7
THE APPLICATION LAYER
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 222222222222222222222222222222222222222222222222222222222222222 1 1 1 1222222222222222222222222222222222222222222222222222222222222222Adversary1 Goal 1 1 Student1 To have fun snooping on people’s email 1 12222222222222222222222222222222222222222222222222222222222222221 1 2222222222222222222222222222222222222222222222222222222222222221 Hacker1 To test out someone’s security system; steal data 1 1 1 1 2222222222222222222222222222222222222222222222222222222222222221 Sales rep1 To claim to represent all of Europe, not just Andorra 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222Businessman1 To discover a competitor’s strategic marketing plan 1 1 Ex-employee1 To get revenge for being fired 1 12222222222222222222222222222222222222222222222222222222222222221 1 2222222222222222222222222222222222222222222222222222222222222221 Accountant1 To embezzle money from a company 1 1 1 1 2222222222222222222222222222222222222222222222222222222222222221 Stockbroker1 To deny a promise made to a customer by email 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222Con man1 To steal credit card numbers for sale 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222Spy1 To learn an enemy’s military strength 1 2222222222222222222222222222222222222222222222222222222222222221 Terrorist1 To steal germ warfare secrets 1
Fig. 7-1. Some people who cause security problems and why.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Passive Active Intruder intruder intruder just can alter listens messages
Encryption Decryption Plaintext, P method method Plaintext
Ciphertext, C = Ek(P) Encryption Decryption key, k key
Fig. 7-2. The encryption model.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall MEGABUCK 74512836 pleasetr Plaintext ans f eron pleasetransferonemilliondollarsto emillion myswissbankaccountsixtwotwo do l l ar s t Ciphertext omyswi ss bankacco AFLLSKSOSELAWAIATOOSSCTCLNMOMANT ESILYNTWRNNTSOWDPAEDOBUOERIRICXB untsixtw o twoabcd
Fig. 7-3. A transposition cipher.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall P-box S-box Product cipher
S1 S5 S9
S2 S6 S10 P1 P2 P3 P4 S3 S7 S11 Encoder: 8 to 3 Decoder: 3 to 8 S4 S8 S12
(a) (b) (c)
Fig. 7-4. Basic elements of product ciphers. (a) P-box. (b) S-box. (c) Product.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 64 bit plaintext L i-1 Ri-1
Initial transposition
Iteration 1
Iteration 2 ⊕ 56 bit key Li-1 f(Ri-1, Ki)
Iteration 16
32 bit swap
Inverse transposition
64 bit ciphertext 32 bits 32 bits Li Ri (a) (b)
Fig. 7-5. The data encryption standard. (a) General outline. (b) Detail of one iteration.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Name Position Bonus
Adams, Les l i e Cl er k $ 10
Bl ack , Rob i n Boss $500 , 000
Co l l i ns , Kim Manager $100 , 000
Davis, Bobbie Janitor $ 5 Bytes 16 8 8
Fig. 7-6. The plaintext of a file encrypted as 16 DES blocks.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall P0 P1 P2 P3 C0 C1 C2 C3
IV # # # # Key D D D D Encryption Decryption box box Key E E E E IV # # # #
Exclusive OR C0 C1 C2 C3 P0 P1 P2 P3
(a) (b)
Fig. 7-7. Cipher block chaining
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 64-bit shift register 64-bit shift register
C2 C3 C4 C5 C6 C7 C8 C9 C2 C3 C4 C5 C6 C7 C8 C9
Encryption Encryption Key E Key E box box C10 C10
Select Select leftmost byte leftmost byte
P10 # C10 C10 # P10 Exclusive OR
(a) (b)
Fig. 7-8. Cipher feedback mode.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall K1 K2 K1 K1 K2 K1
PCE D E CPD E D
(a) (b)
Fig. 7-9. Triple encryption using DES.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 64 bit plaintext Four 16-bit input blocks
Iteration 1 K1 × K2 ++K3 K4 ×
Iteration 2 ##
K5 × +
× Iteration 7 + K6
# ### Iteration 8
Transformation Four 16-bit output blocks
+ 16-Bit addition modulo 216 64 bit ciphertext × 16-Bit multiplication modulo 216 + 1
# 16-Bit EXCLUSIVE OR
(a) (b)
Fig. 7-10. (a) IDEA. (b) Detail of one iteration.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Plaintext (P) Ciphertext (C) After decryption
Symbolic Numeric P3 P3 (mod 33)C7 C7 (mod 33) Symbolic S 19 6859 28 13492928512 19 S U 21 9261 21 1801088541 21 U Z 26 17576 20 1280000000 26 Z A 01 1 1 1 1 A N 14 2744 5 78125 14 N N 14 2744 5 78125 14 N E 05 125 26 8031810176 5 E
Sender's computation Receiver's computation
Fig. 7-11. An example of the RSA algorithm.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 1 A 2 RB 3 KAB (RB) Bob Alice 4 RA 5 KAB (RA)
Fig. 7-12. Two-way authentication using a challenge-response protocol.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall i.71.Asotndtowyatetcto protocol. authentication two-way shortened A 7-13. Fig.
Alice From: optrNetworks Computer r d yAde .Tnnam 96Petc Hall Prentice 1996 © Tanenbaum, S. Andrew by ed. 3rd , 2 3 1 R B, K AB A, R K AB (R (R A B ) A )
Bob i.71.Terfeto attack. reflection The 7-14. Fig.
Trudy From: optrNetworks Computer 2 4 5 3 1 R R B2, B, K r d yAde .Tnnam 96Petc Hall Prentice 1996 © Tanenbaum, S. Andrew by ed. 3rd , AB A, R A, R K K AB (R AB (R B T (R B ) T B ) )
Bob First session Second session First session Alice Bob picks x picks y
1 n, g, gx mod n
2 gy mod n Bob Alice Alice computes Bob computes (gy mod n)x (gx mod n)y = gxy mod n = gxy mod n
Fig. 7-15. The Diffie-Hellman key exchange.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Alice Trudy Bob picks x picks z picks y 1 n, g, gx mod n
2 n, g, gz mod n 3
z Bob Alice
g mod n Trudy
4 gy mod n
Fig. 7-16. The bucket brigade attack.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 1 A, KA (B, KS)
KDC 2 KB (A, KS) Bob Alice
Fig. 7-17. The wide-mouth frog authentication protocol.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 1 RA, A, B
2 KDC KA (RA, B, KS, KB(A, KS)) 3
K (A, K ), K (R ) Bob Alice B S S A2 4 KS (RA2 –1), RB 5 KS (RB –1)
Fig. 7-18. The Needham-Schroeder authentication protocol.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 1 A, B, R, KA (A, B, R, RA)
2 A, KA (A, B, R, RA), B, KB (A, B, R, RB) Bob Alice
KDC 3 4 KB(RB, KS) KA(RA, KS)
Fig. 7-19. The Otway-Rees authentication protocol (slightly sim- plified).
