Download Virtual circuits VC implementation

Network layer
Two Key Network-Layer Functions
❒  transport segment from
application
transport
network
data link
physical
sending to receiving host
❒  on sending side
encapsulates segments
into datagrams
❒  on rcving side, delivers
segments to transport
layer
❒  network layer protocols
in every host, router
❒  router examines header
fields in all IP datagrams
passing through it
network
data link
physical
network
data link
physical
network
data link
physical
packets from
router’s input to
appropriate router
output
network
data link
physical
network
data link
physical
network
network
data link
data link
physical
physical
network
data link
physical
network
data link
physical
❒  forwarding: move
network
data link
physical
network
data link
physical
Network Layer
application
transport
network
data link
physical
4-1
Interplay between routing and forwarding
routing algorithm
value in arriving
packet’s header
route taken by
packets from source
to dest.
❍  routing
of getting through
single interchange
algorithms
Network Layer
4-2
Network layer connection and
connection-less service
host-to-host
choice: network provides one or the other
❍  implementation: in network core
❍  no
Network Layer
4-3
“source-to-dest path behaves much like telephone
circuit”
4-4
a VC consists of:
1. 
performance-wise
network actions along source-to-dest path
❒  call setup, teardown for each call
Network Layer
VC implementation
Virtual circuits
❍ 
❒  forwarding: process
❍  service:
1
3 2
❍ 
planning trip from
source to dest
connectionless service
❒  VC network provides network-layer
connection service
❒  analogous to the transport-layer services,
but:
3
2
2
1
0111
❒  routing: process of
❒  datagram network provides network-layer
local forwarding table
header value output link
0100
0101
0111
1001
❒  routing: determine
analogy:
2. 
3. 
before data can flow
❒  each packet carries VC identifier (not destination host
❒  packet belonging to VC carries VC number
address)
every router on source-dest path maintains “state” for
each passing connection
❒  link, router resources (bandwidth, buffers) may be
allocated to VC (dedicated resources = predictable service)
(rather than dest address)
❒ 
Network Layer
path from source to destination
VC numbers, one number for each link along
path
entries in forwarding tables in routers along
path
❒  VC number can be changed on each link.
❍ 
4-5
New VC number comes from forwarding table
Network Layer
4-6
1
Forwarding table
Virtual circuits: signaling protocols
VC number
22
12
1
Forwarding table in
northwest router:
Incoming interface
1
2
3
1
…
2
32
3
❒  used to setup, maintain teardown VC
❒  used in ATM, frame-relay, X.25
interface
number
Incoming VC #
❒  not used in today’s Internet
Outgoing interface
12
63
7
97
…
Outgoing VC #
3
1
2
3
…
application
transport 5. Data flow begins
network 4. Call connected
data link 1. Initiate call
physical
22
18
17
87
…
6. Receive data application
3. Accept call transport
2. incoming call network
data link
physical
Routers maintain connection state information!
Network Layer
4-7
Datagram networks
4-8
Datagram or VC network: why?
❒  no call setup at network layer
❒  routers: no state about end-to-end connections
❍  no network-level concept of “connection”
Internet (datagram)
❒  data exchange among
❒  packets forwarded using destination host address
❍  packets between same source-dest pair may take
different paths
application
transport
network
data link 1. Send data
physical
Network Layer
application
transport
2. Receive data network
data link
physical
Network Layer
ATM (VC)
❒  evolved from telephony
computers
❒  human conversation:
❍  “elastic” service, no
❍  strict timing, reliability
strict timing req.
requirements
❒  “smart” end systems
❍  need for guaranteed
(computers)
service
❍  can adapt, perform
❒  “dumb” end systems
control, error recovery
❍  telephones
❍  simple inside network,
❍  complexity inside
complexity at “edge”
network
❒  many link types
❍  different characteristics
❍  uniform service difficult
4-9
Router Architecture Overview
Network Layer 4-10
Input Port Functions
Two key router functions:
❒  run routing algorithms/protocol (RIP, OSPF, BGP)
❒ 
forwarding datagrams from incoming to outgoing link
Physical layer:
bit-level reception
Data link layer:
e.g., Ethernet
see chapter 5
Network Layer
4-11
Decentralized switching:
❒  given datagram dest., lookup output port
using forwarding table in input port
memory
❒  goal: complete input port processing at
‘line speed’
❒  queuing: if datagrams arrive faster than
forwarding rate into switch fabric
Network Layer 4-12
2
Three types of switching fabrics
Switching Via Memory
First generation routers:
❒  traditional computers with switching under direct
control of CPU
❒ packet copied to system’s memory
❒  speed limited by memory bandwidth (2 bus
crossings per datagram)
Input
Port
Memory
Output
Port
System Bus
Network Layer 4-13
Network Layer 4-14
Switching Via An Interconnection
Network
Switching Via a Bus
❒  overcome bus bandwidth limitations
❒  datagram from input port memory
❒  Omega networks, other interconnection nets
initially developed to connect processors in
multiprocessor
❒  advanced design: fragmenting datagram into fixed
length cells, switch cells through the fabric.
❒  Cisco 12000: switches 60 Gbps through the
interconnection network
to output port memory via a shared
bus
❒  bus contention: switching speed
limited by bus bandwidth
❒  32 Gbps bus, Cisco 5600: sufficient
speed for access and enterprise
routers
Network Layer 4-15
Output Ports
Network Layer 4-16
Output port queueing
Buffering required when datagrams arrive from
fabric faster than the transmission rate
❒  Scheduling discipline chooses among queued
datagrams for transmission
❒ 
Network Layer 4-17
❒  buffering when arrival rate via switch exceeds
❒ 
output line speed
queueing (delay) and loss due to output port
buffer overflow!
Network Layer 4-18
3
Input Port Queuing
❒  Fabric slower than input ports combined -> queueing
may occur at input queues
❒  Head-of-the-Line (HOL) blocking: queued datagram
at front of queue prevents others in queue from
moving forward
❒  queueing delay and loss due to input buffer overflow!
Network Layer 4-19
4