NBAR chỉ có thể xử lý lên đến 24 URL đồng thời, máy chủ, hoặc MIME loại. NBAR phân tích chỉ có 400 byte đầu tiên của gói tin. NBAR hỗ trợ chỉ CEF và không làm việc nếu một chế độ chuyển đổi được sử dụng. Các gói tin multicast, các gói dữ liệu bị phân mảnh, và | 28 Chapter 2 The TCP IP and OSI Networking Models IP relies on the network interface layer to deliver IP packets across each physical network. IP understands the overall network topology things such as which routers are connected to each other which host computers are connected to which networks and what the IP addressing scheme looks like. However the IP protocol purposefully does not include the details about each of the underlying physical networks. Therefore the Internet layer as implemented by IP uses the services of the network interface layer to deliver the packets over each physical network respectively. The network interface layer includes a large number of protocols. For instance the network interface layer includes all the variations of Ethernet protocols and other LAN standards. This layer also includes the popular WAN standards such as the Point-to-Point Protocol PPP and Frame Relay. The same familiar network is shown in Figure 2-5 with Ethernet and PPP used as the two network interface layer protocols. Figure 2-5 Ethernet and PPP Services Provided to IP To fully appreciate Figure 2-5 first think a little more deeply about how IP accomplishes its goal of delivering the packet from Bob to Larry. Bob wants to send the IP packet to Larry but it must first do so by sending the packet to R2. Bob uses Ethernet to get the packet to R2. At R2 R2 strips the Ethernet header and trailer from the IP packet. To get the IP packet from R2 to R1 R2 does not need to use Ethernet it instead needs to use the PPP serial link. To send the IP packet from R2 to R1 R2 needs to place a PPP header in front of the IP packet and a PPP trailer at the end. Similarly after the packet is received by R1 R1 removes the PPP header and trailer because PPP s job is to get the IP packet across the serial link. R1 then decides that it should forward the packet over the Ethernet to Larry. To do so R1 adds a brand-new Ethernet header and trailer to the packet and forwards it to Larry. In .
