Topics
• Interconnecting LAN segments
– HUB (Physical Layer)
– Bridge (Link layer)
– Layer 2 Switch (multi-port bridge, link layer)
• Interconnecting networks
– Layer 3 Switch (network layer)
– Router (network layer)
• ATM Networks
Csci 232 Computer Networks
Switching and Forwarding
1
Interconnecting LAN Segments
• (Repeating) Hubs (layer 1 devices)
• Bridges (layer 2 devices)
–
–
–
–
Basic Functions
Self learning and bridge forwarding table
Forwarding/filtering algorithm
Bridge looping problem and spanning tree algorithm
• Ethernet Switches
– Remark: switches are essentially multi-port bridges.
– What we say about bridges also holds for switches!
• Readings
– Section 3.2
Csci 232 Computer Networks
Switching and Forwarding
2
Interconnecting with Hubs
• Backbone hub interconnects LAN segments
• Extends max distance between nodes
• But individual segment collision domains become one
large collision domain
– if a node in CS and a node EE transmit at same time: collision
• Can’t interconnect 10BaseT & 100BaseT
– Encoding is different: Manchester vs. 4B/5B
Csci 232 Computer Networks
Switching and Forwarding
Recreates each bit,
boosts its energy strength,
and transmits the bit to
all other interfaces
3
Bridges
• Link layer device
– stores and forwards Ethernet frames
– examines frame header and selectively forwards frame
based on MAC destination address -- filtering
– when frame is to be forwarded on a LAN segment, uses
CSMA/CD to access the LAN segment
• transparent
– hosts are unaware of the presence of bridges
• plug-and-play, self-learning
– bridges do not need to be configured
Csci 232 Computer Networks
Switching and Forwarding
4
Bridges: Traffic Isolation
• Bridge installation breaks LAN into LAN segments
• Bridges filter packets:
– same-LAN-segment frames not usually forwarded onto
other LAN segments
– segments become separate collision domains
collision
domain
collision
domain
bridge
LAN segment
= hub
= host
LAN segment
LAN (IP network)
Csci 232 Computer Networks
Switching and Forwarding
5
Forwarding
How to determine to which LAN segment to
forward frame?
Csci 232 Computer Networks
Switching and Forwarding
6
Self Learning
• A bridge has a bridge (forwarding) table
• Entry in bridge forwarding table:
– <Node LAN Address, Bridge Interface, Time Stamp>
– stale entries in table dropped (TTL can be 60 min)
• Bridges learn which hosts can be reached through
which interfaces
– when frame received, bridge “learns” location of sender:
incoming LAN segment
– records sender/location pair in bridge forwarding table
Csci 232 Computer Networks
Switching and Forwarding
7
Filtering/Forwarding
When bridge receives a frame:
index bridge table using dest MAC address
if entry found for destination
then{
if dest on segment from which frame arrived
then drop the frame
else forward the frame on interface indicated
}
else flood
forward on all but the interface
on which the frame arrived
Csci 232 Computer Networks
Switching and Forwarding
8
Bridge Example
Suppose C sends frame to D and D replies back with
a frame to C.
• Bridge receives frame from C
– notes in bridge forwarding table that C is on interface 1
– because D is not in table, bridge sends frame into interfaces 2
and 3
• frame received by D
Csci 232 Computer Networks
Switching and Forwarding
9
Bridge Learning: Example
• D generates a frame for C, sends
• bridge receives the frame
– notes in bridge forwarding table that D is on interface 2
– bridge knows C is on interface 1, so selectively forwards frame
to interface 1
Csci 232 Computer Networks
Switching and Forwarding
10
Interconnection without Backbone
• Not recommended for two reasons:
- single point of failure at Computer Science hub
- all traffic between EE and SE must path over CS segment
Csci 232 Computer Networks
Switching and Forwarding
11
Backbone Configuration
Recommended !
Csci 232 Computer Networks
Switching and Forwarding
12
Looping and Bridge 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
Disabled
Csci 232 Computer Networks
Switching and Forwarding
13
Bridge Spanning Tree Algorithm:
Algorhyme
I think that I shall never see
A graph more lovely than a tree.
A tree whose crucial property
Is loop-free connectivity.
A tree that must be sure to span
So packets can reach every LAN.
