Cisco Systems, Inc.
J.Y. Le Boudec
An Expedited Forwarding PHB (Per-Hop Behavior)
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright © The Internet Society (2001). All Rights Reserved.
This document defines a PHB (per-hop behavior) called Expedited Forwarding (EF). The PHB is a basic building block in the Differentiated Services architecture. EF is intended to provide a building block for low delay, low jitter and low loss services by ensuring that the EF aggregate is served at a certain configured rate. This document obsoletes RFC 2598.
Table of Contents
1 Introduction ........................................... 2 1.1 Relationship to RFC 2598 ............................... 3 2 Definition of EF PHB ................................... 3 2.1 Intuitive Description of EF ............................ 3 2.2 Formal Definition of the EF PHB ........................ 5 2.3 Figures of merit ....................................... 8 2.4 Delay and jitter ....................................... 8 2.5 Loss ................................................... 9 2.6 Microflow misordering .................................. 9 2.7 Recommended codepoint for this PHB ..................... 9 2.8 Mutability ............................................. 10 2.9 Tunneling .............................................. 10 2.10 Interaction with other PHBs ............................ 10 3 Security Considerations ................................ 10 4 IANA Considerations .................................... 11 5 Acknowledgments ........................................ 11 6 References ............................................. 11 Appendix: Implementation Examples .............................. 12 Authors' Addresses ............................................. 14 Full Copyright Statement ....................................... 16
Network nodes that implement the differentiated services enhancements to IP use a codepoint in the IP header to select a per-hop behavior (PHB) as the specific forwarding treatment for that packet [3, 4]. This memo describes a particular PHB called expedited forwarding (EF).
The intent of the EF PHB is to provide a building block for low loss, low delay, and low jitter services. The details of exactly how to build such services are outside the scope of this specification.
The dominant causes of delay in packet networks are fixed propagation delays (e.g. those arising from speed-of-light delays) on wide area links and queuing delays in switches and routers. Since propagation delays are a fixed property of the topology, delay and jitter are minimized when queuing delays are minimized. In this context, jitter is defined as the variation between maximum and minimum delay. The intent of the EF PHB is to provide a PHB in which suitably marked packets usually encounter short or empty queues. Furthermore, if queues remain short relative to the buffer space available, packet loss is also kept to a minimum.
To ensure that queues encountered by EF packets are usually short, it is necessary to ensure that the service rate of EF packets on a given output interface exceeds their arrival rate at that interface over long and short time intervals, independent of the load of other (non-EF) traffic. This specification defines a PHB in which EF packets are guaranteed to receive service at or above a configured rate and provides a means to quantify the accuracy with which this service rate is delivered over any time interval. It also provides a means to quantify the maximum delay and jitter that a packet may experience under bounded operating conditions.
Note that the EF PHB only defines the behavior of a single node. The specification of behavior of a collection of nodes is outside the scope of this document. A Per-Domain Behavior (PDB) specification  may provide such information.
When a DS-compliant node claims to implement the EF PHB, the implementation MUST conform to the specification given in this document. However, the EF PHB is not a mandatory part of the Differentiated Services architecture - a node is NOT REQUIRED to implement the EF PHB in order to be considered DS-compliant.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 .
1.1. Relationship to RFC 2598
This document replaces RFC 2598 . The main difference is that it adds mathematical formalism to give a more rigorous definition of the behavior described. The full rationale for this is given in .
2. Definition of EF PHB
2.1. Intuitive Description of EF
Intuitively, the definition of EF is simple: the rate at which EF traffic is served at a given output interface should be at least the configured rate R, over a suitably defined interval, independent of the offered load of non-EF traffic to that interface. Two difficulties arise when we try to formalize this intuition:
- it is difficult to define the appropriate timescale at which to measure R. By measuring it at short timescales we may introduce sampling errors; at long timescales we may allow excessive jitter.
- EF traffic clearly cannot be served at rate R if there are no EF packets waiting to be served, but it may be impossible to determine externally whether EF packets are actually waiting to be served by the output scheduler. For example, if an EF packet has entered the router and not exited, it may be awaiting service, or it may simply have encountered some processing or transmission delay within the router.
