Re: ICMP Well-Known Port

["Josh Robb" <josh () fujitsu co nz>]

Off the top of my head (I'll have a go...)

ICMP communications occur at the same level in the protocol stack as TCP and UDP (sort of). (I.e. the network layer of 
the OSI model). TCP and UDP's designers chose to use ports as the endpoint identifiers for their protocols. ICMP 
however is implemented differently using Types and Codes. Each type identifies a major ICMP function (ECHO-REQUEST, 
ECHO-REPLY, TTL-EXPIRED etc), each code allows extra information to be conveyed within the ICMP packet (e.g. a new 
address in a redirect). 

So simply there are no ports for ICMP because it does not use any underliying protocol which supplies such 
functionality. ICMP sits directly ontop of IP, the diagram below shows ICMPs relationship to the TCP/IP stack. Notice 
that TCP/UDP do not enter into the equation.

--------
ICMP - Flow control/Network diagnostics/Basic Routing updates (redirects).
------
IP - Addressing/Routing etc.
--------

ICMP does not need ports because there is no ongoing conversation. It is simply one party sending some 
request/information to another party. This may require a response or it may not.

Regards

Josh
  ----- Original Message ----- 
  From: wpl1 () jps net 
  To: firewall-wizards () nfr net 
  Sent: Wednesday, September 29, 1999 3:28 AM
  Subject: ICMP Well-Known Port


  Evidently I'm really missing the boat.  I've been trying to find the well-known port for ICMP.  Is there no such 
thing?
   
  Dean C. Rowan
  Whittier Public Library