Transcript An Introduction to Computer Networks

An Introduction
to
Computer Networks
Lecture 10: Internetworking
University of Tehran
Dept. of EE and Computer Engineering
By:
Dr. Nasser Yazdani
Univ. of Tehran
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Outline
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Internetworking
Best Effort Service Model
Segmentation and Reassembly (SAR)
Global Addressing Scheme
Packet forwarding.
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Internetworking
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Communication between networks, either directly
connected or switched
Problems:
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Different Networking technologies (Heterogeneity).
So many Networks (Scaling).
Surviving in case of failure.
Different services
etc.,
Some terminologies:


“internetworking” refer to an arbitrary collection of
connected networks.
“Internet” the global internetwork.
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Goals of the DARPA project
0
Connect existing networks

1.
Survivability
-
2.
3.
4.
5.
6.
initially ARPANET and ARPA packet radio network
ensure communication service even in the
presence of network and router failures
Support multiple types of services
Must accommodate a variety of networks
Allow distributed management
Allow host attachment with a low level of effort
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Allow
resource
accountability
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IP Internet
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Concatenation of Networks or
“networks of Networks”.
“R” is routers and “H” is hosts.
Network 1 (Ethernet)
H7
H2
H1
R3
H8
H3
Network 4
(point-to-point)
Network 2 (Ethernet)
R1
R2
H4
Network 3 (FDDI)
H5
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H6
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IP Internet (cont)

Protocol Stack
H1
H8
TCP
IP
IP
ETH

R2
R1
ETH
FDDI
R3
IP
FDDI
IP
PPP
PPP
TCP
IP
ETH
ETH
Everything is running on top of IP, “IP
over everything”
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Service Model
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Connectionless (datagram-based)
Best-effort delivery (unreliable service)
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packets are lost. No recover from 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
8
4
Version HLen
16
TOS
Ident
•Contains all information
for routing of a packet.
TTL
19
Length
Flags
Protocol
Offset
Checksum
SourceAddr
DestinationAddr
Options (variable)
Data
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Introduction to Computer Network
Pad
(variable)
7
Packet Headers
The current Version is 4 or IPv4.
 HLen- the Header Length: from 5-15 in 32-bit
words.
 Length- the total length of the packet including
headers. Max length is 64K.
TTL: Time To Live is expressed in second. It is to
prevent packet from permanently circulating in a
loop.
 Protocol: specify the packet application ex. 1 for
ICMP. It is for demultiplexing to higher layer
protocols.
 Checksum: is a 1-complement error checksum for
the header only.
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Packet Headers (Cont)

TOS: type of Service

Precedence
Specify the priority
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Type of Services
0
2 | 3
Precedence
7
Type of service
D
T
R
C
Specify routing, for instance cheapest, fastest and
more reliable
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D for Delay
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T for Throughput
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R for Reliability
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C for low cost.
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Note: Precedence is only for inside
channel queuing.
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Packet Headers (Cont)

Options
C
Class
Number
If C set, the option will copied to all fragments.
Otherwise, only to the first one.

