Johan J. Lukkien, [e-mail]

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Computer Networks2002/2003

• Connections
• Johan Lukkien

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Connections

• How do we build a network?
• connect to a multi-access wire
• connect two existing networks using a multi-port
  device that forwards packets/messages
• choose the lowest common layer of these
  connectors
• Application layer an application gateway
• Transport layer a gateway
• Network layer a router
• Data Link layer a bridge or (layer 2) switch
• Physical layer a hub (layer 1 switch) or
  repeater
• perform protocol translation (if necessary) in
  this layer
• use a point-to-point connection to an access
  point
• Note gateway is a general term for these
  connecting devices

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Connecting...

• Higher layers dont exist in the connector
• Point-to-access point half this picture

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Hubs, bridges, switches

• A hub
• physical layer (collision domain) or packet based
  (broadcast domain)
• A bridge
• A switch
• broadcast domain concurrent connections

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Bridge

• Connect two networks through frame transport
• transparently
• invisible to connected stations
• not entirely possible in case of different
  standards
• need to buffer frames temporarily
• speed, availability differences
• leads to potential frame dropping
• effectively, constructs a single broadcast domain
  out of several networks
• flooding adjustments
• originally two, but currently many ports
• distinction bridge-switch is floating

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Bridge operation

• Flooding
• Promiscuous listening on all ports
• Upon receipt of a packet via a port p
• transmit packet via each port q, q ltgt p
• Notes
• this bridge does not need a link layer address,
  in principle
• though standard port hardware has it integrated

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Example 802.11 to 802.3
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Bridges Local Internetworking

• Easy method of interconnection
• plug and play
• place of connection irrelevant for higher layers
• Bandwidth sharing
• potentially, minimum of LANs
• Need acyclic topology

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Cycles...

• Note transparency forbids to include aging in
  a frame

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Limit the broadcasting

• Configure
• configuration database in bridge
• assign address ranges to stations in a particular
  way
• e.g. part of the address determines the LAN
• difficult because of the MAC address structure
• conflicts stability, auto-configuration
  requirements
• Learning bridges
• construct dynamically a mapping from MAC
  addresses to ports
• inspect sources in packets and build table
• needs a potentially large amount of memory
• this information is only temporarily valid soft
  state
• same problem with cycles

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Learning bridge operation

• Promiscuous listening on all ports
• Each packet received on a port p
• store (packet source, p) in cache
• search packet destination in cache
• if found, with associated port q
• send packet (only) to port q, if qltgtp
• if not found
• send packet to all ports, except p
• Each cache entry is deleted after an aging period
  has elapsed since last write
• After moving a station
• just send one multicast message

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Spanning tree (802.1d)

• Graph
• nodes the LANs
• edges exist between two nodes if there is a
  bridge connecting the two LANs
• Spanning tree
• acyclic, connected subgraph
• Dynamically established and maintained
• bridges must be able to talk to each-other
• need a communication protocol and an addressing
  mechanism, separate from the regular traffic
• configuration bridge protocol data units ?
• SAP 01000010 (palindrome, 1642)
• destination address special for all bridges

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Spanning Tree Bridges
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Distributed spanning tree algorithm

• Purpose
• decide and maintain a single spanning tree
• in a distributed fashion
• implicit leader election
• the root of the spanning tree
• Context
• Each bridge has an identifier id
• Each bridge knows the cost of each of its links
  (ports)
• cost equal to everything reached via that link
• Each bridge stores per port p a minimal received
  message min(p) (meaning described later)
• this value ages, and must be refreshed
• Assumptions
• bi-directional links
• cost is equal in both directions, and positive

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Variables and invariants

• Root, r
• minimum own id, ids in min(p), all ports
• If rltgtid
• root distance, rd (MIN p min(p).costcost(p))
  
• rd 0, if rid
• root port, rp
• min(rp).cost cost(rp) md
• (min(rp).transmitter_id, min(rp).port_id) is
  minimal
• Bridge is responsible (designated) for forwarding
  on those ports p such that
• (r, md, id, p) lt min(p)

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Distributed spanning tree algorithm

• Algorithm
• For all port p, set min(p) to (8, 8, 8, 0)
• Send (id root , 0 cost , id transmitter
  , p) over all ports p
• Upon receipt of m via p
• if mltmin(p) then
• min(p) m
• for all ports q with (m.id, m.costcost(p), id,
  q) lt min(q)
• send (m.id, m.costcost(p),id, q) via q
• fi

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Using the spanning tree

• For normal operation
• the broadcast (flooding) algorithm restricted to
  ports in the spanning tree
• these are rp and the ones for which the bridge is
  designated
• all other ports are blocked
• The root transmits keep-alive messages
  regularly
• these serve as a means to refresh min(p)
• configuration messages contain an age field

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Re-configuration

• Timeout on any port
• discard min(rp), make it infinite
• compute new messages to be sent to neighbors,
  e.g.
• nothing, or just change root port
• find that a new root is needed may cause new
  connections
• in the spanning tree
• Configuration message of neighbor
• act according to the algorithm
• Issues
• convergence speed include age field in
  configuration message
• avoid temporary loops
• delay before switching from blocked to forwarding
  ... (2 x max. broadcast delay)
• just forward configuration messages during this
  period
• 802.1 listening / learning
• effect on station caches must broadcast topology
  change

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Bridge station caches

• Packets from S get to all networks
• Based on the direction, the caches are adapted
• B3 and B4 have a different idea about the
  direction of S
• Must use different identifications for S

Z
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Transparent?

• Increased probability of
• packet loss
• errors
• packet reordering and duplicates may now occur
• Increase of
• delay
• packet life time
• Maximum packet size is LAN-dependent
• LAN-specific information gets lost
• e.g. ethernet / 802.3
• Assumptions
• MAC address unique
• a receiving station will also transmit

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Remote Bridges

• Tunnel traffic through a long-distance
  point-to-point link half-bridges

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Virtual LANs

1. Four physical LANs organized into two VLANs, gray
   and white, by two bridges.
2. The same 15 machines organized into two VLANs by
   switches.