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1Comnet 2010
Communication NetworksCommunication Networks
Recitation 7Recitation 7
Lookups & NATLookups & NAT
2Comnet 2010
Lookup and Forwarding EngineLookup and Forwarding Engine
header
payload
Packet
Router
Destination Address
Outgoing Port
Destination PortForwarding Table
Routing Lookup Data Structure
65.0.0.0/8
128.9.0.0/16
149.12.0.0/19
3
1
7
3Comnet 2010
0 224 232-1
128.9.0.0/16
65.0.0.0
142.12.0.0/19
65.0.0.0/8
65.255.255.255
Example Forwarding TableExample Forwarding Table
Destination IP PrefixDestination IP Prefix Outgoing PortOutgoing Port65.0.0.0/ 865.0.0.0/ 8 33
128.9.0.0/16128.9.0.0/16 11
142.12.0.0/19142.12.0.0/19 77
IP prefix: 0-32 bits
Prefix length
128.9.16.14
4Comnet 2010
Prefixes can OverlapPrefixes can Overlap
128.9.16.0/21 128.9.172.0/21
128.9.176.0/24
Find the longest matching prefix among all prefixes that match the destination address.
0 232-1
128.9.0.0/16142.12.0.0/1965.0.0.0/8
128.9.16.14
Longest matching prefix
5Comnet 2010
LPM in IP RoutersLPM in IP RoutersUsing 32 exact match algorithms for LPM!Using 32 exact match algorithms for LPM!
Exact matchagainst prefixes
of length 1
Exact matchagainst prefixes
of length 2
Exact matchagainst prefixes
of length 32
Network Address PortPriorityEncodeand pick
6Comnet 2010
LPM in IP RoutersLPM in IP RoutersBinary triesBinary tries
Example Prefixesa) 00001b) 00010c) 00011d) 001e) 0101f) 011g) 100h) 1010i) 1100
j) 11110000
e
f g
h i
j
0 1
a b c
d
7Comnet 2010
LPM in IP RoutersLPM in IP Routers“Patricia” trie“Patricia” trie
Example Prefixesa) 00001b) 00010c) 00011d) 001e) 0101f) 011g) 100h) 1010i) 1100
j) 11110000
e
f g
h i
j Skip 510000
0 1
a b c
d
8Comnet 2010
(exact match)
(range match)
Each prefix is encoded as a range,find narrowest enclosing range.
A
B
Binary Search Binary Search
9Comnet 2010
Private NetworkPrivate Network• Private IP Private IP network is an IP network that is not directly connected network is an IP network that is not directly connected
to the Internetto the Internet
• IP addresses in a private network can be assigned arbitrarily. IP addresses in a private network can be assigned arbitrarily. – Not registered and not guaranteed to be globally uniqueNot registered and not guaranteed to be globally unique
• Generally, private networks use addresses from the following Generally, private networks use addresses from the following experimental address ranges (experimental address ranges (non-routable addressesnon-routable addresses): ): – 10.0.0.0 – 10.255.255.25510.0.0.0 – 10.255.255.255– 172.16.0.0 – 172.31.255.255172.16.0.0 – 172.31.255.255– 192.168.0.0 – 192.168.255.255192.168.0.0 – 192.168.255.255
10Comnet 2010
Private AddressesPrivate Addresses
H1
R1
H2
10.0.1.3
10.0.1.1
10.0.1.2
H3
R2
H4
10.0.1.310.0.1.2
Private network 1
Internet
H5
10.0.1.1Private network 1
213.168.112.3
128.195.4.119 128.143.71.21
11Comnet 2010
Network Address Translation Network Address Translation (NAT)(NAT)
• NAT is a router function where IP addresses (and possibly port NAT is a router function where IP addresses (and possibly port numbers) of IP datagrams are replaced at the boundary of a numbers) of IP datagrams are replaced at the boundary of a private networkprivate network
• NAT is a method that enables hosts on private networks to NAT is a method that enables hosts on private networks to communicate with hosts on the Internetcommunicate with hosts on the Internet
• NAT is run on routers that connect private networks to the public NAT is run on routers that connect private networks to the public Internet, to replace the IP address-port pair of an IP packet with Internet, to replace the IP address-port pair of an IP packet with another IP address-port pair. another IP address-port pair.
