Packet Switch: Intersection where Traffic Flows Meet

1
Packet Switch Intersection where Traffic Flows
Meet
1
1
2
2
? ? ?
? ? ?
N
N

• Inputs contain multiplexed flows from access muxs
  other packet switches
• Flows demultiplexed at input, routed and/or
  forwarded to output ports
• Packets buffered, prioritized, and multiplexed on
  output lines

2
Generic Packet Switch

• Unfolded View of Switch
• Ingress Line Cards
• Header processing
• Demultiplexing
• Routing in large switches
• Controller
• Routing in small switches
• Signalling resource allocation
• Interconnection Fabric
• Transfer packets between line cards
• Egress Line Cards
• Scheduling priority
• Multiplexing

3
Shared Memory Packet Switch
Output Buffering
Ingress Processing
Connection Control
1
1
Queue Control
2
2
3
3
Shared Memory


N
N
Small switches can be built by reading/writing
into shared memory
4
Crossbar Switches
(b) Output buffering
(a) Input buffering
Inputs
Inputs
3
1
1
2
3
8
2
3
3


N
N


1
2
3
N
1
2
3
N
Outputs
Outputs

• Large switches built from crossbar multistage
  space switches
• Requires centralized controller/scheduler (who
  sends to whom when)
• Can buffer at input, output, or both (performance
  vs complexity)

5
Asynchronous Tranfer Mode (ATM)

• Packet multiplexing and switching
• Fixed-length packets cells
• Connection-oriented
• Rich Quality of Service support
• Conceived as end-to-end
• Supporting wide range of services
• Real time voice and video
• Circuit emulation for digital transport
• Data traffic with bandwidth guarantees

6
ATM Networking
Packet
Voice
Video
Packet
Voice
Video
ATM Adaptation Layer
ATM Adaptation Layer
ATM Network

• End-to-end information transport using cells
• 53-byte cell provide low delay and fine
  multiplexing granularity

• Support for many services through ATM Adaptation
  Layer

7
ATM Attributes of TDM Packet Switching

• Packet structure gives flexibility efficiency
• Synchronous slot transmission gives high speed
  density

Packet Header
8
TDM vs. Packet Multiplexing
?
?
?

In mid-1980s, packet processing mainly in
software and hence slow By late 1990s, very
high speed packet processing possible
9
ATM Switching
Switch carries out table translation and routing
ATM switches can be implemented using shared
memory, shared backplanes, or self-routing
multi-stage fabrics
10
ATM Virtual Connections

• Virtual connections setup across network
• Connections identified by locally-defined tags
• ATM Header contains virtual connection
  information
• 8-bit Virtual Path Identifier
• 16-bit Virtual Channel Identifier
• Powerful traffic grooming capabilities
• Multiple VCs can be bundled within a VP
• Similar to tributaries with SONET, except
  variable bit rates possible

Virtual paths
Physical link
Virtual channels
11
VPI/VCI switching multiplexing

• Connections a,b,c bundled into VP at switch 1
• Crossconnect switches VP without looking at VCIs
• VP unbundled at switch 2 VC switching
  thereafter
• VPI/VCI structure allows creation virtual networks

12
MPLS ATM

• ATM initially touted as more scalable than packet
  switching
• ATM envisioned speeds of 150-600 Mbps
• Advances in optical transmission proved ATM to be
  the less scalable _at_ 10 Gbps
• Segmentation reassembly of messages streams
  into 48-byte cell payloads difficult
  inefficient
• Header must be processed every 53 bytes vs. 500
  bytes on average for packets
• Delay due to 1250 byte packet at 10 Gbps 1
  msec delay due to 53 byte cell _at_ 150 Mbps 3
  msec
• MPLS (Chapter 10) uses tags to transfer packets
  across virtual circuits in Internet

13
Multicasting

• Source S sends packets to multicast group G1

14
Multicast Routing

• Multicast routing useful when a source wants to
  transmits its packets to several destinations
  simultaneously
• Relying on unicast routing by transmitting each
  copy of packet separately works, but can be very
  inefficient if number of destination is large
• Typical applications is multi-party conferencing
  over the Internet
• Example Multicast Backbone (MBONE) uses reverse
  path multicasting

