Download Evolution of Data Networks - Electrical & Computer

ELEN Lecture 13
•
•
•
•
LAN Bridges
Routers, Switches, Gateways
Network layer -IP
Reading: 6.7, 8.1-8.3
1
Switches
• A multi-input multi-output device that transfers packets
from one input to an output
• The number of ports on a switch determine the number of
hosts that can be connected.
• Larger # of hosts can be interconnected by interconnecting
switches
• Point-to-point links can be used to extend the geographic
reach
• Adding a new host does not limit or reduce the
performance of other hosts
2
Scalable Networks
• Switch
– forwards packets from input port to output port
– port selected based on address in packet header
T3
T3
STS-1
Input
ports
Switch
T3
T3
STS-1
Output
ports
• Advantages
– cover large geographic area (tolerate latency)
– support large numbers of hosts (scalable bandwidth)
3
Virtual Circuit Switching
• Explicit connection setup (and tear-down) phase
• Subsequence packets follow same circuit
• Sometimes called connection-oriented model
0 Switch 1
1
3
2
5
• Analogy: phone
call
• Each switch
maintains a VC
table
3
11
2 Switch 2
1
0
Host A
7
1
0 Switch 3
3
4
2
Host B
4
Datagram Switching
• No connection setup phase
• Each packet forwarded independently
• Sometimes called connectionless model
Host D
• Analogy: postal
system
3
Host C
• Each switch
maintains a
forwarding (routing)
table
Host E
0 Switch 1
Host F
1
2 Switch 2
2
3
1
0
Host A
Host G
1
0 Switch 3 Host B
3
2
Host H
5
Virtual Circuit Model
• Typically wait full RTT for connection setup before sending first data
packet.
• While the connection request contains the full address for destination,
each data packet contains only a small identifier, making the perpacket header overhead small.
• If a switch or a link in a connection fails, the connection is broken and
a new one needs to be established.
• Connection setup provides an opportunity to reserve resources.
6
Datagram Model
• There is no round trip time 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.
7
Bridges, switches, routers, gateways
• Devices used to interconnect multiple networks
• Bridge: device interconnecting two or more networks at
MAC layer
• Router: device interconnecting two or more networks at
the network layer
• Gateway: device interconnecting two or more networks at
a higher layer
8
A Bridged LAN
S1
S2
S3
LAN1
Bridge
LAN2
S4
S5
S6
9
Interconnection by a Bridge
Network
Network
Bridge
LLC
LLC
MAC
MAC
MAC
MAC
Physical
Physical
Physical
Physical
10
Example Configuration
S1
S2
S3
LAN1
LAN2
port 1
B1
Address
port 2
Port
S5
S4
LAN3
port 1
B2
Address
port 2
Port
11
S1 sends a frame to S5
S1
S1
S2
S3
S5
S4
S5
LAN1
LAN2
B1
port 1
Address
S1
port 2
Port
1
LAN3
B2
port 1
Address
S1
port 2
Port
1
12
S3 sends a frame to S2
S1
S2
S3
S3
LAN1
S5
S4
S2
LAN2
LAN3
B1
B2
port 1
port 2
port 1
port 2
Address
Port
Address
Port
S1
S3
1
2
S1
S3
1
1
13
S4 Sends a frame to S3
S1
S2
S3
S5
S4
S4
LAN1
LAN2
LAN3
B1
B2
port 1
port 2
Address
S1
S3
S4
Port
1
2
2
port 1
port 2
Address
Port
S1
S3
S4
1
1
2
14
S3
S2 sends a frame to S1
S1
S2
S3
S2
S5
S4
S1
LAN1
LAN2
LAN3
Bridge1
Bridge 2
port 1
port 2
port 1
port 2
Address
Port
Address
Port
S1
S3
S4
S2
1
2
2
1
S1
S3
S4
1
1
2
15
Sample Topology
LAN1
(1)
(1)
B1
B2
(1)
(2)
LAN2
(2)
B3