12Comnet 2010
Basic operation of NATBasic operation of NAT
• NAT device has address translation NAT device has address translation tabletable
H1
private address: 10.0.1.2public address: 128.143.71.21
H5
Privatenetwork
Internet
Source = 10.0.1.2Destination = 213.168.112.3
Source = 128.143.71.21Destination = 213.168.112.3
public address: 213.168.112.3NATdevice
Source = 213.168.112.3Destination = 128.143.71.21
Source = 213.168.112.3Destination = 10.0.1.2
PrivateAddress
PublicAddress
10.0.1.2 128.143.71.21
13Comnet 2010
Main uses of NATMain uses of NAT
• Pooling of IP addressesPooling of IP addresses
• Supporting migration between Supporting migration between network service providersnetwork service providers
• IP masqueradingIP masquerading
• Load balancing of serversLoad balancing of servers
14Comnet 2010
Pooling of IP addressesPooling of IP addresses
• Scenario:Scenario: Corporate network has many hosts but only a small Corporate network has many hosts but only a small number of public IP addressesnumber of public IP addresses
• NAT solution:NAT solution:– Corporate network is managed with a private address spaceCorporate network is managed with a private address space– NAT device, located at the boundary between the corporate NAT device, located at the boundary between the corporate
network and the public Internet, manages a pool of public IP network and the public Internet, manages a pool of public IP addresses addresses
– When a host from the corporate network sends an IP When a host from the corporate network sends an IP datagram to a host in the public Internet, the NAT device datagram to a host in the public Internet, the NAT device picks a public IP address from the address pool, and binds picks a public IP address from the address pool, and binds this address to the private address of the hostthis address to the private address of the host
15Comnet 2010
Pooling of IP addressesPooling of IP addresses
H1
private address: 10.0.1.2public address:
H5
Privatenetwork
Internet
Source = 10.0.1.2Destination = 213.168.112.3
Source = 128.143.71.21Destination = 213.168.112.3
public address: 213.168.112.3NATdevice
PrivateAddress
PublicAddress
10.0.1.2
Pool of addresses: 128.143.71.0-128.143.71.30
16Comnet 2010
Supporting migration between Supporting migration between network service providersnetwork service providers
• Scenario:Scenario: In CIDR, the IP addresses in a corporate network are In CIDR, the IP addresses in a corporate network are obtained from the service provider. Changing the service provider obtained from the service provider. Changing the service provider requires changing all IP addresses in the network. requires changing all IP addresses in the network.
• NAT solution:NAT solution:
– Assign private addresses to the hosts of the corporate networkAssign private addresses to the hosts of the corporate network
– NAT device has static address translation entries which bind the NAT device has static address translation entries which bind the private address of a host to the public address. private address of a host to the public address.
– Migration to a new network service provider merely requires an Migration to a new network service provider merely requires an update of the NAT device. The migration is not noticeable to the update of the NAT device. The migration is not noticeable to the hosts on the network. hosts on the network.
Note:Note:
– The difference to the use of NAT with IP address pooling is that the The difference to the use of NAT with IP address pooling is that the mapping of public and private IP addresses is static.mapping of public and private IP addresses is static.
17Comnet 2010
Supporting migration between Supporting migration between network service providersnetwork service providers
H1
private address: 10.0.1.2public address: 128.143.71.21
128.195.4.120
Source = 10.0.1.2Destination = 213.168.112.3
NATdevice
PrivateAddress
PublicAddress
10.0.1.2128.143.71.21128.195.4.120
128.143.71.21
128.195.4.120
Source = 128.143.71.21Destination = 213.168.112.3
Source = 128.195.4.120Destination = 213.168.112.3
ISP 2allocates address block
128.195.4.0/24 to privatenetwork:
Privatenetwork
ISP 1allocates address block
128.143.71.0/24 to privatenetwork:
18Comnet 2010
IP masqueradingIP masquerading
• Also called: Network address and port translation Also called: Network address and port translation (NAPT), port address translation (PAT).(NAPT), port address translation (PAT).
• Scenario:Scenario: Single public IP address is mapped to Single public IP address is mapped to multiple hosts in a private network. multiple hosts in a private network.
