1 Floodless in SEATTLE: A Scalable Ethernet Architecture for Large Enterprises Chang Kim, and Jennifer Rexford http://www.cs.princeton.edu/~chkim Princeton University 2 Goals of Today’s Lecture Reviewing Ethernet bridging (Lec. 10, 11) Flat addressing, and plug-and-play networking Flooding, broadcasting, and spanning tree VLANs New challenges to Ethernet Control-plane scalability Avoiding flooding, and reducing routing-protocol overhead Data-plane efficiency Enabling shortest-path forwarding and load-balancing SEATTLE as a solution Amalgamation of various networking technologies covered so far E.g., link-state routing, name resolution, encapsulation, DHT, etc.2 Quick Review of Ethernet 4 Ethernet Dominant wired LAN technology Covers the first IP-hop in most enterprises/campuses First widely used LAN technology Simpler, cheaper than token LANs, ATM, and IP Kept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketch3 5 Ethernet Frame Structure Addresses: source and destination MAC addresses Flat, globally unique, and permanent 48-bit value Adaptor passes frame to network-level protocol If destination address matches the adaptor Or the destination address is the broadcast address Otherwise, adapter discards frame Type: indicates the higher layer protocol Usually IP6 Ethernet Bridging: Routing at L2 Routing determines paths to destinations through which traffic is forwarded Routing takes place at any layer (including L2) where devices are reachable across multiple hops IP routing (Lec. 13 ~ 15) Overlay routing (Lec. 17) P2P, or CDN routing (Lec. 18) Ethernet bridging (Lec. 10, 11) IP Layer App Layer Link Layer4 7 Ethernet Bridges Self-learn Host Info. Bridges (switches) forward frames selectively Forward frames only on segments that need them Switch table Maps destination MAC address to outgoing interface Goal: construct the switch table automatically switch A!B!C!D!8 Self Learning: Building the Table When a frame arrives Inspect the source MAC address Associate the address with the incoming interface Store the mapping in the switch table Use a time-to-live field to eventually forget the mapping A!B!C!D!Switch learns how to reach A.!5 9 Self Learning: Handling Misses Floods when frame arrives with unfamiliar dst or broadcast address Forward the frame out all of the interfaces … except for the one where the frame arrived Hopefully, this case won’t happen very often A!B!C!D!When in doubt, shout!!10 Flooding Can Lead to Loops Flooding can lead to forwarding loops, confuse bridges, and even collapse the entire network E.g., if the network contains a cycle of switches Either accidentally, or by design for higher reliability6 11 Solution: Spanning Trees Ensure the topology has no loops Avoid using some of the links when flooding … to avoid forming a loop Spanning tree Sub-graph that covers all vertices but contains no cycles Links not in the spanning tree do not forward frames 12 Interaction with the Upper Layer (IP) Bootstrapping end hosts by automating host configuration (e.g., IP address assignment) DHCP (Dynamic Host Configuration Protocol) Broadcast DHCP discovery and request messages Bootstrapping each conversation by enabling resolution from IP to MAC addr ARP (Address Resolution Protocol) Broadcast ARP requests Both protocols work via Ethernet-layer broadcasting (i.e., shouting!)7 13 Broadcast Domain and IP Subnet Ethernet broadcast domain A group of hosts and switches to which the same broadcast or flooded frame is delivered Note: broadcast domain != collision domain Broadcast domain == IP subnet Uses ARP to reach other hosts in the same subnet Uses default gateway to reach hosts in different subnets Too large a broadcast domain leads to Excessive flooding and broadcasting overhead Insufficient security/performance isolation New Challenges to Ethernet, and SEATTLE as a solution8 15 “All-Ethernet” Enterprise Network? “All-Ethernet” makes network mgmt easier Flat addressing and self-learning enables plug-and-play networking Permanent and location independent addresses also simplify Host mobility Access-control policies Network troubleshooting 16 But, Ethernet Bridging Does Not Scale Flooding-based delivery Frames to unknown destinations are flooded Broadcasting for basic service Bootstrapping relies on broadcasting Vulnerable to resource exhaustion attacks Inefficient forwarding paths Loops are fatal due to broadcast storms; uses the STP Forwarding along a single tree leads to inefficiency and lower utilization9 17 State of the Practice: A Hybrid Architecture Enterprise networks comprised of Ethernet-based IP subnets interconnected by routers R R R R Ethernet Bridging - Flat addressing - Self-learning - Flooding - Forwarding along a tree IP Routing (e.g., OSPF) - Hierarchical addressing - Subnet configuration - Host configuration - Forwarding along shortest paths R Broadcast Domain (LAN or VLAN) 18 Motivation Neither bridging nor routing is satisfactory. Can’t we take only the best of each? Architectures Features Ethernet Bridging IP Routing Ease of configuration Optimality in addressing Host mobility Path efficiency Load distribution Convergence speed Tolerance to loop SEATTLE (Scalable Ethernet ArchiTecTure for Larger Enterprises) SEATTLE 10 19 Overview Objectives SEATTLE architecture Evaluation Applications and benefits Conclusions 20 Overview: Objectives Objectives Avoiding flooding Restraining broadcasting Keeping forwarding tables small Ensuring path efficiency SEATTLE architecture Evaluation Applications and Benefits Conclusions11 21 Avoiding Flooding Bridging uses flooding as a routing scheme Unicast frames to unknown destinations are flooded Does not scale to a large
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