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 1 A Login
2 AS KA (KS, KTGS (A, KS))
3 KTGS (A, KS), B, KS (t) Get a 4 TGS Alice ticket KS (B, KAB), KB (A, KAB )
5 KB (A, KAB ), KAB (t) Do the
6 Bob work KAB (t +1)
Fig. 7-20. The operation of Kerberos V4.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 1 EB (A, RA) 2 EA (RA, RB, KS) Bob Alice 3 KS (RB)
Fig. 7-21. Mutual authentication using public-key cryptography.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall A, KA (B, RA, t, P)
K (A, R , t, P, K (A, t, P))
BB B A BB Bob Alice
Fig. 7-22. Digital signatures with Big Brother.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Transmission line Alice's computer Bob's computer
Alice's Bob's Bob's Alice's P private key, public key, private key, public key, P DA EB DB EA
DA(P) EB (DA(P)) DA(P)
Fig. 7-23. Digital signatures using public-key cryptography.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall P, DA (MD (P)) Bob Alice
Fig. 7-24. Digital signatures using message digests.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Generic Countries
int com edu gov mil org net jp us nl . . .
sun yaleacm ieee ac co oce vu
engcs eng jack jill keio nec cs
ai linda cs csl flits fluit
robot pc24
Fig. 7-25. A portion of the Internet domain name space.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 22222222222222222222222222222222222222222222222222222222222222222222 122222222222222222222222222222222222222222222222222222222222222222222Type1 Meaning1 Value 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222SOA1 Start of Authority1 Parameters for this zone 1 122222222222222222222222222222222222222222222222222222222222222222222A1 IP address of a host1 32-Bit integer 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222MX1 Mail exchange1 Priority, domain willing to accept email 1 122222222222222222222222222222222222222222222222222222222222222222222NS1 Name Server1 Name of a server for this domain 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222CNAME1 Canonical name1 Domain name 1 122222222222222222222222222222222222222222222222222222222222222222222PTR1 Pointer1 Alias for an IP address 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222HINFO1 Host description1 CPU and OS in ASCII 1 212222222222222222222222222222222222222222222222222222222222222222222TXT1 Text1 Uninterpreted ASCII text 1
Fig. 7-26. The principal DNS resource record types.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall ; Authoritative data for cs.vu.nl cs.vu.nl. 86400 IN SOA star boss (952771,7200,7200,2419200,86400) cs.vu.nl. 86400 IN TXT "Faculteit Wiskunde en Informatica." cs.vu.nl. 86400 IN TXT "Vrije Universiteit Amsterdam." cs.vu.nl. 86400 IN MX 1 zephyr.cs.vu.nl. cs.vu.nl. 86400 IN MX 2 top.cs.vu.nl. flits.cs.vu.nl. 86400 IN HINFO Sun Unix flits.cs.vu.nl. 86400 IN A 130.37.16.112 flits.cs.vu.nl. 86400 IN A 192.31.231.165 flits.cs.vu.nl. 86400 IN MX 1 flits.cs.vu.nl. flits.cs.vu.nl. 86400 IN MX 2 zephyr.cs.vu.nl. flits.cs.vu.nl. 86400 IN MX 3 top.cs.vu.nl. www.cs.vu.nl.86400 IN CNAME star.cs.vu.nl ftp.cs.vu.nl. 86400 IN CNAME zephyr.cs.vu.nl rowboat IN A 130.37.56.201 IN MX 1 rowboat IN MX 2 zephyr IN HINFO Sun Unix little-sister IN A 130.37.62.23 IN HINFO Mac MacOS laserjet IN A 192.31.231.216 IN HINFO "HP Laserjet IIISi" Proprietary
Fig. 7-27. A portion of a possible DNS database for cs.vu.nl
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Generic Countries
int com edu gov mil org net jp us nl . . .
sun yale acm ieee ac co oce vu
eng cs eng jack jill keio nec cs
ai linda cs csl flits fluit
robot pc24
Fig. 7-28. Part of the DNS name space showing the division into zones.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall VU CS Edu Yale Yale CS Originator name server name server name server name server 1 2 3 4 flits.cs.vu.nl cs.vu.nl edu-server.net yale.edu cs.yale.edu 8 7 6 5
Fig. 7-29. How a resolver looks up a remote name in eight steps.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Management Management process Station Host Router
Printer
Managed node A B A
SNMP protocol Agent A LAN
Bridge
Fig. 7-30. Components of the SNMP management model.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222Primitive type1 Meaning1 Code 1 1 INTEGER1 Arbitrary length integer1 2 1 12222222222222222222222222222222222222222222222222222222222222221 1 1 2222222222222222222222222222222222222222222222222222222222222221 BIT STRING1 A string of 0 or more bits1 3 1 1 1 1 1 2222222222222222222222222222222222222222222222222222222222222221 OCTET STRING1 A string of 0 of more unsigned bytes1 4 1 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222NULL1 A place holder1 5 1 1 1 1 1 2222222222222222222222222222222222222222222222222222222222222221 OBJECT IDENTIFIER1 An officially defined data type1 6 1
Fig. 7-31. The ASN.1 primitive data types permitted in SNMP.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall ccitt (0)iso (1) joint-iso-ccitt (2)
standard (0) registration- member- identified- authority (1) body (2) organization (3)
dod (6)
internet (1)
directory (1)mgmt (2) experimental (3)private (4) security (5) snmpv2 (6)
mib-2 (1)
system (1)interface (2) ip (4) icmp (5) tcp (6) udp (7) egp (8) transmission (10) sample (11)
Fig. 7-32. Part of the ASN.1 object naming tree.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Bits 21 5
Tag Number
00 Universal 0 Primitive type 01 Application 1 Constructed type 10 Context specific 11 Private
Fig. 7-33. The first byte of each data item sent in the ASN.1 transfer syntax.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Tag Tag type Number
Length Value Integer 49 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 1 1 0 0 0 1
Bit String '110' 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 1 1 1 0 0 0 0 0 0
Octet String 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 1 "xy"
NULL 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0
Internet object 0 0 0 0 0 1 1 0 0 0 0 0 0 0 1 1 0 0 1 0 1 0 1 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 1
Gauge 32 14 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 1 0
Fig. 7-34. ASN.1 encoding of some example values.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 22222222222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222Name1 Type1 Bytes1 Meaning 1 1 INTEGER1 Numeric1 41 Integer (32 bits in current implementations) 1 1222222222222222222222222222222222222222222222222222222222222222222222222222221 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222Counter321 Numeric1 41 Unsigned 32-bit counter that wraps 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222Gauge321 Numeric1 41 Unsigned value that does not wrap 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222Integer321 Numeric1 41 32 Bits, even on a 64-bit CPU 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222UInteger321 Numeric1 41 Like Integer32, but unsigned 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222Counter641 Numeric1 81 A 64-bit counter 1 1 TimeTicks1 Numeric1 41 In hundredths of a second since some epoch 1 1222222222222222222222222222222222222222222222222222222222222222222222222222221 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222BIT STRING1 String1 41 Bit map of 1 to 32 bits 1 1 1 1 1 1 21 2222222222222222222222222222222222222222222222222222222222222222222222222222OCTET STRING1 String 1 ≥ 01 Variable length byte string 1 1 1 1 1 1 21 2222222222222222222222222222222222222222222222222222222222222222222222222222Opaque1 String 1 ≥ 01 Obsolete; for backward compatibility only 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222OBJECT ID1 String1 >01 A list of integers from Fig. 7-0 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222IpAddress1 String1 41 A dotted decimal Internet address 1 1 1 1 1 1 21 2222222222222222222222222222222222222222222222222222222222222222222222222222NsapAddress1 String1 < 221 An OSI NSAP address 1
