LAN Ethernet, Multicast

1
LAN (Ethernet), Multicast
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Why Multicast

• When sending same data to multiple receivers
• better bandwidth utilization
• less host/router processing
• quicker participation
• Application
• Video/Audio broadcast (One sender)
• Video conferencing (Many senders)
• Real time news distribution
• Interactive gaming
• Cluster computing

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IP multicast service model

• Invented by Steve Deering (PhD. 1991)
• Its a different way of routing datagrams
• RFC1112 Host Extensions for IP Multicasting -
  1989
• Senders transmit IP datagrams to a "host group"
• Host group identified by a class D IP address
• Members of host group could be present anywhere
  in the Internet
• Members join and leave the group and indicate
  this to the routers
• Senders and receivers are distinct i.e., a
  sender need not be a member
• Routers listen to all multicast addresses and use
  multicast routing protocols to manage groups

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Class D Multicast IP addresses
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IP Multicast Addresses
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IGMP Joining a group

• Example R joins to Group 224.2.0.1
• R sends IGMP Membership-Reportto 224.2.0.1
• DR receives it. DR will start forwarding packets
  for 224.2.0.1 to Network A
• DR periodically sends IGMP Membership-Query to
  224.0.0.1 (ALL-SYSTEMS.MCAST.NET)
• R answers IGMP Membership-Report to 224.2.0.1

IGMP Membership-Report
R
Network A
DR
Data to 224.2.0.1
Network B
R ReceiverDR Designated Router
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IGMP Leaving a group

• Example R leaves from a Group 224.2.0.1
• R sends IGMP Leave-Group to 224.0.0.2
  (ALL-ROUTERS.MCAST.NET)
• DR receives it.
• DR stops forwarding packets for 224.2.0.1 to
  Network A if no more 224.2.0.1 group members on
  Network A.

IGMP Leave-Group
R
Network A
DR
Data to 224.2.0.1
Network B
R ReceiverDR Designated Router
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RPF(reverse path forwarding)

• Simple algorithm developed to avoid duplicate
  packets on multi-access links
• RPF algorithm takes advantage of the IP routing
  table to compute a multicast tree for each
  source.
• RPF check
• When a multicast packet is received, note its
  source (S) and interface (I)
• If I belongs to the shortest path from S,
  forward to all interfaces except I
• If test in step 2 is false, drop the packet
• Packet is never forwarded back out the RPF
  interface!

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Protocol Independent Multicast

• PIM Protocol Independent Multicast
• Independent of particular unicast routing
  protocol
• Most popular multicast routing protocol today
• PIM supports both dense (DM) and sparse (SM) mode
  operation
• Opt out (NACK) type (DM)
• Start with broadcasting then prune brunches
  with no receivers, to create a distribution tree
• Lots of wasted traffic when there are only a few
  receivers and they are spread over wide area
• Opt in (ACK) type (SM)
• Forward only to the hosts which explicitly joined
  to the group
• Latency of join propagation

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PIM DM overview

• Assumes that you have lots of folks who want to
  be part of a group
• Based on broadcast and prune
• Ideal for dense group
• Source tree created on demand based on RPF rule
• If the source goes inactive, the tree is torn
  down
• Easy plug-and-play configuration
• Branches that dont want data are pruned

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PIM DM overview

• Grafts used to join existing source tree
• Asserts used to determine the forwarder for
  multi-access LAN
• Non-RPF point-2-point links are pruned as a
  consequence of initial flooding

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PIM-DM(1)Initial flood of data
S
Source
A
B
G
F
C
D
H
I
E
R1
R2
Receiver 1
Receiver 2
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PIM-DM(2)prune non-RPF p2p link
S
IGMP PIM-Prune
Source
A
B
G
F
C
D
H
I
E
R1
R2
Receiver 1
Receiver 2
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PIM-DM(3) C and D Assert to DetermineForwarder
for the LAN, C Wins
S
IGMP PIM-Assertwith its own IP address
Source
A
B
G
F
C
D
H
I
E
R1
R2
Receiver 1
Receiver 2
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PIM-DM(4)I, E, G send PruneH send Join to
override Gs Prune
S
IGMP PIM-Prune
Source
IGMP PIM-Join
A
B
G
F
C
D
H
I
E
R1
R2
Receiver 1
Receiver 2
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PIM-DM(5)I Gets PrunedEs Prune is Ignored
(since R1 is a receiver)Gs Prune is Overridden
(due to new receiver R2)
S
Source
A
B
G
F
C
D
H
I
E
R1
R2
Receiver 1
Receiver 2
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PIM-DM(6)New Receiver, I send Graft
S
IGMP PIM-Graft
Source
A
B
G
F
C
D
H
I
E
R1
R2
Receiver 1
R3
Receiver 2
Receiver 3
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PIM-DM(6)new branch
S
IGMP PIM-Graft
Source
A
B
G
F
C
D
H
I
E
R1
R2
Receiver 1
R3
Receiver 2
Receiver 3
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Multicast Scope ControlTTL Boundaries

