Indirection

1
Indirection

• Jennifer Rexford
• Advanced Computer Networks
• http//www.cs.princeton.edu/courses/archive/fall08
  /cos561/
• Tuesdays/Thursdays 130pm-250pm

Slides borrowed liberally from Ion Stoicas CS
268 class at UC Berkeley
2
Motivations

• Todays Internet is built around a unicast
  point-to-point communication abstraction
• Send packet p from host A to host B
• This abstraction allows Internet to be highly
  scalable and efficient
• But not appropriate for applications that require
  other communications primitives
• Multicast
• Anycast
• Mobility

3
Why?

• Point-to-point communication
• Implicitly assumes there is one sender
• and one receiver,
• and that they are placed at fixed and
  well-known locations
• Network location tied to network layer identifier
• E.g., a host identified by the IP address
  128.32.xxx.xxx is located in Berkeley

4
IP Solutions

• Extend IP to support new communication
  primitives, e.g.,
• Mobile IP
• IP multicast
• IP anycast
• Disadvantages
• Difficult to implement while maintaining
  Internets scalability (e.g., multicast)
• Require community wide consensus -- hard to
  achieve in practice

5
Application-Level Solutions

• Implement the required functionality at the
  application level, e.g.,
• Application-level multicast
• Application-level mobility
• Disadvantages
• Inefficiency hard to achieve good performance
• Redundancy Each application implements the same
  functionality over and over again
• No synergy each application implements usually
  only one service, services hard to combine

6
Key Observation

• All previous solutions use a simple but powerful
  technique indirection
• Assume a logical or physical indirection point
  interposed between sender(s) and receiver(s)
• Examples
• IP multicast assumes a logical indirection point
  the IP multicast address
• Mobile IP assumes a physical indirection point
  the home agent

7
I3 Solution

• Use overlay network to implement this layer
• Incrementally deployable dont need to change IP

8
Internet Indirection Infrastructure (i3)

• Each packet is associated an identifier id
• To receive packet with identifier id, receiver R
  inserts trigger (id, R) into overlay network

Sender
Receiver (R)
9
Service Model

• API
• sendPacket(p)
• insertTrigger(t)
• removeTrigger(t) // optional
• Best-effort service model (like IP)
• Triggers periodically refreshed by end hosts
• ID length 256 bits

10
Mobility

• Host just needs to update its trigger as it moves
  from one subnet to another

Sender
11
Multicast

• Receivers insert triggers with same identifier
• Dynamically switch between multicast and unicast

id
R1
Receiver (R1)
Sender
id
R2
Receiver (R2)
12
Anycast

• Use longest prefix matching instead of exact
  matching
• Prefix p anycast group identifier
• Suffix si encode application semantics, e.g.,
  location

Receiver (R1)
R1
ps1
R2
ps2
Sender
Receiver (R2)
R3
ps3
Receiver (R3)
13
Service Composition Sender Initiated

• Use a stack of IDs to encode sequence of
  operations to be performed on data path
• Advantages
• Dont need to configure path
• Load balancing and robustness easy to achieve

Transcoder (T)
Receiver (R)
Sender
id
R
idT
T
14
Service Composition Receiver Initiated

• Receiver can also specify the operations to be
  performed on data

Firewall (F)
Receiver (R)
Sender
idF
F
id
idF,R
15
Quick Implementation Overview

• ID space is partitioned across infrastructure
  nodes
• Each node responsible for a region of ID space
• Each trigger (id, R) is stored at the node
  responsible for id
• Use Chord to route triggers and packets to nodes
  responsible for their IDs
• O(log N) hops

16
Example

• IDs0..63 partitioned across five i3 nodes
• Each host knows one i3 node
• R inserts trigger (37, R) S sends packet (37,
  data)

8..20
4..7
42..3
21..35
Sender (S)
36..41
Receiver (R)
17
Optimization Path Length

• Sender/receiver caches i3 node mapping a specific
  ID
• Subsequent packets are sent via one i3 node

8..20
4..7
42..3
21..35
Sender (S)
36..41
Receiver (R)
18
Optimization Triangular Routing

• Use well-known trigger for initial rendezvous
• Exchange a pair of (private) triggers
  well-located
• Use private triggers to send data traffic

8..20
4..7
42..3
21..35
Sender (S)
36..41
Receiver (R)
19
Outline

• Overview
• Security
• Discussion

20
Some Attacks
Eavesdropping
R
id
R
S
21
Constrained Triggers

• hl(), hr() well-known one-way hash functions
• Use hl(), hr() to constrain trigger (x, y)

22
Attacks Defenses
Attack
Defense
23
Design Principles

• Give hosts control on routing
• A trigger is like an entry in a routing table!
• Flexibility, customization
• End-hosts can
• Source route
• Set-up acyclic communication graphs
• Route packets through desired service points
• Stop flows in infrastructure
• Implement data forwarding in infrastructure
• Efficiency, scalability

24
Design Principles (contd)
Infrastructure
Host
Data plane
Internet
Control plane
p2p End-host overlays
Data plane
Control plane
i3
Data plane
Control plane
25
Conclusions

• Indirection key technique to implement basic
  communication abstractions
• Multicast, Anycast, Mobility,
• Building efficient indirection layer on IP
• Triggers, and DHTs to map to the location

26
Discussion

• Value of composition?
• Middlebox functionality
• E.g., transcoding, firewall, caching, etc.
• Efficiency cost?
• Stretch by traversing intermediate node
• Processing and storing of triggers
• Complexity?
• When trying to reduce stretch
• When trying to cache information
• Should indirection be part of future Internet?

27
Next Two Weeks

• Guest lectures
• 11/18 Mike Freedman
• 11/20 Sharon Goldberg
• 11/25 Mung Chiang
• No class on 11/27 due to Thanksgiving
• Ill be in Thailand
• Co-chairing Asia Internet Engineering Conference
• After Thanksgiving
• Programmability and virtualization