15-849: Hot Topics in Networking Mobility
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Transcript of 15-849: Hot Topics in Networking Mobility
15-849: Hot Topics in Networking
Mobility
Srinivasan Seshan
Key Questions
1. Mobility vs. Portability -- Mobile IP tries to ensure that your TCP connections and connectivity survive as we move from network to network. Do we really need this or just need the ability to reconnect anywhere (i.e. portability)
2. Layers again - What layer should we address mobility - network, naming, transport, "link"/local network (i.e. within a bridged LAN/cellular network)? Do we address portability at a different layer?
3. Incremental support -- Should we force all nodes in the Internet to add features to support mobility or do we place the burden on the mobile hosts/mobile networks to "fit in" with the rest of the network?
4. Performance & handoff5. Location privacy
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How to Handle Mobile Nodes?
• Dynamic Host Configuration (DHCP)• Host gets new IP address in new locations• Problems
• Host does not have constant name/address how do others contact host
• What happens to active transport connections?
• Naming• Use DHCP and update name-address mapping
whenever host changes address• Fixes contact problem but not broken transport
connections3
Handling Mobile Nodes (Transport)
• TCP currently uses 4 tuple to describe connection• <Src Addr, Src port, Dst addr, Dst port>
• Modify TCP to allow peer’s address to be changed during connection
• Security issues• Can someone easily hijack connection?
• Difficult deployment both ends must support mobility
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Handling Mobile Node
• Link layer mobility• Learning bridges can handle mobility this is how
it is handled at CMU• Encapsulated PPP (PPTP) Have mobile host act
like he is connected to original LAN• Works for IP AND other network protocols
• Multicast• Solves similar problem how to route packets to
different sets of hosts at different times• Can’t we just reuse same solutions?
• Don’t really have solution for multicast either!5
Handling Mobile Nodes (Routing)
• Allow mobile node to keep same address and name• How do we deliver IP packets when the endpoint
moves?• Why can’t we just have nodes advertise route to their
address?• What about packets from the mobile host?
• Routing not a problem• What source address on packet?
• Key design considerations• Scale• Incremental deployment
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Basic Solution to Mobile Routing
• Same as other problems in Computer Science• Add a level of indirection
• Keep some part of the network informed about current location• Need technique to route packets through this
location (interception)• Need to forward packets from this location
to mobile host (delivery) 7
Interception
• Somewhere along normal forwarding path• At source• Any router along path• Router to home network• Machine on home network (masquerading as
mobile host)• Clever tricks to force packet to particular
destination• “Mobile subnet” – assign mobiles a special address
range and have special node advertise route 8
Delivery
• Need to get packet to mobile’s current location
• Tunnels• Tunnel endpoint = current location• Tunnel contents = original packets
• Source routing• Loose source route through mobile current
location• Network address translation (NAT)
• What about packets from the mobile host?9
Mobile IP (RFC 2290)
• Interception• Typically home agent – hosts on home network
• Delivery• Typically IP-in-IP tunneling• Endpoint – either temporary mobile address or
foreign agent• Terminology
• Mobile host (MH), correspondent host (CH), home agent (HA), foreign agent (FA)
• Care-of-address, home address10
Key Questions
1. Mobility vs. Portability -- Mobile IP tries to ensure that your TCP connections and connectivity survive as we move from network to network. Do we really need this or just need the ability to reconnect anywhere (i.e. portability)
2. Layers again - What layer should we address mobility - network, naming, transport, "link"/local network (i.e. within a bridged LAN/cellular network)? Do we address portability at a different layer?
3. Incremental support -- Should we force all nodes in the Internet to add features to support mobility or do we place the burden on the mobile hosts/mobile networks to "fit in" with the rest of the network?
4. Performance & handoff5. Location privacy
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Outline
• TCP Migrate
• Mobile IP
• I3
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Previous Approaches to Mobility
• Mobility-aware routing (Mobile IP)• Completely transparent to end hosts• Requires a home agent• Often inefficient packet routes
• Endpoint ID (EID) schemes• Retains standard unicast routes, but…• Yet another level of indirection• Also requires changes to transport layer
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The Migrate Approach
• Locate hosts through existing DNS• Secure, dynamic DNS is currently deployed and widely
available (RFC 2137)• Maintains standard IP addressing model
• IP address are topological addresses, not Ids• Fundamental to Internet scaling properties
• Ensure seamless connectivity through connection migration• Notify only the current set of correspondent hosts• Follows from the end-to-end argument
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Migrate Architecture
DNS Server
Mobile Hostfoo.bar.edu
Location Query(DNS Lookup)
Connection Initiation
Location Update(Dynamic DNS Update)
Connection Migration
xxx.xxx.xxx.xxxyyy.yyy.yyy.yyy
CorrespondentHost
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Previous Migration Schemes
• Multi-homed schemes• Require new transport protocols (SCTP)• Often require a priori knowledge of possible
set of IP addresses• Connection-ID schemes
• May not preserve transport semantics• May require a per-packet overhead• Many security and DoS issues
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Our Migration Approach
• Join together two separate connections• By unifying the context space• Reference previous connection with token• Requires minimal transport state machine
changes• Preserve semantics, both internal and
