Computer Networks 2002/2003

1. Computer Networks2002/2003 Connections Johan Lukkien Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

2. 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

3. application gateway gateway router bridge repeater Connecting... • Higher layers don’t exist in the connector • Point-to-access point: half this picture Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

4. Hubs, bridges, switches • A hub • physical layer (collision domain) or packet based (broadcast domain) • A bridge • A switch • broadcast domain: concurrent connections Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

5. 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

6. Bridge operation • Flooding: • “Promiscuous” listening on all ports • Upon receipt of a packet via a port p • transmit packet via each port q, q <> p • Notes • this bridge does not need a link layer address, in principle • though standard port hardware has it integrated Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

7. Example: 802.11 to 802.3 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

8. 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

9. Cycles... • Note: transparency forbids to include “aging” in a frame Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

10. 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

11. 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 q<>p • 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

12. 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, 16#42) • destination address: special for “all bridges Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

13. Spanning Tree Bridges Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

14. 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

15. Variables and invariants • Root, r • minimum own id, id’s in min(p), all ports • If r<>id • root distance, rd = (MIN p :: min(p).cost+cost(p)) • rd = 0, if r=id • 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) < min(p) Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

16. Distributed spanning tree algorithm • Algorithm • For all port p, set min(p) to (∞, ∞, ∞, 0) • Send (id { root }, 0 { cost }, id { transmitter }, p) over all ports p • Upon receipt of m via p • if m<min(p) then • min(p) := m; • for all ports q with (m.id, m.cost+cost(p), id, q) < min(q) • send (m.id, m.cost+cost(p),id, q) via q • fi Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

17. 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

18. 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

19. 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

20. 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

21. Remote Bridges • Tunnel traffic through a long-distance point-to-point link “half-bridges” Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

22. Virtual LANs • Four physical LANs organized into two VLANs, gray and white, by two bridges. • The same 15 machines organized into two VLANs by switches. Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking