Mlppp Packet Fragmentation And Serialization Transmission Latency - Enterasys Security Router X-PeditionTM User Manual

PPP Features

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
applications in the same order.
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
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
Link
Speed
56 kbps
64 kbps
128 kbps
256 kbps
512 kbps
768 kbps
8-6 Configuring PPP
Figure 8-5, to ensure that fragment is reassembled and forwarded to higher layer
Figure 8-5
Type
"Configuring Quality of Service"
Serialization Latency for Different Fragment Size/Link Speed
1 byte 64 bytes
143 us 9 ms
125 us 8 ms
62.5 us 4 ms
31 us 2 ms
15.6 us 1 ms
10 us 640 us
Multilink Header Option Format
Code (Long/Short
Length
Sequence #)
Table
Fragment Size
128 bytes 256 bytes
18 ms 36 ms
16 ms 32 ms
16 ms 32 ms
4 ms 8 ms
2 ms 4 ms
1.28 ms 2.56 ms
# of Suspendable
Classes
on page 12-1 for more information.
8-1.
512 bytes 1024 bytes
72 ms 144 ms
64 ms 126 ms
32 ms 64 ms
16 ms 32 ms
8 ms 16 ms
5.12 ms 10.24 ms
1500 bytes
214 ms
187 ms
93 ms
46 ms
32 ms
15 ms