| Message ID | 20201209033346.83742-1-xie.he.0141@gmail.com |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [net-next] net: x25: Remove unimplemented X.25-over-LLC code stubs | expand |
From: Xie He > Sent: 09 December 2020 03:34 > > According to the X.25 documentation, there was a plan to implement > X.25-over-802.2-LLC. It never finished but left various code stubs in the > X.25 code. At this time it is unlikely that it would ever finish so it > may be better to remove those code stubs. I always wondered about running Class 2 transport directly over LLC2 (rather than Class 4 over LLC1). But the only LLC2 user was netbios - and microsoft's LLC2 was broken. Not to mention the window probing needed to handle systems that said they supported a window of (IIRC) 15 but would discard the 5th back to back frame. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Wed, Dec 9, 2020 at 1:21 PM David Laight <David.Laight@aculab.com> wrote: > > I always wondered about running Class 2 transport directly over LLC2 > (rather than Class 4 over LLC1). > But the only LLC2 user was netbios - and microsoft's LLC2 was broken. > Not to mention the window probing needed to handle systems that > said they supported a window of (IIRC) 15 but would discard the > 5th back to back frame. To me, LLC1 and LLC2 are to Ethernet what UDP and TCP are to IP networks. I think we can use LLC1 and LLC2 wherever UDP and TCP can be used, as long as we are in the same LAN and are willing to use MAC addresses as the addresses. X.25 layer 3 certainly can also run over LLC2. Linux actually has support for LLC1 and LLC2. User space programs can transmit data directly over LLC1 and LLC2 using "AF_LLC" sockets.
From: Xie He > Sent: 09 December 2020 22:54 > > On Wed, Dec 9, 2020 at 1:21 PM David Laight <David.Laight@aculab.com> wrote: > > > > I always wondered about running Class 2 transport directly over LLC2 > > (rather than Class 4 over LLC1). > > But the only LLC2 user was netbios - and microsoft's LLC2 was broken. > > Not to mention the window probing needed to handle systems that > > said they supported a window of (IIRC) 15 but would discard the > > 5th back to back frame. > > To me, LLC1 and LLC2 are to Ethernet what UDP and TCP are to IP > networks. I think we can use LLC1 and LLC2 wherever UDP and TCP can be > used, as long as we are in the same LAN and are willing to use MAC > addresses as the addresses. Except that you don't have any where near enough 'ports' so you need something to demultiplex messages to different applications. We (ICL) always ran class 4 transport (which does error recovery) directly over LLC1 using MAC address (a NUL byte for the network layer). This requires a bridged network and globally unique MAC addresses. Sending out an LLC reflect packet to the broadcast MAC address used to generate a couple of thousand responses (many would get discarded because the bridges got overloaded). > X.25 layer 3 certainly can also run over LLC2. You don't need X.25 layer 3. X.25 layer 2 does error recovery over a point-to-point link. X.25 layer 3 does switching between machines. Class 2 transport does multiplexing over a reliable lower layer. So you normally need all three. However LLC2 gives you a reliable connection between two machines (selected by MAC address). So you should be able to run Class 2 transport (well one of its 4 variants!) directly over LL2. The advantage over Class 4 transport over LLC1 is that there is only one set of retransmit buffers (etc) regardless of the number of connections. But this is all 30 year old history... David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Thu, Dec 10, 2020 at 1:14 AM David Laight <David.Laight@aculab.com> wrote: > > > To me, LLC1 and LLC2 are to Ethernet what UDP and TCP are to IP > > networks. I think we can use LLC1 and LLC2 wherever UDP and TCP can be > > used, as long as we are in the same LAN and are willing to use MAC > > addresses as the addresses. > > Except that you don't have any where near enough 'ports' so you need > something to demultiplex messages to different applications. Yes, LLC only has 256 "ports" compared to more than 60000 for UDP/TCP. > We (ICL) always ran class 4 transport (which does error recovery) > directly over LLC1 using MAC address (a NUL byte for the network layer). > This requires a bridged network and globally unique MAC addresses. > Sending out an LLC reflect packet to the broadcast MAC address used to > generate a couple of thousand responses (many would get discarded > because the bridges got overloaded). Wow, You have a really big LAN! > > X.25 layer 3 certainly can also run over LLC2. > > You don't need X.25 layer 3. > X.25 layer 2 does error recovery over a point-to-point link. > X.25 layer 3 does switching between machines. > Class 2 transport does multiplexing over a reliable lower layer. > So you normally need all three. Yes, I was just saying X.25 layer 3 can run over any reliable point-to-point links, including X.25 layer 2, LLC2 and TCP. > However LLC2 gives you a reliable connection between two machines > (selected by MAC address). > So you should be able to run Class 2 transport (well one of its > 4 variants!) directly over LL2. Yes. > The advantage over Class 4 transport over LLC1 is that there is > only one set of retransmit buffers (etc) regardless of the number > of connections. Right. But nowadays we have big enough memories for many buffers, so it may be preferable to make connections operate independent of each other. This way one lost frame wouldn't affect all connections. This is also why HTTP3 moved to QUIC instead of using TCP. > But this is all 30 year old history... Haha, we are talking about really old technologies.