First, the root must be selected.
By ID, it is elected.
Least cost paths from root are traced.
In the tree, these paths are placed.
A mesh is made by folks like me,
Then bridges find a spanning tree
-- Radia Perlman
Csci 232 Computer Networks
Switching and Forwarding
14
Some Bridge Features
• Isolates collision domains resulting in higher total
max throughput
• “limitless” number of nodes and geographical
coverage
– Scalable? (broadcast, spanning tree algorithm…)
– Heterogeneity (understands one type of LAN address only)
• Can connect different Ethernet types
• Transparent (“plug-and-play”): no configuration
necessary
– Dropping packets? Long latency? Frames reordered?
Csci 232 Computer Networks
Switching and Forwarding
15
Ethernet Switches
• Essentially a multiinterface bridge
• layer 2 (frame) forwarding,
filtering using LAN
addresses
• Switching: A-to-A’ and Bto-B’ simultaneously, no
collisions
• large number of interfaces
• often: individual hosts,
star-connected into switch
– Ethernet, but no collisions!
Csci 232 Computer Networks
Switching and Forwarding
16
Ethernet Switches
• cut-through switching: frame forwarded from
input to output port without awaiting for assembly
of entire frame
– slight reduction in latency
– Cut-through vs. store and forward
• combinations of shared/dedicated, 10/100/1000
Mbps interfaces
Csci 232 Computer Networks
Switching and Forwarding
17
Not an atypical LAN (IP network)
Dedicated
Shared
Csci 232 Computer Networks
Switching and Forwarding
18
A Few Words about VLAN
• Virtual LAN (VLAN) – defined in IEEE 802.1q
– Partition a physical LAN into several “logically separate” LANs
• reduce broadcast traffic on physical LAN!
• provide administrative isolation
– Extend over a WAN (wide area network), e.g.,
via layer 2 tunnels (e.g., L2TP, MPLS) over IP-based WANs!
• Two types: port-based or MAC address-based
–
–
each port optionally configured with a VLAN id
inbound packets tagged with this “VLAN” id
• require change of data frames, carry “VLAN id” tags
• tagged and untagged frames can co-exist
– “VLAN-aware” switches forward on ports part of same VLAN
• More complex ! - require administrative configuration
– static (“manual”) configuration
– some configuration can be learned using GARP and GVRP protocols
– more for info: google search on “VLAN tutorial”
Csci 232 Computer Networks
Switching and Forwarding
19
Summary of LAN
• Local Area Networks
– Designed for short distance
– Use shared media
– Many technologies exist
• Media Access Control: key problem!
– Different environments/technologies-> different
solutions!
• Topology refers to general shape
– Bus
– Ring
– Star
Csci 232 Computer Networks
Switching and Forwarding
20
Summary (continued)
• Address
– Unique number assigned to station
– Put in frame header
– Recognized by hardware
• Address forms
– Unicast
– Broadcast
– Multicast
Csci 232 Computer Networks
Switching and Forwarding
21
Summary (continued)
• Type information
– Describes data in frame
– Set by sender
– Examined by receiver
• Frame format
– Header contains address and type information
– Payload contains data being sent
Csci 232 Computer Networks
Switching and Forwarding
22
Summary (continued)
• LAN technologies
–
–
–
–
Ethernet (bus)
Token Ring
FDDI (ring)
Wireless 802.11
• Wiring and topology
– Logical topology and Physical topology (wiring)
– Hub allows
• Star-shaped bus
• Star-shaped ring
Csci 232 Computer Networks
Switching and Forwarding
23
Summary (cont’d)
• Interconnecting LAN Segments
– (Repeating) Hubs
– Bridges
• Self learning and bridge forwarding table
• Forwarding/filtering algorithm
• Bridge looping problem and spanning tree algorithm
– (Layer-2) Switches
• store and forward switching
• cut-through switching
Csci 232 Computer Networks
Switching and Forwarding
24
•
Switching and Forwarding
Network Layer
Switching and Forwarding
–
–
•
Generic Switch Architecture
Forwarding Tables:
• Bridges/Layer 2 Switches; VLAN
• Routers and Layer 3 Switches
Forwarding in Layer 3 (Network Layer)
– Network Layer Functions
– Network Service Models: VC vs. Datagram
• ATM and IP Datagram Forwarding
Readings: Textbook: Chapter 3: Sections 3.1; 3.3-3.4
Csci 232 Computer Networks
Switching and Forwarding
25
Hubs vs. Bridges vs. Routers
• Hubs (aka Repeaters): Layer 1 devices
– repeat (i.e., regenerate) physical signals
• don’t understand MAC protocols!
