Internet Engineering Task Force Juha Heinanen INTERNET DRAFT Telia Finland Expires March 1999 Fred Baker Cisco Systems Walter Weiss Lucent Technologies John Wroclawski MIT LCS September, 1998 Assured Forwarding PHB Group Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Abstract This document proposes a general use Differentiated Services (DS) [Blake] Per-Hop-Behavior (PHB) Group called Assured Forwarding (AF). The AF PHB group provides delivery of IP packets in four independently forwarded AF classes. Within each AF class, an IP packet can be assigned one of three different levels of drop precedence. A DS node will not reorder IP packets of the same microflow if they belong to the same AF class. 1. Purpose and Overview There is a demand to offer assured delivery of IP packets over the Heinanen Assured Forwarding PHB Group [Page 1] INTERNET DRAFT September, 1998 Internet. In a typical application, a company uses the Internet to connect its geographically distributed sites and wants assured delivery of IP packets within this intranet as long as the aggregate traffic from a site does not exceed the subscribed information rate (profile). It is desirable that the site may exceed the subscribed profile with the understanding that the excess traffic is not delivered with as high a probability as the traffic that is within the profile. It is also important that the network does not reorder packets that belong to the same microflow no matter if they are in or out of the profile. Assured Forwarding (AF) PHB group is a means for a provider DS domain to offer different levels of delivery assurances and, if so desired, also different levels of delay and jitter, for IP packets received from a customer DS domain. IP packets that wish to use the services provided by the AF PHB group are assigned by the customer or the provider DS domain into up to four subscribed AF classes, where each AF class is in each DS node allocated a certain amount of forwarding resources (buffer space, bandwidth). Within each AF class IP packets are marked (again by the customer or the provider DS domain) with one of three possible drop precedence values. In case of congestion, the drop precedence of a packet determines the relative importance of the packet within the AF class. A congested DS node tries to protect packets with a lower drop precedence value from being lost by first discarding packets with a higher drop precedence value. In a DS node, the level of delivery assurance and forwarding delay of an IP packet thus depends on (1) how much forwarding resources has been allocated to the AF class that the packet belongs to, (2) what is the current load of the AF class, and, in case of congestion, (3) what is the drop precedence of the packet. For example, if traffic conditioning actions at the ingress of the provider DS domain make sure that an AF class in the DS nodes is only moderately loaded by packets with the lowest drop precedence value and is not overloaded by packets with the two lowest drop precedence values, then the AF class can offer a high level of delivery assurance for packets that are within the subscribed profile and offer up to two lower levels of delivery assurance for the excess traffic. 2. The AF PHB Group Assured Forwarding (AF) PHB group provides delivery of IP packets in N independent AF classes. Within each AF class, an IP packet can be assigned one of M different levels of drop precedence. An IP packet Heinanen Assured Forwarding PHB Group [Page 2] INTERNET DRAFT September, 1998 that belongs to an AF class i and has drop precedence j is marked with the AF code point AFij, where 1 <= i <= N and 1 <= j <= M. At this point, we define four classes (N=4), with three drop precedences in each class (M=3). A DS node must allocate forwarding resources (buffer space and bandwidth) to AF classes so that, relative to the loads, the AF class k has no more forwarding resources than the AF class l if k < l. Similarly, within an AF class, an IP packet with drop precedence l must not be delivered with greater probability than an IP packet with drop precedence k if k < l. A DS node must not reorder AF packets of the same microflow when they belong to the same AF class regardless of their drop precedence. There is no timing requirements (delay or delay variation) associated with the forwarding of AF packets. The delay properties of an AF class are determined by the forwarding resources and load assigned to the class. The AF PHB group is proposed for general use. It can be used to implement either end-to-end or domain edge-to-domain edge services. 3. Traffic Conditioning Actions If a provider DS domain wants to commit to a customer DS domain that an AF class provides a certain level of delivery and/or delay assurance, the provider DS domain must at the ingress limit the amount and/or kind of traffic that enters the AF class. The traffic conditioning actions that may be used to limit the incoming traffic include packet discarding and packet demotion. Packet demotion means increasing the drop precedence of the packet. A DS node at the edge of a DS domain may also promote a packet if the amount of lower drop precedence traffic in that AF class would otherwise be less than what has been agreed between the two DS domains. Packet promotion means increasing the drop precedence of the packet. 4. Queueing and Discard Behavior A DS node should implement all four AF classes. Packets in one AF class must be forwarded independently from packets in another AF class, i.e., a DS node must aggregate two or more AF classes together. Within each AF class, the three drop precedence values must yield at least two different levels of loss probability. In some networks, particularly in enterprise networks, where transient congestion is a Heinanen Assured Forwarding PHB Group [Page 3] INTERNET DRAFT September, 1998 rare and brief occurrence, it may be reasonable to only support two actual levels of drop precedence. While this may suffice for some networks, two actual levels of drop precedence should be avoided in networks where congestion is a common occurrence. If only two actual levels of drop precedence is supported, then the lowest drop precedence level (AFx1) must be mapped to the one that yields the lower loss probability and the two higher levels (AFx2 and AFx3) to the one that yields the higher loss probability. In order to protect packets within an AF class x that have been marked with a lower drop precedence value from being discarded with greater probability than packets marked with a higher drop precedence value, a DS node must not start discarding AFxi packets before it starts discarding AFxj packets if i < j. Inconsistent discard behaviors lead to inconsistent end-to-end service semantics. It is recommended that the discard mechanism is based on a RED-like algorithm with three configurable levels of drop precedence and a configurable averaging function (interval). Future versions of this document may say more about specific aspects of the desirable behavior. 