2: Application Layer1 Chapter 7 The Application Layer (Cont’d) r Network Security r Domain Name Server r Network management r Applications í File Transfer.
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2: Application Layer 1
Chapter 7The Application Layer (Cont’d)Chapter 7The Application Layer (Cont’d)
Network Security Domain Name Server Network management Applications
File Transfer Protocol, Email, Http, Telnet, ….
© © All rights reserved. No part of these slides may be reproduced, in anyAll rights reserved. No part of these slides may be reproduced, in any form or by any means, without permission in writing from form or by any means, without permission in writing from Professor Wen-Tsuen Chen (email: wtchen@cs.nthu.edu.tw).Professor Wen-Tsuen Chen (email: wtchen@cs.nthu.edu.tw).
2: Application Layer 2
Email SecurityEmail Security PGP: Pretty Good Privacy, by Phil Zimmermaun in 1995. Support text compression, secrecy and digital signatures.
2: Application Layer 3
PGP message formatPGP message format
2: Application Layer 4
PEM: Privacy Enhanced MailPEM: Privacy Enhanced Mail
An official Internet standard described in RFC 1421-1424.
Support privacy and authentication for RFC 822 based email systems.
The message together with its message digest is encrypted using DES with a one-time key that is enclosed along with the message.
The key can be protected with RSA and certified by certification authorities.
2: Application Layer 5
2: Application Layer 6
DNS: Domain Name SystemPeople: many
identifiers: SSN, name, passport #
Internet hosts, routers: IP address (32 bit) -
used for addressing datagrams
“name”, e.g., gaia.cs.umass.edu - used by humans
Q: map between IP addresses and name ?
Domain Name System: A distributed database
implemented in hierarchy of many name servers
An application-layer protocol that allows host, routers, name servers to communicate to resolve names (address/name translation) DNS provides a core
Internet function, implemented as application-layer protocol
DNS is an example of the Internet design philosophy of placing complexity at network’s “edge”
2: Application Layer 7
DNS
Why not centralize DNS? single point of failure traffic volume distant centralized
database maintenance
doesn’t scale!
DNS services Hostname to IP
address translation Host aliasing
Canonical and alias names
Mail server aliasing Load distribution
Replicated Web servers: set of IP addresses for one canonical name
2: Application Layer 8
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: 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 www.amazon.com
2: Application Layer 9
DNS name serversThere are three types of name
servers:(1) local name servers:
each ISP, company has local (default) name server
host DNS query first goes to local name server
(2) root name servers: top level name servers ( to be explained next)
(3) authoritative name servers: for a host: stores that host’s
IP address, name can perform name/address
translation for that host’s name
The DNS is a distributed design
A large number of name servers organized in a hierarchical fashion and distributed around the world
no server has all name-to-IP address mappings
2: Application Layer 10
DNS: Root name servers contacted by local name server that can not resolve name root name server:
gets mapping returns mapping to local name server contacts authoritative name server if name mapping not
known
b USC-ISI Marina del Rey, CAl ICANN Marina del Rey, CA
e NASA Mt View, CAf Internet Software C. Palo Alto, CA
i NORDUnet Stockholm
k RIPE London
m WIDE Tokyo
a NSI Herndon, VAc PSInet Herndon, VAd U Maryland College Park, MDg DISA Vienna, VAh ARL Aberdeen, MDj NSI (TBD) Herndon, VA
13 root name servers worldwide
2: Application Layer 11
TLD and Authoritative Servers 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 TLD
Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail). Can be maintained by organization or service
provider
2: Application Layer 12
Local Name Server
Does not strictly belong to hierarchy Each ISP (residential ISP, company,
university) has one. Also called “default name server”
When a host makes a DNS query, query is sent to its local DNS server Acts as a proxy, forwards query into
hierarchy.
