UDP - User Datagram Protocol
			  ****************************

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


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1. Gral. Info
=============

- RFC 768: UDP Protocol Specification

- UDP Datagram format:

	0				16				32
	+---------------------------------------------------------------+
	|		SRC PORT	|	DST PORT		|
	|---------------------------------------------------------------|
	| UDP datagram length (inc. hdr)|	UDP checksum		|
	|---------------------------------------------------------------|
	~			OPTIONAL DATA				|
	+---------------------------------------------------------------+

  + Maximum UDP Datagram size

    · theoretical limit: 65,535 bytes (16-bit length field)
                         with 20b IP hdr + 8b UDP hdr = 65,507b of user data

    · two limits:       - sockets API limites size of send and receive buffer; 
                          generally 8 kbytes, but you can call a routine to 
		          change this.
		        - TCP/IP implementation - Stevens found various limits 
		          to the sizes - even with loopback interface 
		          (see Stevens, Vol. 1, pg. 159)

    · Hosts are required to handle at least 576 byte IP datagrams, 
      thus lots of protocols limit themselves to 512 bytes or less of data
      ( DNS, TFTP, BOOTP, and SNMP ).

  + UDP checksum: 

    · UDP doesn't require a checksum to be computed, but it's highly recommended
    · The algorithm divides the data to be checksummed into 16-bit fields.
    · Each 16-bit field has a 1's complement operation done on it and each of
      these 1's complements values are added.
    · Validated by the destination host only. 
    · If checksum is wrong, the datagram is discarded silently.
    · The checksum is calculated over the payload (padded to 2-byte boundary) 
      and a pseudo hdr: 
        [ src IP (4b) | dst IP (4b) | 0x00 (1b), proto (1b), udp-length (2b) ]
        [ src port (2b), dst port (2b) | udp length (2b), udp chksum (2b)    ]
        [ data (2-byte padded)                                               ] 

- Characteristics:

	+ Unreliable: there is no guarantee that the datagrams ever
	  reach destination 
	+ Not connection oriented (no sequence numbers)  

- UDP is IP protocol number 17.

- UDP Ports:	0 -> 65535 	[0 -> 1024: Priviledged]
	
  (Windows systems do not have the priviledged port concept)

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2. UDP stimulus - response
==========================

[c = client] [s = server] [r = router] [S = SYN] [R = RESET] [A = ACK] 

+ open port:                     request [c->s] / reply [s->c]
+ closed port:                   request [c->s] / ICMP port unreachable [s->c]
+ host does not exist:           request [c->s] / ICMP host unreachable [r->c]        
+ port blocked by router:        request [c->s] / ICMP host unreachable - admin
                                                 prohibited filter [r->c]
+ port blocked by silent router: S [c->s] / - 


Last Updated: 01/09/2003-06:41:31 - © Copyright 2004, Jess García