The formal definition below takes account of these issues. It assumes that EF packets should ideally be served at rate R or faster, and bounds the deviation of the actual departure time of each packet from the "ideal" departure time of that packet. We define the departure time of a packet as the time when the last bit of that packet leaves the node. The "ideal" departure time of each EF packet is computed iteratively.
In the case when an EF packet arrives at a device when all the previous EF packets have already departed, the computation of the ideal departure time is simple. Service of the packet should (ideally) start as soon as it arrives, so the ideal departure time is simply the arrival time plus the ideal time to transmit the packet at rate R. For a packet of length L_j, that transmission time at the configured rate R is L_j/R. (Of course, a real packet will typically get transmitted at line rate once its transmission actually starts, but we are calculating the ideal target behavior here; the ideal service takes place at rate R.)
In the case when an EF packet arrives at a device that still contains EF packets awaiting service, the computation of the ideal departure time is more complicated. There are two cases to be considered. If the previous (j-1-th) departure occurred after its own ideal departure time, then the scheduler is running "late". In this case, the ideal time to start service of the new packet is the ideal departure time of the previous (j-1-th) packet, or the arrival time of the new packet, whichever is later, because we cannot expect a packet to begin service before it arrives. If the previous (j-1-th) departure occurred before its own ideal departure time, then the scheduler is running "early". In this case, service of the new packet should begin at the actual departure time of the previous packet.
Once we know the time at which service of the j-th packet should (ideally) begin, then the ideal departure time of the j-th packet is L_j/R seconds later. Thus we are able to express the ideal departure time of the j-th packet in terms of the arrival time of the j-th packet, the actual departure time of the j-1-th packet, and the ideal departure time of the j-1-th packet. Equations eq_1 and eq_2 in Section 2.2 capture this relationship.
Whereas the original EF definition did not provide any means to guarantee the delay of an individual EF packet, this property may be desired. For this reason, the equations in Section 2.2 consist of two parts: an "aggregate behavior" set and a "packet-identity-aware" set of equations. The aggregate behavior equations (eq_1 and eq_2) simply describe the properties of the service delivered to the EF aggregate by the device. The "packet-identity-aware" equations (eq_3 and eq_4) enable the bound on delay of an individual packet to be calculated given a knowledge of the operating conditions of the device. The significance of these two sets of equations is discussed further in Section 2.2. Note that these two sets of equations provide two ways of characterizing the behavior of a single device, not two different modes of behavior.
2.2. Formal Definition of the EF PHB
A node that supports EF on an interface I at some configured rate R MUST satisfy the following equations:
d_j <= f_j + E_a for all j > 0 (eq_1)
where f_j is defined iteratively by
f_0 = 0, d_0 = 0 f_j = max(a_j, min(d_j-1, f_j-1)) + l_j/R, for all j > 0 (eq_2)
In this definition:
- d_j is the time that the last bit of the j-th EF packet to depart actually leaves the node from the interface I.
- f_j is the target departure time for the j-th EF packet to depart from I, the "ideal" time at or before which the last bit of that packet should leave the node.
- a_j is the time that the last bit of the j-th EF packet destined to the output I actually arrives at the node.
- l_j is the size (bits) of the j-th EF packet to depart from I. l_j is measured on the IP datagram (IP header plus payload) and does not include any lower layer (e.g. MAC layer) overhead.
- R is the EF configured rate at output I (in bits/second).
- E_a is the error term for the treatment of the EF aggregate. Note that E_a represents the worst case deviation between the actual departure time of an EF packet and the ideal departure time of the same packet, i.e. E_a provides an upper bound on (d_j - f_j) for all j.
- d_0 and f_0 do not refer to a real packet departure but are used purely for the purposes of the recursion. The time origin should be chosen such that no EF packets are in the system at time 0.
- for the definitions of a_j and d_j, the "last bit" of the packet includes the layer 2 trailer if present, because a packet cannot generally be considered available for forwarding until such a trailer has been received.
