This document is titled “SRv6 Network Programming”, which is attention grabbing. It fleshes out how Segment Routing routes IPv6 packets by Segment ID (SID), as well as defining the format of a SID for IPv6. The Introduction states, “An ingress node steers a packet through an ordered list of instructions, called segments.” When one considers this statement along with the document title, it’s apparent that network devices are indeed being given “instructions” in the programming sense, where each “instruction” is encoded in an IPv6 address.

An “instruction” (i.e., SRv6 SID) comprises a locator (which is used to route to a particular network device), and also directs the network device acting as the locator how to process the IPv6 packet. With the help of an IPv6 Segment Routing Header (SRH) header, a series of “instructions” can source route a packet through the network, where each network device acting as a locator learns how to handle the packet before possibly sending it on to the next SID (if any).

The encoding of an IPv6 address as an SRv6 SID includes a locator, a code indicating a certain function, and optionally arguments to that function. An initial set of codes (and their associated algorithms) is defined in this document. Most of the codes describe how the egress router should decapsulate the packet, with might include defining  which routing table the receiving router should look up after decapsulation.

The Security Considerations section points to the Security Considerations of the architecture document and the SRH document. Both documents focus on the fact that SRv6 is intended to be used within a single domain (e.g., provider network) and discuss routing mitigations such as filtering external traffic appropriately. They seem to assume that the boundaries of the domain itself are inviolate such that the domain boundary devices are not subverted, and that there are no bad actors within the domain. These are common assumptions for service provider networks where Segment Routing is intended to be deployed.

However, it cannot be certain that all networks deploying Service Routing to be free of bad actors, and there will certainly be some benefit to a bad actor changing “instructions” encoded in IPv6 addresses. Perhaps there is opportunity for mischief in changing the routing table argument in an End.DT46, for example. Also, as other SRv6 functions are defined (e.g., packet inspection functions), it would be important to ensure that those SIDs are not modified to avoid or decrease the quality of those inspection functions. 

The SRH document does describe an HMAC TLV that is intended to mitigate these kinds of attacks. Since neither of the referenced documents Security Considerations mention it, it would be a good idea to describe here that there are threats to changing SIDs, and point out how to mitigate them with the HMAC TLV.

Also, if I’m not mistaken, when there is just one SID then it is placed in the IPv6 DA and there is no SRH header or HMAC TLV. This is a general problem with ensuring that the DA on packets are not changed in transit, and one could use IPsec to mitigate that issue. It’s probably worthwhile mentioning something like that.