transport protocol level in order to help decrease traffic volumes on the congested network path. Flow Label The Flow Label field was left experimental by RFC 2460 [21], which caused confusion and effectively left the field unused. Most of the implementations simply set the field always to zero, as instructed by RFC 2460 for nodes that do not support the function. The issues related to the field are explained in depth in RFC 6436 'Rationale for Update to the IPv6 Flow Label Specification' [27]. The updated functionality is normatively defined in RFC 6437 'IPv6 Flow Label Specification' [28]. The function that is defined for the Flow Label field is to help networks to perform efficiently various forms of load distribution functions, while helping to ensure that packets belonging to same flow always traverse the same path in order to avoid issues such as packet reordering. The RFC 6437 specification for the Flow Label field recommends hosts to mark each packet belonging to a specific flow, such as to a transport layer session, with a same value. Furthermore, the hosts should select each value pseudorandomly SO that the value for subsequent flows would be difficult to guess. For backwards compatibility reasons the Flow Label value of zero is still supported. While the Flow Label field is not part of the transport layer checksum calculation, nor covered by Internet Protocol security (IPsec) Authentication Header (AH), the specifi- cation forbids networks rewriting the field except in two cases: host has set the value Deploying IPv6 in 3GPP Networks originally to zero, or value rewrite is required for security reasons described in depth in RFC 6437 [28]. In any case, a packet receiver cannot assume that the field value has remained unchanged in its path through the Internet. Therefore, applications should not assume that the Flow Label field can be used for end-to-end signaling purposes. 3.2.2 IPv6 Extension Headers As space in the core IPv6 header is precious, features that did not have to be in the header were exclu