Internet Engineering Task Force R. Bless Internet-Draft Karlsruhe Institute of Technology (KIT) Obsoletes: 3662 (if approved) June 30, 2017 Updates: 4594 (if approved) Intended status: Standards Track Expires: January 1, 2018 A Lower Effort Per-Hop Behavior (LE PHB) draft-ietf-tsvwg-le-phb-02 Abstract This document specifies properties and characteristics of a Lower Effort (LE) per-hop behavior (PHB). The primary objective of this LE PHB is to protect best-effort (BE) traffic (packets forwarded with the default PHB) from LE traffic in congestion situations, i.e., when resources become scarce, best-effort traffic has precedence over LE traffic and may preempt it. There are numerous uses for this PHB, e.g., for background traffic of low precedence, such as bulk data transfers with low priority in time, non time-critical backups, larger software updates, web search engines while gathering information from web servers and so on. This document recommends a standard DSCP value for the LE PHB. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on January 1, 2018. Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. Bless Expires January 1, 2018 [Page 1] Internet-Draft Lower Effort PHB June 2017 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Deployment Considerations . . . . . . . . . . . . . . . . 5 1.3. Requirements Language . . . . . . . . . . . . . . . . . . 6 2. PHB Description . . . . . . . . . . . . . . . . . . . . . . . 6 3. Traffic Conditioning Actions . . . . . . . . . . . . . . . . 7 4. Recommended DS Codepoint . . . . . . . . . . . . . . . . . . 7 5. Remarking to other DSCPs/PHBs . . . . . . . . . . . . . . . . 7 6. Changes to RFC 4594 . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 9.1. Normative References . . . . . . . . . . . . . . . . . . 9 9.2. Informative References . . . . . . . . . . . . . . . . . 10 Appendix A. History of the LE PHB . . . . . . . . . . . . . . . 11 Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . 11 Appendix C. Change History . . . . . . . . . . . . . . . . . . . 11 Appendix D. Note to RFC Editor . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12 Bless Expires January 1, 2018 [Page 2] Internet-Draft Lower Effort PHB June 2017 1. Introduction This document defines a Differentiated Services per-hop behavior [RFC2474] called "Lower Effort" (LE) which is intended for traffic of sufficiently low urgency that all other traffic takes precedence over LE traffic in consumption of network link bandwidth. Low urgency traffic has a low priority for timely forwarding, which does not necessarily imply that it is generally of minor importance. From this viewpoint, it can be considered as a network equivalent to a background priority for processes in an operating system. There may or may not be memory (buffer) resources allocated for this type of traffic. Some networks carry traffic for which delivery is considered optional; that is, packets of this type of traffic ought to consume network resources only when no other traffic is present. Alternatively, the effect of this type of traffic on all other network traffic is strictly limited ("no harm" property). This is distinct from "best- effort" (BE) traffic since the network makes no commitment to deliver LE packets. In contrast, BE traffic receives an implied "good faith" commitment of at least some available network resources. This document proposes a Lower Effort Differentiated Services per-hop behavior (LE PHB) for handling this "optional" traffic in a differentiated services node. 1.1. Applicability A Lower Effort PHB is applicable for many applications that otherwise use best-effort delivery. More specifically, it is suitable for traffic and services that can tolerate strongly varying throughput for their data flows, especially periods of very low throughput or even starvation (i.e., long interruptions due to significant or even complete packet loss). Therefore, an application sending an LE marked flow must be able to tolerate short or (even very) long interruptions due to the presence of severe congestion conditions during the transmission of the flow. Thus, there should be an expectation that packets of the LE PHB may be excessively delayed or dropped when any other traffic is present. The LE PHB is suitable for sending traffic of low urgency across a Differentiated Services (DS) domain or DS region. LE traffic SHOULD be congestion controlled. Since LE traffic may be starved completely for a longer period of time, transport protocols or applications (and their related congestion control mechanisms) SHOULD be able to detect and react to such a situation and should resume the transfer as soon as possible. Congestion control is not only useful to let the flows within the LE behavior aggregate adapt to the available bandwidth that may be highly fluctuating, but also Bless Expires January 1, 2018 [Page 3] Internet-Draft Lower Effort PHB June 2017 in case that LE traffic is mapped to the default PHB in DS domains that do not support LE. Use of the LE PHB might assist a network operator in moving certain kinds of traffic or users to off-peak times. Alternatively, or in addition, packets can be designated for the LE PHB when the goal is to protect all other packet traffic from competition with the LE aggregate while not completely banning LE traffic from the network. An LE PHB SHOULD NOT be used