1. Connections Johan Lukkien
Computer Networks 2002/2003
Connections
Johan Lukkien
2-Jan-19
Johan J. Lukkien,
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
2-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

3. 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,
TU/e Computer Science, System Architecture and Networking

4. 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,
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
2-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

6. 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,
TU/e Computer Science, System Architecture and Networking

7. 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

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
2-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

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

12. 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 = (palindrome, 16#42)
destination address: special for “all bridges
2-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

13. Spanning Tree Bridges 2-Jan-19 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
2-Jan-19
Johan J. Lukkien,
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)
2-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

16. 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,
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
2-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

18. 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,
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
2-Jan-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

20. 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,
TU/e Computer Science, System Architecture and Networking

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