Mobility in the Internet Part I CS 444N, Spring 2002 Instructor: Mary

Mobility in the Internet Part I CS 444N, Spring 2002 Instructor: Mary Baker Computer Science Department Stanford University

Motivation: the changing wireless environment Explosion in wireless services – Some connectivity everywhere – Overlapping, heterogeneous networks Small, portable devices A choice of network connectivity on one device – Sometimes built-in – Sometimes a portable “bridge” between choices Spring 2002 CS444N 2

Opportunity for connectivity New environment gives us opportunity – Continuous connectivity for a mobile host – Seamless movement between networks Examples – Move from office to elsewhere in building – Move outside building, across campus, to cafe Why maintain connectivity? – Avoid restarting applications/networks – Avoid losing “distributed state” Spring 2002 CS444N 3

Different approaches The traditional approach: support in the network – – – – – Intelligence (and expense) is in the network End-points are cheap (handsets) Allows for supporting infrastructure Requires agreements/trust amongst multiple vendors Examples: A link/physical level (many wireless networks) At routing level (Columbia, VIP) – Doesn’t work when switching between technologies and often not between vendors – In Internet would require modifying lots of routers Spring 2002 CS444N 4

Different approaches, continued The Internet approach: end-to-end – – – – Intelligence (and expense) is in the end-points Network is cheap (relatively) and as fast as possible Implies self-support for many activities Less work/trust required amongst multiple vendors End-to-end support at transport/naming/application levels – May be ideal in future, but requires extensive changes – Not currently backwards compatible – TRIAD may be interesting approach Spring 2002 CS444N 5

Different approaches, continued Use end-to-end support at routing level – Makes problem transparent at layers above and below – Current Internet standard: Mobile IP (RFC 2002) TCP/IP network stack: application transport routing link physical Spring 2002 Modify all applications? Modify TCP, UDP, etc.? Modify IP end-points? Modify all device drivers? How dies this work across network technologies? CS444N 6

IP address problem Internet hosts/interfaces are identified by IP address – Domain name service translates host name to IP address – IP address identifies host/interface and locates its network – Mixes naming and location Moving to another network requires different network address – But this would change the host’s identity – How can we still reach that host? Spring 2002 CS444N 7

Routing for mobile hosts MH mobile host CH CH correspondent host Foreign network Home network MH How to direct packets to moving hosts transparently? CH Home network Foreign network MH Spring 2002 CS444N 8

Domains versus interfaces Switching domains & switching interfaces are the same problem at the routing level Network interfaces: Administrative domains: Stanford.edu Mobile host ether 171.64.14.X 171.64.X.X Berkeley.edu radio 42.13.0.X Spring 2002 128.32.X.X CS444N 9

Mobile IP (RFC 2002) Leaves Internet routing fabric unchanged Does not assume “base stations” exist everywhere Simple Correspondent hosts don’t need to know about mobility Works both for changing domains and network interfaces Spring 2002 CS444N 10

Basic Mobile IP – to mobile hosts MH mobile host CH correspondent host HA home agent FA foreign agent (We’ll see later that FA is not necessary or even desirable) CH Foreign network Home network FA HA MH MH registers new “care-of address” (FA) with HA HA tunnels packets to FA FA decapsulates packets and delivers them to MH Spring 2002 CS444N 11

Packet addressing Packet from CH to MH Source address address of CH Destination address home IP address of MH Payload Home agent intercepts above packet and tunnels it Source address address of HA Destination address care-of address of MH Source address address of CH Destination address home IP address of MH Original payload Spring 2002 CS444N 12

When mobile host moves again CH Foreign network #1 Home network FA #1 HA MH Foreign network #2 FA #2 MH MH registers new address (FA #2) with HA & FA #1 HA tunnels packets to FA #2, which delivers them to MH Packets in flight can be forwarded from FA #1 to FA #2 Spring 2002 CS444N 13

Basic Mobile IP - from mobile hosts Mobile hosts also send packets CH Foreign network Home network FA HA MH Mobile host uses its home IP address as source address -Lower latency -Still transparent to correspondent host -No obvious need to encapsulate packet to CH This is called a “triangle route” Spring 2002 CS444N 14