2: Application Layer 13
requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
1
23
4
5
6
authoritative DNS serverdns.cs.umass.edu
78
TLD DNS server
Example
Host at cis.poly.edu wants IP address for gaia.cs.umass.edu
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requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
1
2
45
6
authoritative DNS serverdns.cs.umass.edu
7
8
EDU DNS server
3
Recursive queries
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”
2: Application Layer 15
Recursive and iterated queries
requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
1
23
4
5
6
authoritative DNS serverdns.cs.umass.edu
78
TLD DNS server
iterated queryrecursive query
Typically, all queries are iterated except for the query from the host to the local name server.
2: Application Layer 16
DNS: caching and updating records once (any) name server learns mapping, it
caches mapping cache entries timeout (disappear) after
some time (usually two days) Until recently, the contents of each DNS
servers were configured statically from a configuration file created by a system manager.
More recently, an UPDATE option has been added to the DNS protocol to allow data to be added or deleted from the database via DNS messages.
DNS dynamic update mechanism is specified in RFC 2136
2: Application Layer 17
DNS recordsDNS: distributed database storing resource records (RR)
Type=NS name is domain (e.g.
foo.com) value is IP address 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
“cannonical” (the real) name
www.ibm.com is really servereast.backup2.ibm.com value is cannonical name
Type=MX name is alias name for
some mail server value is the canonical
name of the mail server
2: Application Layer 18
DNS protocol, messagesDNS protocol : query and reply messages, both with same message format
message header identification: 16 bit #
for query, reply to query uses same #
flags: query or reply recursion desired recursion available reply is authoritative
2: Application Layer 19
DNS protocol, messages
Name, type fields for a query
RRs in reponse to queryA hostname can have multiple IP addresses
records for otherauthoritative servers
additional “helpful”information that may be used
e.g. IP address for the canonical hostname of the mail server
2: Application Layer 20
Inserting records into DNS
Example: just created startup “Network Utopia” Register name networkuptopia.com at a registrar
(e.g., Network Solutions) Need to provide registrar with names and IP addresses
of your authoritative name server (primary and secondary)
Registrar inserts two RRs into the com TLD server:
(networkutopia.com, dns1.networkutopia.com, NS)(dns1.networkutopia.com, 212.212.212.1, A)
Put in authoritative server Type A record for www.networkuptopia.com and Type MX record for networkutopia.com
How do people get the IP address of your Web site?
2: Application Layer 21
Network Management SystemsNetwork Management Systems
A network management system is a collection of tools for network monitoring and control.
It has the following key elements: Management station, or manager. Agent in managed nodes, equipments etc. Management Information base (MIB). Network management protocol.
2: Application Layer 22
2: Application Layer 23
The management station will have A set of management applications for data
analysis, fault recovery etc. An Interface by which the network manager
may monitor and control the network. The capability of translating the network
manager’s requirements into the actual monitoring and control of remote elements in the network.
A database of network management information extracted from the database of all the managed entities in the network.
2: Application Layer 24
The agent is an active element residing in hosts, bridges, routers, and hubs etc. that responds to requests for information or actions from a management station, and may provide the management station (through trap) with important but unsolicited information.
An MIB is a collection of objects which are resources in the network that may be managed.
Network management protocol for TCP/IP networks is SNMP (Simple network management protocol), and for OSI-based networks is CMIP (Common Management Information Protocol).
2: Application Layer 25
SNMP: Simple Network Management ProtocolSNMP: Simple Network Management Protocol
First Proposed in 1988, RFC 1028, RFC 1067 Version 1 of SNMP in May 1990, RFC 1157 SNMPv2 issued in 11993, RFCs 1441 to 1452.
SNMP provides the infrastructure for network management applications.
The object definition language ASN.1 (Abstract Syntax Notation One), taken from OSI, is used for defining objects in MIBs.
2: Application Layer 26
Structure of Management InformationStructure of Management Information
The SMI defines the general framework within which an MIB can be defined and constructed.
The SMI identifies the data types (only simple) the scalars and two-dimensional arrays of scalars, called tables) that can be used in the MIB, and how resources within the MIB are represented and named.
2: Application Layer 27
2: Application Layer 28
SNMP ProtocolSNMP Protocol
Provides a basic mechanism for the exchange of management information between manager and agent.
Get-bulk-request, Inform-request, Response, and SNMPv2-trap are SNMPv2 specific.
An SNMPv2-trap is generated by an agent for reporting unusual events.
Inform-request is sent by a manager on behalf of an application, to another manager for providing management information to an application.
The manager receiving an Inform Request acknowledges receipt with Response.
2: Application Layer 29
in lexicographic order
Response Acknowledgement of receipt by Inform-request
2: Application Layer 30
Internet apps: application, transport protocols
Application
e-mailremote terminal access
Web file transfer
streaming multimedia
Internet telephony
Applicationlayer protocol
SMTP [RFC 2821]Telnet [RFC 854]HTTP [RFC 2616]FTP [RFC 959]proprietary(e.g. RealNetworks)proprietary(e.g., Dialpad)
Underlyingtransport protocol
TCPTCPTCPTCPTCP or UDP
typically UDP
2: Application Layer 31
FTP: the file transfer protocol
transfer file to/from remote host client/server model
Client side: the side that initiates transfer (either to/from remote)
Server side: remote host ftp: RFC 959 ftp server: port 21
file transfer FTPserver
FTPuser
interface
FTPclient
local filesystem
remote filesystem
user at host
2: Application Layer 32
FTP: separate control, data connections
FTP client contacts FTP server at port 21, specifying TCP as transport protocol
Client obtains authorization over control connection –- username, password
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.
FTPclient
FTPserver
TCP control connection
port 21
TCP data connectionport 20
Server opens a second TCP data connection to transfer another file.
Control connection: “out of band”
FTP server maintains “state”: current directory, earlier authentication
2: Application Layer 33
FTP commands, responses
Sample commands:sent as ASCII text over control
channel USER username PASS password LIST -- return list of file in
current directory RETR filename --
retrieves (gets) file STOR filename -- stores
(puts) file onto remote host
Sample return codesstatus 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 34
Electronic Mail
Three major components of a mail system:
user agents mail servers simple mail transfer
protocol: SMTP
User Agent Also known as “mail
reader” composing, editing, reading
mail messages e.g., Eudora, Outlook, elm,
Netscape Messenger outgoing, incoming
messages stored on server
user mailbox
outgoing message queue
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
2: Application Layer 35
Electronic Mail: mail servers
Mail Servers mailbox contains
incoming messages for user
message queue of outgoing (to be sent) mail messages
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
mailbox
message queue
user mailbox
outgoing message queue
2: Application Layer 36
Electronic Mail: SMTP [RFC 2821]
Simple Mail Transfer Protocol (SMTP) uses TCP to reliably transfer email message from
client to server, port 25 direct transfer: sending server to receiving 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 37
Scenario: Alice sends message to Bob1) Alice uses user agent to
compose message and “to” bob@someschool.edu
2) Alice’s user agent sends message to her mail server; message placed in message queue
3) Alice’s mail server (Client side) of SMTP opens TCP connection with Bob’s mail server (server side)
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
mailboxmessage queue
user mailbox
outgoing message queue
2: Application Layer 38
Sample SMTP interaction The following transcript begins as soon as the TCP connection is established: S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr> S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu> S: 250 bob@hamburger.edu ... 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
serverclient
2: Application Layer 39
Try SMTP interaction for yourself:
telnet servername 25 see 220 reply from server enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
commands above lets you send email without using email
client (reader)
2: Application Layer 40
SMTP: final words
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 protocol (client’s
point of view) SMTP: push protocol
both have ASCII command/response interaction, status codes
HTTP does not require message to be in 7-bit ASCII
HTTP: one object in one response message
SMTP: multiple objects can be sent in one message
2: Application Layer 41
Mail message format
SMTP: protocol for exchanging email messages
