In a data communications device configured to perform data communications operations within a content delivery network, a method for conveying information between an upstream device (200A, 200B, 205A, 205B, 205C) of the network and a downstream device (200A, 200B, 205A, 205B, 205C) of the network, the method comprising: receiving, from the upstream device (200A, 200B, 205A, 205B, 205C), an initial content element having routing information and content information; generating a marked content element from the initial content element; and sending, to the downstream device (200A, 200B, 205A, 205B, 205C), the marked content element in place of the initial content element based on the routing information, the marked content element having the routing information and the content information of the initial content element, the marked content element further having one or more identifiers (320A, 320B, 330) which are included in a data payload (310, 340) of the content element and identify one or more devices (200A, 200B, 205A, 205B, 205C) of the content delivery network that receive and forward the marked content element, and which: (i) preserve the routing information of the marked content element to enable delivery of the marked element to the downstream device (200A, 200B, 205A, 205B, 205C); (ii) preserve the content information of the marked content element to enable correct content processing of the content information by the downstream device (200A, 200B, 205A, 205B, 205C); and (iii) define a characteristic of the content delivery network to enable diagnosis of content delivery network behaviour. receiving, from the upstream device (200A, 200B, 205A, 205B, 205C), an initial content element having routing information and content information; generating a marked content element from the initial content element; and sending, to the downstream device (200A, 200B, 205A, 205B, 205C), the marked content element in place of the initial content element based on the routing information, the marked content element having the routing information and the content information of the initial content element, the marked content element further having one or more identifiers (320A, 320B, 330) which are included in a data payload (310, 340) of the content element and identify one or more devices (200A, 200B, 205A, 205B, 205C) of the content delivery network that receive and forward the marked content element, and which: (i) preserve the routing information of the marked content element to enable delivery of the marked element to the downstream device (200A, 200B, 205A, 205B, 205C); (ii) preserve the content information of the marked content element to enable correct content processing of the content information by the downstream device (200A, 200B, 205A, 205B, 205C); and (iii) define a characteristic of the content delivery network to enable diagnosis of content delivery network behaviour. (i) preserve the routing information of the marked content element to enable delivery of the marked element to the downstream device (200A, 200B, 205A, 205B, 205C); (ii) preserve the content information of the marked content element to enable correct content processing of the content information by the downstream device (200A, 200B, 205A, 205B, 205C); and (iii) define a characteristic of the content delivery network to enable diagnosis of content delivery network behaviour.