Problems with Foreign Agents Assumption of support from foreign networks – A foreign agent exists in all networks you visit? – The foreign agent is robust and up and running? – The foreign agent is trustworthy? Correctness in security-conscious networks – We’ll see that “triangle route” has problems – MH under its own control can eliminate this problem Other undesirable features – Some performance improvements are harder with FAs We want end-to-end solution that allows flexibility Spring 2002 CS444N 15

Solution Mobile host is responsible for itself -(With help from infrastructure in its home network) -Mobile host decapsulates packets -Mobile host sends its own packets -“Co-located” FA on MH CH Foreign network Home network HA MH MH must acquire its own IP address in foreign network This address is its new “care-of” address Mobile IP spec allows for this option Spring 2002 CS444N 16

Obtaining a foreign IP address Can we expect to obtain an IP address? – – – – – DHCP becoming more common Dynamic IP address binding like some dial-up services Your friend can reserve an IP address for you Various other tricks More support for dynamic IP address binding in IPv6 This assumes less than getting others to run a FA For more information about provisioning networks for visitors, we’ll look at SPINACH later Spring 2002 CS444N 17

Design implications New issues: the mobile host now has two roles: – Home role – Local role - More complex mobile host - Loss of in-flight packets? (This can happen anyway.) Can visit networks without a foreign agent Can join local multicast groups, etc. More control over packet routing more flexibility Spring 2002 CS444N 18

Problems with ingress filtering Home network CH HA Foreign network MH Mobile host uses its home IP address as source address Security-conscious boundary routers will drop this packet Spring 2002 CS444N 19

Solution: bi-directional tunnel Provide choice of “safe” route through home agent both ways Home network CH HA Foreign network MH This is the slowest but most conservative option At the other extreme Spring 2002 CS444N 20

Problem: performance Example: short-lived communication – – – – When accessing a web server, why pay for mobility? Do without location-transparency Unlikely to move during transfer; can reload page Works when CH keeps no state about MH Spring 2002 CS444N 21

Solution: yet more flexibility CH Foreign network Home network MH HA Use current care-of address and send packet directly -This is regular IP! More generally: -MH should have flexibility to adapt to circumstances -A range of options: from slow-but-safe to regular IP -Should be an end-to-end packet delivery decision (no FA) Spring 2002 CS444N 22

Routing options Allow MH to choose from among all routing options Options: – Encapsulate packet or not? – Use home address or care-of address as source address? – Tunnel packet through home agent or send directly? Choice determined by: – – – – Performance Desire for transparent mobility Mobile-awareness of correspondent host Security concerns of networks traversed Equivalent choices for CH sending packets to MH Spring 2002 CS444N 23

Mobility 4x4 Outgoing Indirect, Encapsulated Outgoing Direct, Encapsulated Outgoing Direct, Home Address Incoming Indirect, Encapsulated Most reliable, Requires least efficient decapsulation on CH No securityconscious routers on path Incoming Direct, Encapsulated Requires fully mobile-aware CH No securityconscious routers on path Incoming Direct, Home Address Requires both hosts to be on same net. seg. Incoming Direct, Temp. Address Spring 2002 Outgoing Direct, Temp. Address CS444N Most efficient, no mobility support 24

Implementation Virtual interface (vif): illusion of MH still on home network We hijack the route table lookup Consult Mobile Policy Table in conjunction with route table TCP UDP IPIP MPT IP route lookup Routing Table Network Layer (IP) loopback Spring 2002 ether radio CS444N vif 25

Implementation, continued Traffic back to home net handles boundary routers All web traffic uses regular IP Other traffic uses regular “triangle route” Handles multicast addresses too (bi-directional or regular IP) Destination Netmask Port Number Transparent Mobility? Bi-directional tunneling? a.b.0.0 255.255.0.0 0 Yes Yes 0.0.0.0 0.0.0.0 80 No N/A 0.0.0.0 0.0.0.0 0 Yes No Spring 2002 CS444N 26

Figuring out which to use With bidirectional tunneling – Probe destination using triangle route – If it works, switch to that option With triangle route – If packets aren’t getting through after some number of tries Spring 2002 CS444N 27