5. Tunneling AF packets may be tunneled provided that the PHB of the tunneling packet does not reduce the delivery assurance of the tunneled AF packet and does not cause reordering of AF packets belonging to the same microflow. 6. Recommended Codepoints It is recommended that the AF codepoints AF10, AF20, AF30, and AF40, i.e., the codepoints that denote the lowest drop precedence in each AF class, are mapped to the Class Selector [Nichols] codepoints '010000', '011000', '100000', '101000'. This is done in order to save DS code space, because the forwarding rules associated with these AF codepoints are consistent and compatible with the forwarding rules of the corresponding Class Selector codepoints. The recommended values of the remaining AF codepoints are as follows: AF11 = '010010', AF12 = '010100', AF21 = '011010' AF22 = '011100', AF31 = '100010' AF32 = '100100', AF41 = '101010' and AF42 = '101100'. The table below summarizes the recommended AF codepoint values. Heinanen Assured Forwarding PHB Group [Page 4] INTERNET DRAFT September, 1998 Class 1 Class 2 Class 3 Class 4 +----------+----------+----------+----------+ Low Drop Pref | 010000 | 011000 | 100000 | 101000 | Medium Drop Pref | 010010 | 011010 | 100010 | 101010 | High Drop Pref | 010100 | 011100 | 100100 | 101100 | +----------+----------+----------+----------+ 7. Interactions with Other PHB Groups The AF codepoint mapping recommended above does not interfere with the local use spaces nor does it use the Class Selector codepoints '00x000' and '11x000'. The PHBs selected by those Class Selector codepoints can thus coexist with the AF PHB group, and retain the forwarding behavior and relationships that was defined for them in [Nichols]. In particular, the Default PHB codepoint of '000000' may remain to be used for conventional best effort traffic. Similarly, the codepoints '11x000' can remain to be used for network control traffic. In addition to the Class Selector PHBs, any other PHB groups may co- exist with the AF group within the same DS domain provided that the other PHB groups don't preempt the resources allocated to the AF classes. 8. Security Implications In order to protect itself against denial of service attacks, a provider DS domain must limit the traffic entering the domain to the subscribed profiles. Also, in order to protect a link to a customer DS domain from denial of service attacks, the provider DS domain should allow the customer DS domain to specify how the resources of a link to the customer DS domain are allocated to AF packets. If a service offering requires that traffic marked with an AF codepoint be limited by such attributes as source or destination address, it is the responsibility of the ingress node in a network to verify validity of such attributes. Appendix: Example Services The AF PHB group can be used to implement, for example, the so-called Olympic service, which consists of three service classes: bronze, silver, and gold. Packets are assigned to these three classes so that packets in the gold class experience lighter load (and thus have greater probability for timely delivery) than packets assigned to the silver class. Same kind of relationship exists between the silver class and the bronze class. If desired, packets within each class can be further separated by giving them either low, medium, or high drop precedence. Heinanen Assured Forwarding PHB Group [Page 5] INTERNET DRAFT September, 1998 The bronze, silver, and gold service classes can in the network be mapped to the AF classes 1, 2, and 3. Similarly, low, medium, and high drop precedence can be mapped to AF drop precedence indexes 1, 2, or 3. The drop precedence level of a packet can be assigned, for example, by using a dual leaky bucket traffic policer, which has as its parameters a rate and two burst sizes: a committed burst and an excess burst. If a packet falls within the committed burst, it is assigned low drop precedence. If a packet falls between the committed burst and the excess burst, it is assigned medium drop precedence. And finally, if the packet falls out of the excess burst, it is assigned high drop precedence. Another possibility would be to limit the user traffic of an Olympic service class to a given peak rate and distribute it evenly across each drop precedence. This would yield a proportional bandwidth service, which equally apportions available capacity during times of congestion under the assumption that customers with high bandwidth microflows have subscribed to higher peak rates than customers with low bandwidth microflows. The AF PHB group could also be used to implement a low loss, low delay, and low jitter service using an over provisioned AF class, if the maximum arrival rate to that class is known a priori in each DS node. Specification of the required admission control services, however, is beyond the scope of this document. References [Blake] Blake, Steve, et al., An Architecture for Differentiated Services. Internet draft draft-ietf-diffserv-arch-01.txt, August 1998. [Nichols] Nichols, Kathleen, et al., Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers. Internet draft draft-ietf-diffserv-header-02.txt, August 1998. Author Information Juha Heinanen Telia Finland Myyrmaentie 2 01600 Vantaa, Finland Email: jh@telia.fi Fred Baker Cisco Systems Heinanen Assured Forwarding PHB Group [Page 6] INTERNET DRAFT September, 1998 519 Lado Drive Santa Barbara, California 93111 E-mail: fred@cisco.com Walter Weiss Lucent Technologies 300 Baker Avenue, Suite 100, Concord, MA 01742-2168 E-mail: wweiss@lucent.com John Wroclawski MIT Laboratory for Computer Science 545 Technology Sq. Cambridge, MA 02139 Email: jtw@lcs.mit.edu Heinanen Assured Forwarding PHB Group [Page 7]