An EF-compliant node MUST be able to be characterized by the range of possible R values that it can support on each of its interfaces while conforming to these equations, and the value of E_a that can be met on each interface. R may be line rate or less. E_a MAY be specified as a worst-case value for all possible R values or MAY be expressed as a function of R.
Note also that, since a node may have multiple inputs and complex internal scheduling, the j-th EF packet to arrive at the node destined for a certain interface may not be the j-th EF packet to depart from that interface. It is in this sense that eq_1 and eq_2 are unaware of packet identity.
In addition, a node that supports EF on an interface I at some configured rate R MUST satisfy the following equations:
D_j <= F_j + E_p for all j > 0 (eq_3)
where F_j is defined iteratively by
F_0 = 0, D_0 = 0 F_j = max(A_j, min(D_j-1, F_j-1)) + L_j/R, for all j > 0 (eq_4)
In this definition:
- D_j is the actual departure time of the individual EF packet that arrived at the node destined for interface I at time A_j, i.e., given a packet which was the j-th EF packet destined for I to arrive at the node via any input, D_j is the time at which the last bit of that individual packet actually leaves the node from the interface I.
- F_j is the target departure time for the individual EF packet that arrived at the node destined for interface I at time A_j.
- A_j is the time that the last bit of the j-th EF packet destined to the output I to arrive actually arrives at the node.
- L_j is the size (bits) of the j-th EF packet to arrive at the node that is destined to output I. L_j is measured on the IP datagram (IP header plus payload) and does not include any lower layer (e.g. MAC layer) overhead.
- R is the EF configured rate at output I (in bits/second).
- E_p is the error term for the treatment of individual EF packets. Note that E_p represents the worst case deviation between the actual departure time of an EF packet and the ideal departure time of the same packet, i.e. E_p provides an upper bound on (D_j - F_j) for all j.
- D_0 and F_0 do not refer to a real packet departure but are used purely for the purposes of the recursion. The time origin should be chosen such that no EF packets are in the system at time 0.
- for the definitions of A_j and D_j, the "last bit" of the packet includes the layer 2 trailer if present, because a packet cannot generally be considered available for forwarding until such a trailer has been received.
It is the fact that D_j and F_j refer to departure times for the j-th packet to arrive that makes eq_3 and eq_4 aware of packet identity. This is the critical distinction between the last two equations and the first two.
An EF-compliant node SHOULD be able to be characterized by the range of possible R values that it can support on each of its interfaces while conforming to these equations, and the value of E_p that can be met on each interface. E_p MAY be specified as a worst-case value for all possible R values or MAY be expressed as a function of R. An E_p value of "undefined" MAY be specified. For discussion of situations in which E_p may be undefined see the Appendix and .
For the purposes of testing conformance to these equations, it may be necessary to deal with packet arrivals on different interfaces that are closely spaced in time. If two or more EF packets destined for the same output interface arrive (on different inputs) at almost the same time and the difference between their arrival times cannot be measured, then it is acceptable to use a random tie-breaking method to decide which packet arrived "first".
2.3. Figures of merit
E_a and E_p may be thought of as "figures of merit" for a device. A smaller value of E_a means that the device serves the EF aggregate more smoothly at rate R over relatively short timescales, whereas a larger value of E_a implies a more bursty scheduler which serves the EF aggregate at rate R only when measured over longer intervals. A device with a larger E_a can "fall behind" the ideal service rate R by a greater amount than a device with a smaller E_a.
A lower value of E_p implies a tighter bound on the delay experienced by an individual packet. Factors that might lead to a higher E_p might include a large number of input interfaces (since an EF packet might arrive just behind a large number of EF packets that arrived on other interfaces), or might be due to internal scheduler details (e.g. per-flow scheduling within the EF aggregate).
We observe that factors that increase E_a such as those noted above will also increase E_p, and that E_p is thus typically greater than or equal to E_a. In summary, E_a is a measure of deviation from ideal service of the EF aggregate at rate R, while E_p measures both non-ideal service and non-FIFO treatment of packets within the aggregate.