• NAT solution:NAT solution:– Assign private addresses to the hosts of the Assign private addresses to the hosts of the
corporate networkcorporate network– NAT device modifies the port numbers for NAT device modifies the port numbers for
outgoing trafficoutgoing traffic
19Comnet 2010
IP masqueradingIP masquerading
H1
private address: 10.0.1.2
Private network
Source = 10.0.1.2Source port = 2001
Source = 128.143.71.21Source port = 2100
NATdevice
PrivateAddress
PublicAddress
10.0.1.2/2001 128.143.71.21/2100
10.0.1.3/3020 128.143.71.21/4444
H2
private address: 10.0.1.3
Source = 10.0.1.3Source port = 3020
Internet
Source = 128.143.71.21Destination = 4444
128.143.71.21
20Comnet 2010
Load balancing of serversLoad balancing of servers
• Scenario:Scenario: Balance the load on a set of identical servers, which Balance the load on a set of identical servers, which are accessible from a single IP addressare accessible from a single IP address
• NAT solution:NAT solution:– Here, the servers are assigned private addresses Here, the servers are assigned private addresses – NAT device acts as a proxy for requests to the server from NAT device acts as a proxy for requests to the server from
the public networkthe public network– The NAT device changes the destination IP address of The NAT device changes the destination IP address of
arriving packets to one of the private addresses for a serverarriving packets to one of the private addresses for a server– A sensible strategy for balancing the load of the servers is to A sensible strategy for balancing the load of the servers is to
assign the addresses of the servers in a round-robin fashion. assign the addresses of the servers in a round-robin fashion.
21Comnet 2010
Load balancing of serversLoad balancing of servers
Private network
Source = 213.168.12.3Destination = 128.143.71.21
NATdevice
PrivateAddress
PublicAddress
10.0.1.2 128.143.71.21
Inside network
10.0.1.4 128.143.71.21
Internet128.143.71.21
S1
S2
S3
10.0.1.4
10.0.1.3
10.0.1.2
PublicAddress
128.195.4.120
Outside network
213.168.12.3
Source = 128.195.4.120Destination = 128.143.71.21
22Comnet 2010
Concerns about NATConcerns about NAT
• Performance:Performance:
– Modifying the IP header by changing Modifying the IP header by changing the IP address requires that NAT the IP address requires that NAT boxes recalculate the IP header boxes recalculate the IP header checksumchecksum
– Modifying port number requires that Modifying port number requires that NAT boxes recalculate TCP NAT boxes recalculate TCP checksumchecksum
23Comnet 2010
Concerns about NATConcerns about NAT
• End-to-end connectivity:End-to-end connectivity:– NAT destroys universal end-to-end reachability of NAT destroys universal end-to-end reachability of
hosts on the Internet. hosts on the Internet. – A host in the public Internet often cannot initiate A host in the public Internet often cannot initiate
communication to a host in a private network. communication to a host in a private network. – The problem is worse, when two hosts that are in The problem is worse, when two hosts that are in
a private network need to communicate with each a private network need to communicate with each other.other.
24Comnet 2010
Concerns about NATConcerns about NAT
• IP address in application data:IP address in application data:– Applications that carry IP addresses in the payload Applications that carry IP addresses in the payload
of the application data generally do not work of the application data generally do not work across a private-public network boundary. across a private-public network boundary.
– Some NAT devices inspect the payload of widely Some NAT devices inspect the payload of widely used application layer protocols and, if an IP used application layer protocols and, if an IP address is detected in the application-layer header address is detected in the application-layer header or the application payload, translate the address or the application payload, translate the address according to the address translation table. according to the address translation table.
25Comnet 2010
NAT and FTPNAT and FTP
H1 H2
public address:128.143.72.21
FTP client FTP server
PORT 128.143.72.21/1027
200 PORT command successful
public address:128.195.4.120
RETR myfile
150 Opening data connection
establish data connection
• Normal FTP operationNormal FTP operation
26Comnet 2010
NAT and FTPNAT and FTP
• NAT device with FTP supportNAT device with FTP support
H1
Private network
NATdevice
H2
private address: 10.0.1.3public address: 128.143.72.21
Internet
FTP client FTP server
PORT 10.0.1.3/1027 PORT 128.143.72.21/1027
200 PORT command successful200 PORT command successful
RETR myfile
establish data connection
RETR myfile
150 Opening data connection150 Opening data connection
establish data connection
27Comnet 2010
NAT and FTPNAT and FTP
• FTP in passive mode and NAT.FTP in passive mode and NAT.
H1
Private network
NATdevice
H2
private address: 10.0.1.3public address: 128.143.72.21
Internet
FTP client FTP server
PASV PASV
Entering Passive Mode128.195.4.120/10001
Entering Passive Mode128.195.4.120/10001
public address:128.195.4.120
Establish data connection Establish data connection