Application Layer (Email, DNS, P2P, Socket Programming)
Computer Networks and Applications
Week 3
COMP 3331/COMP 9331
Reading Guide: Chapter 2, Sections 2.4 – 2.7
Announcements
v Lab for Week 3 – Socket Programming Practice v Sample Problem Set
§ Did anyone attempt the first problem set? Please do. Solutions soon.
§ Set for application layer is now available v Remember mid-semester exam in Week 6 v First Programming Assignment released this
week
2
2. Application Layer: outline
2.1 principles of network applications § app architectures § app requirements
2.2 Web and HTTP 2.3 FTP 2.4 electronic mail
§ SMTP, POP3, IMAP 2.5 DNS
2.6 P2P applications 2.7 socket programming
with UDP and TCP
3
4 http://arstechnica.com/business/2016/03/e-mail-inventor-ray-tomlinson-who-popularized-symbol-dies-at-74/
Electronic mail Three major components: v user agents v mail servers v simple mail transfer
protocol: SMTP
User Agent v a.k.a. “mail reader” v composing, editing, reading
mail messages v e.g., Outlook, Thunderbird,
iPhone mail client v outgoing, incoming
messages stored on server
user mailbox
outgoing message queue
mail server
mail server
mail server
SMTP
SMTP
SMTP
user agent
user agent
user agent
user agent
user agent
user agent
5
Electronic mail: mail servers
mail servers: v mailbox contains incoming
messages for user v message queue of outgoing
(to be sent) mail messages v SMTP protocol between mail
servers to send email messages § client: sending mail
server § “server”: receiving mail
server
mail server
mail server
mail server
SMTP
SMTP
SMTP
user agent
user agent
user agent
user agent
user agent
user agent
6
Electronic Mail: SMTP [RFC 2821]
v uses TCP to reliably transfer email message from client to server, port 25
v direct transfer: sending server to receiving server
v three phases of transfer § handshaking (greeting) § transfer of messages § closure
v command/response interaction (like HTTP, FTP) § commands: ASCII text § response: status code and phrase
v messages must be in 7-bit ASCII 7
user agent
Scenario: Alice sends message to Bob
1) Alice uses UA to compose message “to” [email protected]
2) Alice’s UA sends message to her mail server; message placed in message queue
3) client side of SMTP opens TCP connection with Bob’s mail server
4) SMTP client sends Alice’s message over the TCP connection
5) Bob’s mail server places the message in Bob’s mailbox
6) Bob invokes his user agent to read message
mail server
mail server
1
2 3 4 5
6
Alice’s mail server Bob’s mail server
user agent
8
Sample SMTP interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection 9
Try SMTP interaction for yourself:
v telnet servername 25 v see 220 reply from server v enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
commands above lets you send email without using email client (reader) Implication: one could send forged emailed
10
Note: Many SMTP servers will not allow the above interaction without authentication. E.g: try the above with mail.unsw.edu.au @ port 587
How to tell a fake email?
Examine Long Headers or Raw Source
Further reading: http://www.millersmiles.co.uk/identitytheft/spoofemail-060603.htm 11
Securing E-mail
v STARTTLS: upgrade a plain text connection to TLS/SSL instead of using a separate port for encrypted communication § Can be used for SMTP/IMAP/POP
v PGP (later in the course)
12
SMTP: final words
v SMTP uses persistent connections
v SMTP requires message (header & body) to be in 7-bit ASCII
v SMTP server uses CRLF.CRLF to determine end of message
comparison with HTTP: v HTTP: pull v SMTP: push
v both have ASCII command/response interaction, status codes
v HTTP: each object encapsulated in its own response msg
v SMTP: multiple objects sent in multipart msg
13
Mail message format
SMTP: protocol for exchanging email msgs
RFC 822: standard for text message format:
v header lines, e.g., § To: § From: § Subject: different from SMTP MAIL
FROM, RCPT TO: commands!
v Body: the “message” § ASCII characters only
header
body
blank line
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v IF SMTP only allows 7-bit ASCII, how do we send pictures/videos/files via email?
A: We use a different protocol instead of SMTP B: We encode these objects as 7-bit ASCII C: We’re really sending links to the objects, rather than the objects themselves D: We don’t !! You have been lied to !!
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Quiz: E-mail attachments?