For more discussion of these issues see the Appendix and .
2.4. Delay and jitter
Given a known value of E_p and a knowledge of the bounds on the EF traffic offered to a given output interface, summed over all input interfaces, it is possible to bound the delay and jitter that will be experienced by EF traffic leaving the node via that interface. The delay bound is
D = B/R + E_p (eq_5) where
- R is the configured EF service rate on the output interface
- the total offered load of EF traffic destined to the output interface, summed over all input interfaces, is bounded by a token bucket of rate r <= R and depth B
Since the minimum delay through the device is clearly at least zero, D also provides a bound on jitter. To provide a tighter bound on jitter, the value of E_p for a device MAY be specified as two separate components such that
E_p = E_fixed + E_variable
where E_fixed represents the minimum delay that can be experienced by an EF packet through the node.
The EF PHB is intended to be a building block for low loss services. However, under sufficiently high load of EF traffic (including unexpectedly large bursts from many inputs at once), any device with finite buffers may need to discard packets. Thus, it must be possible to establish whether a device conforms to the EF definition even when some packets are lost. This is done by performing an "off-line" test of conformance to equations 1 through 4. After observing a sequence of packets entering and leaving the node, the packets which did not leave are assumed lost and are notionally removed from the input stream. The remaining packets now constitute the arrival stream (the a_j's) and the packets which left the node constitute the departure stream (the d_j's). Conformance to the equations can thus be verified by considering only those packets that successfully passed through the node.
In addition, to assist in meeting the low loss objective of EF, a node MAY be characterized by the operating region in which loss of EF due to congestion will not occur. This MAY be specified, using a token bucket of rate r <= R and burstsize B, as the sum of traffic across all inputs to a given output interface that can be tolerated without loss.
In the event that loss does occur, the specification of which packets are lost is beyond the scope of this document. However it is a requirement that those packets not lost MUST conform to the equations of Section 2.2.
2.6. Microflow misordering
Packets belonging to a single microflow within the EF aggregate passing through a device SHOULD NOT experience re-ordering in normal operation of the device.
2.7. Recommended codepoint for this PHB
Codepoint 101110 is RECOMMENDED for the EF PHB.
Packets marked for EF PHB MAY be remarked at a DS domain boundary only to other codepoints that satisfy the EF PHB. Packets marked for EF PHBs SHOULD NOT be demoted or promoted to another PHB by a DS domain.
When EF packets are tunneled, the tunneling packets SHOULD be marked as EF. A full discussion of tunneling issues is presented in .
2.10. Interaction with other PHBs
Other PHBs and PHB groups may be deployed in the same DS node or domain with the EF PHB. The equations of Section 2.2 MUST hold for a node independent of the amount of non-EF traffic offered to it.
If the EF PHB is implemented by a mechanism that allows unlimited preemption of other traffic (e.g., a priority queue), the implementation MUST include some means to limit the damage EF traffic could inflict on other traffic (e.g., a token bucket rate limiter). Traffic that exceeds this limit MUST be discarded. This maximum EF rate, and burst size if appropriate, MUST be settable by a network administrator (using whatever mechanism the node supports for non- volatile configuration).
3. Security Considerations
To protect itself against denial of service attacks, the edge of a DS domain SHOULD strictly police all EF marked packets to a rate negotiated with the adjacent upstream domain. Packets in excess of the negotiated rate SHOULD be dropped. If two adjacent domains have not negotiated an EF rate, the downstream domain SHOULD use 0 as the rate (i.e., drop all EF marked packets).
In addition, traffic conditioning at the ingress to a DS-domain MUST ensure that only packets having DSCPs that correspond to an EF PHB when they enter the DS-domain are marked with a DSCP that corresponds to EF inside the DS-domain. Such behavior is as required by the Differentiated Services architecture . It protects against denial-of-service and theft-of-service attacks which exploit DSCPs that are not identified in any Traffic Conditioning Specification provisioned at an ingress interface, but which map to EF inside the DS-domain.
