October 07, 2001
Analyzing Frame Relay Statistics
Frame relay is an efficient wide area technology, but this increase is efficiency is countered by a more complex system of network management at the data link layer. When examining the operation of a frame relay network at the packet level, there is a lot of network management functionality that isn't found on a point-to-point WAN link.
Most frame relay networks use a management communication method called Local Management Interface, or LMI. These LMI frames allow the relay switch and the local WAN router to send keep-alive messages and status updates. These LMI frames can also transfer information about the Committed Information Rate (CIR) that is configured on the frame relay switch.
Frame relay routers or access devices are usually configured to send LMI Status Enquiry frames and receive Status frames every ten seconds. These Status Enquiry/Status frames act as the keep-alive method for the frame relay link. Here's a summary of the Status Enquiry and the Status response:
Destination Source Delta Time Protocol Summary DCE.DLCI.1023 DTE.DLCI.1023 0.000.000 LMI Keep Alive Status Enquiry DTE.DLCI.1023 DCE.DLCI.1023 0.000.260 LMI Keep Alive Status
Every sixty seconds, a Full Status message is sent from the frame relay switch. This Status Update frame contains information about each frame relay Permanent Virtual Circuit (PVC), including the channel configuration, active or inactive status, and (optionally) the CIR value that is configured for the PVC in the frame relay switch:
DLC: ----- DLC Header ----- DLC: DLC: DLC: Frame 6 arrived at 11:10:17.6444; frame size is 69 (45 hex) bytes. DLC: Destination = DTE DLC: Source = DCE DLC: FRELAY: ----- Frame Relay ----- FRELAY: FRELAY: Address word = FCF1 FRELAY: 1111 11.. 1111 .... = DLCI 1023 (LMI) FRELAY: .... ..0. .... .... = Response FRELAY: .... .... .... 0... = No forward congestion FRELAY: .... .... .... .0.. = No backward congestion FRELAY: .... .... .... ..0. = Not eligible for discard FRELAY: .... .... .... ...1 = Not extended address FRELAY: LMI: ----- Local Management Interface ----- LMI: LMI: Unnumbered Information LMI: Local Management Interface (LMI) LMI: Call reference LMI: Message type = 7D (Status) LMI: LMI: Information element 01 (Report type) LMI: Report type 00 (Full status message) LMI: LMI: Information element 03 (Keep alive) LMI: Current sequence number = 134 LMI: Last received sequence number = 131 LMI: LMI: Information element 07 (PVC status) LMI: PVC DLCI = 102 LMI: PVC status = X2 LMI: .... 00.. = Channel is present LMI: .... ..1. = Channel is active LMI: .... ...0 = Below buffer threshold LMI: Bandwidth = 32000 bits/second
Examining the information in a Full Status message can be enlightening, especially if your frame relay system is using an LMI type that sends CIR values in the Full Status messages. With this LMI type, it's easy to see if the frame relay provider has configured the PVCs with the correct CIR information.
LMI is a good source of frame relay information, but there is a frame relay header attached to each frame traversing the WAN that contains more interesting information. Each packet's frame relay header contains three important bits that will indicate Forward Explicit Congestion Notification (FECN), Backward Explicit Congestion Notification (BECN), and Discard Eligibility (DE).
FRELAY: ----- Frame Relay ----- FRELAY: FRELAY: Address word = 1841 FRELAY: 0001 10.. 0100 .... = DLCI 100 FRELAY: .... ..0. .... .... = Response FRELAY: .... .... .... 0... = No forward congestion FRELAY: .... .... .... .0.. = No backward congestion FRELAY: .... .... .... ..0. = Not eligible for discard FRELAY: .... .... .... ...1 = Not extended address
The FECN and BECN indicators are changed by the frame relay switches if congestion is encountered in the WAN. The frame relay switches will enable the FECN bit towards the congested traffic's destination and will enable the BECN indicator as traffic moves towards the congestion's source. These bits are seen by the frame relay access device, which then controls any traffic shaping or throttling. Many routers do not modify any traffic patterns, regardless of the number of frames received with a BECN or FECN indicator.
The Discard Eligible (DE) bit is used by the frame relay switch to determine which frames could be removed by the frame relay network if the CIR values are exceeded. If WAN traffic exceeds the configured CIR, then each frame exceeding this utilization rate will have the DE bit set to 1 by the frame relay switch. From this point, the frame will be eligible for discard at any hop along the frame relay cloud. Many frame relay users set their Committed Information Rate for zero kilobits per second, and rely on the frame relay cloud's available bandwidth to get traffic through the link. If the frame relay network becomes busy, these frames are the first to go!
It's almost impossible to manually keep count of all of these bits as each frame traverses the WAN. Almost all WAN analyzers include counters that track these frame relay statistics in real-time. If any FECN or BECN counters are in constant change, then you need to contact your frame relay provider and inquire about the congestion notifications. In some cases, a FECN or BECN issue may simply be an improper buffering configuration in the frame relay network!
Frame relay networks include all of these helpful network management features. Are you taking advantage of your frame relay management tools?
Posted by james_messer at October 7, 2001 11:36 PM
Posted by: Net Observer at September 19, 2006 11:21 AM