Mail access protocols
v SMTP: delivery/storage to receiver’s server v mail access protocol: retrieval from server
§ POP: Post Office Protocol [RFC 1939]: authorization, download
§ IMAP: Internet Mail Access Protocol [RFC 1730]: more features, including manipulation of stored msgs on server
§ HTTP(S): Gmail, Yahoo! Mail, etc.
sender’s mail server
SMTP SMTP mail access
protocol
receiver’s mail server
(e.g., POP, IMAP)
user agent
user agent
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POP3 protocol
authorization phase v client commands:
§ user: declare username § pass: password
v server responses § +OK § -ERR
transaction phase, client: v list: list message numbers v retr: retrieve message by
number v dele: delete v quit
C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off
S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on
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Self Study
POP3 (more) and IMAP more about POP3 v previous example uses
POP3 “download and delete” mode § Bob cannot re-read e-
mail if he changes client
v POP3 “download-and-keep”: copies of messages on different clients
v POP3 is stateless across sessions
IMAP v keeps all messages in one
place: at server v allows user to organize
messages in folders v keeps user state across
sessions: § names of folders and
mappings between message IDs and folder name
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Self Study
v Which of the following is not true?
A. HTTP is pull-based, SMTP is push-based
B. HTTP uses a separate header for each object, SMTP uses a multipart message format
C. SMTP uses persistent connections
D. HTTP uses client-server communication but SMTP does not
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Quiz: HTTP vs SMTP
2. Application Layer: outline
2.1 principles of network applications § app architectures § app requirements
2.2 Web and HTTP 2.3 FTP 2.4 electronic mail
§ SMTP, POP3, IMAP 2.5 DNS
2.6 P2P applications 2.7 socket programming
with UDP and TCP
A nice overview https://webhostinggeeks.com/guides/dns/
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DNS: domain name system
people: many identifiers: § TFN, name, passport #
Internet hosts, routers: § IP address (32 bit) -
used for addressing datagrams
§ “name”, e.g., www.yahoo.com - used by humans
Q: how to map between IP address and name, and vice versa ?
Domain Name System: v distributed database
implemented in hierarchy of many name servers
v application-layer protocol: hosts, name servers communicate to resolve names (address/name translation) § note: core Internet function,
implemented as application-layer protocol
§ complexity at network’s “edge”
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DNS: History v Initially all host-address mappings were in a hosts.txt file (in /etc/
hosts): § Maintained by the Stanford Research Institute (SRI) § Changes were submitted to SRI by email § New versions of hosts.txt periodically FTP’d from SRI § An administrator could pick names at their discretion
v As the Internet grew this system broke down: § SRI couldn’t handle the load; names were not unique; hosts had inaccurate
copies of hosts.txt
v The Domain Name System (DNS) was invented to fix this
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Jon Postel
http://www.wired.com/2012/10/joe-postel/
Example use of DNS
v You type www.cse.unsw.edu.au in the URL window of your web browser
v Your browser must establish a TCP connection with the CSE web server
v To do this, your browser needs to know the IP address of the CSE web server
v How does it obtain the IP address?? § The browser passes on the hostname to the client side of the DNS
application running on your machine (gethostbyname() function in UNIX) § The DNS client sends out a query for mapping the hostname to an IP
address into the DNS hierarchy black box over UDP (destination port number: 53)
§ The DNS client receives a reply with the IP address for www.cse.unsw.edu.au
§ The browser can now initiate a TCP connection with the HTTP server process located at that IP address
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DNS: services, structure why not centralize DNS? v single point of failure v traffic volume v distant centralized database v maintenance
DNS services v hostname to IP address
translation v host aliasing
§ canonical, alias names v mail server aliasing v load distribution
§ replicated Web servers: many IP addresses correspond to one name
§ Content Distribution Networks: use IP address of requesting host to find best suitable server
• Example: closest, least-loaded, etc
A: doesn’t scale!
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Goals
v No naming conflicts (uniqueness) v Scalable
§ many names § (secondary) frequent updates
v Distributed, autonomous administration § Ability to update my own (machines’) names § Don’t have to track everybody’s updates
v Highly available v Lookups should be fast
25
Key idea: Hierarchy
Three intertwined hierarchies § Hierarchical namespace
• As opposed to original flat namespace
§ Hierarchically administered • As opposed to centralised
§ (Distributed) hierarchy of servers • As opposed to centralised storage
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Hierarchical Namespace
v “Top Level Domains” are at the top v Domains are sub-trees
§ E.g: .edu, berkeley.edu, eecs.berkeley.edu v Name is leaf-to-root path
§ instr.eecs.berkeley.edu v Depth of tree is arbitrary (limit 128) v Name collisions trivially avoided
§ each domain is responsible
root
edu com gov mil org net uk fr
berkeley ucla
eecs sims
instr
…
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Hierarchical Administration
root
edu com gov mil org net uk fr
berkeley ucla
eecs sims
instr
root
edu com gov mil org net uk fr
berkeley
eecs sims § A zone corresponds to an administrative authority that
is responsible for that portion of the hierarchy § E.g., UCB controls names: *.berkeley.edu and
*.sims.berkeley.edu
v E.g., EECS controls names: *.eecs.berkeley.edu
Server Hierarchy
v Top of hierarchy: Root servers § Location hardwired into other servers
v Next Level: Top-level domain (TLD) servers
§ .com, .edu, etc. § Managed professionally
v Bottom Level: Authoritative DNS servers § Actually store the name-to-address mapping § Maintained by the corresponding administrative authority
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Server Hierarchy
v Each server stores a (small!) subset of the total DNS database
v An authoritative DNS server stores “resource records” for all DNS names in the domain that it has authority for
v Each server needs to know other servers that are responsible for the other portions of the hierarchy § Every server knows the root § Root server knows about all top-level domains
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DNS Root v Located in Virginia, USA v How do we make the root scale?
