求翻译：通过对Pull模型和Push模型的分析和比较可以看出，尽管在这两种单点登录模型中用户也只进行了一次身份认证，但每次申请新的服务时，Pull模型都要目标站点向源站点发出请求以使用户令牌生效，而Push模型都需要源站点向目标站点发出请求以获取用户令牌。这两种模型的结构是相似的只是运行流程有所不同。从上面的分析可以看出，基于这两种模型的单点登录系统面临如下三个问题：是什么意思？

The pull model and push model analysis and comparison can be seen, although in the single sign-on model of the two users only carried out once authentication, but each time you apply for a new service, the pull model should be the target site The site of origin makes a request to the user token effe

Through the Push Pull model and the model can be analyzed and compared in that, while the two single sign-on model, users can also only a single identity, but every time when applying for a new service, Pull model to target sites are to the source site makes a request to allow the user token, Push m

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By on Pull model and Push model of analysis and comparison can see, despite in this two species single points login model in the user also only for has once identity certification, but each application new of service Shi, Pull model are to target site to source site issued requests to makes user ord