Transcript notes - Academic Csuohio

EEC-484/584
Computer Networks
Lecture 13
Wenbing Zhao
[email protected]
(Part of the slides are based on Drs. Kurose & Ross’s slides
for their Computer Networking book, and on materials supplied by
Dr. Louise Moser at UCSB and Prentice-Hall)
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Outline
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Classful IP address allocation
CIDR
Network address translation
ICMP
IPv6
Spring Semester 2006
EEC-484/584: Computer Networks
Wenbing Zhao
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IP Addresses
• Classful addressing - every host and router has
unique IP address consisting of network number
and host number (2 level hierarchy)
– E.g., Class A: up to 27 = 128 networks with up to 224
= 16,777,216 hosts each
• Network numbers are managed by ICANN
(Internet Corporation for Assigned Names and
numbers) to avoid conflicts
• No longer used, but references to it are still
common
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EEC-484/584: Computer Networks
Wenbing Zhao
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IP Addresses
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Wenbing Zhao
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IP Addresses
• IP address are usually written in dotted decimal
notation
– Each of the 4 bytes is written in decimal, from 0 to 255
– Lowest IP 0.0.0.0, highest 255.255.255.255
• Special IP addresses
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Subnets
• Allow a network to be split into several parts for internal use,
but to act as a single network to outside world
• Take some bits away from host numbers
• Subnet mask – needed by the main router. Indicates split
between network + subnet number and host
– Write the address and the mask as a binary number
– If mask bit is 1, then corresponding bit of address matters
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Wenbing Zhao
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Subnets
• E.g., A class B network can be subnetted into 64 subnets
– Originally 16 bits for host info. Now, 6 bits used for subnet and 10
bits for host numbers
– Subnet mask can be written as 255.255.252.0 or /22
Subnet 1: 10000010 00110010 000001 00 00000001
Subnet 2: 10000010 00110010 000010 00 00000001
Subnet 3: 10000010 00110010 000011 00 00000001
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130.50.4.1
130.50.8.1
130.50.12.1
Wenbing Zhao
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Routing without Subnets
• Each router has a table listing two types of entries:
– (network, 0): tells how to get to distant networks
– (this-network, host): tells how to get to a local host
• When an IP packet arrives, its destination address
is looked up in the routing table
– If the packet is for a distant network, it is forwarded to the
next router on the interface given in the table
– If it is for a local host, it is sent directly to the destination
– If the network is not present, the packet is forwarded to a
default router with more extensive tables
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Wenbing Zhao
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Routing with Subnets
• Three-level hierarchy: entries in a routing table take the
form
– (network, 0)
– (this-network, subnet, 0) and
– (this-network, this-subnet, host)
• A router on subnet k knows how to get to all the other
subnets and also how to get to all the hosts on subnet k
• If a packet is for this network
– Do a Boolean AND of the destination address with the network’s
subnet mask to get rid of the host number
– Look up the resulting address in the routing table
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Routing with Subnets
• For example, a packet address to 130.50.15.6, the
subnet mask is 255.255.252.0/22, AND them, we get
130.50.12.0 and this address is looked up in the routing
table to find out which output line to use
Subnet base address
10000010 00110010 00000100 00000000 Output to line A
10000010 00110010 00001000 00000000 Output to line B
10000010 00110010 00001100 00000000 Output to line C
10000010 00110010 00001111 00000110 (Destination IP Addr: 130.50.15.6)
11111111 11111111 11111100 00000000 (Subnet mask: 255.255.252.0)
10000010 00110010 00001100 00000000 (After AND operation)
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Problems with Classful Addressing
• A class is obviously too large for any organization
• C class is too small (only 256 addresses available)
• B class is requested and allocated, but it is still too
large for most organizations
 Many IP addresses are wasted
 Shortage of IP addresses
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CIDR – Classless InterDomain Routing
• For the remaining IP addresses, classless
allocation is used
– Allocate remaining IP addresses in variable-sized
blocks (must be power of 2), without regard to the
classes
– The starting address must fall on the boundary of
the block size
– E.g., if a site needs, say, 2000 addresses, it is given a
block of 2048 addresses on a 2048-byte boundary
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Classless Allocation – Example
5-59
• Routing tables are updated with the three assigned
entries. Each entry contains a base address and a
subnet mask (in short: base address/subnet mask)
C: 11000010 00011000 00000000 00000000 11111111 11111111 11111000 00000000
E: 11000010 00011000 00001000 00000000 11111111 11111111 11111100 00000000
O: 11000010 00011000 00010000 00000000 11111111 11111111 11110000 00000000
Base address
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Subnet mask
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Classless InterDomain Routing
• Each routing table is extended by giving it a 32-bit mask
• The routing table contains entries of
(IP address, subnet mask, outgoing line) triples
• When a packet comes in, its destination IP address is first
extracted
• Then, the routing table is scanned entry by entry, masking
the destination address and comparing it to the table entry
looking for a match
• If multiple entries (with different subnet mask lengths)
match, the longest mask is used
– E.g., if there is a match for a /20 mask and a /24 mask, the /24
mask is used
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Classless InterDomain Routing:
Example
• If a packet is addressed to 194.24.17.4, in binary
11000010 00011000 00010001 00000100
• First it is Boolean ANDed with the Cambridge mask to get
11000010 00011000 00010000 00000000
• This value does not match the Cambridge base address, so next try
Edinburgh mask, to get
11000010 00011000 00010000 00000000
• This value still does not match, so Oxford is tried, yielding
11000010 00011000 00010000 00000000
• This value matches the Oxford base. If no longer matches are found, the
Oxford entry is used and the packet is sent along the line named in it
C: 11000010 00011000 00000000 00000000 11111111 11111111 11111000 00000000
E: 11000010 00011000 00001000 00000000 11111111 11111111 11111100 00000000
O: 11000010 00011000 00010000 00000000 11111111 11111111 11110000 00000000
Base address
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Subnet mask
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Classless InterDomain Routing
• Aggregate entry – all three new entries can be combined
into a single aggregate entry 194.24.0.0/19 with a binary
address and submask as follows:
11000010 00000000 00000000 00000000 11111111 11111111 11100000 00000000
• By aggregating the three entries, a router has reduced its
table size by two entries
• Aggregation is heavily used throughout the Internet
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NAT – Network Address Translation
• Another workaround for the IP addresses shortage
problem: network address translation
– One public IP address, many private IP addresses
– When a packet exits the private network and goes to the ISP, an
address translation takes place
• Three ranges of IP addresses have been declared as
private:
– 10.0.0.0 – 10.255.255.255 (16,777,216 hosts)
– 172.16.0.0 – 172.31.255.255/12 (1,048,576 hosts)
– 192.168.0.0 – 192.168.255.255/16 (65,536 hosts)
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NAT – Network Address Translation
Placement and operation of a NAT box
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NAT – What about the Incoming Traffic?
