Enterasys Networks Security Router X-PeditionTM - Manual (Page 196)

PPP Features
8-6 Configuring PPP
MLPPP Packet Fragmentation and Serialization Transmission Latency
MLPPP’s packet transport method over multiple member links is made possible by fragmenting
the packet after balancing the load bandwidth to fully utilize the member links’ bandwidth. When
sent over a MLPPP link, each fragment carries a sequence number within the Multilink header, as
shown in
Figure 8-5 , to ensure that fragment is reassembled and forwarded to higher layer
applications in the same order.
Figure 8-5 Multilink Header Option Format
Additionally, each fragment of a sequence stream is assigned a class number in the MLPPP header,
permitting at most four classes for the short and 16 for the long sequence number fragment. The
higher the class number, the higher the priority it is granted over the line. For example, voice,
video and data packets can be assigned high, medium and low priority sequence numbers to
ensure proper QoS. Refer to
“Configuring Quality of Service” on page 12-1 for more information.
Since standard MLPPP allows only a single stream sequence number, the result is that only two
priority level layers can be utilized without breaking the packet order as follows:
• Higher priority layer packets are sent without fragmentation and multilink headers.
• Lower priority layer packets are fragmented and interleaved with the higher priority packet.
MLPPP is marked by the following limitations:
• A higher priority packet can be sent through only one member link since it is not fragmented
and does not contain a sequence number, otherwise the higher priority packet order can not
be guaranteed.
• The bandwidth of the higher priority packet should not exceed the speed of the designated
link used. The result is MLPPP bundle bandwidth is not fully utilized.
Each MLPPP packet holding a fragment is transmitted through the member link with packet
transmission latency occurring as the result of packet size operating against link speed. With a
Multi-link PPP connection, most packets are fragmented into equal size fragments and
transmitted over all member links to balance bandwidth loading over each link with the same or
different speeds. To sum up, fragment size must be controlled in order to minimize transmission
latency. Serialization transmission latency, measured in milliseconds, equals fragment size (in
bits) multiplied by link speed (in Kbps) as shown in
Table 8-1 .
Table 8-1 Serialization Latency for Different Fragment Size/Link Speed
Fragment Size
Link
Speed 1 byte 64 bytes 128 bytes 256 bytes 512 bytes 1024 bytes 1500 bytes
56 kbps 143 us 9 ms 18 ms 36 ms 72 ms 144 ms 214 ms
64 kbps 125 us 8 ms 16 ms 32 ms 64 ms 126 ms 187 ms
128 kbps 62.5 us 4 ms 16 ms 32 ms 32 ms 64 ms 93 ms
256 kbps 31 us 2 ms 4 ms 8 ms 16 ms 32 ms 46 ms
512 kbps 15.6 us 1 ms 2 ms 4 ms 8 ms 16 ms 32 ms
768 kbps 10 us 640 us 1.28 ms 2.56 ms 5.12 ms 10.24 ms 15 ms
Type Length
Code (Long/Short
Sequence #)
# of Suspendable
Classes