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
body
blankline
2: Application Layer 42
Message format: multimedia extensions
MIME (Multipurpose Internet Mail extensions) : multimedia mail extension, RFC 2045, 2056
additional lines in message header declare MIME content type
From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg
base64 encoded data ..... ......................... ......base64 encoded data
multimedia datatype, subtype,
parameter declaration
method usedto encode data
MIME version
encoded data
2: Application Layer 43
MIME typesContent-Type: type/subtype; parameters
Currently, seven types are defined:
(1) Text example subtypes: plain,
html
(2) Image example subtypes: jpeg,
gif
(3) Audio exampe subtypes: basic
(8-bit mu-law encoded), 32kadpcm (32 kbps Adaptive Differential Pulse Code Modulation coding)
(4) Video example subtypes: mpeg,
quicktime
(5)Application other data that must be
processed by reader before “viewable”
example subtypes: msword, octet-stream
(6) Multipart one or more different sets of data
are combined in a single body example subtypes: mixed,
alternative (alternative version of the same information)
(7) Message encapsulate another mail
message example subtypes: rfc822, partial
2: Application Layer 44
Example of 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 --StartOfNextPartDear Bob, Please find a picture of a crepe.--StartOfNextPartContent-Transfer-Encoding: base64Content-Type: image/jpegbase64 encoded data ..... ......................... ......base64 encoded data --StartOfNextPartDo you want the reciple?
2: Application Layer 45
Header line inserted by the receiving server
Received: from crepes.fr by hamburger.edu; 12 Oct 98 15:27:39 GMT
From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg
base64 encoded data ..... ......................... ......base64 encoded data
2: Application Layer 46
Mail access protocols
SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server
POP3: Post Office Protocol, version 3 [RFC 1939]• authorization (agent <--> server) and download
IMAP: Internet Mail Access Protocol [RFC 2060]• more features (more complex)• manipulation of stored messages on server
HTTP: Hotmail , Yahoo! Mail, etc.
useragent
sender’s mail server
useragent
SMTP SMTP Mail accessprotocol
receiver’s mail server
2: Application Layer 47
POP3 protocol(1) Client opens a TCP
connection to the mail server on port 110
(2) authorization phase client commands:
user: declare username pass: password
server responses +OK -ERR
(3) transaction phase, client: list: list message numbers retr: retrieve message by
number dele: delete Quit(4) update phase : mail server
deletes the messages marked for deletion
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
Message size
Message number
2: Application Layer 48
POP3 (more) and IMAP
More about POP3 Previous example uses
“download and delete” mode.
Bob cannot re-read e-mail if he changes client
“Download-and-keep” mode: 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
2: Application Layer 49
Web and HTTPFirst some jargon Web page consists of objects An object is a file such as an HTML file, a JPEG
image, a Java applet, an audio file,… A Web page consists of a base HTML-file and
several referenced objects The base HTML file references the other
objects in the page with the object’s URLs (Uniform Resource Locators)
Example URL:www.someschool.edu/someDept/pic.gif
host name path name
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HTTP overview
HTTP: hypertext transfer protocol
Web’s application layer protocol
client/server model client: browser that
requests, receives, “displays” Web objects
server: Web server sends objects in response to requests
HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2616
PC runningExplorer
Server running
Apache Webserver
Mac runningNavigator
HTTP request
HTTP request
HTTP response
HTTP response
2: Application Layer 51
HTTP overview (continued)
Uses TCP: client initiates TCP
connection (creates socket) to server, port 80
server accepts TCP connection from client
HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)
TCP connection closed
HTTP is “stateless” server maintains no
information about past client requests
Protocols that maintain “state” are complex!
past history (state) must be maintained
if server/client crashes, their views of “state” may be inconsistent, must be reconciled
aside
2: Application Layer 52
HTTP connectionsNonpersistent HTTP At most one object is
sent over a TCP connection.
HTTP/1.0 uses nonpersistent HTTP
Persistent HTTP Multiple objects can be
sent over single TCP connection between client and server.