• LANs connected by hubs belong to same collision domain
• Bridges (and Layer-2 Switches): Layer 2 devices
– store and forward layer-2 frames based on MAC addresses
• speak and obey MAC protocols
• bridges segregate LANs into different collision domains
• Routers (and Layer 3 Switches): Layer 3 devices
– store and forward layer-3 packets based on network layer
addresses (e.g., IP addresses)
• rely on data link layer to deliver packets to (directly
connected) next hop
• network layer addresses are logical (i.e. virtual), need to map
to MAC addresses for packet delivery
Csci 232 Computer Networks
Switching and Forwarding
26
Switching and Forwarding
Bridges and Routers: store-and forward devices!
Function Division:
• input interfaces (input ports):
perform forwarding
– need to know to which output
ports to send frames/packets
– may enqueue packets and
perform scheduling
• switching Fabric:
– move frames or packets from
input ports to output ports
• output interfaces (output ports):
– may enqueue packets and
perform scheduling
– Perform MAC to transmit
frames/packets to next hop
Csci 232 Computer Networks
control
plane
Generic Switch Architecture
Switching and Forwarding
27
Input Port Functions
Physical layer:
bit-level reception
Data link layer:
e.g., Ethernet
Csci 232 Computer Networks
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
Switching and Forwarding
28
Output Ports
Encapsulation)
• Buffering required when datagrams arrive from
fabric faster than the transmission rate
• Scheduling discipline chooses among queued
datagrams for transmission
Csci 232 Computer Networks
Switching and Forwarding
29
Generic Switch Architecture
• Input and output interfaces
are connected through a
switching fabric (backplane)
• A backplane can be
implemented by
input interface
output interface
Interconnection
Medium
(Backplane)
– shared memory
• bridges or low capacity routers
(e.g., PC-based routers)
– shared bus
• E.g., “low end” routers
– point-to-point (switched)
interconnection switching fabric
C
RI
B
RO
C
• high performance switches (e.g.,
as used in high capacity routers
Csci 232 Computer Networks
Switching and Forwarding
30
Three Types of Switching Fabrics
Csci 232 Computer Networks
Switching and Forwarding
31
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
Csci 232 Computer Networks
Switching and Forwarding
32
Switching Via a Bus
• datagram from input port memory
to output port memory via a shared
bus
• bus contention: switching speed
limited by bus bandwidth
• 1 Gbps bus, Cisco 1900: sufficient
speed for access an enterprise
routers (not regional or backbone)
Csci 232 Computer Networks
Switching and Forwarding
33
Switching Via An Interconnection
Network
• overcome bus bandwidth limitations
• Banyan 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 Gbps through the
interconnection network
Csci 232 Computer Networks
Switching and Forwarding
34
Forwarding in Layer 3
Putting in context
• What does layer-3 (network layer) do?
– deliver packets “hop-by-hop” across a network
– rely on layer-2 to deliver between neighboring hops
• Key Network Layer Functions
– Addressing: need a global (logical) addressing scheme
– Routing: build “map” of network, find routes, …
– Forwarding: actual delivery of packets!
• Two basic network layer service models
– datagram: “connectionless”
– virtual circuit (VC): connection-oriented
Csci 232 Computer Networks
Switching and Forwarding
35
What Does Network Layer Do?
• End-to-end deliver
packet from sending to
receiving hosts, “hop-byhop” thru network
application
transport
network
data link
physical
– A network-wide concern!
