Enterprise QoS Solution Reference Network Design Guide

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Teleworker V3PN QoS Considerations Bandwidth Provisioning 6-32 Enterprise QoS Solution Reference Network Design Guide Chapter 6 IPSec VPN QoS Design DOCSIS 1.1 also provides the capabilities to shape traffic at Layer 2 before transmission over the cable network. Although the circuit and frequencies physically are shared, access to the medium can be controlled by the headend so that a device can be guaranteed specified bandwidth. At the time of this writing, the only recommended deployment model for cable is the Integrated Unit + Access Model, as shown in Figure 6-22. Figure 6-22 Teleworker Deployment Model for Cable Integrated Unit + Access Model A few key differences with respect to bandwidth provisioning need to be taken into account for broadband teleworker scenarios, as compared to site-to-site VPNs. The first is that usually only a single call needs to be provisioned for (unless two teleworkers share a single residential broadband connection, which is rarely the case). If bandwidth is low, G.729 codecs are recommended. The second key bandwidth-provisioning consideration is the inclusion of the overhead required by the broadband access technologies, whether DSL or cable. Note Sometimes multicast support is not required in a teleworker environment. For example, routing protocols (which depend on multicast support) might not be required by teleworkers (because default gateways are usually sufficient in this context). In such cases, IPSec tunnel mode (no encrypted IP GRE tunnel) can be used to achieve additional bandwidth savings of 24 bytes per packet. For the remainder of this chapter, IPSec tunnel mode (no encrypted IP GRE tunnel) is the assumed mode of operation. NAT Transparency Feature Overhead IP 831/171X DSL Modem DSL Backbone To Enterprise Beginning in Cisco IOS Release 12.2(13)T, NAT transparency was introduced and is enabled by default, provided that both peers support the feature. It is negotiated in the IKE exchange between the peers. This feature addresses the environment in which IPSec packets must pass through a NAT/pNAT device, and it adds 16 bytes to each voice and data packet. The overhead that this feature adds is shown in Figure 6-23. Version 3.3
Chapter 6 IPSec VPN QoS Design DSL (AAL5 + PPPoE) Overhead Version 3.3 Figure 6-23 NAT Transparency Feature (Layer 3) Overhead IPSec ESP Tunnel Mode UDP Encapsulation 128 Bytes IP Hdr UDP Hdr Non-IKE Mkr ESP Hdr ESP IV IP Hdr UDP RTP Voice Teleworker V3PN QoS Considerations ESP Pad/NH No additional overhead is caused by NAT transparency on a G.729 call over DSL (PPPoE/AAL5) because there is enough AAL5 cell padding to absorb the 16 additional bytes per packet (discussed in more detail in the following section). In cable implementations, these additional 16 bytes increase the number of bits on the wire and, thus, the overall bandwidth consumption. At the headend, NAT transparency increases the bandwidth consumption of VoIP traffic by 1 Mbps for approximately every 82 concurrent G.729 calls; on the teleworker router, NAT transparency increases the bandwidth required by 6.4 kbps per G.729 call. Unless there is a need to implement this feature, the recommendation is to disable it when bandwidth conservation is a requirement. This feature can be disabled with the no crypto ipsec nat-transparency udp-encapsulation global configuration command. For DSL broadband connections, PPPoE is the most commonly implemented deployment. Given the 112-byte, Layer 3 size of an IPSec (only) encrypted G.729 VoIP call, 40 bytes of PPP, PPPoE, Ethernet, and ATM AAL5 headers and trailers are added. Additionally, the pre-ATM Software Segmentation and Reassembly (SAR) engine is required to pad the resulting 152-byte packet to the nearest multiple of 48 (each fixed-length ATM cell can transport only a 48-byte payload). Figure 6-24 shows the resulting 192-byte pre-ATM packet. Figure 6-24 IPSec-Encrypted G.729 Packet Size Through AAL5 + PPPoE Encapsulation The 192 bytes of cell payload are incorporated through ATM SAR into the (48-byte) payloads of four 53-byte ATM cells (192 / 48 = 4 cells). Therefore, the bandwidth required for an IPSec (only) encrypted G.729 VoIP call over DSL is calculated as follows: 53 bytes per cell · 4 cells per IPSec-encrypted VoIP packet 212 bytes per (192-byte) AAL5/PPPoE IPSec VoIP packet · 50 packets per second ESP Auth 20 8 8 8 8 20 8 12 20 2-257 12 LLC Snap 10 This feature adds 16 bytes per packet or 6400 bps per G.729 call. 802.3 Hdr 14 PPP oE PPP 8 IPSec Hdr ESP Hdr ESP IV IP Hdr UDP RTP 20 8 8 20 8 12 Encrypted Authenticated Voice 20 PPPoE + AAL5 Frame - 192 Bytes ESP Pad/NH ESP Auth 12 Enterprise QoS Solution Reference Network Design Guide 4 AAL5 Pad 40 AAL5 Trail 8 6-33
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