CS1652 September 10 th, 2013 The slides are adapted from the publisher’s material All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights.

CS1652September 10th, 2013

The slides are adapted from the publisher’s material All material copyright 1996-2009

J.F Kurose and K.W. Ross, All Rights Reserved

Jack Lange

University of Pittsburgh

1

2: Application Layer 10

Electronic Mail

Three major components: r user agents

r mail servers

r simple mail transfer protocol: SMTP

User Agent

r a.k.a. “mail reader”

r composing, editing, reading mail messages

r e.g., Eudora, Outlook, elm, Mozilla Thunderbird

r outgoing, incoming messages stored on server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP


Electronic Mail: mail servers

Mail Servers r mailbox contains

incoming messages for user

r message queue of outgoing (to be sent) mail messages

r SMTP protocol between mail servers to send email messages client: sending mail

server “server”: receiving mail

server

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP


Electronic Mail: SMTP [RFC 2821]

r uses TCP to reliably transfer email message from client to server, port 25

r direct transfer: sending server to receiving server

r three phases of transfer handshaking (greeting) transfer of messages closure

r command/response interaction commands: ASCII text response: status code and phrase

r messages must be in 7-bit ASCII


Scenario: Alice sends message to Bob1) 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

useragent

mailserver

mailserver user

agent

1

2 3 4 56


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


Try SMTP interaction for yourself:

r telnet servername 25r see 220 reply from serverr enter HELO, MAIL FROM, RCPT TO, DATA, QUIT

commands above lets you send email without using email

client (reader)


SMTP: final words

r SMTP uses persistent connections

r SMTP requires message (header & body) to be in 7-bit ASCII

r SMTP server uses CRLF.CRLF to determine end of message

Comparison with HTTP:

r HTTP: pullr SMTP: pushr both have ASCII

command/response interaction, status codes

r HTTP: each object encapsulated in its own response msg

r SMTP: multiple objects sent in multipart msg


Mail access protocols

r SMTP: delivery/storage to receiver’s server

r Mail access protocol: retrieval from server POP: Post Office Protocol [RFC 1939]

• 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.

useragent

sender’s mail server

useragent

SMTP SMTP accessprotocol

receiver’s mail server

Domain Name System (DNS)

21


DNS: Domain Name System

Internet hosts, routers: IP address (32 bit) -

used for addressing datagrams

“name”, e.g., www.yahoo.com - used by humans

Domain Name System:r distributed database

implemented in hierarchy of many name servers

r application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation)

note: core Internet function, implemented as application-layer protocol

complexity at network’s “edge”


DNS

Why not centralize DNS?r single point of failurer traffic volumer distant centralized

databaser maintenance

doesn’t scale!

DNS servicesr hostname to IP

address translationr host aliasing Canonical, alias names

r mail server aliasingr load distribution replicated Web servers:

set of IP addresses for one canonical name


Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

Client wants IP for www.amazon.com; 1st approx:r client queries a root server to find .com DNS

serverr client queries .com DNS server to get

amazon.com DNS serverr client queries amazon.com DNS server to get IP

address for www.amazon.com


DNS: Root name serversr A.root-servers.net to M.root-servers.net

Each server is a cluster of replicated servers Each IP is shared by many machines (e.g., IP Anycast)

r Responsible for top-level domain NS records

r How do we know the IP addresses of root servers?

13 root name servers worldwideb USC-ISI Marina del Rey, CA

l ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)

i Autonomica, Stockholm (plus 28 other locations)

k RIPE London (also 16 other locations)

m WIDE Tokyo (also Seoul, Paris, SF)

a Verisign, Dulles, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)


TLD and Authoritative Serversr Top-level domain (TLD) servers:

Responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.

