Alcatel-Lucent 7750 Configuration Manual page 58

Port Features
B & E Bits
The B&E bits are used to indicate the epoch of a packet. Ingress packets to the MLPPP process
will have an MTU, which may or may not be larger than the MRRU of the MLPPP network. The
B&E bits manage the fragmentation of ingress packets when it exceeds the MRRU.
The B-bit indicates the first (or beginning) packet of a given fragment. The E-bit indicates the last
(or ending) packet of a fragment. If there is no fragmentation of the ingress packet both B&E bits
are set true (=1).
Sequence Number
Sequence numbers can be either 12 or 24 bits long. The sequence number is zero for the first
fragment on a newly constructed AVC bundle and increments by one for each fragment sent on
that bundle. The receiver keeps track of the incoming sequence numbers on each link in a bundle
and reconstructs the desired unbundled flow through processing of the received sequence numbers
and B&E bits. For a detailed description of the algorithm refer to RFC 1990.
Information Field
The Information field is zero or more octets. The Information field contains the datagram for the
protocol specified in the protocol field.
The MRRU will have the same default value as the MTU for PPP. The MRRU is always
negotiated during LCP.
Padding
On transmission, the Information field of the ending fragment may be padded with an arbitrary
number of octets up to the MRRU. It is the responsibility of each protocol to distinguish padding
octets from real information. Padding must not be added to any but the last fragment (the E-bit set
true).
FCS
The FCS field of each MP packet is inherited from the normal framing mechanism from the
member link on which the packet is transmitted. There is no separate FCS applied to the
reconstituted packet as a whole if transmitted in more than one fragment.
Page 58 7750 SR Interface Configuration Guide