Fig. 7-35. Data types used for SNMP monitored variables.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall lostPackets OBJECT TYPE SYNTAX Counter32-- use a 32-bit counter MAX-ACCESS read-only-- the management station may not change it STATUS current -- this variable is not obsolete (yet) DESCRIPTION "The number of packets lost since the last boot" ::= {experimental 20}
Fig. 7-36. An example SNMP variable.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 22222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222Group1 # Objects1 Description 1 1 System1 71 Name, location, and description of the equipment 1 1222222222222222222222222222222222222222222222222222222222222222222222221 1 1 21 2222222222222222222222222222222222222222222222222222222222222222222222Interfaces1 231 Network interfaces and their measured traffic 1 1 1 1 1 21 2222222222222222222222222222222222222222222222222222222222222222222222AT1 31 Address translation (deprecated) 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222IP1 421 IP packet statistics 1 1 ICMP1 261 Statistics about ICMP messages received 1 1222222222222222222222222222222222222222222222222222222222222222222222221 1 1 21 2222222222222222222222222222222222222222222222222222222222222222222222TCP1 191 TCP algorithms, parameters, and statistics 1 1 1 1 1 21 2222222222222222222222222222222222222222222222222222222222222222222222UDP1 61 UDP traffic statistics 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222EGP1 201 Exterior gateway protocol traffic statistics 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222Transmission1 01 Reserved for media-specific MIBs 1 21 2222222222222222222222222222222222222222222222222222222222222222222222SNMP1 291 SNMP traffic statistics 1
Fig. 7-37. The object groups of the Internet MIB-II.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 22222222222222222222222222222222222222222222222222222222222222222222 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222Message1 Description 1 1 Get-request1 Requests the value of one or more variables 1 1222222222222222222222222222222222222222222222222222222222222222222221 1 222222222222222222222222222222222222222222222222222222222222222222221 Get-next-request1 Requests the variable following this one 1 1 1 1 222222222222222222222222222222222222222222222222222222222222222222221 Get-bulk-request1 Fetches a large table 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222Set-request1 Updates one or more variables 1 1 Inform-request1 Manager-to-manager message describing local MIB 1 1222222222222222222222222222222222222222222222222222222222222222222221 1 222222222222222222222222222222222222222222222222222222222222222222221 SnmpV2-trap1 Agent-to-manager trap report 1
Fig. 7-38. SNMP message types.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Name: Mr. Daniel Dumkopf 32¢ Street: 18 Willow Lane State: NY Mr. Daniel Dumkopf Envelope Zip code: 10604 18 Willow Lane Priority Urgent
White Plains, NY 10604 Envelope Encryption: None
From: United Gizmo United Gizmo Address: 180 Main St. 180 Main St Location: Boston, MA 02120 Boston, MA 02120 Date: Sept. 1, 1996 Sept. 1, 1996 Subject: Invoice 1081
Subject: Invoice 1081 Dear Mr. Dumkopf, Dear Mr. Dumkopf, Our computer records Our computer records show that you still have Message show that you still have not paid the above invoice not paid the above invoice of $0.00. Please send us a of $0.00. Please send us a check for $0.00 promptly.
check for $0.00 promptly. Body Header
Yours truly Yours truly United Gizmo United Gizmo
(a) (b)
Fig. 7-39. Envelopes and messages. (a) Postal email. (b) Elec- tronic email.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 22222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222# 1 Flags 1 Bytes 1 Sender 1 Subject 1 1 1 K 1030 asw Changes to MINIX 1 1222222222222222222222222222222222222222222222222222222222222222222222221 222222222222222222222222222222222222222222222222222222222222222222222221 2 KA 6348 radia Comments on material you sent me 1 1 1 222222222222222222222222222222222222222222222222222222222222222222222221 3 K F 4519 Amy N. Wong Request for information 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222222224 1236 bal Deadline for grant proposal 1 1 5 103610 kaashoek Text of DCS paper 1 1222222222222222222222222222222222222222222222222222222222222222222222221 222222222222222222222222222222222222222222222222222222222222222222222221 6 1223 emily E. Pointer to WWW page 1 1 1 222222222222222222222222222222222222222222222222222222222222222222222221 7 3110 saniya Referee reports for the paper 1 1 1 222222222222222222222222222222222222222222222222222222222222222222222221 8 1204 dmr Re: My student’s visit 1
Fig. 7-40. An example display of the contents of a mailbox.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 2222222222222222222222222222222222222222222222222222222222222222222221 Command1 Parameter1 Description 1 1 h1 #1 Display header(s) on the screen 1 1 1 1 1 1 c1 1 Display current header only 1 2222222222222222222222222222222222222222222222222222222222222222222221 t1 #1 Type message(s) on the screen 1 1 1 1 1 1 s1 address1 Send a message 1 1 f1 #1 Forward message(s) 1 1 1 1 1 2222222222222222222222222222222222222222222222222222222222222222222221 a1 #1 Answer message(s) 1 1 d1 #1 Delete message(s) 1 1 1 1 1 1 u1 #1 Undelete previously deleted message(s) 1 1 m1 #1 Move message(s) to another mailbox 1 1 1 1 1 1 k1 #1 Keep message(s) after exiting 1 2222222222222222222222222222222222222222222222222222222222222222222221 r1 mailbox1 Read a new mailbox 1 1 1 1 1 1 n1 1 Go to the next message and display it 1 1 b1 1 Backup to the previous message and display it 1 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222222g1 #1 Go to a specific message but do not display it 1 2222222222222222222222222222222222222222222222222222222222222222222221 e1 1 Exit the mail system and update the mailbox 1
Fig. 7-41. Typical mail handling commands.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 2222222222222222222222222222222222222222222222222222222222222 1 1 1 12222222222222222222222222222222222222222222222222222222222222Header1 Meaning 1 1 To:1 Email address(es) of primary recipient(s) 1 122222222222222222222222222222222222222222222222222222222222221 1 21 222222222222222222222222222222222222222222222222222222222222Cc:1 Email address(es) of secondary recipient(s) 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222Bcc:1 Email address(es) for blind carbon copies 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222From:1 Person or people who created the message 1 1 Sender:1 Email address of the actual sender 1 122222222222222222222222222222222222222222222222222222222222221 1 21 222222222222222222222222222222222222222222222222222222222222Received:1 Line added by each transfer agent along the route 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222Return-Path:1 Can be used to identify a path back to the sender 1
Fig. 7-42. RFC 822 header fields related to message transport.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 2222222222222222222222222222222222222222222222222222222222222222 1 1 1 12222222222222222222222222222222222222222222222222222222222222222Header1 Meaning 1 1 Date:1 The date and time the message was sent 1 122222222222222222222222222222222222222222222222222222222222222221 1 22222222222222222222222222222222222222222222222222222222222222221 Reply-To:1 Email address to which replies should be sent 1 1 1 1 22222222222222222222222222222222222222222222222222222222222222221 Message-Id:1 Unique number for referencing this message later 1 1 1 1 22222222222222222222222222222222222222222222222222222222222222221 In-Reply-To:1 Message-Id of the message to which this is a reply 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222References:1 Other relevant Message-Ids 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222Keywords:1 User chosen keywords 1 1 1 1 22222222222222222222222222222222222222222222222222222222222222221 Subject:1 Short summary of the message for the one-line display1
