The network layer provides services to the transport layer．It can be based on either (71) ．In both cases，its main job is (72) packets from the source to the destination．

In network layer，subnets can easily become congested，increasing the delay and (73) .

for packets．Network designers attempt to avoid congestion by proper design．Techniques include (74) policy，caching，flowcontrol，and more．

The next step beyond just dealing with congestion is to actually try to achieve a promised quality of service．The methods that can be used for this include buffering at the client，traffic shaping，resource (75) ，and admission control．Approaches that have been designed for good quality of service include integrated services(including RSVP) ，differentiated services， and MPLS．

（71）处填（）。

A．virtual circuits or datagrams

B．TCP or UDP

C．TCP or IP

D．IP or ARP

男，60岁。两个月前自觉中上腹深部偏右疼痛，持续进行性加重，常为钝痛，时有绞痛，体重减轻，皮肤瘙痒，排陶土样便。查体有黄疸，Courvoisier征阳性。

该病人最可能的诊断是

A．肝硬化

B．肝癌

C．胰头癌

D．胃癌