Network Solutions maintains servers for com TLD

Educause for edu TLDr Authoritative DNS servers:

organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web, mail).

can be maintained by organization or service provider


Local Name Server

r does not strictly belong to hierarchyr each ISP (residential ISP, company,

university) has one. also called “default name server”

r when host makes DNS query, query is sent to its local DNS server acts as proxy, forwards query into hierarchy


requesting hostswan.cs.pitt.edu

www.princeton.edu

root DNS server

local DNS serverns1.cs.pitt.edu

1

23

4

5

6

authoritative DNS serverdns.princeton.edu

78

TLD DNS server

DNS name resolution example

r Host at swan.cs.pitt.edu wants IP address for www.princeton.edu

iterated query:r contacted server

replies with name of server to contact

r “I don’t know this name, but ask this server”


requesting hostswan.cs.pitt.edu

www.umass.edu

root DNS server

local DNS serverns1.cs.pitt.edu

1

2

45

6

authoritative DNS serverdns.princeton.edu

7

8

TLD DNS server

3recursive query:r puts burden of name

resolution on contacted name server

r heavy load?


DNS: caching and updating recordsr Once (any) name server learns mapping, it

caches mapping cache entries timeout (disappear) after some

time TLD servers typically cached in local name

servers• Thus root name servers not often visited

r Typical cache hit rate: 80-90% at local DNS server

r Negative caching of DNS queries (RFC 2308) Caches negative responses (e.g., non-existent names)


DNS records

DNS: distributed db storing resource records (RR)

r 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)

r Type=A name is hostname value is IP address

r 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

r Type=MX value is name of mailserver

associated with name


DNS protocol, messagesDNS protocol : query and reply messages, both with same message format

msg headerr identification: 16 bit #

for query, reply to query uses same #

r flags: query or reply recursion desired recursion available reply is authoritative


DNS protocol, messages

Name, type fields for a query

RRs in responseto query

records forauthoritative servers

additional “helpful”info that may be used


Inserting records into DNS

r example: new startup “Network Utopia”r 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)

r create authoritative server Type A record for www.networkuptopia.com; Type MX record for networkutopia.com

Peer-to-Peer Systems

35


Pure P2P architecture

r no always-on serverr arbitrary end systems

directly communicater peers are

intermittently connected and change IP addresses

r Three topics: File distribution Searching for

information Case Study: Skype

peer-peer


File Distribution: Server-Client vs P2PQuestion : How much time to distribute file

from one server to N peers?

us

u2d1 d2

u1

uN

dN

Server

Network (with abundant bandwidth)

File, size F

us: server upload bandwidth

ui: peer i upload bandwidth

di: peer i download bandwidth


File distribution time: server-client

us

u2d1 d2u1

uN

dN

Server


Fr server sequentially sends N copies: NF/us time

r client i takes F/di

time to download

increases linearly in N(for large N)

= dcs = max { NF/us, F/min(di) }i

Time to distribute F to N clients using

client/server approach


File distribution time: P2P

us

u2d1 d2u1

uN

dN

Server


Fr server must send one copy: F/us time

r client i takes F/di time to download

r NF bits must be downloaded (aggregate)

r fastest possible upload rate: us + Σui

dP2P = max { F/us, F/min(di) , NF/(us + Σui) }i


Server-client vs. P2P: example

Client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us


File distribution: BitTorrent

tracker: tracks peers participating in torrent

torrent: group of peers exchanging chunks of a file

obtain listof peers

trading chunks

peer

r P2P file distribution


BitTorrent (1)

r file divided into 256KB chunks.r peer joining torrent:

has no chunks, but will accumulate them over time

registers with tracker to get list of peers, connects to subset of peers (“neighbors”)

r while downloading, peer uploads chunks to other peers.

r peers may come and gor once peer has entire file, it may (selfishly) leave

or (altruistically) remain


BitTorrent (2)

Pulling Chunks

r at any given time, different peers have different subsets of file chunks

r periodically, a peer (Alice) asks each neighbor for list of chunks that they have.

r Alice sends requests for her missing chunks rarest first

Sending Chunks: tit-for-tatr Alice sends chunks to four

neighbors currently sending her chunks at the highest rate

re-evaluate top 4 every 10 secs

r every 30 secs: randomly select another peer, starts sending chunks

newly chosen peer may join top 4

“optimistically unchoke”


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

With higher upload rate, can find better trading partners & get file faster!

