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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 - PowerPoint PPT Presentation
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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