On Jul 28, 2010, at 1:58 PM, Yong Lucy wrote:
> What is flow label usage?
>
> IMO: it enforces that a set of packets with the same flow label has to be
> carried through the networks in the same path or belong to the same
> application at host. Is that correct? Is there other usage of flow label?
My understanding is that it labels a flow. If the right way to carry a flow is
distributed across two network paths, so be it. BTW, a "flow" has many
definitions; the type of flow that Tony was mentioning this morning is a single
application session; to an ISP, a flow is a stream of traffic from an ingress
to an egress, or perhaps the stream of traffic from an ingress to an egress via
a path. This means that the IPv6 Working Group wants the flow label to be a
hash that helps the network find status for a specific session's or set of
sessions' shared' state, while to an ISP it wants to be independent of the
source address - associated with the ingress router.
RFC 1710, the original specification for what later because IPv6, said of the
flow label:
The Flow Label field in the SIPP header may be used by a host to
label those packets for which it requests special handling by SIPP
routers, such as non-default quality of service or "real-time"
service. This labeling is important in order to support applications
which require some degree of consistent throughput, delay, and/or
jitter. The Flow Label is a 28-bit field, internally structured into
three subfields as follows:
RFC 1752 agreed:
* Flow Label - This field may be used by a host to label those
packets for which it is requesting special handling by routers
within a network, such as non-default quality of service or "real-
time" service. (28-bit field)
RFC 2460 reads:
o Flow Labeling Capability
A new capability is added to enable the labeling of packets
belonging to particular traffic "flows" for which the sender
requests special handling, such as non-default quality of
service or "real-time" service.
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