Transcript ipv6 - SNU

IP revisited
Taekyoung Kwon
[email protected]
Courtesy of
Kevin Lai and Ion Stoica with Berkeley
Jim Kurose with Umass
Henning Schulzrinne with Columbia
Network Core: Circuit Switching
network resources (e.g., bandwidth) divided
into “pieces”
• pieces allocated to calls
• resource piece idle if not used by owning call (no
sharing)
• dividing link bandwidth into “pieces”
– frequency division
– time division
Network Core: Packet Switching
each end-end data stream
divided into packets
• user A, B packets share
network resources
• each packet uses full link
bandwidth
• resources used as needed,
Bandwidth division into
“pieces”
Dedicated allocation
Resource reservation
resource contention:
• aggregate resource
demand can exceed
amount available
• congestion: packets
queue, wait for link use
• store and forward:
packets move one hop at
a time
– transmit over link
– wait turn at next link
Network Core: Packet Switching
10 Mbs
Ethernet
A
B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for output
link
D
45 Mbs
E
Packet-switching versus circuit switching: human
restaurant analogy
• other human analogies?
Packet switching versus circuit switching
Packet switching allows more users to use network!
• 1 Mbit link
• each user:
– 100Kbps when “active”
– active 10% of time
• circuit-switching:
– 10 users
• packet switching:
N users
1 Mbps link
– with 35 users,
probability > 10 active
less than .0004
What is statistical multiplexing gain?
Packet switching versus circuit
switching
Is packet switching a “slam dunk winner?”
• Great for bursty data
– resource sharing
– no call setup
• Excessive congestion: packet delay and loss
– protocols needed for reliable data transfer, congestion
control
• Q: How to provide circuit-like behavior?
– bandwidth guarantees needed for audio/video apps
– still an unsolved problem
Packet-switched networks: routing
• Goal: move packets among routers from source to
destination
• datagram network:
– destination address determines next hop
– routes may change during session
– analogy: driving, asking directions
• virtual circuit network:
– each packet carries tag (virtual circuit ID), tag determines
next hop
– fixed path determined at call setup time, remains fixed thru
call
– routers maintain per-call state
Delay in packet-switched networks
packets experience delay on
end-to-end path
• four sources of delay at
each hop
• nodal processing:
– check bit errors
– determine output link
• queueing
transmission
A
propagation
B
nodal
processing
– time waiting at output
link for transmission
– depends on congestion
level of router
queueing
Which delay corresponds to bandwidth?
Delay in packet-switched
networks
Transmission delay:
• R=link bandwidth (bps)
• L=packet length (bits)
• time to send bits into link
= L/R
transmission
A
Propagation delay:
• d = length of physical link
• s = propagation speed in
medium (~2x108 m/sec)
• propagation delay = d/s
Note: s and R are very
different quantities!
propagation
B
nodal
processing
queueing
High-speed network?
High-bandwidth network?
Queueing delay (revisited)
• R=link bandwidth (bps)
• L=packet length (bits)
• a=average packet arrival
rate
traffic intensity = La/R
• La/R ~ 0: average queueing delay small
• La/R -> 1: delays become large
• La/R > 1: more “work” arriving than can be
serviced, average delay infinite!
Internet structure: network of
networks
• roughly hierarchical
• national/international backbone
providers (NBPs) [tier 1]
– e.g. BBN/GTE, Sprint, AT&T, IBM,
UUNet
– interconnect (peer) with each
other privately, or at public
Network Access Point (NAPs)
• regional ISPs [tier 2]
local
ISP
regional ISP
NBP B
NAP
NAP
NBP A
– connect into NBPs
• local ISP [tier 3], company
– connect into regional ISPs
• a point-of-presence (POP) is an
access point from one place to the
rest of the Internet
regional ISP
local
ISP
National Backbone Provider
e.g. Sprint US backbone network
Layered approach
• What is layering?
– A technique to organize a network system into
a succession of logically distinct entities, such
that the service provided by one entity is solely
based on the service provided by the previous
(lower level) entity
Why layering?
Application
Transmission
Media
Telnet
FTP
Coaxial
cable
NFS
Fiber
optic
HTTP
Packet
radio
• No layering: each new application has to be reimplemented for every network technology!
Why Layering?
• Solution: introduce an intermediate layer that
provides a unique abstraction for various network
technologies
Application
Telnet
FTP
NFS
HTTP
Intermediate
layer
Transmission
Media
Coaxial
cable
Fiber
optic
Packet
radio
Layering
• Advantages
– Modularity – protocols easier to manage and maintain
– Abstract functionality: lower layers can be changed
without affecting the upper layers
– Reuse – upper layers can reuse the functionality provided
by lower layers
• Disadvantages
– Information hiding – inefficient implementations
Key Design Decision
• How do you divide functionality across the layers?
