1. 05 - FTP, , and DNS
2: Application Layer

2. FTP: the File Transfer Protocol
user
interface
client
file transfer
FTP
server
user
at host
local file
system
remote file
system
transfer file to/from remote host
client/server model
client: side that initiates transfer (either to/from remote)
server: remote host
ftp: RFC 959
ftp server: port 21
2: Application Layer

3. FTP: separate control, data connections
client
server
TCP control connection
port 21
TCP data connection
port 20
FTP client contacts FTP server at port 21, specifying TCP as transport protocol
Client obtains authorization over control connection
Client browses remote directory by sending commands over control connection.
When server receives a command for a file transfer, the server opens a TCP data connection to client
After transferring one file, server closes connection.
Server opens a second TCP data connection to transfer another file.
Control connection: “out of band”
FTP server maintains “state”: current directory, earlier authentication
Because FTP uses a second connection to send control info, FTP is said to send control info out of band.
2: Application Layer

4. Sample commands sent as ASCII text over control channel Authentication
USER: specify the user name to log in as
PASS: specify the user’s password
Exploring the files
LIST: list the files for the given file specification
CWD: change to the given directory
Downloading and uploading files
TYPE: set type to ASCII (A) or binary image (I)
RETR: retrieve the given file
STOR: upload the given file
Closing the connection
QUIT: close the FTP connection
2: Application Layer

5. Sample return codes status code and phrase (as in HTTP)
331 Username OK, password required
125 data connection already open; transfer starting
425 Can’t open data connection
452 Error writing file
2: Application Layer

6. Why two connections? Avoids need to mark the end of the data transfer
Data transfer ends by closing of data connection
Yet, the control connection stays up
Aborting a data transfer
Can abort a transfer without killing the control connection
… which avoids requiring the user to log in again
Done with an ABOR on the control connection
Third-party file transfer between two hosts
Data connection could go to a different host
… by sending a different client IP address to the server
E.g., user coordinates transfer between two servers
But: this is rarely needed, and presents security issues
2: Application Layer

7. FTP, SFTP FTP is not secure – nothing is encrypted!
SFTP uses SSH, and should be used instead of FTP when possible.
SFTP i.e. SSH with FTP uses public key encryption. SSH2 uses the Diffie-Hellman key exchange (generate a secret key and then use symmetric key encryption). Each message is individually encrpyted.
FTPS or FPT with SSL – uses certificates, public key cryptography and symmetric key – secure connection between client/server.
SSH – UNIX
SSL – Windows
SSL – privacy and trust
SSH – privacy and authentication
2: Application Layer

8. Electronic Mail Three major components: SMTP SMTP SMTP user agents
user mailbox
outgoing
message queue
user
agent
Three major components:
user agents
mail servers
simple mail transfer protocol: SMTP
User Agent
a.k.a. “mail reader”
composing, editing, reading mail messages
e.g., Apple Mail, Outlook, elm
outgoing, incoming messages stored on server
mail
server
user
agent
SMTP
mail
server
user
agent
SMTP
mail
server
SMTP
user
agent
user
agent
user
agent
2: Application Layer

9. Electronic Mail: mail servers
user
agent
Mail Servers
mailbox contains incoming messages for user
message queue of outgoing (to be sent) mail messages
SMTP protocol between mail servers to send messages
client: sending mail server
“server”: receiving mail server
mail
server
user
agent
SMTP
mail
server
user
agent
SMTP
mail
server
SMTP
user
agent
Note that the client and server are both mail-servers.
user
agent
user
agent
2: Application Layer

10. Scenario: Alice sends message to Bob
1) Alice uses UA to compose message and “to”
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
user
agent
user
agent 2 3 6 4 5
2: Application Layer

11. Electronic Mail: SMTP [RFC 2821]
uses TCP to reliably transfer message from client to server, port 25
direct transfer: sending server (client) to receiving server (server)
three phases of transfer
handshaking (greeting)
transfer of messages
closure
command/response interaction
commands: ASCII text
response: status code and phrase
messages must be in 7-bit ASCII
2: Application Layer

