The Ipv6 Datagram Flow Label Field

The Ipv6 Datagram Flow Label Field The flow label is included in IPv6 in order for it to operate in a connection-oriented manner for sending the IPv6 datagram.

The Ipv6 Datagram Flow Label Field

Its size is 20 bits and “may be used by a host to label those packets for which it is requesting special handling by routers within a network” (Stallings and Case, 2013, p. 239). According to Forouzan (2013), “In its simplest form, a flow label can be used to speed up the processing of a packet by a router.

The Ipv6 Datagram Flow Label Field
The Ipv6 Datagram Flow Label Field

In its more sophisticated form, a flow label can be used to support the transmission of real-time audio and video” (p. 676). Routers that support flow labels typically have a flow label table that it consults each time it receives a packet with a flow label to know how to process the packet (Forouzan, 2013).

The Ipv6 Datagram Flow Label Field

An interesting observation about flow label tables is that they are not automatically populated from the flow labels themselves, but rather from other means (Forouzan, 2013). There are many various applications for the flow label. A flow label allows for the router to quickly route the packet using just the flow label table rather than consulting its forwarding table or implementing its routing algorithm to know the packet’s next destination (Forouzan, 2013). Jaeggli (2018) discussed how major operating systems set the flow label to non-zero values; Ethernet, switches, routers, and other devices have implemented the flow label for scalability and as an addition to a hash component.

The Ipv6 Datagram Flow Label Field

Compatible and Mapped Addresses

Compatible addresses are addresses of 96 bits of zero in the IPv6 address followed by the 32 bit IPv4 address. According to Oracle (2010), “The compatible format is used to represent an IPv6 node. This format enables you to configure an IPv6 node to use IPv6 without having a real IPv6 address.” Compatible addressing is used when a computer using IPv6 sends a message to another IPv6 computer (Forouzan, 2013). Mapped addresses, in contrast, are used when one machine that has already moved to using IPv6 sends a message to a machine that still only uses IPv4. Oracle (2010) further stated, “The mapped address format is used to represent an IPv4 node. The only currently defined use of this address format is part of the socket API. An application can have a common address format for both IPv6 addresses and IPv4 addresses. The common address format can represent an IPv4 address as a 128-bit mapped address.”

The Ipv6 Datagram Flow Label Field Combined Protocols

Three protocols that are part of the IPv4 network layer that have been combined into one protocol with IPv6 are ICMP, ARP, and IGMP. The combined protocol in IPv6 is named ICMPv6 (Forouzan, 2013). Because of the combining of three protocols into one, ICMPv6 is more complex. Forouzan (2013) described the categories of messages for the ICMPv6 protocol as error-reporting, informational, neighbor-delivery, and group-membership. This combining of three into one reminds me of the biblical idea of the trinity. Though God was not combined into one God from three separate gods (as is the case with ICMPv6), we nevertheless see three persons in one God. A good example of this is found in the commissioning passage in Matthew 28:18-20. Jesus stated,

“All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age.” Baptism into the triune God is a wonderful illustration of the trinity.

The Ipv6 Datagram Flow Label Field IP Header

The purpose of including the IP header is because it contains the IP address of the original source. The first 8 bytes of the datagram data are included in the header because those bytes detail the first section of the transport layer header included from the payload (Forouzan, 2013). This part of the header contains information about the port numbers and the sequence number. This additional information in the header allows the source to send the error message to the right destination.

IPv6 Address for Home Use

The first three bits of an IPv6 address from an ISP for home use are 111 (Forouzan, 2013). IPv6 addresses for home use are private addresses and correspond to the link-local block.

The Ipv6 Datagram Flow Label Field

Explain the use of the flow field in IPv6.

The book describes the flow label field as “a 20-bit field that is designed to provide special handling for a particular flow of data” p 675 and “can be used to speed up the processing of a packet by a router” (Forouzan, 2013 p.676)

What is the potential application of this field?

“This field will allow routers to treat packets differently via a special routing table called the flow label table.”