– Involves every router, host
in the network
• Compare:
• between two end hosts
– Data link layer
• over a physical link
directly connecting two
(or more) hosts
Switching and Forwarding
network
data link
physical
network
data link
physical
network
data link
physical
– Transport layer
Csci 232 Computer Networks
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
36
Network Layer Functions
• Addressing
– Globally unique address for each routable device
• Logical address, unlike MAC address (as you’ve seen earlier)
– Assigned by network operator
• Need to map to MAC address (as you’ll see later)
• Routing: building a “map” of network
– Which path to use to forward packets from src to dest
• Forwarding: delivery of packets hop by hop
– From input port to appropriate output port in a router
Routing and forwarding depend on network service
models: datagram vs. virtual circuit
Csci 232 Computer Networks
Switching and Forwarding
37
Routing & Forwarding:
Logical View of a Router
5
A
2
1
B
2
D
3
3
1
C
5
1
E
F
2
Csci 232 Computer Networks
Switching and Forwarding
38
Network Service Model
Q: What service model for
The most important
“channel” transporting
abstraction provided
packets from sender to
by network layer:
receiver?
• guaranteed bandwidth?
• preservation of inter-packet
timing (no jitter)?
• loss-free delivery?
• in-order delivery?
• congestion feedback to
sender?
Csci 232 Computer Networks
Switching and Forwarding
? ?
?
virtual circuit
or
datagram?
39
Virtual Circuit vs. Datagram
• Objective of both: move packets through routers from source
to destination
• Datagram Model:
– Routing: determine next hop to each destination a priori
– Forwarding: destination address in packet header, used at
each hop to look up for next hop
• routes may change during “session”
– analogy: driving, asking directions at every corner gas
station, or based on the road signs at every turn
• Virtual Circuit Model:
– Routing: determine a path from source to each destination
– “Call” Set-up: fixed path (“virtual circuit”) set up at “call”
setup time, remains fixed thru “call”
– Data Forwarding: each packet carries “tag” or “label”
(virtual circuit id, VCI), which determines next hop
– routers maintain ”per-call” state
Csci 232 Computer Networks
Switching and Forwarding
40
Virtual Circuit Switching
• Explicit connection setup (and tear-down)
phase
• Subsequence packets follow same circuit
• Sometimes called connection-oriented
model
still packet switching, not circuit switching!
• Analogy:
phone call
• Each switch
maintains a
VC table
0
0
0
3
1
2
3
11 3
1
Switch 1
Switch 2
2
5
0 Switch 3
1
7
3
Host A
Csci 232 Computer Networks
2
Switching and Forwarding
2
4
Host B
41
Datagram Switching
• No connection setup phase
• Each packet forwarded independently
• Sometimes called connectionless model
Host D
• Analogy: postal
system
• Each switch
maintains a
forwarding
(routing) table
0
3
Host C
Host E
Switch 1
1
2
Host F
3
2 Switch 2
1
0
Host A
0 Switch 3
Host G
1
Host B
3
2
Host H
Csci 232 Computer Networks
Switching and Forwarding
42
Forwarding Tables: VC vs. Datagram
• Virtual Circuit
Forwarding Table
a.k.a. VC (Translation) Table
(switch 1, port 2)
• Datagram Forwarding
Table
(switch 1)
Address
A
C
F
G
…
VC In VC Out Port Out
5
6
…
11
8
…
Csci 232 Computer Networks
Port
2
3
1
1
…
1
1
…
Switching and Forwarding
43
More on Virtual Circuits
“source-to-dest path behaves much like telephone
circuit” (but actually over packet network)
• call setup/teardown for each call before data can flow
– need special control protocol: “signaling”
– every router on source-dest path maintains “state” (VCI
translation table) for each passing call
– VCI translation table at routers along the path of a call
“weaving together” a “logical connection” for the call
• link, router resources (bandwidth, buffers) may be
reserved and allocated to each VC
– to get “circuit-like” performance
Csci 232 Computer Networks
Switching and Forwarding
44
Virtual Circuit: Signaling Protocols
• used to setup, maintain teardown VC
• used in ATM, frame-relay, X.25
• used in part of today’s Internet: Multi-Protocol Label
Switching (MPLS) operated at “layer 2+1/2” (between data
link layer and network layer) for “traffic engineering” purpose
application
transport 5. Data flow begins
network 4. Call connected
data link 1. Initiate call
physical
Csci 232 Computer Networks
Switching and Forwarding
6. Receive data application
3. Accept call transport
2. incoming call network
data link
physical
45
Virtual Circuit Setup/Teardown
Call Set-Up:
• Source: select a path from source to destination
– Use routing table (which provides a “map of network”)
• Source: send VC setup request control (“signaling”) packet
– Specify path for the call, and also the (initial) output VCI
– perhaps also resources to be reserved, if supported
• Each router along the path:
– Determine output port and choose a (local) output VCI for the call
• need to ensure that no two distinct VCs leaving the same output
port have the same VCI!
