, broadcast to all ports. Learn SA of incoming frame. Forward frame to outgoing interface. Transmit frame onto link. Copyright 999. All Rights Reserved 9 Lookup in an IP Router Lookup packet DA in forwarding table. If known, forward to correct port. If unknown, drop packet. Decrement TTL, update header Cksum. Forward packet to outgoing interface. Transmit packet onto link. Copyright 999. All Rights Reserved 0 5

6 Data Address ATM and MPLS Switches Direct Lookup VCI Memory (Port, VCI) Copyright 999. All Rights Reserved Bridges and Ethernet Switches Associative Lookups Associative Memory or CAM Advantages: Simple Search Data 48 Network Address Associated Data Associated Data Hit? {Address log N Disadvantages Slow High Power Small Expensive Copyright 999. All Rights Reserved 6

7 Bridges and Ethernet Switches Hashing Search Data 48 Hashing Function 6 Address Memory Data Associated Data Hit? {Address log N Copyright 999. All Rights Reserved Lookups Using Hashing An example Memory Search Data 48 Hashing Function CRC-6 6 # # # #4 # # Associated Data Hit? {Address log N Linked lists # # # Copyright 999. All Rights Reserved 4 7

8 IP Router Lookup H E A D E R Dstn Addr Engine Next Hop Computation Table Destination Next Hop Next Hop Incoming Packet IPv4 unicast destination address based lookup Copyright 999. All Rights Reserved 5 IP Routers Lookup Longest Prefix Matching 65/8 Prefix / / / / /0 4./9 Port Lookup time Storage space Update time Preprocessing time Copyright 999. All Rights Reserved 6 8

9 Ternary CAMs Value Associative Memory Mask R R R R R4 Next Hop Priority Encoder Copyright 999. All Rights Reserved 7 Binary Tries a b d c 0 f g h e i Example Prefixes a) 0000 b) 0000 c) 000 d) 00 e) 00 f) 0 g) 00 h) 00 i) 00 j) 0000 j Copyright 999. All Rights Reserved 8 9

10 Patricia Tree a b d c e 0 f g h i Example Prefixes a) 0000 b) 0000 c) 000 d) 00 Skip=5 e) 00 j f) 0 g) 00 h) 00 i) 00 j) 0000 Copyright 999. All Rights Reserved 9 Switching Fabrics: How does the packet get there? Output and Input Queueing Output Queueing Input Queueing Other non-blocking fabrics Copyright 999. All Rights Reserved 0 0

11 Basic Architectural Components Datapath: per-packet processing. Table Decision. Interconnect. Output Scheduling Table Decision Table Decision Copyright 999. All Rights Reserved Interconnects Two basic techniques Input Queueing Output Queueing Usually a non-blocking switch fabric (e.g. crossbar) Usually a fast bus Copyright 999. All Rights Reserved

12 Interconnects Output Queueing Individual Output Queues Centralized Shared Memory Memory b/w = N.R N Memory b/w = (N+).R N Copyright 999. All Rights Reserved Output Queueing How fast can we make centralized shared memory? Shared Memory 5ns SRAM N 5ns per memory operation Two memory operations per packet Therefore, up to 60Gb/s In practice, closer to 80Gb/s 00 byte bus Copyright 999. All Rights Reserved 4

13 Switching Fabrics Output and Input Queueing Output Queueing Input Queueing Scheduling algorithms Other non-blocking fabrics Combining input and output queues Multicast traffic Copyright 999. All Rights Reserved 5 Input Queueing with Crossbar Memory b/w = R Scheduler Data In configuration Data Out Copyright 999. All Rights Reserved 6

19 Other Non-Blocking Fabrics Clos Network Copyright 999. All Rights Reserved 7 Other Non-Blocking Fabrics Clos Network Expansion factor required = -/N (but still blocking for multicast) Copyright 999. All Rights Reserved 8 9

20 Other Non-Blocking Fabrics Self-Routing Networks Copyright 999. All Rights Reserved 9 Other Non-Blocking Fabrics Self-Routing Networks The Non-blocking Batcher Banyan Network Batcher Sorter Self-Routing Network Fabric can be used as scheduler. Batcher-Banyan network is blocking for multicast. Copyright 999. All Rights Reserved 40 0

Internet Protocols Fall 2004. Outline

Internet Protocols Fall 2004. Outline Internet Protocols Fall 2004 Lecture 7 Andreas Terzis 9/29/04 Outline Internet Protocol CIDR Forwarding CS 449/Fall 04 2 1 Classless Interdomain Routing (CIDR) Addressing The IP address space is broken