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Mobility Why (and How) Things Move Jennifer Rexford COS 461: Computer Networks Lectures: MW 10-­‐10:50am in Architecture N101 hHp://www.cs.princeton.edu/courses/archive/spr12/cos461/ 2
Physical Mobility MulT-­‐Homing WiFi
3G
•  Device aHaches to a new aHachment point •  Device starts using a different aHachment point 3
MigraTon 4
Failover •  Process or virtual machine migraTon •  Backup machine takes over aXer the primary fails 5
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Change Address of Mobile Node? A
Handling Mobility name
link
session
B
address
b1
a
path
?
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Keeping the Address the Same 8
Changing the Address A
b1
a
path
b1
a
•  Mobility is a rou$ng problem name
link
session
B
b1
b2
•  Mobility is a directory problem –  Change the routes to reach the new locaTon –  Challenge: scalability of the rouTng protocol –  Change the mapping of name to address –  Challenge: scalability of directory, updaTng neighbor 9
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Two Internet Design Decisions •  Socket abstracTon –  ConnecTon between a pair of fixed IP addresses and port numbers –  Leads to more emphasis on rouTng soluTons 1.2.3.4
5.6.7.8
•  Interface addresses –  Addresses refer to interfaces (adaptors) –  Not the host, or the service WiFi
RouTng SoluTons Address Stays the Same 3G
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Three Examples Example #1: Ethernet •  Ethernet –  MAC learning of the new locaTon b1
•  IP rouTng a
–  Inject IP address(es) at new locaTon b1
•  Mobile IP –  StaTonary home agent directs traffic to new locaTon •  MAC learning –  Learn b1’s locaTon when b1 sends a frame –  SoX state: Tmeout the cached informaTon 13
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Making Larger Ethernet Segments Pros and Cons •  Ethernet handles mobility •  Advantages –  IP address and MAC address stay the same –  Switches learn to route to the new locaTon –  Seamless mobility, no changes to hosts or apps –  No changes to MAC or IP addresses •  But, larger networks have mulTple segments •  Disadvantages –  Cannot retain your IP address as you move –  Ethernet does not scale –  Long paths, state per MAC address, flooding, … •  SoluTon: virtual local area networks (VLAN) –  Logical Ethernet segment spanning a campus –  E.g., interconnecTng the WiFi access points •  Widely used approach in campus networks 15
Example #2: IP RouTng 16
Boeing Connexion: Wide-­‐Area Mobility •  Node has a persistent address (e.g., 12.34.45.7) •  Injected into rouTng protocol (e.g., OSPF) 12.78.3.0/24
BGP
12.34.45.0/24
12.34.45.7/32
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http://www.nanog.org/meetings/nanog31/
abstracts.php?pt=NTk1Jm5hbm9nMzE=&nm=nanog31
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Pros and Cons Example #3: Mobile IP •  Advantages Home network: permanent “home”
of mobile (e.g., 128.119.40/24)
–  Seamless mobility, no MAC or IP address changes –  Traffic follows an efficient path to new locaTon Home agent: performs mobility
functions on behalf of mobile
•  Disadvantages –  Does not scale to large number of mobile hosts –  More rouTng-­‐protocol messages –  Larger rouTng tables to store smaller address blocks Permanent address: can
always be used to reach
mobile, e.g., 128.119.40.186
wide area
network
Correspondent: wants to
communicate with mobile
correspondent
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Visited Network and Care-­‐of Address Mobility: RegistraTon Visited network: e.g., 79.129.13/24
Care-of-address:
in visited network
2
1
wide area
network
(e.g., 79,129.13.2)
foreign agent contacts home
agent home: “this mobile is
resident in my network”
wide area
network
Foreign agent:
performs mobility
functions for the
mobile.
