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Internet Design: Big Picture
• Internet architectural, design and implementation
principles
– not “scriptures”, but guidelines
– understand pros and cons, trade-offs involves
• Original Internet Design Goals
– what contributed to the huge success?
– what still amiss, biggest weaknesses?
• Internet as a case study
– virtualization., layered architecture, end-to-end
argument, soft-state, fate sharing, …
• Readings: ; [Cla88], [SRC84], [Car96]
– See Reading List
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Internet Design
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Architectural Principles
(not unique to networks!)
Zhi-Li’s version (synthesized from various sources)
• End-to-end argument
– functionality placement
• Modularity
– Increase inter-operability and manage complexity
• vertical partition -> layered architecture
• horizontal partition?
• Keep it simple, stupid (KISS principle)
– Occam’s Razor: choose simplest among many solutions!
• complicated design increases system coupling (interdependence), amplifies errors, ..
• don’t over-optimize!
• Separating policies from mechanisms
– decouple control from data
– “semantics-free”
• Design for scale
– hierarchy, aggregation, …
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Network Architecture
What is (Network) Architecture?
– not the implementation itself
– “design blueprint” on how to “organize” implementations
• what interfaces are supported
• where functionality is implemented
• Two Basic Architectural Principles
– Modularity (e.g., layering)
• how to break network functionality into modules
– End-to-End Argument
• where to implement functionality
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Some Design/Implementation Principles
•
•
•
•
•
•
•
•
virtualization
indirection
soft state vs. hard state
fate sharing
randomization
expose faults, don’t suppress/ignore
caching
……
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Original Internet Design Goals
[Clark’88]
In order of importance:
0
1.
2.
3.
4.
5.
6.
Connect existing networks
– initially ARPANET and ARPA packet radio network
Survivability
- ensure communication service even with network and router
failures
Support multiple types of services
Must accommodate a variety of networks
Allow distributed management
Allow host attachment with a low level of effort
Be cost effective
7. Allow resource accountability
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Priorities
• The effects of the order of items in that list are
still felt today
– E.g., resource accounting is a hard, current research
topic
• Different ordering of priorities would make a
different architecture!
• How well has today’s Internet satisfied these
goals?
• Let’s look at them in detail
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0. Connecting Existing Networks
1974: multiple unconnected networks
–
–
–
–
ARPAnet
data-over-cable networks
packet satellite network (Aloha)
packet radio network
–
–
–
–
–
addressing conventions (i.e., address formats)
packet formats and packet sizes
Performance: bandwidth, latency, loss rate, …
error recovery mechanisms
routing
.. differing in:
• How to inter-network various (heterogeneous) network
technologies?
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Cerf & Kahn: Interconnecting Two Networks
ARPAnet
•
•
•
•
satellite net
“…interconnection must preserve intact the internal operation of each network.”
“ ..the interface between networks must play a central role in the development of
any network interconnection strategy. We give a special name to this interface
that performs these functions and call it a GATEWAY.”
“.. prefer that the interface be as simple and reliable as possible, and deal
primarily with passing data between networks that use different packet-switching
strategies
“…address formats is a problem between networks because the local network
addresses of TCP's may vary substantially in format and size. A uniform
internetwork TCP address space, understood by each GATEWAY and TCP, is
essential to routing and delivery of internetwork packets.”
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Design Alternatives
• Through translation/mapping:
– Map one address format to another: nxn mappings
– Difficulty in dealing with different features supported by
networks
– Scales poorly with # of network types, addition of new types
• Virtualization:
– Provide one common format overlaid on top of “lower-level”
addresses
– Map lower level addresses to common format: nx1 and 1xn
mappings
• role of ARP, encapsulation/decapsulation
– Layering necessary
• but what info from “lower layer” (underlying “physical”
networks) to hide, and what to expose!
