Segment Routing: Inside a New SDN Technology

Terry Slattery explains the new packet-forwarding mechanism and OpenFlow alternative.

Terry Slattery

July 19, 2016

2 Min Read
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IP packet forwarding techniques traditionally have relied on taking the shortest path to the destination IP address. But forcing different traffic types on different paths is often useful depending on application type. For example, real-time UC&C applications typically prefer low-latency, low-jitter paths, while large data applications prefer high-bandwidth paths with low packet loss. Segregating traffic according to application need is very useful. However, policy routing is painful to set up, and is static (or perhaps nearly dynamic, if you have excellent automation tools).

We've also found the need to use policy when arbitrating among multiple applications that want the same service class. Which application gets the bandwidth when a bandwidth allocation for a traffic class is full? How does the network tell the applications that no bandwidth remains in the desired traffic class? That's where software-defined networking (SDN) is desirable -- traffic forwarding selected by an application or by a traffic policy manager (see the International Multimedia Telecommunications Consortium's whitepaper on automating UC quality-of-experience services). The OpenFlow protocol has captured interest for its ability to apply policies dynamically. Now a relatively new technology offers an alternative approach: segment routing (SR).

What Is Segment Routing?
SR is an SDN technology whose packet-forwarding mechanism serves as an alternative to OpenFlow. Created by Cisco, SR is making its way through the IETF standardization process. Several vendors are implementing it, and some customers are using it.

A team of experienced networking professionals took a close look at SR in a recent one-dayTech Field Day event. You can find lots of documents about SR on the Internet, but I found that many of them gloss over important details (like assigning labels to each router/node). For some good, detailed background on SR, see:

SR uses a path-label mechanism (Multi-Protocol Label Switching, or MPLS, labels in IPv4 or a path shim header in IPv6) to specify the route packets must take through a network. The packets must go through each node in the label path, but may go through other intermediary nodes as well, making it function much like the IPv4 loose-source-routing mechanism. The loose-source-routing mechanism reduces the number of labels needed to route a packet to its desired destination. The network forwards packets along the shortest path between nodes in the label list.

Read the rest of this article on NoJitter.

About the Author(s)

Terry Slattery

Principal Architect, NetCraftsmenTerry Slattery is a principal architect at NetCraftsmen, an advanced network consulting firm that specializes in high-profile and challenging network consulting jobs. Terry is currently working on network management, SDN, business strategy consulting, and interesting legal cases. He is the founder of Netcordia, inventor of NetMRI, has been a successful technology innovator in networking during the past 20 years, and is co-inventor on two patents. He has a long history of network consulting and design work, including some of the first Cisco consulting and training. As a consultant to Cisco, he led the development of the current Cisco IOS command line interface. Prior to Netcordia, Terry founded Chesapeake Computer Consultants, which became a Cisco premier training and consulting partner. At Chesapeake, he co-invented and patented the v-LAB system to provide hands-on access to real hardware for the hands-on component of internetwork training classes.Terry co-authored the successful McGraw-Hill text "Advanced IP Routing in Cisco Networks," is the second CCIE (1026) awarded, and is a regular speaker at Enterprise Connect and Interop.

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