12. Sample SMTP interaction
>telnet hamburger.edu 25
S: 220 hamburger.edu
C: HELO crepes.fr
S: 250 Hello crepes.fr, pleased to meet you
C: MAIL FROM:
S: 250 Sender ok
C: RCPT TO:
S: 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
Handshake
2: Application Layer

13. SMTP: final words Comparison with HTTP:
SMTP uses persistent connections
SMTP requires message (header & body) to be in 7-bit ASCII
SMTP server uses CRLF.CRLF to determine end of message
Comparison with HTTP:
HTTP: pull
SMTP: push
both have ASCII command/response interaction, status codes
HTTP: each object encapsulated in its own response msg
SMTP: multiple objects sent in multipart msg
2: Application Layer

14. Mail message format header body
SMTP: protocol for exchanging msgs
RFC 822: standard for text message format:
header lines, e.g.,
To:
From:
Subject:
different from SMTP commands!
body
the “message”, ASCII characters only
header
blank
line
body
2: Application Layer

15. Message format: multimedia extensions
MIME: Multipurpose Internet Mail Extension, RFC 2045, 2056
additional lines in msg header declare MIME content type
From:
To:
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
......base64 encoded data
MIME version
method used
to encode data
multimedia data
type, subtype,
parameter declaration
encoded data
2: Application Layer

16. MIME types Content-Type: type/subtype; parameters
Text
example subtypes: plain, html
Image
example subtypes: jpeg, gif
Audio
example subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding)
Video
example subtypes: mpeg, quicktime
Application
other data that must be processed by reader before “viewable”
example subtypes: msword, octet-stream
2: Application Layer

17. Multipart Type From: alice@crepes.fr To: bob@hamburger.edu
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=StartOfNextPart
--StartOfNextPart
Dear Bob, Please find a picture of a crepe.
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
......base64 encoded data
Do you want the recipe?
2: Application Layer

18. Mail access protocols SMTP SMTP access protocol
user
agent
user
agent
sender’s mail
server
receiver’s mail
server
SMTP: delivery/storage to receiver’s server
Mail access protocol: retrieval from server
POP: Post Office Protocol [RFC 1939]
TCP, port 110
authorization (agent <-->server) and download
IMAP: Internet Mail Access Protocol [RFC 1730]
more features (more complex)
manipulation of stored msgs on server
HTTP: gmail, Hotmail , Yahoo! Mail, etc.
2: Application Layer

19. POP3 protocol authorization phase C: list transaction phase, client:
S: +OK POP3 server ready
C: user bob
S: +OK
C: pass hungry
S: +OK user successfully logged on
authorization phase
client commands:
user: declare username
pass: password
server responses
+OK
-ERR
transaction phase, client:
list: list message numbers
retr: retrieve message by number
dele: delete
Quit
Update phase- server deletes files.
C: list
S: 1 498
S: 2 912
S: .
C: retr 1
S: <message 1 contents>
C: dele 1
C: retr 2
C: dele 2
C: quit
S: +OK POP3 server signing off
2: Application Layer

20. POP3 (more) and IMAP More about POP3
Previous example uses “download and delete” mode.
Bob cannot re-read if he changes client
“Download-and-keep”: copies of messages on different clients
POP3 is stateless across sessions
IMAP
Keep all messages in one place: the server
Allows user to organize messages in folders
IMAP keeps user state across sessions:
names of folders and mappings between message IDs and folder name
Can also download only portions of a message e.g. headers
2: Application Layer

21. Web-based E-mail User agent: browser
Use HTTP to send to server and receive from server.
SMTP between servers.
2: Application Layer

22. DNS: Domain Name System
people: many identifiers:
SSN, name, passport #
Internet hosts, routers:
IP address (32 bit) - used for addressing datagrams
“name”, e.g., - used by humans
Q: map between IP address and name, and vice versa ?
 DNS 

23. DNS DNS services Why not centralize DNS?
hostname to IP address translation
host aliasing
Canonical, alias names
mail server aliasing
Core Internet function implemented as application layer protocol
load distribution
replicated Web servers: set of IP addresses for one canonical name
Why not centralize DNS?
single point of failure
traffic volume
distant centralized database
maintenance
doesn’t scale!

