Download Lesson 07 Bridges, Hub, Switches and Routers

Lesson 07
Bridges, Hub, Switches and Routers
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Until now
The Internet is a graph
‒ Leaves are hosts (laptops, servers, fridges…)
↪ terminate traffic on the Internet
‒ Nodes are routers
↪ get traffic across the Internet
‒ Edges are … but what are edges?
Edges
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Element in graph
Local connectivity
Ethernet
A “layer 2” thingie
Abstraction of a cable
Functional definition
‒ No IP forwarding required in order for any station to reach any other station on same link
‒ A station can, in a single transmission, reach all other stations on the same link
‒ All broadcasts on the same link will reach the same set of stations
Application
Transport
Network
Link
Stuff, more or less faking the
assumptions and expectations of the
IP layer…
Network segment
Starting point… or line…
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Physical Entity
Commonly assumed: a cable
Constrained by physic
Signal attenuation: maximum length
10base5 = 500 m
10base2 = 185 m (200 yd)
1000baseT = 100 m
Propagation delay
Beyond the limits: Repeaters
Two cables can be joined by a repeater
‒ Connect (in general) 2 “cables”
‒ Each up-to max length
‒ Regenerate physical signal
This allows to extend the network segment… beyond any limit?
Multiple Access
Edges are not edges…
What is required from a network segment:
‒ Communication medium shared between all stations “on the
segment”
‒ “Collision domain”
‒ Contention for exclusive access
‒ Medium access control (…MAC)
‒ Mutual exclusion with no “external support”
‒ More stations, more contentions, less throughput and longer delays
Medium sharing using the medium
‒ “Listen while transmit”
‒ If transmission, other than own, received:
↪ COLLISION, back off, retry later
‒ Otherwise, transmission succeeded
Yet… Is it true?
Transmission delay
Otherwise… When is the transmission considered successful?
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One want a way to be sure no collision occurred
Add a limit to the longest possible path between two hosts (232 bits)
Compute the maximum round-time trip (464 bits)
Ensure a minimum packet length so that a packet is longer than the round-time trip plus
some margin to have a noticeable collision (512 bits = 64 bytes)
‒ Under these assumptions, collision only occur while transmitting
Time
Collision
Space
a
b
c
Ethernet
Going wireless
Wireless networks adds a new challenge
↪ Host c may not hear host a
In managed mode, the access point acts as a mediator since it hears every-one
‒ Hosts request medium access with a RTS (request to send)
‒ Access point answers CTS (clear to send) is the medium is free
‒ If the medium is busy, back-off and retry
Physical layer
Cables and repeaters…
Link
‒ Can transmit bits from hosts to others
‒ May detect collisions
‒ Has physical contraints
Application
Transport
Network
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Physical
Can we do better?
Bridges
And bridges were invented…
Link
‒ Bridges can separate collision domains
↪ join two network segments
‒ Forward traffic between segments
‒ Smarter than repeater
‒ Filter on traffic destination
‒ Messages are:
‒ Read for information
‒ Stored in a queue (of bounded size)
Application
Transport
Network
MAC
Physical
Bridge filtering
A bridge connects some network segments
‒ Forwards a message only to the segment the target host belongs to
‒ Learns where hosts are from the messages they send
‒ Records hosts location in the bridging table
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Algorithm:
When the bridge receives:
a message m for host t
received on port p
↪ physical port (contrary to TCP/UDP port)
originated from host s
If the bridge does not know about t:
forward
record (s,p) in the bridging table
Else if (t,p’) is in the bridging table
If p=p’, do nothing
Else forward to port p’
record (s,p) in the bridging table
Do not record broadcast addresses
Link
One segment or some collision domains joined by bridges make a link:
“A set of network interfaces which can communicate directly with each other, without relaying
by an intermediate router”
Quote from previous lesson
Beyond the limits
Cables can be limitedly connected into some collision
domain
Collision domains can be connected into some link
But what is the limit? Why not make the whole Internet of
segments?
‒ Each bridge records, explicitly, all destinations on the
link
‒ Limit is, therefore, memory in the “smallest bridge”
bridges and
Logical and physical addresses
Hosts on the Internet are identified by their IP addresses
Hosts on a link are identified by their MAC addresses
‒ MAC address factory set in network adapter and bridges
↪ (at least for Ethernet) 48 bits
‒ first 24 make the vendor ID
‒ next 24 are uniquely assigned by the vendor
‒ Common prefix not geographically aggregated
↪ not suitable for routing
‒ Way to get MAC address from IP address required
ARP
What happens when a host want to send or forward an IP packet:
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Routing table tells
What is the IP address a of next target for the packet (next-hop or addressee)
On which link this host is located (and directly reachable)
Broadcast “who-as a” on the link
The host whose IP is a replies: “a is-at m”
Encapsulated the IP packet in a Ethernet frame:
Source MAC address: of the sending network interface
Destination MAC address: m
IP header:
Source IP: unchanged, creator of the IP packet
Destination IP: unchanged, addressee of the IP packet
Transmit
MAC addresses are stored on the host in the ARP table
Often, the target of an ARP request checks the MAC address of the requester by another ARP
request
Let us get real
What… This was not real?
‒ A repeater is implemented in an Ethernet hub
↪ but who uses that anymore?
‒ A bridge is implemented in a switch
‒ Yet a bridge do not know anything about IP and do not care about IP headers
‒ Often switches are dealing with IP
‒ IP address for configuration (argh… so confusing)
‒ Some switches are IP aware (e.g. limit a port to a given IP address… reminds you
something?)
‒ Some Cisco switches are routers is disguise (see the switches they lent to BR)
Nowadays
Today’s networks only has switches and no more hubs
‒ Equipment is connected full-duplex one-another
↪ one twisted pair for each direction
‒ No more collision… in wired connections
‒ CSMA/CD still used but only CSMA/CA does some work
Wired packets can still be lost in hosts’ and switches’ queues
Switches and loops
Suppose 1 sends a packet to 2
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B forward to A and C
A and C learns 1 is behind port “to B”
A and C forward to A and C
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Worse if 1 broadcasts…
STP
STP = Spanning Tree Protocol
↪ obsoleted by RSTP = Rapid STP
Distributed algorithm to compute a spanning tree of the “graph” made by switches (nodes) and
collision domains (edges)
Each switch sends BPDU on all their ports:
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Switch ID = smallest MAC address of their ports
Port ID = MAC address of the port
Root ID = lowest known switch ID
Cost = distance to the Root
For the BPDU received from a port:
‒ You are the best if you are the Root or the shortest path to the Root (tie broken by Switch
ID then by Port ID)
‒ As the Root is the smallest known switch, those with wrong Root are disqualified
As all members of a segment receive the same BPDU, all compute the same winner:
‒ It becomes the designated port for this segment (we could say designated switch… unless
there are two ports on the same switch on the same segment)
Each switch updates their status depending of the previous computation done for each of their
ports:
‒ Root updated (the smallest Root among those of the designated port of each segment)
‒ Cost updated (one plus the cost of the designated port corresponding to the Root or 0 for
the Root itself)
‒ Smallest port connected to this segment becomes the root port
After some time of stability, any port which is neither root nor designated is blocked (not
forwarding yet listening for incoming BPDU)
Never say that a switch routes
An analogous role but different protocols and schemes:
‒ Routing table vs. bridging table
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IP address vs. MAC address
Routing protocol vs. STP and learning algorithm
Scalable vs. not so scalable
link can indeed be very large (geographically)
but only contains a small number of hosts
Switch does not manipulate frames while routers changes the TTL and a few other minor things