15-440

Inter-Domain Routing

BGP (Border Gateway Protocol)DNS (Domain Name System)

These slides proudly ripped from Srini Seshan and Dave Anderson and Seth Goldstein, 15-441 F’06 and F’08

Outline

• Internet Structure/Routing Hierarchy

• External BGP (E-BGP)

• Internal BGP (I-BGP)

A Logical View of the Internet?

R

R

R

R R

• After looking at RIP/OSPF descriptions• End-hosts connected to

routers• Routers exchange

messages to determine connectivity

• NOT TRUE!

Internet’s Area Hierarchy

• What is an Autonomous System (AS)?• A set of routers under a single technical administration,

using an interior gateway protocol (IGP) and common metrics to route packets within the AS and using an exterior gateway protocol (EGP) to route packets to other AS’s

• Each AS assigned unique ID• AS’s peer at network exchanges

AS Numbers (ASNs)

ASNs are 16 bit values 64512 through 65535 are “private”

• Genuity: 1 • MIT: 3• CMU: 9• UC San Diego: 7377• AT&T: 7018, 6341, 5074, … • UUNET: 701, 702, 284, 12199, …• Sprint: 1239, 1240, 6211, 6242, …• …

ASNs represent units of routing policy

Currently over 15,000 in use

Example

1 2

3

1.11.2

2.1 2.2

3.1 3.2

2.2.1

44.1 4.2

5

5.1 5.2

EGP

IGP

EGPEGP

IGP

IGP

IGPIGP

EGP

EGP

A Logical View of the Internet?

R

R

R

R R

• RIP/OSPF not very scalable area hierarchies

• NOT TRUE EITHER!• ISP’s aren’t equal

• Size• Connectivity

ISP ISP

A Logical View of the Internet

Tier 1 Tier 1

Tier 2

Tier 2

Tier 2

Tier 3

• Tier 1 ISP• “Default-free” with global

reachability info

• Tier 2 ISP• Regional or country-wide

• Tier 3 ISP• Local

Customer

Provider

Transit vs. Peering

ISP X

ISP Y

ISP Z

ISP P

Transit ($$)

Transit ($$$)

Transit ($$ 1/2)

Transit ($$)

Peering

Transit ($$$)

Transit ($)

Transit ($$)

Transit ($$$)

Policy Impact

• “Valley-free” routing• Number links as (+1, 0, -1) for provider, peer and

customer• In any path should only see sequence of +1, followed

by at most one 0, followed by sequence of -1

• WHY?• Consider the economics of the situation

Outline

• Internet Structure/Routing Hierarchy

• External BGP (E-BGP)

• Internal BGP (I-BGP)

Choices

• Link state or distance vector?• No universal metric – policy decisions

• Problems with distance-vector:• Bellman-Ford algorithm may not converge

• Problems with link state:• Metric used by routers not the same – loops• LS database too large – entire Internet• May expose policies to other AS’s

Solution: Distance Vector with Path

• Each routing update carries the entire path• Loops are detected as follows:

• When AS gets route, check if AS already in path• If yes, reject route• If no, add self and (possibly) advertise route further

• Advantage:• Metrics are local - AS chooses path, protocol ensures

no loops

Interconnecting BGP Peers

• BGP uses TCP to connect peers• Advantages:

• Simplifies BGP• No need for periodic refresh - routes are valid until

withdrawn, or the connection is lost• Incremental updates

• Disadvantages• Congestion control on a routing protocol?• Poor interaction during high load

Hop-by-hop Model

• BGP advertises to neighbors only those routes that it uses• Consistent with the hop-by-hop Internet paradigm• e.g., AS1 cannot tell AS2 to route to other AS’s in a

manner different than what AS2 has chosen (need source routing for that)

• BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS’s

Examples of BGP Policies

• A multi-homed AS refuses to act as transit• Limit path advertisement

• A multi-homed AS can become transit for some AS’s• Only advertise paths to some AS’s

• An AS can favor or disfavor certain AS’s for traffic transit from itself

BGP Messages

• Open• Announces AS ID• Determines hold timer – interval between keep_alive or

update messages, zero interval implies no keep_alive

• Keep_alive• Sent periodically (but before hold timer expires) to

peers to ensure connectivity.• Sent in place of an UPDATE message

• Notification• Used for error notification• TCP connection is closed immediately after notification

BGP UPDATE Message

• List of withdrawn routes• Network layer reachability information

• List of reachable prefixes

• Path attributes• Origin• Path• Metrics

• All prefixes advertised in message have same path attributes

Path Selection Criteria

• Attributes + external (policy) information• Examples:

• Hop count• Policy considerations

• Preference for AS• Presence or absence of certain AS

• Path origin• Link dynamics

LOCAL PREF

• Local (within an AS) mechanism to provide relative priority among BGP routers (e.g. R3 over R4)

R1 R2

R3 R4I-BGP

AS 256

AS 300

Local Pref = 500 Local Pref = 800

AS 100

R5

AS 200

LOCAL PREF – Common Uses

• Peering vs. transit• Prefer to use peering connection, why?