Fig. 7-43. Some fields used in the RFC 822 message header.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 222222222222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222222222222222Header1 Meaning 1 1 MIME-Version:1 Identifies the MIME version 1 12222222222222222222222222222222222222222222222222222222222222222222222222222221 1 21 22222222222222222222222222222222222222222222222222222222222222222222222222222Content-Description:1 Human-readable string telling what is in the message 1 1 1 1 21 22222222222222222222222222222222222222222222222222222222222222222222222222222Content-Id:1 Unique identifier 1 1 1 1 21 22222222222222222222222222222222222222222222222222222222222222222222222222222Content-Transfer-Encoding:1 How the body is wrapped for transmission 1 1 1 1 21 22222222222222222222222222222222222222222222222222222222222222222222222222222Content-Type:1 Nature of the message 1
Fig. 7-44. RFC 822 headers added by MIME.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 22222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222Type1 Subtype1 Description 1 1 1 Plain 1 Unformatted text 1 1 1222222222222222222222222222222222222222222222222222222221 Text 1 222222222222222222222222222222222222222222222222222222222222222222221 1 Richtext Text including simple formatting commands 1 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222Gif 1 Still picture in GIF format 1 1 Image 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222221 Jpeg1 Still picture in JPEG format 1 1 Audio1 Basic1 Audible sound 1 1222222222222222222222222222222222222222222222222222222222222222222221 1 1 222222222222222222222222222222222222222222222222222222222222222222221 Video1 Mpeg1 Movie in MPEG format 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222Octet-stream 1 An uninterpreted byte sequence 1 1 Application 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222221 Postscript1 A printable document in PostScript 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222Rfc822 A MIME RFC 822 message 1 1 1 Message 122222222222222222222222222222222222222222222222222222222Partial 1 Message has been split for transmission 1 1 1 1 1 222222222222222222222222222222222222222222222222222222222222222222221 1 External-body1 Message itself must be fetched over the net 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222Mixed 1 Independent parts in the specified order 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222Alternative 1 Same message in different formats 1 Multipart 1 1 1 Parallel 1 Parts must be viewed simultaneously 1 1 1222222222222222222222222222222222222222222222222222222221 1 222222222222222222222222222222222222222222222222222222222222222222221 1 Digest1 Each part is a complete RFC 822 message 1
Fig. 7-45. The MIME types and subtypes defined in RFC 1521.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall From: [email protected] To: [email protected] MIME-Version: 1.0 Message-Id: <[email protected]> Content-Type: multipart/alternative; boundary=qwertyuiopasdfghjklzxcvbnm Subject: Earth orbits sun integral number of times
This is the preamble. The user agent ignores it. Have a nice day.
--qwertyuiopasdfghjklzxcvbnm Content-Type: text/richtext
Happy birthday to you Happy birthday to you Happy birthday dear
--qwertyuiopasdfghjklzxcvbnm Content-Type: message/external-body; access-type="anon-ftp"; site="bicycle.abc.com"; directory="pub"; name="birthday.snd" content-type: audio/basic content-transfer-encoding: base64 --qwertyuiopasdfghjklzxcvbnm--
Fig. 7-46. A multipart message containing richtext and audio alter- natives.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall S: 220 xyz.com SMTP service ready C: HELO abc.com S: 250 xyz.com says hello to abc.com C: MAIL FROM:
Fig. 7-47. Transferring a message from [email protected] to [email protected]. Message buffer Host 1Gateway Host 2
1 2
12
TCP connection TP4 connection
Network
Fig. 7-48. Transferring email using an application layer email gateway.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall KM : One-time message key for IDEA Bob's public RSA key, EB : Concatenation
Alice's private KM RSA RSA key, DA ASCII text to P1 P1.Z Base the network P MD5 RSA Zip IDEA 64
P1 compressed
Original Concatenation of Concatenation of plaintext P and the signed P1.Z encrypted message hash of P with IDEA and KM from Alice encrypted with EB
Fig. 7-49. PGP in operation for sending a message.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Base64 Message Compressed, encrypted by IDEA key part Signature part Message part
T ID T ID T Sig. y MD5 Msg File of K i of p i Message M hdr m hash hdr name m EB EA e e s e Encrypted by EB DA
Fig. 7-50. A PGP message.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 2222222222222222222222222222222222222222222222222222222222222222222222222 22222222222222222222222222222222222222222222222222222222222222222222222221 Item1 PGP1 PEM 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222Supports encryption?1 Yes1 Yes 1 12222222222222222222222222222222222222222222222222222222222222222222222222Supports authentication?1 Yes1 Yes 1 22222222222222222222222222222222222222222222222222222222222222222222222221 Supports nonrepudiation?1 Yes1 Yes 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222Supports compression?1 Yes1 No 1 12222222222222222222222222222222222222222222222222222222222222222222222222Supports canonicalization?1 No1 Yes 1 22222222222222222222222222222222222222222222222222222222222222222222222221 Supports mailing lists?1 No1 Yes 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222Uses base64 coding?1 Yes1 Yes 1 12222222222222222222222222222222222222222222222222222222222222222222222222Current data encryption algorithm1 IDEA1 DES 1 22222222222222222222222222222222222222222222222222222222222222222222222221 Key length for data encryption (bits)1 1281 56 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222Current algorithm for key managementRSA1 1 RSA or DES 1 12222222222222222222222222222222222222222222222222222222222222222222222222Key length for key management (bits)1 384/512/10241 Variable 1 22222222222222222222222222222222222222222222222222222222222222222222222221 User name space1 User defined1 X.400 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222X.509 conformant?1 No1 Yes 1 12222222222222222222222222222222222222222222222222222222222222222222222222Do you have to trust anyone?1 No1 Yes (IPRA) 1 22222222222222222222222222222222222222222222222222222222222222222222222221 Key certification1 Ad hoc1 IPRA/PCA/CA hierarchy 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222Key revocation1 Haphazard1 Better 1 12222222222222222222222222222222222222222222222222222222222222222222222222Can eavesdroppers read messages?1 No1 No 1 22222222222222222222222222222222222222222222222222222222222222222222222221 Can eavesdroppers read signatures?1 No1 Yes 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222Internet Standard?1 No1 Yes 1 22222222222222222222222222222222222222222222222222222222222222222222222221 Designed by1 Small team1 Standards committee 1
Fig. 7-51. A comparison of PGP and PEM.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 222222222222222222222222222222222222222222222222222222222222222222 2122222222222222222222222222222222222222222222222222222222222222222Name1 Topics covered 1 1 1 1 2122222222222222222222222222222222222222222222222222222222222222222Comp1 Computers, computer science, and the computer industry 1 2122222222222222222222222222222222222222222222222222222222222222222Sci1 The physical sciences and engineering 1 1 1 1 2122222222222222222222222222222222222222222222222222222222222222222Humanities1 Literature and the humanities 1 2122222222222222222222222222222222222222222222222222222222222222222News1 Discussion of USENET itself 1 1 1 1 2122222222222222222222222222222222222222222222222222222222222222222Rec1 Recreational activities, including sports and music 1 2122222222222222222222222222222222222222222222222222222222222222222Misc1 Everything that does not fit in somewhere else 1 2122222222222222222222222222222222222222222222222222222222222222222Soc1 Socializing and social issues 1 1 1 1 2122222222222222222222222222222222222222222222222222222222222222222Talk1 Diatribes, polemics, debates and arguments galore 1 2122222222222222222222222222222222222222222222222222222222222222222Alt1 Alternative tree covering virtually everything 1
