Transcript The Network Layer - Villanova University

The Network Layer
Responsibilities
 Addresses
 Routing
 Fragmentation and reassembly
Network layer addresses
 IP address structure (v4)
 Class A
|0xxxxxxx|yyyyyyyyyyyyyyyyyyyyyyyy|
 Class B
|10xxxxxxxxxxxxxx|yyyyyyyyyyyyyyyy|
 Class C
|110xxxxxxxxxxxxxxxxxxxxx|yyyyyyyy|
 Multicast|1110xxxxxxxxxxxxxxxxxxxx|yyyyyyyy|
 Reserved |1111xxxxxxxxxxxxxxxxxxxx|yyyyyyyy|
IP v4 Class A
 Class A
|0xxxxxxx|yyyyyyyyyyyyyyyyyyyyyyyy|
 27 networks
 each with up to 224 hosts attached
 Not quite. Addresses of all 0 or all 1 are special
cases and not permitted for general use
IP v4 Class B, C
 Class B
|10xxxxxxxxxxxxxx|yyyyyyyyyyyyyyyy|
 214 networks
 each with up to 216 hosts
– - again, not quite.
 Class C
|110xxxxxxxxxxxxxxxxxxxxx|yyyyyyyy|
 221 networks
 each with up to 28 hosts (approximately)
Non unique addresses
 Growth of the Internet has placed demands on the
address space not anticipated originally. There are
more machines than addresses available.
 Some machines do not need a unique address,
because they do not communicate over the
Internet.
 Addresses are set aside to be used as desired for
those machines:
– 10.0.0.0 - 10.255.255.255 (10/8 prefix) {Start with 10, use 8 bits}
– 172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
– 192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
Ref. RFC 1918
Network and Host addresses
 The network address identifies a network
comprised of multiple computers and other
devices.
 Routers deal with network addresses.
 Once the transmission reaches the right network,
the local network protocols deal with delivery to
the correct machine.
 The host address identifies a particular machineto-network connection.
Subnets
 Once upon a time, 254 hosts per network
seemed pretty reasonable
 That was before PCs
 Class C networks are not large enough for
most kinds of organizations
 Multiple Class C networks in a single
organization imposes management overhead
Subnets (2)
 Subnetting allows an organization to
subdivide a network internally. The internal
networks continue to look like a single
network from outside the organization
 Take some bits from the host part of the IP
address and make them part of the network
part for internal routing
The subnet mask
 Allows the routers to know how many bits are part of the
network address and how many are part of the host address
 Example: a class B network is subnetted so that 5 bits of
the host address are part of the network address:
1 10011001
1101000
2
Network
3
Network
00011111
00000110
Host
Host
1 = the 32 bit address
2 = network/host division without subnetting
3 = network/host division with subnetting
Mask tells which bits to consider part of the network address:
1 in each net address position; 0 elsewhere.
Mask for the example is 11111111 11111111 11111000 00000000
Dotted decimal representation:
255
255
248
0
Subnets (3)
The old network addresses for some of our machines
 Tiger:
 wild:
 renoir:
 camille:
 tanner:
 hawk:
 cassel:
 smurfs:
153.104. 7.161
153.104. 1. 10
153.104. 7.174
153.104. 7. 1
153.104. 7.178
153.104. 8. 50
153.104. 7.181
153.104.24. 32
What class network?
153 = 10011001 => Class B network
Any indication of subnetting?
Current subnetting
 Mendel (CSC)
 Within VU’s 153.104
 IP address range start 200.1
 IP address range end 203.254
 What is the subnet mask?
255.255.252.0
Subnetting and DSL
 Some DSL providers offer static IP
addresses in groups of 8 (really?)
 What does that mean in terms of IP
subnetting?
 One possibility:
– A class C network is divided among a group of
subscribers. Each gets a subnet mask that
allows 8 addresses.
– Addresses 000 and 111 are not legal IP
addresses, though.
IP v6
 128 bit addresses
– written as 8 parts, separated by :
– each part is 6 bits, expressed in hex
– (no more dotted decimal)
 Notes:
– space reserved for other address schemes
– place to imbed the local link address
– multicast, anycast, no broadcast
IP V6 packet layout
Flow Label
Version Priority
Payload Length
Next Header
Hop Limit
Source Address
Destination Address
Version = 6 Flow label = connect packets from the same source
Payload = Packet size in bytes
Next header = Next layer up connection (Protocol)
Hop Limit = time to live in hops
Routing - Link State overview
 Each routing node obtains
– the information concerning the immediate
neighbors of each other node in the network
– Once this information is available, the node
constructs a graphical representation of the
internet
Routing - Link State details
 Enter self into table
 Enter data from immediate neighbors
– mark this data tentative (T)
 For each node marked T in the table,
examine the connection information about
that node and enter it into the table.