– Update VCI translation table (“forwarding table”)
• add an entry, establishing an mapping between incoming VCI & port
no. and outgoing VCI & port no. for the call
Call Tear-Down: similar, but remove entry instead
Csci 232 Computer Networks
Switching and Forwarding
46
green call
four “calls” going thru
the router, each entry
corresponding one call
purple call
blue call
orange call
VCI translation table (aka “forwarding table”), built at call set-up phase
1
2
3
2
1
1
1
2
During data packet forwarding phase, input VCI is used to look up the table,
and is “swapped” w/ output VCI (VCI translation, or “label swapping”)
Csci 232 Computer Networks
Switching and Forwarding
47
Virtual Circuit: Example
“call” from host A to host B along path:
host A router 1 router 2  router 3  host B
•each router along path
maintains an entry for
the call in its VCI
translation table
• the entries piece
together a “logical
connection” for the call
Router 4
0 Router 1
1
3
2 Router 2
2
5
3
11
0
Host A
• Exercise: write down
the VCI translation table
entry for the call at each
router
Csci 232 Computer Networks
1
7
0 Router 3
1
3
4
2
Switching and Forwarding
Host B
48
Virtual Circuit Model: Pros and Cons
• Full RTT for connection setup
– before sending first data packet.
• Setup request carries full destination
address
– each data packet contains only a small identifier
• If a switch or a link in a connection fails
– new connection needs to be established.
• Provides opportunity to reserve resources.
Csci 232 Computer Networks
Switching and Forwarding
49
ATM Networks
• Asynchronous Transfer Mode
– Single technology for handling voice,video, and data
• Connection-oriented service using virtual
circuits
– In-sequence but unreliable
• Cell switching using fixed-size cells: 53
bytes
– Statistical multiplexing of cells of different circuits
• Provide QoS guarantees/assurance
– Variety of services such as CBR, VBR, ABR etc
Csci 232 Computer Networks
Switching and Forwarding
50
Variable vs Fixed-Length Packets
• No optimal length
– if small: high header-to-data overhead
– if large: low utilization for small messages
• Fixed-Length easier to switch in hardware
– simpler
– enables parallelism
Csci 232 Computer Networks
Switching and Forwarding
51
Big vs Small Packets
• Small Improves Queue behavior
– finer-grained pre-emption point for scheduling link
•
•
•
•
•
maximum packet = 4KB
link speed = 100Mbps
transmission time = 4096 x 8/100 = 327.68us
high priority packet may sit in the queue 327.68us
in contrast, 53 x 8/100 = 4.24us for ATM
•
•
•
•
•
two 4KB packets arrive at same time
link idle for 327.68us while both arrive
at end of 327.68us, still have 8KB to transmit
in contrast, can transmit first cell after 4.24us
at end of 327.68us, just over 4KB left in queue
– near cut-through behavior
Csci 232 Computer Networks
Switching and Forwarding
52
Big vs Small (cont)
• Small improves latency (for voice)
–
–
–
–
voice digitally encoded at 64KBps (8-bit samples at 8KHz)
need full cell’s worth of samples before sending cell
example: 1000-byte cells implies 125ms per cell (too long)
smaller latency implies no need for echo cancellors
• ATM Compromise: 48 bytes = (32+64)/2
Csci 232 Computer Networks
Switching and Forwarding
53
ATM Cell Format
Csci 232 Computer Networks
Switching and Forwarding
54
More on Cell Format
• User-Network Interface (UNI)
4
8
16
3
1
8
384 (48 bytes)
GFC
VPI
VCI
T ype
CLP
HEC (CRC-8)
Payload
–
–
–
–
–
host-to-switch format
GFC: Generic Flow Control (still being defined)
VCI: Virtual Circuit Identifier
VPI: Virtual Path Identifier
Type: management, congestion control, AAL5 (later, type field
contains a user signaling bit to identify the end of a PDU )
– CLPL Cell Loss Priority
– HEC: Header Error Check (CRC-8)
• Network-Network Interface (NNI)
– switch-to-switch format
– GFC becomes part of VPI field
Csci 232 Computer Networks
Switching and Forwarding
55
Virtual Paths and VP Switch
• Why use Virtual Paths (VPs)?
