Internet: Technologie, Protokolle und Dienste

Advantages:

Allows to extend a network (e.g., beyond the physical limits of an Ethernet) without renumbering or installing routing software, simply by modifying a single router. Splits a network in several parts, reducing broadcast and multicast traffic (even for switched Ethernet networks).(1).

Disadvantage:

Assumes trusting relationship; only works for broadcast networks (1).

step	N	d(B),p(B)	d(C),p(C)	d(D),p(D)	d(E),p(E)
1	A	1,A	1,A	-,-	-,-
2	A B	1,A	2,B	9,A	4,B
3	A B C	1,A	2,B	9,A	3,C
4	A B C E	1,A	2,B	8,E	3,C
5	A B C E D	1,A	2,B	8,E	3,C

(per mistake: -1)

The UDP and TCP checksum, a one's complement addition of 16-bit words, will not detect if 16-bit words are swapped or if the same bit position is wrong in two different words. Note that the UDP and TCP checksum use the same algorithm as the IP header checksum, but cover the whole payload, plus the pseudo-header.(2)

Normal termination (orderly release): Client (for example) sends FIN, which is acknowledged by server with ACK. The client goes to state FIN_WAIT_1, until it receives the FIN from the server, which goes to state LAST_ACK. The client acknowledges the server FIN and goes to state TIME_WAIT. (State names are not necessary.) (2)

Abortive release: The client or server sends a RST (reset). There is no answer from the other side, including no ACK. (2)

With abortive release, the sender has no idea how many bytes have been received by the other side (except if all outstanding bytes have been acknowledged). (1)

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
  int s;
  struct sockaddr_in sin;
  char msg[80] = "root\n";
  int n;

  if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
    perror("socket"); return -1;
  }
  sin.sin_family      = AF_INET;
  sin.sin_port        = htons(79);
  sin.sin_addr.s_addr = inet_addr("127.0.0.1");
  if (connect(s, (struct sockaddr *)&sin, sizeof(sin)) < 0) {
    perror("connect"); return -1;
  }
  if (write(s, msg, strlen(msg)+1) < 0) {
    perror("write"); return -1;
  }
  while ((n = read(s, msg, sizeof(msg))) > 0) {
    msg[n] = '\0';
    fputs(msg, stdout);
  }
  if (close(s) < 0) { perror("close"); return -1; }
  return 0;
}

(Sequence: 4; socket/address: 2)

By looking at the Received: header to determine the last hops before reaching the destination. This field is set by the server MTA based on the HELO SMTP command issued by the client. Servers should thus check that the IP address of the client agrees with the host name included in the HELO command. Some servers refuse to accept bogus HELO host names, others produce something like:

Return-path: 
Delivery-date: Mon, 22 Jul 1996 17:12:46 +0200
Received: from whitehouse.gov (actually lupus.fokus.gmd.de)
          by ceres.fokus.gmd.de with SMTP (PP-ICR1v5);
          Mon, 22 Jul 1996 17:12:13 +0200
To: schulzrinne@fokus.gmd.de
From: Jane Smith 
Date: Mon, 22 Jul 1996 16:52:19 +0200 (MET DST)

Some servers (ns.research.att.com) blindly accept whatever host name is given in the HELO command and provide no indication of mismatches between name and address, so this method is not foolproof.

Also, Return-path: might provide a clue, although that is easily spoofed when simulating sendmail via telnet, as seen in the example.(2)

http://www.tu-berlin.de:8000/pub/meier/book.html#sec1

(http/host, port, directory/fragment: 1 each)

• HTML does not support macros (functions that translate into actual formating instructions).
• HTML describes the structure rather than the formatting of text (although LaTeX does this to some extent as well).
• HTML has no notion of pages of a certain size ("scroll-like").
• HTML supports anchors for hyperlinks.
• HTML allows embedding of interaction (forms) and applets.

(1 each)

Criteria	ftp	NFS	HTTP
Data type support	ASCII, binary, EBCDIC	just raw bytes, no type indication	any MIME type (not just text or HTML!)
What happens when data changes on server?	Client won't notice (local copy) and has to manually check directory listing to see if the document has changed.	Client gets new data with next access (server cache).	Client tests if data has expired (HEAD, If-modified-since) and retrieves again (client cache).
File operations?	retrieval (RETR), storage (STOR), delete (DELE), directory list (LIST), ...	standard Unix file operations	currently (V1.0), just retrieval (GET), storage (POST), header (HEAD), but others planned
Number of connections	2 (control, data)	none (UDP), 1 (TCP)	1
transport protocols	TCP	UDP or TCP	TCP
Protection against eavesdropping?	none (but: extensions); password in the clear	none (but: extensions); no password	SSL; challenge-response authentication

(1 each)

WWW retrieval	Internet telephony
no delay bound	delay bound (hundreds of ms)
use any available bandwidth	fixed bandwidth
loss-sensitive (needs TCP)	somewhat loss-tolerant
unicast only	unicast or multicast
TCP and HTTP	UDP and RTP
client-server	symmetric (but: caller and callee)

Both applications are reasonably insensitive to packet re-ordering, but the transport protocol (TCP) or application (Internet telephony) resequences packets.

See lecture slides.