Verisign, Dulles, VA
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DNS Root Servers v 13 root servers (labeled A-M; see http://www.root-servers.org/)
B USC-ISI Marina del Rey, CA L ICANN Los Angeles, CA
E NASA Mt View, CA F Internet Software Consortium Palo Alto, CA
I Autonomica, Stockholm
K RIPE London
M WIDE Tokyo
A Verisign, Dulles, VA C Cogent, Herndon, VA D U Maryland College Park, MD G US DoD Vienna, VA H ARL Aberdeen, MD J Verisign
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DNS Root Servers
B USC-ISI Marina del Rey, CA L ICANN Los Angeles, CA
E NASA Mt View, CA F Internet Software Consortium, Palo Alto, CA (and 37 other locations)
I Autonomica, Stockholm (plus 29 other locations)
K RIPE London (plus 16 other locations)
M WIDE Tokyo plus Seoul, Paris, San Francisco
A Verisign, Dulles, VA C Cogent, Herndon, VA (also Los Angeles, NY, Chicago) D U Maryland College Park, MD G US DoD Vienna, VA H ARL Aberdeen, MD J Verisign (21 locations)
l 13 root servers (labeled A-M; see http://www.root-servers.org/) l Replicated via any-casting
33 Root Server health: https://www.ultratools.com/tools/dnsRootServerSpeed
Anycast in a nutshell
v Routing finds shortest paths to destination
v If several locations are given the same address, then the network will deliver the packet to the closest location with that address
v This is called “anycast” § Very robust § Requires no modification to routing algorithms
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TLD, authoritative servers
top-level domain (TLD) servers: § responsible for com, org, net, edu, aero, jobs, museums,
and all top-level country domains, e.g.: uk, fr, ca, jp § Network Solutions maintains servers for .com TLD § Educause for .edu TLD
authoritative DNS servers: § organization’s own DNS server(s), providing authoritative
hostname to IP mappings for organization’s named hosts § can be maintained by organization or service provider
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Local DNS name server v does not strictly belong to hierarchy v each ISP (residential ISP, company, university) has one
§ also called “default name server” v Hosts configured with local DNS server address (e.g., /etc/
resolv.conf) or learn server via a host configuration protocol (e.g., DHCP)
v Client application § Obtain DNS name (e.g., from URL) § Do gethostbyname() to trigger DNS request to its local DNS server
v when host makes DNS query, query is sent to its local DNS server § has local cache of recent name-to-address translation pairs
(but may be out of date!) § acts as proxy, forwards query into hierarchy
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requesting host wagner.cse.unsw.edu.au
gaia.cs.umass.edu
root DNS server
local DNS server cse.unsw.edu.au
1
2 3
4 5
6
authoritative DNS server dns.cs.umass.edu
7 8
TLD DNS server
DNS name resolution example
v host at wagner.cse.unsw.edu.au wants IP address for gaia.cs.umass.edu
iterated query: v contacted server
replies with name of server to contact
v “I don’t know this name, but ask this server”
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4 5
6 3
recursive query: v puts burden of name
resolution on contacted name server
requesting host wagner.cse.unsw.edu.au
gaia.cs.umass.edu
root DNS server
local DNS server cse.unsw.edu.au
1
2 7
authoritative DNS server dns.cs.umass.edu
8
DNS name resolution example
TLD DNS server
Applet: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/dns/dns.html 38
39
Quiz: Which one would you use? Why?
