RFC 3248
A Delay Bound alternative revision of RFC 2598
RFC 3248: A Delay Bound alternative revision of RFC 2598 🎀
Network Working Group, Category: Informational, March 2002 💖
Authors, all in the header: G. Armitage (Swinburne University of Technology), B. Carpenter (IBM), A. Casati (Lucent Technologies), J. Crowcroft (University of Cambridge), J. Halpern (Consultant), B. Kumar (Corona Networks Inc.), and J. Schnizlein (Cisco Systems) 🫶
Obsoletes and updates: the header doesn't list anything this RFC obsoletes or updates. It's an alternative revision of RFC 2598 though, so keep that in mind 👀
Status of this Memo 📌
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited ✨
Copyright Notice ©️
Copyright (C) The Internet Society (2001). All Rights Reserved. 🔒
Abstract 🌸
Okay so, for historical interest, this document keeps the EF Design Team's proposed solution. The original authors of RFC 2598 preferred it, but the working group didn't adopt it in December 2000. The original definition of EF was based on comparing forwarding on an unloaded network. This experimental Delay Bound (DB) PHB needs a bound on the delay of packets caused by other traffic in the network 🧚
At the Pittsburgh IETF meeting in August 2000, the Differentiated Services working group had serious questions about RFC 2598, the group's standards track definition of the Expedited Forwarding (EF) Per Hop Behavior (PHB). So an 'EF Design Team' volunteered to re-express RFC 2598 while keeping those issues in mind. At the San Diego IETF meeting in December 2000, the DiffServ working group decided to pursue an alternative re-expression of the EF PHB. The main character moment is that this one didn't win, but it's still iconic 👑
Specification of Requirements 📖
This document is for Informational purposes only. If implementors choose to experiment with the DB PHB, key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are interpreted as described in RFC 2119 [3]. 🔥
1 Introduction 🦋
RFC 2598 was the Differentiated Services (DiffServ) working group's first standards track definition of the Expedited Forwarding (EF) Per Hop Behavior (PHB) [1]. As part of the DiffServ working group's ongoing refinement of the EF PHB, various issues were raised with the text in RFC 2598 [2] 💅
After the Pittsburgh IETF meeting in August 2000, a volunteer 'EF design team' was formed (the authors of this document) to propose a new expression of the EF PHB. The rest of this Informational document shares our feedback to the DiffServ working group from the San Diego IETF in December 2000. Our solution focussed on a Delay Bound (DB) based re-expression of RFC 2598 that met the goals of RFC 2598's original authors. The DiffServ working group ultimately chose an alternative re-expression of the EF PHB text. The authors of [2] developed it, and they revised it to also cover our model described here ✨
Our proposed Delay Bound solution is archived for historical interest. Section 2 covers the minimum, necessary and sufficient description of what we believed qualifies as 'DB' behavior from a single node. Section 3 then discusses a number of issues and assumptions made to support the definition in section 2 🎀
2. Definition of Delay Bound forwarding 🩷
For a traffic stream not exceeding a particular configured rate, the goal of the DB PHB is a strict bound on the delay variation of packets through a hop 💜
This section begins with the goals and necessary boundary conditions for DB behavior. Then it gives a descriptive definition of DB behavior itself, talks about what it means to conform to the DB definition, and assigns the experimental DB PHB code point 🧚
2.1 Goal and Scope of DB 🎯
For a traffic stream not exceeding a configured rate, the goal of the DB PHB is a strict bound on the delay variation of packets through a hop 💖
Traffic MUST be policed and/or shaped at the source edge (for example, on ingress to the DS-domain as discussed in RFC 2475 [5]) in order to get such a bound. However, specific policing and/or shaping rules are outside the scope of the DB PHB definition. Such rules MUST be defined in any per-domain behaviors (PDBs) composed from the DB PHB 🔒
A device (hop) delivers DB behavior to appropriately marked traffic received on one or more interfaces (marking is specified in section 2.4). A device SHALL deliver the DB behavior on an interface to DB marked traffic meeting (i.e. less than or equal) a certain arrival rate limit R ✨
If more DB traffic arrives than is acceptable, the device is NOT REQUIRED to deliver the DB behavior. However, although the original source of DB traffic will be shaped, aggregation and upstream jitter mean that the traffic arriving at any given hop can't be assumed to be shaped. So a DB implementation SHOULD have some tolerance for burstiness: the ability to provide EF behavior even when the arrival rate exceeds the rate limit R 🌈
Different DB implementations are free to show different tolerance to burstiness. (Burstiness MAY be characterized in terms of the number of back-to-back wire-rate packets to which the hop can deliver DB behavior. However, since the goal of characterizing burstiness is to allow useful comparison of DB implementations, vendors and users of DB implementations MAY choose to use other burstiness metrics.) 🍓
The DB PHB definition does NOT mandate or recommend any particular method for achieving DB behavior. Instead, it identifies parameters that bound the operating range(s) over which an implementation can deliver DB behavior. Implementors characterize their implementations using these parameters, and network designers and testers use them to assess how useful different DB implementations are 💅
2.2 Description of DB behavior 🧠
To keep things simple, the definition is explained using an example where traffic arrives on only one interface and is destined for another (single) interface 🦋
The crux of this definition is that the difference in time between when a packet might have been delivered and when it is delivered will never exceed a specifiable bound. That's the whole vibe 😭
Given an acceptable (not exceeding arrival rate limit R) stream of DB packets arriving on an interface:
There is a time sequence E(i) when these packets would be delivered at the output interface in the absence of competing traffic. That is, E(i) are the earliest times that the packets could be delivered by the device.