Fig. 7-52. USENET hierarchies in order of decreasing signal-to- noise ratio.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 222222222222222222222222222222222222222222222222222222222222222222222222222222222 1222222222222222222222222222222222222222222222222222222222222222222222222222222222Name1 Topics covered 1 1 1 1 1 Comp.ai1 Artificial intelligence 1 1 Comp.databases1 Design and implementation of database systems 1 1 Comp.lang.c1 The C programming language 1 1 Comp.os.minix1 Tanenbaum’s educational MINIX operating system 1 1222222222222222222222222222222222222222222222222222222222222222222222222222222222Comp.os.ms-windows.video1 Video hardware and software for Windows 1 1 1 1 1 Sci.bio.entomology.lepidoptera1 Research on butterflies and moths 1 1 Sci.geo.earthquakes1 Geology, seismology, and earthquakes 1 1222222222222222222222222222222222222222222222222222222222222222222222222222222222Sci.med.orthopedics1 Orthopedic surgery 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222222222222222222Humanities.lit.authors.shakespeare1 Shakespeare’s plays and poetry 1 1 News.groups1 Potential new newsgroups 1 2222222222222222222222222222222222222222222222222222222222222222222222222222222221 News.lists1 Lists relating to USENET 1 1 Rec.arts.poems1 Free poetry 1 1 1 1 1 Rec.food.chocolate1 Yum yum 1 1 Rec.humor.funny1 Did you hear the joke about the farmer who ... 1 2222222222222222222222222222222222222222222222222222222222222222222222222222222221 Rec.music.folk1 Folks discussing folk music 1 1 Misc.jobs.offered1 Announcements of positions available 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222222222222222222Misc.health.diabetes1 Day-to-day living with diabetes 1 1 Soc.culture.estonia1 Life and culture in Estonia 1 1 Soc.singles1 Single people and their interests 1 2222222222222222222222222222222222222222222222222222222222222222222222222222222221 Soc.couples1 Graduates of soc.singles 1 1 1 1 1 Talk.abortion1 No signal, all noise 1 1222222222222222222222222222222222222222222222222222222222222222222222222222222222Talk.rumors1 This is where rumors come from 1 1 Alt.alien.visitors1 Place to report flying saucer rides 1 1 Alt.bermuda.triangle1 If you read this, you vanish mysteriously 1 1 1 1 1 Alt.sex.voyeurism1 Take a peek and see for yourself 1 2222222222222222222222222222222222222222222222222222222222222222222222222222222221 Alt.tv.simpsons1 Bart et al. 1
Fig. 7-53. A small selection of the newsgroups.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 22222222222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222Smiley1 Meaning1 Smiley1 Meaning1 Smiley1 Meaning 1 1 :-) 1 I’m happy 1 =|:-) 1 Abe Lincoln 1 :+) 1 Big nose 1 1222222222222222222222222222222222222222222222222222222222222222222222222222221 1 1 1 1 1 212222222222222222222222222222222222222222222222222222222222222222222222222222:-( 1 I’m sad/angry 1 =):-) 1 Uncle Sam 1 :-)) 1 Double chin 1 1 1 1 1 1 1 1 212222222222222222222222222222222222222222222222222222222222222222222222222222:-| 1 I’m apathetic 1 *<:-) 1 Santa Claus 1 :-{) 1 Mustache 1 1 1 1 1 1 1 1 212222222222222222222222222222222222222222222222222222222222222222222222222222;-) 1 I’m winking 1 <:-( 1 Dunce 1 #:-) 1 Matted hair 1 1 1 1 1 1 1 1 122222222222222222222222222222222222222222222222222222222222222222222222222222:-(O) 1 I’m yelling 1 (-: 1 Australian 1 8-) 1 Wears glasses 1 1 1 1 1 1 1 1 212222222222222222222222222222222222222222222222222222222222222222222222222222:-(*) 1 I’m vomiting 1 :-)X 1 Man with bowtie 1 C:-) 1 Large brain 1
Fig. 7-54. Some smileys.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall From: [email protected] Message-Id: <[email protected]> Subject: Bird Sighting Path: cs.vu.nl!sun4nl!EU.net!news.sprintlink.net!in2.uu.net!pc144.nyu.edu!news Newsgroups: rec.birds Followup-To: rec.birds Distribution: world Nntp-Posting-host: nuthatch.bio.nyu.edu References: Organization: New York University Lines: 4 Summary: Guess what I saw
I just saw an ostrich on 52nd St. and Fifth Ave. in New York. Is this their migration season? Did anybody else see it?
Jay Vogel
Fig. 7-55. A sample news article.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 2222222222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222222Command1 Meaning 1 1 LIST1 Give me a list of all newsgroups and articles you have 1 122222222222222222222222222222222222222222222222222222222222222222222222222221 1 21 222222222222222222222222222222222222222222222222222222222222222222222222222NEWGROUPS date time1 Give me a list of newsgroups created after date/time 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222222222222222222GROUP grp1 Give me a list of all articles in grp 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222222222222222222NEWNEWS grps date time1 Give me a list of new articles in specified groups 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222222ARTICLE id1 Give me a specific article 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222222POST1 I have an article for you that was posted here 1 1 IHAVE id1 I have article id. Do you want it? 1 122222222222222222222222222222222222222222222222222222222222222222222222222221 1 21 222222222222222222222222222222222222222222222222222222222222222222222222222QUIT1 Terminate the session 1
Fig. 7-56. The principal NNTP commands for news diffusion.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall S: 200 feeder.com NNTP server at your service (response to new connection) C: NEWNEWS soc.couples 960901 030000 (any new news in soc.couples?) S: 230 List of 2 articles follows S: <[email protected]> (article 1 of 2 in soc.couples is from Berkeley) S: <[email protected]> (article 2 of 2 in soc.couples is from AOL) S: . (end of list) C: ARTICLE <[email protected]> (please give me the Berkeley article) S: 220 <[email protected]> follows S: (entire article <[email protected]> is sent here) S: . (end of article) C: ARTICLE <[email protected]> (please give me the AOL article) S: 220 <[email protected]> follows S: (entire article <[email protected]> is sent here) S: . (end of article) C: NEWNEWS misc.kids 960901 030000 (any new news in misc.kids?) S: 230 List of 1 article follows S: <[email protected]> (1 article from Rice) S: . (end of list) C: ARTICLE <[email protected]> (please give me the Rice article) S: 220 <[email protected]> follows S: (entire article <[email protected]> is sent here) S: . (end of article) C: NEWGROUPS 960901 030000 S: 231 2 new groups follow S: rec.pets S: rec.nude S: . C: NEWNEWS rec.pets 0 0 (list everything you have) S: 230 List of 1 article follows S: <[email protected]> (1 article from fido.net) S: . (end of list) C: ARTICLE <[email protected]> (please give me the fido.net article) S: 220 <[email protected]> follows S: (entire article is sent here) S: . C: POST S: 340 (please send your posting) C: (article posted on wholesome.com sent here) S: 240 (article received) C: IHAVE <[email protected]> S: 435 (I already have it, please do not send it) C: QUIT S: 205 (Have a nice day)