requesting host wagner.cse.unsw.edu.au
gaia.cs.umass.edu
root DNS server
local DNS server cse.unsw.edu.au
1
2 3
4 5
6
authoritative DNS server dns.cs.umass.edu
7 8
TLD DNS server
4 5
6 3
gaia.cs.umass.edu
root DNS server
local DNS server cse.unsw.edu.au
1
2 7
8
requesting host wagner.cse.unsw.edu.au
Iterated queries Recursive queries
DNS: caching, updating records v once (any) name server learns mapping, it caches
mapping § cache entries timeout (disappear) after some time (TTL) § TLD servers typically cached in local name servers
• thus root name servers not often visited
v Subsequent requests need not burden DNS v cached entries may be out-of-date (best effort
name-to-address translation!) § if name host changes IP address, may not be known
Internet-wide until all TTLs expire
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v The TTL value should be:
A. Short, to make sure that changes are accurately reflected
B. Long to avoid re-queries of higher level DNS servers
C. Something else
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Quiz: DNS Record TTL
DNS records
DNS: distributed db storing resource records (RR)
type=NS § name is domain (e.g.,
foo.com) § value is hostname of
authoritative name server for this domain
RR format: (name, value, type, ttl)
type=A § name is hostname § value is IP address
type=CNAME § name is alias name for some
“canonical” (the real) name § www.ibm.com is really servereast.backup2.ibm.com
§ value is canonical name
type=MX § value is name of mailserver
associated with name
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DNS protocol, messages v query and reply messages, both with same message
format
msg header v identification: 16 bit # for
query, reply to query uses same #
v flags: § query or reply § recursion desired § recursion available § reply is authoritative
identification flags
# questions
questions (variable # of questions)
# additional RRs # authority RRs
# answer RRs
answers (variable # of RRs)
authority (variable # of RRs)
additional info (variable # of RRs)
2 bytes 2 bytes
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name, type fields for a query
RRs in response to query
records for authoritative servers
additional “helpful” info that may be used
identification flags
# questions
questions (variable # of questions)
# additional RRs # authority RRs
# answer RRs
answers (variable # of RRs)
authority (variable # of RRs)
additional info (variable # of RRs)
DNS protocol, messages
2 bytes 2 bytes
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bash-3.2$ dig www.cse.unsw.edu.au ; <<>> DiG 9.6-ESV-R4 <<>> www.cse.unsw.edu.au ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 26306 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 2, ADDITIONAL: 4 ;; QUESTION SECTION: ;www.cse.unsw.edu.au. IN A ;; ANSWER SECTION: www.cse.unsw.edu.au. 300 IN CNAME albeniz.orchestra.cse.unsw.edu.au. albeniz.orchestra.cse.unsw.edu.au. 86400 IN A 129.94.242.51
;; AUTHORITY SECTION: orchestra.cse.unsw.edu.au. 86400 IN NS beethoven.orchestra.cse.unsw.edu.au. orchestra.cse.unsw.edu.au. 86400 IN NS maestro.orchestra.cse.unsw.edu.au. ;; ADDITIONAL SECTION: maestro.orchestra.cse.unsw.edu.au. 86400 IN A 129.94.242.33 beethoven.orchestra.cse.unsw.edu.au. 86400 IN A 129.94.208.3 beethoven.orchestra.cse.unsw.edu.au. 86400 IN A 129.94.242.2 beethoven.orchestra.cse.unsw.edu.au. 86400 IN A 129.94.172.11 ;; Query time: 1 msec ;; SERVER: 129.94.242.2#53(129.94.242.2) ;; WHEN: Mon Mar 18 12:36:28 2013 ;; MSG SIZE rcvd: 195 45
Your 2nd lab
Application Layer
Inserting records into DNS
v example: new startup “Network Utopia” v register name networkuptopia.com at DNS registrar
(e.g., Network Solutions) § provide names, IP addresses of authoritative name server
(primary and secondary) § registrar inserts two RRs into .com TLD server: (networkutopia.com, dns1.networkutopia.com, NS)
(dns1.networkutopia.com, 212.212.212.1, A) v create authoritative server type A record for
www.networkuptopia.com; type MX record for networkutopia.com
v Q: Where do you insert these type A and type MX records?
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Reliability
v DNS servers are replicated (primary/secondary) § Name service available if at least one replica is up § Queries can be load-balanced between replicas
v Usually, UDP used for queries § Need reliability: must implement this on top of UDP § Spec supports TCP too, but not always implemented
v Try alternate servers on timeout § Exponential backoff when retrying same server
v Same identifier for all queries § Don’t care which server responds
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DNS provides Indirection
v Addresses can change underneath § Move www.cnn.com to 4.125.91.21 § Humans/Apps should be unaffected
v Name could map to multiple IP addresses § Enables
• Load-balancing • Reducing latency by picking nearby servers
v Multiple names for the same address § E.g., many services (mail, www, ftp) on same machine § E.g., aliases like www.cnn.com and cnn.com
v But, this flexibility applies only within domain! 48
DNS Load Balancing
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• (Author i t a t i ve ) DNS ser ver monitors the load of the multiple web servers
• Replies back with the IP address for the least loaded server
• Can also consider the location of the client (e.g., a user from Sydney directed to Australian server)
DNS for Content Distribution
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0: cnn.com distributes images to Akamai server 1: An end user types in www.cnn.com in their browser 2: index page contains link to the a73.g.akamai.net 3: local DNS server asks root DNS server to resolve a73.g.akamai.net 4: root DNS replies with IP address of high-level Akamai DNS server, akamai.net 5: local DNS server queries this nameserver to resolve a73.g.akamai.net 6: high-level DNS server replies with IP address of the DNS server for g.akamai.net 7: local DNS queries this nameserver to resolve a73.g.akamai.net 8: low level DNS server takes into consideration the client’s location and replies with the IP address of the closest Akamai web server that hosts the image 9: end user’s machine sends a GET request for the image to this closest Akamai web server 10: Akamai webserver responds with the image
Reverse DNS
v IP address -> domain name v Special PTR record type to store reverse DNS
entries v Where is reverse DNS used?
