Transcript Connection Migration: Why & How

Connection Migration: Why & How
Hari Balakrishnan
Networks and Mobile Systems Group
MIT Lab for Computer Science
http://nms.lcs.mit.edu/
Joint work with Alex Snoeren & Dave Andersen
Anatomy of a connection
Some socket on
breeze.lcs.mit.edu
18.31.0.83:5678
www.cnn.com’s
HTTP service (port)
207.25.71.23:80
• Connection defined by IPA:PortA  IPB:PortB
• An IP address does not identify a host; it only
identifies a network interface
• Is this a good definition of a connection?
Problem #1: Host mobility
• Cerf’s comment from DoD Internet paper:
“If a host were to move, its network (and host) addresses would
change and this would affect the connection identifiers used by
the TCP. This is rather like a problem called "dynamic
reconnection" which has plagued network designers since the
inception of the ARPANET project in 1968.”
• Two options today for connections:
 Terminate
and retry
 Somehow preserve IP address and continue
• “Horizontal” mobility isn’t quite enough...
Vertical mobility: Seamless
inter-provider movement
Regional-Area/
”wireless cable”
Metro-Area
Campus-Area
Packet Radio
In-building & In-room
Problem #2: Unreliable
components
cnn.com
• Individual components rather unreliable
• Replicate for improved reliability and availability
• What happens to a bound connection on failure or
unresponsiveness?
Possible solutions
• 1. Force constant IP address for end-point
 Mobile
IP
 Layer-N switches with “Virtual IPs”
• 2. Make names routable
 All
packets identify destination by name, which
serves as routing identifier
 Intentional naming (late binding), TRIAD
• 3. In-band migration
 Don’t
confound end-point and routing identifiers!
Address constancy: Mobile IP
Correspondent
Hosts
D
Foreign Agent (FA)
“Detunnel” to D
on addr dtmp
D
Mobile Host
D
Home Agent
Intercepts
“Tunnel” to FA
pkts
dFA
D
Temporary address dtmp
changes with mobility
Why Mobile IP isn’t right
• Requires additional network support and
infrastructure (HA, FA, authentication,…)
• Triangle routing even for “local” interactions
• Many types of mobile applications
 Connections
that don’t care for seamlessness
 Connection initiators
 Both initiators and responders
• Ingress filtering  reverse tunneling too!
• Vertical mobility can’t be properly handled
• Applications can’t be made aware of mobility
Address constancy: Layer-N
switching
With work can solve:
Local distribution
Client
But we want:
Global distribution
Name-based routing (example)
Intentional name
[service = camera]
[building = NE43
[room = 510]]
Lookup
image
Intentional name resolvers
form an overlay network
Late binding: integrate resolution and message routing
What should a connection be?
• Between communicating applications, not
network interfaces
• Should be possible for an application to easily
change network interface of connection:
 While
preserving good unicast routes
 Securely
• Should not require a priori knowledge of valid
network interfaces
• Dynamism should not affect semantics or
correctness, nor worsen reliability
If done right, can solve both problems at once!
Migrate overview
Location Query
(DNS Lookup)
Location Update
(Dynamic DNS Update)
Name server
Connection Initiation
Connection Migration
Fixed
Host
Migrating Host
foo.bar.edu
yyy.yyy.yyy.yyy
xxx.xxx.xxx.xxx
Problems
• Consistency of name mapping
• Correctness
 Handling
packet losses around time of movement
 What if someone else gets your old address?
• Security
 Connection
hijacking
 Denial-of-service protection
• Semantics
 How
to maintain semantics of connection
sequence across different nodes?
Dynamic name mappings
• Zero TTL on A-records for migratable names
• Use Dynamic DNS (RFC 2136, 2137) for
Internet names
• Potential problems:
 Race
condition between name update and
movement
• Simply retry! This isn’t a new failure mode
 What about old BIND implementations?
• Pray that they’ll vanish off the face of the earth
 What
about extra DNS load?
• What load? Ask Akamai!