Distributed Hash Table (DHT)

r DHT = distributed P2P databaser Database has (key, value) pairs;

key: ss number; value: human name key: content type; value: IP address

r Peers query DB with key DB returns values that match the key

r Peers can also insert (key, value) peers

DHT Identifiers

r Assign integer identifier to each peer in range [0,2n-1]. Each identifier can be represented by n bits.

r Require each key to be an integer in same range.

r To get integer keys, hash original key. eg, key = h(“Led Zeppelin IV”) This is why they call it a distributed “hash” table

How to assign keys to peers?

r Central issue: Assigning (key, value) pairs to peers.

r Rule: assign key to the peer that has the closest ID.

r Convention in lecture: closest is the immediate successor of the key.

r Ex: n=4; peers: 1,3,4,5,8,10,12,14; key = 13, then successor peer = 14 key = 15, then successor peer = 1

1

3

4

5

810

12

15

Circular DHT (1)

r Each peer only aware of immediate successor and predecessor.

r “Overlay network”

Circle DHT (2)

0001

0011

0100

0101

10001010

1100

1111

Who’s resp

for key 1110 ?I am

O(N) messages

on avg to resolve

query, when there

are N peers

1110

1110

1110

1110

1110

1110

Define closestas closestsuccessor

Circular DHT with Shortcuts

r Each peer keeps track of IP addresses of predecessor, successor, short cuts.

r Reduced from 6 to 2 messages.r Possible to design shortcuts so O(log N) neighbors,

O(log N) messages in query

1

3

4

5

810

12

15

Who’s resp

for key 1110?

Peer Churn

r Peer 5 abruptly leavesr Peer 4 detects; makes 8 its immediate

successor; asks 8 who its immediate successor is; makes 8’s immediate successor its second successor.

r What if peer 13 wants to join?

1

3

4

5

810

12

15

•To handle peer churn, require

each peer to know the IP address

of its two successors. • Each peer periodically pings its two successors to see if they are still alive.


Chapter 2: Summary

r application architectures client-server P2P hybrid

r application service requirements: reliability, bandwidth,

delay

r Internet transport service model connection-oriented,

reliable: TCP unreliable, datagrams:

UDP

our study of network apps now complete!

r specific protocols: HTTP FTP SMTP, POP, IMAP DNS P2P: BitTorrent, Skype

r socket programming


Chapter 2: Summary

r typical request/reply message exchange: client requests info or

service server responds with

data, status code

r message formats: headers: fields giving

info about data data: info being

communicated

Most importantly: learned about protocols

Important themes: r control vs. data msgs in-band, out-of-bandr centralized vs.

decentralized r stateless vs. statefulr reliable vs. unreliable

msg transfer r “complexity at network

edge”


P2P Case study: Skype

r inherently P2P: pairs of users communicate.

r proprietary application-layer protocol (inferred via reverse engineering)

r hierarchical overlay with SNs

r Index maps usernames to IP addresses; distributed over SNs

Skype clients (SC)

Supernode

(SN)

Skype login server


Peers as relays

r Problem when both Alice and Bob are behind “NATs”. NAT prevents an

outside peer from initiating a call to insider peer

r Solution: Using Alice’s and Bob’s

SNs, Relay is chosen Each peer initiates

session with relay. Peers can now

communicate through NATs via relay


Mail message format

SMTP: protocol for exchanging email msgs

RFC 822: standard for text message format:

r header lines, e.g., To: From: Subject:

different from SMTP commands!

r body the “message”, ASCII

characters only

header

body

blankline


POP3 protocol

authorization phaser client commands:

user: declare username pass: password

r server responses +OK -ERR

transaction phase, client:

r list: list message numbersr retr: retrieve message by

numberr dele: deleter 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


POP3 (more) and IMAPMore about POP3r Previous example uses

“download and delete” mode.

r Bob cannot re-read e-mail if he changes client

r “Download-and-keep”: copies of messages on different clients

r POP3 is stateless across sessions

IMAPr Keep all messages in

one place: the serverr Allows user to

organize messages in folders

r IMAP keeps user state across sessions:

names of folders and mappings between message IDs and folder name

CS1652 September 10 th, 2013 The slides are adapted from the publisher’s material All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights.

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