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Interconnection: Switching and Bridging. CS 4251: Computer Networking II Nick Feamster Spring 2008. In This Lecture. How hosts find each other on a subnet Address Resolution Protocol (ARP) Broadcast Interconnecting subnets Switches: Forwarding and filtering Self-learning bridges

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interconnection switching and bridging

Interconnection: Switching and Bridging

CS 4251: Computer Networking IINick FeamsterSpring 2008

in this lecture
In This Lecture
  • How hosts find each other on a subnet
    • Address Resolution Protocol (ARP)
    • Broadcast
  • Interconnecting subnets
    • Switches: Forwarding and filtering
    • Self-learning bridges
    • Spanning tree protocols
  • Switches vs. Hubs
  • Swtiches vs. Routers
  • Can Ethernet scale to a million nodes?
    • VLANs
    • Other alternatives
bootstrapping networks of interfaces

frame

frame

Bootstrapping: Networks of Interfaces
  • LAN/Physical/MAC address
    • Flat structure
    • Unique to physical interface (no two alike)…how?

datagram

receiver

link layer protocol

sender

adapter

adapter

  • Frames can be sent to a specific MAC address or to the broadcast MAC address

What are the advantages to separating network layer from MAC layer?

arp ip addresses to mac addresses
ARP: IP Addresses to MAC addresses
  • Query is IP address, response is MAC address
  • Query is sent to LAN’s broadcast MAC address
  • Each host or router has an ARP table
    • Checks IP address of query against its IP address
    • Replies with ARP address if there is a match

Potential problems with this approach?

  • Caching on hosts is really important
    • Try arp –a to see an ARP table
life of a packet on a subnet
Life of a Packet: On a Subnet
  • Packet destined for outgoing IP address arrivesat network interface
    • Packet must be encapsulated into a frame with the destination MAC address
  • Frame is sent on LAN segment to all hosts
  • Hosts check destination MAC address against MAC address that was destination IP address of the packet
interconnecting lans
Interconnecting LANs
  • Receive & broadcast (“hub”)
  • Learning switches
  • Spanning tree (RSTP, MSTP, etc.) protocols
interconnecting lans with hubs
Interconnecting LANs with Hubs
  • All packets seen everywhere
    • Lots of flooding, chances for collision
  • Can’t interconnect LANs with heterogeneous media (e.g., Ethernets of different speeds)

hub

hub

hub

hub

problems with hubs no isolation
Problems with Hubs: No Isolation
  • Scalability
  • Latency
    • Avoiding collisions requires backoff
    • Possible for a single host to hog the medium
  • Failures
    • One misconfigured device can cause problems for every other device on the LAN
improving on hubs switches
Improving on Hubs: Switches
  • Link-layer
    • Stores and forwards Ethernet frames
    • Examines frame header and selectively forwards frame based on MAC dest address
    • When frame is to be forwarded on segment, uses CSMA/CD to access segment
  • Transparent
    • Hosts are unaware of presence of switches
  • Plug-and-play, self-learning
    • Switches do not need to be configured
switch traffic isolation
Switch: Traffic Isolation
  • Switch breaks subnet into LAN segments
  • Switch filters packets
    • Same-LAN-segment frames not usually forwarded onto other LAN segments
    • Segments become separate collision domains

switch

collision domain

hub

hub

hub

collision domain

collision domain

filtering and forwarding

LAN B

B

A

C

LAN C

LAN A

Filtering and Forwarding
  • Occurs through switch table
  • Suppose a packet arrives destined for node with MAC address x from interface A
    • If MAC addressnot in table, flood (act like a hub)
    • If MAC addressmaps to A, do nothing (i.e. drop the packet - packet destined for same LAN segment)
    • If MAC address maps to another interface, forward
  • How does this table get configured?
advantages vs hubs
Advantages vs. Hubs
  • Better scaling
    • Separate collision domains allow longer distances
  • Better privacy
    • Hosts can “snoop” the traffic traversing their segment
    • … but not all the rest of the traffic
  • Heterogeneity
    • Joins segments using different technologies
disadvantages vs hubs
Disadvantages vs. Hubs
  • Delay in forwarding frames
    • Bridge/switch must receive and parse the frame
    • … and perform a look-up to decide where to forward
    • Storing and forwarding the packet introduces delay
    • Solution:cut-through switching (faster, but more errors are possible).
  • Need to learn where to forward frames
    • Bridge/switch needs to construct a forwarding table
    • Ideally, without intervention from network administrators (which creates overhead)
    • Solution:self-learning
motivation for self learning
Motivation For Self-Learning
  • 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

B

A

C

switch

D

self learning bridges
(Self)-Learning Bridges
  • Switch is initially empty
  • For eachincoming frame, store:
    • The incoming interface from which the frame arrived
    • The time at which that frame arrived
    • Delete the entry if no frames with a particular source address arrive within a certain time

B

Switch learns how to reach A.

