8.7 Summary

In this chapter we discussed the following:

Customers negotiate Service-Level Agreements (SLAs) with service providers. The SLA specifies what services the customers will receive. SLAs can be static or dynamic. For static SLAs, customers can transmit data at any time. For dynamic SLAs, customers must use a signaling protocol such as RSVP to request services on demand before transmitting data. SLA monitoring is still an immature technology, and it is not clear what a carrier will or will not accept as proof of a failing SLA. For an ISP or carrier SLA monitoring is not an option, but an end-user network needs to consider the cost-benefit of implementing such tools or outsourcing this activity.
The ingress routers of ISPs are configured with classification, policing, and remarking rules. The egress routers of ISP networks are configured with reshaping rules. Such rules may be configured manually by network administrators or dynamically by some protocol such as LDAP or RSVP. ISPs must implement admission control in order to support dynamic SLAs. Classification, marking, policing, and shaping/reshaping are only done at the boundary routers. Core routers are shielded from the signaling process. They need only implement two queues with strict priority. They process packets based solely on their DS fields.
Constraint-Based Routing (CBR) can be used to compute the routes subject to QoS and policy constraints. The goal is to meet the QoS requirements of traffic and to improve utilization of the networks. QoS offers a more sophisticated approach to traffic engineering by taking into account multiple constraints and attempting to distribute traffic more evenly over the available bandwidth while maintaining the QoS requirements for a particular flow. The advantages of CBR over traditional routing are the ability to satisfy QoS requirements of flows and improved network utilization. The disadvantages of CBR are increased communication, computation and storage overheads, longer paths consuming more resources, and potential routing instability. CBR is similar to the dynamic/adaptive routing techniques in telephone networks and ATM networks. CBR must be implemented and deployed with care; otherwise, the cost of instability and increased complexity may outweigh the benefits.
With MPLS, Label-Switched Paths (LSPs) are established between each ingress-egress pair. At the ISP ingress routers, labels and Class of Service (CoS) fields are determined from the classification and routing results. MPLS headers are then inserted into the packets. Core routers process packets based on their labels and COS fields only. Labels are removed before packets leave an MPLS domain. MPLS and constraint-based routing can be used together to control the path of traffic so as to avoid congestion and improve the utility of the networks.
The relatively recent ratification of 802.1p is enabling QoS convergence in the LAN environment. Setting priority bits and implementing class-based forwarding are significant steps toward multimedia convergence and policy-enabled networking. While most vendors today agree that 802.1p/Q is the mechanism to tag frames for prioritization, there is no uniform approach to implementing the underlying queuing mechanisms to control priority flows.
One of the first attempts to provide QoS through the Internet was via a signaling protocol called Resource Reservation Protocol (RSVP). RSVP uses end-to-end signaling to set up and tear down IP connections, each having specific service characteristics. Intermediate nodes (such as routers and switches) are expected to reserve the resources necessary to satisfy a particular application service specification. RSVP is now part of an overall architecture for QoS developed by the IETF, called Integrated Services (IS). RSVP only defines the connection setup mechanisms; other technologies are required to allocate and ensure that the resources are in place.
There are still major issues to resolve with the DS model if is to be deployed across the global Internet. RSVP must be deployed on all routers in the end-to-end path of a flow. RSVP session processes are complex, and RSVP is difficult to scale, since it places more processing load on heavily utilized backbone nodes. Moreover, the pricing model and intercarrier billing mechanisms for RSVP are far from resolved. These issues are considerable barriers to widespread IS over public backbones, and there is some consensus that IS is perhaps better employed in enterprise environments, mapping onto DS at the backbone edge.
Differentiated Services (DS) is an alternative end-to-end QoS model proposed by the IETF. A key advantage of DS over the IS model is that DS provides a scalable service for the Internet without the need for per flow state and signaling at every router hop. It eliminates the need to store information about each flow and can be deployed without requiring expensive and complicated end-to-end signaling (e.g., RSVP). DS networks classify packets according to one of a small number of aggregated flows, based on the setting of bits in a ToS field in each packet's IP header.

References

[1] T. Kenyon, High-Performance Network Design: Design Techniques and Tools (Woburn, MA: Digital Press, 2001).

[2] A Framework for QoS-based Routing in the Internet, RFC 2386, August 1998.

[3] Blake et. al., Architecture for Differentiated Services. Internet draft, draft-ietf-diffserv-arch-02.txt, October 1998.

[4] D. Clark, "The Design Philosophy of the DARPA Internet Protocols," in Proceedings of ACM SIGCOMM '88, August 1988.

[5] Cisco Systems, Tag Switching Architecture Overview, RFC 2105, February 1997.

[6] Internet Protocol, RFC 791, September 1981.

[7] Type of Service in the Internet Protocol Suite, RFC 1349, July 1992. (obsoleted by RFC 2474).

[8] Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers, RFC 2474, December 1998.

[9] ANSI T1S1, DSSI Core Aspects of Frame Rely, March 1990.

[10] Traffic Management Specification version 4.0. The ATM Forum Technical Committee, af-tm-0056.000, April 1996.

