Using Stored Procedure Send Web Request Failed

I am tring to use a stored procedure to run a function POST some data through HTTP, but failed. Here are the codes I used. Would appreciate if anyone can help me about that. Thank you very much.

Database Stored Procedure:

CREATE PROCEDURE [dbo].[USP_HTTP_POST]
 @URL NCHAR(255),
 @CONTENT NCHAR(1024),
 @Result bit OUTPUT
AS
BEGIN
/***** Call Function to POST *****/
set @Result = NULL
EXECUTE @Result = [dbo].[udfHttpPost]
  @URL,
 @Content
END

Database Function:

CREATE FUNCTION [dbo].[udfHttpPost]
(@Url [nchar](255),
@Content [nchar](1024))
RETURNS [bit] WITH EXECUTE AS CALLER
AS
EXTERNAL NAME [SqlServices].[SqlServices.HttpServices].[HttpPost]

VC# Assembly Code

using System;
using System.Configuration;
using System.Collections.Generic;
using System.Text;
using System.Net;
using System.Security;
using System.Security.Permissions;
using System.IO;
using Microsoft.SqlServer.Server;

namespace SqlServices
{
 public class HttpServices
 {
  [SqlFunction(DataAccess = DataAccessKind.None)]
  public static bool HttpPost(string Url, string Content)
  {
   Url = Url.TrimEnd();
   Content = Content.TrimEnd();
   string rawOutput = "";

   //Get Access Right to Web
   WebPermission p = new WebPermission(NetworkAccess.Connect,Url);
   p.Assert();

   //Prepare the WebRequest
   WebRequest Req = (WebRequest)WebRequest.Create(Url);
   Req.Timeout = 60000;
   Req.Method = "POST";
   Req.ContentLength = Content.Length;
   Req.ContentType = "application/x-www-form-urlencoded";

   StreamWriter PostWriter = new StreamWriter(Req.GetRequestStream());
   PostWriter.Write(Content);
   PostWriter.Close();

   WebResponse Resp = Req.GetResponse();
   StreamReader sr = new StreamReader(Resp.GetResponseStream());
   rawOutput = sr.ReadToEnd();
   sr.Close();
   return true;
  } 
 }
}

System Exception:

A .NET Framework error occurred during execution of user defined routine or aggregate 'udfHttpPost':
System.Security.SecurityException: Request for the permission of type 'System.Security.Permissions.SecurityPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.
System.Security.SecurityException:
   at System.Security.CodeAccessSecurityEngine.CheckNReturnSO(PermissionToken permToken, CodeAccessPermission demand, StackCrawlMark& stackMark, Int32 unrestrictedOverride, Int32 create)
   at System.Security.CodeAccessSecurityEngine.Assert(CodeAccessPermission cap, StackCrawlMark& stackMark)
   at System.Security.CodeAccessPermission.Assert()
   at SqlServices.HttpServices.HttpPost(String Url, String Content)