§ Troubleshooting tools such as traceroute and ping § “Received” trace header field in SMTP e-mail § SMTP servers for validating IP addresses of originating
servers § Internet forums tracking users § System logging or monitoring tools § Used in load balancing servers/content distribution to
determine location of requester
51
Do you trust your DNS server?
v Censorship
v Logging § IP address, websites visited, geolocation data and more § E.g., Google DNS:
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https://developers.google.com/speed/public-dns/privacy
https://wikileaks.org/wiki/Alternative_DNS
Attacking DNS DDoS attacks v Bombard root servers
with traffic § Not successful to date § Traffic Filtering § Local DNS servers cache
IPs of TLD servers, allowing root server to be bypassed
v Bombard TLD servers § Potentially more dangerous
Redirect attacks v Man-in-middle
§ Intercept queries v DNS poisoning
§ Send bogus replies to DNS server, which caches
Exploit DNS for DDoS v Send queries with spoofed
source address: target IP v Requires amplification
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Want to dig deeper? http://www.networkworld.com/article/2886283/security0/top-10-dns-attacks-likely-to-infiltrate-your-network.html
DNS Cache Poisoning v Suppose you are a bad guy and you control the name server
for drevil.com. Your name server receives a request to resolve www.drevil.com. and you respond as follows:
;; QUESTION SECTION: ;www.drevil.com. IN A ;; ANSWER SECTION: www.drevil.com 300 IN CNAME 129.45.212.42
;; AUTHORITY SECTION: drevil.com 86400 IN NS dns1.drevil.com. drevil.com 86400 IN NS google.com ;; ADDITIONAL SECTION: google.com 600 IN A 129.45.212.222
v Solution: Do not allow DNS servers to cache IP address mappings unless they are from authoritative name servers
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A drevil.com machine, not google.com
Dig deeper?
DNS Cache Poisoning Test https://www.grc.com/dns/dns.htm DNSSEC: DNS Security Extensions, http://www.dnssec.net
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2. Application Layer: outline
2.1 principles of network applications § app architectures § app requirements
2.2 Web and HTTP 2.3 FTP 2.4 electronic mail
§ SMTP, POP3, IMAP 2.5 DNS
2.6 P2P applications 2.7 socket programming
with UDP and TCP
56
Pure P2P architecture v no always-on server v arbitrary end systems
directly communicate v peers are intermittently
connected and change IP addresses
examples: § file distribution
(BitTorrent) § Streaming (KanKan) § VoIP (Skype)
57
File distribution: client-server vs P2P
Question: how much time to distribute file (size F) from one server to N peers? § peer upload/download capacity is limited resource
us
uN
dN
server
network (with abundant bandwidth)
file, size F
us: server upload capacity
ui: peer i upload capacity
di: peer i download capacity u2 d2
u1 d1
di
ui
58
File distribution time: client-server
v server transmission: must sequentially send (upload) N file copies: § time to send one copy: F/us § time to send N copies: NF/us
increases linearly in N
time to distribute F to N clients using
client-server approach Dc-s > max{NF/us,,F/dmin}
v client: each client must download file copy § dmin = min client download rate § min client download time: F/dmin
us
network di
ui
F
59
File distribution time: P2P
v server transmission: must upload at least one copy § time to send one copy: F/us
time to distribute F to N clients using
P2P approach
us
network di
ui
F
DP2P > max{F/us,,F/dmin,,NF/(us + Σui)}
v client: each client must download file copy § min client download time: F/dmin
v clients: as aggregate must download NF bits § max upload rate (limiting max download rate) is us + Σui
… but so does this, as each peer brings service capacity increases linearly in N …
60
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20 25 30 35
N
Min
imum
Dis
tribu
tion
Tim
e P2PClient-Server
Client-server vs. P2P: example
client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
61
P2P file distribution: BitTorrent
tracker: tracks peers participating in torrent
torrent: group of peers exchanging chunks of a file
Alice arrives …
v file divided into 256Kb chunks v peers in torrent send/receive file chunks
… obtains list of peers from tracker … and begins exchanging file chunks with peers in torrent
62
.torrent files
v Contains address of trackers for the file § Where can I find other peers?