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Gateway Alternative
• Translation
– Difficulty in dealing with different features supported
by networks
– Scales poorly with number of network types (N^2
conversions)
• Standardization/Virtualization
– “IP over everything”
– Minimal assumptions about network
– Hourglass design
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Design of Original Internet via Gateways
(cf. Cerf and Kahn)
Internetwork layer:
• addressing: internetwork
appears as a single, uniform
entity, despite underlying local
network heterogeneity
• network of networks
Gateway:
• “embed internetwork packets in
local packet format or extract
them”
• route (at internetwork level) to
next gateway
gateway
ARPAnet
CSci5221:
Internet Design
satellite net
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Historical Aside:
Proposed Internetwork packet in 1974:
local source dest.
byte
seq. #
header address address
count
network
TCP
identifier
8
16
CSci5221:
Internet Design
flag
field
text
checksum
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Cerf & Kahn’s Internetwork
Architecture
What is virtualized?
• two layers of addressing: internetwork
and local network
• new layer makes everything homogeneous
at internetwork layer
• underlying local network technology (cable,
satellite, 56K modem) is “invisible” at
internetwork layer
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1. Survivability
1. As long as the network is not partitioned, two
endpoints should be able to communicate
2. Failures (excepting network partition) should not
interfere with endpoint semantics (why?)
• Maintain state only at end-points
–
–
•
•
Fate-sharing, eliminates network state restoration
stateless network architecture (no per-flow state)
Routing state is held by network (why?)
No failure information is given to end-points
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Survivability (cont’d)
• If network disrupted and reconfigured:
– Communicating entities (“end systems”) should not care!
– No higher-level state reconfiguration
• How to achieve such reliability?
– Where can communication state be stored?
Network
Host
Failure handling
Replication
“Fate sharing”
Switches
Maintain only
routing state;
otherwise
“stateless”
Maintain perflow/per-connection
state
Host trust
Less
More
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Fate Sharing
Connection
State
No State
State
• Lose connection state information for an
entity if (and only if?) the entity itself is
lost
• Examples:
– OK to lose TCP state if one endpoint crashes
• NOT okay to lose if an intermediate router reboots
– Is this still true in today’s network?
• NATs, firewalls and other middle-boxes
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Soft-State
• Basic behavior
– Announce state
– Refresh state
– Timeout state
• Penalty for timeout – poor performance
• Robust way to identify communication
flows
– Possible mechanism to provide non-best effort service?
• Helps survivability
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End-to-End Argument
• Deals with where to place functionality
– Inside the network (in switching elements) ?
– At the edges (end-points) ?
• Argument:
– There are functions that can only be correctly
implemented by the endpoints – do not try to completely
implement these elsewhere
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Discussion
• Is there any need to implement reliability
at lower layers?
• Yes, but only to improve performance
• If network is highly unreliable
– Adding some level of reliability helps performance, not
correctness
– Don’t try to achieve perfect reliability!
– Implementing a functionality at a lower level should have
minimum performance impact on the applications that do
not use the functionality
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Design Challenges and Trade-offs
• Install functions in network that aid application
performance….
• …without limiting the application flexibility of the
network
• Trade-offs:
– application has more information about the data and semantics
of required service (e.g., can check only at the end of each data
unit)
– lower layer has more information about constraints in data
transmission (e.g., packet size, error rate)
• Note: these trade-offs are a direct result of
layering!
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Do These Belong in the Network?
•
•
•
•
Multicast?
Routing?
Quality of Service (QoS)?
Name resolution? (is DNS “in the
network”?)
• Web or media caches?
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2. Types of Service
•
•
•
•
Best effort delivery
All packets are treated the same
Relatively simple core network elements
Building block from which other services (such as
reliable data stream) can be built
• Contributes to scalability of network
• No QoS support assumed from below
– Accommodates more networks
– Hard to implement without network support
– QoS is an ongoing debate…
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Types of Service (cont’d)
• TCP vs. UDP
–
–
–
–
Elastic apps that need reliability: remote login or email
Inelastic, loss-tolerant apps: real-time voice or video
Others in between, or with stronger requirements
Biggest cause of delay variation: reliable delivery
• Today’s net: ~100ms RTT
• Reliable delivery can add seconds.
