X-P2P-PeerDist header: where does it come from?

Question

I'm seeing this HTTP header on some requests made to my server:

X-P2P-PeerDist: Version=1.0

I'm aware of this: http://www.faqs.org/patents/app/20110016220#ixzz3g3X8lSYF, but I would like to know what are the known clients sending that header.

Any idea?

UPDATE: as requested, i'm including the other headers sent with the request (I've obfuscated some private stuff that are related to our customer with ***):

GET http://***.com/ HTTP/1.1
Accept: application/x-ms-application, image/jpeg, application/xaml+xml, image/gif, image/pjpeg, application/x-ms-xbap, */*
Connection: Keep-Alive
Accept-Language: en-US
User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1; Win64; x64; Trident/7.0; .NET CLR 2.0.50727; SLCC2; .NET CLR 3.5.30729; .NET CLR 3.0.30729; InfoPath.3; .NET4.0C; .NET4.0E)
X-P2P-PeerDist: Version=1.0
UA-CPU: AMD64
Accept-Encoding: gzip, deflate, peerdist
Host: ***.com
Cookie: SMSESSION=***; OrgName=***; authCookie=***; ASP.NET_SessionId=***
Pragma: no-cache
Cache-Control: no-cache

The user agent seems to indicate IE11 (Trident/) in IE7 mode (compatible; MSIE 7.0), but I'm not getting the X-P2P-PeerDist header when I'm using this config, so I don't think it's directly related.

Answer 1

It is one of the headers used in the PeerDist peer content caching protocol. It has never been standardized and are not even registered with the IANA registry for HTTP headers.

It is known to be sent from some recent versions of Internet Explorer, .NET HTTP clients and SilverLight HTTP clients.

Answer 2

I’m relatively certain that this header is send when if the client is configured to use BranchCache and a request is made using WinHTTP. About BranchCache:

To optimize WAN bandwidth, BranchCache copies content from your main office content servers and caches the content at branch office locations, allowing client computers at branch offices to access the content locally rather than over the WAN. (Source).

When testing here locally, a client that has BranchCache enabled sends the X-P2P-PeerDist header, while a brand new client (no BranchCache configured) does not sent it.

The underlying protocol seems for is Peer Content Caching and Retrieval (MS-PCCRTP), as the section Message Syntax from the protocol description defines X-P2P-PeerDist as one of the HTTP header extension it uses.

However, the document BranchCache Deployment Guide for Windows Server 2008 R2 and Windows 7 lists other MS-PCCx protocols but not MS-PCCRTP. So I don't have an external evidence that this header is caused by a BranchCache enabled client.

Answer 3

I have seen this in IE11 + compatibility mode on Windows 7, connected to an intranet application and downloading a docx file. The request only appears when downloading a document. All requests are going through an F5 load balancer. In other non-download requests on the same browser and same application, the X-p2p-peerdist header is missing.