v Contain a list of file chunks and their cryptographic hashes § This ensures that chunks are not modified
63
v peer joining torrent: § has no chunks, but will
accumulate them over time from other peers
§ registers with tracker to get list of peers, connects to subset of peers (“neighbours”)
P2P file distribution: BitTorrent
v while downloading, peer uploads chunks to other peers v peer may change peers with whom it exchanges chunks v churn: peers may come and go v once peer has entire file, it may (selfishly) leave or
(altruistically) remain in torrent
64
BitTorrent: requesting, sending file chunks
requesting chunks: v at any given time, different
peers have different subsets of file chunks
v periodically, Alice asks each peer for list of chunks that they have
v Alice requests missing chunks from peers, rarest first
sending chunks: tit-for-tat v Alice sends chunks to those
four peers currently sending her chunks at highest rate § other peers are choked by Alice
(do not receive chunks from her) § re-evaluate top 4 every10 secs
v every 30 secs: randomly select another peer, starts sending chunks § “optimistically unchoke” this peer § newly chosen peer may join top 4
65
BitTorrent: tit-for-tat (1) Alice “optimistically unchokes” Bob (2) Alice becomes one of Bob’s top-four providers; Bob reciprocates (3) Bob becomes one of Alice’s top-four providers
higher upload rate: find better trading partners, get file faster !
Original Research Paper on BitTorrent linked to lecture notes page 66
Getting rid of the server
v Distribute the tracker information using a Distributed Hash Table (DHT)
v A DHT is a lookup structure § Maps keys to an arbitrary value § Works a lot like, well …. hash table
67
Title End-systems that have it House of Cards Season 4 Michonne, Rick, Chuck, Sansa Walking Dead Season 6 Frank, Mike, Tyrion, Gregor Game of Thrones Season Saul, Mike, Glenn Better Call Saul Season 2 Khal, Jon, Daryl
Getting rid of the server
v Distribute the tracker information using a Distributed Hash Table (DHT)
v A DHT is a lookup structure § Maps keys to an arbitrary value § Works a lot like, well …. hash table
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Title End-systems that have it House of Cards Season 4 124.45.66.128, 88.45.34.214 Walking Dead Season 6 111.111.111.111, 432.125.43.43 Game of Thrones Season 42.12.43.32 Better Call Saul Season 2 123.134.2.123
Recall: Hash Function
v Mapping of any data to an integer § E.g. , md5sum, sha1, etc. § md5: 04c3416cadd85971a129dd1de86cee49
v With a good (cryptographic) hash
§ Hash values likely to be unique, although duplicates are possible
§ Very difficult to find collisions (hashes spread out)
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Recall: Hash Table
v N buckets
v Key-value pair is assigned bucket i § i = HASH(key)%N
v Easy to look up value based on key
v Multiple key-value pairs assigned to each bucket
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Distributed Hash Table (DHT)
v DHT: a distributed P2P database v database has (key, value) pairs; examples:
§ key: TFN number; value: human name § key: file name; value: BT tracker peer(s)
v Distribute the (key, value) pairs over the (millions of peers)
v a peer queries DHT with key § DHT returns values that match the key
v peers can also insert (key, value) pairs 71
Overlay network
v A network made up of “virtual” or logical links
v Virtual links map to one or more physical links (i.e. paths)
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Overlay Network
• A network made up of “virtual” or logical links
• Virtual links map to one or more physical links
Challenges
v How do we assign (key, value) pairs to nodes?
v How do we find them again quickly?
v What happens if nodes join/leave?
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Q: how to assign keys to peers?
v basic idea: § convert each key to an integer § Assign integer to each peer § put (key,value) pair in the peer that is closest to the
key
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DHT identifiers: Consistent Hashing
v assign integer identifier to each peer in range [0,2n-1] for some n-bit hash function § E.g., node ID is hash of its IP address
v require each key to be an integer in same range v to get integer key, hash original key
§ e.g., key = hash(“House of Cards Season 4”) § this is why its is referred to as a distributed “hash”
table
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Assign keys to peers
v rule: assign key to the peer that has the closest ID.
v common convention: closest is the immediate successor of the key.
v e.g., n=4; peers: 1,3,4,5,8,10,12,14; § key = 13, then successor peer = 14 § key = 15, then successor peer = 1
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1
3
4
5
8 10
12
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Circular DHT (1)
v each peer only aware of immediate successor and predecessor.
v “overlay network” 77
0001
0011
0100
0101
1000 1010
1100
1111
Who’s responsible for key 1110 ? I am
1110
1110
1110
1110
1110
1110
Define closest as closest successor
Circular DHT (2)
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v Given N nodes, what is the complexity (number of messages) of finding a value when each peer knows its successor?
A. O (log n)
B. O (n) C. O (n2) D. O (2n)
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Quiz: DHT Complexity
Circular DHT with shortcuts
v each peer keeps track of IP addresses of predecessor, successor, short cuts
v reduced from 6 to 2 messages. v possible to design shortcuts so O(log N) neighbours, O(log N)
messages in query
1
3
4
5
8 10
12
15
Who’s responsible for key 1110?