Fig. 7-57. How wholesome.com might acquire news articles from its newsfeed.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 2222222222222222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 1 WELCOME TO THE UNIVERSITY OF EAST PODUNK’S WWW HOME PAGE 1 1 1 1 d Campus Information 1 1 5 Admissions22222222222222222222 information 1 1 5 Campus22222222222 map 1 1 5 Directions222222222222222222 to campus 1 1 5 2222222222222222222 1 1 The UEP student body 1 1 1 1 d Academic Departments 1 1 5 Department22222222222222222222222222222 of Animal Psychology 1 1 5 Department2222222222222222222222222222 of Alternative Studies 1 1 5 222222222222222222222222222222 1 1 Department of Microbiotic Cooking 1 1 5 Department2222222222222222222222222222222 of Nontraditional Studies 1 1 5 Department2222222222222222222222222222 of Traditional Studies 1 1 1 22222222222222222222222222222222222222222222222222222222222222222222222222222222221 [email protected] 1 (a) 22222222222222222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 1 THE DEPARTMENT OF ANIMAL PSYCHOLOGY 1 1 1 1 d Information2222222222222222222222222222222 for prospective majors 1 1 d Personnel 1 1 5 Faculty2222222222222222 members 1 1 5 Graduate22222222222222222 students 1 1 5 22222222222222222 1 1 Nonacademic staff 1 1 d Research22222222222222222 Projects 1 1 d Positions22222222222222222 available 1 1 d Our most popular courses 1 1 5 Dealing2222222222222222222222 with herbivores 1 1 5 222222222222222222 1 1 Horse management 1 1 5 Negotiating22222222222222222222222 with your pet 1 1 5 User-friendly222222222222222222222222222222222 doghouse construction 1 1 d Full22222222222222222 list of courses 1 1 1 212222222222222222222222222222222222222222222222222222222222222222222222222222222222Webmaster@animalpsyc.eastpodunk.edu 1 (b)
Fig. 7-58. (a) A Web page. (b) The page reached by clicking on Department22222222222222222222222222222 of Animal Psychology Server Server Client abc.com xyz.com Current page displayed by browser Hyperlink
to abc.com Hyperlink
Browser to xyz.com program
Disk HTTP Disk HTTP Server Server HTTP used over this TCP connection
The Internet
Fig. 7-59. The parts of the Web model.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall C: telnet www.w3.org 80 T: Trying 18.23.0.23 ... T: Connected to www.w3.org. T: Escape character is ’ˆ]’. C: GET /hypertext/WWW/TheProject.html HTTP/1.0 C: S: HTTP/1.0 200 Document follows S: MIME-Version: 1.0 S: Server: CERN/3.0 S: Content-Type: text/html S: Content-Length: 8247 S: S:
S: The World Wide Web Consortium
S: S: The World Wide Web is the universe of network-accessible information. S: The World Wide Web Consortium S: exists to realize the full potential of the Web.
S: S: W3C works with the global community to produce S: specifications and S: reference software . S: W3C is funded by industrial S: members S: but its products are freely available to all.
S: S: In this document: S:
S:
S:
W3C is hosted by the S: Laboratory for Computer Science at S: MIT , and S: in Europe by INRIA . S:
Fig. 7-60. A sample scenario for obtaining a Web page. FTP Request HTTP FTP Server Browser FTP Reply
HTTP HTTP Request FTP Request FTP FTP Proxy Server Browser HTTP Reply FTP Reply
Fig. 7-61. (a) A browser that speaks FTP. (b) A browser that does not.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 2222222222222222222222222222222222222222222222222222222222222 1 1 1 21 222222222222222222222222222222222222222222222222222222222222Method1 Description 1 1 GET1 Request to read a Web page 1 21 2222222222222222222222222222222222222222222222222222222222221 1 21 222222222222222222222222222222222222222222222222222222222222HEAD1 Request to read a Web page’s header 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222PUT1 Request to store a Web page 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222POST1 Append to a named resource (e.g., a Web page) 1 1 DELETE1 Remove the Web page 1 122222222222222222222222222222222222222222222222222222222222221 1 21 222222222222222222222222222222222222222222222222222222222222LINK1 Connects two existing resources 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222UNLINK1 Breaks an existing connection between two resources 1
Fig. 7-62. The built-in HTTP request methods.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222Name1 Used for1 Example 1 1 http1 Hypertext (HTML)1 http://www.cs.vu.nl/~ast/ 1 12222222222222222222222222222222222222222222222222222222222222222221 1 1 1222222222222222222222222222222222222222222222222222222222222222222ftp1 FTP1 ftp://ftp.cs.vu.nl/pub/minix/README 1 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222file1 Local file1 /usr/suzanne/prog.c 1 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222news1 News group1 news:comp.os.minix 1 1 news1 News article1 news:[email protected] 1 12222222222222222222222222222222222222222222222222222222222222222221 1 1 1222222222222222222222222222222222222222222222222222222222222222222gopher1 Gopher1 gopher://gopher.tc.umn.edu/11/Libraries 1 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222222222mailto1 Sending email1 mailto:[email protected] 1 1 1 1 1 21 22222222222222222222222222222222222222222222222222222222222222222telnet1 Remote login1 telnet://www.w3.org:80 1
Fig. 7-63. Some common URLs.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall
Welcome to AWI’s Home Page
We are so happy that you have chosen to visit Amalgamated Widget’s home page. We hope you will find all the information you need here.
Below we have links to information about our many fine products. You can order electronically (by WWW), by telephone, or by fax.
Product information
Telephone numbers
- By telephone: 1-800-WIDGETS
- By fax: 1-415-765-4321
Welcome to AWI's Home Page
We are so happy that you have chosen to visit Amalgamated Widget's home page. We hope you will find all the information you need here. Below we have links to information about our many fine products. You can order electronically (by WWW), by telephone, or by FAX.
Product Information Big widgets Little widgets
Telephone numbers 1-800-WIDGETS 1-415-765-4321 (b)
Fig. 7-64. (a) The HTML for a sample Web page. (b) The format- ted page. 2222222222222222222222222222222222222222222222222222222222222222222 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222Tag1 Description 1 1 ... 1 Declares the Web page to be written in HTML 1 122222222222222222222222222222222222222222222222222222222222222222221 1 12222222222222222222222222222222222222222222222222222222222222222222
... 1 Delimits the page’s head 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222- ...
- ...
1 Force a break here 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222
1 Start of paragraph 1 1
1 Horizontal rule 1 122222222222222222222222222222222222222222222222222222222222222222221 1 12222222222222222222222222222222222222222222222222222222222222222222
...1 Preformatted text; do not reformat 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222
Fig. 7-65. A selection of common HTML tags. Some have addi- tional parameters.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall
| Item | HTML 1.0 | HTML 2.0 | HTML 3.0 |
|---|---|---|---|
| Active Maps and Images | x | x | |
| Equations | x | ||
| Forms | x | x | |
| Hyperlinks x | x | x | |
| Images | x | x | x |
| Lists | x | x | x |
| Toolbars | x | ||
| Tables | x |
222222222222222222222222222222222222222222222222222222222222Some Differences between HTML Versions 2222222222222222222222222222222222222222222222222222222222221 Item1 HTML 1.01 HTML 2.01 HTML 3.0 1 1 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222Active Maps and Images1 1 x1 x 1 1222222222222222222222222222222222222222222222222222222222222Equations1 1 1 x 1 2222222222222222222222222222222222222222222222222222222222221 Forms1 1 x1 x 1 1 1 1 1 1 1222222222222222222222222222222222222222222222222222222222222Hyperlinks1 x1 x1 x 1 1222222222222222222222222222222222222222222222222222222222222Images1 x1 x1 x 1 2222222222222222222222222222222222222222222222222222222222221 Lists1 x1 x1 x 1 2222222222222222222222222222222222222222222222222222222222221 1 1 1 1 1 Toolbars1 1 1 x 1 2222222222222222222222222222222222222222222222222222222222221 Tables1 1 1 x 1
Fig. 7-66. (a) An HTML table. (b) A possible rendition of this table.