diff --git a/Documentation/networking/x25.rst b/Documentation/networking/x25.rst index 00e45d384ba0..e11d9ebdf9a3 100644 --- a/Documentation/networking/x25.rst +++ b/Documentation/networking/x25.rst @@ -19,13 +19,11 @@ implementation of LAPB. Therefore the LAPB modules would be called by unintelligent X.25 card drivers and not by intelligent ones, this would provide a uniform device driver interface, and simplify configuration. -To confuse matters a little, an 802.2 LLC implementation for Linux is being -written which will allow X.25 to be run over an Ethernet (or Token Ring) and -conform with the JNT "Pink Book", this will have a different interface to -the Packet Layer but there will be no confusion since the class of device -being served by the LLC will be completely separate from LAPB. The LLC -implementation is being done as part of another protocol project (SNA) and -by a different author. +To confuse matters a little, an 802.2 LLC implementation is also possible +which could allow X.25 to be run over an Ethernet (or Token Ring) and +conform with the JNT "Pink Book", this would have a different interface to +the Packet Layer but there would be no confusion since the class of device +being served by the LLC would be completely separate from LAPB. Just when you thought that it could not become more confusing, another option appeared, XOT. This allows X.25 Packet Layer frames to operate over diff --git a/net/x25/af_x25.c b/net/x25/af_x25.c index d41fffb2507b..ff687b97b2d9 100644 --- a/net/x25/af_x25.c +++ b/net/x25/af_x25.c @@ -211,11 +211,7 @@ static int x25_device_event(struct notifier_block *this, unsigned long event, if (!net_eq(dev_net(dev), &init_net)) return NOTIFY_DONE; - if (dev->type == ARPHRD_X25 -#if IS_ENABLED(CONFIG_LLC) - || dev->type == ARPHRD_ETHER -#endif - ) { + if (dev->type == ARPHRD_X25) { switch (event) { case NETDEV_REGISTER: case NETDEV_POST_TYPE_CHANGE: diff --git a/net/x25/x25_dev.c b/net/x25/x25_dev.c index 25bf72ee6cad..5259ef8f5242 100644 --- a/net/x25/x25_dev.c +++ b/net/x25/x25_dev.c @@ -160,10 +160,6 @@ void x25_establish_link(struct x25_neigh *nb) *ptr = X25_IFACE_CONNECT; break; -#if IS_ENABLED(CONFIG_LLC) - case ARPHRD_ETHER: - return; -#endif default: return; } @@ -179,10 +175,6 @@ void x25_terminate_link(struct x25_neigh *nb) struct sk_buff *skb; unsigned char *ptr; -#if IS_ENABLED(CONFIG_LLC) - if (nb->dev->type == ARPHRD_ETHER) - return; -#endif if (nb->dev->type != ARPHRD_X25) return; @@ -212,11 +204,6 @@ void x25_send_frame(struct sk_buff *skb, struct x25_neigh *nb) *dptr = X25_IFACE_DATA; break; -#if IS_ENABLED(CONFIG_LLC) - case ARPHRD_ETHER: - kfree_skb(skb); - return; -#endif default: kfree_skb(skb); return; diff --git a/net/x25/x25_route.c b/net/x25/x25_route.c index ec2a39e9b3e6..9fbe4bb38d94 100644 --- a/net/x25/x25_route.c +++ b/net/x25/x25_route.c @@ -124,12 +124,7 @@ struct net_device *x25_dev_get(char *devname) { struct net_device *dev = dev_get_by_name(&init_net, devname); - if (dev && - (!(dev->flags & IFF_UP) || (dev->type != ARPHRD_X25 -#if IS_ENABLED(CONFIG_LLC) - && dev->type != ARPHRD_ETHER -#endif - ))){ + if (dev && (!(dev->flags & IFF_UP) || dev->type != ARPHRD_X25)) { dev_put(dev); dev = NULL; }
According to the X.25 documentation, there was a plan to implement X.25-over-802.2-LLC. It never finished but left various code stubs in the X.25 code. At this time it is unlikely that it would ever finish so it may be better to remove those code stubs. Also change the documentation to make it clear that this is not a ongoing plan anymore. Change words like "will" to "could", "would", etc. Cc: Martin Schiller <ms@dev.tdt.de> Signed-off-by: Xie He <xie.he.0141@gmail.com> --- Documentation/networking/x25.rst | 12 +++++------- net/x25/af_x25.c | 6 +----- net/x25/x25_dev.c | 13 ------------- net/x25/x25_route.c | 7 +------ 4 files changed, 7 insertions(+), 31 deletions(-)