Class 0 for control
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Class 2 for debugging and measurement.
Options are rarely used in today except for
‘loose’ and ‘strict’ source routing parameters.
‘loose’ and ‘strict’ source option sometimes, is
used for IP encapsulation in another IP or
“Tunneling”
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Fragmentation and Reassembly
0
7|0
7|0
Identification
4
7|0
Flags
 Flags
 DF: Don’t Fragment
 MF: More Fragment coming
7
Fragment Offset
0
1
2
0
DF
MF
In fragmentation, IP copy the original header and only modify
The length, which is the new length, and offset.
Offset is used for reassembly.
 Note: Fragmentation may degrade the network performance.
That is why the IP packet should be the same of TCP packets
Modern TCP implement “Path MTU discovery”.
It start with large packet and with DF set flag, if it passed
TCP keeps the same packet size, otherwise, it reduces it.
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Fragmentation and Reassembly
(cont)
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Each network has a Maximum Transfer Unit size,
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
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Example
• Packet delivery from host H1 to host H8
H1
R1
ETH IP(1400)
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R2
FDDIIP(1400)
R3
H8
PPP IP(512)
ETH IP(512)
PPP IP(512)
ETH IP(512)
PPP IP(376)
ETH IP(376)
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Example (cont)
The packets are fragmented as:
Start of header
Ident = x
1
Offset = 0
Rest of header
Start of header
Ident = x
0
512 data bytes
Offset = 0
Rest of header
1400 data bytes
Start of header
Ident= x
1 Offset= 512
Rest of header
512 data bytes
Start of header
Ident = x
0 Offset= 1024
Rest of header
376 data bytes
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Addressing
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Each host in the network is identified by an
address having the following property.
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globally unique
hierarchical: network + host
Address Classes
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Class D for Multicasting
Class E for experiments
A:
B:
7
24
0 Network
1 0
Host
14
16
Network
Host
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Address Notation
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C:
1 1 0
Network
8
Host
10.3.2.4
128.96.33.81
192.12.69.77
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IP Addresses
Example:
Class “A” address
www.mit.edu
18.181.0.31
(18<128 => Class A)
Class “B” address
Class C)
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mekong.stanford.edu
171.64.74.155
(128<171<128+64 =>
www.ece.ut.ac.ir
194.225.
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Addressing in IP
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IP addresses are names of interfaces
Domain Name System (DNS) names are
names of hosts
DNS binds host names to interfaces
Routing binds interface names to paths
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How to assign IP Addresses?
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Manually
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Uniqueness
Too much and tedious job
Dynamically: use DHCP: Dynamic Host
Configuration Protocol.
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Making a Forwarding Decision
IP Address Space
Class A
Class B
Class A
212.17.9.4
Class B
Class C
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Class C
D
Routing Table:
Exact
Match
212.17.9.0
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212.17.9.0 Port 4
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Forwarding Datagrams
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Every datagram contains a destination
address.
“Network ID” uniquely identifies a
physical network.
All hosts and routers sharing a Network
ID share same physical network.
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Forwarding an IP Router
• Lookup packet DA in forwarding table.
– If known, forward to correct port.
– If unknown, drop packet.
• Decrement TTL, update header Checksum.
• Forward packet to the outgoing interface.
• Transmit packet onto link.
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Address Translation
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Map IP addresses into physical addresses
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Techniques
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destination host
next hop router
encode physical address in host part of IP address
table-based
ARP
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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
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ARP Details
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Request Format
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HardwareType: type of physical network (e.g., Ethernet)
ProtocolType: type of higher layer protocol (e.g., IP)
HLEN & PLEN: length of physical and protocol addresses
Operation: request or response
Source/Target-Physical/Protocol addresses
Notes
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table entries timeout in about 10 minutes
update table with source when you are the target
update table if already have an entry
do not refresh table entries upon reference
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ARP Packet Format
0
8
16
Hardware type = 1
HLen = 48
31
ProtocolType = 0x0800
PLen = 32
Operation
SourceHardwareAddr (bytes 0 – 3)
SourceHardwareAddr (bytes 4
SourceProtocolAddr (bytes 2
– 5) SourceProtocolAddr (bytes 0
– 3)
– 1)
TargetHardwareAddr (bytes 0 – 1)
TargetHardwareAddr (bytes 2
TargetProtocolAddr (bytes 0
– 5)
– 3)
•HLen = Hardware Address Length
•PLen = Protocol Address Length
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Internet Control Message
Protocol (ICMP)
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Echo (ping)
Redirect (from router to source host)
Destination unreachable (protocol, port,
or host)
TTL exceeded (so datagrams don’t cycle
forever)
Checksum failed
Reassembly failed
Cannot fragment
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Tunneling
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Big companies having different networks want to
connect them together.
Virtual Private Network (VPN)
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Use leased line to connect networks.
Use Internet (Shared line).
Network 1
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Sloution
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R1
Internet
R2
Network 2
Virtual line
Encapsulate packets in R1 in IP packets for destination
R2.
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IP Address Problem (1991)
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Inefficient use of Hierarchical Address Space
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class C with 2 hosts (2/255 = 0.78% efficient)
class B with 256 hosts (256/65535 = 0.39%
efficient)
Address space depletion
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In danger of running out of classes A and B
Class C too small for most domains
Very few class A – IANA (Internet Assigned
Numbers Authority) very careful about giving
Class B – greatest problem
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IP Address Utilization (‘98)
http://www.caida.org/outreach/resources/learn/ipv4space/
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Solutions for IP Address
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Better utilize address space
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Subnetting
Supperneting
Locally use unofficial IP addresses (NATing)
Use wider IP addresses
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New version of IP, IPv6.
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Subnetting
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Add another level to address/routing hierarchy:
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subnet
Subnet masks define variable partition of host part
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Subnets visible only within site
Network number
Host number
Class B address
1111111111111111111
0000000000000000
Subnet mask (255.255.0.0)
Network number
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Subnet ID
Subnetted address
Host ID
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Subnet Example
Subnet
Net
host
Subnet mask: 255.255.255.128.
Subnet number: 128.96.34.0
128.96.34.15
H1
111….1.0xxx….x
128.96.34.1
R1
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.128
128.96.34.130
128.96.34.139
128.96.34.129
H3
128.96.33.14
H2
R2
128.96.33.1 Forwarding
Subnet mask: 255.255.255.0
Subnet number: 128.96.33.0
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table at router R1
Subnet #
128.96.34.0
128.96.34.128
128.96.33.0
Subnet Mask
255.255.255.128
255.255.255.128
255.255.255.0
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Next Hop
interface 0
interface 1
R2
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Forwarding Algorithm
D = destination IP address
for each entry (SubnetNum, SubnetMask, NextHop)
D1 = SubnetMask & D
if D1 = SubnetNum
if NextHop is an interface
deliver datagram directly to D
else
deliver datagram to NextHop
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Use a default router if nothing matches
Not necessary for all 1s in subnet mask to be
contiguous
Can put multiple subnets on one physical network
Subnets
not visible
fromtothe
rest
of the Internet 32
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Supernetting
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Assign block of contiguous network numbers to
nearby networks
Called CIDR: Classless Inter-Domain Routing
Represent blocks with a single pair
(first_network_address, count)
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Restrict block sizes to powers of 2
Use a bit mask (CIDR mask) to identify block size
All routers must understand CIDR addressing
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Classless Addressing
CIDR
Class-based:
A
0
Classless:
B
128.9.0.0
65/8
0
C
D
232-1
142.12/19
128.9/16
216
232-1
128.9.16.14
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Classless Addressing
CIDR
128.9.19/24
128.9.25/24
128.9.16/20 128.9.176/20
128.9/16
0
232-1
128.9.16.14
Most specific route = “longest matching prefix”
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Forwarding Datagrams
128.17.20.1
R2
1
R1 2
3
R3
R4
128.17.16.1
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e.g. 128.9.16.14 => Port 1
Prefix
Next-hop
Port
65/8
128.9/16
128.9.16/20
128.9.19/24
128.9.25/24
128.9.176/20
142.12/19
128.17.16.1
128.17.14.1
128.17.14.1
128.17.10.1
128.17.14.1
128.17.20.1
128.17.16.1
3
2
1
7
2
1
3
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Default Routing
R1
Default
Routing
R2
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R3
Requires
Routing
Table
R4
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R5
Default
Routing
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Inside a Router
1.
Forwarding
Table
2.
3.
Output
Scheduling
Interconnect
Forwarding
Decision
Forwarding
Table
Forwarding
Decision
Forwarding
Table
Forwarding
Decision
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NAT: Network Address
Translation