for a customer's "normal internet" traffic nor should packets be "downgraded" to the LE PHB instead of being dropped, particularly when the packets are unauthorized traffic. The LE PHB is expected to have applicability in networks that have at least some unused capacity at certain periods. The LE PHB allows networks to protect themselves from selected types of traffic as a complement to giving preferential treatment to other selected traffic aggregates. LE should not be used for the general case of downgraded traffic, but may be used by design, e.g., to protect an internal network from untrusted external traffic sources. In this case there is no way for attackers to preempt internal (non LE) traffic by flooding. Another use case in this regard is forwarding of multicast traffic from untrusted sources. Multicast forwarding is currently enabled within domains only for specific sources within a domain, but not for sources from anywhere in the Internet. A main problem is that multicast routing creates traffic sources at (mostly) unpredictable branching points within a domain, potentially leading to congestion and packet loss. In case multicast packets from untrusted sources are forwarded as LE traffic, they will not harm traffic from non-LE behavior aggregates. A further related use case is mentioned in [RFC3754]: preliminary forwarding of non- admitted multicast traffic. There is no intrinsic reason to limit the applicability of the LE PHB to any particular application or type of traffic. It is intended as an additional traffic engineering tool for network administrators. For instance, it can be used to fill protection capacity of transmission links that is otherwise unused. Some network providers keep link utilization below 50% to ensure that all traffic is forwarded without loss after rerouting caused by a link failure. LE marked traffic can utilize the normally unused capacity and will be preempted automatically in case of link failure when 100% of the link capacity is required for all other traffic. Ideally, applications mark their packets as LE traffic, since they know the urgency of flows. Example uses for the LE PHB: Bless Expires January 1, 2018 [Page 4] Internet-Draft Lower Effort PHB June 2017 o For traffic caused by world-wide web search engines while they gather information from web servers. o For software updates or dissemination of new releases of operating systems. o For backup traffic or non-time critical synchronization or mirroring traffic. o For content distribution transfers between caches. o For preloading or prefetching objects from web sites. o For Netnews and other "bulk mail" of the Internet. o For "downgraded" traffic from some other PHB when this does not violate the operational objectives of the other PHB or the overall network. o For multicast traffic from untrusted (e.g., non-local) sources. 1.2. Deployment Considerations In order to enable LE support, DS nodes typically only need o A BA classifier (Behavior Aggregate classifier, see [RFC2475]) that classifies packets according to the LE DSCP o A dedicated LE queue o A suitable scheduling discipline, e.g., simple priority queueing Alternatively, implementations may use active queue management mechanisms instead of a dedicated LE queue, e.g., dropping all arriving LE packets when certain queue length or sojourn time thresholds are exceeded. Internet-wide deployment of the LE PHB is eased by the following properties: o No harm to other traffic: since the LE PHB has the lowest forwarding priority it does not consume resources from other PHBs. Deployment across different provider domains with LE support causes no trust issues or attack vectors to existing (non LE) traffic. Thus, providers can trust LE markings from end-systems, i.e., there is no need to police or remark incoming LE traffic. Bless Expires January 1, 2018 [Page 5] Internet-Draft Lower Effort PHB June 2017 o No PHB parameters or configuration of traffic profiles: the LE PHB itself possesses no parameters that need to be set or configured. Similarly, since LE traffic requires no admission or policing, it is not necessary to configure traffic profiles. o No traffic conditioning mechanisms: the LE PHB requires no traffic meters, droppers, or shapers. See also Section 3 for further discussion. DS domains that cannot or do not want to support the LE PHB should be aware that they violate the "no harm" property of LE. DS domains without LE PHB support SHOULD NOT drop LE marked packets, but rather map them to the default PHB and keep the LE DSCP. See also Section 5 for further discussion of forwarding LE traffic with the default PHB instead. 1.3. Requirements Language 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 [RFC2119]. 