A new connection need not be set up for the transfer of each Web object
HTTP/1.1 uses persistent connections in default mode – can be configured to use nonpersistent connection
2: Application Layer 53
Nonpersistent HTTPSuppose user enters URL www.someSchool.edu/someDepartment/home.index
1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80
2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index
1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client
3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket
time
(contains text, references to 10
jpeg images)
2: Application Layer 54
Nonpersistent HTTP (cont.)
5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 references to the 10 jpeg objects
6. Steps 1-5 repeated for each of 10 jpeg objects
4. HTTP server closes TCP connection.
time
2: Application Layer 55
Response time modeling
Definition of RRT: time to send a small packet to travel from client to server and back.
Response time: one RTT to initiate TCP
connection one RTT for HTTP request
and first few bytes of HTTP response to return
file transmission timetotal = 2RTT+transmit time
time to transmit file
initiate TCPconnection
RTT
requestfile
RTT
filereceived
time time
2: Application Layer 56
Persistent HTTP
Nonpersistent HTTP issues: requires 2 RTTs per object OS must work and allocate
host resources for each TCP connection
but browsers often open parallel TCP connections to fetch referenced objects
Persistent HTTP server leaves connection
open after sending response
subsequent HTTP messages between same client/server are sent over connection
Persistent without pipelining: client issues new request
only when previous response has been received
one RTT for each referenced object
Persistent with pipelining: default in HTTP/1.1 client sends requests as
soon as it encounters a referenced object
as little as one RTT for all the referenced objects
Two versions of persistent connections:
2: Application Layer 57
HTTP request message
two types of HTTP messages: request, response
HTTP request message: ASCII (human-readable format)
GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr
(extra carriage return, line feed)
request line(GET, POST,
HEAD commands)
header lines
Carriage return, line feed
indicates end of message
2: Application Layer 58
Explanation of the example
GET /somedir/page.html HTTP/1.1 -- Request to return the object /somedir/page.html -- The browser implements version HTTP/1.1
Host: www.someschool.edu -- Specifies the host on which the object resides
User-agent: Mozilla/4.0 -- Specifies the browser type that is making the request
Connection: close -- Indicates that the connection SHOULD NOT be considered
`persistent‘. It wants the server to close the connection after the current request/response is complete
Accept-language:fr -- Indicates that the user prefers to receive a French
version of the object
2: Application Layer 59
HTTP request message: general format
2: Application Layer 60
Method types
HTTP/1.0 GET : Return the object POST : Send
information to be stored on the server
HEAD Return only information
about the object, such as how old it is, but not the object itself
HTTP/1.1 GET, POST, HEAD PUT
Uploads a new copy of existing object in entity body to path specified in URL field
DELETE deletes object
specified in the URL field
2: Application Layer 61
Uploading form input
Post method: Web page often
includes form input Input is uploaded to
server in entity body
URL method: Uses GET method Input is uploaded in
URL field of request line:
www.somesite.com/animalsearch?monkeys&banana
2: Application Layer 62
HTTP response messageAn HTTP response consists of the following: 1. A status line, which indicates the success or failure of the request2. Header lines: A description of the information in the response. This
is the metadata or meta information3. The actual information requested
HTTP/1.1 200 OK Connection closeDate: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ...
status line(protocol status code status
phrase)
header lines
data, e.g., requestedHTML file
blank line
2: Application Layer 63
HTTP response status codes
200 OK request succeeded, requested object later in this
message
301 Moved Permanently requested object moved, new location specified later
in this message (Location:)
400 Bad Request request message not understood by server
404 Not Found requested document not found on this server
505 HTTP Version Not Supported
In first line in server -> client response message.A few sample codes:
2: Application Layer 64
Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server:
Opens TCP connection to port 80(default HTTP server port) at www.eurecom.fr.Anything typed is sent to port 80 at www.eurecom.fr
telnet www.eurecom.fr 80
2. Type in a GET HTTP request:
GET /~ross/index.html HTTP/1.0 By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to HTTP server
3. Look at response message sent by HTTP server!
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