• In general, customer > peer > provider• Use LOCAL PREF to ensure this

AS_PATH

• List of traversed AS’s

AS 500

AS 300

AS 200 AS 100

180.10.0.0/16 300 200 100170.10.0.0/16 300 200

170.10.0.0/16 180.10.0.0/16

Multi-Exit Discriminator (MED)

• Hint to external neighbors about the preferred path into an AS • Non-transitive attribute

• Different AS choose different scales

• Used when two AS’s connect to each other in more than one place

MED

• Hint to R1 to use R3 over R4 link• Cannot compare AS40’s values to AS30’s

R1 R2

R3 R4

AS 30

AS 40

180.10.0.0MED = 120

180.10.0.0MED = 200

AS 10

180.10.0.0MED = 50

MED

• MED is typically used in provider/subscriber scenarios• It can lead to unfairness if used between ISP because it

may force one ISP to carry more traffic:

SF

NY

• ISP1 ignores MED from ISP2• ISP2 obeys MED from ISP1• ISP2 ends up carrying traffic most of the way

ISP1

ISP2

Decision Process

• Processing order of attributes:• Select route with highest LOCAL-PREF• Select route with shortest AS-PATH• Apply MED (if routes learned from same neighbor)

Important Concepts

• Wide area Internet structure and routing driven by economic considerations• Customer, providers and peers

• BGP designed to:• Provide hierarchy that allows scalability• Allow enforcement of policies related to structure

• Mechanisms• Path vector – scalable, hides structure from neighbors,

detects loops quickly

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What is DNS?

• DNS (Domain Name Service) is primarily used to translate human readable names into machine usable addresses, e.g., IP addresses.

• DNS goal:• Efficiently locate resources.

E.g., Map name IP address• Scale to many users over a large area• Scale to many updates

Lecture 13

How resolve name IP addr?

Lecture 13 15-441 © 2008 37

15-441 © 2008 38

Obvious Solutions (1)

Why not centralize DNS?• Single point of failure• Traffic volume• Distant centralized database• Single point of update

• Doesn’t scale!

Lecture 13

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Obvious Solutions (2)

Why not use /etc/hosts?• Original Name to Address Mapping

• Flat namespace• /etc/hosts • SRI kept main copy• Downloaded regularly

• Mid 80’s this became untenable. Why?• Count of hosts was increasing: machine per

domain machine per user• Many more downloads• Many more updates /etc/hosts still exists.

Lecture 13

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Domain Name System Goals• Basically a wide-area distributed database

(The biggest in the world!)• Scalability• Decentralized maintenance• Robustness• Global scope

• Names mean the same thing everywhere

• Don’t need all of ACID• Atomicity• Strong consistency

• Do need: distributed update/query & Performance

Lecture 13

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Programmer’s View of DNS

• Conceptually, programmers can view the DNS database as a collection of millions of host entry structures:

• in_addr is a struct consisting of 4-byte IP addr

• Functions for retrieving host entries from DNS:•gethostbyname: query key is a DNS host name.•gethostbyaddr: query key is an IP address.

/* DNS host entry structure */ struct hostent { char *h_name; /* official domain name of host */ char **h_aliases; /* null-terminated array of domain names */ int h_addrtype; /* host address type (AF_INET) */ int h_length; /* length of an address, in bytes */ char **h_addr_list; /* null-termed array of in_addr structs */ };

Lecture 13

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DNS Message Format

Identification

No. of Questions

No. of Authority RRs

Questions (variable number of answers)

Answers (variable number of resource records)

Authority (variable number of resource records)

Additional Info (variable number of resource records)

Flags

No. of Answer RRs

No. of Additional RRs

Name, type fields for a query

RRs in response to query

Records for authoritative servers

Additional “helpful info that may be used

12 bytes

Lecture 13

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DNS Header Fields

• Identification• Used to match up request/response

• Flags• 1-bit to mark query or response• 1-bit to mark authoritative or not• 1-bit to request recursive resolution• 1-bit to indicate support for recursive resolution