Widget Order Form
(a)Widget Order Form
Name
Street address
City State Country
Credit card # Expires M/C Visa
Widget size Big Little Ship by express courier
Submit order
Thank you for ordering an AWI widget, the best widget money can buy! (b)
Fig. 7-67. (a) The HTML for an order form. (b) The formatted page.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall tsatp c h rwe f()atrrnigfrawhile. a for running after (b) browser of Java-based browser A The (b) (c) browser. generation startup. first at A (a) 7-68. Fig. HTTP FTP Operating system HTML interpreter SMTP From: NNTP optrNetworks Computer (a) Gopher Telnet PostScript GIF JPEG r d yAde .Tnnam 96Petc Hall Prentice 1996 © Tanenbaum, S. Andrew by ed. 3rd , MPEG-1 (No protocolorimage interpreters builtin) Operating system Java interpreter (b)
HTML applet HTTP applet Operating system
Java interpreter GIF applet Audio applet
(c) MPEG-2 applet 2222222222222222222222222222222222222222222222222222222222 1 1 1 1 22222222222222222222222222222222222222222222222222222222221 Type1 Size1 Description 1 1 Byte1 1 Byte1 A signed integer between −128 and +127 1 122222222222222222222222222222222222222222222222222222222221 1 1 22222222222222222222222222222222222222222222222222222222221 Short1 2 Bytes1 A signed 2-byte integer 1 1 1 1 1 22222222222222222222222222222222222222222222222222222222221 Int1 4 Bytes1 A signed 4-byte integer 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222Long1 8 Bytes1 A signed 8-byte integer 1 1 Float1 4 Bytes1 A 4-byte IEEE floating-point number 1 122222222222222222222222222222222222222222222222222222222221 1 1 22222222222222222222222222222222222222222222222222222222221 Double1 8 Bytes1 An 8-byte IEEE floating-point number 1 1 1 1 1 22222222222222222222222222222222222222222222222222222222221 Boolean1 1 Bit1 The only values are true and false 1 1 1 1 1 22222222222222222222222222222222222222222222222222222222221 Char1 2 Bytes1 A character in Unicode 1
Fig. 7-69. The basic Java data types.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 2222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222222222222Statement1 Description1 Example 1 1 Assignment1 Assign a value1 n = i + j; 1 21 2222222222222222222222222222222222222222222222222222222222222222222221 1 1 21 222222222222222222222222222222222222222222222222222222222222222222222If1 Boolean choice1 if (k < 0) k = 0; else k = 2*k; 1 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222222222222Switch1 Select a case1 switch (b) {case 1: n++; case 2: n−−;} 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222For1 Iteration1 for (i = 0; i < n; i++) a[i] = b[i]; 1 1 While1 Repetition1 while (n < k) n += i; 1 122222222222222222222222222222222222222222222222222222222222222222222221 1 1 21 222222222222222222222222222222222222222222222222222222222222222222222Do1 Repetition1 do {n = n + n} while (n < m); 1 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222222222222Break1 Exit statement1 break label; 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222Return1 Return1 return n; 1 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222Continue1 Next iteration1 continue label; 1 21 222222222222222222222222222222222222222222222222222222222222222222222Throw1 Raise exception1 throw new IllegalArgumentException(); 1 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222222222222Try1 Exception scoping1 try { ... } catch (Exception e) {return −1}; 1 1 1 1 1 21 222222222222222222222222222222222222222222222222222222222222222222222Synchronized1 Mutual exclusion1 synchronized void update(int s) { ... } 1
Fig. 7-70. The Java statements. The notation { ... } indicates a block of code.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall class Factorial { /* This program consists of a single class with two methods. */
public static void main (int argc, String args[]) {// main program long i, f, lower = 1, upper = 20; // declarations of four longs
for (i = lower; i <= upper; i++) { // loop from lower to upper f = factorial(i); // f = i! System.out.println(i + " " + f); // print i and f } }
static long factorial (long k) { // recursive factorial function if (k == 0) return 1; // 0! = 1 else return k * factorial(k-1); // k! = k * (k-1)! } }
Fig. 7-71. A Java program for computing and printing 0! to 20!.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall class ComplexNumber { // Define a subclass of Object called ComplexNumber // Hidden data. protected double re, im; // real and imaginary parts
// Five methods that manage the hidden data. public void Complex(double x, double y) {re = x; im = y;} public double Real() {return re;} public double Imaginary() {return im;} public double Magnitude() {return Math.sqrt(re*re + im*im);} public double Angle() {return Math.atan(im/re);} } class test { // A second class, for testing ComplexNumber public static void main (String args[]) { ComplexNumber c; // declare an object of class ComplexNumber
c = new ComplexNumber(); // actually allocate storage for c c.Complex(3.0, 4.0); // invoke the Complex method to initialize c System.out.println("The magnitude of c is " + c.Magnitude() ); } }
Fig. 7-72. A package defining two classes.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall import ComplexNumber; // import the ComplexNumber package class HairyNumber extends ComplexNumber {// define a new class public void AddTo(ComplexNumber z) {// with one method re = re + z.Real(); im = im + z.Imaginary(); } } class test2 { // test program for HairyNumber public static void main(String args[]) { HairyNumber a, h; // declare two HairyNumbers
a = new HairyNumber();// allocate storage for a h = new HairyNumber();// allocate storage for h a.Complex(1.0, 2.0); // assign a value to a h.Complex(-1.5, 4.0); // assign a value to h h.AddTo(a); // invoke the AddTo method on h System.out.println("h = (" + h.Real() + "," + h.Imaginary() + ")" ); } }
Fig. 7-73. A subclass of ComplexNumber defining a new method.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 2222222222222222222222222222222222222222222222222222222222222222222222222 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222Package1 Example functionality 1 1 Java.lang1 Classes, threads, exceptions, math, strings 1 122222222222222222222222222222222222222222222222222222222222222222222222221 1 22222222222222222222222222222222222222222222222222222222222222222222222221 Java.io1 I/O on streams and random access files, printing 1 1 1 1 22222222222222222222222222222222222222222222222222222222222222222222222221 Java.net1 Sockets, IP addresses, URLs, datagrams 1 1 1 1 22222222222222222222222222222222222222222222222222222222222222222222222221 Java.util1 Stacks, hash tables, vectors, time, date 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222Java.applet1 Getting and displaying Web pages, audio, Object class 1 1 1 1 12222222222222222222222222222222222222222222222222222222222222222222222222Java.awt1 Events, dialog, menus, fonts, graphics, window management 1 1 Java.awt.image1 Colors, image cropping, filtering, and conversion 1 122222222222222222222222222222222222222222222222222222222222222222222222221 1 22222222222222222222222222222222222222222222222222222222222222222222222221 Java.awt.peer1 Access to the underlying window system 1
