Datagram reassembly

A method of processing a datagram received subdivided into multiple fragments, the method comprising the steps of:

• storing fragment list entries, wherein a fragment list entry comprises a pointer to a linked list of fragments and an accumulated size for the linked list of fragments;
• performing an atomic access to a selected fragment list entry in response to reception of a fragment, wherein the selected fragment list entry is selected in dependence on at least one datagram identifier in the fragment, wherein the atomic access comprises a read access followed by a write access;
• calculating an updated accumulated size as the accumulated size plus a size of the fragment;
• and
• on completion of the read access, performing the write access to the selected fragment list entry,
• wherein, when the selected fragment list entry comprises a size of the datagram and the updated accumulated size is at least equal to the size of the datagram, the write access comprises setting the selected fragment list entry to a null entry and, when the write access is complete, attempting a datagram reassembly process on the fragment and the fragments indicated by the linked list of fragments pointed to by the pointer of the selected fragment list entry,
• and wherein, when the updated accumulated size is less than the size of the datagram or the selected fragment list entry does not comprise the size of the datagram, the write access comprises updating the selected fragment list entry to include the fragment in the linked list of fragments and with the updated accumulated size and the datagram reassembly process is not attempted until at least a further fragment has been received for which the selected fragment list entry is selected.

Apparatus comprising:

• means for receiving a datagram, wherein the datagram is received subdivided into multiple fragments;
• means for storing fragment list entries, wherein a fragment list entry comprises a pointer to a linked list of fragments and an accumulated size for the linked list of fragments;
• and
• means for processing fragments to perform an atomic access to a selected fragment list entry in response to reception of a fragment, wherein the selected fragment list entry is selected in dependence on at least one datagram identifier in the fragment, wherein the atomic access comprises a read access followed by a write access;
• means for calculating an updated accumulated size as the accumulated size plus a size of the fragment,
• wherein the means for processing fragments is responsive to completion of the read access to perform the write access to the selected fragment list entry,
• wherein, when the selected fragment list entry comprises a size of the datagram and the updated accumulated size is at least equal to the size of the datagram, the write access comprises setting the selected fragment list entry to a null entry and when the write access is complete the means for processing fragments attempting a datagram reassembly process on the fragment and the fragments indicated by the linked list of fragments pointed to by the pointer of the selected fragment list entry,
• and wherein, when the updated accumulated size is less than the size of the datagram or the selected fragment list entry does not comprise the size of the datagram, the write access comprises updating the selected fragment list entry to include the fragment in the linked list of fragments and with the updated accumulated size and the datagram reassembly process is not attempted until at least a further fragment has been received for which the selected fragment list entry is selected.