Lecture 13

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DNS RecordsRR format: (class, name, value, type, ttl)

• DB contains tuples called resource records (RRs)• Classes = Internet (IN), Chaosnet (CH), etc.• Each class defines value associated with type

For “IN” class:

• Type=A• name is hostname• value is IP address

• Type=NS• name is domain (e.g. foo.com)• value is name of authoritative name

server for this domain

• Type=CNAME• name is an alias name for

some “canonical” name• value is canonical name

• Type=MX• value is hostname of

mailserver associated with name

Lecture 13

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Properties of DNS Host Entries

Different kinds of mappings are possible:

• 1-1 mapping between domain name and IP addr:provolone.crcl.cs.cmu.edu maps to 128.2.218.81

• Multiple domain names maps to the same IP addr:www.scs.cmu.edu and www.cs.cmu.edu both

map to 128.2.203.164

• Single domain name maps to multiple IP addresses:aol.com and www.aol.com map to multiple IP addrs.

• Some valid domain names don’t map to any IP addr:crcl.cs.cmu.edu doesn’t have a host

Lecture 13

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DNS Design: Hierarchy Definitions

root

edunetorg ukcom

gwu ucb cmu bu mit

cs ece

crcl

• Each node in hierarchy stores a list of names that end with same suffix

• Suffix = path up tree• E.g., given this tree, where

would following be stored:• Fred.com• Fred.edu• Fred.cmu.edu• Fred.crcl.cs.cmu.edu• Fred.cs.mit.edu

Lecture 13

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DNS Design: Zone Definitions

Single node

Subtree

Complete Tree

• Zone = contiguous section of name space

• E.g., Complete tree, single node or subtree

• A zone has an associated set of name servers

• Must store list of names and tree links

root

edunetorg ukcom

gwu ucb cmu bu mit

cs ece

crcl

Lecture 13

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DNS Design: Cont.• Zones are created by convincing owner node to

create/delegate a subzone• Records within zone stored in multiple redundant name

servers• Primary/master name server updated manually• Secondary/redundant servers updated by zone transfer of

name space• Zone transfer is a bulk transfer of the “configuration” of a

DNS server – uses TCP to ensure reliability

• Example:• CS.CMU.EDU created by CMU.EDU admins• Who creates CMU.EDU or .EDU?

Lecture 13

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DNS: Root Name Servers

• Responsible for “root” zone• 13 root name servers

• Currently{a-m}.root-servers.net

• Local name servers contact root servers when they cannot resolve a name

• Why 13?

Lecture 13

Not really 13!

Lecture 13 15-441 © 2008 50

10/08, from www.root-servers.org Check out anycast)

So Far• Database structure

• Hierarchy of labels x.y.z• Organized into zones• Zones have nameservers (notice plural!)

• Database layout• Records which map

namesnames, namesip,etc.

• Programmer API: gethostbyname, …

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Servers/Resolvers

• Each host has a resolver• Typically a library that applications can link to• Local name servers hand-configured (or DHCP)

(e.g. /etc/resolv.conf)

• Name servers• Either responsible for some zone or…• Local servers• Do lookup of distant host names for local hosts• Typically answer queries about local zone

Lecture 13

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Typical Resolution

ClientLocal

DNS server

root & edu DNS server

ns1.cmu.edu DNS server

www.cs.cmu.edu

NS ns1.cmu.eduwww.cs.cmu.edu

NS ns1.cs.cmu.eduA www=IPaddr

ns1.cs.cmu.eduDNS

server

Hmm: Notice root server returned NS ns1.cmu.edu

Lecture 13

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Typical Resolution

• Steps for resolving www.cmu.edu• Application calls gethostbyname() (RESOLVER)

• Resolver contacts local name server (S1)

• S1 queries root server (S2) for (www.cmu.edu)

• S2 returns NS record for cmu.edu (S3)

• What about A record for S3?

• This is what the additional info section is for (PREFETCHING)

• S1 queries S3 for www.cmu.edu

• S3 returns A record for www.cmu.edu

• Can return multiple A records What does this mean?

Lecture 13

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Lookup MethodsRecursive query:• Server goes out and searches

for more info• Only returns final answer

or “not found”

Iterative query:• Server responds with

as much as it knows.• “I don’t know this

name, but ask this server”

Workload impact on choice?• Root/distant server does iterative• Local server typically does recursive

requesting host

surf.eurecom.frgaia.cs.umass.ed

u

root name server

local name server

dns.eurecom.fr1

2

34

5 6authoritative name server

dns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

iterated query

Lecture 13

How to manage workload?