• VCs of different VPs can have same VCIs
• VPI/VCI translation
– Cells are routed using VPI/VCI pairs in the header
• VP Switch
– Routing based on VPI only, VCI not translated
Csci 232 Computer Networks
Switching and Forwarding
56
Segmentation and Reassembly
• ATM Adaptation Layer (AAL)
– Sets above ATM layer and below the layer with variable
length frame
– AAL 1 and 2 designed for applications that need
guaranteed rate (e.g., voice, video)
– AAL 3/4 designed for packet data
– AAL 5 is an alternative standard for packet data
AAL
AAL
■■■
■■■
ATM
Csci 232 Computer Networks
ATM
Switching and Forwarding
57
AAL 3/4
• Convergence Sublayer Protocol Data Unit
(CS-PDU) – encapsulation before
segmentation
–
–
–
–
8
8
16
CPI
Btag
BASize
< 64 KB
User data
0─24
8
8
16
Pad
0
Etag
Len
CPI: common part indicator (version field)
Btag/Etag: beginning and ending tag
BAsize: hint on amount of buffer space to allocate
Length: size of whole PDU
Csci 232 Computer Networks
Switching and Forwarding
58
AAL 3/4 Cell Format
40
ATM header
2
4
10
Type
SEQ
MID
352 (44 bytes)
Payload
6
10
Length
CRC-10
• Add AAL 3/4 header and trailer to bring up to 48B
– Type
•
•
•
•
BOM (10): beginning of message
COM (00): continuation of message
EOM (01): end of message
SSM (11): Single-segment message
– SEQ: sequence of number
– MID: multiplexing id or message id
– Length: number of bytes of PDU in this cell
Csci 232 Computer Networks
Switching and Forwarding
59
Encapsulation and Segmentation
for AAL 3/4
Csci 232 Computer Networks
Switching and Forwarding
60
AAL5
• CS-PDU Format
< 64 KB
0─ 47 bytes
16
16
32
Data
Pad
Reserved
Len
CRC-32
– pad so trailer always falls at end of ATM cell
– Length: size of PDU (data only)
– CRC-32 (detects missing or misordered cells)
• Cell Format
– end-of-PDU bit in Type field of ATM header
Csci 232 Computer Networks
Switching and Forwarding
61
Encapsulation and Segmentation
for AAL5
Csci 232 Computer Networks
Switching and Forwarding
62
Datagram Networks:
the Internet model
• no call setup at network layer
• routers: no state about end-to-end connections
– no network-level concept of “connection”
• packets forwarded using destination host address
– packets between same source-dest pair may take
different paths, when intermediate routes change!
application
transport
network
data link 1. Send data
physical
Csci 232 Computer Networks
application
transport
2. Receive data network
data link
physical
Switching and Forwarding
63
Datagram Model
• There is no round trip delay waiting for connection
setup; a host can send data as soon as it is ready.
• Source host has no way of knowing if the network
is capable of delivering a packet or if the
destination host is even up.
• Since packets are treated independently, it is
possible to route around link and node failures.
• Since every packet must carry the full address of
the destination, the overhead per packet is higher
than for the connection-oriented model.
Csci 232 Computer Networks
Switching and Forwarding
64
Network Layer Service Models:
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
• Internet model being extended: MPLS, Diffserv
Csci 232 Computer Networks
Switching and Forwarding
65
Datagram or VC: Why?
Internet
ATM
• evolved from telephony
• data exchange among
• human conversation:
computers
– strict timing, reliability
– “elastic” service, no strict
requirements
timing req.
– need for guaranteed
service
• “smart” end systems
(computers)
• “dumb” end systems
– telephones
– can adapt, perform
control, error recovery
– complexity inside
network
– simple inside network,
complexity at “edge”
• many link types
– different characteristics
– uniform service difficult
Csci 232 Computer Networks
Switching and Forwarding
66
Forwarding and Switching
Network Layer Summary
• Switching and Forwarding
– Generic Switch Architecture
– Forwarding Tables:
• Bridges/Layer 2 Switches; VLAN
• Routers and Layer 3 Switches
• Network Service (Forwarding) Models
– Virtual Circuit vs. Datagram
– Virtual Circuit Model: ATM example
• VC set-up/tear-down
• data forward operations
Csci 232 Computer Networks
Switching and Forwarding
67
More on Router Architecture
Three Typical Architectures
• Output queued
• Input queued
• Combined Input-Output queued
Csci 232 Computer Networks
Switching and Forwarding
68
How to Speed Up Forwarding?
