Part I: Core networking concepts

Naming & Addressing

Names and addresses

• Names are identifiers– Used by

• end users / applications to interact with your system• system components to interact with each other

– Name operators• compare, resolve, bind/un-bind

• Addresses: names that locate objects• Good names should be decoupled from addresses

Names or addresses?

• NYU ID• /home/jinyang/doc/lec2.ppt• www.nytimes.com• 199.239.137.245• http://www.nytimes.com/world• 00:18:8B:06:DC:CB• BitTorrent: f22bd0823..c86a5

Addresses

Design considerations

• Addresses are used by routers to forward packets to an endpoint

• Should be uniquely allocated

• Don’t have to be user-friendly

• Should enable scalable routing

IP address evolution

• Original scheme: – 8-bit net (area) / 24-bit host (intra-area)

• Why distinguishing net and host?

• Why’s wrong with 8-bit net?– 256 is not enough nets– Most networks don’t have 16 million hosts

Class-based IP address

MIT 18.*.*.* Apple 17.*.*.*

NYU 128.122.*.* Microsoft 207.46.*.*

Forwarding based on class-based address

1. Examine first 1/2/3 bits,

2. Perform a lookup according to net #

Class-based --> CIDR

• Why not class-based addresses?– Class A is wasteful!– Too many organizations are > C, but < B– Too many entries at routers

• CIDR: classless inter-domain routing– Represent net size explicitly 216.239.32.0/19 61.135.0.0/16– Allocate appropriate size– Allocate hierarchically

Hierarchical allocation

Sprint At&t

ISPAnother ISP

12.0.0.0/8

12.4.0.0/16

12.4.240.0/20

Forwarding w/ CIDR addresses

• Longest prefix match– 12.4.225.69 matches 12.24.225.0/20 instead

of 12.0.0.0/8

• Non-trivial– 10-100 millions pkts/sec – Memory latency 5-10 ns

Still not enough IP address?

• NAT (Network address Translator)

• Maps external address/port pairs to internal address/port pairs– Rewrites src/dst addresses!

• NAT breaks– global reachability– Protocols that identify host w/ IP addresses

IPv6

• 128-bit addresses– Different classes of addresses– Hierarchically allocated addresses like CIDR– Lower 64-bits are interface ID

• Simplified header format– 40 bytes as opposed to 20 in IPv4

IPv6 deployment options

• Embed v4 addresses in low bits of IPv6

• Tunnel IPv6 packets over IPv4 networks

• Applications must be dual-stacked or use a v4-to-v6 translator

IPv6 deployment status

Names

Design Considerations

• Ensuring uniqueness1. Central naming authority2. Hierarchical delegation3. Pseudo-randomly4. Content hashes

• Intended audience: humans or machines?

DNS

• Why domain names? – IP addresses are not user friendly– Need topology-independent names

• Early 80s: hosts.txt file, maps host name IP• DNS: distributed service, maps domain name IP

– Record types: A, NS, MX, CNAME, PTR …

Deep hierarchy

Hierarchical names enable delegation

.com .edu .gov .cn .uk

.nyu

.cs

.news

.

flat

www

Resolving hierarchical names

Stub resolver

applicationcs.nyu.edu

DNSserver

rootname server

.comname server

.google.comname server

• Root servers might become bottlenecks?• Long latency?

Query: www.google.comResponse: .com NS a.gtld-servers.nett

Q: www.google.comR: google.com NS ns1.google.com

Q: www.google.comR: www.google.com A 216.239.32.10

Replicating servers for capacity/availability

• Each sub-tree (zone) is kept at 2 name servers• 13 root servers

– [A-M].root-servers.net– Geographically diverse: VA, CA, MD, Japan etc.

• Another 13 name servers for .com, .net

Caching

Stub resolver

cs.nyu.eduDNS

server

rootname server

comname server

googlename server

Query: www.google.comResponse: .com NS a.gtld-servers.nett

Q: www.google.comR: .google.com NS ns1.google.com

Q: www.google.comR: www.google.com A 216.239.32.10

.com NS

.google.com NS www.google.com A

• All record types are cached according to TTL• Caching NS records is effective at reducing latency

Stub resolver

Stub resolver

Caching, continued

• Cache negative response– 10-42% lookups result in a neg answer – Most neg answers are for reverse IP lookups

• Setting low TTL for A records harmful?– Not really [Jung et. al. 2002]– Most DNS cache hits happen in short succession

• Sharing DNS caches at multiple sites useful?– Not really– Names follow zipf distribution, misses are for rare names

“Innovative uses” of DNSload balancing/server selection

• DNS server returns different A records to different clients at different times

• Short TTL: e.g. 60 sec for Akamai

“Innovative” uses of DNSspam blacklisting

• Is 125.191.168.35 a spam source?

• Resolve name 35.168.191.125.bl.spamcop.net

Problems with current naming/addressing

A layered naming architecture

“Almost every problem in computer science can be solved by another level of indirection”

-- David Wheeler 70s

LNA Proposal overviewUser level descriptor (ULD)

e.g. email, search string

SID

EID

IP

Youtube -> (SID_a5f4)

SID_a5f4 -> (EID_365a, TCP, port 80)

EID_365a -> IP_12.4.224.3

Claimed Advantage #1: Host mobility

• Authors’ claim– TCP breaks if hosts

change IPs– Difficult to initiate

connection to mobile host

• How LNA solves it?

• Devil’s advocate

Claimed Advantage #2: Service/data migration/replication• Authors’ claim

– URL-based links break if domain name changes

– No name for replicated data

• How LNA solves it?

• Devil’s advocate

Claimed Advantage #3: Accommodating middle boxes

• Authors’ claim– No explicit support

for network-level middle boxes

– No explicit support for application-level middle boxes

• Devil’s advocate