• to keep multicast traffic within an
  administrative domain, e.g., for privacy or
  resource reasons

the rest of the Internet
TTL threshold set oninterfaces to these
links,greater than the diameterof the admin.
domain
an administrative domain
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Direct connection broadcast

• Shared media

Metcalfes Ethernet Sketch (1973)

• Ethernet dominant LAN technology
• cheap 30 for 100Mbs!
• first widely used LAN technology
• simpler, cheaper than token LANs and ATM
• kept up with speed race 10, 100, 1000, 10000
  Mbps
• wireless options

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10Mb/s Ethernet Physical Layer

• Each bit has a transition
• Allows clocks in sending and receiving nodes to
  synchronize to each other
• no need for a centralized, global clock among
  nodes!

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Ethernet Format Framing

• Preamble (synchronization)
• 8 bytes, allows sender/receiver clocks to
  synchronize
• Destination/Source Address (hey Paul, Tom here)
• 6 bytes each
• Type
• 2 bytes, indicates higher layer protocol
• 0x0800 is IP, 0x0806 is ARP
• Data 46-1500 bytes
• FCS (CRC)
• catches most transmission errors - errored frames
  dropped

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Ethernet Packet Structure

• 14 byte header
• 2 addresses

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Ethernet Addressing

• 6 byte address (unique to each adapter)
• Example 08-0b-db-e4-b1-02
• 248 281 trillion can produce 100 million LAN
  devices every day for 2000 years!
• Interpretation of address
• Upper 24 bits OUI (Organizationally Unique
  Identifier)
• Lower 24 bits Organization-assigned portion
• Unicast lowest bit of first byte is 0
• Multicast lowest bit of first byte is 1
• Broadcast ff-ff-ff-ff-ff-ff
• Adaptor accept frame if and only if
• Destination address matches adapter address, or
• Destination address is broadcast, or
• Destination address is multicast and adapter has
  been configured to accept it

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Ethernet Media sharing

• CSMA/CD (the polite conversationalist)
• carrier sense dont transmit if you sense
  someone else transmitting
• collision detection abort your transmission if
  you sense someone else transmitting
• random access wait random time before attempting
  a retransmission

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Ethernet Technologies

• 10Base2
• 10Mbps, 200 meters max cable length
• thin coaxial cable in a bus topology
• repeaters connect multiple segments
• 10BaseT / 100BaseT fast ethernet
• 10/100Mbps, Twisted pair
• Nodes connect to a hub in star topology
• Gigabit Ethernet
• 1Gbps, fibre or copper
• Extending from LAN to MAN
• 10 Gbps Ethernet available
• High data speed larger distance increasing
  number of devices per LAN gt switching

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Twisted Pair Wire Map

• EIA/TIA 568B (UGA Standard)

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Standard vs Crossover Cables
Card-to-Hub Wiring (Standard Cable)
RD
TD
TD-
RD-
RD
TD
TD-
RD-
Card-to-Card (Hub-to-Hub) Wiring (Crossover Cable)
TD (RD)
TD (RD)
TD- (RD-)
TD- (RD-)
RD (TD)
RD (TD)
RD- (TD-)
RD- (TD-)
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Power over Ethernet (PoE)
http//www.nwfusion.com/news/2003/1124infrapoe.htm
l
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Ethernet

• Most popular LAN technology nowadays 10Mb/s -
  1Gb/s
• Each host has unique 48bit MAC address (factory
  assigned)
• Frames sent to MAC addresses
• To find destination MAC address, ARP protocol is
  used

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ARP finding the MAC Address
RFC 826 Address Resolution Protocol, 1982
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ARP frame format
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Multicast one to many communication

• Application level one to many communication
• multiple unicasts

• IP multicast

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IP Ethernet Multicast Address Mapping

• IP multicast addresses (class D) range from
  224.0.0.1 to 239.255.255.255 and map to Ethernet
  destination MAC addresses as shown below

32-bit Class D IP Address
Low-order 23 bits of multicast
Not mapped
Group ID copied to Enet address
48-bit Ethernet Address
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Multicast Addresses

• Multicast revises addresses to be protocol
  specific high byte, least bit is 1 if
  multicast.
• Applications that use multicast
• One-to-many IP video broadcasting
• Computing clusters in Grids

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Ethernet Multicast Addresses

• 01-00-5E-00-00-00

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Switching (same as Bridging)

• Goals
• traffic isolation
• transparent operation
• plug-and-play
• Operation
• store and forward Ethernet frames
• examine frame header and selectively forward
  frame based on MAC dest address
• when frame is to be forwarded on segment, uses
  CSMA/CD to access segment

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Switching Tables
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Spanning Tree Protocol
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Spanning tree protocol (IEEE 802.1d)