• C – input/output link capacity
• RI – maximum rate at which an
input interface can send data
into backplane
• RO – maximum rate at which an
output can read data from
backplane
• B – maximum aggregate
backplane transfer rate
• Back-plane speedup: B/C
C
• Input speedup: RI/C
• Output speedup: RO/C
Csci 232 Computer Networks
input interface
Switching and Forwarding
output interface
Interconnection
Medium
(Backplane)
RI
B
RO
C
69
Output Queued (OQ) Routers
• Only output interfaces
store packets
– buffering when arrival rate
via switch exceeds output
line speed
– queueing (delay) and loss
input interface
output interface
Backplane
due to output port buffer
overflow!
• Advantages
– Easy to design algorithms: only one congestion point
• Disadvantages
B
RO
C
– Requires an output speedup of N, where N is the
number of interfaces  not feasible
Csci 232 Computer Networks
Switching and Forwarding
70
Input Queued Routers: Pros & Cons
• Advantages
– Easy to built
input interface
output interface
• Store packets at inputs if
contention at outputs
– Relatively easy to design algorithms
Backplane
• Only one congestion point, but not
output…
• need to implement backpressure
• Disadvantages
– Head-of-line (HOL) blocking
– In general, hard to achieve high
utilization
Csci 232 Computer Networks
C
Switching and Forwarding
RI
B
C
RO
71
Input Queued (IQ) Routers
• 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 – achieve 59% of max throughput
• queueing delay and loss due to input buffer overflow!
Csci 232 Computer Networks
Switching and Forwarding
72
Combined Input-Output Queueing
(CIOQ) Routers
• Both input and output
interfaces store packets
• Advantages
input interface
– Utilization 1 can be achieved
with limited input/output
speedup (<= 2)
output interface
Backplane
• Disadvantages
– Harder to design algorithms
• two congestion points
• need to design flow control
– An input/output speedup of 2,
a CIOQ can emulate any workconserving OQ scheduling algo.
Csci 232 Computer Networks
Switching and Forwarding
RO
RI
C
B
73
Backplane
• Point-to-point switch allows to
simultaneously transfer a packet between
any two disjoint pairs of input-output
interfaces
• Goal: come-up with a schedule that
– Meet flow QoS requirements
– Maximize router throughput
• Challenges:
– Address head-of-line blocking at inputs
– Resolve input/output speedups contention
– Avoid packet dropping at output if possible
• Note: packets are fragmented in fix sized cells
(why?) at inputs and reassembled at outputs
– In Partridge et al, a cell is 64 bytes (cf. ATM,
trade-offs?)
Csci 232 Computer Networks
Switching and Forwarding
74
Head-of-Line Blocking Revisited
• The cell at head of an input queue cannot be
transferred, thus blocking the following cells
Cannot be transferred because
is blocked by red cell
Input 1
Output 1
Input 2
Output 2
Input 3
Csci 232 Computer Networks
Cannot be
transferred
because output
buffer full
Output 3
Switching and Forwarding
75
Solution to Avoid Head-of-line Blocking
• Maintain at each input N virtual queues, i.e., one
per output
Input 1
Output 1
Input 2
Output 2
Output 3
Input 3
• Need smart algorithms to schedule cell transfer
to avoid input/output contentions, overflow output
buffer, emulate output queuing mechanisms, ……
Csci 232 Computer Networks
Switching and Forwarding
76
Generic Architecture of a High
Speed Router Today
• Combined Input-Output Queued
Architecture
– Input/output speedup <= 2
• Input interface
– Perform packet forwarding (and classification)
• Output interface
– Perform packet (classification and) scheduling
• Backplane
– Point-to-point (switched) bus; speedup N
– Schedule packet transfer from input to output
Csci 232 Computer Networks
Switching and Forwarding
77