Internetworking
Outline
Internet Architecture
Best Effort Service Model
Global Addressing Scheme
Fall 2001
CS 640
1
Layering revisited (cause it’s real important)
• Recall pros and cons of packet switched networks
– Pros: High utilization, low startup overhead
– Cons: No guarantees on delay and loss
• But files are transmitted without “holes”?
• Reliable transfer is a function of a specific protocol layer
(TCP)
• Distinct functions are separated into layers
– lower layers as black boxes (like C library)
• Layering simplifies description of functions and enables
interoperability in heterogeneous environment
Fall 2001
CS 640
2
ISO Architecture
End host
End host
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Network
Network
Data link
Data link
Data link
Data link
Physical
Physical
Physical
Physical
One or more nodes
within the network
Fall 2001
CS 640
3
Internet Architecture
•
Defined by Internet Engineering Task Force (IETF)
1. Application: interacts with user to initiate data transfers (browser,
media player, command line)
2. Transport: reliable, in-order delivery of data (TCP and UDP)
3. Network: addressing and routing (IP)
4. Data Link: defines how hosts access physical media (Ethernet)
5. Physical: defines how bits are represented on wire (Manchester)
•
Information is passed between layers via encapsulation
– Header information is attached to data passed down layers
•
•
•
Multiplexing between layers
Layers access other layers via API’s (eg. sockets)
Communication at a specific layer is enabled by a protocol
Fall 2001
CS 640
4
Hourglass Design
• Single protocol at network level insures packets will
get from source to destination while allowing for
flexibility
FTP
HTTP
NV
TFTP
UDP
TCP
IP
NET1
Fall 2001
NET2
CS 640
…
NETn
5
IP Internet
• Concatenation of Networks
Network 1 (Ethernet)
H7
H2
H1
R3
H8
H3
Network 4
(point-to-point)
Network 2 (Ethernet)
R1
R2
H4
• Protocol Stack
Network 3 (FDDI)
H5
H6
H1
H8
TCP
R1
IP
IP
ETH
Fall 2001
R2
ETH
R3
IP
FDDI
FDDI
IP
PPP
CS 640
PPP
TCP
IP
ETH
ETH
6
IP Service Model
• Connectionless (datagram/packet-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
Fall 2001
CS 640
7
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
Fall 2001
Network Number
1
2
3
4
CS 640
Next Hop
R3
R1
interface 1
interface 0
8
Forwarding Tables
• Suppose there are n possible destinations, how
many bits are needed to represent addresses in a
routing table?
– log2n
• So, we need to store and search n * log2n bits in
routing tables?
– We’re smarter than that!
Fall 2001
CS 640
9
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:
Fall 2001
0
7
24
Network
Host
1 0
1 1 0
CS 640
14
16
Network
Host
21
8
Network
Host
10