In the presence of competing traffic, the packets will be delayed to some later time D(i).
Competing traffic includes all DB traffic arriving at the device on other ports, and all non-DB traffic arriving at the device on any port 👀
DB is defined as the behavior which ensures, for all i, that:
D(i) - E(i) <= S * MTU/RMTU is the maximum transmission unit (packet size) of the output. R is the arrival rate that the DB device is prepared to accept on this interface 🫶
Note that D(i) and E(i) simply refer to the times of what can be thought of as "the same packet" under the two treatments, with and without competing traffic 💜
The score, S, is a characteristic of the device at the rate, R, in order to meet this defined bound. This score, preferably a small constant, depends on the scheduling mechanism and configuration of the device 🎀
2.3 Conformance to DB behavior ✅
An implementation need not conform to the DB specification over an arbitrary range of parameter values. Instead, implementations MUST specify the rates, R, and scores S, for which they claim conformance with the DB definition in section 2.2, and the implementation-specific configuration parameters needed to deliver conformant behavior. An implementation SHOULD document the traffic burstiness it can tolerate while still providing DB behavior 🌸
The score, S, and configuration parameters depend on the implementation error from an ideal scheduler. Discussion of the ability of any particular scheduler to provide DB behavior, and the conditions under which it might do so, is outside the scope of this document 🧚
The implementor MAY define additional constraints on the range of configurations in which DB behavior is delivered. These constraints MAY include limits on the total DB traffic across the device, or total DB traffic targeted at a given interface from all inputs 🍓
This document does not specify any requirements on DB implementation's values for R, S, or tolerable burstiness. These parameters will be bounded by real-world considerations, such as the actual network being designed and the desired PDB 🌈
2.4 Marking for DB behavior 🏷️
One or more DiffServ codepoint (DSCP) values may be used to indicate a requirement for DB behavior [4] 💖
By default, we suggest an 'experimental' DSCP of 101111 be used to indicate that DB PHB is required ✨
3. Discussion 💬
This section discusses some issues that might not be immediately obvious from the definition in section 2 🫶
3.1 Mutability 🦋
Packets marked for DB PHB MAY be remarked at a DS domain boundary only to other codepoints that satisfy the DB PHB. Packets marked for DB PHBs SHOULD NOT be demoted or promoted to another PHB by a DS domain 🔥
3.2 Tunneling 🚇
When DB packets are tunneled, the tunneling packets must be marked as DB 🎀
3.3 Interaction with other PHBs 🧩
Other PHBs and PHB groups may be deployed in the same DS node or domain with the DB PHB, as long as the requirement of section 2 is met 🌈
3.4 Output Rate not specified 📉
The definition of DB behavior given in section 2 is quite explicitly given in terms of input rate R and output delay variation D(i) - E(i). A scheduler's output rate doesn't need to be specified, since (by design) it will be whatever is needed to achieve the target delay variation bounds 💅
3.5 Jitter 🎢
Jitter is not the bounded parameter in DB behavior. Jitter can be understood in a number of ways, for example as the variability in inter-packet times from one inter-packet interval to the next. However, DB behavior aims to bound a related but different parameter: the variation in delay between the time packets would depart in the absence of competing traffic, E(i), and when they would depart in the presence of competing traffic, D(i) 👑
Part 2 is coming with the rest of the doc, so stay tuned bestie 💖🧚
3.6 Multiple Inputs and/or Multiple Outputs 🦋
okay so this one is about when DB traffic comes in through more than one port and all of it converges on the same output port 🚪. the definition of 'competing traffic' in section 2.2 covers both the simple single-in/single-out case AND the messier converging case, so there are no gaps 💅. when you're checking whether a DB device can give DB behavior to traffic arriving on one port, any DB traffic showing up on the other ports gets counted as competing traffic. it's giving fair-share energy 🫶
and when DB traffic from a single input is leaving through multiple ports? it's clear the behavior is no worse than if ALL of that traffic could leave through each one of those ports individually (subject to limits on how much is permitted). iconic 👑
3.7 Fragmentation and Rate 🧩
where an ingress link has a higher MTU than an egress link, packets could get fragmented as they pass through a Diffserv hop, ngl 💀. but fragmentation is super unpredictable, and that's lowkey the complete opposite of controllable QoS 😭. so we assume fragmentation of DB packets is being avoided (either through some form of Path MTU discovery, or configuration), and it doesn't need its own special handling in the DB behavior definition ✨
3.8 Interference with other traffic 🚨
if the DB PHB is implemented with a mechanism that allows unlimited preemption of other traffic (e.g., a priority queue), the implementation MUST include some means to limit the damage DB traffic could inflict on other traffic. this gets reflected in the DB device's burst tolerance from section 2.1. boundaries matter, bestie 🙅♀️
