Question

Show how an MPLS packet is encapsulated in a frame. Find out what should be the value of the type field when the protocol is the Ethernet.

Answer 1

Frame format

MPLS carries a very small amount of information mainly composed of the label, a TTL and the EXP field, used for QoS. Below is the frame format for one label. A MPLS header, called "label stack", is composed of 1 or more labels.

Stacking labels can be used to provide additional services. For example the Virtual Private Networks based on MPLS use an additional label to identify the 'service' packets belong to additionally to the first layer of labels that are used for switching packets.

EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the payload of the frame. The same field is also used to indicate the size of some Ethernet frames. EtherType was first defined by the Ethernet II framing standard, and later adapted for the IEEE 802.3 standard. This field is used by the data link layer to determine which protocol to hand over the payload to on the receiving end.

In modern implementations of Ethernet, the field within the Ethernet frame used to describe the EtherType also can be used to represent the size of the payload of the Ethernet Frame. Historically, depending on the type of Ethernet framing that was in use on an Ethernet segment, both interpretations were simultaneously valid, leading to potential ambiguity. Ethernet II framingconsidered these octets to represent EtherType while the original IEEE 802.3 framing considered these octets to represent the size of the payload in bytes.

In order to allow Ethernet II and IEEE 802.3 framing to be used on the same Ethernet segment, a unifying standard, IEEE 802.3x-1997, was introduced that required that EtherType values be greater than or equal to 1536. That value was chosen because the maximum length (MTU) of the data field of an Ethernet 802.3 frame is 1500 bytes. Thus, values of 1500 and below for this field indicate that the field is used as the size of the payload of the Ethernet frame while values of 1536 and above indicate that the field is used to represent an EtherType. The interpretation of values 1501–1535, inclusive, is undefined.

The end of a frame is signaled by loss of carrier or by a special symbol or sequence in the line coding scheme for a particular Ethernet physical layer, so the length of the frame does not always need to be encoded as a value in the Ethernet frame. However, as the minimum payload of an Ethernet frame is 46 bytes, a protocol which uses EtherType must include its own length field if that is necessary for the recipient of the frame to determine the length of short packets (if allowed) for that protocol.