Spring 2002CMSC 3321 Application Level Protocols Outline Simple Mail Transfer Protocol (SMTP)...

Spring 2002 CMSC 332 1

Application Level Protocols

OutlineSimple Mail Transfer Protocol (SMTP)HyperText Transport Protocol (HTTP)


Three Points• Application protocols are not application

programs– HTTP is a protocol used to fetch web pages, while

Netscape and Mosaic are programs that use HTTP• Both protocols examined here use request/reply

paradigm, yet they are not implemented on top of RPC (but instead on TCP)

• Both have a companion protocol that specifies the format of data that can be exchanged– RFC 822 and Multipurpose Internet Mail Extensions

(MIME) are companion to SMTP– HTML is companion that specifies form of web pages


RFC 822

• Defines header and body, both ASCII– Augmented by MIME to allow message body to carry

all sorts of data, but data still represented as ASCII text• Header

– Series of <CRLF> terminated lines (<CRLF> stands for carriage return + linefeed ASCII characters)

– Separated from body by blank line– Each header line has form <type>:<value>– Ex. To:, Subject:, Date:, From:


MIME

• RFC 822 extended in 1993 & 1996 to allow email to carry video, Word files, etc.

• Three basic pieces– Header lines augmenting RFC 822 – Set of content types and subtypes– Encoding schemes for various data types to allow

shipment as ASCII


MIME (cont.)

• Header extensions describe data being carried by message body– MIME-Version:– Content-Description: human readable

description of what’s in message– Content-Type: type of data– Content-Transfer-Encoding: how data is

encoded


MIME Content Types

• image/gif, image/jpeg• text/plain, text/rich: “marked up” text

(I.e. special fonts, etc)• application/

– application/postscript, etc• multipart: how a message carrying more than

one data type is structured– Like a programming language with base types and

structs, etc


MIME Encoding

• Ex. JPEG: Each byte can assume one of 256 values, not all of which are valid ASCII characters– Must use ASCII encoding because gateways can

corrupt non-ASCII portions– Solution is the base64 encoding scheme:

• Three bytes of binary data converted to four ASCII characters• 24 bits divided into four groups of 6 bits, each of which maps

to a valid ASCII character (so note that such an encoding only contains alphanumeric and + and – characters)

• Regular text can be encoded using 7-bit ASCII


ExampleMIME-Version: 1.0Content-Type: multipart/mixed; boundary=“-------417CA6E2DE4ABCAFBC5”From: Doug Szajda <[email protected]>To: [email protected]: grades for CMSC 332 class this semesterDate: Thurs, 11 April 2002 12:22:19 -0400

-------417CA6E2DE4ABCAFBC5Content-Type: text/plain; charset=us-asciiContent-Transfer-Encoding: 7 bit

Dear Ron,

I’ve decided to fail them all! It will save me the trouble ofhaving to grade their finals. I’ve also included the picture yourequested, and a copy of my latest paper.

Doug


Example-------417CA6E2DE4ABCAFBC5Content-Type: image/jpegContent-Transfer-Encoding: base64.. Unreadable encoding of a jpeg image.

-------417CA6E2DE4ABCAFBC5Content-Type: application/postscript; name=“draft.ps”Content-Transfer-Encoding: 7bit

.

. Readable encoding of a PostScript document

.


SMTP• SMTP is the protocol that handles the actual

transfer• The players:

– mail reader: program with which user interacts (I.e. Pine, Elm, Netscape Mail, etc.). Transfers messages to/from mail daemon

– mail daemon: process running on each host. Uses SMTP over TCP to transfer message to mail daemon running on other host. Though any code implementing SMTP could be used, typical daemon is descendent of original Berkeley implementation of sendmail


SMTP

• Players (cont.)– mail gateway: similar to an IP gateway (what we have

called a router), but willing to buffer messages and retry sending for days


Why Gateways?• I.e. why not just send message directly to

receiver’s host?• Receiver doesn’t want to include specific host on

which he/she reads email– May read mail on several hosts– Host on which mail is read may not be connected to

network• Solution: have mail delivered to a mail gateway at

destination (I.e. at umiacs.umd.edu)– Mail then forwarded (using another SMTP/TCP

connection) to user (after dbase lookup) OR– User fetches mail from server using something like

Post Office Protocol (POP3)


SMTP (cont.)

• Independent SMTP connection used between each mail gateway.

• SMTP sessions are dialog between two daemons, one acting as client, the other as server

• SMTP is ASCII based (makes sense since RFC 822 defines messages using this base representation)– Means a human at a keyboard can pretend to be an SMTP

client program


Example SMTP DialogueHELO richmond.edu250 Hello [email protected] [141.166.127.54]MAIL FROM:<[email protected]> 250 OKRCPT TO:<[email protected]> 250 OKDATA354 Start mail input; end with <CRLF>.<CRLF>Hi Doug,

I realize they whine a lot,and they probably deserve to fail, but they’ll be alums someday and that means money! And thanks for the picture.

Ron<CRLF>.<CRLF>250 OKQUIT221 Closing connection


SMTP (cont.)

• Client posts commands: HELO, MAIL, etc• Server responds with a numeric code along with human

readable explanation• Note the server verifies that name supplied in HELO

command corresponds to IP address being used for TCP connection

• Mail daemon parses message for info needed to run SMTP (info is called envelope for message). Daemon then uses this to parameterize SMTP exchange. sendmail popular because no on wanted to reimplement parsing function!


HTTP

• Assume you know and understand the following terms– Universal Resource Locator (URL)– Web browser– Hypertext link

• Basic Web mechanism– User “selects” page, browser (client) fetches page from

server using HTTP running over TCP


HTTP (cont.)