3.9 Micro flow awareness 🔍
some DB implementations might choose to do queuing and scheduling at a finer granularity (for example, per micro flow) than what the packet's DSCP alone tells you 🧚. that kind of behavior is NOT precluded by the DB PHB definition. however, that behavior is ALSO NOT part of the DB PHB definition. implementors are free to characterize and publicize the extra per micro flow capabilities of their DB implementations however they see fit 💜
3.10 Arrival rate 'R' 📈
without any extra info, R is assumed to be limited by the slowest interface on the device. ✨
on top of that, a DB device can be characterized by different values of R for different traffic flow scenarios (for example, for traffic aimed at different ports, total incoming R, and possibly total per output port incoming R across all incoming interfaces). so R isn't one-size-fits-all, it's giving customizable 🎀
4. IANA Considerations 📝
this doc suggests one experimental codepoint, 101111 🍓. because the DSCP is taken from the experimental code space, it may be re-used by other experimental or informational DiffServ proposals 🌈
5. Conclusion. 🎀
okay so this doc defines DB behavior in terms of a bound on delay variation for traffic streams that are rate shaped on ingress to a DS domain 🦋. two parameters, capped arrival rate (R) and a 'score' (S), are defined and related to the target delay variation bound ✨. all claims of DB 'conformance' for specific implementations of DB behavior are made with respect to particular values for R, S, and the implementation's ability to tolerate small amounts of burstiness in the arriving DB traffic stream 💖
Security Considerations 🔐
okay, this part is SERIOUS bestie 😭🔥. to protect itself against denial of service attacks, the edge of a DS domain MUST strictly police all DB marked packets to a rate negotiated with the adjacent upstream domain (for example, some value less than or equal to the capped arrival rate R). packets in excess of the negotiated rate MUST be dropped. if two adjacent domains have not negotiated a DB rate, the downstream domain MUST use 0 as the rate (i.e., drop all DB marked packets). no exceptions, periodt 💅
since PDBs constructed from the DB PHB will require that the upstream domain police and shape DB marked traffic to meet the rate negotiated with the downstream domain, the downstream domain's policer should never have to drop packets. thus these drops (or a summary of these drops) SHOULD be noted (e.g., via rate-limited SNMP traps) as possible security violations or serious misconfiguration 🚩
overflow events on a DB queue MAY also be logged as indicating possible denial of service attacks or serious network misconfiguration 👀
Acknowledgments 🫶
this document is the product of the volunteer 'EF Resolve' design team, building on the work of V. Jacobson, K. Nichols, K. Poduri [1] and clarified through discussions with members of the DiffServ working group (particularly the authors of [2]). the non-contentious text (such as the use of DB with tunnels, the security considerations, etc.) was drawn directly from equivalent text in RFC 2598. big thanks to the crew 💜
Intellectual Properties Considerations ⚖️
to establish whether any considerations apply to the idea expressed in this document, readers are encouraged to review notices filed with the IETF and stored at:
References 📚
[1] Jacobson, V., Nichols, K. and K. Poduri, "An Expedited Forwarding PHB", RFC 2598, June 1999.
[2] Davie, B., Charny, A., Baker, F., Bennett, J.C.R., Benson, K., Le Boudec, J.Y., Chiu, A., Courtney, W., Davari, S., Firoiu, V., Kalmanek, C., Ramakrishnan, K. and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[4] 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.
[5] Black, D., Blake, S., Carlson, M., Davies, E., Wang, Z. and W. Weiss, "An Architecture for Differentiated Services", RFC 2475, December 1998.
Authors (volunteer EF Design Team members) 👯♀️
Grenville Armitage
Center for Advanced Internet Architectures
Swinburne University of Technology,
Melbourne, Australia
EMail: garmitage@swin.edu.auBrian E. Carpenter (team observer, WG co-chair)
IBM Zurich Research Laboratory
Saeumerstrasse 4
8803 Rueschlikon
Switzerland
EMail: brian@hursley.ibm.comAlessio Casati
Lucent Technologies
Swindon, WI SN5 7DJ United Kingdom
EMail: acasati@lucent.comJon Crowcroft
Marconi Professor of Communications Systems
University of Cambridge
Computer Laboratory
William Gates Building
J J Thomson Avenue
Cambridge
CB3 0FD
Phone: +44 (0)1223 763633
EMail: Jon.Crowcroft@cl.cam.ac.ukJoel M. Halpern
P. O. Box 6049
Leesburg, VA 20178
Phone: 1-703-371-3043
EMail: jmh@joelhalpern.comBrijesh Kumar
Corona Networks Inc.,
630 Alder Drive,
Milpitas, CA 95035
EMail: brijesh@coronanetworks.comJohn Schnizlein
Cisco Systems
9123 Loughran Road
Fort Washington, MD 20744
EMail: john.schnizlein@cisco.comFull Copyright Statement 🌸
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.
The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement 💖
Funding for the RFC Editor function is currently provided by the Internet Society. ✨
📦 where this page comes from
- ietf: rfc3248 2002-03
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms