Enterprise QoS Solution Reference Network Design Guide

Chapter 6 IPSec VPN QoS Design
Version 3.3
Teleworker V3PN QoS Considerations
summarize the results of these tests: Asymmetrical links are preferred (over symmetrical links) as long
as QoS is enabled on the uplink, provided that the lower of the two speeds (the uplink) is adequate for
transporting both voice and data.
Some service providers for business-class services offer symmetrical links in an effort to compete with
Frame Relay providers; 384 kbps/384 kbps and 768 kbps/768 kbps are examples. With no QoS enabled
on the service provider edge, this offering is not optimal for the enterprise. An asymmetrical link such
as 384 kbps/1.5 kbps is a better choice for the V3PN networks.
Broadband Serialization Mitigation Through TCP Maximum Segment Size
Tuning
The majority of broadband deployments are DSL with PPPoE and cable with DOCSIS 1.0. As previously
noted, neither of these technologies includes any mechanisms to fragment data packets and interleave
voice packets at Layer 2 to minimize the impact of serialization and blocking delay on voice packets
(which is a recommended requirement on link speeds £ 768 kbps).
The DOCSIS 1.1 specification for cable includes fragmentation and interleaving support, and DSL
providers can implement MLP over ATM (which includes MLP LFI support). However, these do not
represent the majority of the currently deployed base of broadband networks.
An alternative way to mitigate serialization delay on broadband circuits is provided by adjusting the TCP
Maximum Segment Size (MSS). The TCP MSS value influences the resulting size of TCP packets and
can be adjusted with the ip tcp adjust-mss interface command. Because the majority of large data
packets on a network are TCP (normal UDP application packets, such as DNS and NTP, average less
than 300 bytes and do not create a significant serialization delay issue), this command effectively can
reduce serialization delay in most teleworker scenarios when no Layer 2 fragmentation and interleaving
mechanism is available.
Note It is not recommended to run UDP-based video applications on broadband links £ 768 kbps that do not
support Layer 2 LFI in a V3PN teleworker scenario. This is because such applications regularly generate
large UDP packets that, obviously, are not subject to TCP MSS and thus can cause significant and
unmitigatable serialization delays to VoIP packets.
The recommended TCP MSS value of 542 was calculated to eliminate the IPSec crypto algorithm
(3DES) padding and the ATM AAL5 padding in DSL implementations (cable implementations have
3DES padding but no AAL5). Therefore, a TCP MSS value of 542 bytes is valid for cable but is
optimized for DSL. Figure 6-26 illustrates a TCP packet with an MSS size of 542.
Note Both the ESP and AAL5 pad lengths are 0. A TCP packet with a TCP MSS value of 542 fits exactly into
14 ATM cells, with no wasted bytes because of cell padding.
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