Enterasys Networks Security Router X-PeditionTM - Manual (Page 197)

PPP Features
XSR User’s Guide 8-7
The overall serialization latency for a fragment over a synchronous/ asynchronous Serial or T1
link should be multiplied by the size of the transmission queue. To control latency, both the
transmission queue size and fragment size must be controlled.
Fragment Interleaving Over the Link
Transmitting a higher priority packet, either voice or video, with minimum and controlled
latency, especially when lower priority packets such as data packets are present, is performed by
the XSR automatically interleaving the fragment of the higher priority packet into the stream of
the fragment of the lower priority packet. Fragment interleaving ensures that the transmission
latency of a higher priority packet will not exceed the maximum value as determined by the sum
of the fragment and transmission queue sizes.
To sum up, Multi-Class MLPPP performs as follows:
• Each packet is designated with a class number based on the classification result of the packet
type. Packet classification is performed by the external method like QoS, which classifies the
packet according to the ACL and assigns a class number according to priority as follows: the
smaller the class number, the lower the priority. The current class number ranges from 1 to 4,
with 1 for lower priority packets and 4 for the highest. Class 0 is reserved for a packet in the
fair-queue class.
• Each packet is split into the small size. The maximum fragment size is defined as the
maximum delay allowed over the slowest speed link in the bundle.
• Each fragment is appended with the MLPPP header, which includes the class information and
sequence number dedicated to this class and transmitted over the member links. Fragments of
different classes are interleaved.
• Fragments with a higher class number suspend the transmission of fragments with a lower
class number that is pending for transmission, which is the interleaving of the fragments with
different class numbers.
• When fragments are received, they are re-assembled based on class and sequence number
with the order of the packet within the same class preserved.
Multilink Head Format Negotiation
The Multilink Head Format is negotiated between peers based on whether an MRRU, short
sequence number, and code are present, as listed in
Table 8-2 on page 8-7:
1536 kbps 5 us 320 us 640 us 1.28 ms 2.56 ms 5.12 ms 10.24 ms
2024 kbps 4 us 256 us 512 us 1 ms 2 ms 4 ms 6 ms
Table 8-1 Serialization Latency for Different Fragment Size/Link Speed (continued)
Fragment Size
Table 8-2 Multi-Class MLPPP Negotiation
Option Type
MRRU Short Sequence # Multilink Head Format Negotiation Result
present nil nil MLPPP with Long Sequence # & No Multi-Class
present present MLPPP with Short Sequence # & No Multi-Class
present nil present/Code = 2 MLPPP with Long Sequence # & Multi-Class