Re: DLT value for IP over IB (Infiniband)

[Guy Harris <guy () alum mit edu>]

On Jul 27, 2011, at 3:02 AM, Darren Reed wrote:

> With Solaris, the interfaces available from the driver and protocol stack prohibit access to actual packets at the 
> link layer. I don't know if this is or will be possible with Linux, but if the link layer header for IPoIB on Linux 
> is 12 bytes, then no, the data before the IP header that is exposed by Infiniband on Linux is not the link layer 
> header. Furthermore, the comments that I've received suggest that this type of access to network packets is not 
> possible with Infiniband.
>
> For ARP packets, the influence of Infiniband is simply on the size of the address placed in the ARP packets.
>
> The address used in ARP packets for Infiniband is the same across all implementations of IPoIB.
>
> So whilst the pre-IP header is different on Solaris and Linux for Infiniband packets, the Infiniband address placed 
> in the ARP packets is an Infiniband address and is not dependent on the implementation of IPoIB.
>
> Thus mapping ARPHRD_INFINIBAND to 32 will be fine for both Linux and Solaris.

Presumably "for Solaris" means that, for libpcap on Solaris 11, you have a choice of using BPF (which returns DLT_ 
values), PF_PACKET sockets (which returns ARPHRD_ values), and DLPI (which returns DL_ values)?  If it doesn't support 
using PF_PACKET sockets for capturing, libpcap-on-Solaris has no reason to care about ARPHRD_anything.

> For the DLT values, I'm going to use the names DLT_IPOIB and LINKTYPE_SOLARIS_IPOIB for Solaris 11. If a pair of 
> numbers can be assigned in the next 24 or so hours, I'll use those, otherwise it'll be DLT_USER15 for both. If I 
> understand correctly, the design is such that libpcap on Linux would then map DLT_IPOIB to LINKTYPE_LINUX_IPOIB

No.  As there are APIs in libpcap that are expected to return DLT_ values for savefiles, and as savefiles have 
LINKTYPE_ values in them (because there are some cases where different BSDs use different numerical values for the same 
DLT_ definitions - and, in at least some of those cases, BSD #1 uses a given numerical value for DLT_xxx and BSD #2 
uses that numerical value for DLT_yyy and a different numerical value for DLT_xxx - so we need a single LINKTYPE_xxx 
numerical value to correspond to all of the different numerical values of DLT_xxx), so there would have to be different 
DLT_s for LINKTYPE_SOLARIS_IPOIB and LINKTYPE_LINUX_IPOIB.

So Linux would, presumably, when opening an Infiniband interface, map ARPHRD_INFINIBAND (32) to DLT_LINUX_IPOIB, just 
as Solaris BPF would just return get DLT_SOLARIS_IPOIB and, if there's DLPI access to those interfaces, libpcap on 
Solaris's DLPI code would map DL_IPOIB or whatever to DLT_SOLARIS_IPOIB (if they have the same link-type header 
format).  libpcap on *all* platforms, and WinPcap on Windows, would map LINKTYPE_SOLARIS_IPOIB in a capture file to 
DLT_SOLARIS_IPOIB and would map LINKTYPE_LINUX_IPOIB in a capture file to DLT_LINUX_IPOIB to be returned by 
pcap_datalink().-
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