dix ethernet and IEEE original ethernet standard

[sarahr202]

Hi every body!

According to my book,Dix ethernet frame v2 has following feilds:

Preamble

destination

source

type

data

fcs.

what value was used in" type"field to specify ip packet in data field?

======================================

My new book by Wendell Odom shows " IEEE 802.3(original)" standard as :

preamble

SFD

destination

source

length

data

fcs

IEEE replaced" type" field in Dix ethernet by " length"field in IEEE802.3 original standards.

My question is since there is no " type"

field,how the different pay load such as ip packet, ipx is supported over such standard.

The old book by Wendell Odom shows the IEEE 802.3 original standard as :

preamble

sd

dest address

sourceaddress

length

dsap

ssap

control

data

fcs.

The above has "dsap and ssap "fields to specify type of pay load which is not shown in IEEE 802.3 original in the new book.

Which one is correct?

thanks a lot !

[Giuseppe Larosa · Accepted solution]

Hello Sarah,

I have my pc connected to an ADSL router here it is the txt of a capture while I was pinging a website

cat example_icmp1pack.txt

No. Time Source Destination Protocol Info

4 0.122055 213.92.16.191 192.168.2.2 ICMP Echo (ping) reply

Frame 4 (98 bytes on wire, 98 bytes captured)

Ethernet II, Src: 00:11:50:7e:c3:04, Dst: 00:0c:6e:d3:6c:45

Internet Protocol, Src Addr: 213.92.16.191 (213.92.16.191), Dst Addr: 192.168.2.2 (192.168.2.2)

Internet Control Message Protocol

Actually, for IPv4 ethernet format is used otherwise as you noted you should use :

LLC header (3 bytes)

+

SNAP header (5 bytes)

in last two bytes of SNAP the ethertype 0x800 should be used.

1997 802.3 revision simply accepts as a standard what had become standard in real world :

ethernet II format + IPv4 packet

if you check your PC you should see something similar:

ethernet II frame format with ethertype 0x800 and then the packet.

Here I put the hex dump of same packet

000 00 0c 6e d3 6c 45 00 11 50 7e c3 04 >>08 00<< 45 00 ..n.lE.. P~....E.

0010 00 54 c6 31 00 00 37 01 14 b2 d5 5c 10 bf c0 a8 .T.1..7. ...\....

0020 02 02 00 00 7c 1d a5 13 00 01 da ce 2a 4a e4 b1 ....|... ....*J..

as you can see after two MAC address ethertype 0800 follows the next byte has value 45 and it is the beginning of ipv4 header

Of course NAT allows my private ip address to reach the internet.

Hope to help

Giuseppe

[sarahr202]

Thanks Giuseppe!

IEEE 802.3 1997 standard only takes the format of dix v2 when length/field is used

as a type. just a quick review.

dixv2:

preamble

destination

source

type

data

fcs

802.3 1997standard:

preamble

sfd

destination

source

type ( when length/type is used as type

data

fcs

=================================

The two are almost identical.

But 802.3 1997 only takes the DIXv2 format when length/field is used as a

type which means length/type > 1536.

Since the maximum ip packet size can be only 1500 bytes. So 802.3 1997 can not be used for ip. The solution is to use 802.3 with snap. Since in your reply, i don't see any 802.3 snap frame. All i saw is ethernetII .

It appears to me NIC' vendors are using ethernet II frame format . They are using ethernet II format and Mac and physical layer from the IEEE standards .

So such NIC are not 802.3 compliant 100 percents. For an nic to be 100 % IEEE compliant , It must implement the whole standards not just MAC layer and physical layer.

thanks a lot Giuseppe for your patience and impressive know-how. It will take me a decade to know as much as you know!

[Laurent Aubert · Accepted solution]

1) As I said 802.3 specification doesn't describe the full Data-link layer. you need 802.2 to have full implementation. That's why 100% of the original 802.3 frame before 1997 requires the LLC header.

2)Before 1997, you couldn't differentiate an Ethernet II frame from a 802.3 frame and so both formats couldn't co-exist

After 1997 as Giuseppe already explained both field had been merged and they made sure no overlapping can occur. Now both formats can be supported on the same LAN.

Laurent.