Connections Johan Lukkien Computer Networks 2002/2003 Connections Johan Lukkien 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

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

Connecting... Higher layers don’t exist in the connector Point-to-access point: half this picture application gateway router bridge repeater 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Hubs, bridges, switches A hub A bridge A switch physical layer (collision domain) or packet based (broadcast domain) A bridge A switch broadcast domain: concurrent connections 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

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

Bridge operation Flooding: Notes “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 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Example: 802.11 to 802.3 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

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

Cycles... Note: transparency forbids to include “aging” in a frame 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

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

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

Spanning tree (802.1d) Graph Spanning tree 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 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Spanning Tree Bridges 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

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

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

Distributed spanning tree 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 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

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

Re-configuration Timeout on any port Configuration message of neighbor 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 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

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

Transparent? Increased probability of Increase 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 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

Remote Bridges Tunnel traffic through a long-distance point-to-point link “half-bridges” 2-Jan-19 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

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