• Does root nameserver do recursive lookups?• What about other zones?• What about imbalance in popularity?

• .com versus .dj• google.com versus bleu.crcl.cs.cmu.edu?

• How do we scale query workload?

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Workload and Caching

• DNS responses are cached • Quick response for repeated translations• Other queries may reuse some parts of lookup

• E.g., NS records for domains

• DNS negative queries are cached• Don’t have to repeat past mistakes• E.g., misspellings, search strings in resolv.conf

• How do you handle updates?

Lecture 13

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Workload and Caching

• DNS responses are cached • Quick response for repeated translations• Other queries may reuse some parts of lookup

• E.g., NS records for domains

• DNS negative queries are cached• Don’t have to repeat past mistakes• E.g., misspellings, search strings in resolv.conf

• Cached data periodically times out• Lifetime (TTL) of data controlled by owner of data• TTL passed with every record

Lecture 13

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Typical Resolution

ClientLocal

DNS server

root & edu DNS server

ns1.cmu.edu DNS server

www.cs.cmu.edu

NS ns1.cmu.eduwww.cs.cmu.edu

NS ns1.cs.cmu.eduA www=IPaddr

ns1.cs.cmu.eduDNS

server

Lecture 13

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Subsequent Lookup Example

ClientLocal

DNS server

root & edu DNS server

cmu.edu DNS server

cs.cmu.eduDNS

server

ftp.cs.cmu.edu

A ftp=IPaddr

ftp.cs.cmu.edu

Lecture 13

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Reliability

• DNS servers are replicated• Name service available if ≥ one replica is up• Queries can be load balanced between replicas

• UDP used for queries• Need reliability must implement this on top of UDP!• Why not just use TCP?

• Try alternate servers on timeout• Exponential backoff when retrying same server

• Same identifier for all queries• Don’t care which server responds

Lecture 13

So far

• Hierarchial name space

Lecture 13 15-441 © 2008 62

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Reverse DNS

• Task• Given IP address, find its

name

• Method• Maintain separate hierarchy

based on IP names• Write 128.2.204.27 as

27.204.2.128.in-addr.arpa• Why is the address

reversed?

• Managing• Authority manages IP

addresses assigned to it• E.g., CMU manages name

space 2.128.in-addr.arpa

edu

cmu

cs

bleu128.2.204.27

crcl

unnamed root

arpa

in-addr

128

2

204

27

Arpa: backronym Address and Routing Parameter Area

Lecture 13

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.arpa Name Server Hierarchy

• At each level of hierarchy, have group of servers that are authorized to handle that region of hierarchy

128

2

204

bleu

128.2.204.27

in-addr.arpaa.root-servers.net • • • m.root-servers.net

chia.arin.net(dill, henna, indigo, epazote, figwort, ginseng)

cucumber.srv.cs.cmu.edu,t-ns1.net.cmu.edut-ns2.net.cmu.edu

mango.srv.cs.cmu.edu(peach, banana, blueberry)

Lecture 13

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Prefetching• Name servers can add additional data to response• Why would they?

Lecture 13

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Prefetching• Name servers can add additional data to response• Why would they?• Typically used for prefetching

• CNAME/MX/NS typically point to another host name• Responses include address of host referred to in

“additional section”

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Mail Addresses

• MX records point to mail exchanger for a name• E.g. cmu.edu. 2590 IN MX 10 CMU-MX4.ANDREW.cmu.edu.

cmu.edu. 2590 IN MX 10 CMU-MX5.ANDREW.cmu.edu.

• Addition of MX record type proved to be a challenge• How to get mail programs to lookup MX record for mail

delivery?• Needed critical mass of such mailers• Could we add a new one now?

Lecture 13

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Outline

• DNS Design

• DNS Today

Lecture 13

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Root Zone• Generic Top Level Domains (gTLD)

= .com, .net, .org, etc…• Country Code Top Level Domain (ccTLD)

= .us, .ca, .fi, .uk, etc…• Root server ({a-m}.root-servers.net) also used to

cover gTLD domains• Load on root servers was growing quickly!• Moving .com, .net, .org off root servers was clearly

necessary to reduce load done Aug 2000

• How significant an effect would this have?• On load?• On performance?