Is it fast enough to be seamless? Interval Packet between loss packets (common case) Cold Ether ether 10 ms switch Ether radio 250 ms Packet loss (worst case) Time in transition 0 1 10 ms 1 4 1.25 s 0 1 0.5 s Radio ether Hot Ether radio 250 ms switch Radio ether Spring 2002 CS444N 28

Mobile IP issues on local network Host visiting local network with foreign agent – No real presence on local network Host visiting local network with its own IP address – – – – Has a role on local network Reverse name lookups through special name? Or do you change the DNS entry? Its IP address / HW address gets into local hosts’ ARP caches – Which IP address should go into cache? – How do you update caches if host moves again? Spring 2002 CS444N 29

Local ARP cache problem ARP caches store (IP address, HW address) pairs MH host visits foreign network Wants to talk directly back and forth to local hosts – If it wants to maintain connectivity with them after moving Use home IP address Other hosts address MH by HW address on local link But if MH moves again, ARP cache entries are wrong – If it doesn’t care Use local IP address If MH moves, ARP cache is wrong, but nobody cares Spring 2002 CS444N 30

Multiple Network Interfaces – Why? Want to probe hosts through all active interfaces – Example: register with HA through new interface before switching to it – Helps with smooth handoff between types of networks Want transparent mobility for more than one interface Example: – One application users cheap/slow interface while another uses expensive/fast interface – Move to new network(s) or lose contact with one network – Don’t want to restart either application Spring 2002 CS444N 31

Why is this hard? System support missing in at least two areas Need “next hop” info for more than one interface – Need to be able to send packets beyond local subnet for more than one interface – Current support only uses gateway info for one interface Mobile IP doesn’t separate traffic flows to different interfaces – (This isn’t the Mobile IP “simultaneous binding” feature) – Current HA won’t keep different bindings for more than one interface per host based on traffic flow Spring 2002 CS444N 32

Solution for next hop Backwards-compatible extension to routing table – Add “next-hop” info for more than one interface – Take advantage of “metric” field for priority of interface – This maintains backwards compatible default route Destination Gateway Netmask Flags Metric Iface a.b.0.0 0.0.0.0 255.255.0.0 U 0 eth0 c.d.0.0 0.0.0.0 255.255.0.0 U 0 st0 127.0.0.0 0.0.0.0 255.0.0.0 U 0 lo 0.0.0.0 a.b.0.1 0.0.0.0 UG 1 eth0 0.0.0.0 c.d.0.1 0.0.0.0 UG 100 st0 Spring 2002 CS444N 33

Solution for Mobile IP Extend home agent Mobile host registers flow-to-interface bindings Home Agent flow 1 flow 2 flow 1 CoA1 flow 2 CoA2 Mobile Host Correspondent Host Spring 2002 CS444N 34

Performance overhead Flow binding demultiplexing cost Flow Bindings Demultiplexing Time ( s) Cost ( s) Per flow ( s) 0 2.1 (0.30 std. dev.) N/A N/A 1 2.3 (0.45 std. dev.) 0.2 0.20 2 2.7 (0.30 std. dev.) 0.6 0.30 10 3.9 (0.30 std. dev.) 1.8 0.18 20 4.7 (0.46 std. dev.) 2.6 0.13 30 5.3 (0.46 std. dev.) 3.2 0.11 40 6.7 (0.64 std. dev.) 4.6 0.12 60 9.2 (0.40 std. dev.) 7.1 0.12 Spring 2002 CS444N 35

Flexible connectivity management Need to manage this extra flexibility through adaptivity – – – – – Monitor availability of various interfaces System detects & configures interfaces automatically Applications can express interest in types of service System (or application) can choose best interface System feedback necessary: system notifies application of changes as conditions warrant Spring 2002 CS444N 36

Connectivity management, continued Must address protocol interaction when connecting – Is DHCP available? – Is this a frequently visited network? (probe for gateways) If so, can use pre-determined address – Must the host use a foreign agent here? If it’s broken, how do we find what’s wrong & fix it? – Cable loose? – Battery in radio dead? – Home agent dead? Strong need for “no-futz” computing on mobile hosts Spring 2002 CS444N 37