Post on 30-Dec-2015
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Chapter 4 - Internetworking
Networks are built with different technology We want to be able to connect these differing
networks This chapter is about the problems of
interconnecting different networks. Basically layer 3 – the Network layer Routers are the main network device
Main Issues
IP or Internet protocol Finding efficient, loop-free paths through the
constituent networks Internet problems: address space, large
routing tables, scaleable networks Multicast
What is an Internetwork?
Look over figures 4.1and 4.2 They are graphically describing the issue of
interconnecting differing networks and a simple way of looking at the overall changing of protocols
IP service model: best effort
R2
R1
H4
H5
H3H2H1
Network 2 (Ethernet)
Network 1 (Ethernet)
H6
Network 3 (FDDI)
Network 4(point-to-point)
H7 R3 H8
R1
ETH FDDI
IPIP
ETH
TCP R2
FDDI PPP
IP
R3
PPP ETH
IP
H1
IP
ETH
TCP
H8
IP Packet Structure
Note in particular: source address destination address TTL (time-to-live) Protocol (higher level-TCP, UDP etc.)
Version HLen TOS Length
Ident Flags Offset
TTL Protocol Checksum
SourceAddr
DestinationAddr
Options (variable) Pad(variable)
0 4 8 16 19 31
Data
H1 R1 R2 R3 H8
ETH IP (1400) FDDI IP (1400) PPP IP (512)
PPP IP (376)
PPP IP (512)
ETH IP (512)
ETH IP (376)
ETH IP (512)
IP Addresses
Examples: www.cnn.com 64.236.16.20
Registered to billionaire Ted Turner, one big site!! www.nvc.cs.vt.edu 208.22.18.79
CS department site, a small to medium site www.somuchdata.com hosted at 216.40.247.57
Registered to me, William May (used to advertise a book I wrote three years ago), a very small site!
IP Addresses and Classes
Class A, B and C addresses Shown in figure 4.6 For most medium to large organizations like
companies, universities, government offices etc. Class A is way too big and Class C is too small
Class B is where the crunch came
Network Host
7 24
0(a)
Network Host
14 16
1 0(b)
Network Host
21 8
1 1 0(c)
Datagram forwarding in IP
Every IP datagram contains the destination IP address
The “network” part of the address uniquely identifies a single physical network
All hosts that share the same network part of the address, are connected to the same physical network and can communicate with each other by sending frames over that network
So just how does the datagram get forwarded?
Two parts Routers forward the datagram based on the
network part of the address At the end there has to be a (local) table of
address pairs – that is a table that maps IP addresses into physical addresses ARP (Address Resolution Protocol)
And how does a sender determine where to send the datagram?
A host needs the address of the first router to send a datagram on its way
Called the default router DHCP is one common way of learning which
is your default router
DHCPrelay
DHCPserver
Other networks
Unicast to server
Broadcast
Host
Virtual Networks and Tunnels
Figure 4.12 explains it best, another level of encapsulation
IP header ,Destination = 2.x
IP payload
IP header ,Destination = 10.0.0.1
IP header ,Destination = 2.x
IP payload
IP header ,Destination = 2.x
IP payload
Network 1 R1 Internetwork Network 2R2
10.0.0.1
Routing
Routing table – generally contains mappings from network numbers to next hops (which are labeled as output ports on the router)
“Routing is the process by which forwarding tables are built.” (page 281)
Routing Protocols
Routers talking to other routers (very roughly-experts would shoot me for this
analogy!) the router analog of the spanning tree procedure for switches
Routing Protocols
We will look at several: Static routes (work fine for small internetworks) RIP (most widely used, simple but can be used on
reasonably large internetworks) OSPF (scaleable, good for campus-sized
internetworks) BGP (used by ISPs, very complicated, we will
only discuss in passing)
RIP
The ICND book has a great presentation of this protocol
I recommend going over that presentation, it is mainly pictures of how the routing table changes
Distance vector – RIP counts “hops”
4
3
6
21
9
1
1D
A
FE
B
C
D
G
A
F
E
B
C
A
C
1
2
3
B
D
4
5
6
Address of net 2
Distance to net 2
Command Must be zero
Family of net 2 Address of net 2
Family of net 1 Address of net 1
Address of net 1
Distance to net 1
Version
0 8 16 31
OSPF
Link State Figure 4.17 (next slide) show the basics of
how the routers talk to each other
X A
C B D
(a)
X A
C B D
(b)
X A
C B D
(c)
X A
C B D
(d)
D
A
B
C
5 3
211
10
OSPF Hierarchy
Hierarchy is one of the main tools to make systems more scaleable
OSPF allows more hierarchy to be imposed by partitioning a domain into areas
This means: a router within a domain does not necessarily need to know how to reach every network within that domain – cuts down on the information that has to be stored and processed
R9 R7R8
R5R6
R4
Area 1Area 0
Area 3
Area 2
R1 R3
R2
Authentication
Version Type Message length
Checksum Authentication type
SourceAddr
AreaId
0 8 16 31
LS Age Options Type=1
0 Flags 0 Number of links
Link type Num_TOS Metric
Link state ID
Advertising router
LS sequence number
Link ID
Link data
Optional TOS information
More links
LS checksum Length
The Internet
1990 form in figure 4.23 Today’s is shown next
Backbone service provider
Peeringpoint
Peeringpoint
Large corporation
Large corporation
Smallcorporation
“Consumer ” ISP
“Consumer ” ISP
“ Consumer ” ISP
Exhaustion of IP Addresses
~4,000,000,000 (232 addresses) are not enough!
Class B particularly bad off Subnetting and CIDR (Classless InterDomain
Routing) are a temporary solution
Network number Host number
Class B address
Subnet mask (255.255.255.0)
Subnetted address
111111111111111111111111 00000000
Network number Host IDSubnet ID
Subnet mask: 255.255.255.128Subnet number: 128.96.34.0
128.96.34.15128.96.34.1
H1
R1
128.96.34.130 Subnet mask: 255.255.255.128Subnet number: 128.96.34.128
128.96.34.129128.96.34.139
R2H2
128.96.33.1128.96.33.14
Subnet mask: 255.255.255.0Subnet number: 128.96.33.0
H3
BGP
Additional level of hierarchy: AS or Autonomous Systems
Page 322 explains three major problems with interdomain routing and the Internet. BGP is designed to help with these
A very hard protocol, the book only skims over it
Border gateway(advertises path to
11000000000001)
Regional network
Corporation X(11000000000001000001)
Corporation Y(11000000000001000000)
R1
Autonomous system 1R2
R3
Autonomous system 2R4
R5 R6
Backbone network(AS 1)
Regional provider A(AS 2)
Regional provider B(AS 3)
Customer P(AS 4)
Customer Q(AS 5)
Customer R(AS 6)
Customer S(AS 7)
128.96192.4.153
192.4.32192.4.3
192.12.69
192.4.54192.4.23
IPv6
Aimed at solving several of the problems that we have seen
The book describes the packet at a high level but doesn’t focus on what IPv6 will do for the Internet of the future
Multicast
Motivation: there are applications that want to send a packet to more than one destination host
Forcing the source to send a separate packet to each destination host wastes resources big time!
Multicast (a work still in progress) is aimed at making this procedure more efficient by not forcing separate packets on the sender
AR1 R2
B
R3 R4 R5
CR6 R7
Source
AR1 R2
B
R3 R4 R5
CR6 R7
Source
AR1
R2
B
R3 R4 R5
CR6 R7
Source
AR1 R2
B
R3 R4 R5
CR6 R7