Lecture 13

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gTLDs• Unsponsored

• .com, .edu, .gov, .mil, .net, .org• .biz businesses• .info general info• .name individuals

• Sponsored (controlled by a particular association)• .aero air-transport industry• .cat catalan related• .coop business cooperatives• .jobs job announcements• .museum museums• .pro accountants, lawyers, and physicians• .travel travel industry

• Starting up• .mobi mobile phone targeted domains• .post postal • .tel telephone related

• Proposed• .asia, .cym, .geo, .kid, .mail, .sco, .web, .xxx• Whatever you want!

Is there anything special about .com?What about adding .goldstein as a gTLD?

Lecture 13

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New Registrars

• Network Solutions (NSI) used to handle all registrations, root servers, etc…• Clearly not the democratic (Internet) way• Large number of registrars that can create new

domains However NSI still handles A root server

Lecture 13

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Measurements of DNS• No centralized caching per site

• Each machine runs own caching local server• Why is this a problem?• How many hosts do we need to share cache? recent

studies suggest 10-20 hosts

• “Hit rate for DNS:1 - (#DNS/#connections)80%• Is this good or bad?• Most Internet traffic was Web with HTTP 1.0

• What does a typical page look like? average of 4-5 imbedded objects needs 4-5 transfers

• This alone accounts for 80% hit rate!

• Lower TTLs for A records does not affect performance

• DNS performance really relies more on NS-record caching

Lecture 13

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Measurements of DNS• No centralized caching per site

• Each machine runs own caching local server• Why is this a problem?• How many hosts do we need to share cache? recent

studies suggest 10-20 hosts

• “Hit rate for DNS:1 - (#DNS/#connections)80%• Is this good or bad?• Most Internet traffic was Web with HTTP 1.0

• What does a typical page look like? average of 4-5 imbedded objects needs 4-5 transfers

• This alone accounts for 80% hit rate!

• Lower TTLs for A records does not affect performance

• DNS performance really relies more on NS-record caching

Lecture 13

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Tracing Hierarchy (1)• Dig Program

• Allows querying of DNS system• Use flags to find name server (NS)• Disable recursion so that operates one step at a time

• All .edu names handled by set of servers

unix> dig +norecurse @a.root-servers.net NS kittyhawk.cmcl.cs.cmu.edu

;; AUTHORITY SECTION:edu. 172800 IN NS L3.NSTLD.COM.edu. 172800 IN NS D3.NSTLD.COM.edu. 172800 IN NS A3.NSTLD.COM.edu. 172800 IN NS E3.NSTLD.COM.edu. 172800 IN NS C3.NSTLD.COM.edu. 172800 IN NS F3.NSTLD.COM.edu. 172800 IN NS G3.NSTLD.COM.edu. 172800 IN NS B3.NSTLD.COM.edu. 172800 IN NS M3.NSTLD.COM.

ZoneTTL

Class

Type

Value

Lecture 13

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Tracing Hierarchy (2)

• 3 servers handle CMU names

unix> dig +norecurse @e3.nstld.com NS kittyhawk.cmcl.cs.cmu.edu

;; AUTHORITY SECTION:cmu.edu. 172800 IN NS CUCUMBER.SRV.cs.cmu.edu.cmu.edu. 172800 IN NS T-NS1.NET.cmu.edu.cmu.edu. 172800 IN NS T-NS2.NET.cmu.edu.

Lecture 13

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Tracing Hierarchy (3 & 4)

• 4 servers handle CMU CS names

• Quasar is master NS for this zone

unix> dig +norecurse @t-ns1.net.cmu.edu NS kittyhawk.cmcl.cs.cmu.edu

;; AUTHORITY SECTION:cs.cmu.edu. 86400 IN NS MANGO.SRV.cs.cmu.edu.cs.cmu.edu. 86400 IN NS PEACH.SRV.cs.cmu.edu.cs.cmu.edu. 86400 IN NS BANANA.SRV.cs.cmu.edu.cs.cmu.edu. 86400 IN NS BLUEBERRY.SRV.cs.cmu.edu.

unix>dig +norecurse @blueberry.srv.cs.cmu.edu NS kittyhawk.cmcl.cs.cmu.edu

;; AUTHORITY SECTION:cs.cmu.edu. 300 IN SOA QUASAR.FAC.cs.cmu.edu.

Lecture 13

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DNS (Summary)• Motivations large distributed database

• Scalability• Independent update• Robustness

• Hierarchical database structure• Zones• How lookups are done

• Caching/prefetching and TTLs• Reverse name lookup• What are the steps to creating your own domain?

Lecture 13