- Enterasys Security Router User's Guide

PPP Features
XSR User’s Guide 8-5
• Fragmentation/reassembly
• Detection of fragment loss
• Optimal buffer usage
• MTU size determination
• Management of MLPPP bundles
• MIB support for network management
• Up to four T1/E1 lines can be aggregated running MLPPP
• Multi-class MLPPP for up to five multiple sequence number streams over one MLPPP bundle
Multi-Class MLPPP
The Multi-Class extension to Multi-link PPP, as defined by RFC-2686, provides a means of
transmitting multiple sequence traffic streams over one Multi-link PPP bundle on a Multilink or
Dialer interface. Multi-Class offers best-effort Quality of Service (QoS) to minimize delay and fully
utilize bandwidth over each member link ensuring that high priority traffic such as real-time voice
and video packets is transmitted with minimum delay. The XSR’s implementation of Multi-Class
MLPPP supports the following features:
• Multilink Maximum Received Reconstructed Unit (MRRU). This non-configurable feature is
set to 1500 bytes by default.
• Multilink Short Sequence Number Header Format is non-configurable but allows lower
priority traffic classes to be suspended in favor of higher priority classes when necessary. The
XSR defaults to the long sequence number header format but is passive - if a peer requests the
short format the router provides a short sequence number. The Suspendable (class) level for
negotiation is defaulted to 5. The XSR will accept any lower Suspendable (class) level
negotiation and reject any larger levels.
• Endpoint Discriminator is configurable to specify a class with the
ppp multipoint endpoint
command.
• Multilink Header Format is enabled with the
ppp multilink multi-class command.
The benefits of operating Multi-Class MLPPP are as follows:
• Fragment interleaving with different classes (priorities).
• Multiple suspension (class) layers to accommodate multiple priority classifications and packet
interleaving.
• Full bandwidth utilization across the bundle since all fragments with different priorities can
be sent over any member link without violating the order of packets sharing the same
classification.
Multi-Class is limited in that it does not provide a method for packet classification, relying on an
external method - QoS - to prioritize the output packet stream with certain criteria. QoS currently
supports up to four types of classification via ACLs. While MLPPP supports sending packets
outside MLPPP with no fragmentation and no MLPPP header by default, the total level of
prioritization supported is five for QoS, the same as that (four suspendable levels) available in
Multi-Class MLPPP.
If the negotiated Multi-Class level is less than the number of classes set by QoS, MLPPP will fit the
QoS class type into the MLPPP Multi-Class number according to the principle that the higher
priority QoS class type will fit into a higher Multi-Class class until the Multi-Class number 0,
which will contain any remaining QoS low priority classes.