A

C

D

cut through switching
Cut-Through Switching
  • Buffering a frame takes time
    • Suppose L is the length of the frame
    • And R is the transmission rate of the links
    • Then, receiving the frame takes L/R time units
  • Buffering delay can be a high fraction of total delay, especially over short distances

A

B

switches

cut through switching1
Cut-Through Switching
  • Start transmitting as soon as possible
    • Inspect the frame header and do the look-up
    • If outgoing link is idle, start forwarding the frame
  • Overlapping transmissions
    • Transmit the head of the packet via the outgoing link
    • … while still receiving the tail via the incoming link
    • Analogy: different folks crossing different intersections

A

B

switches

limitations on topology
Limitations on Topology
  • Switches sometimes need to broadcast frames
    • Unfamiliar destination: Act like a hub
    • Sending to broadcast
  • Flooding can lead to forwarding loops and broadcast storms
    • E.g., if the network contains a cycle of switches
    • Either accidentally, or by design for higher reliability

Worse yet, packets can be duplicated and proliferated!

solution spanning trees
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 treedo not forward frames
constructing a spanning tree
Constructing a Spanning Tree
  • Elect a root
    • The switch with the smallest identifier
  • Each switch identifies if its interfaceis on the shortest path from the root
    • And it exclude from the tree if not
    • Also exclude from tree if same distance,but higher identifier
  • Message Format: (Y, d, X) i.e.,
  • (Root-Distance-Node)
    • From node X
    • Claiming Y as root
    • Distance is d

root

One hop

Three hops

steps in spanning tree algorithm
Steps in Spanning Tree Algorithm
  • Initially, every switch announces itself as the root
    • Example: switch X announces (X, 0, X)
  • Switches update their view of the root
    • Upon receiving a message, check the root id
    • If the new id is smaller, start viewing that switch as root
  • Switches compute their distancefrom the root
    • Add 1 to the distance received from a neighbor
    • Identify interfaces not on a shortest path to the root and excludethose ports from the spanning tree
example from switch 4 s viewpoint
Example From Switch # 4’s Viewpoint

1

  • Switch #4 thinks it is the root
    • Sends (4, 0, 4) message to 2 and 7
  • Switch #4 hears from #2
    • Receives (2, 0, 2) message from 2
    • … and thinks that #2 is the root (lower id)
    • And realizes it is just one hop away
  • Switch #4 hears from #7
    • Receives (2, 1, 7) from 7
      • (i.e., 2 is the root, it’s 1 hop from “me”, I’m node 7.)
    • And realizes this is a longer path
      • because it adds 1 to the path from 7 – 2, wh/is already 1 hop, so 4-7-2 = 2 hops
    • So, prefers its own one-hop path (4-2 =1hop)
    • And removes 4-7 link from the tree

3

5

2

4

6

7

robust spanning tree algorithm
Robust Spanning Tree Algorithm
  • Algorithm must react to failures
    • Failure of the root node
      • Need to elect a new root, with the next lowest identifier
    • Failure of other switches and links
      • Need to recompute the spanning tree
  • Root switch continues sending messages
    • Periodically reannouncing itself as the root (1, 0, 1)
    • Other switches continue forwarding messages
  • Detecting failures through timeout
  • Switch waits to hear from others
    • Eventually times out and claims to be the root
extension virtual lans
Extension: Virtual LANs
  • Partition a single switched LAN into several virtual ones
    • Switched LANs do not scale well to large networks
    • Spanning tree algorithm has linear scaling behavior
    • Some frames are broadcast
  • Group users/hosts based on organizational structure, rather than physical location
    • Improve privacy and isolation
    • Exploit locality
  • Avoid physical rewiring
  • More in Lec. 12 (Plus, Network Layers as Link Layers)
switches vs routers
Switches vs. Routers

Switches

  • Switches are automatically configuring
  • Forwarding tends to be quite fast, since packets only need to be processed through layer 2

Routers

  • Router-level topologies are not restricted to a spanning tree
    • Can even have multipath routing
scaling ethernet
Scaling Ethernet
  • Main limitation: Broadcast
    • Spanning tree protocol messages
    • ARP queries
  • High-level proposal: Distributed directory service
    • Each switch implements a directory service
    • Hosts register at each bridge
    • Directory is replicated
    • Queries answered locally
  • …are there other ways to do this?
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