• Original Internet model: “TCP/IP” one layer
– First app was remote login…
– But then came debugging, voice, etc.
– These differences caused the layer split, added UDP
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3. Varieties of Networks
• Minimum set of assumptions for underlying net
– Minimum packet size
– Reasonable delivery odds, but not 100%
– Some form of addressing unless point to point
• Important non-assumptions:
–
–
–
–
Perfect reliability
Broadcast, multicast
Priority handling of traffic
Internal knowledge of delays, speeds, failures, etc.
• Much engineering then only has to be done once
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The “Other” goals
• 4. Management
– Each network owned and managed separately
– Will see this in BGP routing especially
• 5. Attaching a host
– Not awful; DHCP and related autoconfiguration
technologies helping.
• 6. Cost effectiveness
– Economies of scale won out
– Internet cheaper than most dedicated networks
– Packet overhead less important by the year
• But…
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7. Accountability
• Huge problem!
• Accounting
– Billing? (mostly flat-rate. But phones are moving that
way too - people like it!)
– Inter-provider payments
• Hornet’s nest. Complicated. Political. Hard.
• Accountability and security
– Huge problem!
– Worms, viruses, etc.
• Partly a host problem. But hosts very trusted.
– Authentication
• Purely optional. Many philosophical issues of privacy
vs. security.
– Greedy sources aren’t handled well
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Other IP Design Weaknesses
• Weak administration and management tools
• Incremental deployment difficult at times
– Result of no centralized control
– No more “flag” days
– Are active networks the solution?
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Internet Motto
We reject kings , presidents, and voting. We
believe in rough consensus and running
code.”
David Clark
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Real Goals
1.
2.
3.
4.
5.
6.
7.
8.
9.
Something that works…..
Connect existing networks
Survivability (not nuclear war…)
Support multiple types of services
Accommodate a variety of networks
Allow distributed management
Easy host attachment
Cost effective
Allow resource accountability
CSci5221:
Internet Design
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Summary: Internet
Architecture
• Packet-switched
datagram network
• IP is the “compatibility
layer”
– Hourglass architecture
– All hosts and routers run IP
• Stateless architecture
– No per flow state inside
network
CSci5221:
Internet Design
TCP
UDP
IP
Ethernet
Cellular
WiFi
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Summary: Minimalist Approach
• Dumb network
– IP provide minimal functionalities to support connectivity
• Addressing, forwarding, routing
• Smart end system
– Transport layer or application performs more
sophisticated functionalities
• Flow control, error control, congestion control
• Advantages
– Accommodate heterogeneous technologies (Ethernet,
modem, satellite, wireless)
– Support diverse applications (telnet, ftp, Web, X
windows)
– Decentralized network administration
• Beginning to show age
– Unclear what the solution will be  probably IPv6?
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Questions
• What priority order would a commercial
design have?
• What would a commercially invented
Internet look like?
• What goals are missing from this list?
• Which goals led to the success of the
Internet?
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Requirements for Today’s Internet
Some key requirements (“-ities”)
• Availability and reliability
– “Always on”, fault-tolerant, fast recovery from failures, …
• Quality-of-service (QoS) for applications
– fast response time, adequate quality for VoIP, IPTV, etc.
• Scalability
– millions or more of users, devices, …
• Mobility
– untethered access, mobile users, devices, …
• Security (and Privacy?)
– protect against malicious attacks, accountability of user actions?
• Manageability
– configure, operate and manage networks
– trouble-shooting network problems
• Flexibility, Extensibility, Evolvability, ……?
– ease of new service creation and deployment?
– evolvable to meet future needs?
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