In the IP Precedence section, you had mentioned something like:
“With the old 5-bit type of service bits, you could flip some switches and have an IP packet that requested low delay and high throughput. With the “newer” 4-bit type of service bits, you have to choose one of the 5 options. Good thinking, but the type of service bits have never been really used…”
Can you tell me what the term “you could flip some switches” means?
Also can you explain why is it a problem when type of service bits 3 and 4 are set to a value of 1?
bit 3: 1= Low Delay
bit 4: 1= High Throughput.
Now, I might be wrong but I wanted to understand when Throughput is higher, the delay in forwarding a packer is lower? or am I confusing myself?
The term “you could flip some switches” that Rene used in the lesson is just a metaphorical way of saying “you could change some settings.” In this context, it refers to changing the value of the ToS bits in the IP header of a packet to request specific service characteristics. The point of the comment is that with the old way, you can actually create a scenario where you can set the priorities to have bits 3 and 4 set to 1, which is essentially an IP packet that requests low delay and high throughput.
The problem with setting both bit 3 and bit 4 to a value of 1 is that it might create a contradiction in the network. Low Delay and High Throughput are often seen as mutually exclusive in networking.
High throughput means that the network is trying to maximize the amount of data that can be sent over a given period of time, which might involve using techniques like buffering or packet scheduling that could introduce delays. On the other hand, low delay means that the network is trying to minimize the time it takes for a packet to travel from the source to the destination, which might involve bypassing these techniques and therefore reducing the overall throughput.
In an ideal scenario, high throughput would indeed mean low delay as more packets are being forwarded efficiently. However, in a real-world scenario, achieving high throughput might involve techniques that could potentially introduce delays. Hence, it’s generally considered a best practice to prioritize either throughput or delay, but not both. Does that make sense?
With IPP, a lot of my resources say that values 6 and 7 are reserved. However, there doesn’t seem to be anything preventing me from using 6 and 7 in IPP to mark my traffic
Why are packets like OSPF marked by default? These markings don’t really do much if QoS isn’t implemented, or?
And even if we did implement QoS, I doubt we would use a marking like “CS6” specifically.
Why were Class Selector markings even created? If the idea is to provide backwards compatibility with IPP-enabled devices, wouldn’t the other DSCP values already achieve that?
If an IPP device was to read this packet, it would only check the first 3 left-most bits of the ToS byte, or not? So:
And the final thing, how exactly would I define what DiffServ is? I understand the model but I can’t think of a good definition. In IntServ, we can have each device make a bandwidth reservation. This model where we configure classes, markings, and all the other QoS tools such as shaping, policing, congestion avoidance, queuing, etc on our devices, is called DiffServ?. Is that a good way to define it?
Also, what do the terms “Integrated” and “Differentiated” exactly mean when it comes to these models?
Hmm, I haven’t seen precedence values 6 and 7 being reserved. Values for the set ip precedence command range from 0 to 7, and values 6 and 7 correspond to “internetwork control” and “network control”. However, bits 6 and 7 of the Type of Service (ToS) bits are reserved for future use. Could that be what you mean?
OSPF packets are marked by default to ensure they receive priority handling by the network. Note that these markings are applied to control plane traffic (OSPF messages) and not data plane or user traffic. Even if QoS isn’t explicitly configured on network devices, many switches, and some newer routers already have some default settings for CoPP which helps to ensure that control plane traffic is given the appropriate priorities. As for the use of “CS6”, it’s a common marking for network control traffic such as OSPF messages.
Class Selector markings were created to ensure backward compatibility with older devices that only understand IPP. While it’s true that an IPP device would only check the first 3 left-most bits of the ToS byte, the Class Selector markings provide a convenient way to explicitly map the DSCP values to IPP values rather than relying simply on the reading of the first tree bits. This ensures that even if a packet is forwarded through a network that only supports IPP, the QoS information will not be completely lost.
To answer this question, let me try to give a definition of “integrated” vs “differentiated” services so that we can understand the difference and understand the specific definitions…
The “Integrated Services” term refers to integrating resource reservation into the network’s operation. This means that each flow explicitly reserves resources (bandwidth, buffer, etc.) along its path using a signaling protocol like RSVP.
The term “Differentiated Services” refers to differentiating traffic by class rather than treating each flow individually. Traffic is grouped and handled based on predefined policies, offering “differentiated” levels of service.
If you need any further clarification or if you would like to further discuss any of these responses, please let me know!
I understand the confusion. First of all, let me reiterate that bits 6 and 7 of the ToS field are indeed defined as “reserved for future use” and cannot be used. These should not be confused with the decimal values of 6 and 7 defining precedence (i.e. represented by the first three bits of the ToS field).
Now having said that, the values of IP precedence ranging from 0 to 7 that can be seen in the 3-bit precedence field have very specific purposes as defined way back in the first RFC describing IPv4. Specifically, within RFC 791 in the section concerning these values, it states the following. Note that value 6 is known as the network control precedence designation, while a value of 7 is known as the internetwork control designation:
The Network Control precedence designation is intended to be used
within a network only. The actual use and control of that
designation is up to each network. The Internetwork Control
designation is intended for use by gateway control originators only.
If the actual use of these precedence designations is of concern to
a particular network, it is the responsibility of that network to
control the access to, and use of, those precedence designations.
The precedence values defined here are simply an expression of how they are intended to be used. What you end up using them for is completely up to you i.e. the responsibility of each network administrator.
Now Cisco and other vendors may say that values 6 and 7 are reserved “for network control information” but that doesn’t prevent you from using them. Cisco may recommend not using them because Cisco devices automatically set these precedence values for particular control information such as routing protocol messages, and if you use them for other traffic, this may cause some confusion, but again, this doesn’t mean you can’t use them.
So I think that it’s more a matter of the terminology used in documentation, and even in videos like the one you shared, rather than a strict enforcement of these rules. Does that make sense?