– Consider T nodes in order of cost to get there,
least costly first
– Previously unknown nodes are added
– Previously known nodes are examined to see if
a better route is found
Routing - Distance Vector
 Each router node knows about itself
– the distance to itself = 0
• first entry in the routing table
 Each router knows about its directly
connected neighbors
– the distance to a direct neighbor = 1
• next set of entries
 Exchanging information with neighbors
extends the diameter of the known universe
to each router
Our sample network
A
B
E
C
D
H
J
G
M
F
K
I
A special problem
What happens if we apply the link state protocol to the
following special situation:
C
A
B
1. Determine the routing table entries for each router: A, B, C
2. Assume the connection between B and C is broken
3. Show the steps by which A, B revise their tables
This is the “counting to infinity” problem
Border Gateway Protocol
 See
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm
– For complete information on BGP
 BGP is a link state protocol
 BGP is run between autonomous systems, rather than within
autonomous systems
 Instead of using a cost metric, the BGP messages contain an entire
route to the destination
Routing within the VU domain
Connection to our service provider
Internal routers
153.104.0.18
153.104.0.249
153.104.0.254
153.104.0.19
...
153.104.0.1
153.104.200.1
153.104.201.1
153.104.202.1
153.104.203.1
How would you fill in the missing numbers?
Routing from Renoir out
1 153.104.200.1 (153.104.200.1) 0.825 ms 0.631 ms 0.590 ms
2 153.104.0.1 (153.104.0.1) 1.024 ms 0.724 ms 0.701 ms
3 153.104.0.254 (153.104.0.254) 1.053 ms 1.382 ms 1.801 ms
4 207.68.14.11 (207.68.14.11) 6.086 ms 9.067 ms 6.155 ms
5 205.171.38.85 (205.171.38.85) 8.062 ms 10.089 ms 12.455 ms
6 nyc-core-03.inet.qwest.net (205.171.17.121) 11.345 ms 10.354 ms 10.395 ms
7 nyc-core-01.inet.qwest.net (205.171.17.82) 10.308 ms 17.639 ms *
8 wdc-core-02.inet.qwest.net (205.171.5.235) 19.174 ms 16.058 ms 17.888 ms
9 wdc-core-03.inet.qwest.net (205.171.24.6) 20.636 ms 20.425 ms 21.594 ms
10 hou-core-01.inet.qwest.net (205.171.5.187) 36.128 ms 43.064 ms 44.321 ms
11 hou-edge-07.inet.qwest.net (205.171.23.14) 37.849 ms 41.555 ms 41.659 ms
12 205.171.36.154 (205.171.36.154) 52.102 ms 50.555 ms 52.055 ms
13 192.12.10.60 (192.12.10.60) 49.084 ms 49.554 ms 46.130 ms
14 ser9-msfc1.gw.utexas.edu (128.83.2.9) 50.420 ms 50.396 ms 46.334 ms
15 128.83.37.18 (128.83.37.18) 49.908 ms 57.542 ms 50.448 ms
16 cs.utexas.edu (128.83.139.9) 50.164 ms 46.581 ms *
traceroute Christie.netlab.csc.villanova.edu
traceroute to Christie.netlab.csc.villanova.edu (153.104.203.200), 30 hops max, 38 byte packets
1 pm40.iwaynet.net (198.30.105.210) 117.453 ms 109.666 ms 119.863 ms
2 icg-gw.iwaynet.net (198.30.105.193) 119.719 ms 109.765 ms 139.856 ms
3 oeb7-sl0-0-0c10.columbus.oar.net (199.18.98.37) 129.763 ms 118.785 ms 109.832 ms
4 oeb9-atm1-0.columbus.oar.net (199.18.202.19) 119.748 ms 129.768 ms 109.871 ms
5 208.46.62.49 (208.46.62.49) 139.748 ms 139.751 ms 149.855 ms
6 chi-core-03.inet.qwest.net (205.171.20.33) 129.769 ms 129.782 ms 159.867 ms
7 chi-core-02.inet.qwest.net (205.171.20.29) 159.762 ms 139.801 ms 119.864 ms
8 nyc-core-02.inet.qwest.net (205.171.5.249) 149.749 ms 139.759 ms 149.839 ms
9 205.171.17.118 (205.171.17.118) 139.753 ms 169.741 ms 159.854 ms
10 205.171.38.62 (205.171.38.62) 149.753 ms 159.793 ms 205.171.38.86 (205.171.38.86)
159.861 ms
11 207.68.14.50 (207.68.14.50) 159.701 ms 629.814 ms *
12 153.104.0.249 (153.104.0.249) 179.816 ms 159.723 ms 199.836 ms
13 153.104.0.18 (153.104.0.18) 169.751 ms 159.807 ms 169.850 ms
14 * * 153.104.0.18 (153.104.0.18) 1339.845 ms !H
15 * * 153.104.0.18 (153.104.0.18) 1889.932 ms !H
Routing to Christie 16 * * 153.104.0.18 (153.104.0.18) 1869.955 ms !H
17 * * *
attempt when netlab
18 153.104.0.18 (153.104.0.18) 1759.827 ms !H * *
was disconnected
19 153.104.0.18 (153.104.0.18) 1849.827 ms !H
Routing - scale
 How big is a routing table?
