Hubs, Bridges and Switches
description
Transcript of Hubs, Bridges and Switches
Lecture 3 #1
Hubs, Bridges and Switches
Lecture 3
Lecture 3 #2
Interconnecting LANs
Q: Why not just one big LAN? Limited amount of supportable traffic: on
single LAN, all stations must share bandwidth limited length: 802.3 (Ethernet) specifies
maximum cable length (need small a) large “collision domain” (can collide with many
stations) limited number of stations: 802.5 (token ring)
have token passing delays at each station
Lecture 3 #3
Hubs Physical Layer devices: essentially repeaters
operating at bit levels: repeat received bits on one interface to all other interfaces
Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top
Lecture 3 #4
Hubs (more)
Each connected LAN referred to as LAN segment Hubs do not isolate collision domains: node may
collide with any node residing at any segment in LAN
Hub Advantages: simple, inexpensive device Multi-tier provides graceful degradation: portions
of the LAN continue to operate if one hub malfunctions
extends maximum distance between node pairs (100m per Hub)
Lecture 3 #5
Hub limitations
single collision domain results in no increase in max throughput multi-tier throughput same as single
segment throughput individual LAN restrictions pose limits on
number of nodes in same collision domain and on total allowed geographical coverage
cannot connect different Ethernet types (e.g., 10BaseT and 100baseT) Why?
What happens to a?
Lecture 3 #6
Bridges
Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit
Lecture 3 #7
Bridges (more)
Bridge advantages: Isolates collision domains resulting in higher
total max throughput, and does not limit the number of nodes nor geographical coverage
Can connect different type Ethernet since it is a store and forward device
Transparent: no need for any change to hosts LAN adapters
Lecture 3 #8
Backbone Bridge
Lecture 3 #9
Interconnection Without Backbone
Not recommended for two reasons:- single point of failure at Computer Science hub- all traffic between EE and SE must pass over CS segment
Lecture 3 #10
Bridges: frame filtering, forwarding
bridges filter packets same-LAN -segment frames not forwarded
onto other LAN segments forwarding:
how to know on which LAN segment to forward frame?
Lecture 3 #11
Bridge Filtering
bridges learn which hosts can be reached through which interfaces: maintain filtering tables when frame received, bridge “learns” location
of sender: incoming LAN segment records sender location in filtering table
filtering table entry: (Node LAN Address, Bridge Interface, Time
Stamp) stale entries in Filtering Table dropped (TTL can
be 60 minutes)
Lecture 3 #12
Bridge Operation
bridge procedure(in_MAC, in_port,out_MAC)Set filtering table (in_MAC) to in_port /*learning*/lookup in filtering table (out_MAC) receive out_portif (out_port not valid) /* no entry found for destination */
then flood; /* forward on all but the interface on which the frame arrived*/
if (in_port = out_port) /*destination is on LAN on which frame was received */
then drop the frame
Otherwise (out_port is valid) /*entry found for destination */
then forward the frame on interface indicate
Lecture 3 #13
Bridge Learning: example
Suppose C sends frame to D and D replies back with frame to C
C sends frame, bridge has no info about D, so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D
Lecture 3 #14
Bridge Learning: example
D generates reply to C, sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1, so selectively
forwards frame out via interface 1
C 1
Lecture 3 #15
What will happen with loops?Incorrect learning
A
B
1 1
22
A , 1 A , 122
Lecture 3 #16
What will happen with loops?Frame looping
A
C
1 1
22
C,?? C,??
Lecture 3 #17
What will happen with loops?Frame looping
A
B
1 1
22
B,2 B,1
Lecture 3 #18
Loop-free: tree
A
B
C
A message from Awill mark A’s location
Lecture 3 #19
Loop-free: tree
A
B
C
A message from Awill mark A’s location
A:
Lecture 3 #20
Loop-free: tree
A
B
CA:
A:
A message from Awill mark A’s location
Lecture 3 #21
Loop-free: tree
A
B
CA: A:
A:
A:
A:
A message from Awill mark A’s location
Lecture 3 #22
Loop-free: tree
A
B
CA: A:
A:
A:
A:
A message from Awill mark A’s location
Lecture 3 #23
Loop-free: tree
A
B
C
A:
A: A:
A:
A:
So a message toA will go by marks…
A message from Awill mark A’s location
Lecture 3 #24
Bridges Spanning Tree for increased reliability, desirable to have redundant,
alternative paths from source to dest with multiple paths, cycles result - bridges may
multiply and forward frame forever solution: organize bridges in a spanning tree by
disabling subset of interfaces
Disabled
Lecture 3 #25
Introducing Spanning Tree
Allow a path between every LAN without causing loops (loop-free environment)
Bridges communicate with special configuration messages (BPDUs)
Standardized by IEEE 802.1D
Note: redundant paths are good, active redundant paths are bad (they cause loops)
Lecture 3 #26
How to construct a spanning tree? Bridges run a distributed spanning tree
algorithm Select what ports (and bridges) should
actively forward frames Standardized in IEEE 802.1 specification
Lecture 3 #27
Overview of STP
We make a series of simplifications: Build a ST of bridges (in fact, need to
span LAN segments!) Assume that we are given a root
bridgeSo we solve in order:1. How to find a root bridge?2. How to compute a ST of bridges?3. How to compute a ST LAN segments?
