Chapter 1: Internet Protocol Operations Fundamentals

Cisco Press

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Other examples include: IP packets containing options in their header field, IP packets requiring fragmentation, and IP multicast packets used to create state. There are other exceptions as well, and these vary between router platforms.

Non-IP Packets

The other group of exception packets includes non-IP packets. In general, there are two groups of non-IP packets that routers may need to process. The first group includes the Layer 2 packets that are generated by the routers themselves to construct and maintain the network. Examples of packets of this type include:

  • Layer 2 keepalives: Cisco HDLC, Frame Relay, ATM Operation, Administration, and Maintenance (OAM), and other Layer 2 protocols typically send periodic L2 messages to convey interface up/down status between devices.

  • Link Control Protocol (LCP): LCP is an integral part of PPP and Multilink PPP (MLP), and provides automatic configuration of the interfaces such as setting datagram size, escaped characters, and magic numbers, and selecting (optional) authentication. LCP can also detect a looped-back link and other common misconfigurations, and terminate the link.

  • Cisco Discovery Protocol (CDP): CDP is a proprietary protocol that transmits router hardware, software, and interface status information between adjacent routers via multicast Layer 2 frames.

The preceding examples use purely Layer 2 frames, which are handled as exceptions by the router (punted and handled by the router CPU).


Note - All of the Layer 2 packets just described are local packets, meaning point-to-point packets that are processed by the local router CPU. This distinguishes them from Layer 2 packets that are tunneled (for example, AToM, VPLS, and L2TPv3).


The other group of non-IP packets includes all Layer 3 "non-IP" packets that may be configured to run on the router concurrently with IP.

Examples of non-IP Layer 3 protocols include:

  • Intermediate System-to-Intermediate System (IS-IS): An IGP used by many large service providers to maintain routing information within their own network administrative domain (instead of OSPF) to support reachability between BGP next-hops. IS-IS operates at Layer 3 like IP, but is a separate protocol that was originally developed by the International Organization for Standardization (ISO) as a routing protocol for Connectionless Network Protocol (CLNP) as part of Connectionless Network Services (CLNS). It was later extended to support IP routing, and is referred to as Integrated IS-IS.

  • Address Resolution Protocol (ARP): Used by hosts to find the corresponding Layer 2 (hardware) address to an IP network (Layer 3) address.

  • Multiprotocol Label Switching (MPLS): A data-carrying mechanism that emulates some of the properties of a circuit-switched network. MPLS is generally considered to operate between the traditional definitions of Layer 2 and Layer 3 protocols.

Other examples of non-IP Layer 3 protocols include: Novell Corporation's Internetwork Packet Exchange (IPX) and Apple Corporation's AppleTalk protocol.

As you have just seen, four distinct traffic types must be handled by routers: transit traffic, receive traffic, exception IP traffic, and non-IP traffic. The primary reason these four types of traffic are described separately here is that routers process these packets in different ways. Router vendors, such as Cisco, build hardware and software to handle all types of traffic within acceptable performance bounds appropriate for a given cost structure. At the same time, network architects and operators must be aware of the interactions between these four traffic types and understand the effects each may have on router and network performance and availability. For example, certain denial-of-service (DoS) attacks may be based on the purposeful manipulation of IP protocol exception packets. Routers and network infrastructure must be designed and built to efficiently forward "normal" traffic, while at the same time handle exception traffic and mitigate attack traffic without adverse impact.

IP Traffic Planes

Sufficient background has been covered to now fully explore the concepts of IP traffic planes. What types of IP traffic planes are there? Why should network traffic be segmented into IP traffic planes? What types of traffic are found in each traffic plane? These are the questions answered here.

Traffic planes are logical separations used to classify traffic based on the function it performs in the network. This approach is used for several reasons. First, it provides a consistent basis from which security policies can be developed. Second, it provides the basis for transforming these security policies into actual network control functions that can be implemented on various network elements.

As you saw in the previous discussion, depending on where a router is in the network, it will have a different perspective on what type of packet it is processing (transit vs. receive, for example). However, whether a packet is transit or receive does not automatically give any indication as to the function each packet is ultimately supporting. It is the concept of IP traffic planes that provide this end-to-end framework. Packets in each traffic plane have certain requirements that must be enforced, regardless of where they are within the network. Four distinct IP traffic planes are defined: the data plane, the control plane, the management plane, and the services plane. Each has its own distinctive characteristics, and its own security requirements. The four IP traffic planes are described in detail next.

Data Plane

The data plane is the logical entity containing all "customer" application traffic. In this context, customer traffic refers to traffic generated by hosts, clients, servers, and applications that are intended to use the network as transport only. Thus, data plane traffic should never have destination IP addresses that belong to any networking devices (routers, switches), but rather should be sourced from and destined to other devices, such as PCs and servers, that are supported by the network. The primary job of the router in the case of the data plane is simply to forward these packets downstream as quickly as possible. Figure 1-6 illustrates the basic concepts of the data plane.