24. DNS name servers no server has all name-to-IP address mappings
DNS is a distributed database implemented in hierarchy of many name servers
no server has all name-to-IP address mappings
local name servers:
each ISP, company has local (default) name server
host DNS query first goes to local name server
authoritative name server:
for a host: stores that host’s IP address, name
can perform name/address translation for that host’s name
2: Application Layer

25. Distributed, Hierarchical Database
Root DNS Servers
com DNS servers
org DNS servers
edu DNS servers
poly.edu
DNS servers
umass.edu
yahoo.com
amazon.com
pbs.org
client wants IP for 1st approx:
client queries a root server to find com DNS server
client queries com DNS server to get amazon.com DNS server
client queries amazon.com DNS server to get IP address for
Com DNS servers  Top-level domain (TLD) severs
Amazon DNS servers  authoritative DNS servers

26. DNS: Root name servers
contacted by local name server that can not resolve name
root name server:
contacts authoritative name server if name mapping not known
gets mapping
returns mapping to local name server
b USC-ISI Marina del Rey, CA
l ICANN Marina del Rey, CA
e NASA Mt View, CA
f Internet Software C. Palo Alto, CA
i NORDUnet Stockholm
k RIPE London
m WIDE Tokyo
a NSI Herndon, VA
c PSInet Herndon, VA
d U Maryland College Park, MD
g DISA Vienna, VA
h ARL Aberdeen, MD
j NSI (TBD) Herndon, VA
13 root name servers worldwide
(actually > 80 using anycasting)
Zonefile stored at a root server
2: Application Layer

27. authoritative name server
Simple DNS example
root name server
host surf.eurecom.fr wants IP address of gaia.cs.umass.edu
1. contacts its local DNS server, dns.eurecom.fr
2. dns.eurecom.fr contacts root name server, if necessary
3. root name server contacts authoritative name server, dns.umass.edu, if necessary 2 4 3 5
local name server
dns.eurecom.fr
authoritative name server
dns.cs.umass.edu 1 6
requesting host
surf.eurecom.fr
gaia.cs.umass.edu
2: Application Layer

28. DNS example Root name server: may not know authoritative name server
may know intermediate name server: who to contact to find authoritative name server 2 6 7 3
local name server
dns.eurecom.fr
intermediate name server
dns.umass.edu 4 5 1 8
authoritative name server
dns.cs.umass.edu
requesting host
surf.eurecom.fr
gaia.cs.umass.edu
2: Application Layer

29. DNS: iterated queries recursive query: iterated query:
root name server
recursive query:
puts burden of name resolution on contacted name server
heavy load?
iterated query:
contacted server replies with name of server to contact
“I don’t know this name, but ask this server”
iterated query 2 3 4 7
local name server
dns.eurecom.fr
intermediate name server
dns.umass.edu 5 6 1 8
authoritative name server
dns.cs.umass.edu
requesting host
surf.eurecom.fr
gaia.cs.umass.edu
2: Application Layer

30. DNS: caching and updating records
once (any) name server learns mapping, it caches mapping
cache entries timeout (disappear) after some time
If the TLD servers and intermediate DNS servers perform their functions correctly, the root servers will rarely be contacted.
2: Application Layer

31. RR format: (name, value, type, ttl)
DNS records
DNS: distributed db stores resource records (RR)
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
is really
servereast.backup2.ibm.com
value is canonical name
Type=NS
name is domain (e.g. foo.com)
value is IP address of authoritative name server for this domain
Type=MX
value is name of mailserver associated with name
Use “nslookup” and “dig” to see RRs.
2: Application Layer