Lecture 3 #28
1. Choosing a root bridge
Assume each bridge has a unique identifier
Each bridge remembers smallest ID seen so far (my_root_ID)
Periodically, send my_root_ID to all neighbors (“flooding”)
When receiving ID, update if necessary Is that enough?!
Lecture 3 #29
2. Compute ST Given a root
Idea: each node finds its shortest paths to the root shortest paths tree
Output: At each node, parent pointer (and distance)
How: Bellman-Ford algorithm
Lecture 3 #30
Distributed Bellman-Ford
Assumption: There is a unique root node s
Idea: Each node, periodically, tells all its neighbors what is its distance from s
But how can they tell? s: easy. dists = 0 always! Another node v:
Mark neighbor with least distance as “parent”
Lecture 3 #31
Why does this work?
Suppose all nodes start with distance , and suppose that updates are sent every time unit.
CD
B
E
F
G
A 0
Lecture 3 #32
Why does this work?
Suppose all nodes start with distance , and suppose that updates are sent every time unit.
CD
B
E
F
G
A 0
1 1
1
1
Lecture 3 #33
Why does this work?
Suppose all nodes start with distance , and suppose that updates are sent every time unit.
CD
B
E
F
G
A 0
1 1
2
1
1
2
Lecture 3 #34
Why does this work?
Suppose all nodes start with distance , and suppose that updates are sent every time unit.
CD
B
E
F
G
A 0
1 1
2
1
1
3
2
Lecture 3 #35
Bellman-Ford: properties
Works for any non-negative link weights w(u,v):
Works when the system operates asynchronously.
Works regardless of the initial distances! (later...)
Lecture 3 #36
3. ST of LAN segments
Assumption: given a ST of the bridgesIdea: Each segment has at least one bridge
attached. Only one of them should forward packets! Choose bridge closest to root. Break ties by bridge ID
(and then by port ID...)
Implementation: Bridges listen to all distance announcement on each port. Mark port as “designated port” iff best on that port’s LAN
Lecture 3 #37
Spanning Tree Concepts:Path Cost A cost associated with each port on
each bridge (“weight” of the segment) default is 1
The cost associated with transmission onto the LAN connected to the port Can be manually or automatically assigned Can be used to alter the path to the root
bridge
Lecture 3 #38
Spanning Tree Concepts:Root Port Each non-root bridge has a Root port:
The port on the path towards the root bridge parent pointer
The root port is part of the lowest cost path towards the root bridge
If port costs are equal on a bridge, the port with the lowest ID becomes root port
Lecture 3 #39
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation:1. Pick a root2. Each bridge picks a
root port
B8
Lecture 3 #40
Example Spanning Tree
B3
B5
B7B2
B1
B4B6
Root
B4 B5 B6
B8
B1
Spanning Tree:
root port
B3
B7B2
B8
Lecture 3 #41
Spanning Tree Concepts: Designated Port Each LAN has a single designated port This is the port reporting minimum cost
path to the root bridge for the LAN Only designated and root ports remain
active!
Lecture 3 #42
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Forwarding Tree:
Designated Bridge
root port
Note: B3, B6 forward nothing
Lecture 3 #43
Spanning Tree Requirements
Each bridge has a unique identifierA broadcast address for bridges on
a LANA unique port identifier for all ports
on all bridgesBridge id + port number
Lecture 3 #44
Spanning Tree Algorithm:ImplementationKeep pumping a single message:
(my root ID, my cost to root, my ID)BPDU: Bridge Protocol Data UnitUpdate var’s when receiving: My_root_ID: smallest seen so far My_cost_to_root: smallest received to
my_root_ID + link cost Break ties by ID
That’s enough!
Lecture 3 #45
Spanning Tree Algorithm:Select Designated Bridges
Bridges send BPDU frames to its attached LANssender port IDbridge and port ID of the bridge the sending
bridge considers rootroot path cost for the sending bridge
3. Best bridge wins, and it knows it (and winning port) (lowest ID/cost/priority)
Lecture 3 #46
Forwarding/Blocking State
1. Only root and designated ports are active for data forwarding
Other ports are in the blocking state: no forwarding!
If bridge has no designated port, no forwarding at all block root port too.