Figure 1.6

Figure 1-6

Data Plane

Networks are built and operated to support data plane traffic. Without the data plane, there is no need for a network. First and foremost, the data plane must be "available." As you will see shortly, the data plane depends on the control plane and, to a certain extent, the management plane. Thus, interdependencies exist between these planes and they must be considered. In addition, there may be a "confidentiality" requirement, which may be satisfied via data separation (as would be provided by Frame Relay or MPLS VPNs, for example) or encryption. This is discussed further in the "Services Plane" section.

Data plane traffic always includes transit packets. Under normal conditions, transit traffic should account for a large percentage of all data plane traffic. This is precisely why routers often use specialized forwarding hardware and algorithms to accomplish this forwarding function as quickly as possible. That does not imply that all transit packets belong to the data plane, or that the data plane consists only of transit packets. There are exceptions, and in this case, routers may be required to perform some additional work to forward certain data plane packets. Hence, the data plane may also include certain (transit) exception packets. When this occurs, additional router resources are required to forward data plane traffic. Two examples will help clarify this point:

  • Example 1: A packet enters the router's interface, and the router determines that it is a transit packet that needs to be delivered to a host on a directly connected Ethernet LAN segment. However, the router does not have an ARP entry for the destination IP address. In this case, the router must use its control plane to "ARP" for the destination MAC address. Once the MAC address has been obtained, the packet (and all subsequent packets destined to this IP address) can be forwarded directly without further "exceptions."

  • Example 2: A packet enters an interface on the router that has a maximum transmission unit (MTU) of 1500 bytes. The router determines that the transit packet should be forwarded out an interface with an MTU of 1300 bytes. This requires the router to fragment the packet. Thus, the router must determine whether this is allowable by first checking the DF (Don't Fragment) bit in the IP header (see Figure 1-4). If the DF bit is set to 0, the packet must be fragmented by the router and then forwarded. If the DF bit is set to 1, the router must drop the packet and then generate an error message of ICMP Type 3, Code 4 (Fragmentation Needed, Don't Fragment Set) and send it to the packet source. Either event causes additional router processing resources to be consumed.

As you can see even with just these two examples, legitimate data plane traffic can impact the performance of a router or a network by causing exception conditions that the router must fulfill through special processing. Most security books describe methods for protecting data plane traffic from various attacks. There is also the need to protect the router and network from data plane traffic under exception conditions. An effective data plane security policy must accomplish both goals.

Data plane traffic must be separated and controlled to protect the router and network against many threats. These threats can come from legitimate traffic and malicious traffic, and the data plane security policy must be prepared for either case. When the router or network performance is impacted, does it matter whether malicious traffic or legitimate traffic caused the problem? Not to the other users of the network. Thus, data plane security must ensure the delivery of customer traffic, and ensure that customer traffic, whether legitimate, malformed, or malicious, does not interfere with the proper operation of the network. Chapter 2 provides additional discussion on some of the threats to the data plane. Chapter 4, "Data Plane Security," provides detailed descriptions of the current best practices for securing the data plane.

Control Plane

The control plane is the logical entity associated with routing processes and functions used to create and maintain the necessary intelligence about the state of the network and a router's interfaces. The control plane includes network protocols, such as routing, signaling, and link-state protocols, that are used for communication between network elements, and other control protocols that are used to build network services. Thus, the control plane is how the network gets dynamically built, and provides the mechanisms for routers to understand forwarding topologies and the operational state of the network. Without the control plane, no other traffic planes would function. Figure 1-7 illustrates the basic concepts of the control plane.

Figure 1.7

Figure 1-7

Control Plane Example

The control plane always includes receive packets. Receive packets are both generated and consumed by various control processes running on the router. These may include Layer 3 packets for routing protocol processes such as OSPF and BGP, or for other processes that maintain the forwarding state such as Protocol Independent Multicast (PIM), Label Distribution Protocol (LDP), and Hot Standby Routing Protocol (HSRP).

The control plane also includes transit packets. For example, multihop eBGP packets traverse several intermediate routers between peers, and thus have transit characteristics from the perspective of the intermediate routers along their path. These eBGP packets are not destined for processes running on the intermediate routers, yet they are undoubtedly part of the control plane for the overall network. Other examples include mechanisms such as OSPF virtual-link and Resource Reservation Protocol (RSVP). ICMP is the part of the control plane that typically generates messages in response to errors in IP datagrams or for diagnostic or routing purposes.

The control plane also includes certain Layer 3 non-IP packets, such as the routing protocol IS-IS, and ARP, and the Layer 2 packets such as Layer 2 keepalives, CDP, ATM OAM, and PPP LCP frames.


Note - The control plane is typically associated with packets generated by the network elements themselves. End users typically do not interact with the control plane. The ICMP ping application is one exception where a control plane protocol may be directly employed by end users. The ping application allows end users to directly interact with the control plane to determine network reachability information.


Securing the control plane is critical to both router and network operations. If the control plane is compromised, nothing can be guaranteed about the state of the network. Compromises in the control plane may adversely affect the data plane, management plane, and services plane. This could lead to the following:

  • Service disruption: Data not being delivered

  • Unintended routing: Data traversing adversary networks for packet sniffing, rogue DNS use, and Trojan/malware insertion, for example

  • Management integrity issues: Billing, service theft, and so forth

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