• Like SMTP, HTTP is text-oriented• HTTP message form

– START_LINE <CRLF> :request or response– MESSAGE_HEADER <CRLF> :as in email messages– <CRLF> :blank line that terminates header– MESSAGE_BODY <CRLF> :typically empty for

requests


HTTP Request Messages• START_LINE specifies:

– Operation to be performed (see next slide)• GET: retrieve and display a web page• HEAD: test validity of a hypertext link or check if page has been

modified since last fetched

– Web page operation should be performed on– Version of HTTP being used– Ex. GET http:www.mathcs.richmond.edu/index.html HTTP/1.1

– Ex. GET index.html HTTP/1.1 Host: www.mathcs.richmond.edu


HTTP Request Operations

OPTIONS: request information about available optionsGET: retrieve document identified in URLHEAD: retrieve metainformation about document identified in URLPOST: give information (e.g. annotation) to serverPUT: store document under specified URLDELETE: delete specified URLTRACE: loopback request messageCONNECT: for use by proxies


HTTP Response Message

• START_LINE specifies:– Version of HTTP being used– Three digit code indicating success/failure of request– Text string giving reason for response– Ex. HTTP/1.1 202 Accepted– Ex. HTTP/1.1 404 Not Found


HTTP Response Message

• Possible header lines:– Location: requested URL available at another location– Content-length– Expires– Last-modified: time at which contents last modified at

server• HTTP/1.1 301 Moved Permanently Location:http://www.mathcs.richmond.edu/cs/index.html


HTTP Result CodesCode Type Example Reasons

1xx Informational Request received, continuing process

2xx Success Action successfully received, understood, and accepted

3xx Redirection Further action must be taken to complete the request

4xx Client error Request contains bad syntax or cannot be filled

5xx Server error Server failed to fulfill an apparently valid request


HTTP and TCP

• HTTP Version 1.0– Separate TCP connection established for each data item

retrieved from server• Connection setup and teardown required even if just checking

on page status• Text plus a dozen icons requires more than 13 separate

connections being established and closed

– Needless to say, efficiency concerns


HTTP and TCP• HTTP Version 1.1 (latest)

– Persistent connections: client and server can exchange multiple request/response messages over same TCP connection

• Eliminate connection setup/teardown, reducing load on server, load on network, and delay perceived by user

• TCP congestion window operates more efficiently, because it is not necessary to go through slow start phase for each page

– The cost (there always is one): Neither client nor server knows how long to keep connection open.

• Especially problematic for server• Both sides must watch for signal to close their side of

connection


Caching Web Pages• Active area of research (and entrepreneurship)• Advantages:

– Pages can be displayed faster is fetched from closer server– Reduced server load since less requests to handle

• Where to cache?– User’s browser– Sitewide cache (user browsers configured to check this proxy cache

first)– ISPs: router snoops packets on way in and way out

• Various HTTP design features support this– Server assigns expiration date to each page it sends to client– Various cache directives (can a document be caches, how long, etc)


Simple Network Management Protocol (SNMP)

• As sysadmin, might want to know about state info at any of dozens of routers, hundreds of hosts– Address translation tables– Routing tables– TCP connection state– Number of IP datagram reassemblies aborted (does

timeout that garbage collects IP fragments need to be tuned?)

– Loads on various nodes


SNMP

• Allows reading and writing of various pieces of state information on different nodes

• Specialized request/response protocol (that runs over UDP) supporting – GET– SET– GET-NEXT (explained below)

• Typically, sysadmin uses browser like interface, which turns requests into SNMP operations, which are in turn fulfilled by SNMP server running on host in question


Management Information Base (MIB)

• Two questions– How does client indicate which piece of info it wants to

retrieve?– How does server know which variable in memory to

read to satisfy request?• MIB is a companion specification that defines

specific pieces of information (MIB variables) that can be retrieved from network node

• Current version: MIB-II


MIB-II• Variables organized in 10 groups

– System: general system (node) parameters (name, location, etc)

– Interfaces: address(es), how many packets sent/received– Address translation: ARP info, including ARP table– IP: routing table entries, how many packets forwarded,

stats about datagram reassembly, dropped packets, etc.– TCP: number of open connections, timeouts, default

timeout settings. Also per-connection info while connection exists

– UDP: total number of UDP datagrams sent/received– Also groups for Internet Control Message Protocol

(ICMP) , EGP, SNMP, “different media”


Two More Problems• Precise syntax used to state which MIB variables

wanted• Precise representation for values returned by

server• ASN.1/BER: Abstract Syntax Notation One/ Basic

Encoding Rules (Recall text Section 7.1)– Takes care of second problem– ASN.1/BER also defines an object identification

system (not described in text) and this is used to give globally unique ID to all MIB variables


Example

• 1.3.6.1.2.1.4.3 is unique ASN.1 identifier for IP related MIB variable ipInReceives– 1.3.6.1.2: prefix identifies MIB database (ASN.1 object

ID’s are for all possible objects in world)– 4: IP grou– 3 for third variable in this group


SNMP Summary

• SNMP client puts ASN.1 identifier for MIB variable into request message, sends this to server

• Server maps identifier into a local variable, retrieves value held in variable, uses ASN.1/BER to encode value it sends back to client

• GET-NEXT: Many MIB variables are structs or tables. GET-NEXT, when applied to a variable ID, returns the value of variable plus the ID of the next variable (next table entry or field in struct). Thus helps walk through data structures.

Spring 2002CMSC 3321 Application Level Protocols Outline Simple Mail Transfer Protocol (SMTP)...

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