Migrating a connection
• Initiate migration from new network address
 Identify
previous connection with token, on SYN
 Secure token to protect against hijacking
 Requires some state machine changes to
guarantee correctness
• Preserves service model to application
• Handles “middle boxes”
 Works
with most NATs, PEPs, stateful firewalls
• Requires changes to transport protocol
 Kernel
TCP, SCTP, RTP (linked library)
TCP
Connection
Migration
1. Initial SYN
2. SYN/ACK
3. ACK (with data)
4. Normal data transfer
5. Migrate SYN
6. Migrate SYN/ACK
7. ACK (with data)
TCP
Connection
Migration
1. Initial SYN
2. SYN/ACK
3. ACK (with data)
4. Normal data transfer
5. Migrate SYN
6. Migrate SYN/ACK
7. ACK (with data)
TCP
Connection
Migration
1. Initial SYN
2. SYN/ACK
3. ACK (with data)
4. Normal data transfer
5. Migrate SYN
6. Migrate SYN/ACK
7. ACK (with data)
Two correctness issues
• SYN uses 1 byte of sequence space;
what should SYN ACK value be set to?
 Needed
to correctly handle lost segments
• What if someone else gets your previous
address?
 Peer
TCP will reset connection
Correctness:
SYN ACK
corresponds
to data
1. Initial SYN
2. SYN/ACK
3. ACK (with data)
4. Normal data transfer
5. Migrate SYN
6. Migrate SYN/ACK
7. ACK (with data)
• 2 new transitions
between existing states
- and • 1 new state
handles potential race
condition due to rapid
readdressing
recv: SYN (migrate T, R)
send: SYN, ACK
Modified
TCP State
Machine
MIGRATE_WAIT
2MSL timeout
Securing the Migration
• Problem: Increased vulnerability to hijacking
filtering (RFC 2827) doesn’t help
 Attacker only needs token and sequence space
 Ingress
• Solution: Keep the token secret
 Negotiate
it using Diffie-Hellman exchange
(Elliptic-Curve DH)
 Use sequence numbers to prevent replay
 Complete crypto exchange in SYN handshake
• Result: Connections are as secure as
standard TCP
 Use
IPsec or SSH for real security
Semantics of multi-machine
migration
ACK 9000
7801-9000
• Sequence spaces across different machines may not
have same application-layer semantics
One solution: Soft-state
synchronzation
foo.mp3
ISN = I
Token = T
caddr = Ca
cport = Cp
preload-tcb
Health
Monitor
ACK 9000
7801-9000
• Technique for static content (e.g., file)
• Information about mapping between filename and
TCP initial sequence periodically disseminated
Implementation
• Use application-specific stream mapper to
map between sequence space and app (e.g.,
HTTP range requests)
Client
request
HTTP GET parser/creator
HTTP header parser/
Response
stripper
handler
Data relay
To client
HTTP range
request
Backend
HTTP
Response server
• Stream mapper involved in initial connection
processing and in re-establishment
Experiment #1: Mobility
Mobile client initiates a transfer…
Mobile
Location 1
19.2Kbps
Modem
Fixed
Server
100Mbps Ethernet
Fixed
Basestation
19.2Kbps
Modem
…then moves to a new location
Mobile
Location 2
Migration Trace
SYN/ACK
Buffered
Packets
(old address)
Migrate SYN
A Lossy Trace with SACK
SYN/ACK
Buffered
Packets
(old address)
ACK
w/SACK
Migrate SYN
Experiment #2: Failover works!
0.000 cl.1065 > sA.8080: . ack 0505 win 31856
----> (Erroneous) sA death pronouncement issued
0.080 sA.8080 > cl.1065: P 0505:1953(1448) ack 1 win 31856
----> Successful connection migration to sB
0.095 sB.1033 > cl.1065: S 0:0(0) win 0 <migrate PRELOAD 1>
0.096 cl.1065 > sB.1033: S 0:0(0) ack 1953 win 32120
0.142 sB.1033 > cl.1065: . ack 1 win 32120
----> Continued data transmission from sA (Reset by client)
0.174 sA.8080 > cl.1065: P 0505:1953(1448) ack 1 win 31856
0.174 cl.1065 > sA.8080: R 1:1(0) win 0
----> Resumed data transmission from sB
0.241 sB.1033 > cl.1065: P 1953:3413(1460) ack 1 win 32120
...
Oscillations aren’t a problem
Summary
• Host mobility and service failover are
examples of the same fundamental problem:
 Connections
must be between applications
• The Migrate architecture enables connections
to be separated from, and move between, IP
addresses
• Mobility & service failover are both really
end-to-end issues!
• Got code?
Networks and Mobile Systems
Migrate code for Linux 2.2 available from:
http://nms.lcs.mit.edu/software/
Migrate project Web page:
http://nms.lcs.mit.edu/projects/migrate/