2. All ports send BPDU messages
To adjust to changes
B3B5
B7B2
B1
B6 B4
Root
B8
Designated Bridge
root port
Lecture 3 #47
Spanning Tree Protocol: Execution
B3
B5
B7B2
B1
B6 B4
B8
(B1,root=B1, dist=0)(B1,root=B1,dist=0)
(B4, root=B1, dist=1)(B6, Root=B1dist=1)
Lecture 3 #48
Bridges vs. Routers both store-and-forward devices
routers: network layer devices (examine network layer headers) bridges are Link Layer devices
routers maintain routing tables, implement routing algorithms
bridges maintain filtering tables, implement filtering, learning and spanning tree algorithms
Lecture 3 #49
Routers vs. Bridges
Bridges + and - + Bridge operation is simpler requiring less
processing- Topologies are restricted with bridges: a
spanning tree must be built to avoid cycles - Bridges do not offer protection from broadcast
storms (endless broadcasting by a host will be forwarded by a bridge)
Lecture 3 #50
Routers vs. Bridges
Routers + and -+ arbitrary topologies can be supported, cycling is
limited by TTL counters (and good routing protocols)+ provide firewall protection against broadcast storms- require IP address configuration (not plug and play)- require higher processing
bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)
Lecture 3 #51
Ethernet Switches
layer 2 (frame) forwarding, filtering using LAN addresses
Switching: A-to-B and A’-to-B’ simultaneously, no collisions
large number of interfaces often: individual hosts,
star-connected into switch Ethernet, but no
collisions!
Lecture 3 #52
Ethernet Switches
cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frameslight reduction in latency
combinations of shared/dedicated, 10/100/1000 Mbps interfaces
Lecture 3 #53
Ethernet Switches (more)Dedicated
Shared
Lecture 3 #54
Optional: Wireless LAN and PPP
Lecture 3 #55
IEEE 802.11 Wireless LAN wireless LANs: untethered (often mobile)
networking IEEE 802.11 standard:
MAC protocol unlicensed frequency spectrum: 900Mhz,
2.4Ghz Basic Service Set (BSS)
(a.k.a. “cell”) contains: wireless hosts access point (AP):
base station BSS’s combined to
form distribution system (DS)
Lecture 3 #56
Ad Hoc Networks Ad hoc network: IEEE 802.11 stations can
dynamically form network without AP Applications:
“laptop” meeting in conference room, car
interconnection of “personal” devicesbattlefield
IETF MANET (Mobile Ad hoc Networks) working group
Lecture 3 #57
IEEE 802.11 MAC Protocol: CSMA/CA802.11 CSMA: sender- if sense channel idle for
DISF sec. then transmit entire frame
(no collision detection)-if sense channel busy
then binary backoff
802.11 CSMA receiver:if received OK return ACK after SIFS
Why?
Lecture 3 #58
IEEE 802.11 MAC Protocol
802.11 CSMA Protocol: others
NAV: Network Allocation Vector
802.11 frame has transmission time field
others (hearing data) defer access for NAV time units
Lecture 3 #59
Hidden Terminal effect
hidden terminals: A, C cannot hear each other obstacles, signal attenuation collisions at B
goal: avoid collisions at B CSMA/CA: CSMA with Collision Avoidance
Lecture 3 #60
Collision Avoidance: RTS-CTS exchange CSMA/CA: explicit
channel reservation sender: send short
RTS: Request To Send receiver: reply with
short CTS: Clear To Send
CTS reserves channel for sender, notifying (possibly hidden) stations
avoid hidden station collisions
Lecture 3 #61
Collision Avoidance: RTS-CTS exchange
RTS and CTS short: collisions less likely, of
shorter duration end result similar to
collision detection IEEE 802.11 allows:
CSMA CSMA/CA: reservations polling from AP
Lecture 3 #62
Point to Point Data Link Control one sender, one receiver, one link:
easier than broadcast link:no Media Access Controlno need for explicit MAC addressinge.g., dialup link, ISDN line
popular point-to-point DLC protocols:PPP (point-to-point protocol)HDLC: High level data link control
(Data link used to be considered “high layer” in protocol stack!)
Lecture 3 #63
PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer
datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency: must carry any bit pattern in the data field
error detection (no correction) No error management (ack) required
connection liveness: detect, signal link failure to network layer
network layer address negotiation: endpoint can learn/configure each other’s network address
Lecture 3 #64
PPP non-requirements
no error correction/recovery no flow control out of order delivery OK no need to support multipoint links
(e.g., polling)
Error recovery, flow control, data re-ordering all relegated to higher layers!!!
Note the difference with 802.11
Reason:
different
error rates
Lecture 3 #65
PPP Data Frame
Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible
multiple control fields Protocol: upper layer protocol to which frame
delivered (eg, PPP-LCP, IP, IPCP, etc)
Lecture 3 #66
PPP Data Frame
info: upper layer data being carried check: cyclic redundancy check (CRC)
for error detection
Lecture 3 #67
Byte Stuffing “data transparency” requirement: data field
must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag?
Sender: adds (“stuffs”) extra < 01111101> byte before each < 01111110> or <01111101> data byte
Receiver: Receive 01111101
• discard the byte, • Next byte is data
Receive 01111110: flag byte
Lecture 3 #68
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
Lecture 3 #69
PPP Data Control ProtocolBefore exchanging network-
layer data, data link peers must
configure PPP link (max. frame length, authentication)
learn/configure network layer information
for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address
Lecture 3 #70
Data Link: Summary
principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing, ARP
various link layer technologies Ethernet hubs, bridges, switches IEEE 802.11 LANs PPP
Chapter 5 Kurose and Ross
Lecture 3 #71
Configuration Messages: BPDU