blank.gif (43 bytes)

Church Of The
Swimming Elephant

Packet Reception Connected: An Internet Encyclopedia
Packet Reception

Up: Connected: An Internet Encyclopedia
Up: Requests For Comments
Up: RFC 826
Prev: Packet Generation
Next: Why is it done this way??

Packet Reception

Packet Reception When an address resolution packet is received, the receiving Ethernet module gives the packet to the Address Resolution module which goes through an algorithm similar to the following. Negative conditionals indicate an end of processing and a discarding of the packet.

?Do I have the hardware type in ar$hrd?
Yes: (almost definitely)
  [optionally check the hardware length ar$hln]
  ?Do I speak the protocol in ar$pro?
    [optionally check the protocol length ar$pln]
    Merge_flag := false
    If the pair  is
        already in my translation table, update the sender
	hardware address field of the entry with the new
	information in the packet and set Merge_flag to true. 
    ?Am I the target protocol address?
      If Merge_flag is false, add the triplet  to
	  the translation table.
      ?Is the opcode ares_op$REQUEST?  (NOW look at the opcode!!)
	Swap hardware and protocol fields, putting the local
	    hardware and protocol addresses in the sender fields.
	Set the ar$op field to ares_op$REPLY
	Send the packet to the (new) target hardware address on
	    the same hardware on which the request was received.

Notice that the triplet is merged into the table before the opcode is looked at. This is on the assumption that communcation is bidirectional; if A has some reason to talk to B, then B will probably have some reason to talk to A. Notice also that if an entry already exists for the pair, then the new hardware address supersedes the old one. Related Issues gives some motivation for this.

Generalization: The ar$hrd and ar$hln fields allow this protocol and packet format to be used for non-10Mbit Ethernets. For the 10Mbit Ethernet takes on the value <1, 6>. For other hardware networks, the ar$pro field may no longer correspond to the Ethernet type field, but it should be associated with the protocol whose address resolution is being sought.

Next: Why is it done this way??

Connected: An Internet Encyclopedia
Packet Reception


Protect yourself from cyberstalkers, identity thieves, and those who would snoop on you.
Stop spam from invading your inbox without losing the mail you want. We give you more control over your e-mail than any other service.
Block popups, ads, and malicious scripts while you surf the net through our anonymous proxies.
Participate in Usenet, host your web files, easily send anonymous messages, and more, much more.
All private, all encrypted, all secure, all in an easy to use service, and all for only $5.95 a month!

Service Details

Have you gone to church today?
All pages ©1999, 2000, 2001, 2002, 2003 Church of the Swimming Elephant unless otherwise stated
Church of the Swimming Elephant©1999, 2000, 2001, 2002, 2003 is a wholly owned subsidiary of Packetderm, LLC.

Packetderm, LLC
210 Park Ave #308
Worcester, MA 01609