Layering: Hop-by-Hop vs. End-to-End
• Think twice before implementing a functionality
that you believe that is useful to an application
at a lower layer
• If the application can implement a functionality
correctly, implement it a lower layer only as a
performance enhancement
Example: Reliable File Transfer
Host A
Host B
Appl.
OS
Appl.
OK
OS
• Solution 1: make each step reliable, and then
concatenate them
• Solution 2: end-to-end check and retry
Discussion
• Solution 1 not complete
– What happens if the sender or/and receiver misbehave?
• The receiver has to do the check anyway!
• Thus, full functionality can be entirely
implemented at application layer; no need for
reliability from lower layers
• Is there any need to implement reliability at
lower layers?
Discussion
• Yes, but only to improve performance
• Example:
– Assume a high error rate on communication network
– Then, a reliable communication service at data link layer
might help
Trade-offs
• Application has more information about the data
and the semantic of the service it requires (e.g.,
can check only at the end of each data unit)
• A 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!
Rule of Thumb
• Implementing a functionality at a lower level
should have minimum performance impact on the
application that do not use the functionality
Internet & End-to-End Argument
• Provides one simple service: best effort datagram
(packet) delivery
• Only one higher level service implemented at
transport layer: reliable data delivery (TCP)
– Performance enhancement; used by a large variety of
applications (Telnet, FTP, HTTP)
– Does not impact other applications (can use UDP)
• Everything else implemented at application level
Key Advantages
• The service can be implemented by a large variety
of network technologies
• Does not require routers to maintain any fined
grained state about traffic. Thus, network
architecture is
– Robust
– Scalable
What About Other Services?
• Multicast?
• Quality of Service (QoS)?
Summary: Layering
• Key technique to implement communication
protocols; provides
– Modularity
– Abstraction
– Reuse
• Key design decision: what functionality to put in
each layer?
Summary: End-to-End Argument
• If the application can do it, don’t do it at a lower
layer -- anyway the application knows the best
what it needs
– Add functionality in lower layers iff it is (1) used and
improves performances of a large number of applications,
and (2) does not hurt other applications
• Success story: Internet
Summary
• Challenge of building a good (network) system: find the right
balance between:
Reuse, implementation effort
(apply layering concepts)
Performance

End-to-end argument
No universal answer: the answer depends on the
goals and assumptions!
How Internet started?
The Problem
• Before Internet
– different packet-switching networks (e.g., ARPANET, ARPA
packet radio)
– only nodes on the same physical/link layer network could
communicate
– want to share room-size computers, storage to reduce
expense
The Challenge
• Interconnect existing networks
• … but, packet switching networks differ
widely
– different services
• e.g., degree of reliability
– different interfaces
• e.g., length of the packet that can be transmitted,
address format
– different protocols
• e.g., routing protocols
Possible solutions
• Reengineer and develop one global packet
switching network standard
– not economically feasible
– not deployable
• Have every host implement the protocols of any
network it wants to communicate with
– Complexity/node = O(n)
– O(n2) global complexity
Solution
• Add an extra layer: inter-networking layer
– hosts:
• understand one network protocol
• understand one physical/link protocol
– gateways:
• understand one network protocol
• understand the physical/link protocols of the networks they
gateway
– Complexity to add a node/network: O(1) with respect to
number of existing nodes
Solution
Gateways
Common Intermediate Representation
• Examples:
– telnet, IP, strict HTML, I-mode cHTML
• Who ignored this:
– US cell phone providers (pairwise roaming agreements)
– IE HTML, Netscape HTML, etc.
– WAP (WML same purpose as HTML, but not compatible)
• network value = O(n2), (Metcalfe's Law)
• pairwise translation: cost = O(n2), utility = O(1)
• CIR: cost = O(n), utility = O(n)
Challenge 1: Different Address Formats
• Options:
– Map one address format to another. Why not?
– Provide one common format
• map lower level addresses to common format
• Format:
– Initially: 8b network 16b host 24b total
– Before Classless InterDomain Routing (CIDR):
• 7b/24b, 14b/16b, or 21b/8b 32b total
– After CIDR: Arbitrary division 32b total
– NAT: 32b + 16b simultaneously active
– IPv6: 128b total
Address Formats
•
•
•
•
•
•
256 networks? What were they thinking?
Why CIDR?
What happens if they run out before IPv6?
Why IPv6?
Why 128b for IPv6? 248=281 trillion.
Why not variable length addresses?
Challenge 2: Different Packet Sizes
• Need to define maximum packet size
• Options:
– Take the minimum of the maximum packets sizes over all
networks
– Implement fragmentation/reassembly
• Flexibility to adjust packet sizes as new technologies arrive
• IP: fragment at routers, reassemble at host
• Why not reassemble at routers?
– Still stuck with 1500B as de facto maximum
Other Challenges
• Errors  require end-to-end reliability
– Thought to be rarely invoked, but necessary
• Different (routing) protocols  coordinate these
protocols
• Accounting
– Did not envision script kiddies
• Quality of Service
– Not addressed
Transmission Control Program
• Original TCP/IP (Cerf & Kahn)
– no separation between transport (TCP) and network (IP)
layers
– one common header (vestige?)
– flow control, but not congestion control (why not?)
– fragmentation handled by TCP
• Today’s TCP/IP
–
–
–
–
separate transport (TCP) and network (IP) layer (why?)
split the common header in: TCP and UDP headers
fragmentation reassembly done by IP
congestion control
Devil’s Advocate
• Who cares about resource sharing?
– 1974:
cheap
– 2002:
– 1974:
– 2002:
sell
cycles, storage, bandwidth expensive, people
resources cheap, people expensive
Share computer resources
Communicate with people, access documents, buy,
• Does it still make sense to make processes the
endpoint?
Back to the big picture
Goals (Clark’88)
0 Connect existing networks
– initially ARPANET and ARPA packet radio
network
1. Survivability
-
ensure communication service even in the
presence of network and router failures
2. Support multiple types of services
3. Must accommodate a variety of networks
4.
5.
6.
7.
Allow distributed management
Allow host attachment with a low level of effort
Be cost effective
Allow resource accountability
1. Survivability
• continue to operate even in the presence of
network failures (e.g., link and router
failures)
– failures (excepting network partition) should be
transparent to endpoints
• maintain state only at end-points (fatesharing)
– no need to replicate and restore router state
– disadvantages?
• Internet: stateless network architecture
– no per-flow state, still have state in address
allocation, DNS
2. Types of Services
• Add UDP to TCP to better support other types of
applications
– e.g., “real-time” applications
• Probably main reason for separating TCP and IP
• Provide datagram abstraction: lower common
denominator on which other services can be built
– service differentiation considered (ToS header bits)
– was not widely deployed (why?)
Application Assumptions
• Who made them:
– Telephone network: voice (web, video?)
– Cable: broadcast (2-way?)
– X.25: remote terminal access (file transfer?)
– BBS: centralized meeting place (web, p2p?)
– NAT: client/server model (p2p, IM, IP Telephony?)
• Who didn't: Internet
– Caveat: best-effort, unicast, fixed location (real-time,
multicast, mobility?)
• Allows development of unforeseen applications:
– Web, p2p, distributed gaming
3. Variety of Networks
• Very successful
– because the minimalist service; it requires from
underlying network only to deliver a packet with a
“reasonable” probability of success
• …does not require:
– reliability, in-order delivery, single delivery, QoS
guarantees
• The mantra: IP over everything
– Then: ARPANET, X.25, DARPA satellite network..
– Now: ATM, SONET, WDM, PPP, USB, 802.11b, GSM, GPRS,
DSL, cable modems, power lines
Internet Architecture
• Packet-switched
datagram network
• IP is the glue
• Hourglass architecture
– all hosts and routers run
IP
• Common Intermediate
Representation
TCP
UDP
IP
Satellite
Ethernet ATM
Other Goals
• Allow distributed management
– each network can be managed by a different
organization
– different organizations need to interact only at
the boundaries
– doesn’t work so well for routing, accounting
• Cost effective
– sources of inefficiency
•
•
•
header overhead
retransmissions
routing
– …but routers relatively simple to implement
(especially software side)
Other Goals (Cont)
• Low cost of attaching a new host
– not a strong point  higher than other architecture
because the intelligence is in hosts (e.g., telephone vs.
computer)
• Moore’s law made this moot point, both <$100
– bad implementations or malicious users can produce
considerably harm (remember fate-sharing?)
• DDoS possibly biggest threat to Internet
• Accountability
– very little so far
What About the Future?
• Datagram not the best abstraction for:
– resource management, accountability, QoS
• A new abstraction: flow?
• Routers require to maintain per-flow state (what is
the main problem with this raised by Clark?)
– state management
• Solution
– soft-state: end-hosts responsible to maintain the state
Summary: Minimalist Approach
• Dumb network
– IP provide minimal functionalities to support connectivity
– addressing, forwarding, routing
• Smart end system
– transport layer or app does more sophisticated functionalities
– flow control, error control, congestion control
• Advantages
– accommodate heterogeneous technologies
– support diverse applications (telnet, ftp, Web, X windows)
– decentralized network administration
• Disadvantages
– poor realtime performance
– poor accountability
Textbook Internet vs. real Internet
end-to-end (application
only in 2 places)
middle boxes (proxies, ALGs,
…)
permanent interface
identifier (IP address)
time-varying (DHCP)
globally unique and
routable
network address translation
(NAT)
multitude of L2 protocols dominance of Ethernet, but
(ATM, ARCnet, Ethernet, FDDI,
also L2’s not designed for
modems, …)
networks (1394 Firewire, Fibre
Channel, …)
Textbook Internet vs. real Internet
mostly trusted end users
hackers, spammers, con
artists, pornographers, …
small number of
manufacturers, making
expensive boxes
Linksys, Dlink, Netgear, …,
available at Radio Shack
technical users, excited about
new technology
grandma, frustrated if email
doesn’t work
4 layers (link, network,
transport, application)
layer splits
transparent network
firewalls, L7 filters,
“transparent proxies”
The Internet
Protocol
Hourglass
(Deering)
email WWW phone...
SMTP HTTP RTP...
TCP UDP…
IP
ethernet PPP…
CSMA async sonet...
copper fiber radio...
Why the hourglass architecture?
• Why an internet layer?
– make a bigger network
– global addressing
– virtualize network to isolate end-to-end
protocols from network details/changes
• Why a single internet protocol?
– maximize interoperability
– minimize number of service interfaces
• Why a narrow internet protocol?
– assumes least common network functionality
to maximize number of usable networks
Deering, 1998
Putting on
Weight
email WWW phone...
SMTP HTTP RTP...
TCP UDP…
IP + mcast
+ QoS +...
ethernet PPP…
CSMA async sonet...
copper fiber radio...
• requires more
functionality
from underlying
networks
Mid-Life
Crisis
email WWW phone...
SMTP HTTP RTP...
TCP UDP…
IP4
IP6
ethernet PPP…
CSMA async sonet...
copper fiber radio...
• doubles number
of service
interfaces
• requires changes
above & below
• major interoperability issues
Layer splitting
• Traditionally, L2 (link), L3 (network = IP), L4
(transport = TCP), L7 (applications)
• Layer 2: Ethernet  PPPoE (DSL)
• Layer 2.5: MPLS, L2TP
• Layer 3: tunneling (e.g., GPRS)
• Layer 4: UDP + RTP
• Layer 7: HTTP + real application
Internet acquires presentation layer
• All learn about OSI 7-layer model
• OSI: ASN.1 as common rendering of application
data structures
– used in LDAP and SNMP (and H.323)
• Internet never really had presentation layer
– approximations: common encoding (TLV, RFC 822 styles)
• Now, XML as the design choice by default
Internet acquires session layer
• Originally, meant for data sessions
• Example (not explicit): ftp control connection
• Now, separate data delivery from session setup
– address and application configuration
– deal with mobility
– E.g., RTSP, SIP and H.323
Standards
•
Mandatory vs. voluntary
•
Telecommunications and networking always focus of standardization
•
Five major organizations:
– Allowed to use vs. likely to sell
– Example: health & safety standards UL listing for electrical appliances, fire codes
– 1865: International Telegraph Union (ITU)
– 1956: International Telephone and Telegraph Consultative Committee (CCITT)
–
–
–
–
–
ITU for lower layers, multimedia collaboration
IEEE for LAN standards (802.x)
IETF for network, transport & some applications
W3C for web-related technology (XML, SOAP)
ISO for media content (MPEG)
Who makes the rules? - ITU
• ITU = ITU-T (telecom standardization) + ITU-R
(radio) + development
– http://www.itu.int
– 14 study groups
– produce Recommendations:
• E: overall network operation, telephone service (E.164)
• G: transmission system and media, digital systems and
networks (G.711)
• H: audiovisual and multimedia systems (H.323)
• I: integrated services digital network (I.210); includes ATM
• V: data communications over the telephone network (V.24)
• X: Data networks and open system communications
• Y: Global information infrastructure and internet protocol
aspects
ITU
• Initially, national delegations
• Members: state, sector, associate
– Membership fees (> 10,500 SFr)
• Now, mostly industry groups doing work
• Initially, mostly (international) telephone services
• Now, transition from circuit-switched to packetswitched universe & lower network layers (optical)
• Documents cost SFr, but can get three freebies for
each email address
IETF
• IETF (Internet Engineering Task Force)
– see RFC 3233 (“Defining the IETF”)
• Formed 1986, but earlier predecessor organizations (1979-)
• RFCs date back to 1969
• Initially, largely research organizations and universities, now mostly
R&D labs of equipment vendors and ISPs
• International, but 2/3 United States
– meetings every four months
– about 300 companies participating in meetings
• but Cisco, Ericsson, Lucent, Nokia, etc. send large delegations
IETF
•
•
Supposed to be engineering, i.e., translation of well-understood technology
 standards
– make choices, ensure interoperability
– reality: often not so well defined
Most development work gets done in working groups (WGs)
–
–
–
–
–
specific task, then dissolved (but may last 10 years…)
typically, small clusters of authors, with large peanut gallery
open mailing list discussion for specific problems
interim meetings (1-2 days) and IETF meetings (few hours)
published as Internet Drafts (I-Ds)
•
•
•
•
anybody can publish draft-somebody-my-new-protocol
also official working group documents (draft-ietf-wg-*)
versioned (e.g., draft-ietf-avt-rtp-10.txt)
automatically disappear (expire) after 6 months
IETF process
• WG develops  WG last call  IETF last call 
approval (or not) by IESG  publication as RFC
• IESG (Internet Engineering Steering Group)
consists of area directors – they vote on proposals
– areas = applications, general, Internet, operations and
management, routing, security, sub-IP, transport
• Also, Internet Architecture Board (IAB)
– provides architectural guidance
– approves new working groups
– process appeals
IETF activities
• general (3): ipr, nomcom, problem
• applications (25): crisp, geopriv, impp, ldapbis, lemonade, opes,
provreg, simple, tn3270e, usefor, vpim, webdav, xmpp
• internet (18) = IPv4, IPv6, DNS, DHCP: dhc, dnsext, ipoib, itrace,
mip4, nemo, pana, zeroconf
• oam (22) = SNMP, RADIUS, DIAMETER: aaa, v6ops, netconf, …
• routing (13): forces, ospf, ssm, udlr, …
• security (18): idwg, ipsec, openpgp, sasl, smime, syslog, tls,
xmldsig, …
• subip (5) = “layer 2.5”: ccamp, ipo, mpls, tewg
• transport (26): avt (RTP), dccp, enum, ieprep, iptel, megaco,
mmusic (RTSP), nsis, rohc, sip, sipping (SIP), spirits, tsvwg
RFCs
•
•
•
•
•
Originally, “Request for Comment”
now, mostly standards documents that are well settled
published RFCs never change
always ASCII (plain text), sometimes PostScript
anybody can submit RFC, but may be delayed by review
(“end run avoidance”)
• see April 1 RFCs (RFC 1149, 3251, 3252)
• accessible at http://www.ietf.org/rfc/ and http://www.rfceditor.org/
IETF process issues
• Can take several years to publish a standard
– see draft-ietf-problem-issue-statement
• Relies on authors and editors to keep moving
– often, busy people with “day jobs”  spurts three times a year
• Lots of opportunities for small groups to delay things
• Original idea of RFC standards-track progression:
– Proposed Standard (PS) = kind of works
– Draft Standard (DS) = solid, interoperability tested (2 interoperable
implementations for each feature), but not necessarily widely used
– Standard (S) = well tested, widely deployed
IETF process issues
• Reality: very few protocols progress beyond PS
– and some widely-used protocols are only I-Ds
• In addition: Informational, Best Current Practice
(BCP), Experimental, Historic
• Early IETF: simple protocols, stand-alone
– TCP, HTTP, DNS, BGP, …
• Now: systems of protocols, with security,
management, configuration and scaling
– lots of dependencies  wait for others to do their job
Other Internet standards
organizations
• ISOC (Internet Society)
– legal umbrella for IETF, development work
• IANA (Internet Assigned Numbers Authority)
– assigns protocol constants
• NANOG (North American Network Operators Group)
(http://www.nanog.org)
– operational issues
– holds nice workshop with measurement and “real world” papers
• RIPE, ARIN, APNIC
– regional IP address registries  dole out chunks of address
space to ISPs
– routing table management
ICANN
• Internet Corporation for Assigned Names and
Numbers
– manages IP address space (at top level)
– DNS top-level domains (TLD)
•
•
•
•
ccTLD (country codes): .us, .uk, .kr, …
gTLDs (generic): .com, .edu, .gov, .int, .mil, .net, and .org
uTLD (unsponsored): .biz, .info, .name, and .pro
sTLD (sponsored): .aero, .coop, and .museum
• actual domains handled by registrars