(Forouzan, 2013 p.676)  The book suggests usages such as real-time audio and video because of the technical needs of the data “high bandwidth, large buffers, long processing times…” which is a noble idea, give preference in routing to the technologies that need preference, however, were I managing the allotting of this preferential treatment I would auction the preferential treatment to the highest bidder. provides a similar explanation “When routers receive the first packet of a new flow, they can process the information carried by the IPv6 header, Routing header, and Hop-by-Hop extension headers, and store the result in a cache memory and use the result to route all other packets belonging to the same flow” (IPv6 Header Deconstructed, 2019).  If I were Comcast or Verizon or some other ISP, I would price out the fastest lines and sell the preferred treatment to business customers.

A real world example is in Virginia Interstate 95 from Fredericksburg to Washington DC has High Occupancy Toll (HOT) lanes where the cost of riding in the lanes, with fewer than three people in the car, is based on the amount of traffic at any given time, the more traffic the higher the toll, which reminds me a great deal of the flow label.  If you want to skip the traffic and congestion you have to pay the toll.  The highest toll I have seen is $29.85 to go 8 miles one way one time.

Being a former commuter, I found this same practice being implemented by the Virginia Railway Express (VRE) commuter train in Virginia.  Many an afternoon I spent 30 minutes or more sitting going nowhere waiting for the Tropicana 2 mile long train to go by because that train had preferred access to the rail road…it did not wait…period and Tropicana paid a premium for that treatment.

The Ipv6 Datagram Flow Label Field

Interestingly the Obama administration tried to prevent the potential benefits of the flow label field, via the FCC and “Net Neutrality”, which is a colloquialism whose goal is as follows “Internet service providers (ISPs) should treat all data that travels over their networks fairly, without improper discrimination in favor of particular apps, sites or services” (Net Neutrality, n.d.)…it seems like the goal is directly opposed to the goal of the IPv6 flow label field.

While treating all data fairly appears a noble pursuit “The last thing we should want is [a president] or a government agency picking winners and losers on the Internet. And enforcing net neutrality is picking winners and losers even if it looks like it is just “leveling the playing field.” …it completely blocks certain business models and stops any possible innovation that might emerge if given the option of seeking differential access to bandwidth.” (Dorfman, 2014).

Consider the cost of physically installing the wires underground or under the ocean and the expectations of the rewards of this effort, as Psalms 128:2 tells us, “For thou shalt eat the labor of thine hands…”, and then a government or other authority structure of man steps in and denies you that benefit for no other reason than to be “fair” to someone else who put forth none of that effort

The Ipv6 Datagram Flow Label Field

Distinguish between compatible and mapped addresses and explain their applications.

The course book reads “during the IPv4 to IPv6 transition hosts can use their IPv4 address embedded in an IPv6 address.  To support these two formats have been design “compatible and mapped”

A Compatible Address is an address of 96 bits of zero followed by 32 bits of IPv4 address.  It’s used when a computer using

IPv6 wants to send a message to another computer using IPv6

A Mapped Address is used when a computer already migrated to version 6 wants to send an address to a computer still using version. 4” (Forouzan, 2013 p.671)

This approach reminds me of when the US Postal Service switched from a 5 digit Zip Code to a 9 Digit Zip Code.

The Ipv6 Datagram Flow Label Field

Coincidentally those extra 4 digits function similarly to protocol fields “The sixth and seventh digits point to a delivery sector, which may be several blocks, a group of streets, a group of Post Office boxes, several office buildings, a single high-rise office building, a large apartment building or a small geographic area. The last two digits represent a delivery segment, which might be one floor of an office building, a side of a street, specific departments in an office, or specific Post Office boxes.” (, n.d.)

List 3 protocols in the IPv4 network layer that are combined into a single protocol in IPv6.

The IGMP, ICMPv4, and ARP protocol from in IPv4 have been combined into a single protocol ICMPv6 in IPv6 (Forouzan, 2013 p.679)

What is the purpose of including the IP header and the first 8 bytes of datagram data in the error-reporting ICMP messages?

The reason that the IP header and first 8 bytes of a datagram data in the error reporting ICMP message is included is to help you understand what conditions might have created the error message.  (Forouzan, 2013 p.576)

The Ipv6 Datagram Flow Label Field

If you are assigned an IPv6 address by your ISP for your personal computer at home, what should be the first (leftmost) 3 bits of this address?

“The block in the address space that is used for unicast (one-to-one) communication is called…the global unicast address block” (Forouzan, 2013 p.668) looking at the table in the course text book Global Unicast is 001 so the leftmost 3 bits would be 001.

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