• Solution is based on the assumption all traffic is
TCP/UDP
• TCP/UDP has two port fields, one for source port, the other
for destination port, each 16 bits wide
• The source port is used as an index to an internal table
maintained by the NAT box
• The internal sender’s private IP and original port info are
stored in the table
• When the reply comes back, it will carry the index as the
destination port, the NAT box then translates the address
back
• For both outgoing and incoming address translations, the
TCP/UDP and IP header checksums are recomputed
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Drawback of NAT
• NAT violates the architectural model of IP, which states that every IP
address uniquely identifies a single machine worldwide
• NAT box must maintain mapping info for each connection passing
through it. This changes the Internet from a connectionless network to
a kind of connection-oriented network
• NAT violates the most fundamental rule of protocol layering: layer k
may not make any assumptions about what layer k+1 has put into the
payload field
• NAT only support UDP/TCP traffic
• NAT has problem supporting apps that include local IPs in payload,
such as FTP and H.323
• Each NAT box can support at most 65,536 (216) hosts
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EEC-484/584: Computer Networks
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Internet Control Message Protocol
• ICMP messages are sent using the basic IP header
• The first byte of the data portion of the datagram is a
ICMP type field
– The type field determines the format of the remaining data
• Typical format: type, code plus first 8 bytes of IP
datagram causing error
0
1
2
3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
Type
|
Code
|
Checksum
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
unused
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
Internet Header + 64 bits of Original Data Datagram
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Destination Unreliable Message Format
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Internet Control Message Protocol
• When something unexpected occurs in Internet,
the event is reported by routers using ICMP
• Principal ICMP message types
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Internet Protocol Version 6
• IPv4 current version
• IPv5 experimental real-time stream protocol
• IPv6
– Longer addresses than IPv4 – 16 bytes
– Fixed-length 40 byte header
• No checksum field
• No fragmentation (by intermediate routers) allowed. Only
sender is allowed to fragment a packet, using fragment
extension
– Better support for options
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The Main IPv6 Header
8000:0000:0000:0000:0123:4567:89AB:CDEF
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The Main IPv6 Header
• Version 6
• Priority
– 0-7 slow down in event of congestion
– 8-15 real-time traffic
• Flow label – allows source and destination to set up
pseudo-connection with particular properties and
requirements
• Payload length (as opposed to total length in IPv4)
• Next header – additional optional extension header
• Hop limit (time to live in IPv4)
• Source/destination address – 128 bits (32 bits in IPv4)
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IPv6 Extension Headers
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IPv6 Extension Headers
http://www.tcpipguide.com/free/t_IPv6DatagramExtensionHeaders-2.htm
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IPv6 Extension Headers
• Two types of extension header formats
– Fixed format
– Variable number of variable-length fields
• Each item is encoded as a (type, length, value)
tuple
• Type: 1-byte field telling which option this is
• Length: 1-byte field telling how long the value is (0255 bytes)
• Value: any info required
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Wenbing Zhao
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IPv6 Extension Headers
• Hop-by-hop options: used for info that all routers
along the path must examine
– One option has been defined to support datagrams
exceeding 64K
• Next header: 1-byte field telling type of header
• Length field: 1-byte field telling how long the hop-by-hop header
is in bytes, excluding the first 8 bytes, which are mandatory
• 1-byte field indicating that this option defines the datagram size
• 1-byte field telling the size is a 4-byte number
• 4-byte field: size of datagram
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IPv6 Extension Headers
• Routing option header: lists one or more routers
that must be visited on the way to destination
– Routing type field: 1-byte giving format of the rest of the
header
– Segments left field: 1-byte keeping track of how many of
the addresses in the list have not yet been visited
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Wenbing Zhao
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Exercise - CIDR
• According to Classless InterDomain Routing, the
remaining IP addresses are allocated in variable-sized
blocks, without regard to the classes. However, the
starting address must fall on the boundary of the block
size allocated. Assuming that a large number of
consecutive IP address are available starting at
194.24.0.0. Suppose that three organizations, A, B, and
C, request 4000, 1000, and 2000 addresses,
respectively, and in that order. For each of these, give
the first IP address assigned, the last IP address
assigned, both must be in dotted decimal form, and the
mask in the w.x.y.z/s notation.
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Exercise - CIDR
• A router has just received the following new IP
addresses: 57.6.96.0/21, 57.6.104.0/21,
57.6.112.0/21, and 57.6.120.0/21. If all of them
use the same outgoing line, can they be
aggregated? If so, to what? If not, why not?
Spring Semester 2006
EEC-484/584: Computer Networks
Wenbing Zhao