Alcatel-Lucent 7710 SR OS, Interface Configuration Manual

Page: 38 / 436

Port Features
Page 38 7710 SR OS Interface Configuration Guide
Figure 2: MLPPP 12-bit Fragment Format
The required and default format for MP is the 24-bit format. During the LCP state the 12-bit
format can be negotiated. The SR-series routers can support and negotiate the alternate 12-bit
frame format.
Protocol Field (PID)
The protocol field is two octets its value identifies the datagram encapsulated in the Information
field of the packet. In the case of MP the PID also identifies the presence of a 4-octet MP header
(or 2-octet, if negotiated).
A PID of Ox003d identifies the packet as MP data with an MP header.
The LCP packets and protocol states of the MLPPP session follow those defined by PPP in RFC
1661,
The Point-to-Point Protocol (PPP) . The options used during the LCP state for creating an
MLPPP NCP session are described below.
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
1
1
0 1 2 3 4 5 6 7 8 9
1
0 1
2 1
3 1
4 1
5 1
7 1
6 1
8 1
9 2
0 2
1 2
3 2
2 2
4 2
5 2
6 2
7 2
9 2
8 3
0 3
1
Control 0x03 PID 0x003d Address 0xff
B E
Fragment Data...
FCS Fragment Data
Sequence
Fragment Data ...