4. IANA Considerations
This document allocates one codepoint, 101110, in Pool 1 of the code space defined by .
This document was the result of collaboration and discussion among a large number of people. In particular, Fred Baker, Angela Chiu, Chuck Kalmanek, and K. K. Ramakrishnan made significant contributions to the new EF definition. John Wroclawski provided many helpful comments to the authors. This document draws heavily on the original EF PHB definition of Jacobson, Nichols, and Poduri. It was also greatly influenced by the work of the EFRESOLVE team of Armitage, Casati, Crowcroft, Halpern, Kumar, and Schnizlein.
 Jacobson, V., Nichols, K. and K. Poduri, "An Expedited Forwarding PHB", RFC 2598, June 1999.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.  Black, D., Blake, S., Carlson, M., Davies, E., Wang, Z. and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998.  Black, D., "Differentiated Services and Tunnels", RFC 2983, October 2000.  Charny, A., Baker, F., Davie, B., Bennett, J.C.R., Benson, K., Le Boudec, J.Y., Chiu, A., Courtney, W., Davari, S., Firoiu, V., Kalmanek, C., Ramakrishnan, K.K. and D. Stiliadis, "Supplemental Information for the New Definition of the EF PHB (Expedited Forwarding Per-Hop Behavior)", RFC 3247, March 2002.  Nichols K. and B. Carpenter, "Definition of Differentiated Services Per Domain Behaviors and Rules for their Specification", RFC 3086, April 2001.
Appendix: Implementation Examples
This appendix is not part of the normative specification of EF. However, it is included here as a possible source of useful information for implementors.
A variety of factors in the implementation of a node supporting EF will influence the values of E_a and E_p. These factors are discussed in more detail in , and include both output schedulers and the internal design of a device.
A priority queue is widely considered as the canonical example of an implementation of EF. A "perfect" output buffered device (i.e. one which delivers packets immediately to the appropriate output queue) with a priority queue for EF traffic will provide both a low E_a and a low E_p. We note that the main factor influencing E_a will be the inability to pre-empt an MTU-sized non-EF packet that has just begun transmission at the time when an EF packet arrives at the output interface, plus any additional delay that might be caused by non- pre-emptable queues between the priority queue and the physical interface. E_p will be influenced primarily by the number of interfaces.
Another example of an implementation of EF is a weighted round robin scheduler. Such an implementation will typically not be able to support values of R as high as the link speeds, because the maximum rate at which EF traffic can be served in the presence of competing traffic will be affected by the number of other queues and the weights given to them. Furthermore, such an implementation is likely to have a value of E_a that is higher than a priority queue implementation, all else being equal, as a result of the time spent serving non-EF queues by the round robin scheduler.
Finally, it is possible to implement hierarchical scheduling algorithms, such that some non-FIFO scheduling algorithm is run on sub-flows within the EF aggregate, while the EF aggregate as a whole could be served at high priority or with a large weight by the top- level scheduler. Such an algorithm might perform per-input scheduling or per-microflow scheduling within the EF aggregate, for example. Because such algorithms lead to non-FIFO service within the EF aggregate, the value of E_p for such algorithms may be higher than for other implementations. For some schedulers of this type it may be difficult to provide a meaningful bound on E_p that would hold for any pattern of traffic arrival, and thus a value of "undefined" may be most appropriate.
It should be noted that it is quite acceptable for a Diffserv domain to provide multiple instances of EF. Each instance should be characterizable by the equations in Section 2.2 of this specification. The effect of having multiple instances of EF on the E_a and E_p values of each instance will depend considerably on how the multiple instances are implemented. For example, in a multi- level priority scheduler, an instance of EF that is not at the highest priority may experience relatively long periods when it receives no service while higher priority instances of EF are served. This would result in relatively large values of E_a and E_p. By contrast, in a WFQ-like scheduler, each instance of EF would be represented by a queue served at some configured rate and the values of E_a and E_p could be similar to those for a single EF instance.
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