Fig. 7-74. The packages included in the standard API.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Pointers Pointers Overflow to URLS to titles Hash chains String storage code URL 0 2 1 Title 2 4 3 URL 5 19 6 21 44 Title
n Url_table Heap Hash table
Fig. 7-75. Data structures used in a simple search engine.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 1.00 0.75 0.50 0.25 0 1 TTT1 –0.25 2 T 2 T –0.50 –0.75 –1.00 (a) (b) (c)
Fig. 7-76. (a) A sine wave. (b) Sampling the sine wave. (c) Quan- tizing the samples to 3 bits.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall The next field Scan line painted Scan line start here on the screen
1 3 5 7 9 11
13 Time 15 . Horizontal . Vertical retrace . retrace 483
Fig. 7-77. The scanning pattern used for NTSC video and televi- sion.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Pixel value 4 pixels
160 160 161 160 160 0 1 0
161 165 166 158 1 5 6 –2
160 167 165 161 0 7 5 1
159 160 160 160 –1 0 1 0
(a) (b)
Fig. 7-78. (a) Pixel values for part of an image. (b) A transforma- tion in which the upper left-hand element is subtracted from all elements except itself.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Square with many pixels Code book
0 0 1 0 0 0 0 1 0 2 2 0 3 2 2 0 0 4 0 0
2 2 2 0 1
3 2 2 0 2
0 4 0 0 3
4
(a) (b) (c)
Fig. 7-79. An example of vector quantization. (a) An image divided into squares. (b) A code book for the image. (c) The encoded image.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Input Discrete Run- Statistical Output Block Differential cosine Quantization length output preparation quantization transform encoding encoding
Fig. 7-80. The operation of JPEG in lossy sequential mode.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall i.78.()RBiptdt.()Atrbokpreparation. block After (b) data. input RGB (a) 7-81. Fig. 480 From: optrNetworks Computer RGB 640 a (b) (a) 24-Bit pixel r d yAde .Tnnam 96Petc Hall Prentice 1996 © Tanenbaum, S. Andrew by ed. 3rd ,
480 1 Block Block 4799 8-Bit pixel 640 YI
240 240 320 Q i.78.()Oeboko the of block One (a) 7-82. Fig. Y/I/Q Amplitude
y From: optrNetworks Computer xFx r d yAde .Tnnam 96Petc Hall Prentice 1996 © Tanenbaum, S. Andrew by ed. 3rd , Y arx b h C coefficients. DCT The (b) matrix.
DCT
Fy DCT Coefficients Quantized coefficients
150 80 40 14 4 2 1 0 150 80 20 4 1 0 0 0 92 75 36 10 6 1 0 0 92 75 18 3 1 0 0 0 52 38 26 8 7 4 0 0 26 19 13 2 1 0 0 0 12 8 6 4 2 1 0 0 3 2 2 1 0 0 0 0 4 3 2 0 0 0 0 0 1 0 0 0 0 0 0 0 2 2 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Quantization table
1 1 2 4 8 16 32 64 1 1 2 4 8 16 32 64 2 2 2 4 8 16 32 64 4 4 4 4 8 16 32 64 8 8 8 8 8 16 32 64 16 16 16 16 16 16 32 64 32 32 32 32 32 32 32 64 64 64 64 64 64 64 64 64
Fig. 7-83. Computation of the quantized DCT coefficients.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 150 80 20 4 1 0 0 0
92 75 18 3 1 0 0 0
26 19 13 2 1 0 0 0
3 2 2 1 0 0 0 0
1 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
Fig. 7-84. The order in which the quantized values are transmitted.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Audio signal Audio encoder
System MPEG –1 output Clock multiplexer
Video signal Video encoder
Fig. 7-85. Synchronization of the audio and video streams in MPEG-1.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Fig. 7-86. Three consecutive frames.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Packetized elementary stream
Video Video PS PS Packetizer encoder Multi- (Variable-length packets, plexor common time base)
Clock
Audio Video TS TS Packetizer encoder Multi- (Fixed-length packets, plexor no common time base)
Packetized elementary stream
Fig. 7-87. Multiplexing of two streams in MPEG-2.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Video Switch server Customer's house
Fiber Local spooling ATM or SONET server backbone network
Audio Local distribution server network
Fig. 7-88. Overview of a video-on-demand system.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall RAM
RAID
Optical disk
Tape archive
Fig. 7-89. A video server storage hierarchy.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Movie Local Local Main CPU . . . CPU Cache RAM RAM RAM (RAM)
High-speed bus
Optical Magnetic Tape Network disk disk controller interface controller controller
To ATM Switch
Tape archive Optical juke box RAID
Fig. 7-90. The hardware architecture of a typical video server.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall i.79.Ds ufrn tteserver. the at buffering Disk 7-91. Fig. Megabytes data Buffered From: optrNetworks Computer read fromdisk Amount ofdata r d yAde .Tnnam 96Petc Hall Prentice 1996 © Tanenbaum, S. Andrew by ed. 3rd , Time Transmission ASDL 1.536 Mbps ASDL network 16 Kbps subscription unit 4 KHz analog unit
Fiber This connection uses the connection existing twisted pair to the end office
Fig. 7-92. ADSL as the local distribution network.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall Applications
MPEG audio MPEG video MPEG PS TS
AAL 1 AAL 5
ATM ATM layer
ATM physical layer
Fig. 7-93. A video-on-demand protocol stack.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall CPU ROM RAM
I/O MPEG Network
To network
Remote control
Television set
Fig. 7-94. The hardware architecture of a simple set-top box.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall 222222222222222222222222222222222222222222222222222222222222222 1 1 1222222222222222222222222222222222222222222222222222222222222222What technology will the backbone use (SONET, ATM, SONET + ATM)? 1 1 What speed will the backbone run at (OC-3, OC-12)? 1 12222222222222222222222222222222222222222222222222222222222222221 2222222222222222222222222222222222222222222222222222222222222221 How will local distribution be done (HFC, FTTC)? 1 1 1 2222222222222222222222222222222222222222222222222222222222222221 How much upstream bandwidth will there be (16 kbps, 1.5 Mbps)? 1 1 1 1222222222222222222222222222222222222222222222222222222222222222Will movies be encrypted, and if so, how? 1 1 Will error correction be present (mandatory, optional, absent)? 1 12222222222222222222222222222222222222222222222222222222222222221 2222222222222222222222222222222222222222222222222222222222222221 Who will own the set-top box (user, network operator)? 1 1 1 2222222222222222222222222222222222222222222222222222222222222221 Will telephony be part of the system (analog, N-ISDN)? 1 1 1 2222222222222222222222222222222222222222222222222222222222222221 Will high-resolution hypertext applications be supported (e.g., WWW)? 1
Fig. 7-95. A few areas in which standards are needed.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall M router B C D
Multicast island LAN Workstation G A
E F
Fig. 7-96. MBone consists of multicast islands connected by tun- nels.
From: Computer Networks, 3rd ed. by Andrew S. Tanenbaum, © 1996 Prentice Hall