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Peer churn
example: peer 5 abruptly leaves v peer 4 detects peer 5 departure; makes 8 its immediate successor; asks 8 who its immediate successor is; makes 8’s immediate successor its second successor.
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3
4
5
8 10
12
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handling peer churn: v peers may come and go (churn) v each peer knows address of its two successors v each peer periodically pings its two successors to check aliveness v if immediate successor leaves, choose next successor as new immediate successor
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v What happens to 5’s value? § A. Lost forever § B. Lost until 5 comes back § Still recoverable (how?)
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Quiz: Peer Churn 1
3
4
5
8 10
12
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v Which of the following is not true?
A. DHT distributes portions of a hash table across peers.
B. The key corresponding to an object (e.g., movie) depends on the current number of peers.
C. Which peer is responsible for an object depends on the current number of peers.
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Quiz: DHT
More DHT info
v How do nodes join?
v How does cryptographic hashing work?
v How much state does each node store?
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Research Papers (on the lectures page): Chord: A Scalable Peer-to-Peer Lookup Service for Internet Applications Dynamo: Amazon’s Highly Available Key-value Store
DHT: Applications v File sharing and backup [CFS, Ivy, OceanStore, PAST,
Pastiche …] v Web cache and replica [Squirrel, Croquet Media Player] v Censor-resistant stores [Eternity] v DB query and indexing [PIER, Place Lab, VPN Index] v Event notification [Scribe] v Naming systems [ChordDNS, Twine, INS, HIP] v Distributed BitTorrent tracker [Kademlia, Vuze] v Communication primitives [I3, …] v Host mobility [DTN Tetherless Architecture]
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2. Application Layer: outline
2.1 principles of network applications § app architectures § app requirements
2.2 Web and HTTP 2.3 FTP 2.4 electronic mail
§ SMTP, POP3, IMAP 2.5 DNS
2.6 P2P applications 2.7 socket programming
with UDP and TCP Note: 6th Edition uses Python. Lecture notes show examples of both Java and Python
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Socket programming
goal: learn how to build client/server applications that communicate using sockets
socket: door between application process and end-end-transport protocol
Internet
controlled by OS
controlled by app developer
transport
application
physical
link
network
process
transport
application
physical
link
network
process socket
Self Study
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Socket programming
Two socket types for two transport services: § UDP: unreliable datagram § TCP: reliable, byte stream-oriented
Application Example: 1. Client reads a line of characters (data) from its
keyboard and sends the data to the server. 2. The server receives the data and converts
characters to uppercase. 3. The server sends the modified data to the client. 4. The client receives the modified data and displays
the line on its screen.
Self Study
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Socket programming with UDP
UDP: no “connection” between client & server v no handshaking before sending data v sender explicitly attaches IP destination address and
port # to each packet v rcvr extracts sender IP address and port# from
received packet
UDP: transmitted data may be lost or received out-of-order
Application viewpoint: v UDP provides unreliable transfer of groups of bytes
(“datagrams”) between client and server
Self Study
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Client/server socket interaction: UDP (Java)
close clientSocket
Server (running on hostid)
read reply from clientSocket
create socket,
clientSocket = DatagramSocket()
Client
Create, address (hostid, port=x, send datagram request using clientSocket
create socket, port=x, for incoming request: serverSocket = DatagramSocket()
read request from serverSocket write reply to serverSocket specifying client host address, port number
Self Study
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Example: Java client (UDP) import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
Create input stream
Create client socket
Translate hostname to IP
address using DNS
Self Study
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Example: Java client (UDP), cont. DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); }
}
Create datagram with data-to-send,
length, IP addr, port Send datagram
to server
Read datagram from server
Self Study
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Example: Java server (UDP) import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
Create datagram socket
at port 9876
Create space for received datagram
Receive datagram
Self Study
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Example: Java server (UDP), cont String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase(); sendData = capitalizedSentence.getBytes(); DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } }
}
Get IP addr port #, of
sender
Write out datagram to socket End of while loop,
loop back and wait for another datagram
Create datagram to send to client
Self Study
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Client/server socket interaction: UDP (Python)
close clientSocket
read datagram from clientSocket
create socket: clientSocket = socket(AF_INET,SOCK_DGRAM)
Create datagram with server IP and port=x; send datagram via clientSocket
create socket, port= x: serverSocket = socket(AF_INET,SOCK_DGRAM)
read datagram from serverSocket
write reply to serverSocket specifying client address, port number
server (running on serverIP) client
Self Study
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Example app: UDP client
from socket import * serverName = ‘hostname’ serverPort = 12000 clientSocket = socket(socket.AF_INET, socket.SOCK_DGRAM) message = raw_input(’Input lowercase sentence:’) clientSocket.sendto(message,(serverName, serverPort))
