Subj : Knock Knock
To   : Lawrence Garvin
From : Alejandro Pelaez
Date : Sat Jun 02 2001 02:00 am

Hola Lawrence Garvin, ¨como est s?

El 30-May-01 a las 20:56:46, Lawrence Garvin escribi˘ a Alejandro Pelaez
          Tema: Knock Knock

 LG> Alejandro wrote to Carol at 03:37 27 May:

 AP>> Erm, this echo is alive?.
 LG> Most definitely, Alejandro!
Oh!

 AP>> Can you explain me what happend with the source and destination
 AP>> mac address while a packet is being forwarding through routers?.
 AP>> What happend with the mac address when the packet go thorough an
 AP>> ethernet interface, to a hdlc interface?. HDLC frame don't have 6
 AP>> byte mac address, so how the arp request are forwarded?. Or they
 AP>> aren't forwarded?. Hmmmm

 LG> The MAC address is stripped from the source packet when it is
 LG> converted from an Ethernet frame (or Token Ring frame) to whatever
 LG> framing type is used to transport the packet between the routers
 LG> -- Frame Relay, HDLC, ATM, etc

 LG> If the router is linking two LAN segments, the outbound Ethernet
 LG> frame contains the MAC address of the outbound Ethernet (or Token
 LG> Ring) MAC address

 LG> When the final router receives the packet and places it on the
 LG> destination LAN, the Ethernet (or Token Ring) frame that is
 LG> created by that router contains the MAC address of the Ethernet
 LG> (or Token Ring) interface connecting the router to the LAN
Ah, so the router only have mac addresses of devices directly connected?.

And i have another question for you. Is about Ethernet: when a computer
sends a frame trough the network, the last 4 bytes of the frame are the
checksum. In the header there is a lenght field that tells you the size
of the packet, so the receiving computer knows where the packet ends.
That is in Ethernet 802.3, but in Ethernet II there are no lenght field,
only Type field. So how the receiving computer knows the lenght of the
header?. I know that if the type of the header is 0x8000 it is a IP packet, so
i can look at the lenght header of the IP packet. What i don't understand
is that all devices have to look at the IP header, even if they are layer 2
devices, in order to know the size of the frame?.




 -=> Muchos saludos, Alejandro Pelaez <=-

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