Simplifying Firewall Rules with Network Programming and SRH Metadata

The Segment Routing architecture is defined in . The IPv6 instantiation of Segment Routing, also known as SRv6, leverages the Segment Routing Header (SRH) defined in to encode a list of segments, as well as some complementary information in an IPv6 header. builds upon the base SRv6 definition and introduces the concept of network programming. In a sense, the list of segments in the SRH is the source code of a network program, while the SRH TLVs represent the memory of that program. Furthermore, describes how segments can be associated with Service Functions and defines SRH TLVs specifically designed for carrying service metadata. Together, these documents define an integrated solution for underlay, overlay and SFC that uses a single header and does not require any per-flow state in the network fabric.

In an SR domain, firewall policies are applied to control how the various endpoints, users or applications are allowed to communicate between each other. These entities are categorized into classes for the purpose of applying policies to pools rather than individual entities. For example, the endpoints in Class1 may be allowed to communicate with those in either Class3 or Class4, but Class2 is can only communicate with Class4, and Class5 cannot communicate with any other class. A reference diagram is depicted on . An SRv6-enabled network interconnects 4 classes (Class1..4) and a firewall appliance is in charge of enforcing the network policies.

| +-|Class4 | +-------+ | | +-------+ +--------------------------------+ ]]> Node A is configured to steer the traffic coming from Class1 or Class2 and headed to Class3 or Class4 into an SRv6 service policy to Node B, via the firewall F1. As part of the steering process, Node A identifies the source and destination classes, encapsulates the traffic and attaches an SRH that contains the SR Policy SID-list, as well as the class information in the SRH TLVs. The procedure to identify the traffic classes is out of the scope of this document. Node B is similarly configured to handle flows in the reverse direction. The firewall F1 reads the SRH TLVs and decides to forward or drop the traffic based on the combination of the source and destination classes. The availability of class metadata allows the firewall rule-set size to scale with the number of valid (source class, destination class) pairs. This drastically simplifies the firewall configuration and operation compared to a traditional 5-tuple-based model with tens of thousands of entries. In , a traffic flow from Class1 to Class3 is steered into the SRv6 Policy <B:F1:A::, B:B:D3::>, where B:F1:A:: represents a service SID instantiated on the firewall F1 and B:B:D3:: is an End.DX4 SID on the egress node B that sends the inner packet to Class3. The SRH S-class and D-class TLVs respectively represent the source and destination class identifiers. This traffic flow is allowed to traverse the firewall and reaches its final destination in Class3.

| ---------------------> | --> | | +--------------------------------+ +--------------+ +--------------+ +--------------+ +--------------+ |IP4(10.0.1.12,| | IP6(A, F1) | | IP6(A, B) | |IP4(10.0.1.12,| | 10.3.0.34)| +--------------+ +--------------+ | 10.3.0.34)| +--------------+ |SRH(B:B:D3::, | |SRH(B:B:D3::, | +--------------+ |B:F1:A::;SL=1;| |B:F1:A::;SL=0;| | S-class=Cl1;| | S-class=Cl1;| | D-class=Cl3)| | D-class=Cl3)| +--------------+ +--------------+ |IP4(10.0.1.12,| |IP4(10.0.1.12,| | 10.3.0.34)| | 10.3.0.34)| +--------------+ +--------------+ ]]> In , a traffic flow from Class2 to Class3 is steered into the exact same SRv6 Policy <B:F1:A::, B:B:D3::>. The SRH S-class and D-class TLVs are similarly populated with the source and destination class identifiers. However, S-class=Cl2 and D-class=Cl3 does not match an authorized class combination on the firewall. The traffic is considered as invalid and dropped at F1.

| --------> X | | | +--------------------------------+ +--------------+ +--------------+ |IP4(10.0.1.12,| | IP6(A, F1) | | 10.3.0.34)| +--------------+ +--------------+ |SRH(B:B:D3::, | |B:F1:A::;SL=1;| | S-class=Cl2;| | D-class=Cl3)| +--------------+ |IP4(10.0.1.12,| | 10.3.0.34)| +--------------+ ]]>