Wow!! Thank you very much.
>From: "Priscilla Oppenheimer" >Reply-To: "Priscilla Oppenheimer" >To: [EMAIL PROTECTED] >Subject: Re: TOS (TOS 0)routes?? [7:32242] >Date: Thu, 17 Jan 2002 14:17:27 -0500 > >The At 08:48 PM 1/16/02, Cisco Nuts wrote: > >Hello, > >I have been trying to find out what exactly does TOS(TOS0) routes mean in > >OSPF but am having no luck. Tried CCO, Doyle books, CCNP books etc. > >EG: > >RTA#sh ip ospf > >Routing Process "ospf 1" with ID 15.15.15.1 > >Supports only single TOS(TOS0) routes > >It's referring to the Type of Service (TOS) bits within the Type of Service >field in the IP header. These bits were originally intended to help a >router select a routing path when multiple paths to a particular >destination were available. That meaning for the bits is obsolete now. The >field is now called the Differentiated Services field. > >Early OSPF RFCs said that OSPF should support routing based on the setting >of the Type of Services bits. Newer OSPF RFCs don't say that anymore. It >proved impractical. > >TOS 0 means that the router doesn't pay attention to the setting of the >bits. The OSPF process supports only one type of service, which could be >called type 0, and is just the ordinary default behavior, without reference >to the setting of the TOS bits. > >Here's some background info from my up-and-coming book: > >The IP Type of Service field specifies both precedence and type of service. >Precedence helps a router determine which packet to send when several >packets are queued for transmission to the same output interface. Type of >service helps a router select a routing path when multiple paths to a >particular destination are available. > > > >The 4-bit Type-of-Service subfield supports four types of service. (Note >that the subfield has the same name as the major field, which can be >confusing.) > >The goal of the Type of Service subfield is to help a router select a route >from a set of routes to a particular destination, especially when the >routes have different characteristics. Routing protocols attempt to >determine the best route to a destination, but there are several >definitions of best -- cheapest, fastest, most reliable, least delay, and >so on. > >According to RFC 1349, which clarified the use of the Type of Service byte >(but is now obsolete), the Type of Service subfield has four bits that >specify the routing service that an application wishes to receive from a >router. An application or host can set one (and only one) of the four bits: > >1 The delay bit (D) tells routers to minimize delay >2 The throughput bit (T) tells routers to maximize throughput >3 The reliability bit (R) tells routers to maximize reliability >4 The cost bit (C) tells routers to minimize monetary cost > >Setting the D bit is suitable for interactive applications, such as Telnet >and Rlogin. When the D bit is set, a router should select a path that >minimizes delay, for example, a dedicated high-speed leased line instead of >a shared Frame Relay link. File transfer applications, or any applications >that send bulk data, can set the T bit. Setting the T bit tells routers to >select high-throughput links. Network management and mission-critical >applications can set the R bit. Setting the R bit tells routers to select >reliable paths. Applications for which delay, throughput, or reliability >are not critical -- but a low monetary cost is important -- set the C bit. >Some implementations of the Network News Transfer Protocol (NNTP), which >reads UseNet news, set the C bit, presumably because reading news is not a >critical activity and should not use a lot of monetary resources. > >In practice, routing protocols and routers never had good methods for >handling these bits. Selecting a path based on the setting of these bits >proved to be impractical. Although early versions of the Open Shortest Path >First (OSPF) and Border Gateway Protocol (BGP) routing protocols specified >support for the bits, newer versions of OSPF and BGP specifications do not >require support for routing based on the setting of the bits. > >Differentiated Services >RFC 1349 was declared obsolete with the publication of RFC 2474, >"Definition of the Differentiated Services Field (DS Field) in the IPv4 and >IPv6 Headers," and RFC 2475, "An Architecture for Differentiated Services." >Per these newer RFCs, the Type of Service field became the DS field. The DS >field has two currently unused (CU) bits at the end. The first six bits are >called the Differentiated Services Codepoint (DSCP). > > > >And that's probably way more than you ever wanted to know. > >Priscilla > > > > >Can someone help? > > > >Thank you. > > > >_________________________________________________________________ > >Join the worlds largest e-mail service with MSN Hotmail. > >http://www.hotmail.com >________________________ > >Priscilla Oppenheimer >http://www.priscilla.com _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.msn.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=32362&t=32242 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
