I know that in TCP/IP model the data link layer provides connection less service.

While reading IPv6 Header. 'The flow label is used for connection oriented servie'.

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How does TCP/IP model provide connection oriented service in data link layer.

It's a bit confusing.

  • "How does TCP/IP model provide connection oriented service in data link layer." It does not. The data-link and network layers are connectionless. Even many transport protocols are connectionless. For example, UDP is connectionless, while TCP is connection-oriented.
    – Ron Maupin
    Nov 24 '19 at 15:18

The flow label is a (potentially random) number to help identify flows, ie. series of packets belonging together, not necessarily connection-oriented.

The flow label is placed in the IPv6 header so that networking devices don't have to look into and interpret L4 headers. It is not used for holding a connection together nor can it be used that way.

The IPv6 flow label is specified in RFC 6437:

From the viewpoint of the network layer, a flow is a sequence of packets sent from a particular source to a particular unicast, anycast, or multicast destination that a node desires to label as a flow. From an upper-layer viewpoint, a flow could consist of all packets in one direction of a specific transport connection or media stream. However, a flow is not necessarily 1:1 mapped to a transport connection.

Traditionally, flow classifiers have been based on the 5-tuple of the source address, destination address, source port, destination port, and the transport protocol type. However, some of these fields may be unavailable due to either fragmentation or encryption, or locating them past a chain of IPv6 extension headers may be inefficient. Additionally, if classifiers depend only on IP-layer headers, later introduction of alternative transport-layer protocols will be easier.

The usage of the 3-tuple of the Flow Label, Source Address, and Destination Address fields enables efficient IPv6 flow classification, where only IPv6 main header fields in fixed positions are used.

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