[11] Integrated Services in the Internet Architecture: An Overview, RFC 1633, June 1994.

[12] Y. Bernet, R. Yavatkar, P. Ford, et al., A Framework for Use of RSVP with DiffServ Networks. Internet draft <draft-ietf-diffserv-rsvp-00.txt>, June 1998.

[13] ISO/IEC 8802-5 Information Technology—Telecommunications and Information Exchange between Systems—Local and Metropolitan Area Networks—Common Specifications—Part 5: Token Ring Access Method and Physical Layer Specifications (also ANSI/IEEE Std 802.5-1995), 1995.

[14] ISO/IEC Final CD 15802-3 Information Technology—Telecommunications and Information Exchange between Systems—Local and Metropolitan Area Networks—Common Specifications—Part 3: Media Access Control (MAC) Bridges, (current draft available as IEEE P802.1D/D15), September 1998.

[15] www.frforum.com, Frame Relay Forum home page.

[16] www.tmforum.org, Telemanagement Forum home page.

[17] C. Labovitz, G. R. Malan, F. Jahanian, "Internet Routing Instability," IEEE/ACM Transactions on Networking, vol. 6, no. 5, October 1998, 515–528.

[18] OSPF version 2, RFC 2178, July 1997.

[19] C. Villamizar, and T. Li, IS-IS Optimized Multipath (IS-IS OMP). Internet draft: draft-villamizar-isis-omp-00.txt, October 1998.

[20] S. Chen, "Routing Support for Providing Guaranteed End-to-End Quality of Service." Ph.D. dissertation, University of Illinois, May 1999.

[21] R. Guerin, S. Kamat, A. Orda, et al., QoS Routing Mechanisms and OSPF Extensions. Internet draft, draft-guerin-QoS-routing-ospf-03.txt, January 1998.

[22] Z. Zhang, C. Sanchez, B. Salkewicz, et al,. QoS Extensions to OSPF. Internet draft: draft-zhang-qos-ospf-01, September 1997.

[23] R. Callon, P. Doolan, N. Feldman, et al., A Framework for Multiprotocol Label Switching. Internet draft, draft-ietf-mpls-framework-02.txt, November 1997.

[24] M. Seaman, A. Smith, and E. Crawley, Integrated Service Mappings on IEEE 802 Networks (work in progress). Internet Draft, draft-ietf-issll-is802-svc-mapping-01.txt, November 1997.

[25] W. Lidinsky, IEEE Standard P802.1D Information Technology—Telecommunications and Information Exchange between Systems—Common Specifications—Part 3: Media Access Control (MAC) Bridges: Revision, 24.11.1997, 1997.

[26] A. Ghanwani, J. W. Pace, V. Srinivasan, et al., A Framework for Integrated Services Over Shared and Switched IEEE 802 LAN Technologies. Internet draft, draft-ietf-issll-is802-framework-07.txt, June 1999.

[27] R. YavatkarD. HoffmanY. Bernet Y, et al., SBM (Subnet Bandwidth Manager): A Protocol for RSVP-based Admission Control over IEEE 802-style Networks. Internet draft: draft-ietf-issll-is802-sbm-07.txt, November 1998.

[28] ISO/IEC 15802-3 Information Technology—Telecommunications and Information Exchange between Systems—Local and Metropolitan Area Networks—Specific Requirements—Supplement to Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications—Frame Extensions for Virtual Bridged Local Area Network (VLAN) Tagging on 802.3 Networks, IEEE Std 802.3ac-898 (supplement to IEEE 802.3 1998 edition), 1998.

[29] ISO/IEC 8802-5 Information Technology—Telecommunications and Information Exchange between Systems—Local and Metropolitan Area Networks—Common Specifications—Part 5: Token Ring Access Method and Physical Layer Specifications (also ANSI/IEEE Std 802.5-1995), 1995.

[30] ISO/IEC 15802-3 Information Technology—Telecommunications and Information Exchange between Systems—Local and Metropolitan Area Networks—Common Specifications—Part 3: Media Access Control (MAC) Bridges (also ANSI/IEEE Std 802.1D-1998), 1998.

[31] IEEE Standards for Local and Metropolitan Area Networks: Demand Priority Access Method, Physical Layer, and Repeater Specification for 100-Mbps Operation, IEEE Std 802.12-1995, 1995.

[32] Specification of Guaranteed Quality of Service, RFC 2212, September 1997.

[33] Specification of the Controlled Load Network Element Service. RFC 2211. September 897.

[34] Resource Reservation Protocol (RSVP)—version 1, Functional Specification, RFC 2205, September 1997.

[35] E. Bernet et al., A Framework for Use of RSVP with DiffServ Networks, draft-ietf-diffserv-rsvp-00.txt, June 1998.

[36] Signaled Preemption Priority Policy Element, RFC 2751, January 2000 (proposed standard).

[37] RSVP Extensions for Policy Control, RFC 2750 (proposed standard).

[38] Identity Representation for RSVP, RFC 2752, January 2000 (proposed standard).