IP revisited Taekyoung Kwon tkkwon@snu.ac.kr Courtesy of Kevin Lai and Ion Stoica with Berkeley Jim...

IP revisited

Taekyoung Kwontkkwon@snu.ac.kr

Courtesy of Kevin Lai and Ion Stoica with Berkeley

Jim Kurose with UmassHenning 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,

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 linkBandwidth division into

“pieces”Dedicated allocationResource reservation

Network Core: Packet Switching

Packet-switching versus circuit switching: human restaurant analogy

• other human analogies?

A

B

C10 MbsEthernet

1.5 Mbs

45 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link

Packet switching versus circuit switching

• 1 Mbit link• each user:

– 100Kbps when “active”– active 10% of time

• circuit-switching: – 10 users

• packet switching: – with 35 users,

probability > 10 active less than .0004

Packet switching allows more users to use network!

N users

1 Mbps link

What is statistical multiplexing gain?

Packet switching versus circuit switching

• 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

Is packet switching a “slam dunk winner?”

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– time waiting at output

link for transmission – depends on congestion

level of routerA

B

propagation

transmission

nodalprocessing 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

Propagation delay:• d = length of physical link• s = propagation speed in

medium (~2x108 m/sec)• propagation delay = d/s

A

B

propagation

transmission

nodalprocessing queueing

Note: s and R are very different quantities!

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 provi

ders (NBPs) [tier 1]– e.g. BBN/GTE, Sprint, AT&T, IBM, UU

Net– interconnect (peer) with each other pr

ivately, or at public Network Access Point (NAPs)

• regional ISPs [tier 2]– 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

NBP A

NBP B

NAP NAP

regional ISP

regional ISP

localISP

localISP

National Backbone Providere.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?

• No layering: each new application has to be re-implemented for every network technology!

Telnet FTP NFS

Packetradio

Coaxial cable

Fiberoptic

Application

TransmissionMedia

HTTP

Why Layering?

• Solution: introduce an intermediate layer that provides a unique abstraction for various network technologies

Telnet FTP NFS

Packetradio

Coaxial cable

Fiberoptic

Application

TransmissionMedia

HTTP

Intermediate layer

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

• Solution 1: make each step reliable, and then concatenate them

• Solution 2: end-to-end check and retry

OS

Appl.

OS

Appl.

Host A Host B

OK

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 perf

ormances 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)

End-to-end argumentPerformance

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: cycles, storage, bandwidth expensive, people

cheap– 2002: resources cheap, people expensive– 1974: Share computer resources– 2002: Communicate with people, access documents, buy,

sell

• 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 services3. Must accommodate a variety of networks4. Allow distributed management5. Allow host attachment with a low level of effort6. Be cost effective7. 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 (fate-sharing)– 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, c

able modems, power lines

Internet Architecture

• Packet-switched datagram network

• IP is the glue • Hourglass architecture

– all hosts and routers run IP

• Common Intermediate Representation

IP

TCP UDP

ATM

Satellite

Ethernet

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 in

telligence 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 mai

n 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 (ATM, ARCnet, Ethernet, FDDI, modems, …)

dominance of Ethernet, but also L2’s not designed for 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”

email WWW phone...

SMTP HTTP RTP...

TCP UDP…

IP

ethernet PPP…

CSMA async sonet...

copper fiber radio...

The Internet Protocol Hourglass(Deering)

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

email WWW phone...

SMTP HTTP RTP...

TCP UDP…

IP + mcast

+ QoS +...

ethernet PPP…

CSMA async sonet...

copper fiber radio...

Putting on Weight

• requires more functionality from underlying networks

email WWW phone...

SMTP HTTP RTP...

TCP UDP…

IP4 IP6

ethernet PPP…

CSMA async sonet...

copper fiber radio...

Mid-Life Crisis

• doubles number of service interfaces

• requires changes above & below

• major interoper-ability 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– Allowed to use vs. likely to sell– Example: health & safety standards UL listing for electrical appliances, fire codes

• Telecommunications and networking always focus of standardization– 1865: International Telegraph Union (ITU)– 1956: International Telephone and Telegraph Consultative Committee (CCITT)

• Five major organizations:– 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 packet-

switched 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, sim

ple, 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 avoi

dance”)• see April 1 RFCs (RFC 1149, 3251, 3252)• accessible at http://www.ietf.org/rfc/ and http://www.rfc-editor.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 implementation

s 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