modifiedMessage, serverAddress = clientSocket.recvfrom(2048) print modifiedMessage clientSocket.close()
Python UDPClient include Python’s socket library
create UDP socket for server
get user keyboard input
Attach server name, port to message; send into socket
print out received string and close socket
read reply characters from socket into string
Self Study
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Example app: UDP server
from socket import * serverPort = 12000 serverSocket = socket(AF_INET, SOCK_DGRAM) serverSocket.bind(('', serverPort)) print “The server is ready to receive” while 1: message, clientAddress = serverSocket.recvfrom(2048) modifiedMessage = message.upper() serverSocket.sendto(modifiedMessage, clientAddress)
Python UDPServer
create UDP socket
bind socket to local port number 12000
loop forever
Read from UDP socket into message, getting client’s address (client IP and port)
send upper case string back to this client
Self Study
Socket programming with TCP client must contact server v server process must first be
running v server must have created
socket (door) that welcomes client’s contact
client contacts server by: v Creating TCP socket,
specifying IP address, port number of server process
v when client creates socket: client TCP establishes connection to server TCP
v when contacted by client, server TCP creates new socket for server process to communicate with that particular client § allows server to talk with
multiple clients § source port numbers used
to distinguish clients (more in Chap 3)
TCP provides reliable, in-order byte-stream transfer (“pipe”) between client and server
application viewpoint:
Self Study
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TCP Sockets Self Study
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Client/server socket interaction: TCP (in Java)
wait for incoming connection request connectionSocket = welcomeSocket.accept()
create socket, port=x, for incoming request:
welcomeSocket = ServerSocket()
create socket, connect to hostid, port=x
clientSocket = Socket()
close connectionSocket
read reply from clientSocket close clientSocket
Server (running on hostid) Client
send request using clientSocket read request from
connectionSocket write reply to connectionSocket
TCP connection setup
Self Study
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Example: Java client (TCP) import java.io.*; import java.net.*; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); Socket clientSocket = new Socket("hostname", 6789); DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
Create input stream
Create client socket,
connect to server Create
output stream attached to socket
Self Study
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Example: Java client (TCP), cont.
BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); sentence = inFromUser.readLine(); outToServer.writeBytes(sentence + '\n'); modifiedSentence = inFromServer.readLine(); System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close(); } }
Create input stream
attached to socket
Send line to server
Read line from server
Self Study
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Example: Java server (TCP) import java.io.*; import java.net.*; class TCPServer { public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept(); BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
Create welcoming socket
at port 6789
Wait, on welcoming socket for contact
by client Create input
stream, attached to socket
Self Study
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Example: Java server (TCP), cont DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream()); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; outToClient.writeBytes(capitalizedSentence); } } }
Read in line from socket
Create output stream, attached
to socket
Write out line to socket
End of while loop, loop back and wait for another client connection
Self Study
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Example app: TCP client
from socket import * serverName = ’servername’ serverPort = 12000 clientSocket = socket(AF_INET, SOCK_STREAM) clientSocket.connect((serverName,serverPort)) sentence = raw_input(‘Input lowercase sentence:’) clientSocket.send(sentence) modifiedSentence = clientSocket.recv(1024) print ‘From Server:’, modifiedSentence clientSocket.close()
Python TCPClient
create TCP socket for server, remote port 12000
No need to attach server name, port
Self Study
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Example app: TCP server
from socket import * serverPort = 12000 serverSocket = socket(AF_INET,SOCK_STREAM) serverSocket.bind((‘’,serverPort)) serverSocket.listen(1) print ‘The server is ready to receive’ while 1: connectionSocket, addr = serverSocket.accept() sentence = connectionSocket.recv(1024) capitalizedSentence = sentence.upper() connectionSocket.send(capitalizedSentence) connectionSocket.close()
Python TCPServer
create TCP welcoming socket
server begins listening for incoming TCP requests
loop forever
server waits on accept() for incoming requests, new socket created on return
read bytes from socket (but not address as in UDP)
close connection to this client (but not welcoming socket)
Self Study
Application Layer: summary
v application architectures § client-server § P2P
v application service requirements: § reliability, bandwidth, delay
v Internet transport service model § connection-oriented,
reliable: TCP § unreliable, datagrams: UDP
our study of network apps now complete!
v specific protocols: § HTTP § FTP § SMTP, POP, IMAP § DNS § P2P: BitTorrent, DHT
v socket programming: TCP, UDP sockets
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v typical request/reply message exchange: § client requests info or
service § server responds with
data, status code v message formats:
§ headers: fields giving info about data
§ data: info being communicated
important themes: v control vs. data msgs
§ in-band, out-of-band v centralized vs. decentralized v stateless vs. stateful v reliable vs. unreliable msg
transfer v “complexity at network
edge”
Application Layer: summary most importantly: learned about protocols!
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Solve all sample problems + those posted with lectures Reading Exercise for next week: Chapter 3: 3.1 – 3.4