Outline

• Transparent Learning Bridges
  • learning function
  • distributed spanning tree protocol
• IP Routing Protocols
  • IP address properties
  • RIP
  • OSPF
  • BGP

Learning Bridges

• plug-and-play
• can connect distant sites via point-to-point links
• distributed spanning tree for redundant connections: IEEE 802.1

Learning Bridge Algorithm

1. keep table of <host, port, time> entries
2. entry expires after a few minutes
3. time and port are refreshed whenever packet is received from host
4. when packet for host is received:
   • if host is in table, forward on port
   • otherwise, forward on all ports (flood)

Distributed Spanning Tree Algorithm

1. broadcast configuration messages containing:
   • sender ID
   • ID of root of spanning tree
   • distance in hops from sender to root
2. first message is <self, self, 0>
3. when receiving message, keep new info (adding 1 to distance) if message has:
   • root with smaller ID than our root, or
   • same root but shorter distance, or
   • same root and distance, smaller sender ID
   otherwise, discard message
4. only broadcast packets along spanning tree

In-Class exercise

• form groups of 3-5 people
• each person in the group is connected by a point-to-point link to every other person in the group
• run the IEEE 802.1 algorithm to compute a spanning tree
• use name as bridge ID, and alphabetical comparison to determine which name is "least"
• once the tree is built, one person should send a broadcast packet

IP Address Properties

• Identifier: only one host in Internet has a given IP address at a given time
• Locator: the IP address provides some guidance to the location of the internet host
• Structure: the IP address can be subdivided into a network part, identifying/locating the network that the host is connected to, and a host part, used to identify the individual host within the network (analogy: last name tells us something about geographic area of origin, first name identifies individual).
• Routing is concerned with getting a datagram to the "last-hop" router, i.e. to the destination network. Network-dependent "routing" (e.g. ARP) is used from the last-hop router to the final destination.

IP Routing: RIP

• Routing Information Protocol
• Intradomain routing only (e.g. within hawaii.edu)
• distance vector using hop count as metric
• hop count 16 represents infinity
• RIP v2 allows specification of netmask: bits in netmask are
  • 1 if corresponding address bit is in the network part of the address
  • 0 if corresponding address bit is host part
  example netmask is 255.255.255.0
• routers exchange messages every 30 seconds

IP Routing: OSPF

• Open Shortest Path First
• Link-State protocol
• authentication
• hierarchy: each domain is partitioned into areas
• load balancing: multiple routes to the same destination
• message types:
  • hello: tracking of neighbors' state
  • request state
  • send state
  • acknowledge message

IP Routing: BGP-4

• Border Gateway Protocol
• Internet is arbitrarily connected set of autonomous systems:
  • Stub AS has single connection to Internet
  • Multihomed AS has multiple connections but refuses to carry transit traffic
  • Transit AS will carry transit traffic
• try to find a route, don't try too hard to find the optimal route
• one router in AS is BGP speaker, advertises reachable networks
• not DV or LS: instead, distribute complete paths to destinations
• explicit cancellation: withdrawn routes

IP Routing: Characterization

• summarize, summarize, summarize
• use hierarchy whenever possible:
  • route to networks, not hosts
  • OSPF: route to areas, not networks
  • BGP: route to autonomous systems
• routing protocols can evolve faster than the underlying IP transport