2. PHB Description The LE PHB is defined in relation to the default PHB (best-effort). A packet forwarded with the LE PHB SHOULD have lower precedence than packets forwarded with the default PHB, i.e., in case of congestion, LE marked traffic SHOULD be dropped prior to dropping any default PHB traffic. Ideally, LE packets SHOULD be forwarded only if no packet with any other PHB is awaiting transmission. A straightforward implementation could be a simple priority scheduler serving the default PHB queue with higher priority than the lower- effort PHB queue. Alternative implementations may use scheduling algorithms that assign a very small weight to the LE class. This, however, may sometimes cause better service for LE packets compared to BE packets in cases when the BE share is fully utilized and the LE share not. If a dedicated LE queue is not available, an active queue management mechanism within a common BE/LE queue could also be used. This could drop all arriving LE packets as soon as certain queue length or sojourn time thresholds are exceeded. Since congestion control is also useful within the LE traffic class, Explicit Congestion Notification [RFC3168] SHOULD be used for LE packets, too. Bless Expires January 1, 2018 [Page 6] Internet-Draft Lower Effort PHB June 2017 3. Traffic Conditioning Actions If possible, packets SHOULD be pre-marked in DS-aware end systems by applications due to their specific knowledge about the particular precedence of packets. There is no incentive for DS domains to distrust this initial marking, because letting LE traffic enter a DS domain causes no harm. Thus, any policing such as limiting the rate of LE traffic is not necessary at the DS boundary. As for most other PHBs an initial classification and marking can be also performed at the first DS boundary node according to the DS domain's own policies (e.g., as protection measure against untrusted sources). However, non-LE traffic (e.g., BE traffic) SHOULD NOT be remarked to LE on a regular basis without consent or knowledge of the user. See also remarks with respect to downgrading in Section 1.1. 4. Recommended DS Codepoint The RECOMMENDED codepoint for the LE PHB is '000010'. Earlier specifications [RFC4594] recommended to use CS1 as codepoint (as mentioned in [RFC3662]). This is problematic since it may cause a priority inversion in DiffServ domains that treat CS1 as originally proposed in [RFC2474], resulting in forwarding LE packets with higher precedence than BE packets. Existing implementations SHOULD therefore use the unambiguous LE codepoint '000010' whenever possible. 5. Remarking to other DSCPs/PHBs "DSCP bleaching", i.e., setting the DSCP to '000000' (default PHB) is NOT RECOMMENDED for this PHB. This may cause effects that are in contrast to the original intent in protecting BE traffic from LE traffic (no harm property). In case DS domains do not support the LE PHB, they SHOULD treat LE marked packets with the default PHB instead (by mapping the LE DSCP to the default PHB), but they SHOULD do so without remarking to DSCP '000000'. The reason for this is that later traversed DS domains may then have still the possibility to treat such packets according the LE PHB. However, operators of DS domains that forward LE traffic within the BE aggregate should be aware of the implications, i.e., induced congestion situations and quality-of-service degradation of the original BE traffic. In this case, the LE property of not harming other traffic is no longer fulfilled. In order to limit the impact in such cases, traffic policing of the LE aggregate may be used. In case LE marked packets are effectively carried within the default PHB (i.e., forwarded as best-effort traffic) they get a better Bless Expires January 1, 2018 [Page 7] Internet-Draft Lower Effort PHB June 2017 forwarding treatment than expected. For some applications and services, it is favorable if the transmission is finished earlier than expected. However, in some cases it may be against the original intention of the LE PHB user to strictly send the traffic only if otherwise unused resources are available, i.e., LE traffic may compete with BE traffic for the same resources and thus adversely affect the original BE aggregate. In some cases users want to be sure that their LE marked traffic actually fulfills the "no harm" property. One possible solution for a clear distinction in such cases would be to use two different codepoints, "LE-min = LE, better treatment allowed", "LE-strict = LE, better treatment NOT allowed". However, since DSCPs are a scarce resource, applications that want to ensure the lower precedence compared to BE traffic SHOULD use additionally a corresponding Lower-than-Best-Effort transport protocol [RFC6297], e.g., LEDBAT [RFC6817]. A DS domain that still uses DSCP CS1 for marking LE traffic (including Low Priority-Data as defined in [RFC4594] or the old definition in [RFC3662]) MUST remark traffic to the LE DSCP '000010' at the egress to the next DS domain. This increases the probability that the DSCP is preserved end-to-end, whereas a CS1 marked packet may be remarked by the default DSCP if the next domain is applying DiffServ-intercon [RFC8100]. 6. Changes to RFC 4594 [RFC4594] recommended to use CS1 as codepoint in section 4.10, whereas CS1 was defined in [RFC2474] to have a higher precedence than CS0, i.e., the default PHB. Consequently, DiffServ domains implementing CS1 according to [RFC2474] will cause a priority inversion for LE packets that contradicts with the original purpose of LE. Therefore, every occurrence of the CS1 DSCP is replaced by the LE DSCP. Changes: o The Low-Priority Data row in Figure 3 is updated as follows: |---------------+---------+-------------+--------------------------| | Low-Priority | LE | 000010 | Any flow that has no BW | | Data | | | assurance | ------------------------------------------------------------------ o The Low-Priority Data row in Figure 4 is updated as follows: Bless Expires January 1, 2018 [Page 8] Internet-Draft Lower Effort PHB June 2017 |---------------+------+-------------------+---------+--------+----| | Low-Priority | LE | Not applicable | RFCXXXX | Rate | Yes| | Data | | | | | | ------------------------------------------------------------------ o Section 4.10: The RECOMMENDED DSCP marking is LE (Lower Effort). o [RFC4594] recommended to remark Low-Priority Data to DSCP '000001' inside a DS domain that uses IP precedence marking. By using the herein defined LE DSCP such remarking is not necessary, so even if Low-Priority Data is unsupported (i.e., mapped to the default PHB) the LE DSCP should be kept across the domain as RECOMMENDED in Section 5. 