CODEWITHSUNDEEP
cnetwork-programming
Peng Zhang
Peng Zhang

Reputation: 3585

Is the "P/Q tag" field before the "Length/Type" field in the Ethernet frame header?

I need to handle Ethernet frame directly in C. And I got confused by the order of fields in Ethernet frame header. This is a programming question, so please do not mark it as offtopic.

The following figure is from Page 85 of book "TCP/IP Illustrated Volume 1, 2nd Edition". IEEE Packet Structure

As it shows, the "length or type" field precedes the "P/Q Tag" field (if there is any).

However, after hexdumping the frame I received, I realized the frame I received has "length or type" follows "P/Q Tag". See below 

0100 5e00 007b Destination MAC (multicast)
000f 5325 fb00 Source MAC 
8100           Q-tagged frame. (802.1q standard uses it to denote VLAN).    
0065           Prio (3 bits), CFI (1bit), VLAN ID(12 bit)
0800           EtherType (Type field)
4500           IP packet starts from here.

Question 1: What is the order of these two fields.
Question 2: How can I check the size of Ethernet frame header correctly. Need to know where the IP packet starts.

Upvotes: 2

Views: 2491

Answers (1)

Peng Zhang
Peng Zhang

Reputation: 3585

It is an error in the textbook (only in the 2nd edition, see EJP's comment). Not only my code shows the Q-Tag is before the "Length/Type" field. Also see the following open source code from OpenOnload.

openonload-201405-u1/src/include/ci/net/ethernet.h

typedef struct ci_ether_hdr_s {
  ci_uint8   ether_dhost[ETH_ALEN];
  ci_uint8   ether_shost[ETH_ALEN];
  ci_uint16  ether_type;
} ci_ether_hdr;

typedef struct {
  ci_uint8  ether_dhost[ETH_ALEN];  /* destination eth addr   */
  ci_uint8  ether_shost[ETH_ALEN];  /* source ether addr      */
  ci_uint16 ether_vtype;            /* vlan type field 0x8100 */
  ci_uint16 ether_vtag;             /* vlan tag               */ 
  ci_uint16 ether_type;             /* packet type ID field   */
} ci_ethhdr_vlan_t;

Upvotes: 2

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