– Assume the current IP v4 address scheme
– Assume that subnets are internal and not the
problem of internet routers
– What is the potential load on a router?
Classless Inter-Domain Routing
 First pass at hierarchical routing in the
Internet
– Assign addresses in clumps that are not
dependent on the old Class A, B, C scheme.
– Much more flexible in the allocation of space
and able to serve the needs of users more
efficiently.
CIDR address assignments
CIDR Block Prefix
/27
/26
/25
/24
/23
/22
/21
/20
/19
/18
/17
/16
/15
/14
/13
Equivalent Class C # of Host Addresses
1/8th of a Class C 32 hosts
1/4th of a Class C
64 hosts
1/2 of a Class C
128 hosts
1 Class C
256 hosts
2 Class C
512 hosts
4 Class C
1,024 hosts
8 Class C
2,048 hosts
16 Class C
4,096 hosts
32 Class C
8,192 hosts
64 Class C
16,384 hosts
128 Class C
32,768 hosts
256 Class C
65,536 hosts (= 1 Class B)
512 Class C
131,072 hosts
1,024 Class C
262,144 hosts
2,048 Class C
524,288 hosts
A case …
Currently, big blocks of addresses are assigned to the large Internet
Service Providers (ISPs) who then re-allocate portions of their address
blocks to their customers. For example, Pacific
Bell Internet has been assigned a CIDR address block with a
prefix of /15 (equivalent to 512 Class C addresses or 131,072 host
addresses) and typically assigns its customers CIDR
addresses with prefixes ranging from /27 to /19. These customers,
who may be smaller ISPs themselves, in turn re-allocate portions of their
address block to their users and/or customers.
However, in the global routing tables all these different networks
and hosts can be represented by the single Pacific Bell Internet route
entry. In this way, the growth in the number of
routing table entries at each level in the network hierarchy has
been significantly reduced. Currently, the global routing tables have
approximately 35,000 entries.
Ref: http://public.pacbell.net/dedicated/cidr.html
Network Address Translation
 Primary source for information: RFC1631
 Goal: Stand between the local network
environment and the rest of the Internet
The Internet
Router
IP address
Local network
environment
Why use NAT
 Non unique addresses on the internal
network work fine for communication that
does not involve the global Internet.
 To provide communication between a
machine with a non unique address and the
global Internet, the address must be
translated into a globally unique address.
How dynamic NAT works
192.168.0.1
192.168.0.2
153.104.203.220
153.104.203.220
153.104.203.220
192.168.0.3
Internal
network has
non-unique
IP addresses
153.104.203.220
NAT box has an
address translation
table and a set of
assigned IP
addresses that can be
used in the Internet
Internal host external connection
192.168.0.1
192.168.0.2
192.168.0.3
153.104.203.220
153.104.203.220
153.104.203.220
192.168.0.3
Internal host
requests
connection to an
external host
153.104.203.220
NAT associates the
internal address with a
globally unique address
and makes the connection
with the external host
Response from external host
192.168.0.1
192.168.0.2
192.168.0.3
192.168.0.3
153.104.203.220
153.104.203.220
153.104.203.220
153.104.203.220
Response from external
host connected to the
right internal host
Once there has been an exchange of
messages, the table has the mapping needed
and further communications are just passed
through.
Overloading
192.168.0.1
192.168.0.2
192.168.0.3
192.168.0.3
Port 23
192.168.0.2/25
153.104.203.220
Port 2000
Port 2001
Port 2002
Port 2003
Port 2004
When there are not as many unique IP
addresses as internal hosts who may
want to access external hosts, add the
use of port numbers in the table
Variable length subnet masks
 Originally, subnet masks were of a fixed
length
– Clearly inefficient for an organization that has
logical subnets of varying size
– Recent revisions of the routing protocol
implementations allow variable length subnet
masks
Fragmentation, reassembly
 Routers connect networks
– pass messages from one network to another
 Network characteristics are not all the same
– maximum packet size varies
 Routers must break up large packets to
allow them to go into networks where the
maximum allowed size is smaller
 Question: Where to reassemble?
Reassembly question
 Should a router join packets to make larger
ones when a fragmented transmission is
leaving a network?
– Large packets require fewer routing decisions;
they are more efficient
– Reassembly and then later fragmentation are
time consuming; these activities should be
minimized.
Network layer summary
 Addressing
– current most common is IP v4
– subnetting adds flexibility to network sizes
 Routing
– Link State and Distance Vector
 Fragmentation/Reassembly
– dealing with the restrictions of individual
networks.