(3)
(2)
(1)
B4
(2)
LAN3
(1)
B5
(2)
LAN4
16
Spanning Tree
LAN1
D (1)
R (1)
B1
B2
R (1)
D (2)
(2)
B3
LAN2
(3)
D
D (2)
R (1)
B4
(2)
LAN3
R (1)
B5
(2)
LAN4
17
Frame Format for Source Routing
Routing
Control
2 bytes
Destination
Address
Route-1
Designator
Route-2
Designator
Route-m
Designator
2 bytes
2 bytes
2 bytes
Source
Address
Routing
Information
Data
FCS
18
LAN interconnection with source routing
bridges
S2
S1
B1
LAN
1
LAN
2
B4
LAN
4
B3
B5
B7
LAN
3
B6
LAN
5
B2
S3
19
Routes followed by single-route broadcast frames
LAN1
B3
LAN3
B4
LAN4
B6
LAN5
B1
20
Routes of all-routes broadcast frames
B2
B6
LAN3
B3
B5
LAN1
LAN2
LAN4
LAN5
B3
B1
LAN2
B1
LAN1
B4
LAN4
B4
LAN2
B4
LAN4
B5
B7
B2
B5
B7
B1
LAN1
B2
B3
B7
B1
B4
B7
B2
LAN3
B2
B5
B6
B1
LAN1
LAN2
LAN4
B5
LAN1
B3
B5
B6
B1
LAN3
B3
LAN3
B2
LAN1
B3
LAN2
B6
B1
B4
B3
B4
B2
LAN2
LAN1
21
IP Internet
• Concatenation of Networks
Network 1 (Ethernet)
H7
H2
H1
R3
H8
H3
Network 4
(point-to-point)
Network 2 (Ethernet)
R1
R2
• Protocol Stack
H4
Network 3 (FDDI)
H5
H6
H1
H8
TCP
R1
IP
IP
ETH
R2
ETH
R3
IP
FDDI
FDDI
IP
PPP
PPP
TCP
IP
ETH
ETH
22
Service Model
• Connectionless (datagram-based)
• Best-effort delivery (unreliable service)
– packets are lost
– packets are delivered out of order
– duplicate copies of a packet are delivered
– packets can be delayed for a long time
• Datagram format
0
4
Version
8
HLen
16
TOS
31
Length
Ident
TTL
19
Flags
Protocol
Offset
Checksum
SourceAddr
DestinationAddr
Options (variable)
Pad
(variable)
Data
23
Fragmentation and Reassembly
• Each network has some MTU
• Strategy
– fragment when necessary (MTU < Datagram)
– try to avoid fragmentation at source host
– re-fragmentation is possible
– fragments are self-contained datagrams
– use CS-PDU (not cells) for ATM
– delay reassembly until destination host
– do not recover from lost fragments
24
Example
Start of header
Ident= x
0
Offset= 0
Rest of header
1400 data bytes
Start of header
Ident= x
H1
R1
R2
R3
H8
1
Offset= 0
Rest of header
512 data bytes
Start of header
ETH IP (1400)
FDDI IP (1400)
PPP IP (512)
ETH IP (512)
PPP IP (512)
ETH IP (512)
Rest of header
PPP IP (376)
ETH IP (376)
512 data bytes
Ident= x
1 Offset= 512
Start of header
Ident= x
0 Offset= 1024
Rest of header
376 data bytes
25
Global Addresses
• Properties
– globally unique
– hierarchical: network + host
• Dot Notation
– 10.3.2.4
– 128.96.33.81
– 192.12.69.77
A:
B:
C:
0
7
24
Network
Host
1 0
1 1 0
14
16
Network
Host
21
8
Network
Host
26
Datagram Forwarding
• Strategy
– every datagram contains destination’s address
– if directly connected to destination network, then forward
to host
– if not directly connected to destination network, then
forward to some router
– forwarding table maps network number into next hop
– each host has a default router
– each router maintains a forwarding table
• Example (R2)
Network Number
Next Hop
1
2
3
4
R3
R1
interface 1
interface 0
27
Address Translation
• Map IP addresses into physical addresses
– destination host
– next hop router
• Techniques
– encode physical address in host part of IP address
– table-based
• ARP
– table of IP to physical address bindings
– broadcast request if IP address not in table
– target machine responds with its physical address
– table entries are discarded if not refreshed
28