• Every bridge has bridge-id
• bridge-id 2-byte priority 6-byte MAC addr
• MAC address is 00A0C5123456
• bridge ID is 800000A0C5123456
• Every port of bridge has
• port-id 1-byte priority 1-byte port-number
• port-cost inversely proportional to link speed
• Bridge with lowest bridge-id is root bridge
• On each LAN segment, bridge with lowest path cost
  to root is designated bridge (use bridge-id and
  port-id to break ties)
• A bridge forwards frames through a port only if
  it is a designated bridge for that LAN segment

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STP terminology

• Port roles
• Root port (switch port leading to root)
• Designated port (LAN port leading to root)
• Alternate / backup port (anything else)
• Port states
• Blocking (no send/rcv, except STP bpdus)
• Listening (prepare for learning/forwarding)
• Learning (learn MAC addr but no forwarding)
• Forwarding (send/rcv frames)
• Can disable STP on port or switch
• All frames are forwarded
• BPDUs?

43
STP operation

• BPDU carries 4-tuple
• ltroot-id, root-cost, bridge-id, port-idgt
• Store rcvd and send 4-tuple for each port
• port with best rcvd 4-tuple is root port
• root bridge has no such port
• if send 4-tuple better than rcv 4-tuple, port is
  designated port
• rest of the ports are alternate/backup ports
• Various timers

44
Spanning tree example
DP
DP
DP
RP
DP
RP
RP
DP
DP
DP
DP
DP
root
RP
DP
DP
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New Spanning Tree Protocol versions

• Implementation of
• Rapid Spanning Tree Protocol 802.1w (RSTP)
• Per VLAN Spanning Tree 802.1q (PVST )
• Multiple Spanning Tree 802.1s (MST)
• Load balancing across links
• Uni-Directional Link Detection (UDLD)

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802.1w Rapid Spanning Tree Protocol

• The IEEE 802.1w specification, Rapid Spanning
  Tree Protocol, provides for subsecond
  reconvergence of STP after failure of one of the
  uplinks in a bridged environment.
• 802.1w provides the structure on which the 802.1s
  features such as multiple spanning tree operates.
• There are only three port states left in RSTP
  corresponding to the three possible operational
  states Learning ,Forwarding and Discarding.
• Rapid Transition to Forwarding State is the most
  important feature introduced by 802.1w
• RSTP actively confirms safe port transition to
  forwarding without relying on timers
• There is now a real feedback mechanism that
  takes place between RSTP-compliant bridges.
• In order to achieve fast convergence on a port,
  the protocol relies upon two new variables edge
  ports and link type.

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Virtual LANs

• LAN (broadcast domain) grows large
• departments or workgroups not happy with big
  broadcast domain
• Security (eavesdropping)
• Bandwidth consumed by flooding/multicasting
• Split LAN into multiple broadcast domains
• Multiple physical LANs?
• Too expensive!
• People move all the time!
• VLAN logical partition of LAN

48
Virtual LANs
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VLANs IEEE 802.1q
destination addr
source addr
data
FCS
type
3-bit priority 1-bit CFI 12-bit VLAN id
VLAN protocol id 0x8100

• Tagged Ethernet frames contain VLAN-id
• Switch adds/removes tag when forwarding frames
  between trunk and non-trunk ports
• Complications
• Hosts and legacy switches do not understand VLAN
  tags
• Tag insertion/removal requires FCS recomputation
• Frame length increases beyond legacy MTU

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VLAN Standard IEEE 802.1q
CFI-Canonical Format Identifier
(Ethernet/TokenRing)
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The 802.3 (legacy) and 802.1Q Ethernet frame
formats
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L2 Tunneling
The default system MTU for traffic on the switch
is 1500 bytes. You can configure the switch to
support larger frames by using the system mtu
global configuration command. Because the 802.1Q
tunneling feature increases the frame size by 4
bytes when the metro tag is added, you must
configure all switches in the service-provider
network to be able to process larger frames by
increasing the switch system MTU size to at least
1504 bytes. The maximum allowable system MTU for
Catalyst 3550 Gigabit Ethernet switches is 2000
bytes the maximum system MTU for Fast Ethernet
switches is 1546 bytes.
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Some Switches Support Priorities
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802.1p Prioritization

• Eight levels of prioritization - p0 (lowest)
  through p7 (highest)
• 802.1p example

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Gigabit Ethernet over Fiber
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Switch Configuration Example

• interface GigabitEthernet2/9
• description NISN/NASA
• mtu 9216
• no ip address
• speed nonegotiate
• switchport
• switchport trunk encapsulation dot1q
• switchport trunk allowed vlan 210-213,217-226,231
  ,232
• switchport mode trunk
• switchport nonegotiate
• interface FastEthernet0/7
• description ASSA
• switchport access vlan 210
• no ip address
• speed 10

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IGMP Snooping

• Internet Group Management Protocol (IGMP - RFC
  2236) used to manage IP multicast traffic
• Application wishing to receive traffic for
  specific IP multicast address sends out an ICMP
  join request (or a leave request to stop
  receiving multicast)
• Switches that employ IGMP snooping listen for
  IGMP join/leave requests to decide when to send a
  specific multicast frame to a port