external to the connection• Implicit address assignment• Works with NATs, PEPs, all middle boxes
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An Application: TCP
• Provide special Migrate option• Sent on SYN packets of new connection• Indicates new connection should be joined to a
previous one• Use previous sequence space
• Works with SACK, FACK, Snoop…• Preserve three-way SYN handshake
• Works with statefull firewalls18
TCP ConnectionMigration1. Initial SYN
2. SYN/ACK
3. ACK (with data)
4. Normal data transfer
5. Migrate SYN
6. Migrate SYN/ACK
7. ACK (with data) 19
TCP ConnectionMigration1. Initial SYN
2. SYN/ACK
3. ACK (with data)
4. Normal data transfer
5. Migrate SYN
6. Migrate SYN/ACK
7. ACK (with data) 20
TCP ConnectionMigration1. Initial SYN
2. SYN/ACK
3. ACK (with data)
4. Normal data transfer
5. Migrate SYN
6. Migrate SYN/ACK
7. ACK (with data)(Note typo in proceedings) 21
Securing the Migration
• Problem: Increased vulnerability to hijacking• Ingress filtering doesn’t help• Attacker only needs token and sequence space
• Solution: Keep the token secret• Negotiate it using Diffie-Hellman exchange• Use sequence numbers to prevent replay
• Resulting connections are as secure as standard TCP (not very)• Use IPsec or SSH for real security
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Preventing DoS Attacks
• Migrate SYNs are heavyweight• Require real computation (SHA-1 hash)• Thus Migrate SYN floods are more dangerous
than standard SYN floods• A pre-computable token guards against
frivolous computation• Refreshing tokens after each successful
migration makes replay window very small23
Benefits & Limitations
• Exposes address changes to end hosts• Agile applications can adapt to changing conditions
for better performance• Mobility per connection, not just per host
• Preserves IP addressing semantics• No changes to the routing infrastructure
• Minimal penalty for mobility support• Obtain optimal unicast packet routing
• End hosts can’t move “simultaneously”• Relatively rare in non ad-hoc environments
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Outline
• TCP Migrate
• Mobile IP
• I3
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Mobile IP (MH at Home)
Mobile Host (MH)
Visiting Location
Home
Internet
Correspondent Host (CH)
Packet
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Mobile IP (MH Moving)
Visiting Location
Home
Internet
Correspondent Host (CH)
Packet
Home Agent (HA) Mobile Host (MH)I am here
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Mobile IP (MH Away – Foreign Agent)
Visiting Location
Home
Internet
Correspondent Host (CH)
Packet
Home Agent (HA) Foreign Agent (FA)Encapsulated
Mobile Host (MH)
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Mobile IP (MH Away - Collocated)
Visiting Location
Home
Internet
Correspondent Host (CH)
Packet
Home Agent (HA) Mobile Host (MH)Encapsulated
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Other Mobile IP Issues
• Route optimality• Resulting paths can be sub-optimal• Can be improved with route optimization
• Unsolicited binding cache update to sender
• Authentication• Registration messages• Binding cache updates
• Must send updates across network• Handoffs can be slow
• Problems with basic solution• Triangle routing• Reverse path check for security
Outline
• TCP Migrate
• Mobile IP
• I3
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Multicast
S1
C1 C2
S2
R RP RR
RR
RP: Rendezvous Point
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Mobility
HA FA
Home Network
Network 5
5.0.0.1 12.0.0.4
Sender
Mobile Node
5.0.0.333
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i3: Motivation
• Today’s Internet based on point-to-point abstraction
• Applications need more:• Multicast• Mobility• Anycast
• Existing solutions:• Change IP layer• Overlays
So, what’s the problem?A different solution for each service
The i3 solution• Solution:
• Add an indirection layer on top of IP• Implement using overlay networks
• Solution Components:• Naming using “identifiers” • Subscriptions using “triggers”• DHT as the gluing substrate
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IndirectionEvery problem
in CS …
Only primitiveneeded
i3: Rendezvous Communication
• Packets addressed to identifiers (“names”)• Trigger=(Identifier, IP address): inserted by
receiver
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Sender Receiver (R)
ID R
trigger
send(ID, data)send(R, data)
Senders decoupled from receivers
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i3: Service Model• API
• sendPacket(id, p);• insertTrigger(id, addr);• removeTrigger(id, addr); // optional
• Best-effort service model (like IP)• Triggers periodically refreshed by end-hosts• Reliability, congestion control, and flow-
control implemented at end-hosts
i3: Implementation
• Use a Distributed Hash Table • Scalable, self-organizing, robust• Suitable as a substrate for the Internet
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Sender Receiver (R)
ID R
trigger
send(ID, data)send(R, data)
DHT.put(id)
IP.route(R)
DHT.put(id)
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Mobility and Multicast
• Mobility supported naturally• End-host inserts trigger with new IP
address transparent to sender• Robust and supports location privacy
• Multicast• All receivers insert triggers under same ID• Sender uses that ID for sending• Can optimize tree construction to balance
load
Mobility
• The change of the receiver’s address • from R to R’ is transparent to the
sender
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Multicast
• Every packet (id, data) is forwarded to each receiver Ri that inserts the trigger (id, Ri)
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Service Composition• Receiver mediated: R sets up chain and
passes id_gif/jpg to sender: sender oblivious
• Sender-mediated: S can include (id_gif/jpg, ID) in his packet: receiver oblivious
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Sender(GIF)
Receiver R(JPG)
ID_GIF/JPG S_GIF/JPGID R
send((ID_GIF/JPG,ID), data)
S_GIF/JPG
send(ID, data) send(R, data)
Public, Private Triggers
• Servers publish their public ids: e.g., via DNS
• Clients contact server using public ids, and negotiate private ids used thereafter
• Useful:• Efficiency -- private ids chosen on “close-by”
i3-servers• Security -- private ids are shared-secrets 44