A Burst Allowance Is Not a Second Network Bitrate

Read the rate and the two burst capacities

A network offer can state an information rate alongside a burst allowance. The useful question is how much arriving traffic can receive each treatment, given its packet sizes and recent history. A larger allowance can change a short burst's eligibility without changing the replenishment rate.

Use a named contract: RFC 2697, A Single Rate Three Color Marker, published in September 1999 as Informational rather than an Internet Standard. Its CIR, or Committed Information Rate, counts IP bytes per second, including the IP header and excluding link-specific headers. Committed Burst Size, CBS, and Excess Burst Size, EBS, are byte capacities for buckets C and E. The examples below use its color-blind mode; color-aware operation also considers an incoming packet's previous color.

RFC 3290's May 2002 Informational model explains the underlying distinction: tokens accumulate up to a finite maximum. Idle time can restore spent credit, but credit stops growing at that maximum. A capacity expressed in bytes therefore answers a different question from a rate expressed in bytes per second.

References: [TB1][TM1]

Follow one shared refill stream

RFC 2697 starts both buckets full. Its formal model generates CIR one-byte refill opportunities per second. Each opportunity adds one token to C if C is below CBS; otherwise it adds one to E if E is below EBS. When both are full, that opportunity adds nothing. Two buckets consequently do not receive two independent CIR refills.

For an arriving packet, first test whether C can cover its complete IP length. If so, mark it green and subtract that length from C. Otherwise test E: a complete fit produces yellow and subtracts the length from E. If neither fits, mark the packet red and leave both counts unchanged.

These are credit decisions at arrival. In particular, an insufficiently funded packet is not held by this meter until credit returns.

References: [TB1][TA1]

Trace four arrivals before the next refill

Consider a wholly fictional meter with CIR = 3,000 IP bytes/s, CBS = 3,000 bytes and EBS = 1,500 bytes. Begin with C = 3,000 and E = 1,500. Four ordered packets, each exactly 1,500 IP bytes, arrive before any refill opportunity. This stipulates an accounting sequence without assigning a physical link rate or reporting a measurement.

The first packet is green: C falls from 3,000 to 1,500 while E remains 1,500. The second is also green: C falls to zero. The third cannot fit C, but fits E exactly; it is yellow and leaves E at zero. The fourth is red, with both counts still zero.

The sequence contains 6,000 arriving IP bytes. Of these, 3,000 are green, 1,500 yellow and 1,500 red. Adding CBS and EBS gives 4,500 bytes of initial credit across two colors, not 4,500 bytes of green eligibility. Nothing in this arithmetic identifies which packets are ultimately delivered.

That distinction matters when comparing an offer that increases CBS with one that increases only EBS. In an initially full meter, the extra credit is available at different stages of the color decision.

References: [TB1]

Separate green recovery from full burst recovery

Continue from C = 0 and E = 0 after the four arrivals. With no intervening packets, exactly 1,500 refill opportunities take 1,500 / 3,000 = 0.5 seconds in the stipulated formal model. Every one credits C, giving C = 1,500 and E = 0. A 1,500-byte packet arriving after those updates is green and empties C again.

Now make a separate recovery calculation from both buckets empty, with no packet arrivals throughout recovery. The first 3,000 opportunities fill C, taking 1 second. The next 1,500 fill E, taking another 0.5 seconds. Restoring the original full state takes 4,500 opportunities, or 1.5 seconds. Counting an independent 3,000-byte/s refill for E would incorrectly shorten that recovery.

The half-second green result therefore does not mean the original green-plus-yellow burst allowance has returned. Continued consumption can postpone E's recovery further. For example, starting empty and repeatedly allowing exactly 1,500 updates before each 1,500-byte arrival produces a green packet each time, while E stays at zero: C never reaches its full capacity between arrivals.

For a repeated-burst requirement, the idle interval and intervening traffic are as consequential as the advertised capacities.

References: [TB1]

A positive total balance can still leave a packet red

Take a separate snapshot of the same fictional meter with C = 1,000 bytes and E = 1,000 bytes. A 1,500-byte packet cannot fit either bucket. It is red, even though their combined balance is 2,000 bytes. Both counts remain unchanged. The decision cannot spend 1,000 from C and borrow the remaining 500 from E.

If the next packet is instead 1,000 bytes and no refill occurs between arrivals, it fits C exactly and is green. C becomes zero while E remains 1,000. Thus a red arrival neither proves that every token was exhausted nor predicts the color of a later, smaller packet.

Packet size belongs in the comparison. Even after an indefinitely long idle period, C cannot exceed CBS; an IP packet larger than CBS cannot be green under these rules. RFC 3290 distinguishes this whole-packet, strict-conformance approach from loose token accounting that can borrow future credit. The generic phrase token bucket leaves that choice unresolved.

References: [TB1][TM1]

Follow the color into the service's forwarding policy

RFC 2475's December 1998 Informational Diffserv architecture distinguishes metering from marking, shaping and dropping. A meter evaluates traffic against a profile. A marker sets a packet's differentiated-services codepoint. A shaper delays traffic to meet a profile, normally using finite packet storage; a dropper discards traffic according to the applicable policy.

A color result needs that next policy to become a forwarding claim. In the fictional sequence, the third packet being yellow does not establish that it is queued, discarded or delivered. Likewise, the fourth packet's red result does not itself report a loss. Obtain the offered service's treatment of each result.

The linked shared-buffer article addresses whether waiting packets can occupy memory. The linked deficit-scheduling article addresses which queued competitor receives output service. Neither quantity can be calculated from the 4,500-byte initial credit balance. Green eligibility alone also provides no application completion time or end-to-end throughput guarantee.

References: [TA1]

Compare eligibility and recovery under the same contract

For two offers described as single-rate, first establish whether both actually use RFC 2697 color-blind behavior. Identify which traffic shares the meter, then keep that population, packet lengths, units, initial credit and arrival spacing consistent. An isolated burst after a long idle period and the same burst following recent traffic are different comparisons.

With the fictional CIR unchanged, increasing CBS can permit more initially green whole packets and also lengthen the empty-to-full C recovery. Increasing EBS alone can provide more initially yellow credit, but C still receives refill priority. Neither change creates a second sustained replenishment rate. A short observation interval can include spending previously accumulated credit, so its green-plus-yellow byte total should not be presented as a new sustained entitlement.

For actual equipment, obtain the documented algorithm, mode, accounted packet length, effective capacities, refill behavior and color actions for the exact hardware and software release. RFC 3290's strict-versus-loose distinction is one reason a label alone is insufficient. Have the appointed network owner assess those particulars against the required traffic pattern; the fictional timings are arithmetic for the stated model.

References: [TB1][TM1]

Carry approved connection changes into a physical scope

When the selected equipment requires new or changed physical connections, the approved endpoint, assembly and route schedule becomes a separate installation input. Leviathan's Structured Cabling & Fiber Installation service can discuss agreed routing, dressing, labeling, physical checks and connection records with customer-furnished or separately sourced materials. Traffic-meter policy and network configuration remain with the appointed network owner.

Use the related service and project-inquiry links to share the approved connections, materials responsibilities, access window and required physical handoff records. The meter's burst capacities supply no cable quantity or physical acceptance result.

References: [L1]