Correspondent
visited network
home network
Permanent address: remains
constant (e.g., 128.119.40.186)
mobile contacts
foreign agent on
entering visited
network
•  Foreign agent knows about mobile •  Home agent knows locaTon of mobile 21
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Mobility via Indirect RouTng foreign agent
receives packets,
forwards to mobile
home agent intercepts
packets, forwards to
foreign agent
home
network
Pros and Cons •  Advantages –  Seamless to the remote end-­‐point –  No rouTng-­‐protocol overhead visited
network
•  Disadvantages 3
–  Overhead of running home and foreign agents –  Inefficient “triangle rouTng” (high “stretch”) –  Foreign agent sends “spoofed” IP source address wide area
network
correspondent
addresses packets
using home address
of mobile
1
2
4
mobile replies
directly to
correspondent
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Three Examples •  Ethernet –  Gratuitous ARP to change the MAC address associated with an IP address Directory SoluTons •  Dynamic DNS –  DNS updates to change the IP address(es) associated with a domain name Change the mapping of name to address •  Various recent proposed designs –  UpdaTng the remote end-­‐point (e.g., end host, edge switch) to use a new address 25
Example #1: Ethernet IP 1.2.3.4
MAC m1
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Ethernet MulT-­‐Homing IP 1.2.3.4
MAC m1
•  Backup machine floods “gratuitous ARP” response IP 1.2.3.4
MAC m2
•  Gratuitous ARP IP 1.2.3.4
MAC m2
–  Balance traffic over two interfaces –  Fail over from one interface to the other –  Associates the IP address with a new MAC address –  Hosts update their ARP cache 27
Pros and Cons 28
Example #2: Dynamic DNS Name: www.nbc.com
IP: 1.2.3.4
•  Advantages –  Seamless change from one MAC address to another •  Disadvantages –  Works only within a single Ethernet subnet –  Scalability limitaTons of Ethernet Name: www.nbc.com
IP: 5.6.7.8
•  Dynamically update DNS •  Used in data-­‐center networks –  Change the mapping of domain name to IP address –  Future DNS requests get the new addres –  But doesn’t help with smart phones homed to mulTple administraTve domains 29
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ApplicaTons of Dynamic DNS Pros and Cons •  Advantages •  Replicated services –  Direct future requests to a different replica –  E.g., for failover, load balancing, performance, etc. •  Services on dynamically-­‐assigned IP addresses –  No new infrastructure –  Leverages exisTng DNS servers •  Disadvantages –  Only helps for new connecTons –  Overheads of updaTng DNS servers –  Stymied by DNS caching –  ResidenTal user with a dynamic IP address –  Directs clients to the server’s current address •  “Fast flux” in botnets –  Hiding phishing and malware delivery servers –  … behind constantly changing IP addresses 31
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Example #3: UpdaTng the End-­‐Points UpdaTng the Edge Switches 1.2.3.4
1.2.3.4
8.9.10.11
5.6.7.8
5.6.7.8
10.0.0.2
10.0.0.1
8.9.10.11
•  Mobile node updates the remote end-­‐point 10.0.0.1
–  Sends the remote end-­‐point the new IP address –  Allowing ongoing connecTon to conTnue –  Can be used in conjuncTon with Dynamic DNS •  Update the switches –  Hosts retain their addresses –  Switches rewrite the addresses, or encapsulate –  Used in some data-­‐center networks 33
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Pros and Cons Mobility Today •  Limited network support for mobility •  Advantages –  E.g., within a single Ethernet subnet –  E.g., among base staTons on a campus –  Scalability of hierarchical addressing –  Efficiency of rouTng along short paths •  ApplicaTons increasingly robust to mobility •  Disadvantages –  Robust to changes in IP address, and disconnecTons –  E.g., e-­‐mail client contacTng the e-­‐mail server –  … and allowing reading/wriTng while disconnected –  Changes to the end host (e.g., apps, TCP, etc.) –  … or support from the edge switches –  Difficulty when both end-­‐points move at once •  Work in progress –  Used in some data centers, recent standards/projects 35
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Mobility Tomorrow •  Increasing demand for seamless IP mobility –  E.g., conTnue a VoIP call while on the train –  E.g., virtual machine migraTon within and between data centers •  Increasing integraTon of WiFi and cellular –  E.g., mulT-­‐homed cell phones that can use both networks –  E.g., servers with mulTple interface cards •  Need beHer mobility & mulT-­‐homing soluTons! 37
Conclusions •  Mobility – Change is hard – RouTng and directory soluTons – Mobility is sTll a moving target…  •  Friday’s precept: IP routers and assignment #2 •  Midterm next week – Midterm next Wednesday during lecture Tme – In Frist 302, not in the lecture hall 38
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