IP addresses need to be unique when we are going
outside of our network.


We can use whatever IP address as long as we are
functioning inside our own network.
Translate to a valid IP address when going
out. Do reverse when coming in.
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Mobile IP: Motivation

Connectivity everywhere
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

Overlapping, heterogeneous networks
Small, portable devices
Maintaining ongoing connections as the
user moves
Why maintain connectivity?


Avoid restarting applications/networks
Avoid losing “distributed state”
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Problems?



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The IP address associated with a mobile
host is network dependent!
When user connects to another network,
IP address needs to change
Packets belonging to ongoing connections
somehow need to be delivered to the
mobile host
Solutions?

Any solution should be transparent
Intuitive Solution




Take up the analogy of you moving from
one apartment to another
What do you do?
Leave a forwarding address with your old
post-office!
The old post-office forwards mails to your
new post-office, which then forwards
them to you
Basic Mobile IP
MH = mobile host
CH = correspondent host
HA = home agent
FA = foreign agent
(We’ll see later that FA
is not necessary or even
desirable)
CH
Home network
HA
Foreign network
FA
MH
•MH registers new “care-of address” (FA) with HA
•HA tunnels packets to FA
•FA decapsulates packets and delivers them to MH
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Packet Tunneling
Packet from CH to MH
Source address = address of CH
Destination address = home IP address of MH
Payload
Home agent intercepts above packet and tunnels it
Source address = address of HA
Destination address = care-of address of MH
Source address = address of CH
Destination address = home IP address of MH
Original payload
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host moves again
CH
Home network
HA
Foreign network #1
FA #1
MH
Foreign network #2
FA #2
MH
•MH registers new address (FA #2) with HA & FA #1
•HA tunnels packets to FA #2, which delivers them to MH
•Packets in flight can be forwarded from FA #1 to FA #2
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Basic Mobile IP (cont)
Mobile hosts also send packets
CH
Home network
HA
Foreign network
FA
MH
•Mobile host uses its home IP address as source address
-Lower latency
-Still transparent to correspondent host
-No obvious need to encapsulate packet to CH
•This is called a “triangle route”
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Mobile IP (RFC 2002)
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Leaves Internet routing fabric unchanged
Does assume “foreign Agent ” exist
everywhere
Simple
Correspondent hosts don’t need to know
about mobility
Works both for changing domains and
network interfaces
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IP Version 6

Features

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128-bit addresses (classless)
multicast
real-time service
authentication and security
autoconfiguration
Any cast address
protocol extensions
Header


40-byte “base” header
extension headers (fixed order, mostly fixed length)




No fragmentation
source routing
authentication and security
other options
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IPV6: Packet format
0
4
Version
12
16
24
31
FlowLabel
TrafficClass
PayloadLen
NextHeader
HopLimit
…
SourceAddr (16 bytes)
…
DestinationAddr (16 bytes)
Next header/ Data

Next header: IP option and protocol fields in IPv4. If options
(i.e. fragmentation) indicated by this field, otherwise, it is
protocol fields.
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