The Innovation of Session Bridge Process

VoIP and other applications, such as video conferencing and networked online gaming, require many small packets to minimize latency and prevent callers from speaking over each other. Today, each packet carries the same large header set.

Current approaches to compression in routed networks largely focus on payload compression and ignore the headers despite the fact that in some low-latency applications such as VoIP, the header is actually bigger than the payload. One-way streaming applications (e.g. Netflix) allow larger payloads, but bandwidth savings are still possible using the same Session Bridge process.

Two ingrained assumptions have held back progress on header compression:

• IP Headers are needed to transport traffic on a network

• All headers at the receiving end device must be exactly the same as the headers from the origin device [Ho].

These historical assumptions led all compression attempts to maintain a single end-to-end session. That is, the delivered decompressed header must be exactly the header that was compressed when it was received at the network entry.

Session Definition
A 'session' is a logical relationship between devices that exchange data. A 'protocol stack' is a collection of software that performs the work of translating the information from an application into packets on the physical network. Stacks have multiple layers; 1. PHYsical, 2. Link, 3. NETwork, 4. session, ... 7. application. Each layer will add its own header to a payload; e.g., 2. Ethernet, 3. IP, 4. UDP, 7. RTP. Some headers maintain a session.

The Session Bridge process starts with several different assumptions:

1. Full headers, while important for upstream and downstream applications, are not needed to transport packet payloads.

2. An end device can't tell if the headers it receives are exactly the same as those the other device sent.

3. A Virtual Circuit such as a Label Switched Path delivers packets in sequence. The forced order removes the need for sequence numbers in the packets and reduces the need for buffering which is typically used by the receiver to re-order packets.

Because the original headers are not needed to route/transport, they can be stripped at the origin Session Bridge appliance and replaced with a short 8-byte bridge tag. Because the end-user device doesn’t need the exact header, the terminating Session Bridge appliance can recreate the header information as the packet exits. As a result of these breakthroughs, the Session Bridge does not need to maintain a single session across the network. Both end-user devices are unaware of the compression.

A Session Bridge module terminates protocol sessions at the front of a network, removes headers entirely, and reformats the payload with a new bridge tag to route and identify the packet stream. Rather than compress and preserve headers, SB appliances operate separate sessions at each end. The origination appliance adds the unique bridge tag to transport the packet (across the bridge) and to identify how the headers are to be recreated as the packet exits the network. Transport is simpler and more efficient when headers need not be identical. The amount of “state” information to remember is much less.

Session Bridge compression increases the efficiency of small-payload traffic (Figure 1). When combined with standard voice compression (legacy G.711 payload changed to G.729), the Session Bridge appliance quadruples the capacity of a link; the Transparent Link Session Bridge can double that capacity again. Video payloads are mostly longer, reducing but not eliminating the benefit of header compression.

MultiProtocol Label Switching (MPLS)
Routing a packet involves comparing the destination IP address in the header of each arriving packet to data in a local forwarding table that identifies the optimal way to send that packet onward. To simplify forwarding, Label Edge Routers (LERs) add a 4-byte header (the Label) before the IP address. Label Switch Routers (LSRs) process packets based on the Label alone, ignoring IP addresses. Labels in forwarding tables of network nodes define the logical Label Switched Path (LSP)--a virtual circuit. A Session Bridge instance acts as a LER when it adds a tag that includes a Label.

 
Figure 1: The Session Bridge replaces the full headers with smaller bridge tags. Two payloads per packet at 25 packets per second reduces the effective size of both header and payload.

To forward packets without traditional headers, session bridging splits the traditional single session between end devices (e.g., two phones) into two bridged sessions. As packets enter a Session Bridge appliance, sessions terminate there. A separate session (VoIP/LAN) extends from the far-end appliance to the end device, placing new headers on each exiting payload. While on the network, the packet is guided using only the tag (MPLS label, virtual circuit identifier, or Ethernet address). Appliances hide the bridging operation from the end-customer equipment.

To accomplish this feat, the Session Bridge instance:

1. Terminates the session at the Entry appliance which strips the headers from each packet payload.

2. Transports packet payloads between appliances on a Label Switched Path, Virtual Circuit, or Ethernet connection (The Bridge). Packets have much smaller bridge tags that direct the packet across the bridge. A Session Bridge instance acts as a Label Edge Router when it inserts a bridge tag that includes an MPLS Label.

3. Uses the tag to ensure the Exit appliance adds the correct headers as packets leave the appliance toward an end device.

The terms Entry and Exit refer to the direction of the flow of packets. Every appliance acts in both capacities across two unidirectional bridge channels to maintain a full-duplex, bidirectional connection.

The trade-off for efficiency from splitting the protocol sessions is the loss of error correction, also known as reliable delivery. Because media sessions can't replace lost or damaged packets quickly enough to be of use, nobody tries. Session bridging has no impact on voice or video. Reliable data transport requires TCP or similar protocol, end-to-end.

Today's routers and switches require Ethernet for transport. That is, an Ethernet header is required on any packet to cross a link. Recent announcements of programmable chips to interface with links means that legacy Ethernet requirements no longer need apply. The Session Bridge process in the NIC will replace the Ethernet header with a smaller Layer 2 header. At the link level, software reduces the header size for every packet, not limited to streaming media. Separation between protocol layers allows both forms of Session Bridge compression to operate independently or together.