7. IANA Considerations This document assigns the Differentiated Services Field Codepoint (DSCP) '000010' from the Differentiated Services Field Codepoints (DSCP) registry (https://www.iana.org/assignments/dscp-registry/dscp- registry.xml) to the LE PHB. IANA is requested to update the registry as follows: o Name: LE o Value (Binary): 000010 o Value (Decimal): 2 o Reference: [RFC number of this memo] 8. Security Considerations There are no specific security exposures for this PHB. Since it defines a new class of low forwarding priority, remarking other traffic as LE traffic may lead to quality-of-service degradation of such traffic. Thus, any attacker that is able to modify the DSCP of a packet to LE may carry out a downgrade attack. See the general security considerations in [RFC2474] and [RFC2475]. 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . Bless Expires January 1, 2018 [Page 9] Internet-Draft Lower Effort PHB June 2017 [RFC2474] 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, DOI 10.17487/RFC2474, December 1998, . [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, DOI 10.17487/RFC2475, December 1998, . 9.2. Informative References [draft-bless-diffserv-lbe-phb-00] Bless, R. and K. Wehrle, "A Lower Than Best-Effort Per-Hop Behavior", draft-bless-diffserv-lbe-phb-00 (work in progress), September 1999, . [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001, . [RFC3662] Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort Per-Domain Behavior (PDB) for Differentiated Services", RFC 3662, DOI 10.17487/RFC3662, December 2003, . [RFC3754] Bless, R. and K. Wehrle, "IP Multicast in Differentiated Services (DS) Networks", RFC 3754, DOI 10.17487/RFC3754, April 2004, . [RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration Guidelines for DiffServ Service Classes", RFC 4594, DOI 10.17487/RFC4594, August 2006, . [RFC6297] Welzl, M. and D. Ros, "A Survey of Lower-than-Best-Effort Transport Protocols", RFC 6297, DOI 10.17487/RFC6297, June 2011, . [RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind, "Low Extra Delay Background Transport (LEDBAT)", RFC 6817, DOI 10.17487/RFC6817, December 2012, . Bless Expires January 1, 2018 [Page 10] Internet-Draft Lower Effort PHB June 2017 [RFC8100] Geib, R., Ed. and D. Black, "Diffserv-Interconnection Classes and Practice", RFC 8100, DOI 10.17487/RFC8100, March 2017, . Appendix A. History of the LE PHB A first version of this PHB was suggested by Roland Bless and Klaus Wehrle in 1999 [draft-bless-diffserv-lbe-phb-00]. After some discussion in the DiffServ Working Group Brian Carpenter and Kathie Nichols proposed a bulk handling per-domain behavior and believed a PHB was not necessary. Eventually, Lower Effort was specified as per-domain behavior and finally became [RFC3662]. More detailed information about its history can be found in Section 10 of [RFC3662]. Appendix B. Acknowledgments Since text is borrowed from earlier Internet-Drafts and RFCs the co- authors of previous specifications are acknowledged here: Kathie Nichols and Klaus Wehrle. David Black and Ruediger Geib provided helpful comments and suggestions. Appendix C. Change History This section briefly lists changes between Internet-Draft versions for convenience. Changes in Version 02: o Applied many editorial suggestions from David Black o Added Multicast traffic use case o Clarified what is required for deployment in section 1.2 (Deployment Considerations) o Added text about implementations using AQMs and ECN usage o Updated IANA section according to David Black's suggestions o Revised text in the security section o Changed copyright Notice to pre5378Trust200902 Changes in Version 01: o Now obsoletes RFC 3662. Bless Expires January 1, 2018 [Page 11] Internet-Draft Lower Effort PHB June 2017 o Tried to be more precise in section 1.1 (Applicability) according to R. Geib's suggestions, so rephrased several paragraphs. Added text about congestion control o Change section 2 (PHB Description) according to R. Geib's suggestions. o Added RFC 2119 language to several sentences. o Detailed the description of remarking implications and recommendations in Section 5. o Added Section 6 to explicitly list changes with respect to RFC 4594, because this document will update it. Appendix D. Note to RFC Editor This section lists actions for the RFC editor during final formatting. o Please replace the occurrence of RFCXXXX in Section 6 with the assigned RFC number for this document. o Delete Appendix C. o Delete this section. Author's Address Roland Bless Karlsruhe Institute of Technology (KIT) Kaiserstr. 12 Karlsruhe 76131 Germany Phone: +49 721 608 46413 Email: roland.bless@kit.edu Bless Expires January 1, 2018 [Page 12]