15
Reverse-Path Broadcasting (RPB)

• Fact Set of shortest paths to the source node S
  forms a tree that spans the network
• Approach Follow paths in reverse direction
• Assume each router knows current shortest path to
  S
• Upon receipt of a multicast packet, router
  records the packets source address and the port
  it arrives on
• If shortest path to source is through same port
  (parent port), router forwards the packet to
  all other ports
• Else, drops the packet
• Loops are suppressed each packet forwarded a
  router exactly once
• Implicitly assume shortest path to source S is
  same as shortest path from source
• If paths asymmetric, need to use link state info
  to compute shortest paths from S

16
Example Shortest Paths from S

• Spanning tree of shortest paths to node S and
  parent ports are shown in blue

17
Example S sends a packet
?
G1
G1
1
2
7
3
2
4
2
3
4
2
1
5
1
2
5
G1
3
3
8
4
2
1
1
4
S
G1
1
3
5
4
2
2
4
6
3
1
3
2
1
1
G2
3
4
3
G3
G3

• S sends a packet to node 1
• Node 1 forwards to all ports, except parent port

18
Example Hop 1 nodes broadcast
?
?
G1
G1
1
2
7
3
2
4
2
3
4
2
1
5
1
?
2
5
G1
3
3
8
4
2
1
?
1
4
S
G1
1
3
5
4
2
2
4
6
3
1
3
2
1
1
G2
3
4
3
G3
G3

• Nodes 2, 3, 4, and 5 broadcast, except on parent
  ports
• All nodes, not only G1, receive packets

19
Example Broadcast continues
G1
G1
1
2
7
3
2
4
2
3
4
2
1
5
1
2
5
G1
3
3
8
4
2
1
1
4
S
G1
1
3
5
4
2
2
4
6
3
1
3
2
1
1
G2
3
4
3
G3
G3

• Truncated RPB (TRPB) Leaf routers do not
  broadcast if none of its attached hosts belong to
  packets multicast group

20
Internet Group Management Protocol (IGMP)

• Internet Group Management Protocol
• Host can join a multicast group by sending an
  IGMP message to its router
• Each multicast router periodically sends an IGMP
  query message to check whether there are hosts
  belonging to multicast groups
• Hosts respond with list of multicast groups they
  belong to
• Hosts randomize response time cancel response if
  other hosts reply with same membership
• Routers determine which multicast groups are
  associated with a certain port
• Routers only forward packets on ports that have
  hosts belonging to the multicast group

21
Reverse-Path Multicasting (RPM)

• Reverse Path Multicasting (RPM) relies on IGMP to
  identify multicast group membership
• The first packet to a given (source, group), i.e.
  (S,G) is transmitted to all leaf routers using
  TRPB
• Each leaf router that has no hosts that belong to
  this group on any of its ports, sends a prune
  message to its upstream router to stop sending
  packets to (S, G)
• Upstream routers that receive prune messages from
  all their downstream routers, send prune messages
  upstream
• Prune entries in each router have finite lifetime
• If a host requests to join a group, routers can
  cancel previous pruning with a graft message

22
Example Pruning for G1
G1
G1
1
2
7
3
2
4
2
3
4
2
1
5
1
2
5
G1
3
3
8
4
2
1
1
4
S
G1
1
3
5
4
2
2
4
6
3
1
3
2
1
1
G2
3
4
3
G3
G3

• Routers 3, 4, and 6 send prune messages upstream

23
Example RPM Multicast Tree
G1
G1
1
2
7
3
2
4
2
3
4
2
1
5
1
2
5
G1
3
3
8
4
2
1
1
4
S
G1
1
3
5
4
2
2
4
6
3
1
3
2
1
1
G2
3
4
3
G3
G3

• RPM multicast tree after pruning

24
Example Graft from Router 6
G1
G1
1
2
7
3
2
4
2
3
4
2
1
5
1
2
5
G1
3
3
8
4
2
1
1
4
S
G1
1
3
Graft
5
4
2
2
4
6
3
1
3
2
1
1
G1
3
4
3
G3
G3

• Graft message flows upstream to router 1

25
Example RPM Tree after Graft
G1
G1
1
2
7
3
2
4
2
3
4
2
1
5
1
2
5
G1
3
3
8
4
2
1
1
4
S
G1
1
3
5
4
2
2
4
6
3
1
3
2
1
1
G1
3
4
3
G3
G3